|Year : 2017 | Volume
| Issue : 2 | Page : 83-108
Pathologic anatomy of the soft palate, part 2: The soft tissue lever arm, pathology, and surgical correction
Michael H Carstens
Division of Plastic Surgery, Saint Louis University, St. Louis, MO, USA; Department of Surgery Universidad Nacional Autónoma de Nicaragua, Leon, Nicaragua
|Date of Web Publication||11-Aug-2017|
Michael H Carstens
160 South Virginia Avenue, Falls Church City, VA 22046, USA
Source of Support: None, Conflict of Interest: None
Pathologic anatomy of the soft palate, part 2: The soft tissue lever arm, pathology, and reconstruction. In part two, we consider the soft tissue components of the soft palate: Epithelium, fascia, muscles, arterial supply, and innervation. These velar tissues constitute a functional “lever arm” for control of speech and swallowing. Fascia and peripheral nerves arise neural crest originating from rhombomeres 2–7. Muscles arise from paraxial mesoderm (PAM) of somitomeres 4, 6, and 7. Lateral plate mesoderm lying outside of PAM provides the building blocks of the circulatory system. Neurovascular analysis discloses the soft palate to have three developmental zones with distinct sources of neurovascular supply. Emphasis is placed on the anterior third of the palatine aponeurosis; this critical structure determines where the levator complex will insert. The basic field defect of soft palate clefts arises from insufficiency of the lesser palatine neurovascular pedicle affecting the posterior palatine shelf and anterior 1/3 of the palatine aponeurosis. This leads to forward displacement of the levator complex and pathologic insertion onto the bony margin of the cleft site. Soft-tissue disruption will then be presented in terms of the simple genetic loop between bone morphogenetic protein 4 (BMP-4) and Sonic hedgehog. The migration of soluble factors such as BMP-4 from their origin with developing bone to the free border of the epithelium permitting fusion of adjacent structures.
Keywords: Alveolar extension, buccinator, cleft palate, neuromere, palatoplasty
|How to cite this article:|
Carstens MH. Pathologic anatomy of the soft palate, part 2: The soft tissue lever arm, pathology, and surgical correction. J Cleft Lip Palate Craniofac Anomal 2017;4:83-108
|How to cite this URL:|
Carstens MH. Pathologic anatomy of the soft palate, part 2: The soft tissue lever arm, pathology, and surgical correction. J Cleft Lip Palate Craniofac Anomal [serial online] 2017 [cited 2019 May 22];4:83-108. Available from: http://www.jclpca.org/text.asp?2017/4/2/83/212829
| Soft Tissues of the Palate: the Lever Arm|| |
The following references are useful for an overall orientation to palate anatomy and soft palate development. This paper makes use of the neuromeric model that relates developmental units of the CNS genetically defined by homeotic genes to out-lying tissue units of neural crest, mesoderm, and the pharyngeal arches. These relationships, described in Part 1 of this series, are summarized in [Figure 1].
|Figure 1: Review of neuromeric model. Central nervous system encoded into developmental units, neuromeres, each with specific neuroanatomic content. Anatomic boundaries of neuromeres determined by a unique expression pattern of homeotic genes. Each neuromere shares its “barcode” with neural crest cells arising from the neural folds in the anatomic territory of the neuromere|
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Mucosa/fascia, and sensory innervation
The submucosa of the soft palate is analogous to the dermis. It is supplied by r2 neural crest on the nasal surface and a mixture of r2 and r6 neural crest on the oral surface. This explains the neuroanatomic distribution of fibers for soft palate pain (V2) and the gag reflex (XI). The fascia of tensor veli palatini comes from r3 neural crest. Proprioception from this muscle reports back to V3. All remaining palate muscles from Sm7 are ensheathed by r6–r7 neural crest fascia. Proprioception is referred to the pharyngeal plexus through the glossopharyngeal nerve. Taste buds represent the sole contribution of 2nd arch to the soft palate. Epithelial precursors from r5-r6 are incorporated into the mucosa.
Palatine aponeurosis and muscles
The palatine aponeurosis is a neural crest structure arising from rhombomeres r2 and r6-r7 associated, respectively, with the 1st and 3rd pharyngeal arches. It consists of genetically-encoded zones that “instruct” soft palate muscles to insert in a fixed temporal and spatial order. The soft palate thus constitutes a functional “lever arm” designed to make contact with the pharynx for purposes of swallowing and speech. The anatomy of the aponeurosis is depicted in [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7] (below).,,,
|Figure 2: Pinball model. The palate is a functional unit consisting of a bony platform and paired (fused) soft tissue lever arm designed to control speech and swallowing|
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|Figure 3: Size discrepancy. The function of the soft palate depends upon its ability to close a critical contact distance with the pharynx. In cleft palate, the platform to which the lever arm is attached (bone and anterior aponeurosis) is foreshortened. Despite an otherwise technically sound repair, if this tissue loss exceeds the critical contact distance, velophranyngeal incompetence can result. In this circumstance, tissue interposition may be required|
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|Figure 4: Palatine aponeurosis. Anterior 1/3 is nonmuscle-bearing. Tensor inserts into its lateral border only. Middle 1/3 bears the insertions of the muscles. Posterior 1/3 is mostly glands|
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|Figure 5: Aponeurosis is an autonomous neural crest structure - not a continuation of tensor veli palatini. Soft palate clefts always have some degree of deficit of the palatine bone. Absence of posterior nasal spine is pathognomonic. Aponeurosis is always deficient. Foreshortened aponeurosis brings levator complex into contact with hard palate|
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|Figure 6: Sagittal section of soft palate shows clamp on the levator veli palatini, the tensor veli palatini is lateral to sp. aponeurosis and the anterior 1/3 of which is muscle-free|
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|Figure 7: Spatial-temporal order of muscle insertions into aponeurosis. Nasal layer: Tensor veli palatini, palatoglossus, palatopharyngeus (nasal lamina) oral layer: Levator veli palatine, palatopharyngeus (oral lamina covers levator veli palatine). Note that uvulus is nasal to the levator complex|
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The aponeurosis is often described as an “extension” of the tendon of tensor veli palatini (TVP). This is a developmental impossibility because the aponeurosis is an r2 neural crest structure, whereas the tendon of TVP is r3 enclosing a mesodermal muscle belly from Sm4. The aponeurosis results from an epithelial–mesenchymal interaction between the r2 oral epithelium and r2 neural crest submucosa; it covers the entire surface area of the soft palate oral epithelium. The maturation gradient of aponeurotic mesenchyme is lateral-to-medial and proximal-to-distal. Palatal muscles move into position following these gradients. Thus uvulus, being last, is located in the midline and must arc over (nasal to) the levator sling.
The blastema of SM4 (TVP) is first seen lateral to the soft palate processes at Stage 20. At this stage, the shelves are vertical and demonstrate a mesenchymal condensation. The blastema of Sm7 (palatoglossus [PG], palatopharyngeus [PP], and levator veli palatine [LVP]) appears one stage later. By Stage 23 (8 weeks), the aponeurosis is present as a separate structure. Insertion of TVP into aponeurosis is complete at 9 weeks. The remaining 3rd arch muscles follow. Uvulus is last to appear.
Neural crest fasciae covering the individual muscles of the soft palate recognize signals produced by the aponeurosis. The muscles insert in a fixed spatial and temporal sequence according to the biologic “maturity” of the target. Those muscles arriving first insert anteriorly; those that come later must accept a posterior binding site. The direction of spread of the muscles (differentiation process) is progressively lateral to medial.
The soft palate is suspended in space by four muscle slings: two nasal (TVP and LVP) and two oral (PG and PP). The first nasal sling is TVP. Its insertion is limited, stopping short at the anterolateral border of the aponeurosis. PG forms the first oral sling. This muscle is rather thin; it inserts just behind the “bare area” of aponeurosis. The more substantial PP forms the second nasal sling. It covers 2/3 of the remaining muscular aponeurosis. LVP swoops downward as the second nasal sling of the soft palate. It normally inserts into the middle 1/3 of the aponeurosis. The second layer of PP overlaps the fibers of LVP. Uvulus is attached to the oral mucosa posterior to the raphe. It then arcs over the levator and reattached to the oral mucosa anterior to the raphe at the posterior nasal spine (PNS). The presence of uvulus physically precludes any contact between levator and the posterior margin of the palatine bone. In the soft palate cleft, the PNS is absent: Uvulus cannot insert. Instead, its two head remain divided and run along both cleft margins.
Sensory innervation of the soft palate
The oral and nasal mucosa of the proximal 1/3–1/2 the soft palate are supplied by V2. Application of a cotton swab will elicit touch, but no gag. The posterior half of the soft palate is supplied by IX. Stimulation comes the trigeminal nucleus a levels r6–r7 and will initiate a gag reflex from motor neurons in r6–r7 traveling through the IX and pharyngeal plexus of X.
Clinical significance of the palatine aponeurosis
As we shall see in our subsequent discussion of the vascular zones of the soft palate, the anterior zone is supplied by the lesser palatine artery (LPA) associated with the 1st arch. Into its lateral border are inserted the fibers of tensor veli palatini. The aponeurosis is a distinct structure; it does not represent any transverse extension of TVP.
Soft plate dissections by Vacher et al. comparing normal cadavers and cleft palate demonstrate several important points. (1) In normal specimens, the posterior edge of the hard palate has no muscular insertions save a few tendinous fibers of TVP located at the lateral margin. (2) The orientation of the muscles fibers posterior to the “bare area” of the palatine aponeurosis is transverse. (3) The palatine aponeurosis is not found to be a continuation of the TVP tendon but was a separate anatomic entity. The aponeurosis related to the oral mucosa but was attached to the posterior palatine margin, from which it could be readily detached. (4) The muscular sling that controls soft palate position is located in the middle 1/3 of the soft palate, as was previously described by Huang et al.
In the Vacher study, cleft palate patients displayed three critical differences from the normal. (1) The overall orientation of muscular fibers was not transverse, but anteroposterior. They were observed sweeping forward to insert into the palatine shelf. (2) The palatal aponeurosis (the nonmuscular part) was not observed in any of the cleft specimens. (3) These same findings were repeated in isolated soft palate clefts and in submucous clefts.
The basic defect in soft palate clefts is a deficiency state in the neuroangiosome that supplies the horizontal palatine shelf and the anterior “bare area” of the palatine aponeurosis. The purpose of this paper is to provide an in-depth biologic explanation for a phenomenon that has been correctly described in the past. Veau, in his groundbreaking 1931 monograph, attributed the shortening of the soft palate in patients with cleft palate to the absence of the aponeurosis interposed between the palatine bones and the muscles of the soft palate sling. Kriens defined the anterior aponeurosis as a distinct entity, not an extension of TVP. Furthermore, he defined the cleft palate state as (1) the incomplete development or absence of the anterior palatine aponeurosis; (2) forward displacement of anterior components of the musculoaponeurotic velum; and (3) pathologic fusion with the remaining bony elements.
As we shall see, these concepts generate a new algorithm for management of soft palate clefts with two distinct approaches. Tradition techniques rearrange the existing structures, attempting to gain length with the tissues at hand. The anterior palatine aponeurotic defect is not addressed. If velopharyngeal insufficiency (VPI) results, pharyngeal tissues are used to reduce the space. Fat grafts to the soft palate or pharynx have been described. Developmental field techniques reconstruct the missing Lego® piece, the aponeurosis with interposition buccal flaps. This approach is a logical response to VPI after an otherwise satisfactory primary repair.
Muscles of the soft palate
First pharyngeal arch: Mastication and ear drainage
Tensor veli palatini
Tensor veli palatini and medial pterygoid are considered to arise from a common embryologic anlage.,,,,[Figure 8], [Figure 9], [Figure 10]. Thus, TVP is a muscle of mastication. By stabilizing the soft palate, it assists in swallowing. The muscle arises from somitomere 4 and is innervated by V3 palatine branch of the nerve to medial pterygoid. Its fascia is made up of r3 neural crest. TVP has two distinct heads: scaphoid and tubal.
|Figure 8: Tensor veli palatini shown inserting into aponeruosis (lilac). All other muscles are removed. Soft palate muscles. Note “bare area” of anterior 1/3 palatine aponeurosis|
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|Figure 9: Stage 20: Tensor veli palatini appears with medial pterygoid. Palatine shelf: Vertical with tissue condensation = palatine aponeurosis|
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|Figure 10: Stage 24: Tensor veli palatini appears with medial pterygoid. Palatine shelf: Vertical with tissue condensation = palatine aponeurosis|
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The scaphoid TVP gains access to the soft palate by passing over superior constrictor. Its secondary insertion is through a tendon passing around the pterygoid hamulus but it does not insert into the hamulus. The tendon of TVP subsequently fans out into the anterolateral palatine aponeurosis. About 33% of cases, TVP forms an attachment to the maxillary tuberosity. Under normal circumstances, TVP does not insert into the palatine shelf.
Although TVP inserts into palatine aponeurosis, it does not contribute to the aponeurosis, these are developmentally distinct structures (De la Cuadra). In patients with cleft palate, abnormalities of TVP insertion have been documented. These include attachments to the horizontal palatine plate and even into the lateral pterygoid process, including hamulus. It is thought that such a slackened or misdirected insertion of TVP may predispose to auditory tube dysfunction.
TVP is the first muscle to develop in the soft palate and is enveloped by r3 neural crest fascia. The palatine aponeurosis is also a neural crest structure into which all the remaining four palatal muscles insert. Deficiency states of r2 and the lesser palatine neuroangiosomes that affect the horizontal palatine plate are concomitantly shared with the anterior palatine aponeurosis – making it foreshortened. In this case, LVP, which normally inserts into the middle 1/3 of the aponeurosis, posterior to the fibers from TVP, will be displaced forward. The deficiency of the aponeurosis is readily confirmed when placing interposition buccinator flaps. The resultant gap is often 1.5–2 cm.
The tubal TVP has its primary insertion along the membranous anterolateral wall of the pharyngotympanic tube including the isthmus, where the bony lateral 1/3 meets the cartilaginous medial 2/3. Some studies describe TVP as exerting traction on the tube to open it. This makes sense because the lateral half of the Eustachian tube originates from the 1st arch. Maneuvers such as jaw opening and swallowing to alleviate ear pain from changes in pressure when flying are based on mechanical traction on to open the tube. TVP and tensor tympani (TT) do not represent a digastric muscle. They are anatomically distinct.
TT also arises from somitomere 4 and is innervated by V3 palatine branch of the nerve to medial pterygoid. Its fascia is made up of r3 neural crest. TT is related in function to muscles of mastication acting on the mandible. This is because the bone fields of the primitive tetrapod mandible (derived from r3 neural crest) become incorporated into the middle ear. Angular bone forms the ectotympanic ring around the eardrum. Prearticular become the goniale. Articular forms the actual malleus; it is in physical contact with quadrate, a derivative of the primitive maxilla known as the palatoquadrate cartilage. As one would expect, quadrate becomes the incus, with which malleus articulates. In practical terms, TT acts to stabilize and protect the eardrum.
Second pharyngeal arch: Embryologic linkage between the 2nd and 3rd arches
Levator veli palatini
Several lines of evidence point to LVP as having components from both the 2nd and 3rd archs. As such, LPV forms part of the 2nd arch muscles of deep investing fascia, muscles involved with mastication such as posterior digastric and stylohyoid [Figure 11] and [Figure 12].,, LVP is also functionally related to TVP at the eustachian tube. Note that directly next to stylohyoid is the 3rd arch stylopharyngeus. Thus, LVP arises from myoblasts form the most posterior sector of Sm6 and the most anterior sector of Sm7. For this reason, the facial artery of 2nd arch gives off an ascending palatine branch to the soft palate. Thus the innervation of LVP included fibers from facial nerve transported via greater petrosal nerve of VII to the lesser palatine nerve.
|Figure 11: Levator veli palatini: Sm7 innervated by X from rhombomere 6. Inserts middle 1/3 of palatine aponeurosis; note intervening bare zone of aponeruosis between horizontal plate and levator veli palatine. Note: Tensor veli palatini seen, palatoglossus, palatopharyngeus, uvulus removed|
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|Figure 12: Soft palate muscle sling occupies the middle 1/3 of the aponeurosis. Palatoglossus, palatopharyngeus, uvulus all originate from somitomere 7. Palatoglossus and nasal layer of palatopharyngeus form slings posterior to tensor fibers. Palatoglossus is deep to palatopharyngeus oral layer of palatopharyngeus overlaps levator. Only muscle truly inserted into horizontal plate is uvulus|
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Buccinator from 2nd arch and superior constrictor (SC) m from 3rd arch abut against each other with an intervening raphe to form a common oropharyngeal sphincter. This juxtaposition may imply that myoblasts of distal levator veli palatini are derived from somitomere 6 and retain a motor relationship with facial nerve as the most posterior component of the buccinator complex.
This relationship explains the contribution of the ascending palatine branch from facial artery – a derivative of the 2nd aortic arch artery. It also explains the association of levator weakness (not paralysis) with microtia, the latter condition being primarily a 2nd arch defect. Recall that the posterior three cartilaginous hillocks of the auricle are the 2nd arch derivatives. When these are absent, the ipsilateral LVP does not elevate equal with that of the opposite side.
Innervation of LVP by facial nerve has been described. Stimulation of VII nerve produces movement in the upper nasopharynx while stimulation of IX/X causes upper movement of the velum.
Third pharyngeal arch: The workhorse of the soft palate
Levator veli palatini
Levator veli palatini is only present in mammals. It has a dual motor supply.,,,, Primary motor control is from r6 and through cranial nerves IX and X to the pharyngeal plexus (see below). It is also supplied by lesser palatine nerve, a branch of the V2 complex. However, left peroneal nerve is not purely sensory; it receives two types of efferent fibers from the facial nucleus of cranial nerve VII to the soft palate. (1) Visceral motor fibers bring parasympathetic control to the palatine salivary glands. (2) Branchial motor fibers supply the distal portion of LVP these fibers originate in facial nucleus travel to the geniculate ganglion and then are conducted through the superior petrosal nerve to the pterygopalatine ganglion, at which point they gain access to V2 and then travel through LVP to the soft palate. The unappreciated contribution of VII to the soft palate causes lateral motion in the palatal plane, a function perhaps related to speech.
Levator veli palatini and superior constrictor are considered to arise as a common embryonic anlage from somitomere 7. The muscle has been identified by computed tomography (CT) scan at Stage 21. It becomes readily identifiable by Stage 23.
LVP and superior constrictor (SC) share a common motor supply with three variations: pharyngeal branch of IX, communication branch between pharyngeal branch of IX and X, and the pharyngeal branch of X. In all cases, the cell bodies for these nerves are located in the anterior sector of the nucleus ambiguous corresponding to r6–r7.
The primary insertion of LVP is from the inferior surface of petrous temporal bone just in front of the carotid foramen. It is also inserted into the inferior/medial cartilaginous part of the pharyngotympanic tube. This makes sense because the Eustachian tube – representing the first pharyngeal pouch – is buried on its medial side by the 3rd arch. Contraction of LVP displaces the cartilage superior, medial, and posterior. Its secondary insertion passes through the fibers of PP to join with its contralateral muscle. LVP is above (nasal) to the bulk of PP fibers. In so doing, the levator sling attaches to the palatine aponeurosis in middle 50% of the soft palate (including the uvulus). LVP does not normally insert anteriorly into bone. However in cleft palate states, LVP is uniformly pulled forward because the palatine aponeurosis is foreshortened. As a consequence, LVP is pathologically inserted into the horizontal lamina of the palatine bone.
PG has its primary insertion in the midsection of the palatine aponeurosis, forming a sling anterior to PP and posterior to the TVP insertion zone into the aponeurosis. It is in direct contact with the oral surface, deep to PP and levator veli palatini (Kluber, Fara). Toward the midline, fibers of PG pass upward to intertwine with those of PP and LVP. As it leaves the palate, it passes downward, forming the lateral boundary of the tonsillar fossa, and inserts into the intrinsic transverse muscle fibers of the tongue. The insertion is superficial/cranial to that of styloglossus and of glossopharyngeus. The function of PG is depression of the soft palate and to a lesser extent elevation of the tongue. This assists in the latter stages of swallowing to direct the food bolus to the pharynx (Kuehn).
Like LVP and PP, PP arises from somitomere 7 and shares a common innervation with LVP. Like LVP, PP forms a common anlage with superior constrictor, with which it forms a common sheet. In the lateral and posterior walls of the pharynx, dense interconnections between the two muscles have been described. A unique feature of this muscle is two primary insertions into two distinct planes. The split to embrace LVP. The anterior fasciculus is thicker and runs longitudinally. It is attached to the palatine aponeurosis medial-to-TVP and below (oral) to levator. The posterior fasciculus is (nasal) to levator and unites transversely in the posterior midline with the contralateral muscle. At the posterolateral margin of the soft palate, the layers converge and descend, forming the medial boundary of the tonsillar fossa. The fibers are closely associate with stylopharyngeus. The two muscles attach to the posterior thyroid cartilage (4th arch derivative). Thus, PP spans the entire posterior border of the 3rd arch to insert into the boundary with 4th arch.
These thin, paired muscles arise from somitomere 7. [Figure 13], [Figure 14], [Figure 15].,,, They extend backward from the PNS along the midline of the palate to gain a secondary insertion into the palatine aponeurosis. The muscles arch of the palatal muscul in the dorsal midline. Immediately deep to the palatal muscles and in parallel with musculus. Uvulae is the fibrous palatine raphe, formed by the midline fusion of the two mesenchymal masses of the soft palate. uvulae run at right angles to levator and are thought to create a “levator prominence” that assists the levator in closing off the nasopharynx.
|Figure 13: Musculus uvulae consists of paired muscle bellies running forward in the midline, arching over levator and inserting into the posterior nasal spine. Blood supply to this zone is an anastomosis between lesser palatine artery (anterior 1/3) and ascending palatine branch of the facial artery with additional supply from ascending pharyngeal. Lesser palatine artery represents 1st arch, APF represents the 2nd arch, and ascending pharyngeal is derived from 3rd arch. Recall that 1st and 2nd arches are a composite unit. The character of the soft palate changes with ascending pharyngeal|
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|Figure 14: Coronal section of middle 1/3 shows the midline longitudinal raphe on the oral side, the intervening transverse fibers of levator and paired uvulus muscles on the nasal side|
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|Figure 15: Relationship of uvulus and the palatine raphe. Uvulus arches over the levator complex|
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In cleft situations, where no PNS is present, wisps of uvulus are always seen running all the way forward along the medial border of the cleft and attaching to bone. It is important to release them because they tend to tether the levator at the cleft margin. The fact that, in cleft palate, musculus uvulae retains its bulk at the uvulus while becoming attenuated at the bone margin attests to the overall posterior-to-anterior growth pattern of the muscle. The absence of PNS creates the conditions for a false insertion of levator into the palatine bone.
The insertion patterns of all three constrictors follow from the biologically “older” zone of pharyngeal arch structures dorsally to the biologically more recent cervical zones. These are neuromerically more distal. The thinnest of the three constrictors, the superior constrictor (SC) arises within somitomere 7. Its motor supply comes primarily from glossopharyngeal or from IX via X. Its primary insertion is bony (to the pterygoid hamulus) and more caudally, to the fibrous structures of the pterygomandibular raphe and the mylohyoid line of the mandible. Its secondary insertion sweeps backward to the pharyngeal tubercle of the occipital bone and follows the median pharyngeal raphe. Thus, SC spans from between r2 and r3 (buccinator) back to levels c1–c4. Its upper border with the skull base transmits tensor veli palatini and the Eustachian tube. Its lower border with middle constrictor contains stylopharyngeus and the glossopharyngeal nerve evidence for the 3rd arch relationship of this muscle.
Migration of this Sm7 muscle from takes place after that of the superior constrictor. Its primary insertions are therefore more caudal: (1) superior fibers track external to SC to seek out the hyoid bone, attaching to the superior and inferior cornua, (2) middle fibers go out transversely, and (3) the inferior fibers insert into the pharynx, passing deep to inferior constrictor. The secondary insertion of MC tracks dorsal to SC and extends inferior to it along the median pharyngeal raphe. Thus, MC spans from r5 (lesser cornu) and r6 (greater cornu) distally to levels c3–c6. The space between middle constrictor and inferior constrictor transmits the internal branch of laryngeal nerve and the laryngeal branch of superior thyroid artery (the primary blood supply for the 4th arch. This space is the boundary between the 3rd arch and the 4th arch. Nota bene: (1) pharyngeal plexus lies along the lateral surface of the middle constrictor. The muscle may also receive fibers from IX via X. (2) At the hyoid cornu, MC lies deep to hyoglossus, a derivative of occipital somite 2. This indicates that as the tongue muscles migrate from somites 2–5 they must skirt around the pharyngeal arches before accessing the floor of the mouth.
Fourth pharyngeal arch: Separation of the gut from the airway
Though not technically part of the soft palate, a few comments are worthwhile to better understand the anatomic boundaries of the 3rd and 4th arches. These are best mapped out from a sensory standpoint. The base of the tongue and the valleculae arise from the 3rd arch. Their mucosa leads downward to the larynx and is supplied by IX, as are the tastebuds of the posterior 1/3 of the tongue. The epiglottis is a 4th arch structure has taste fibers supplied by X. The entire laryngeal cartilage is a derivative of r8–r9 neural crest. Vagus from r8 to r9 sector of NA supplies the external laryngeal nerve that is motor to both inferior constrictor and cricothyroid.
The muscle arises from somitomere 8 and relates to the 4th and 5th pharyngeal arches. Its motor supply is exclusively vagus. Inferior constrictor has two parts. (1) Thyropharyngeus relates to the 4th arch. It inserts into the oblique line of the thyroid cartilage. Its secondary insertion is into the pharyngeal raphe dorsal and distal to its predecessor. Thus, inferior constrictor spans from r8–r11 backward to attach at levels c5–c8. (2) Cricopharyngeus relates to the 5th arch. Its primary insertion is the attachment between the articular facet of the thyroid cornu and the cricothyroid cartilage. The caudal margin of inferior constrictor is embraced by the thyroid and abuts against esophagus. At this point, roughly at neuromeric level c7, the pharyngeal raphe terminates. So, cricopharyngeus inserts into the esophagus.
Two zones of cricopharyngeus have been described. These represent potential anatomic weak spots. The upper pars oblique continues to insert into the pharyngeal raphe, whereas the lower pars fundiformis merely forms a circular band around the esophagus – without the reinforcement of the raphe. Killian's triangle lies between these two zones. Laimer's triangle is located inferior to pars fundiformis and the circular esophageal fibers. At both these sites, diverticula can occur. Note that external laryngeal nerve to the larynx descends superficial to IC and penetrates it to supply cricothyroid, whereas internal laryngeal nerve supplying the arytenoid muscles lies deep to IC. Thus, IC constitutes a plane separating motor nerves supplying 4th arch muscles from 5th arch muscles.
Blood supply to the soft palate
The soft palate is divided into distinct developmental fields, each of which is defined by a separate neurovascular pedicle. Fields defined in this way are called neuroangiosomes, based on the pioneering work of Taylor and Palmer. Mercer demonstrated the predominant neuroangiosomes of the soft palate to be the ascending palatine brance of the facial artery (APF) and ascending pharyngeal artery (APhA). These findings were confirmed by Maistry's examination of the vascular anatomy of the soft palate in 100 specimens.
Three divisions were described by Maistry. (1) Proximal soft palate runs from the posterior border of the hard palate to the point of attachment of superior margin of levator veli palatini and the origin of the superior border of PG. The proximal soft palate thus consists of insertions of TVP into its anterolateral borders and a bare area of fascial without a muscle sling, save for midline fibers of uvulus. The predominant blood supply is (2) middle soft palate lies between the superior and inferior margins of LVP and PG. Its blood supply is mixed between ascending branch of facial (2nd arch) and ascending pharyngeal (APh) (3rd arch). (3) Distal soft palate lies inferior to the inferior margins of insertion of LVP and PG. Very importantly, the vessels run with the nerves on the oral surface of the soft palate. Its predominant supply is APh (3rd arch). These findings are consistent with developmental studies by Grimaldi et al. and with the origin of soft palatal musculature in somitomere 7 as mapped by Noden and Francis-West.
Neuroangiosomes of the soft palate (emphasis on the ascending palatine branch of facial and on ascending pharyngeal)
Lesser palatine artery
Recall that all StV2 and StV3 branches arise from a common trunk that is distal to the anastomosis of the extracranial stapedial system to the stump of the internal maxillo-manidbular artery (IMMA). This branch supplies all 1st a pharyngeal arch structures of non-neural crest derivation. Distal to this anastomosis, the hybrid maxillo-mandibular artery runs forward to the sphenopalatine fossa where it gives rise to multiple branches, each of which follows a branch of V2. LPA supplies the palatine aponeurosis of tensor veli palatini, along with TVP proper, the sole 1st arch muscle in the soft palate. Thus, its territory of distribution is specific to the proximal soft palate, but it shares this with ascending palatine branch of the facial artery. LPA does not supply the levator per se LPA selectively supplies the oral surface of the soft palate mucosa. Anastomoses between descending palatine artery (DPA) and APh enhance the perfusion of the latter [Figure 16].
|Figure 16: Descending palatine gives off greater and lesser palatine arteries. The lesser palatine artery supplies tissues via multiple foramina directed posteriorly|
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Ascending palatine branch of facial artery
Ascending palatine artery (APF) arises from the proximal facial artery [Figure 17]. It is not a stapedial derivative. Facial artery serves 2nd pharyngeal arch structures and anastomoses with 1st arch structures as well. It represents a remodeling of the intermediate part of the ventral pharyngeal artery, itself derived from the original 2nd aortic arch artery.
|Figure 17: Ascending branch of facial embraces the posterior aspect of tonsil and proceeds to the soft palate. Tonsil sandwiched between palatopharyngeus and palatoglossus. Mucosa in front of tonsil is 1st arch (V3), behind tonsil is 3rd arch (IX). Tonsil: Endoderm in caudal 2nd pharyngeal pouch between 2nd arch and 3rd arch. Levator veli palatini has dual innervation: VII + IX|
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APA passes upward along stylopharyngeus (a 2nd arch muscle) until it reaches LVP, at which point it divides. One branch is directed to the middle third of the soft palate. The other proceeds upward to supply superior constrictor. This attests to a continuous fascial plane extending backward from buccinator (2nd arch) to superior constrictor. The space between superior and middle constrictor admits the passage of IX and therefore constitutes a biologic boundary between the neural associated with r4–r5 (2nd arch) and that of r6–r7 (3rd arch). APA is not part of the vascular territory of middle and inferior constrictor – these belong to APh.
With the exception of LVP, the soft palate does not contain 2nd arch muscles, so why should it be irrigated by APA? The answer may lie in the neurologic components of the soft palate. From the geniculate ganglion of the 7th cranial nerve, preganglionic paraympathetic autonomic nervous system (PANS) motor fibers for salivary glands and special sensory fibers for taste travel forward in the greater petrosal nerve; this eventually targets the pterygopalatine ganglion. From this site, postganglionic PANS fibers and taste fibers flow downward into the palate through the lesser palatine nerve (sensory V2). Thus, the soft palate receives motor supply for the salivary glands and taste receptors all the way down its the oral surface.
Greater petrosal nerve conveys motor fibers as well. These have been demonstrated in the distal levator veli palatini. LVP thus has a small component contributed by somitomere 6 and represents a posterior connection between the 2nd arch buccinator the 3rd arch superior constrictor. The motor response of the soft palate to isolated seventh cranial nerve stimulation suggests an additional form of control useful for speech.
Ascending pharyngeal artery
APhA is the artery of the 3rd pharyngeal arch [Figure 18] and [Figure 19]., It represents the remodeling of the most proximal part of the ventral pharyngeal artery, itself derived from the 3rd aortic arch artery. APhA arises from the medial aspect of external carotid artery. The rationale for APhA in the soft palate is based on the origin of the soft palate muscles (save TVP) which arise from somitomere 7 and (subsequently) populate the 3rd pharyngeal arch. Shortly after its take-off from the external carotid, APh splits into two main trunks. Careful study of this anatomy provides valuable insights into the derivatives of the 3rd arch.
|Figure 18: Ascending pharyngeal artery divides into pharyngeal and neuromeningeal trunks. The anastomosis with descending palatine (via lesser palatine) is critical the anterior zone of aponeurosis and the horizontal plate of palatine bone|
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|Figure 19: Blood supply to the palate. Descending palatine (2) giving lesser palatine and (1) greater palatine. Ascending palatine branch of facial (3) anastomoses with lesser palatine. Ascending pharyngeal not shown but supplies middle 1/3 and posterior 1/3|
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APhA selectively supplies the nasal surface of the soft palate mucosa. It functions in a gradient with ascending palatine from the facial. APF and APhA both supply the muscular zone of the middle third of the soft palate. The posterior third is selectively supplied by APhA.
The pharyngeal trunk is anterior and extracranial. It gives off three divisions: The superior, middle and inferior branches are distributed into the pharyngeal submucosa space. These supply, respectively, the three corresponding constrictor muscles... inferior constrictor receiving superior thyroid artery as well. The constrictors arise from Sm7, Sm8 and possible the 1st somite. Superior or medial pharyngeal artery supplies the medial wall of the Eustachian tube, the site of primary insertion for levator veli palatini. The lateral wall of the Eustachian tube is the primary insertion of tensor veli palatini. Thus, the auditory tube is a sandwich, with 1st arch elements lateral and 3rd arch elements medial. The distal extension of superior pharyngeal, the pterygo-vaginal artery, makes a clinically important anastomosis with DPA. In LeFort osteotomies, trauma to the DPA is compensated by the palatovaginal artery, providing an important source of blood supply for adequate wound healing.
The neuromeningeal trunk is posterior and intracranial, entering the skull through the foramen magum, where it divides into hypoglossal and jugular branches. Hypoglossal branch is distributed to the posterior fossa through the hypoglossal canal where it supplies the meninges and the vasa nervorum of cranial nerve XII. It also sends a descending branch to the odontoid process and richly supplies the dens. It supplies cervical vertebrae C1, C2, and C3, where it anastomoses with the vertebral artery.
Readers familiar with the material in previous work by this author will recall the evolutionary significance of the absorption of proatlas (originally a 5th somite derivative) into the skull base. This explains anatomic features such as the condyles (the transverse processes of proatlas). It is also the rationale for the seeming contradiction of 8 cervical somites but only 7 cervical vertebrae. The “lost” vertebral body, the proatlas, now incorporated into the mammalian skull base, is faithfully served by this system. Thus, the interface between the APhA and the vertebral system represents the union of derivatives from the pharyngeal arch and occipital somites with those derived from cervical somites. Jugular branch also is distributed to the posterior fossa through the jugular foramen. It supplies the vasa nervorum of cranial nerves IX, X, and XI. A small branch serves the internal auditory canal. The inferior tympanic branch provides an interesting anastomosis between the petrosquamous branch of middle meningeal (1st arch derivative) and the stylomastoid artery (2nd arch derivative) supplying the facial nerve.
| Development of the Soft Palate|| |
The sequence of the embryologic events forming soft palate is summarized by Carnegie stages in [Table 1] below.
The primary and secondary palatal processes begin to develop at Stage 17. This is well demonstrated in the monograph by Hinrichsen. The biologic signal for tissues to accumulate and to project outward as the maxillary shelves of the hard palate is located at the interface between two neuroangiosomes. The nasal side is the territory of lateral nasopalatine, whereas the oral side is the territory of the greater palatine branch of DPA. The biologic signal for the palatine shelves of the hard palate is also an interface between three neurovascular axes. The territory of lesser palatine branch of DPA (1st arch) projects the mixed territory of ascending palatine of facial (2nd arch), and APh (3rd arch). It should be emphasized that the representation of 2nd arch in the midportion of the soft palate is minimal. Intraoral representation of 2nd arch is virtually nonexistent.
The tissues of the soft palate also reflect the apposition of 1st and 3rd arch derivatives. Representation of 2nd arch is minimal, being relegated to taste buds and glands. This is seen in the juxtaposition of TVP versus LVP. Halfway back in the soft palate the arterial supply to the muscles becomes mixed as well between lesser palatine and APh. Innervation of the soft palate mucosa is also mixed between somatic sensory from V2 and visceral sensory from IX.
Bone growth drives soft tissue positioning
In conjunction with the medial growth of the horizontal palatine shelf, the mucoperiosteum of the bone drags attached pharyngeal mucosa into the oral cavity. Myoblasts that have accumulated in the lateral pharyngeal wall are spatially positioned according to their final relationships. These are brought along passively. Craniofacial muscles masses develop in conjunction with a fascial envelope or epithelium which provide the program for their boundaries. All their connections to the skull base, Eustachian tube, pharynx, and the 3rd arch base of the tongue are already present. During the process of bone formation, bone morphogenetic protein 4 (BMP-4) signals are elaborated, as previously described. These diffuse outward through the soft palate, permitting epithelial breakdown and fusion to occur in anterior-to-posterior gradient.
The key concept to bear in mind regarding the soft palate muscles is that their spatial relationships are predetermined within their somitomeres of origin. They migrate in a predetermined spatial-temporal order and assemble themselves at the “jump-off point” in the lateral pharyngeal wall behind the palatine bone. From here, they migrate into place over the palatine aponeurosis where they each encounter a binding site and assume their final functional relationships.
Three-dimensional relationships of palatal muscles
As the horizontal palatine shelf begin to proliferate into the oral cavity, five groups of myoblasts are arranged along the lateral pharyngeal wall, awaiting their turn to be positioned into space. They have the same spatial order in the wall and bear the same attachments as in their final state. Uvulus is embedded in the midline. Just lateral, the tensory/levator complex is attached to the primitive pharyngotympanic tube. PG extends forward, and PP is directed backward. Initially, when the palatine shelf first buds inward, these five myoblast groups are all clustered together. But with rapid embryonic growth, they all are transported away from each other, like an expanding supernova. Thus, the palatal muscles in their final configuration are strung out like guy wires suspending the palate in space to the structures that surround it.
All five palate muscles attach to the palatine aponeurosis. Tensor attaches first to the lateral margin. Its fibers of insertion are localized at that site. Although it exerts traction on the aponeurosis, it does not form a sling per se three muscle slings of the 3rd arch (LVP, PG, PP) all insert into the mid-portion of the palate at about the same time. PG lies deep and anterior to PP. The anterior margin of PG respects the “bare area” of the aponeurosis. PP is much broader and has two insertions which enclose levator. The deep insertion makes direct contact with aponeurosis. The superficial insertion is admixed with levator. Levator swoops downward in the middle 1/3 of the palate to attach to a diamond-shaped zone, its fibers intermingling with those of PG and PP. It can be argued that levator has more of a muscular insertion into itself and its companions that into the palatine aponeurosis per se.
Deep to the three muscle slings on the oral side, a midline condensation of fibrous tissue, the palatine raphe, runs all the way forward to the palatine bone but does not insert into it. Superficial to the slings on the nasal side, paired uvulus muscles run the length of the soft palate to insert into the PNS and the anterior and posterior raphe.
Thus, the final order of spatial-temporal development of soft palate muscles is anterior-to-posterior, lateral-to-medial, and oral-to-nasal: Tensor, PG, PP – admixed with levator, uvulus.
Two muscles of the soft palate are directly attached to the palatine shelf. Tensor sends a few fibers very laterally to the maxillary tuberosity. Uvulus inserts into PNS. With the failure of the PNS to develop, uvulus cannot attach, leaving an opening for levator to insert opportunistically into the bone.
How do mesenchymal deficiencies of bone fields occur?
Each bone field is supplied by a neuroangiosome. It includes a genetically-paired nerve and artery that run in tandem with each other. Each one has a growth cone with stem cells at the tip. The neural growth cone secretes vascular endothelial growth factor while the arterial growth cone secretes nerve growth factor. As tissue development proceeds, “crosstalk” between the nerve and the artery enables them to grow forward into space. As they do so, they continuously nourish the mesenchymal tissues around them. They act as a “spillway” allowing continuous expansion of the tissues they support. In fact, the anatomy of neuroangiosome is just a reflection of how tissues arrive at their destination.
If, for some reasons, growth cone failure occurs from either component of the neuroangiosome, tissue development will be arrested at that very point. Endogenous causes include inadequate stem cell numbers or function, so the growth cones stop advancing. Exogenous causes can be due to hypoxia or toxins. Regardless of etiology, growth failure always follows a distal-to-proximal gradient. The most distal zone of a field is the most vulnerable. As we have seen, the pattern of hard palate development demonstrates these concepts perfectly. As the artery develops, it progressively adds mesenchyme. The greater palatine axis keeps adding branches from anterior to posterior. Thus, mesenchymal insufficiency of the maxillary shelf shows up posteriorly and medially and progresses forward. The lesser palatine axis supplying the horizontal plate adds bone from lateral to medial and from anterior to posterior. Thus, PNS is the most vulnerable to deficiency.
| Neurology of the Soft Palate|| |
Innervation of the soft palate is reviewed by Logjes et al. The classification of pharyngeal muscles based on innervation is discussed by Sakamoto.
A note to the reader
This section, despite its seemingly arcane detail, is extremely important to understand the structural layout of the palate… and indeed, the entire pharynx. We have all learned that each pharyngeal arch has its own assigned cranial nerve. This system seems to hold true for the 1st and 2nd arches – although from a functional standpoint, the VII has no somatic sensory role. The 3rd arch marks a critical transition point. We have seen that striated muscle contents of the 3rd arch, originating in Sm7, are much more extensive than appreciated. The entire sensory supply of the 3rd arch mucosa and glands is IX, yet only a single muscle is considered to be innervated by the glossopharyngeal nerve. Instead, we find the motor supply for these muscles to be ascribed to the vagus. Yet, the vagus nerve is officially assigned to the 4th and 5th arches. Hence, we have a dilemma. Now that we know where the soft palate muscles come from, how can they be innervated by the 4th arch?
The answer to this resides in inadequacies of conventional neuroanatomy which was conceived before the era of molecular neuroembryology. These contradictions are corrected and explained by the neuromeric model. They help us to understand that organization of tissues supplied by the medulla r6–r11. To get oriented, recall that neural crest from r6–r7 supplies the 3rd arch, all the muscles of which arise from somitomere 7. Neural crest from r8–r11 supplies the 4th and 5th arches and flows into the occipital somites (S1–S4) as well.
Many anatomic changes take place at level r8: The medulla is a very busy place
- Changes in segmentation – The nonsegmented cephalic mesoderm of somitomeres Sm1–Sm7 gives way to the segmented somite system. This starts with the conversion of the 8th somitomere into the 1st somite, a process that continues all the way backward to the tail of the embryo
- Somites are more anatomically complex. Somitomeres are simple balls of PAM. Somites are compartmentalized. The occipital somites (S1–S4) have sclerotomes which are assembled into the chondral bones of the posterior cranial fossa and myotomes dedicated to muscles of the tongue. Dermatomes do not appear until the 2nd cervical somite, S6
- Positioned just lateral to medulla are two different mesenchymal structures – Pharyngeal arches 4 and 5 and the first four occipital somites. They have different innervations and different blood supplies
- Neural crest migrates in two ways. Neural crest emanating from r8–r11 flow into the 4th and 5th pharyngeal arches, using a 2:1 ratio. Thus, rhombomeres 8 and 9 send their crest to the 4th arch, whereas rhombomeres 10–11 supply the 5th arch. Within the arches, the crest cells organize the mesoderm into distinct muscle masses. Neural crest enters somites on a 1:1 ratio. From rhombomere 8 backward, each neuromere supplies its own designated somite. Within the somites, neural crest also organizes myoblasts within the myotome
- The medulla innervates two functional groups of muscles, innervated by two spatially distinct motor columns. (1) Branchiomeric muscles arise when PAM migrates outward at the somitomeric stage, before the somite transition). They originate from somitomeres 7–9. Those from Sm8–Sm9 probably develop and migrate before the somitomere-somite transition. This mesoderm populates arches 3, 4, and 5 to form the muscles of the palate, pharynx, and larynx. Their nuclei are located in the lateral motor column of the brainstem, the NA; it supports cranial nerves IX, X, and XI. (2) The second population of PAM organizes slightly later within somites 2–8. These myoblasts constitute the hypobranchial cord that migrates downward and forward in the midline beneath the arches to form the strap muscles and the tongue. These are supplied by the medial motor column of the brainstem, hypoglossal nucleus which runs down into the spinal cord as low as C4. The medial motor column supports cranial nerve XII and the motor branches of the cervical plexus, C1–C4.
The hypobranchial muscles arise from seven consecutive somites and consist of two groups which behave in similar fashion. Tongue muscles originate in somites S2–S5 and are supplied by r9–r11 and c1. They sweep downward, and skirt laterally around and beneath preexisting pharyngeal arches and thenceforward in the midline to access the floor of the mouth. The tongue muscles are strap muscles originate from somites S6–S8 and are supplied by c2–c4. They descend to the interclavicle (which becomes manubrium) and thenceforward in the midline to attach to the anterior surface of the 3rd and 4th arches.
Hypobranchial first appeared with the primitive jawed fishes. They represent a new application of hypaxial trunk muscles for jaw control. In sharks these includes two groups: Prehyoid muscles refer to those that insert anterior to the 2nd arch. Their generic precursor in cartilaginous fishes (sharks) is coracomandibularis. In tetrapods, this breaks into the genioglossus group (control of the tongue from the 1st arch mandible) and the geniohyoid group (control of the tongue from the 2nd and 3rd arch structures, styloid process and hyoid bone). Post-hyoid muscles refer to those inserting caudal to the 2nd arch. In sharks, these are represented by coracohyoideus. In tetrapods, this complex becomes rectus cervicis and eventually the strap muscles.
Origin and insertion of muscles: The functional anatomy of strap muscles
Questions for the curious: Why do strap muscles exist? How do they “know” where to insert?First, let's clarify some definitions the origin of a muscle corresponds exactly to the zone or zones of mesoderm in which it develops… this in turn is corresponded to the neuromeric levels of its motor nerve. The insertion of a muscle refers to its attachment into a structure such as bone or cartilage when that structure elaborates a signal, such as BMP-4, that acts like the landing lights of an airport and instructs the fascial envelope surrounding the muscle to make an attachment. The primary insertion of a muscle is always to a binding site on a structure that is produced from the same neuromeric level as the muscle. The secondary insertion takes place to the next available binding site in an adjacent structure.
The process of primary and secondary insertions is strictly mathematical. The motor nerves to the strap muscles are C1–C3 so these muscles all arise from the first three cervical somites (S5–S7). The clavicle arises develops from neural crest that originates at the first three neuromeres immediately after the hindbrain, that is, from levels c1–c3. It is part of the pectoral girdle. The strap muscle myoblasts migrate outward from their somites and make primary insertions into sternum and clavicle because these structures come from the same neuromeric levels. They then travel forward beneath the arches to seek out the inferior border of hyoid bone, a 3rd arch structure.
Why does this peculiar arrangement make sense? The system first appears in the primitive bony fishes, such as Polypterus, in which jaw opening is accomplished through the primitive hypobranchial strap muscle muscle complex rectus cervicis, (coracohyoideus, and the coracobranchials). These muscles connect the pectoral girdle with the hyoid, and branchial arches; they function as depressors of the lower jaw and the gills. The piscine pectoral girdle consists of a series of dermal bones fixed proximally to the skull by posttemporal bone and extending outward: Posttemporal, supracleithrum, postcleithrum, cleithrum, and finally clavicle. Attached to cleithrum is the chondral scapulocoracoid to which the fin is attached.
Over the course of evolution, the multiple bones of the pectoral girdle disappear. With the emergence of tetrapods on land, the attachment of the pectoral girdle to the skull is lost, creating mobile neck with great advantages for food gathering. At the same time, paired ventral bone units, the interclavicles, united in the midline below the neck to stabilize the pectoral girdles. These persist in birds as the furcula (wishbone). In mammals, the interclavicles morphed into the manubrium sterni. The three ossification centers seen in the clavicle (one medial at the manubrial joint and two laterally along the shaft) may represent contributions of three neural crest populations, c1–c3, with c1 being assigned to interclavicle.
In sum, modern strap muscles originate from somites C1–C3 (S5–S7). They are enclosed by neural crest fascia from c1–c3. Their attachments include both manubrium (interclavicle) and the clavicle. The unusual serial arrangement of sternothyroid and thyrohyoid (both sharing an attachment to the oblique line of 4th arch thyroid cartilage with ultimate insertion into 3rd arch hyoid) can be explained by the original branchial arch insertions of rectus cervicis, coracobranchialis.
Cranial nerves and their targets
Cranial nerves of the midbrain and hindbrain are laid out in individual nuclei and columns. These are arranged from medial-to-lateral according to their function. A simple model of the motor nuclei and functional columns is summarized in [Table 2] below.
The term visceral is a historic misnomer from when arch muscles were thought to lateral plate mesoderm of the gut. We know all craniofacial muscles are striated and originate from PAM.
The 1st arch receives its muscles contents from somitomere 4. All are innervated by V3, the nucleus of which resides in r3. Only one, tensor veli palatini, is assigned to the palate. TVP is considered a muscle of mastication. It shares a common blastema with medial pterygoid and is supplied by the palatine branch of the nerve to medial pterygoid.
The 2nd arch is supplied with muscle precursors from somitomeres 5 and 6. These are innervated by VII, the nuclei of which are located in r4 and r5. Neural crest originating from the two rhombomeres may represent as the upper and lower divisions of the facial nerve. The facial muscles are organized in two planes, superficial and deep. Buccinator is a deep plane muscle in continuity with superior constrictor from the 3rd arch. Tensor veli palatine and superior constrictor develop from a common anlage. A small component of LVP receives innervation from the geniculate ganglion through the greater petrosal nerve which connects to lesser palatine nerve. It is possible that Sm 6 contributes to the muscle mass of the palate [Figure 20].
|Figure 20: Facial nerve supplies visceral motor (parasympathetic) fibers to glands and somatic motor to the lower (palatal) aspect of levator|
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Nota bene: The eye has a number of accessory protective muscles; these arise in the 5th somitomere. These include avian quadratus nictitans, the mammalian retractor bulbi, and lateral rectus. Innervation for this blastema comes from r4 and r5. This blastema is physically separate from the 2nd arch. The nuclei reside in the medial motor column but are not in continuity with the other extraocular muscles because rhombomeres r1–r3 of the pons constitute a “skip area” where medial motor column simply does not exist. Recall that the facial muscles develop in the 6th somitomere (also innervated from r4–r5). This may explain the unusual course of the 7th cranial nerve which has to loop forward around adbucens to exit the brain stem.
The muscle contents of the 3rd arch are more extensive than generally appreciated. Somitomere 7 produces stylopharyngeus, levator veli palatini, PG, PP, superior constrictor, and middle constrictor. The neural crest fascia for stylopharyngeus comes from r6; all remaining muscles are supplied from r7. The motor supply for the 3rd, 4th, and 5th arches originates from the nucleus ambiguus (NA), a nucleus for somatic muscles that extends throughout entire caudal hindbrain from rhombomere 6 to rhomobomere 11.
Nucleus ambiguus demystified: Putting an end to ambiguity
NA richly deserves its name because its anatomy is poorly understood [Figure 21] and [Figure 22]. Three nerves emerge from this nucleus, each of which has a contradiction. From the rostral end glosspharyngeal is said to be the unique cranial nerve for the 3rd arch. Although this is true from a sensory standpoint, IX provides motor control for only one muscle, PP. Vagus nerve exits from the midpoint and appears to supply via the pharyngeal plexus, the soft palate, constrictors, and superior laryngeal muscle to cricothyroid/pharyngeus. However, the sensory representation of vagus is exclusive to the 4th and 5th arches. Emerging from caudal portion of NA are two distinct nerves. Inferior (recurrent) laryngeal nerve belongs to vagus and supplies the intrinsic muscles of phonation. The very same zone gives rise to the cranial roots of accessory nerve. These immediately fuse with the vagus, but are considered the possible source of the inferior laryngeal. Nevertheless, XI has no sensory distribution in the pharynx so it cannot be considered the motor nerve to the 5th arch. On the other hand, XI supplies two muscles demonstrated to arise from the occipital somites, sternocleidomstoid and trapezius. As these muscles span from the occipital skull to the pectoral girdle, they do not qualify as pharyngeal arch derivatives.
|Figure 21: Neuromeric map of cranial nerves IX and X. Nucleus ambiguus has three motor zones: Anterior 1/3 (r6–r7) supplies soft palate, superior constrictor. It has nuclei for both IX and X. Middle 1/3 (r8–r9) supplies superior laryngeal nerve to 4th arch and somatic motor to middle constrictor. Posterior 1/3 (r10–r11 supplies 5th arch and inferior constrictor. Fibers from r6–r7 and shared between these two nerves. Thus, motor supply to soft palate really based on rhombomeres and can travel via either IX via X. Conclusion: Do not code the soft palate as IX or X, just code it as r6–r7. Nuclei of XI are extensive (r10-c6) and supply transitional muscles sternocleidomastoid and trapezius|
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|Figure 22: Motor and sensory columns of the brainstem. Yellow: Eye muscles (Sm1–Sm3, Sm5) are not pharyngeal arch derivatives; tongue muscles come from somites 2-5. Brown: Pharyngeal arch muscles, V, VII, IX, X, XI upper 1/3 nucleus ambiguus = soft palate|
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How can these contradictions be resolved?First, NA as being strictly dedicated to motor control of the later motor conveniently divided into three functional sections containing, at distinct levels, three cranial nerves: Glossopharyngeal (r6–r7), vagus (r7–r11), and accessory (r7–r11). The motor supply for the 3rd arch is found in the cranial portion of NA, r6–r7. It supplies the muscles of the soft palate, and two constrictors, upper and middle. Located in r6 are the motor fibers glossopharyngeal per se which supplies only one muscle in Sm7, palatopharypharyngeus. Located in r7, in addition to IX, vagus is also represented. This is the most anterior nucleus of vagus and it is also dedicated to the 3rd arch. It gains access to all its target muscles through the pharyngeal plexus. Thus, the muscles of the 3rd arch are under control from r6–r7 via two nerves, IX and X. The motor supply for the 4th arch resides in middle portion of NA, r8–r9. It supplies the muscles of the Sm8: Cricothyroid of the larynx (superior laryngeal nerve) and inferior constrictor. The motor supply to the muscles of the 5th arch is located in the inferior portion of NA, r10–r11. These neuromere contains vagus to the muscles of Sm9: Internal laryngeal and arytenoid muscles (inferior laryngeal nerve).
Two structures are critical for understanding the neuroanatomy of the soft palate: (1) the pharyngeal plexus, and cranial nerve XI (the accessory nerve). The pharyngeal plexus is responsible for sensory innervation of the oropharynx and the laryngopharynx. It is defined by the sensory distribution of IX and X. The plexus is also responsible conveying somatic motor fibers from NA (r6–r11) to target muscles. Gray's 40th edition considers these fibers to have their origin in XI. Specifically, the cranial part of NA is distributed to the muscles of somitomeres 7 and 8: The soft palate, constrictors, and the cricopharyngeus (superior laryngeal nerve). The caudal part of NA is distributed to the muscles of somitomere 9: Intrinsic laryngeal muscles (inferior laryngeal nerve).
One can ask the question as to why fibers from one nerve connect with those of another, as in the pharyngeal plexus. In the case of the soft palate, motor nuclei that supply three different nerves, IX, X, and XI co-exist within the NA. When cell bodies for nerves of similar function reside within a common neuromere, the possibility exists for an extracranial communication between those nerves.
Recall that the sensory “identity” of oral epithelium is derived from the neural crest that forms the submucosa. Neural crest-derived from r6 and r7 defines the sensory contribution of the 3rd arch to the oropharynx. The innervation boundary between r6 and r7 corresponds roughly to superior versus middle constrictor. Soft palate submucosa likely originates from r6. Neural crest from r8 and r9 determines the sensory distribution of 4th arch. The 5th arch is in register with r10 and r11.
Given the intimate relationship between cranial nerves IX and X to the 3rd arch it is not surprising that their muscle targets are closely related. PP (r7, X) passes downward from the soft palate posterior to the tonsil and joins with stylopharyngeus (r6, IX) to insert into posterior thyroid cartilage (arch 4). Furthermore, at the boundary between arches 3 and 4 is Waldeyer's ring (the insertion of lymphoid tissues) which in turn corresponds to the site of the buccopharyngeal membrane. The track of the buccopharyngeal membrane passes through the tonsillar pillars and thence moves backward between superior and middle constrictors. Thus, the tonsillar pillars serve as a boundary between the cranial pharyngeal arches (PA1–PA3) and the caudal arches associated with the pharynx and larynx (PA4–PA5).
To test out these relationships between IX and X, put your finger on the hard palate (sensory supply V3). There is no gag reflex. Now pass it backward to the oral surface of the soft palate (sensory supply V3 and IX) and you will elicit a gag reflex with a vagus-mediated reflex arc.
Conclusions of this section (yes, you survived)
- All pharyngeal arches are in register with, receive neural crest from, and are innervated by paired rhombomeres
- The functional neuroanatomy of the medulla is better understood on the basis of rhombomeres than by individual nerves
- The NA is no ambiguous at all – in fact, it is the key to the medulla
- Muscles supplied by the medial motor column have an unerring mission to move forward in the midline into their insertion sites
- The cranial nerve that can by physically identified as providing motor supply to pharyngeal arches 3–5 is vagus through the pharyngeal plexus and separate superior and inferior laryngeal nerves
- With the exception of PP, motor fibers from vagus innervate muscles from somitomeres 7–9
- The best way to understand the cranial nerves to pharyngeal arches 3–5 is not on the basis of anatomic structures outside the brainstem but on the original neuromeric anatomy of the NA
- True vagus motor neurons may be strictly visceral, that is, parasympathetic, not somatic
- The source of somatic motor neurons traveling via vagus, based on function, is accessory nerve.
| Pathologies of Soft Palate Cleft|| |
Key concept: DPA supplies the hard palate vies greater palatine artery (GPA) and the soft palate via LPA. Secondary hard palate clefts represent pathology of the GPA field while soft palate clefts represent pathology of the LPA field.
Recall that the neural crest source of the soft palate is from 1st arch and 3rd arch with some 2nd arch represented as well. It is not surprising that the blood supplies of the three zones of the soft palate reflect this gradient: anterior zone (1st + 2nd), middle zone (2nd + 3rd), and posterior zone (3rd) [Figure 23].
|Figure 23: CP muscle insertions. Aponeurosis absent. Tensor veli palatini does not have transverse fibers. Uvulus at the margin, flanked by levator veli palatine|
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Abnormal platform/normal muscles
Lesser palatine neuroangiosome
Isolated cleft palate is characterized by a reduction in the horizontal plate of palatine bone, an r2 neural crest derivative. Its neurovascular axis is the LPA. LPA supplies: (1) horizontal plate of the palatine bone, (2) anterior 1/3 of the palatine aponeurosis. As previously discussed, developmental of the horizontal plate involved the progressive deposition of mesenchyme by the LPA neuroagiosome in a medial-to-lateral and anterior-to-posterior sequence. The most medial and distal structure of the horizontal plate to form is the PNS.
When a deficiency state occurs in the LPA neuroangiosome, the pattern of tissue loss is exactly the reverse of its developmental gradient. Thus, the PNS is the very first structure to be knocked out. The absence of the PNS is pathognomonic for isolated CP. Further mesenchymal loss reduces the horizontal plate from its initial rectangular shape to that of a triangle. The palatine aponeurosis, being of the same angiosome, is reduced or absent. This causes displaced forward of LVP, bringing it into contact with the posterior edge of palatine and facilitating its insertion into the bone. Because aponeurosis is an r2 structure, it may itself be intrinsically reduced, bringing the entire palatal muscle complex further forward into space.
The soft palate is a form of “muscle sandwich.” The spatial order of the soft palate muscles reflects the timing with which these muscles come into position. In every case TVP arrives first and is associated with the oral mucosa. LVP follows next and assumes an intermediate position. PG and PP are last. These muscles lie oral/cranial to LVP; they are associated with the nasal mucosa. Uvulus is anomalous but is associated with a secondary insertion into the PNS.
Insertion is a process by which the fascial envelope of a muscle “recognizes” a biologic signal, such as BMP-4, elaborated by bone or fascia. Under normal circumstances, bone or aponeurosis may express BMP-4 signals in a spatio-temporal order as its various sectors achieve developmental maturity. Much as landing lights signal, the presence of an airport runway, so does BMP-4 guide a fascia-muscle unit into its appropriate insertion site. Muscle units arrive in a fixed sequence. Each muscle will respond opportunistically, choosing an insertion site according to its spatial position and the timing with which the “landing lights” are turned on.
Under normal circumstances, uvulus leaps over the levator complex and the “bare area” of aponeurosis to insert into the PNS. Recall that each bone has a spatio-temporal maturation sequence. As soon as one zone is mature, it stops producing BMP-4. Muscle insertion will then take place at the next available maturation site. In the case of the palatine bone the vertical plate is the “oldest,” and its ossification sequence leads to the synthesis of the horizontal plate, from lateral-to-medial, and from anterior-to-posterior. The PNS is the “newest” zone and consequently, the most vulnerable. Under normal circumstances, LVP assumes its insertion site into at a time when the development of the horizontal lamina is complete, leaving only the PNS as a potential insertion site – which will be occupied by uvulus. When the developmental sequence of palatine bone is altered, the PNS is gone, the intervening anterior aponeurosis is absent, LVP is positioned forward, and the horizontal lamina expresses BMP-4. Uvulus makes an opportunistic insertion into the edge of the bony cleft, followed by LVP. Nota bene: These two muscles can be distinguished. Uvulus is seen as distinct, wispy longitudinal fibers. Levator is just medial and is distinctly more bulky.
This concept explains why, in the submucous cleft, levator is malpositioned, even when the apparent volume of the palatine bone can appear normal. The absence of PNS is the “give-away.” It tells us that the developmental sequence of palatine bone is aberrant. And the defect in aponeurosis always precedes that of the bone. Thus, even though only PNS is absent, soft palate muscles are forward-positioned. As the deficit in the palatine bone becomes more pronounced, the normally rectangular horizontal lamina becomes triangular. Ultimately, a pronounced notch will be present. As the palatine pathology worsens, the levator insertion streams further and further forward. If the horizontal plate is sufficiently attenuated medially, as in the case of a maxillary cleft, levator fibers will actually pass forward to insert into the posteriomedial margin of the maxilla.
Mechanism of the soft tissue cleft
Under such circumstances, why should a cleft form in the soft palate? The answer has to do with the epithelial stability of the mucosal envelope. As previously discussed, muscles assigned to the soft palate fill out a potential envelope of mucosa located at the interface of two neuroangiosomes just behind the maxilla. The boundary between r6 mucosa innervated by IX and r2 mucosa supplied by lesser palatine nerve constitutes the biologic “exit sign” for the myoblasts to move into position. The two edges of the mucosal envelope break down and fusion takes place.
Zhang et al. describe an elegant model for soft tissue fusion that depends upon the production of soluble protein products by adjacent developing bone fields [Figure 24]. The epithelial integrity of the mucosal envelope depends on the local production of sonic hedgehog (SHH) gene products. BMP-4 inhibits SHH and permits fusion to take place. BMP-4 is a byproduct of membranous bone production. Thus, the horizontal palatine shelf produces a fixed amount of BMP that diffuses backward through the tissue of the soft palate. Any reduction in bone mass of the horizontal plate will reduce the amount of BMP-4 available for SHH inhibition. When the deficiency of BMP-4 reaches a critical point, persistent SHH in the epithelium will prevent the fusion mechanism from taking place. Because the source of BMP-4 is anterior, the most vulnerable zone of the soft palate is distal, at the uvulus. Worsening BMP-4 deficits simply cause the soft tissue cleft to advance forward until it finally reaches the bony shelf.
|Figure 24: Bone morphogenetic protein 4/Sonic hedgehog loop. Membranous sp. palatine bone formation produces bone morphogenetic protein 4 which diffuses through soft palate to reach the epithelial margins. Here, under normal circumstances it inhibits the stabilizing effect of sonic hedgehog on the epithelium. Fusion can then take place. Quantitative reduction in (bone morphogenetic protein 4) causes epithelium to remain intact and not fuse. Process proceeds from distal to proximal|
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Greater palatine neuroangisome
If the biologic problem involves the GPA neuroangiosome as well, a forward extension of this very mechanism will be seen. If the maxillary hard palate gap is narrow, mucosal fusion can still take place, leaving a palpable defect in the bone, but no visible cleft. However if the critical contact distance is exceeded, if the maxillary shelves are too much reduced, soft tissue closure becomes impossible to achieve. A final variation can occur when both shelves are perfectly normal but are related to a vomer (actually, paired vomerine fields) that is deficient. A very narrow midline cleft can occur. This is perhaps the only situation in which and intact palatine hard palate can be found in the presence of such a midline maxillary hard palate cleft.
Abnormal bony platform/abnormal muscles
This situation obtains when one or more muscles are dysfunctional. The cause can be mesenchymal, that is, pathology affecting the mass or quality of myoblasts within their somitomere of origin before migration. It can also be neurologic, with an otherwise normal muscle but poor to absent motor innervation.
Tensor veli palatini develops from somitomere 4; its fascia originates from r3 neural crest. Problems with may be a manifestation of a more global neural crest problem affecting both r2 and r3. TVP weakness manifests itself as the inadequate opening of the pharyngotympanic tube and predisposition to otitis media. In such cases, cleft palate repair cannot be expected to correct a dysfunctional TVP.
Palatopharyngeal muscles develop from somitomere 7; their fasciae originate from r7 neural crest. These include superior and middle constrictors. Dysfunction of these muscles may lead to inadequate palate closure. Hypoplasia or paresis of LVP and the remaining muscles of the 3rd arch can be seen in Treacher-Collins and Goldenhar syndrome. See chapter on pharyngeal arches and pharyngeal arch syndromes.
The physiologic response of soft palate muscles to muscle testing is summarized in [Table 3].
Note that although stylopharyngeus is also a derivative of Sm7 its fascia is r6. This is consonant with its glossopharyngeal innervation form cranial nerve IX with its nucleus in the r6 component of the NA.
Normal bony platform/abnormal muscles
This is the situation seen most cases of noncleft VPI. The presence or absence of the PNS determines if the palatine bone is considered borderline deficient. If the ANS to PNS distance is normal, but VPI is present, the diagnosis will rest on which muscles are involved. Abnormalities of Sm4 and Sm7 exist independently, but in the latter case, all of its derivatives are usually involved.
A curious observation is seen in cases of microtia when the hard palate is otherwise intact. These demonstrate a near universal weakness of ipsilateral levator function (but not the remaining Sm7 muscles). Recall, cf. chapter on pharyngeal arch syndromes and ear development, that the Eustachian tube is trilaminar. The lateral tube, being a derivative of the first arch, provides attachment for Sm4 tensor. The medial tube derived from the third arch, provide attachment for Sm7 levator. The tube itself is the buried remnant of the 1st pharyngeal pouch. Recall that virtually all representation of 2nd arch within the oral cavity is obliterated by a combination of involution of 2nd arch mucosa and overgrowth of mucosa from the 1st and 3rd arches.
| Summary of Pathology by Neuroangiosome|| |
Greater palatine artery affected/lesser palatine artery unaffected
It is theoretically possible to have a minor GPA deficiency state with and intact soft palate. This presupposes that the maxillary palatal shelves cannot achieve the critical contact distance required to fuse with each other and/or with the vomer. Such rare defects can occur at any place along the length of the maxillary hard palate with one caveat. As long as the width of the maxillary shelves is sufficient posteriorly so that the palatine shelves can close the soft palate will remain intact. PNS will be present.
If GPA is significantly affected, it will entrap the mesenchyme of palatine bone even if the LPA pedicle is biologically intact. PNS will be absent. If the mesenchymal masses are physically separated beyond the critical contact distance a soft palate cleft will result.
Lesser palatine artery affected/greater palatine artery unaffected
This is the typical scenario for isolated soft palate clefts. In such cases the defect in the palatine shelves can run forward as far as the posterior border of the maxillary shelves, but no farther.
Lesser palatine artery affected/greater palatine artery affected
This describes the situation of palate clefts involving a complete separation of the palatine shelves with the defect extending forward into the maxillary hard palate. Since the deposition of bone mesenchyme by the GPA neuroangiosome is anterior to posterior, the most recent zone, and the one affected first by any GPA deficiency state will be the posterior margin. Thus, combined clefts of the palatine and maxillary hard palate extend forward toward the incisive foramen with greater degrees of mesenchymal insufficiency.
| Surgical Correction of Cleft Palate: Developmental Field Algorithm|| |
Is velopharyngeal insufficiency predetermined?
It should be apparent at this point that in most cases of soft palate clefts the muscles are normal in size and function. Proper spatial repositioning at the appropriate age should lead to a good outcome. An exception to this is velocardiofacial syndrome in which 3rd arch pathology affects the palatal musculature itself. Assuming normal neuromuscular development, why does VPI occur in the face of an otherwise uncomplicated repair? Excluding issues of surgical trauma and inadequate dissection, several questions remain: Is the physical position of the soft palate in space inadequate from the very start? What is the developmental sequence of the bony palate?
Alveolar extension palatoplasty
The literature of cleft palate is vast. Anatomic studies by Kriens remain helpful., Management of the hard palate cleft using alveolar extension palatoplasty (AEP) technique uses and embryologic dissection of the entire greater palatine neuroangiosome to generate longer flaps and preventing fistulae [Figure 25], [Figure 26], [Figure 27], [Figure 28], [Figure 29], [Figure 30], [Figure 31], [Figure 32], [Figure 33]. Unlike the two-flap technique of Bardach, the AEP incision is not placed at the junction of the alveolus with the palatal shelf. The flaps are incised just off the midline of the alveolar ridge, thus preserving all lateral vessels ascending from the GPA to supply the stem cells within the biosynthetic mucoperiosteum responsible for bone growth of the lingual alveolus. The technique is nicely illustrated by Bennun and Monasterio Aljaro.
|Figure 25: Two-flap palatoplasty (Bardach). Lateral incision at border of hard palate with alveolus. Alveolar extension palatoplasty (AEP, Carstens). Incision just lingual to the midline of the alveolus|
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|Figure 26: Two-flap palatoplasty flaps intrinsically short and narrow. Cannot reach anterior palate. Greater palatine artery supply to lingual mucoperiosteum interrupted – alveolar growth. Denuded areas fill with scar|
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|Figure 27: Alveolar extension palatoplasty versus Veau-Wardill. Expanded length and surface area: (1) anterior coverage, (2) no lateral raw areas, (3) release site at maxillary tuberosity|
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|Figure 28: Anterior fistula results from subdividing the greater palatine artery angiome. Greater palatine artery injury: Incision at junction of hard palate and alveolus spares alveolus. Vascular anatomy of greater palatine artery is sharply demarcated|
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|Figure 29: Geometric expansion of alveolar extension palatoplasty flaps versus Veau-Wardill flaps. Augmentation of both width and height translates to (1) significantly larger anterior reach; and (2) ability to cover lateral releasing incisions without a raw area. Only open site is at the maxillary tuberosity|
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|Figure 30: Alveolar– extension palatoplasty “extension effect.” Veau-Wardill (red): Short of incisive foramen. Alveolar extension palatoplasty (purple): +30% gain… greater segment exceeds nasal floor|
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|Figure 31: Alveolar extension palatoplasty flaps demonstrating gain in length from using the entire neuroangiosome. Noncleft flap extends 0.5 cm beyond the alveolar cleft. Cleft-side flap reaches anteriorly immediately behind the alveolar cleft. Midline flap closure is S-shaped|
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|Figure 32: Alveolar extension palatoplasty closure of primary palate no lateral raw areas|
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|Figure 33: Alveolar extension palatoplasty at age 5 showing normal eruption. Paramedian incisions have no sequelae provided dissection, once through the mucosa shifts medially to avoid and dental lamina and becomes immediately subperiosteal|
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We have covered a great deal of information in this chapter. All of it boils down to a very simple model. The palate is a functional structure designed to assist in the management of food and to control airflow for proper speech. It includes a static bony platform to which is attached a dynamic muscle effector arm. To function properly, the muscular soft palate must be physically capable of making effective contact with the pharynx. To do so, it must be: (1) positioned properly in space, and (2) functionally capable of lifting and extending as required.
The management of isolated soft palate clefts depends on the answers to two fundamental questions. First, are the dimensions of the hard palate of proper proportions such that the soft palate is correctly positioned in space? Second, are the muscles of the soft palate biologically capable of fulfilling their roles? The answers to these two questions will determine surgical strategy.
The physical position of the soft palate with respect to the posterior pharynx is absolutely determined by the relative sizes of the bony components to which it is attached. It is imperative to know if the platform is normal or short. Can we deduce an insufficiency anteroposterior state before surgery? If the palatal platform is foreshortened, what can be done about it and when?
The length of the bony palate can be quantitated in several ways. Physical examination reveals the geometry of the horizontal palatine plate. A notch or defect can be assessed in terms of its dimensions. Caliper measurement at surgery can give the answer as well. The dimensions and shape of the palatine bone can be readily assessed by palpation. When the ANS-PNS distance falls short of normal by 1.5 cm, consideration can be given to immediate versus delayed interposition soft tissue reconstruction.
Imagining studies such as three-dimensional CT scanning in older patients provides an accurate way to measure dimensions of bone and soft tissue. In an ideal world, a study of normal subjects would determine age-related norms of distances in the palatal plane for ANA-PNS, and ANS to the posterior pharyngeal wall (PPW). Similar studies of cleft palate subjects would demonstrate the discrepancies between the two groups. In isolated cleft palate subjects, PNS to PPW would be either normal or excessive. The latter situation could be due to foreshortening of the palatine shelf, the maxillary shelf or both. The diagnosis would rest in assessing the shape of the palatine bone. Thus, as foreshortening increases, so does the risk of VPI, even in the face of technically perfect surgery. Unfortunately, such technologies have limited usefulness for diagnosis in very young patients due to requirements for sedation and radiation exposure. Further advances in imaging may ameliorate this situation.
The dimensions of the anterior palatine aponeurosis determine the physical position of the levator complex with respect to the bony platform. Both the palatine bone and the palatine aponeurosis develop from a common embryologic tissue source: Neural crest from the 2nd rhombomere. An r2-based deficiency state affects both bone and aponeurosis. All soft palate muscles insert into the aponeurosis at programmed binding sites. The anterior 1/3 of the aponeurosis is muscle-free (except for uvulus). In the presence of an r2 osseous deficiency, the anterior aponeurosis becomes foreshortened or absent, dragging the otherwise normal soft palate muscles forward into space. This brings TVP into abnormally close contact with the horizontal plate. This is true, even in cases when the length of the hard palate is normal.
In cases of a foreshortened palate, there will always be deficiency or absence of the anterior palatine aponeurosis. This situation may demand reconstruction, either at primary surgery to prevent VPI or as the first option in cases of established VPI. If the gap is modest, reconstruction with a single buccinator flap is rational. If the gap is significant buccinator flaps can be overlapped in the midline to allow for retroposition of the muscle envelope without resorting to extensive separation of muscle components from their normal relationships to aponeurosis and mucosa. If VPI supervenes, revisionary surgery consist of intervelar veloplasty can be carried out. This sequence can also be reversed, the drawback being that intervelar scarring has been created by the first procedure.
Mesenchymal deficiency in the horizontal plate of the palatine bone unmasks a muscle-binding site into which LVP forms an opportunistic false insertion. Tethering of LVP prevents normal palatal function. Because the fibers of PP are intermingled with levator, its function is also impaired. Complete disinsertion from the bone is required. However, LVP has an otherwise normal relationship to aponeurosis. Thus, if the anterior aponeurotic deficit is not addressed, separation of the levator from the palatine aponeurosis (intervelar veloplasty) may simply introduce scarring with little increase in length. Repairs using z-plasty flaps obliterate otherwise normal embryologic relationships.
The muscles of the soft palate arise from different mesodermal structures in the embryo. The functional status of each muscle must be determined before surgery. A perfect anatomic repair in the face of a dysplastic levator sling or an unresponsive superior constrictor will result in failure. Muscle stimulation can be carried out in a cooperative patient with anesthetic spray to the palate mucosa. Alternatively, it can be performed under anesthesia. This information is of prognostic and therapeutic significance. Thus, patients undergoing primary repair can be assessed for their potential to require an additional procedure for VPI. Closure patterns can be reliably reproduced in a noneffort-dependent manner to determine the best procedure for management of VPI, should it arise.
Given a good anatomic repair of the levator sling and normal musculature, the functional status of the soft palate musculature can only be ascertained after surgery. The patient will demonstrate the extent of brain-palate coordination. Speech therapy is vital to avoid or correct errors. Sometimes even an apparently short palate can achieve closure by coordination with surrounding pharyngeal elements.
When postoperative VPI is present, buccinator myomucosal flap interposition, either single or bilateral, achieves palatal lengthening by addressing the primary problem: Anterior palatine aponeurosis deficiency. The procedure is reliably achieves 1.5–2.5 cm of retropositioning with minimal donor site morbidity. Re-operation of an otherwise good palate repair is avoided. It is the logical first-line of defense for VPI correction.
Surgical algorithm for soft palate cleft
- Check the hard palate length
- Check the soft palate function: Muscle stimulator
- Primary surgery goals:
- Anatomically sound muscle repair – reserve z-plasty for secondary cases
- Observe postoperative function
- Secondary surgery goals:
- Gain additional length:First address the aponeurosis, then z-pasty
- Prevent unnecessary operation and fibrosis of the muscular soft palate.
Buccinator myomucosal flap: Anterior and posterior design and applicationsand applications
Technical aspects of the buccinator procedure are demonstrated in [Figure 34], [Figure 35], [Figure 36], [Figure 37], [Figure 38], [Figure 39].,,,
|Figure 34: Buccinator anatomy: Type 3 flap with extensive anastomoses. Anterior – facial artery. Posterior – buccal artery: Much more extensive. Can be elevated as an island or with posterior muscle pedicle|
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|Figure 35: Buccinator design – left cheek. Important to complete mucosal incison posteriorly. Muscle remains intact as posterior pedicle|
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|Figure 36: Buccinator (posteriorly-based) turned down behind molar. Transverse coverage = reconstruction of palatine aponeurosis. Mucosa sectioned circumferentially – the posterior muscle is left intact|
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|Figure 38: Transverse buccinators – designed for both nasal and oral coverage in a case of velopharyngeal insufficiency due to missing palatine aponeurosis|
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|Figure 39: Midline fistula reconstruction with bilateral buccinators placed side-to-side. 38a shows defect and the two flaps in place. 38b shows flap design|
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Trigeminal block, either extraoral or intraoral, with 0.25% bupivacaine with max dose of 1 cc/kg is administered. This is without epinephrine. With the addition of epinephrine, the dose is higher. For further details, see chapter regarding blocks. A transverse incision is made down to the level of the nasal mucosa. If, in one's assessment of the prior repair, further muscle retropositioning and alignment is needed (cases performed elsewhere) this can be done, exposing an intact zone of nasal mucosa. Buccinator interposition is then carried out. If the repair is deemed adequate but short, the muscle complex is freed from the nasal mucosa for a distance of 3–4 mm. The nasal mucosa is then divided transversely; the proximal cuff of tissue will serve for proper inset.
The donor site is suspended with two sutures to give it planarity and injected with lidocaine and epinephrine solution for hemostasis. An elliptical design is drawn and the mucosa incised with a knife or cutting cautery. The underlying muscle is incised from proximal to distal and the fat plane superficial to the muscle is entered. The muscle has two pedicles. Anteriorly, the facial artery gives off two to three branches running longitudinally into the muscle from the external (facial) aspect. These are cauterized without compromising the facial artery itself.
The dissection moves distally gentle spreading of the fat and cautery as required. No issues with facial nerve are present. The flap, involving the middle 50% of the muscle (or more) will be deinnervated. In the posterior 1/3 of the flap, a series of vessels coming from the buccal artery are seen. These are preserved. Nota bene: In an alternative technique, Robert Mann has shown that simple preservation of flow through the muscle is sufficient to preserve the flap.
Dissection then proceeds posteriorly, to the tail of the flap. This is liberated completely, taking care to sever any attachments to the periosteum of the maxillary tuberosity. Using two forceps, the flap is rotated into position, usually with the proximal tip being brought anteriorly and into the midline – this rotation will the tail of the flap distal to the vascular pedicle. Suture should be placed below the bite plane of the third molar to achieve recession of the flap. Rotation is accompanied by spreading in the plane of the vessels to achieve tension-free inset. In the Mann technique, the posterior muscle remains intact and constitutes a bridge behind molars. Often this remains insignificant, but it at times, it may require surgical division as a secondary procedure.
The flap or flaps are first inset distally, into the soft palate. The proximal margin thus remains well visualized. The posterior edges are inset. The tail is sutured last. Donor site closure is carried out from proximal to distal using 4-0 chronic or vicryl. After the mouth gag is removed, with the cheek on stretch, it is usually possible to achieve primary closure of the donor site incision. If a raw area remains, it will mucosalize. Over time, with an active soft palate, the buccinator flaps can stretch out substantially.
| Conclusions|| |
- The biologic defect in cleft palate involves neural crest produced by the neural folds at the level of the 2nd rhombomere
- The migration pattern of r2 neural crest follows the individual branches of V2 to create neuroangiosomes. These nourish the various developmental fields of the nasal cavity and maxilla
- Deficiency of r2 neural crest affects synthesis of the maxillary and palatine shelves. The gradient is always anterior to posterior and lateral to medial
- Palatine bone insufficiency is always accompanied by a reduction in the palatine aponeurosis, eliminating the anterior 25% nonmuscle binding zone, and bringing the muscle-bearing mid-portion into contact with the palatine shelves
- Loss of the PNS exposes a binding site for muscles along the horizontal palatine shelf
- Loss of mesenchymal mass of the horizontal palatine shelf results in a decreased production of BMP-4
- BMP-4 normally diffuses through the soft palate tissue to reach the epithelial border. There is inhibits SHH, a gene product responsible for maintaining epithelial integrity – and preventing fusion
- Reduced concentration of BMP-4 at the epithelial edges fails to repress SHH resulting in failure of fusion and a soft tissue cleft
- Muscle defects from somitomere 4 (TVP) or somitomere 7 (LVP et al.) can affect soft palate function in the presence of a normal bony platform.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21], [Figure 22], [Figure 23], [Figure 24], [Figure 25], [Figure 26], [Figure 27], [Figure 28], [Figure 29], [Figure 30], [Figure 31], [Figure 32], [Figure 33], [Figure 34], [Figure 35], [Figure 36], [Figure 37], [Figure 38], [Figure 39]
[Table 1], [Table 2], [Table 3]