10: Applied Surgical Anatomy of the Head and Neck

Applied Surgical Anatomy of the Head and Neck

Outline

Because traumatic injuries disrupt the anatomy, the surgeon who is to repair and replace these traumatized structures must have an in-depth knowledge of normal anatomy. In addition, the operator must consider possible variations of normal and other associated structures that may be in close relationship to the traumatized area. Although numerous texts have been written on basic anatomy,1–11 it is thought that for completeness, this textbook should include a review of major head and neck anatomy. Details on specific problems and treatment modalities are found in the appropriate chapters. It is the intent of this chapter to discuss the general anatomy, its inherent relationships, and some technical problem areas that should be considered in the management of traumatic facial injuries.

Skin Lines and Lines of Langer

The natural skin lines and wrinkles are major factors in determining the final soft tissue aesthetic result for the patient with facial trauma. The character and aesthetics of a scar are affected by its relationship to the location and direction of normal skin lines.

The natural skin lines and wrinkle lines are different from the lines of Langer, which denote the direction of the collagen fibers within the dermis. Langer believed that the skin was less extensible in the direction of the lines of tension that cross them.2,12 In the face, Langer’s lines have been shown to run across natural creases and flexion lines, thus making scars generated by incisions and trauma along those lines more noticeable. It is therefore recommended that elective incisions be made in or parallel to the lines of facial expression or the natural skin lines, when possible (Fig. 10-1).13 Skin wrinkles provide an excellent template for areas of relaxed skin tension. Old scar lines and hairlines can also direct a surgeon to the most appropriate placement of an incision. Considerations as the ethnicity of the patient must also be taken into account because hyperpigmentation and hypopigmentation issues might arise.

Scalp

The scalp is made up of five layers, three of which are closely bound together. These are the skin, dense connective tissue, and galea aponeurotica. Beneath these layers are the loose connective tissue and the periosteum or pericranial layer.1,6,8 The scalp bleeds freely because the vessels are bound firmly in the dense connective tissue layer (Fig. 10-2). This firm union and the extensive blood supply frequently make bleeding excessive and often difficult to control rapidly with hemostats. Pressure usually controls the open bleeders, and the rapid application of Raney’s clips controls full-thickness lacerations or elective incisions. Because of the nature of the loose connective tissue layer, dissection of the scalp is rather easy in this tissue plane. In a similar manner, however, the effusion of fluid spreads rapidly in this plane, leading to a boglike edema.

The innervation of the scalp comes from the trigeminal nerve anteriorly and laterally and from the cervical nerves (C2 and C3) posteriorly.1,4 If dissection is kept within the loose connective tissue layer, these nerves are avoided. In the supraorbital region, the superior orbital branch of the trigeminal nerve passes through a notch or foramen to innervate this area of the scalp. The supratrochlear nerve is located slightly medially and innervates the upper lid and the medial area of the forehead.14 Care should be taken when elevating flaps and managing lacerations in this area. As with most areas of the anatomy, when the skeleton makes angles or muscles insert, there is a denser attachment of the skin and soft tissue. In the scalp, this attachment is most notable in the glabella and supraorbital regions.

Skin of the Face

The skin of the face becomes specialized in the area of the eyelids, which are comprised of two structural lamellae: the external lamellae formed by the orbicularis muscle and its overlying skin and the internal lamellae of the tarsal plate and conjunctiva.15 The skin of the eyelid is extremely thin and delicate and contains small lacrimal, sweat, and sebaceous glands and hair follicles (Fig. 10-3).16

The skin of the nose is tightly attached to the lower lateral cartilage in the tip area. In other areas, the skin is less tightly adhered to the underlying infrastructure. The skin is thin in the nasal root and tip areas and thicker in the supratip region.4

Osteology

The bones will be considered in the traditional facial thirds (Fig. 10-4).3,5

Midface

The maxilla, zygoma, lacrimal, nasal, palatine, inferior nasal concha, and vomer bones are collectively referred to as the middle third of the facial skeleton.4,17,18 Although the sphenoid, frontal, and ethmoid bones are not classically facial skeleton bones, they are frequently traumatized in midfacial fractures and thus should be considered in the midfacial skeleton. The bones will be discussed separately, but their interconnections are of utmost importance.

Maxilla

The maxilla (Figs. 10-5 and 10-6) is a paired bone of the upper jaw,* fused to form one bone, and is the central focus of the middle third of the face. Each hemimaxilla contains a large pyramid-shaped body, the maxillary sinus (antrum of Highmore), and four prominent processes—the frontal, alveolar, zygomatic, and palatine processes.

The body of the maxilla is hollow and contains the maxillary sinus. The anterior wall of the sinus is the facial surface of the maxilla and is usually thin. The medial wall is the lateral nasal wall. The sinus opens superiorly and medially into the nasal cavity at the semilunar hiatus in the middle meatus. The superior wall or roof of the sinus is the orbital floor, and the floor of the sinus is the palatine and alveolar processes of the maxilla.

The frontal process arises from the anteromedial corner of the body and articulates with the frontal bone to form the medial orbital rim. The medial portion of the frontal process fuses with the nasal bone and may therefore be termed the nasofrontal process. Posteriorly, the process articulates with the lacrimal bone to form the anterior portion of the medial orbital wall. This area of articulation with the frontal bone, nasal bone, and lacrimal bone is prominent in the facial skeleton and is frequently fractured by blunt trauma.

The inferiorly extending portion of the maxilla is the alveolar process, which contains the maxillary teeth. The teeth are key to the accurate management of many midfacial fractures. The alveolar process may be fractured by direct trauma and therefore may be functionally separate from other portions of the maxilla.

The palatine process arises horizontally from the lower edge of the medial surface of the body. It joins the opposite process and forms the major portion of the hard palate.

The zygomatic process of the maxilla arises from the anterolateral corner of the maxilla and articulates laterally with the zygoma. Together, they form the inferior orbital rim and the greatest portion of the orbital floor. The infraorbital foramen is on the anterior surface of the zygomatic process of the maxilla.

Surgical Note: The classic Le Fort I fracture passes through the anterior wall of the maxilla, extending posteriorly to the pterygoid plates. It is important to remember that this is a paired bone and, even though it is fused in the midline, in adults it behaves like two separate bones when manipulated. It may often be separated along the midpalatal suture in the more extreme facial fractures.

Zygoma

The zygoma (zygomatic bone, malar bone) is a paired bone that makes up the essence of the cheek prominence (Fig. 10-7; see also Fig. 10-4).* This thick, strong, diamond-shaped bone forms the lateral and anterior projections to the midface and is composed of four processes. The frontal process forms the lateral orbital wall and articulates with the frontal bone at the frontozygomatic suture. It is this articulation that is separated or rotated in isolated zygomatic fractures. The temporal process forms the zygomatic arch and articulates with the temporal bone. The maxillary process articulates with the maxilla to form the infraorbital rim and part of the floor of the orbit. Finally, the fourth process joins the maxilla on the lateral wall, producing the zygomatic eminence. This is an area of thickened bone that is usually available for fixation in the treatment of zygomaticomaxillary complex (ZMC) fractures. Along the crest of the zygoma, on the inferior aspect, is the insertion of the masseter muscle. The direction of force for this muscle is down and backward and its contraction contributes to displacement of the complex fracture of the zygoma, which may precipitate redisplacement in the improperly fixated fracture.

The zygoma articulates with the sphenoid bone on the posterior aspect of the frontal process. This articulation is with the greater wing of the sphenoid bone and forms the lateral wall of the orbit.

The only foramina of the zygomatic bone are the zygomaticofacial foramen, which opens from the orbital surface of the bone and passes to the eminence, and the zygomaticotemporal foramen, which opens to the infratemporal fossa. The zygomaticofacial and zygomaticotemporal branches of the second division of the trigeminal nerve pass from within the orbit to the surface and give sensory innervation to the associated structures.

Surgical Note: The classic zygomaticomaxillary fracture (tripod, zygomaticomaxillary complex, and trimalar) involves this bone and its articulations. Details on the fracture and its management are found in Chapter 16. The options of surgical manipulation for reduction are discussed in the following sections.

Temporal Fossa Approach.

The Gillies approach is made via an incision in the hair-bearing area of the scalp, approximately 2 cm above and 1 cm anterior to the ear.14,2023 This incision is carried down to the level of the temporalis fascia. The only structure of anatomic importance is the superficial temporal artery. This vessel courses across the area of the incision and can be identified through palpation and thus avoided by the properly placed incision. If it is encountered, it can be ligated and cut without complications. The temporalis fascia runs to the arch of the zygoma. An incision is made through this fascia to expose the muscle and develop a path for the passage of instruments to manipulate the zygoma. The main anatomic concern is being too superficial to the fascia, thus introducing the elevator lateral to the arch.

Lateral Brow Approach.

The lateral brow incision* is made to allow easy access to the frontozygomatic suture, because this area frequently requires open inspection, manipulation, and stabilization. Again, no major anatomic structures lie in close approximation. The incision is made full thickness to the bone, and the periosteum is reflected to expose the fracture. Generally, some form of elevator is placed posteriorly in the infratemporal fossa to aid in elevation of the fracture. Care should be taken not to lever against the temporal bone and displace any nondiagnosed, nondisplaced skull fractures. The incision is generally placed in the hair of the brow. The usual rule of making an incision perpendicular to the skin margin can be altered in this area because the incision should be made with the long axis of the hair follicle to prevent damaging the follicles, which could prevent the regrowth of hair. This hair should not be shaved for numerous reasons. The hair will grow back, but slowly, and it will be of a different texture and may often be sparse. The most notable problem is that of aligning the hair-bearing skin margins during suturing. If this is not performed properly, there will be an unsightly step in the brow when the hair does regrow.

Approaches to the Inferior Rim.

The multiple surgical approaches to the inferior rim and articulation of the zygoma and maxilla are described in Chapter 16. These approaches provide access to the inferior orbital rim, orbital floor, lacrimal duct area and, in some cases, medial and lateral orbital walls. There are few anatomic problems if care is used to locate the layers of the inferior lid, most specifically the orbicularis oculi muscle, orbital septum, and inferior orbital rim. The approaches from the skin include the infraciliary incision and a lower incision through an existing skin crease. The infraciliary incision is aesthetically pleasing but may result in excessive and prolonged edema of the eyelid. The lateral extent of this incision, combined with the frequently needed lateral brow incision, compromises the lymphatic drainage of the lower lid. The skin is thin in this area and the skin flap must be developed carefully. The incision through the orbicularis oculi muscle is performed longitudinally to expose the inferior orbital rim. The incision through the periosteum should be on the facial aspect of the bone but above the infraorbital nerve.

Transoral Approach.

The floor of the orbit and infratemporal fossa can be approached through a buccal mucosa incision in the posterior maxillary vestibule.15,1719 The eminence of the zygoma becomes available for stabilization via this approach. The maxillary sinus can be entered and the floor of the orbit, inferior rim, and eminence of the zygoma can be elevated from within. The infratemporal fossa can be entered posteriorly and superiorly, and the zygomatic arch and zygomatic body may be elevated. The buccal fat pad often interferes with visualization but is generally of no notable anatomic concern.

Transconjunctival Approach.

The final approach to the orbital floor is via the transconjunctival incision.5,14,21 Its only advantage is the avoidance of the slight facial scar from the infraciliary approaches. In the case of trauma, this is usually not a major factor. The incision is made through the conjunctiva at the lower border of the inferior tarsal plate, with the lower lid retracted inferiorly. A preseptal or retroseptal dissection can be made, although the preseptal dissection offers better control of the orbital fat. The approach to the orbital floor is otherwise the same and there are no notable anatomic problems associated with any of these approaches.

Nasal Bones

The nasal bones (Fig. 10-8; see also Fig. 10-4)1,2,4,6 are rectangular and articulate with the frontal bone superiorly and with each other at the midline. At the superior articulation, they are relatively thick, but inferiorly, they are much thinner. It is in this area that most fractures occur. The nasal bones articulate posteriorly with the frontal process of the maxilla.

Ethmoid Bone

The ethmoid bone is an unpaired bone that is central to the facial structure (Fig. 10-9),* and it is an integral part of the nasal structure, to both orbits and to the anterior cranial base. The perpendicular plate forms the superior and anterior portions of the nasal septum and attaches to the cribriform plate. It articulates posteroinferiorly with the vomer and posterosuperiorly with the sphenoid bone.

The cribriform plate articulates anteriorly and laterally with the frontal bone and posteriorly with the sphenoid bone. Hanging bilaterally from the cribriform plate are the superior and middle nasal conchae. The middle concha has thin-walled ethmoidal air cells, which extend laterally to it. The multiple septa, which pass relatively perpendicular to the conchae, extend laterally to the thin plate of bone that constitutes most of the medial orbital wall. This bone is the lamina orbitalis of the ethmoid bone. It is extremely thin; hence the term lamina papyracea.

Surgical Note: The thin lamina orbitalis may be fractured in blunt orbital trauma.11,15,17 The anterior ethmoid artery is a point for ligation as it passes from the orbital to the nasal aspect of the ethmoid bone. Because this artery is one of the terminal branches of the ophthalmic artery, which is a branch of the internal carotid artery, it is not affected by the usual measures to control facial bleeding and may require direct ligation via a medial canthal approach. The anterior ethmoid foramen is approximately 1.5 cm deep, measured from the medial orbital rim. Rarely is any surgical manipulation of this bone necessary or possible.

Vomer

The vomer (see Fig. 10-8)1,4,7 is a plow-shaped bone that is located in the midline of the nasal fossa and forms the posterior portion of the nasal septum. It articulates with the palatine, maxillary, and ethmoid bones and rarely is of notable concern in the primary management of facial trauma.

Palatine Bones

The paired palatine bones connect the maxilla with the sphenoid bone (Fig. 10-10; see also Figs. 10-5A and 10-6).1,2,4,5 This extremely irregularly shaped bone is composed of a major horizontal portion and vertical perpendicular plates. The horizontal plate articulates anteriorly with the maxilla and with the palatine bone of the opposite side in the midline to form the posterior aspect of the hard palate.

The vertical plate passes superiorly behind the maxilla and articulates posteriorly with the lateral pterygoid plate of the sphenoid bone. A ledge of the vertical plate terminates in a small contribution to the orbital floor at the posteromedial aspect. The junction of the sphenoid and palatine bones forms the sphenopalatine foramen. This foramen attaches the posterior aspect of the nasal cavity with the pterygopalatine fossa.

Surgical Note: Manipulation of the maxilla generally accomplishes adequate reduction of the palatine bones.8,17 It is important to remember the small contribution of the palatines to the orbital floor because extreme trauma to the maxilla and palate may cause some displacement or involvement of the orbital contents.

Inferior Nasal Concha

The inferior nasal concha is a paired bone2 that forms the bony support of the inferior turbinate bilaterally. It is of surgical importance only when it obstructs the inferior meatus and the nasal lacrimal duct.

Frontal Bone

The frontal bone (Fig. 10-11; see also Fig. 10-4)* is a cranial bone that is unpaired and forms the anterior portion of the calvaria. The importance of this bone in facial trauma is its relationship to the anterior midfacial skeleton and the paranasal sinuses. The frontal bone articulates laterally with the zygoma and medially with the maxilla and nasal bones. Inferiorly and deep in the middle of the face, it articulates with the ethmoid and lacrimal bones and posteroinferiorly articulates with the wings of the sphenoid bone. The frontal bone articulates posterolaterally with the parietal bones.

The frontal bone forms a great portion of the roof of the orbit. Its thickened projections articulate laterally with the zygoma at the frontozygomatic suture and form the lateral orbital walls. The thickening of the frontal bone in the anterior region forms the supraorbital ridges. These curved elevations connect the zygomatic portion of the frontal bone with its midportion, articulating with both the maxilla and nasal bones. The supraorbital notch or foramen crosses this rim and transmits the frontal vessels and nerves.

The frontal sinus lies in the frontal bone in an area superior to the articulation with the nasal bones. Approximately 4% of the population do not have a frontal sinus. The sinus is not a simple chamber but rather is subdivided into compartments or recesses by incomplete bony partitions. There is usually an intrasinus septum that divides the left from the right. Drainage into the nose is by a well-formed duct, the nasofrontal duct. The duct itself is soft tissue and may follow a serpentine course to the anterior middle meatus of the nose, where it empties. The frontal sinuses are protected somewhat from injury by the supraorbital ridges. The anterior wall of the sinus has low resistance, but the ridges are highly resistant.

Surgical Note: There are multiple incisions and techniques of management for the frontal sinus. The major anatomic point of concern is the inner table, which, when fractured, demands a neurosurgical evaluation. Other areas of concern are the supraorbital nerves, which can usually be saved with careful dissection and removal from the supraorbital foramen by the use of a small osteotome. The neurovascular bundle can then be retracted with the orbital contents.

Sphenoid Bone

The sphenoid bone (Fig. 10-12)* is a single midline bone situated at the base of the skull that creates the anteroinferior extent of the cranial base and the posterior transition from facial bones to cranial bones. This complex bone has many processes that have delicate articulations with the adjacent cranial and facial bones.

The sphenoid bone articulates with the temporal and occipital bones to form the cranial base. It joins the parietal and frontal bones anteriorly and superiorly to complete the cranial complex. It meets the vomer and ethmoid bones in the midline anteriorly and meets the zygoma, palatine bones, and sometimes the tuberosity of the maxilla to complete its articulation with the facial skeleton.

The body of the sphenoid bone is hollow and forms two cavities separated by a thin bony septum. The hollow cavities are the sphenoidal sinuses; these drain into the sphenoethmoid recess above and behind the superior nasal concha. Although air-fluid levels can frequently be noted on radiographs, surgical management in the trauma patient is rarely necessary.

Lower Face

Mandible

Despite the fact that the mandible (Figs. 10-13 to 10-15)* is the largest and strongest facial bone by virtue of its position on the face and its prominence, it is commonly fractured when maxillofacial trauma has been sustained. Mandibular fractures occur twice as often as midfacial fractures29,32; however, it has been shown in cadaver experiments that almost four times as much force is required to fracture the mandible versus the maxilla.30 The osteology of the mandible, various muscle attachments and their influence, and the presence of developing or completed dentition all play a notable role in producing inherent weaknesses. Therefore, fractures are seen more frequently in certain isolated areas.

The mandible is composed of the body and two rami, with their junction or angle forming the prominent gonion. The angle formed may vary between 110 and 140 degrees (mean, 125 degrees).15 The angle decreases slightly during growth because of changes in the condylar process, shape, and size. With aging, the angle becomes more obtuse.30 The body is U-shaped and has an external and internal cortical surface. The external cortical plate is thickest at the mental protuberance and in the region of the third molar. There is also a thickened triangular mental protuberance bounded laterally by the mental tubercles. The mental foramen is located on the external surface in the vicinity of the root apices of the first and second premolars. There are variations in the exact location of the foramen, as noted by Tebo and Telford.33 The opening is directed backward and laterally and transmits the mental nerves and vessels.18,34 The oblique line runs from just inferior to the mental foramen posteriorly and superiorly to the ascending ramus.

The internal cortical surface is elevated in the midline near the inferior border by the mental spine. Associated with this may be two pairs of discrete bone prominences termed the genial tubercles. They represent the origin of the geniohyoid muscles inferiorly and the genioglossus muscles superiorly. Running horizontally and slightly superior from front to back is an oblique ridge, the mylohyoid line, which represents the attachment of the mylohyoid muscle. Below this is the shallow depression created by the submandibular gland, called the submandibular fossa. Superior to the mylohyoid line and located anteriorly is the sublingual fossa, where the sublingual gland is found in close approximation.

The ramus of the mandible, when viewed from the side, is a quadrilateral structure. The lateral surface may be rough and thickened in the region of the angle by the insertion of the masseter muscle. On the medial surface is the mandibular foramen, which leads downward and forward into the mandibular canal and transmits the inferior alveolar nerve and vessels. The lingula is a medial bony projection to which the sphenomandibular ligament is attached. The mylohyoid groove extends from the lingula and runs anteriorly and inferiorly to the submandibular fossa. Below this is a roughened area created by insertion of the medial pterygoid muscle.

The mandibular notch is located on the superior edge of the ramus. It is bounded anteriorly by the coronoid process and its temporalis attachments, while also bound posteriorly by the neck and head of the mandibular condyle. A detailed description of condylar head anatomy is given later (see “Temporomandibular Joint”). Attached anteriorly to the neck of the condyle is the insertion of the lateral pterygoid muscle and attached laterally is the lateral ligament.

The body of the mandible supports the alveolus and dental structures. The body and alveolus have dense cortical outer and inner tables of bone, with central spongy or cancellous bone.

The strengths of the mandible are apparent when one evaluates the thick, round inferior border and the mental protuberance. The periodontal ligament and bone alveolus also combine with the trabecular pattern in the cancellous bone and are directed in a parallel fashion up the ramus to transmit pressures up to the condylar region.35,36 The thickening on the inner aspect of the condylar neck or crest of the neck apparently acts as a main buttress of the mandible as it transmits pressure to the temporomandibular joint (TMJ) and the base of the skull.29,32 The temporal crest runs from the coronoid process to the retromolar triangle distal to the terminal molar. The thickened posterior border of the mandible may act as an additional crest.28

Major structural forces are created at the angle of the mandible because of the cantilevered nature of its shape. The bone height at this angle is therefore critical in determining its strength and the presence of the perfectly aligned muscle sling created by the masseter and medial pterygoid muscles.37 Thus, aging, with its potential for bone and alveolar resorption, weakens this area.

Areas that exhibit weakness include the area lateral to the mental protuberance, mental foramen, mandibular angle, and condylar neck.29 If teeth are present, the socket is a weak zone, especially if teeth are impacted or unerupted. This would seem to indicate that a child in the mixed dentition stage may be highly susceptible; the fact that the child’s bones are so resilient and flexible offsets the disadvantage of the unerupted teeth.30

Temporomandibular Joint

The TMJ (Figs. 10-16 and 10-17)* is a freely movable synovial joint located between the glenoid fossa of the temporal bone and the head of the mandibular condyle below. The anatomic classification of the TMJ is a diarthrodial joint, with independent discontinuous movement between the two bones. An articular disc (meniscus) divides the joint into two cavities. The superior compartment permits hinge or rotational movement, whereas the inferior joint space permits translatory motion. The bony surfaces within the joint spaces are lined by synovial membrane, which is responsible for secreting synovial fluid that functions as nutritional support and a lubricating medium. This membrane is extremely thin, smooth, and highly vascular. Cells in the synovial membrane have the ability to differentiate into chondrocytes and provide the synovium’s ability to regenerate following injury. The articular surfaces of the joint and condyle and the central portion of the meniscus are composed of collagen. This feature differentiates this joint from most other articulations because the surfaces are not covered by hyaline cartilage. Histologically, this avascular fibrous tissue may contain cartilage cells and therefore may be termed fibrocartilage.10,17 This tissue has the ability to regenerate and to remodel under functional stress or following anatomic alteration, such as fracture.

The condylar head is a semicylindroid process 15 to 20 mm long and 8 to 10 mm thick. The long axis of the condyle is related to the position of the ramus of the mandible and not to the skeletal or frontal plane. The angle formed by the two condylar axes varies between 145 and 160 degrees. The articulating surface of the condylar head faces superiorly and forward, giving the condylar neck an appearance of being bent forward. When viewed anteriorly, the condylar head notably projects medially to the inner surface of the ramus, but less so laterally.

The temporal bone provides the articulating surface to the skull. This bone is located anterior to the tympanic bone. The fossa is composed of a posterior slope and the convex part of the articular eminence. The squamotympanic suture forms the boundary between the tympanic bone and temporal squama. The posterior part of the fossa has a raised crest joining the articular tubercle and postglenoid process. The roof is relatively thin, separating the fossa from the middle cranial fossa. The anterior portion of the fossa or articular eminence is a broad horizontal ledge that is convex in the anteroposterior direction and concave in the transverse direction.

The fibrous capsule surrounding the joint is thickest on the lateral surface and is considered a separate, distinct ligament—the temporomandibular ligament. It extends from the tubercle on the root of the zygoma to the lateral surface of the neck of the mandible, behind and below the lateral pole of the condyle. The temporomandibular ligament has a superficial fan-shaped layer of obliquely oriented connective tissue fiber and a deeper narrow band of fibers that are horizontally oriented.

The articular capsule in general attaches from the temporal bone to the neck of the condyle. There is a loose attachment between the temporal bone and meniscus, with considerable ability to move. The attachment from the meniscus to the condyle, however, is much stiffer and is reinforced medially and laterally. These paired ligaments act to restrict movement of the meniscus away from the condyle. This stabilizes the complex and aids in synchronizing the condyle and meniscus. Other ligaments to consider in this region are the stylomandibular and sphenomandibular ligaments, which serve as passive restraints to mandibular movement. The stylomandibular ligament originates from the styloid process and extends downward to the posterior border of the ramus of the mandible. The sphenomandibular ligament runs from the spine of the sphenoid bone and squamotympanic fissure to the lingula on the medial aspect of the mandibular ramus. The maxillary artery, its middle meningeal and inferior alveolar branches, and auriculotemporal and inferior alveolar nerves pass between this ligament and the mandible.

The articular disc or meniscus, a dense fibrocartilaginous structure, histologically may show varying degrees and locations of cartilage cells.1 The meniscus is nonvascular and has no sensory innervation. When viewed from the lateral direction, there are three anatomic divisions or zones—the anterior band, intermediate zone, and posterior band. The thin intermediate or central zone corresponds to the functional area between the mandibular condyle and posterior slope of the articular eminence. When viewed from above, the meniscus is an oval biconcave structure that is thicker posteriorly. The posterior tissues are highly vascularized and innervated and are referred to as the bilaminar zone. The superior elastic fibers of the retrodiscal tissue attach to the tympanic plate and function to restrict anterior meniscus movement during extreme translation.43 The inferior fibers of the retrodiscal tissue are composed of collagen and attach to the articular surface of the condyle. They act as a check ligament to prevent extreme rotation of the meniscus during rotational movement of the mandible. The anterior attachment of the meniscus is contiguous with the lateral pterygoid muscle. The overall shape and size of the meniscus, therefore, vary depending on age, functional remodeling, and pathologic condition.

The vascular supply to the TMJ arises anteriorly from the masseteric artery and posteriorly from branches of the superficial temporal and maxillary arteries. The retrodiscal region has a rich venous plexus that fills and empties under the influence of mandibular movement.

The nerve supply to the joint is principally from the auriculotemporal nerve, with some additional innervation via the masseteric nerve and the deep temporal nerve.

Mandibular Fracture Location

The mandible has various strengths and weaknesses, as noted.* The common sites of mandibular fracture, therefore, are the mandibular condyle region, mandibular angle region (especially in the presence of an impacted or semierupted third molar), mental foramen or body region, mandibular parasymphysis, and any component of the dental alveolus. In the very young and older patient with mandibular atrophy,30 other factors such as tooth buds developing in the child or a decrease in the cancellous-to-cortical ratio in older adults come into play. Because of the U shape of the mandible, eccentric forces often create bilateral fractures—one at the site of injury and the other contralaterally. Nahum36 has shown that forces in excess of 800 lb are required to fracture the symphysis and both condylar necks, and further demonstrated that the mandible is more sensitive to lateral impact than to impact from the front.

Anatomic Factors in Fracture Displacement

The direction of the causative blow, the direction of the line of fracture, and muscle pull all influence the amount and direction of bone displacement. Muscle forces acting in the anterior region of the mandible, including those inserting in the mental region on the inner surface, are the geniohyoid, genioglossus, digastric, and mylohyoid muscles. These forces act to displace anterior segments inferiorly and posteriorly, with some possible medial component.13,41

In the posterior mandible, the muscles of mastication generally cause upward and forward displacement. This observation is especially true in the pterygomasseteric sling region. The medial pterygoid muscle also creates a medial component of pull. The temporalis muscle has two components of attachment and creates elevating forces and retraction forces. In a similar way, the lateral pterygoid muscle has two attachments. The internal component is responsible for superior, anterior, and medial forces, whereas the external component pulls the condyle down, anteriorly, and medially. If the balance is disrupted because of a fracture, displacement results.

The fracture angulation or direction of the mandibular angle and body region can vary. Depending on its orientation, muscle influence may be enhanced or prevented from actively displacing the proximal segment (Fig. 10-18). A fracture is regarded as favorable if it is in a downward and forward direction horizontally because of locking effects at the fracture site. If the horizontal direction is downward and posterior, the active pull of the posterior elevator muscles, such as the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles, will displace the proximal segment superiorly. The vertical angulation of the fracture or bevel will inhibit the medial forces of the elevator group if the fracture direction is posterior and medial. This condition is regarded as vertically favorable. The opposite is true of the vertically unfavorable fracture traveling anteriorly and medially.

Other factors affecting the amount of displacement are the presence or absence of teeth in occlusion, muscle protection, such as in the pterygomasseteric sling region, and the exact relationship of the fracture position to muscle insertion. This last factor is especially apparent in condylar neck injuries and their relationship to the lateral pterygoid muscle insertion. If a fracture occurs above the insertion of this muscle on the neck of the condyle, little displacement occurs. If a lower level fracture occurs, displacement of the condyle will be medial and anterior because of the action of the lateral pterygoid muscle.

Surgical Note: Fractures that may compromise the patient’s airway are important to the treating surgeon. Whenever the fracture creates an unstable situation for the tongue, consideration must be given to immediate temporary stabilization, intubation, or other means of supporting the airway. Mandibular fractures that may create airway problems include bilateral subcondylar fractures, bilateral parasymphysis fractures, and any maxillofacial trauma with massive edema or oral lacerations with subsequent bleeding.

Mandibular Surgical Approaches

Mandibular surgical approaches are as follows (Fig. 10-19): extraoral approaches include the (1) Risdon, (2) condylar, and (3) symphysis approaches; intraoral approaches are the (1) angle and (2) mental or parasymphysis approaches.

Extraoral Surgical Approaches

An absolute description of various surgical procedures is possible in an elective situation. However, when dealing with a trauma patient, each approach is designed while taking into consideration the location of the injury, extent of the injury, method of stabilization to be used, possible coexisting soft tissue injuries, and potential anatomic factors.* The basic principles of making soft tissue incisions in natural skin folds may be more difficult to apply in the presence of severe edema and lacerations. In dealing with mandibular trauma in this section, the more common surgical approaches used for open reduction and exploration are discussed.

Preauricular and Condylar Approaches

Several soft tissue incision designs have been described, including the inverted L, as described by Blair,38 the T, as described by Wakely,46 and the endaural approach advocated by Lempert.47 Dingman and Moorman1 slightly modified Lempert’s approach and reported it in 1951. The facial nerve, when damaged, poses the most notable obstacle and potentially the most serious complications. A detailed description of the facial nerve appears later in this chapter (see “Facial Nerve”). Other anatomic structures that must be considered in the overall dissection in the preauricular approach include the parotid gland, superficial temporal vessels, and auriculotemporal nerves.

The parotid gland is located below the zygomatic arch in front of the external auditory meatus, with the superficial pole of the gland lying directly on the TMJ lateral capsule. The parotid gland is enclosed in fascia derived from the superficial layer of the deep cervical fascia or parotideomasseterica fascia.

The superficial temporal vessels, along with the auriculotemporal nerve, emerge from the superior aspect of the parotid gland. Bleeding from these vessels can be brisk and often requires ligation. The vein lies superficial and posterior to the artery; the auriculotemporal nerve is located posterior and superficial to the artery. By keeping dissection close to the cartilaginous portion of the external auditory meatus, trauma to the auriculotemporal nerve can be minimized.

The endaural incision is started in the skin crease immediately adjacent to the anterior helix and is carried downward to the level of the tragus. The incision can then be placed in the gap between the spine of the helix and tragus, which is filled with fibrous attachments for the lamina of the tragus. (While in the auditory meatus, the incision remains in contact with the bony tympanic plate.) This incision results in a better cosmetic appearance. It is important not to extend the incision or dissection inferiorly because damage to the facial nerve as it exits the stylomastoid foramen may result.

The initial incision is made through skin and subcutaneous connective tissues, which include the temporoparietal fascia to the depth of the superficial layers of the temporalis fascia. In the upper aspect of the incision, the superficial temporal vessels may be encountered, as may the auriculotemporal nerve. The nerve is retracted and the vessels are retracted or ligated. The temporalis fascia is incised by an oblique incision above the zygomatic arch. The intervening fat is visualized between the two layers of temporalis fascia and blunt dissection is carried out inferiorly beneath the superficial layer of the temporalis muscle. The periosteum is stripped off the zygomatic arch from above. A sharp incision posteriorly along the plane of the initial incision can safely be made down to the periosteal level. The flap is elevated anteriorly, exposing the articular eminence. The temporomandibular capsule is now visualized totally. The condyle is palpated with the help of manual movements of the body of the mandible. Scissors or a scalpel can be used to enter the upper joint space horizontally, if necessary, to evaluate the condylar head surface or meniscus integrity. Depending on the extent of the dissection necessary at this point, the operator should be cognizant of the medial structures, including the maxillary artery, middle meningeal artery, auriculotemporal nerve inferiorly, and pterygoid plexus of veins lying anteromedially. It may be necessary in some cases to use a Risdon approach as well when performing an open reduction in low fractures.

Jan 14, 2015 | Posted by in Oral and Maxillofacial Surgery | Comments Off on 10: Applied Surgical Anatomy of the Head and Neck
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