Embryology

The Scar Solution Natural Scar Removal

Scar Solution Book By Sean Lowry

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The embryologic origins of the various tissues composing the oral cavity can be traced back to the mesoderm of the paraxial and lateral plates, the ectodermal placodes, and neural crest tissue (1). However, the most straightforward organization of these derivatives is to consider their origins from the pharyngeal arches, which form in the fourth and fifth weeks of gestation. Each arch contains all three germ cell layers and gives rise to a major arterial derivative, a bone or cartilaginous derivative, a cranial nerve, and the musculature associated with that cranial nerve (2). Although the arches are separated by clefts externally and pouches internally, the pouches and clefts never communicate, as in the branchia of lower animals (2,3).

The first pharyngeal arch consists of a dorsal maxillary process and a ventral mandibular process. The Meckel's cartilage of the first arch gives rise to the bone of the palate (maxilla and premaxilla) and the mandible. In addition, the first arch also forms the zygoma, part of the temporal bone, and the incus and malleus. The first arch also gives rise to the trigeminal nerve, which in turn supplies the muscles of mastication (medial and lateral pterygoid, temporalis, masseter, anterior belly of the digastric, mylohyoid, and tensor veli palatini) (2,3).

The second arch cartilage (Reichert's cartilage) gives rise to the upper part of the hyoid body, the lesser horns of the hyoid, the styloid process, and the stapes. The facial nerve arises from the second pharyngeal arch and supplies the muscles of facial expression, the posterior belly of the digastric muscle, the stylohyoid muscle and ligament, and the stapedeus and auricular musculature (2,3).

The third pharyngeal arch cartilage gives rise to the lower portion of the hyoid body and its greater horns. Its cranial nerve is the glossopharyngeal, which innervates the stylopharyngeous muscle. The fourth pharyngeal arch is associated with the superior laryngeal branch of the vagus, which supplies the levator veli palatini as well as the cricothyroid muscle (1,2). Because they are in close proximity to the oral cavity, the first pharyngeal pouch gives rise to the Eustachian tube and middle ear cavity, and the second pharyngeal pouch to the palatine tonsils.

The tongue begins to develop at four weeks gestation as a single medial tongue bud (tuberculum impar) and two laterally positioned tongue buds, all of which arise from the first pharyngeal arch. The two lateral tongue buds overgrow the medial tongue bud and fuse at the midline, forming the anterior two-thirds of the tongue. Posteriorly, the copula is formed from fusion of ventromedial aspects of the second pharyngeal arches. The hypobranchial eminence, formed from the ventromedial aspects of the third and fourth pharyngeal arches, eventually overgrows the copula and develops into the posterior third of the tongue. The anterior and posterior aspects of the tongue fuse at the terminal sulcus. The tongue musculature develops from myocytes derived from the occipital myotomes and is innervated by the hypo-glossal nerve (2,3).

The facial prominences also begin to develop during the fourth week of gestation. Paired maxillary prominences develop lateral to the stomodeum (primordial mouth), paired mandibular prominences caudal to it, and the frontonasal prominence rostral to it. The mandibular prominences fuse across the midline forming the lower jaw, lower lip, and lower cheeks. Nasal placodes develop on the inferior border of the frontonasal prominence. These invaginate, producing nasal pits and medial and lateral nasal prominences. The maxillary prominences grow medially causing the lateral and medial nasal prominences to fuse with each other and the maxillary prominences, thus forming the upper cheeks and maxilla. The fused maxillary prominences form the intermaxillary segment, which gives rise to the philtrum of the upper lip, four incisors, premaxillary aspect of the maxilla, and the primary palate. Outgrowths from the maxillary prominences, known as palatine shelves, migrate into a horizontal position and fuse at the midline to produce the secondary palate. The secondary palate fuses anteriorly with the primary palate, separated by the incisive fossa. Bone develops in the primary palate and then migrates posteriorly to the anterior portion of the secondary palate (1,2).

The salivary glands develop between the sixth and twelfth weeks of gestation. The parotid glands arise first, in the sixth week of development, as invaginations of the ectodermal epithelium at the angles of the stomodeum. They migrate posteriorly to the ascending rami of the mandible. During this migration, the facial nerves migrate anteriorly. The submandibular glands develop late in the sixth week of gestation from the endoderm in the floor of the stomodeum. Then the sublingual glands develop in the eighth week from endodermal buds in the paralingual sulcus. The minor salivary glands develop last, in the twelfth week of gestation (1,3).

ORAL COMPETENCE Normal

The primary function of the lips and the corresponding musculature is to provide oral competence. The basic structure of the lips appears simple with a musculature core covered with skin on its outer surface and mucosa on its inner surface. However, the sensory and motor functions of the lips are complex (Fig. 2, chap. 1). The primary muscle involved is the orbicularis oris, which is composed of four independent quadrants, each separated into a pars peripheralis and a pars marginalis (4). From a functional standpoint, it is more straightforward to consider the obicularis oris muscle a solid ring of muscle divided into a deep and superficial component. The deep component provides the lips with sphincteric action, which includes a unique association between the obicularis fibers of the upper and lower lips where the upper lip fibers split and the lower lip fibers pass through the split. Upon contraction, this forms a seal at the angles of the mouth (5). Contraction of the deep muscle fibers of the orbicularis oris allows the lips to form a seal around eating utensils in order to bring food into the mouth, prevent food from exiting the oral cavity anteriorly, and prevent drooling (6). The orbicularis oris receives motor innervation via the buccal and marginal mandibular divisions of the facial nerve (4).

The superficial muscle fibers of the orbicularis oris function in the finer movements of the lips, as do many muscles of facial expression (4). The incisivus super-ioris and inferioris are considered to be accessory muscles to the orbicularis oris, serving to press the upper lip against the teeth. The levator anguli oris raises the corners of the mouth; the depressor anguli oris moves the corners of the mouth down and laterally; the risorius retracts the angle of the mouth laterally. The zygomaticus major brings the angle of the mouth superiorly and laterally. Collectively, these muscles (orbicularis oris, incisvus superioris and inferioris, levator anguli oris, depressor anguli oris, risorius, and zygomaticus major), along with the buccinator, converge at the angle of the mouth, forming the modiolus. Another group of muscles (levator labii superioris, levator labii superioris alaeque nasi, and zygomaticus minor) enters the upper lip. The levator labii superioris is the main elevator of the lip but also causes slight eversion of the upper lip. The levator labii superioris alaeque nasi dilates the nostril and elevates and everts the upper lip, whereas the zygomaticus minor elevates and curls the upper lip. Finally, the muscle group comprised of the depressor labii inferioris, mentalis, and platysma enters the lower lip. The depressor labii inferioris depresses and pulls the lower lip slightly laterally. The mentalis maintains the position of the lower lip and chin while at rest, but upon contraction protrudes and elevates the lower lip. Most of the muscles in the lips are paired, with the exception of the mentalis muscle of the lower lip, which provides the lower lip with greater speed and force than the upper lip (5). Although the lips often act simultaneously, they can be activated separately, as proven by phonation of sounds such as "f" (4). The platysma assists in opening the mouth and may play a role in depression and lateral movement of the mouth (4,5). Sensory innervation of the upper lip is provided by the infraorbital nerve (V2), whereas lower lip sensation is supplied by the mental nerve (terminal component of inferior alveolar branch of V3) (5).

Abnormal

Difficulties in oral competence can take several forms. Loss of sensory input can result in loss of competence even with an intact muscular sphincter. For example, Sandstedt and Sorensen sent out a questionnaire to individuals who had suffered trigeminal nerve damage (6,7). Of the 226 individuals who responded, greater than 70% had sensory disturbances. Of those with sensory deficits in the lower lip, 54% had issues of insecurity; 56% had decreased oral competence, manifested by drooling; 50% complained of biting their lip.

Sacrifice of the mental nerve (lower lip) or inferior alveolar nerve (upper lip) might occur with resection of soft tissue alone or with resection of the mandible or maxilla, respectively. Loss of upper lip sensation alone does not result in oral incompetence. Mental nerve loss, however, is more problematic and with nerve grafting, some of this lost sensation can be recovered. When a lateral or anterior segmental resection has been performed for carcinoma of the mandible, the proximal and distal inferior alveolar nerve stumps are typically sampled using frozen section to rule out perineural tumor involvement. Assuming a clear margin, the remaining proximal nerve stump within the mandible is in a poor position for grafting, especially if an osseous flap is being used for reconstruction. However, the remaining nerve can usually be pulled out of the mandible with gentle traction and brought through the inferior alveolar canal. This provides a more easily approached proximal nerve stump for placement of an interposition cable nerve graft to re-establish connection between this proximal inferior alveolar nerve stump and the distal mental nerve stump.

Loss of muscular control of the oral sphincter can occur with loss of the lower division of the facial nerve or with loss of the soft tissues including the obicularis muscle of the lower lip. The marginal mandibular and buccal branches supply the obicularis oris and the additional musculature of the lower lip. Loss of the marginal mandibular nerve is a more likely event than loss of the buccal branches. With loss of the marginal mandibular nerve, one loses function of the depressor musculature of the lower lip, resulting in elevation of the ipsilateral lower lip and consequent asymmetry of the smile. Loss of the marginal mandibular nerve alone typically does not result in significant loss of oral competence; however, when coupled with mental nerve loss, significant incompetence often occurs. Such losses of oral competence can be addressed with re-innervation or suspension in the form of passive or active sling procedures.

When the oral sphincter is impaired by significant resection, again, typically involving the lower lip, re-establishment of the oral sphincter is most successful when innervated lip tissues can be utilized. In general, lip-switch procedures and use of local- or distant-tissue transfers for lip reconstruction will produce an asensate lower lip segment, which can result in incompetence due to loss of sensory and motor function. However, there is some initial return of sensory function along with partial motor function (8). As expected, this is less important with upper lip reconstruction. In contrast, advancement of lip tissues with an intact neurovascular pedicle, like the Karapanzic flap, is preferred because sensory and motor functions are preserved (9).

Another issue affecting oral competence is that of lip height. When malignancies of the anterior mandibular arch involve the mucosa and soft tissue of the lower lip, our reconstructive efforts must allow the re-establishment of lower lip height. The physician should consider the potential for contracture to reduce the reconstructed lip height. Given the thickness of the cutaneous component of the flap relative to normal lip thickness, persons undergoing osteocutaneous free-flap reconstruction of this area will often have what appears to be excessive bulk of the lower lip early in their recovery. Adequate wound maturation should be allowed prior to debulking, because efforts to thin this tissue or to prepare the site for dental implants can result in further wound contracture and loss of lip height.

SALIVATION Normal

The role of saliva is to moisten and protect oral and pharyngeal mucosa, lubricate, transmit taste information, buffer chemicals, initiate carbohydrate digestion, act as an antimicrobial agent, prevent dental caries, and participate in enamel formation (10,11). The oral cavity houses three major paired salivary glands [parotid, submandibular (submaxillary), and sublingual] and hundreds of minor salivary glands. The minor salivary glands are primarily located in the buccal, labial, palatal, and lingual regions, but are also found at the superior pole of the tonsils, tonsillar pillars, and base of the tongue (10). Together, the major and minor salivary glands produce 1000-1500 mL of saliva per day, equivalent to an average salivary flow rate of 1 mL/min, depending on salivary gland stimulation (12). Each gland at rest secretes at a rate of 0.001-0.2mL/min, and secretion increases to 0.18-1.7mL/min when the glands are stimulated (10). The gland distribution of salivary production also depends on whether a stimulus is present: the parotid gland produces approximately two-thirds of the total saliva when stimulated, but, under resting conditions, is responsible for only 26% of total production (13,14). At rest, the submandibular and sublingual glands produce 69% and 5% of total saliva, respectively (14). Minor salivary gland output varies little with stimulation and accounts for less than 10% of total overall salivary production (15). The parotid gland consists mainly of serous acinar cells that produce watery saliva, whereas the sublingual gland has mainly mucous acinar cells that produce viscous saliva. The submandibular glands have both mucous and serous acinar cells and produce saliva of intermediate viscosity (10).

Once saliva is produced, it travels by ducts to enter the oral cavity. Stensen's duct exits the parotid gland from its anterior border, travels anterior to the masseter muscle and through the buccinator muscle to enter the oral cavity through a papilla opposite the second upper molar. The Wharton's duct orifice exits from the medial surface of the submandibular gland and opens to the oral cavity lateral to the lingual frenulum. Saliva from the sublingual glands travels through multiple (usually 10) separate ducts, collectively referred to as Ducts of Rivinus, which open into the sublingual fold or plica of the mouth floor. Occasionally, a number of these ducts converge to form a Bartholin's duct, which empties into Wharton's duct (10).

Both sympathetic and parasympathetic nerve stimulation cause salivary secretion. Sympathetic nerve fibers from the superior cervical ganglion enter the glands with the arterial supply (external carotid artery to the parotid gland, facial artery to submandibular gland, and lingual artery to sublingual gland). Such stimulation produces a small volume of viscous saliva, which ceases after prolonged stimulation. Parasympathetic nerve stimulation is provided through cranial nerves 7 and 9. Preganglionic parasympathetic fibers from the glossopharyngeal nerve reach the otic ganglion via the lesser superficial petrosal nerve, and the postgan-glionic fibers continue to the parotid gland via the auriculotemporal nerve (branch of V3). Preganglionic fibers in the chorda tympani travel with the lingual nerve to the submandibular ganglion (Fig. 6, chap. 1). Postganglionic fibers then cause stimulation of the submandibular gland. Some of these postganglionic fibers travel in the lingual nerve to the sublingual glands. Parasympathetic stimulation produces a large volume of watery saliva that is secreted throughout the entire time of stimulation (11).

The composition of saliva varies according to a number of factors including rate of secretion, stimulus duration, dominant gland involved, time of day, and season of the year (17). The average pH of saliva is between 6.2 and 7.4, consisting chiefly of water (99.5%) and organic and inorganic solvents (0.5%). Saliva also contains sodium (10 mEq/L) potassium (26 mEq/L) chlorine (10 mEq/L), and bicarbonate (30mEq/L) (16). Saliva also contains proteins, calcium, magnesium, and phosphate (17,18). With a slight increase in salivary flow rate, sodium and bicarbonate concentration and pH increase while potassium, calcium, phosphate, chloride, and protein content decrease. When flow rate is greatly increased, concentrations of sodium, calcium, chloride, bicarbonate, protein, and pH increase. At the highest flow rate, phosphate concentration decreases and potassium concentration remains unchanged. An increased duration of stimulation is known to increase protein, calcium, and bicarbonate concentrations and pH while chloride concentration decreases (17).

Abnormal

Although, resection of major salivary glands does negatively impact overall saliva production, in most patients, there will be few complaints of xerostomia with resection alone. In contrast, those who have undergone either primary or post-operative radiotherapy to the oral cavity will usually complain of some degree of xerostomia. Often patients find that xerostomia is their most problematic long-term side effect of treatment. Although, salivary substitutes and parasympathetic agonists help somewhat, most patients opt for keeping their oral cavity moist with regular and frequent small sips of water. Without this regular moistening of the oral cavity, speech becomes distorted by adherence of oral mucous membranes to one another. Ease of mastication is obviously dependent on the water content of the foodstuff in question, and as expected, increased water intake is required for drier foods. These difficulties with xerostomia are further exacerbated by the presence of cutaneous surfaces within the oral cavity. The keratin produced by the skin has poorer clearance in patients with xerostomia.

MASTICATION Normal

Mastication involves coordination of complex movements of the mandible, tongue, and teeth. The muscles of mastication (masseter, temporalis, and medial and lateral pterygoids) control the movements of the mandible. The masseter elevates and protrudes the mandible while the temporalis elevates it further (19). When unilaterally contracted, the medial and lateral pterygoids move the mandible laterally, but bilateral contraction causes the medial pterygoids to elevate the mandible while the lateral pterygoids cause depression and protrusion (17). Motor innervation of these muscles is provided by the mandibular division of the trigeminal nerve (V3). The paired mylohyoid and digastric muscles also have a role in mandibular motion, as both contribute to depression of the mandible (5). Motor innervation of the mylohyoid and anterior belly of the digastric is provided by the mylohyoid nerve, a branch of the inferior alveolar nerve (5). The posterior belly of the digastric is innervated by the facial nerve (5). The first phase of jaw closure is caused by elastic recoil of the stretched jaw-closing muscles in coordination with relaxation of jaw-opening muscles. Jaw closure then continues with the medial pterygoids, followed by the anterior temporalis and then the posterior temporalis and masseter muscles (18). Jaw opening is initiated by the mylohyoid and continues with the digastric and lateral pterygoid muscles (18). The masseter muscles also contribute to jaw opening as they relax and are stretched under the weight of the mandible. Rotational movement of the mandible is produced by the pterygoids (18). During opening and closing of the jaw, the muscles are in an isotonic state, but during occlusion or when crushing a bolus, the muscles are in an isometric state (18).

The tongue musculature consists of both intrinsic and extrinsic muscles. The intrinsic muscles (superior longitudinal, inferior longitudinal, vertical, and transverse) have no bony connections and serve to alter the form of the tongue and receive their motor innervation via the hypoglossal nerve. The extrinsic muscles (genioglossus, hyoglossus, styloglossus, and palatoglossus) cause movement of the tongue relative to other oral cavity structures. The extrinsic muscles also receive motor innervation via the hypoglossal nerve except for the palatoglossus, which is innervated via the pharyngeal plexus (18).

The palate is controlled by numerous muscles which receive motor innervation from the pharyngeal plexus (18). The palatoglossus raises the tongue, and the pala-topharyngeous elevates the larynx and pharynx, all of which narrow the oropharyn-geal opening. The musculus uvulae alters the uvula, and the levator veli palatini elevates the soft palate to bring it in contact with the posterior pharyngeal wall. The tensor veli palatini moves the soft palate laterally, producing rigidity.

The average adult mouth houses 32 permanent teeth, obviously varying greatly among individuals. Each quadrant of the jaw typically contains two incisors (maxillary larger than mandibular), one canine, two premolars, and three molars (decreasing in size distally). An individual tooth is comprised of a crown, coated with a 1.5 mm layer of enamel, and a root, covered with cement and separated by the cervical margin. The body of the tooth is made of dentine and contains a pulp cavity at its core. The root is surrounded by alveolar bone, which undergoes resorption when teeth are lost, and a 0.2-mm thick periodontal ligament separates the two structures. Gingiva surrounds the tooth, periodontal ligament, and bone near the cervical margin (5).

During central occlusion, teeth achieve maximum contact. In this position, the mandibular canines and post-canines are slightly in front of their maxillary counterparts. The mandibular incisors make contact with the lingual aspect of their maxillary counterparts during mastication (5). Mastication is neither completely voluntary or involuntary: masticatory movements can be voluntarily initiated, but these motions continue without further voluntary input. This involuntary activity is thought to be mediated by the chewing reflex as shown by Sherrington in his work with decerebrate animals (19). In this study, with downward forces on the jaw, possibly due to gravity or food, the stretching of muscles activates muscle spindle receptors, which in turn lead to contraction of the jaw-closing musculature. As the jaw closes, food in the oral cavity comes in contact with the teeth, gingiva, or hard palate, inhibiting further contraction of these jaw-closing muscles and stimulating contraction of jaw-opening muscles. As this cycle continues, the stimuli grow weaker, decreasing inhibition of the jaw-closing musculature. A chewing cycle lasts approximately two-thirds of a second and the bolus is in contact with the teeth for

20% of this time (20). This reflex is unilateral, reacting only to the side of the mouth containing the food bolus (19).

The rate of chewing is quite variable but is thought to have an average rate of 1-2 strokes/sec. The time necessary for mouth opening is not significantly different than that for mouth closing: the mouth opens at a rate of about 7-8 cm/sec, with a rotary downward movement toward the side with the bolus, and when closing, the mandible returns to the position of central occlusion at a rate of over 10 cm/sec. Chewing generally takes place bilaterally for the first 3-4 strokes, after which the food bolus is moved to a preferred side (19).

We are capable of producing chewing forces much greater than those needed for a normal human diet. In 1948, Howell and Manly measured the maximal force between incisors to be 130-240 N and that between molars to be 22-881 N in tested subjects (n = 4) (21). A more recent study has shown the mean maximal bite force of the molar and premolar area to be 738 N ± 209 N whereas investigators in a similar study reported that the mean maximal bite force of the molar area is 847 N in men and 597 N in women (22,23). However, these maximal forces are 25-30% less in individuals with full dentures (18). The actual force used during mastication varies according to the properties of the food, with harder foods necessitating higher pressures (20).

Chewing efficiency is dependent on a number of factors including dental occlusion, producible force, number and position of teeth, strength of teeth, and their periodontal structure, function of musculature involved in mastication, and time given to chew. Molars are important to chewing efficiency because the first and second molar can provide up to 70% of the chewing surface area. Some individuals have decreased efficiency secondary to limited chewing tolerance due to factors such as pain.

It has also been shown that chewing efficiency is best in individuals with normal teeth and better in those with partials than complete dentures. Duration of mastication is prolonged in patients with partials compared to complete dentures, but this is thought to be due to the increased periodontal proprioception in this group (24). Also, persons with partial dentures tend to seek regular dental follow-up because they have a high desire to retain their remaining natural teeth. In contrast, patients with complete dentures tend to neglect dental follow-up. The mandibular alveolus undergoes a slow but progressive resorption in the edentulous patient, and as this resorption becomes more dramatic over time, the ability to use a tissue-borne denture is eventually lost because the alveolar ridge necessary for stabilization is progressively shallower.

Abnormal

Discussion of abnormal mastication will be limited to impairments in the movement of the mandible. Although resections of the tongue can negatively effect mastication, these will be discussed as impairments in bolus formation and propulsion in a later section of this chapter.

Resection of the lateral oral cavity soft tissues is often performed with preservation of the ipsilateral mandible. In this setting, scar contracture can result in significant limitation in temporomandibular joint (TMJ) range of motion with resultant trismus. This is particularly true with larger resections of the buccal soft tissues and muscles of mastication.

The amount of scar contracture that forms is governed by multiple issues, including the volume of resection, the amount and location of pterygoid or masseter muscle loss, the addition of post-operative radiotherapy, and the type of reconstruc tion employed. For example, one would expect the maximal amount of contracture with healing by secondary intent, some improvement with a split-thickness skin graft, more with a full-thickness graft and the best result with vascularized tissue such as a radial forearm, free-tissue transfer. Heavier and thicker flaps will produce excessive bulk, which can prevent adequate mastication by placing soft tissue between the occlusal surfaces of the mandibular and maxillary alveolus. Often, even with a well-planned and ultimately successful reconstruction, the temporary swelling associated with the inflammatory phase of healing will produce this difficulty with mastication.

Although, replacement with a vascularized flap is helpful, a second important component of minimizing trismus is the use of aggressive post-operative physical therapy. This therapy should be initiated early to counteract the contraction that occurs during maturation of the wound.

Most difficulties with mastication are the result of segmental resections of the mandible. Resection of the lateral mandible creates an unbalanced set of forces in the pterygoid and masseter muscles. The pterygoids are greater in bulk and stronger than the masseters and will therefore overpower the masseter and pull the remaining intact mandible inwardly or medially. Some physicians think that this medial displacement lessens the cosmetic deformity produced by resection of the hemimandible (25). This jaw shift produces a cross-bite malocclusion. However, with guide training, the patient can be taught to correct much of this malocclusion (25). With a lateral mandibular defect, most dentulous patients will resume chewing and often have surprisingly little deterioration in their diet when compared to their preoperative diet status. In edentulous patients, the loss of masticatory ability with lateral resection is also surprisingly minor. Even so, these unbalanced movements produce an abnormal set of forces on the TMJ. In younger patients with anticipated longevity, TMJ arthritis is more likely.

Reconstruction of the lateral mandible with an osteocutaneous free flap or with a soft-tissue flap and plate will maintain the original size and contour of the oral cavity. Heavier bone flaps such as the iliac crest or fibula will also allow for implant-borne dental restoration. However, in most cases these osteocutaneous flaps also bring in a cutaneous soft tissue component, which remains largely asensate and adynamic. In some cases, this cutaneous component becomes problematic with bolus formation or food retention in this portion of the oral cavity. This is most likely to occur in areas with a natural concavity, which might harbor foodstuffs, such as the floor of mouth (FOM) and labial/buccal sulcus. In contrast, in the non-reconstructed patient, the oral cavity can often be closed primarily. The size of the oral cavity can then be significantly decreased, because the mandibular arch no longer restricts it. This avoids the previously mentioned difficulties that can be encountered with asensate skin in the oral cavity. Also, with these primary closures, the slope of the closure tends to slant toward the unresected side, which helps keep food on the "normal" side of the mouth. Typically, in patients with a higher performance status, the advantages of reconstruction of the lateral mandible will outweigh these negatives. A comparison of pectoralis major or radial forearm flap plus plate reconstructions demonstrated a slight reduction in plate extrusion with the free forearm group. Also, no real difference in oral deglutition or facial contour was seen when these soft-tissue-plus-plate options were compared with osteocutaneous free-flap reconstruction of the lateral mandible (26). However, only the latter reconstruction allows for consideration of implants.

Loss of the anterior mandibular arch produces profound impairment in mastication and all other oral functions. Even those who are less inclined toward free-tissue transfer will agree that osteocutaneous free flaps are the best reconstruction option for these defects. However, even with bony free-flap reconstruction, aberrations in mastication can occur. For example, loss of the anterior musculature of the mandible (mylohyoid and digastric muscles) results in impairment in jaw opening. In addition, loss of this portion of the mandible results in loss of hyoid suspension with consequent downward displacement of the larynx and resultant propensity for aspiration. As a result, hyoid suspension is helpful with anterior mandibular reconstruction.

BOLUS FORMATION AND PROPULSION Normal

During oral preparation, the stage in which pleasure for food and eating is experienced, food is manipulated to a texture that is appropriate for swallowing (27). The incisors are used for vertical movements such as biting, during which the lips and anterior portion of the tongue manipulate the food (18,21). The anterior third of the tongue is responsible for the manipulation of food such as taking the food off a utensil, lapping, or licking, etc. (28). The molars use lateral and vertical movements to crush and grind food (18), whereas the cheek and the body of the tongue manipulate food into the correct position (29). The middle third of the tongue is used to maintain appropriate placement of food during mastication. This portion of the tongue moves the food into the lateral food channels positioned over the sides of the tongue and between the teeth (28). The tongue is also used to crush food against the hard palate (17), manipulating food into particles that are only a few cubic millimetre in volume (20). Facial tone from contraction of the buccal musculature prevents food from entering anterior and lateral sulci, between the lips or buccal mucosa and the mandible, thus keeping the food in the medial oral cavity (27). During this phase, the palatoglossus muscle descends the soft palate to prevent food from prematurely entering the oropharynx as well as to increase the width of the nasal airway (27,30).

After the oral preparatory stage of the swallow just described, the next stage of swallowing is known as the oral phase, which takes approximately 0.7-1.2 seconds (31). This stage is voluntary and causes the food bolus to move from the oral cavity to the oropharynx. Once a bolus is formed, it can be held between the dorsal surface of the tongue and the hard palate or between the tongue and the floor of the mouth (27). If the bolus is in the latter position, the tongue must maneuver the bolus to its dorsal surface prior to bolus propulsion (27). The intrinsic muscles of the tongue move the bolus to the dorsal surface of the tongue, which is referred to as the preparatory position (18). The elevating movement of the anterior and lateral portions of the tongue is also used to separate the 5-15 cm3 portion of the bolus that is an acceptable texture to be swallowed from other oral cavity contents (20,29). If a smaller volume of only several cubic centimetre is swallowed, the oral cavity does not close around this small volume as it does with a larger bolus. Instead, assuming an anterior seal (i.e., competent oral sphincter) there is significant admixture with air such that, for small bolus sizes, there is significant aerophagia (31).

Two distinct but not mutually exclusive theories are thought to explain the neural control of the oral and pharyngeal stages of swallowing. The first, the reflex chain hypothesis, suggests that the act of swallowing is a chain of reflexes in which one action (movement of the bolus) stimulates the next. The second hypothesis, the central pattern generator hypothesis, states that deglutition is controlled by the medullary swallowing center (32). Thus, once swallowing is initiated, the process continues independent of sensory feedback (32).

After the bolus is formed, it is necessary to move it from the oral cavity to the oropharynx, which is known as bolus propulsion. The mandible is kept in a closed position to stabilize the tongue musculature (32). The posterior third of the tongue plays a major role in bolus propulsion, especially when the bolus volume is large (28,31). The posterior third of the tongue acts like a wedge once food has entered the oropharynx by opposing the soft palate and pharyngeal constrictors, which, in turn, pushes the bolus downward. With smaller bolus volumes, the pharyngeal constrictors play a larger role in this propulsion, because a bolus of this size can simply be spilled into the oropharynx (29,31). The anterior tip of the tongue comes in contact with the hard palate and the lateral aspects of the tongue contact the alveolar ridge or the pharyngeal wall, which provides the pressure necessary to propel the bolus into the pharynx (30). The central groove of the tongue then undergoes centripetal (contraction of the genioglossus muscle and vertical and transverse intrinsic muscles of the tongue) and centrifugal motion, causing a wave-like or rolling movement, which acts to transport the bolus (31). The mylohyoid elevates the floor of the mouth (16). The soft palate is then elevated by musculus uvulae, levator veli palatini, and tensor veli palatini to close off the nasopharynx, thereby preventing the generated pressure from dissipating through the nasopharynx and preventing the bolus from being refluxed into the nasopharynx (18). When the bolus reaches the facial arches, sensory receptors are triggered and the pharyngeal swallow reflex is initiated (27). Pharyngeal pressure generators activate when the pharyngeal swallow is initiated (30). In general, that part of a food bolus which falls behind the glosso-palatal junction will be swallowed, where as that component anterior to this junction remains in the oral cavity (31).

The frequency of deglutition varies among individuals and also according to the activity being performed. An average value for number of swallows/hr was determined to be 24, which is equivalent to about 585 swallows/day (5). This frequency of swallowing increases 7- to 12-fold during the act of eating (33). When an individual is asleep, the rate of swallowing decreases to an average of 5.8-7.5 swallows/hr, and this activity chiefly occurs during the processes of falling asleep or waking or during periods associated with movement arousals (32,34).

Abnormal

Abnormalities in bolus formation and propulsion can occur with disease processes or anatomical changes created by surgical manipulation that promote food retention, result in loss of bolus manipulation and propulsion, and/or result in bolus escape.

The creation of asensate concavities or pockets within the oral cavity produces problems with food retention for different reasons depending on the reconstruction options. For example, the pectoralis major flap has excess weight and bulk, which can pull the oral soft tissues downward. The lighter platysma flap has a pedicle, which is shorter along its medial skin edge than along the lateral edge; therefore, when a platysma flap is brought into the FOM, the medial edge between the ventral tongue and the platysma skin will pull downward, creating a "pedicle pull.'' This problem can be avoided with the similar but better vascularized submental island skin flap or other free-tissue transfer options such as the radial forearm or lateral arm flap for FOM and tongue reconstruction. However, when using the radial forearm flap, it is easy to place too much tissue in the FOM, so limiting the flap size to the size of the resected area provides enough soft tissue to avoid limitation in tongue mobility, but avoids excess tissue that would result in bolus retention and stasis.

Another problem associated with reconstruction is re-establishing sensory input through re-innervation, which may improve overall function but does not fully restore normal function. Logemann and Bytell (35) found that patients with anterior FOM resection had significant problems with anterior stasis of the bolus with consequent oral incompetence and drooling.

Generally, most problems arising from reconstruction efforts can be eliminated via smaller revision procedures. Pedicles can be divided when adequate neovascular-ization has occurred, flaps can be debulked, and areas of cutaneous surface redundancy can be eliminated. When problems of food retention do occur despite the best efforts, patients can work with a swallowing therapist to develop strategies to clear the material during the act of eating. Also, with more fastidious oral care, problems with halitosis can be avoided.

Problems of bolus formation relate primarily to loss of anterior tongue function. As described above, the intrinsic musculature allows the shape of the tongue to be altered while the extrinsic musculature moves the tongue relative to the remainder of the oral cavity. In persons undergoing anterior tongue resection, reconstruction should focus on maintaining the sensory and motor function of the remaining anterior tongue. Often when less than half of the anterior tongue is involved, and the resection does not extend into the FOM, primary closure is the best method.

Although the patient will have less tongue bulk, the remaining tongue will be fully sensate and retain good mobility. In contrast, when FOM and alveolar subunits are also involved, reconstruction with skin grafts or vascularized flaps is required to retain tongue mobility; although, even this point is open to debate. A prospective analysis of speech and swallowing was conducted in patients (n = 284) who were divided into three groups based on the method of oral cavity and oropharyngeal resection (primary closure, myocutaneous flap, and free-tissue transfer). The study results suggested that, contrary to popular belief, primary closure resulted in equal or better function than the use of flap reconstruction in patients with a comparable locus of resection and percentage of oral tongue and tongue base resection (36). In general, patients often compensate better than expected with oral tongue reconstruction. It is important to emphasize the significant improvements that occur over time in those discouraged by initially poor speech and bolus formation/propulsion performance. This concept was demonstrated in a simple study by McFarland and colleagues, in which significant adaptive improvements were made in a very short interval (15 minutes) after patients received a palatal obturator (37).

In patients who have undergone near total or total glossectomy, the problems are somewhat different. Logemann and Bytell (35) found that oral transit time increased as the amount of resected lingual tissue increased. This finding was reinforced by studies that examined compensatory features of patients who have undergone near total or total glossectomy. Specifically, Kothary and Desouza clarified functional compensatory swallowing mechanisms in patients (n = 25) with total glossectomy. Because the patients lack a tongue for bolus propulsion, the food was passed by quickly tilting the head posteriorly, which placed the food in the oropharynx to initiate the pharyngeal phase of the swallow (38). A liquid or pureed diet is often most appropriate for total glossectomy patients in as much as the absence of tongue movement makes bolus formation and propulsion very difficult.

Restoration of form and bulk in patients who have undergone total glossect-omy can greatly improve swallowing function. If the reconstructed oral mound (residual tongue or flap tissues) can make contact with the palate, the food bolus can be pushed posteriorly rather than simply using the head tilt maneuver previously described. The importance of apposition of the residual or reconstructed tongue with the palate was also proved by Robbins et al., who found improvements in speech with a palatal augmentation prosthesis that allowed improved contact of the oral mound with the palate and thus allowed improvement in articulation, voice speed, and intelligibility (39).

With regard to extent of resection and its relation to risk of aspiration, persons with good pulmonary reserve who undergo total glossectomy with resection of both hypoglossal nerves can be taught to swallow safely if their palatal and supraglottic sensation is preserved. Similarly, those who undergo resection of the entire tongue base and the epiglottis can be taught to swallow safely. In contrast, those who undergo total or near total glossectomy and also require epiglottic resection or resection of lateral oropharyngeal/palatal soft tissues will have significant aspiration difficulties. Similarly, those patients who undergo resection of the entire tongue base in conjunction with the entire supraglottic larynx will also perform poorly. Thus, the surgeon must determine when evidence suggests post-surgical swallowing failure, and he/she must attempt to prevent or manage the problem.

When deciding which patients will tolerate total glossectomy, the criterion for open supraglottic laryngectomy can be considered. First, the patient should have pulmonary function indices of at least 70% of predicted values. Next, the patient should understand that longer recovery time is required, that swallowing retraining may be necessary and is highly likely, and that a perioperative gastrostomy tube is generally necessary. The patient's age should also be considered. In general, younger patients do relatively well and those over the age of 70 do more poorly, even when preoperative functional status indicators look favorable.

Use of laryngeal suspension may also be beneficial. When near-total or total glossectomy is performed, the suprahyoid musculature has been divided and therefore the larynx descends inferiorly. Suspending the hyoid from the anterior mandible brings the larynx anteriorly and superiorly. In Weber et al.'s review of total and near-total glossectomy patients, none of the patients who had undergone laryngeal suspension had persistent aspiration, whereas 20% of those who did not undergo larygneal suspension had serious problems with aspiration (40).

TASTE Normal

Taste buds are the receptors for the sensation of taste. These taste receptors are mainly located on the tongue, but can also be found on the palate, epiglottis, larynx, pharynx, uvula, proximal third of the esophagus, lips, and cheeks. On the tongue, taste buds are contained in three of the four varieties of papillae, which include fungiform, circum-villate, and foliate papillae; filiform papillae do not contain taste buds. Fungiform papillae are located on the anterior two-thirds of the tongue and each house 1-18 taste buds. The circumvillate papillae form a "V" on the posterior aspect of the tongue, just anterior to the sulcus terminalis. Foliate papillae can be found on the lateral aspect of the tongue, just anterior to the circumvillate papillae (41). Taste receptors undergo atrophic changes with age, generally beginning after the age of 45 (18).

Three cranial nerves are involved in the sensation of taste. The anterior two-thirds of the tongue (fungiform papillae, anterior foliate papillae) and the soft palate are innervated by the chorda tympani (branch of the facial nerve), which travels in the lingual nerve sheath. The posterior third of the tongue (circumvillate papillae and posterior foliate papillae) is innervated by the glossopharyngeal nerve. The vagus nerve innervates the tongue base, pharynx, larynx, uvula, and epiglottis. The tongue has four basic taste dimensions, each of which is sensed best at a specific location on the tongue. Sweet, bitter, sour, and salty tastes are discriminated at the tip, base, middle and lateral, and tip of the tongue, respectively (18).

Abnormal

The sense of smell and taste together are responsible for conveying the appeal of food. Loss or distortions in olfaction or the sense of taste will have negative impacts on quality of life and even on overall health. Persons who have an adequate swallowing mechanism but significant hypoguesia or dysguesia may become nutritionally depleted and even cachectic due to the loss of interest in food.

Abnormalities in taste occur in persons who have undergone operations that result in loss of olfaction such as anterior craniofacial resection and laryngectomy. Surprisingly, unless an individual has had a total glossectomy, most persons do not complain of hypoguesia or dysguesia as a result of surgical resection of the tongue alone. In contrast, those who have also undergone radiotherapy to the oral cavity have significant difficulties with loss of taste or more commonly, impairments of taste.

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