Face and scalp

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CHAPTER 29 Face and scalp

SKIN

The scalp and buccolabial tissues are described here. The structure of the eyelids is described in Chapter 39.

SCALP

The scalp extends from the top of the forehead in front to the superior nuchal line behind. Laterally it projects down to the zygomatic arch and external acoustic meatus. It consists of five layers: skin, subcutaneous tissue, occipitofrontalis (epicranius) and its aponeurosis, subaponeurotic loose areolar tissue and periosteum of the skull (pericranium).

The skin of the scalp is hairy and rich in sebaceous glands: it is the commonest site for sebaceous cysts. The dense subcutaneous connective tissue has the richest cutaneous blood supply in the body. The anterior and posterior muscular bellies of occipitofrontalis are connected by a tough, fibrous, epicranial aponeurosis, and this layer is therefore often called the aponeurotic layer (galea aponeurotica). These three upper layers of the scalp can easily slide on the underling layer of loose connective tissue. A scalp flap can be raised within the plane between the galea and the pericranium without compromising either the blood or nerve supply of the scalp, because all of these structures lie in the subcutaneous layer (superficial fascia). Anteriorly based subgaleal scalp flaps (bicoronal) provide excellent access to the craniofacial skeleton for the correction of congenital deformity such as craniosynostoses; treatment of craniofacial fractures involving the frontal bone, nasoethmoid complex, orbit or zygomatic arch; skull base surgery or craniotomies. Pericranial flaps can be used to separate the frontal sinus floor from the nasal cavity in the management of fractures of the posterior wall of the frontal sinus (frontal sinus cranialization). Traumatic scalp avulsion may occur if hair becomes trapped in moving machinery or a shearing force is applied in the subgaleal plane during a road traffic accident or fall injury.

The arterial blood supply to the scalp is particularly rich, and there are free anastomoses between branches of the occipital and superficial temporal vessels. Scalp lacerations continue to bleed profusely because the elastic fibres of the underlying galea aponeurotica prevent initial vessel retraction. Their repair requires a two-layer closure technique to approximate the galea aponeurotica and skin layers. The pericranial layer, if involved, cannot usually be closed because it retracts.

BUCCOLABIAL TISSUE

Lips

The lips are two fleshy folds surrounding the oral orifice. The centre of each lip contains a thick fibrous strand, consisting of parallel bundles of skeletal muscle fibres (orbicularis oris, together with incisivus superior and inferior, and the direct labial tractors), and their attachments to skin, mucosa or other muscle fibres. The free external surface of each lip is covered by a thin keratinized epidermis, and is continuous with the mucosa at the vermilion (red) zone of the lip. The dermis is well vascularized and contains numerous hair follicles (many of them large in the male), sebaceous glands (Fordyce spots) and sweat glands. Subcutaneous adipose tissue is scanty. The internal mucous surfaces are lined with a thick non-keratinizing stratified squamous epithelium. The submucosa is well vascularized and contains numerous minor salivary glands, which may be harvested for histological confirmation of Sjögren’s connective tissue disease.

Between the skin and mucosa, the vermilion zone is covered with a specialized keratinized stratified squamous epithelium which is thin near the skin, increases in thickness slightly as the mucosa is approached, and then thickens abruptly when true mucosa is reached. The epithelium is covered with transparent, dead squames and its deep surface is highly convoluted, interdigitating with abundant long dermal papillae. The latter carry a rich capillary plexus which imparts a dusky red colour. These surfaces are hairless, their dermis lacks sebaceous, sweat or mucous glands, and they are moistened with saliva by the tip of the tongue. The dense innervation of the lips is reflected in their acute sensitivity to light touch sensation, attributable mainly to the increased density of Meissner’s corpuscles in the dermal papillae.

The size and curvature of the exposed red lip surfaces is subject to considerable individual, gender, and ethnic variation. The line of contact between the lips, the oral fissure, lies just above the incisal edges of the anterior maxillary teeth. On each side a labial commissure forms the angle (corner) of the mouth, usually near the first premolar tooth. The labial epithelia and internal tissues radiate over the boundaries of the commissure to become continuous with those of the cheek. With age, buccolabial (labiomarginal) grooves appear at the corners of the mouth. On each side, the upper lip is separated from the cheek laterally by the nasolabial groove and is continuous above the nasal ala with the circumalar groove (sulcus). The lower lip is separated from the chin by the mentolabial groove (sulcus).

Externally, the central region of the upper lip presents a shallow vertical groove, the philtrum, which is limited above by its attachment to the columella of the nose, and ends below in a slight tubercle limited by lateral ridges. The lower lip shows a small depression in the midline that corresponds to the tubercle. The junction between the external, hair-bearing skin and the red, hairless surface of the upper lip almost invariably takes the form of a double-curved Cupid’s bow. From the centre it rises rapidly on each side to an apex that corresponds to the lower end of each ridge of the philtrum, and then slopes gently downwards towards the angle of the mouth. The line of contact between the red lip surfaces is typically almost horizontal. The Cupid’s bow is interrupted in cleft lip anomalies.

In the upper lip, a narrow band of smooth tissue related to the subnasal maxillae marks the point at which labial mucosa becomes continuous with gingival mucosa. The corresponding reflexion in the lower lip coincides approximately with the mentolabial sulcus, and here the lip is continuous with mental tissues. The upper and lower lips differ in cross-sectional profile in that neither is a simple fold of uniform thickness. The upper lip has a bulbous asymmetrical profile: the skin and red-lip have a slight external convexity, and the adjoining red-lip and mucosa a pronounced internal convexity, creating a mucosal ridge or shelf that can be wrapped around the incisal edges of the parted teeth. The lower lip is on a more posterior plane than the upper lip. In the position of neutral lip contact, the external surface of the lower lip is concave, and there is little or no elevation of the internal mucosal surface. The profile of the lips can be modified by muscular activity.

RELAXED SKIN TENSION LINES AND SKIN FLAPS ON THE FACE

The direction in which facial skin tension is greatest varies regionally. Skin tension lines which follow the furrows formed when the skin is relaxed are known as ‘relaxed skin tension lines’ (Borges & Alexander 1962). In the living face, these lines frequently (but not always) coincide with wrinkle lines (Fig. 29.1) and can therefore act as a guide in planning elective incisions.

When lesions on the face such as scars, pigmented lesions and skin cancers are excised, the dimensions of these lesions often require excision as an ellipse, so that the resulting defect can be closed as a straight line. If the resulting scar is to be aesthetically acceptable it is important to make the long axis of the ellipse parallel to the natural relaxed skin tension lines, so that the scar will look like a natural skin crease. If the excision line runs contrary to the skin tension lines, the scar may be more conspicuous and will tend to stretch transversely as a result of natural expressive facial movements.

When larger lesions are excised it may be necessary to advance or rotate adjacent soft tissue to fill the defect. The ability to raise these skin flaps is entirely dependent on the regional blood supply and both random pattern and axial pattern skin flaps are used surgically. Because of the richness of the subdermal plexus in the face, random pattern flaps can be raised with a greater length : breadth ratio than in any other area of the body.

The following are examples of axial pattern flaps that can be used to reconstruct defects on the face and scalp. Supratrochlear/supraorbital arteries support forehead flaps that are useful for nasal reconstruction: there is usually enough skin laxity to allow the majority of the donor site to be closed directly. The frontal branch of the superficial temporal artery anastomoses in the midline with its opposite number, and consequently the entire forehead skin can be raised on a narrow pedicle based on just one of the superficial temporal arteries. These flaps can be used to repair many facial defects and also intraoral defects, but the donor site defect cannot be closed directly and must be covered by a skin graft. The parietal branch of the superficial temporal artery and the occipital artery can support hair-bearing flaps from the scalp which are useful for reconstructing defects involving the scalp. The nasolabial flap utilizes the lax skin just lateral to the nasolabial groove. It is not supplied by a named axial artery but rather its blood supply is provided by many small branches from the underlying facial artery. These branches run perpendicular to the skin surface. Nasolabial flaps can be either superiorly or inferiorly based.

SOFT TISSUE

FASCIAL LAYERS

Fascial layers and tissue planes in the face

On the basis of gross dissection and complementary histological studies, four distinct tissue planes are recognized on the face superficial to the plane of the facial nerve and its branches. From superficial to deep, these layers are the skin; a subcutaneous layer of fibro-adipose tissue; the superficial musculo-aponeurotic system (SMAS); and the parotid–masseteric fascia.

Parotid fascia (capsule)

The parotid gland is surrounded by a fibrous capsule called the parotid fascia or capsule. Traditionally this has been described as an upward continuation of the investing layer of deep cervical fascia in the neck which splits to enclose the gland within a superficial and a deep layer. The superficial layer is attached above to the zygomatic process of the temporal bone, the cartilaginous part of the external acoustic meatus, and the mastoid process. The deep layer is attached to the mandible, and to the tympanic plate, styloid and mastoid processes of the temporal bone. The prevailing view is that the deep layer of the parotid gland is derived from the deep cervical fascia. However, the superficial layer of the parotid capsule appears to be continuous with the fascia associated with platysma, and is now regarded as a component of the SMAS (Mitz & Peyronie 1976; Wassef 1987; Gosain et al 1993). It varies in thickness from a thick fibrous layer anteriorly to a thin translucent membrane posteriorly. It may be traced forwards as a separate layer which passes over the masseteric fascia (itself derived from the deep cervical fascia), separated from it by a cellular layer which contains branches of the facial nerve and the parotid duct. Histologically, the parotid fascia is atypical in that it contains muscle fibres which parallel those of platysma, especially in the lower part of the parotid capsule. Although thin fibrous septa may be seen in the subcutaneous layer at the histological level, macroscopically there is little evidence of a distinct layer of superficial fascia.

The deep fascia covering the muscles forming the parotid bed (digastric and styloid group of muscles) contains the stylomandibular and mandibulostylohyoid ligaments. The stylomandibular ligament passes from the styloid process to the angle of the mandible. The more extensive mandibulostylohyoid ligament (angular tract) passes between the angle of the mandible and the stylohyoid ligament for varying distances, generally reaching the hyoid bone. It is thick posteriorly but thins anteriorly in the region of the angle of the mandible. There is some dispute as to whether the mandibulostylohyoid ligament is part of the deep cervical fascia (Ziarah & Atkinson 1981), or lies deep to it (Shimada & Gasser 1988). The stylomandibular and mandibulostylohyoid ligaments separate the parotid gland region from the superficial part of the submandibular gland, and so are landmarks of surgical interest.

Temporo-parietal and temporal fasciae

Above the level of the zygomatic arch, on the lateral side of the head, the temporo-parietal fascia (superficial temporal fascia) constitutes a fascial layer which lies in the same plane as, but is not continuous with, the SMAS. It is quite separate from, and superficial to, the temporal fascia (deep temporal fascia). More superiorly, it blends with the galea aponeurotica. The plane between the temporo-parietal fascia and the underlying deep temporal fascia contains loose areolar tissue and a small amount of fat. This tissue plane, the temporo-parietal fat pad, is continuous superiorly with the subgaleal plane of loose areolar tissue in the scalp. Running superiorly in the temporo-parietal fascia or just deep to it are the superficial temporal vessels, the auriculotemporal nerve and its branches, and the temporal branches of the facial nerve. When raising a bicoronal flap, identification of the temporoparietal fat pad helps to separate these two fascial layers; subsequent dissection in a plane deep to the temporoparietal fascia protects the temporal branch of the facial nerve. The temporal fascia is a dense aponeurotic layer which lies deep to the temporo-parietal fat pad and covers temporalis: the deep surface of the fascia affords attachment to the superficial fibres of temporalis. Above, it is a single layer attached along the length of the superior temporal line, blending with the periosteum. Below, at approximately the level of the superior orbital rim, it splits into superficial and deep laminae which run downwards to attach to the lateral and medial margins of the upper surface of the zygomatic arch respectively. These fascial attachments have a clinical application in the reduction of fractures of the zygomatic complex via a Gillies approach: an instrument is inserted deep to the deep lamina of temporalis fascia through a scalp incision and used to elevate depressed zygomatic complex fractures. The fat enclosed between these two layers is termed the superficial temporal fat pad; it contains the zygomatico-orbital branch of the superficial temporal artery and a cutaneous nerve, the zygomatico-temporal branch of the maxillary nerve. The temporal fascia is overlapped by auriculares anterior and superior, the epicranial aponeurosis and part of orbicularis oculi, and the superficial temporal vessels and auriculotemporal nerve ascend over it.

BONES OF THE FACIAL SKELETON AND CRANIAL VAULT

The skull consists of the facial skeleton and cranial vault (calvarium) attached at the skull base. The cranial vault encloses and protects the brain. The facial skeleton is the anterior part of the skull and includes the mandible. The bones of the nasoethmoidal and zygomaticomaxillary complexes are described here. The mandible is described in Chapter 30.

PARIETAL BONE

The two parietal bones form most of the cranial roof and sides of the skull. Each is irregularly quadrilateral and has two surfaces, four borders and four angles (Fig. 29.2).

The external surface is convex and smooth, with a central parietal tuber (tuberosity). Curved superior and inferior temporal lines cross it and form posterosuperior arches. The temporal fascia is attached to the superior line or arch and temporalis is attached to the inferior line or arch. The epicranial aponeurosis lies above these lines, and part of the temporal fossa lies below. Posteriorly, close to the sagittal (superior) border, an inconstant parietal foramen transmits a vein from the superior sagittal sinus and sometimes a branch of the occipital artery.

The internal surface is concave and marked by impressions of cerebral gyri and by grooves for the middle meningeal vessels. The latter ascend, inclining backwards, from the sphenoidal (anteroinferior) angle and posterior half (or more) of its inferior border. A groove for the superior sagittal sinus lies along the sagittal border, and is completed by the groove on the opposite parietal bone. The falx cerebri is attached to the edges of the groove. Granular foveolae for arachnoid granulations flank the sagittal sulcus, and are most pronounced in old age.

The dentated sagittal border, longest and thickest, articulates with the opposite parietal bone at the sagittal suture. The anterior part of the squamosal (inferior) border is short, thin and truncated, bevelled externally and overlapped by the greater wing of the sphenoid. The middle part of the inferior border is arched, bevelled externally and overlapped by the squamous part of the temporal bone. The posterior part of the inferior border is short, thick and serrated for articulation with the mastoid part.

The frontal border is deeply serrated, bevelled externally above, internally below, and articulates with the frontal bone to form one half of the coronal suture. The occipital border, deeply dentated, articulates with the occipital bone, forming one half of the lambdoid suture.

The frontal (anterosuperior) angle, which is approximately 90°, is at the bregma, where sagittal and coronal sutures meet, and marks the site of the anterior fontanelle in the neonatal skull. The sphenoidal (anteroinferior) angle lies between the frontal bone and greater wing of the sphenoid. Its internal surface is marked by a deep groove or canal that carries the frontal branches of the middle meningeal vessels. The frontal, parietal, sphenoid and temporal bones usually meet at the pterion, which marks the site of the sphenoidal fontanelle in the embryonic skull. The frontal bone sometimes meets the squamous part of the temporal bone, in which case the parietal bone fails to reach the greater wing of the sphenoid bone. The rounded occipital (posterosuperior) angle is at the lambda, the meeting of the sagittal and lambdoid sutures, which marks the site of the posterior fontanelle in the neonatal skull. The blunt mastoid (posteroinferior) angle articulates with the occipital bone and the mastoid portion of the temporal bones at the asterion. Internally it bears a broad, shallow groove for the junction of the transverse and sigmoid sinuses.

FRONTAL BONE

The frontal bone is like half a shallow, irregular cap forming the forehead or frons (Fig. 29.3). It has three parts, and contains two cavities, the frontal sinuses.

Squamous part

The squamous part forms the major portion of the frontal bone. Its external surface has a rounded frontal tuber (tuberosity) approximately 3 cm above the midpoint of each supraorbital margin. These tubera vary, but are especially prominent in young skulls and more so in adult females than males. Below them and separated by a shallow groove, are two curved superciliary arches, medially prominent and joined by a smooth median elevated glabella. The arches are more prominent in males; prominence depends partly on the size of the frontal sinuses, but is occasionally associated with small sinuses. The curved supraorbital margins of the orbital openings lie inferior to the superciliary arches. The lateral two-thirds of each margin are sharp, the medial third rounded; a supraorbital notch or foramen, which transmits the supraorbital vessels and nerve, lies at the junction between them. A small frontal notch or foramen lies medial to the supraorbital notch in 50% of skulls. Both features show sexual dimorphism. The supraorbital margin ends laterally in a strong, prominent zygomatic process that articulates with the zygomatic bone. A line curves posterosuperiorly from the process and divides into superior and inferior temporal lines, which are continued on the squamous part of the temporal bone. The area of the frontal bone below and behind the temporal lines is known as the temporal surface and forms the anterior part of the temporal fossa. The parietal (posterior) margin is thick, deeply serrated, and bevelled internally above and externally below. Inferiorly it becomes a rough, triangular surface that articulates with the greater wing of the sphenoid.

The internal surface of the frontal bone is concave. Its upper, median, part displays a vertical sulcus whose edges unite below as the frontal crest. The sulcus contains the anterior part of the superior sagittal sinus. The crest ends in a small notch which is completed by the ethmoid bone to form a foramen caecum. The anterior portion of the falx cerebri is attached to the margins of the sulcus and to the frontal crest. The internal surface shows impressions of cerebral gyri, small furrows for meningeal vessels, and granular foveolae for arachnoid granulations near the sagittal sulcus.

Orbital parts

Most of the frontal bone is thick, and consists of trabecular tissue lying between two compact laminae. In contrast, the orbital plates consist entirely of compact bone and are thin and often translucent posteriorly, indeed they may be partly absorbed in old age.

The orbital plates form the largest part of the orbital roofs and are two thin, curved, triangular laminae separated by a wide ethmoidal notch. The orbital surface of each plate is smooth and concave, and bears a shallow anterolateral fossa for the lacrimal gland. The trochlear fovea (or spine) for attachment of a fibrocartilaginous trochlea through which the tendon of superior oblique plays, lies below and behind the medial end of the supraorbital margin, midway between the supraorbital notch and frontolacrimal suture. The convex cerebral surface is marked by frontal gyri and faint grooves for meningeal vessels.

The quadrilateral ethmoidal notch is occupied by the cribriform plate of the ethmoid bone. Inferior to its lateral margins, the bone articulates with the labyrinths of the ethmoid bone and impressions of the ethmoidal air cells can be seen on this surface. Two transverse grooves across each margin are converted into anterior and posterior ethmoidal canals by articulation with the ethmoid bone: these canals open on the medial orbital wall and transmit the anterior and posterior ethmoidal nerves and vessels. The posterior borders of the orbital plates are thin and serrated and articulate with the lesser wings of the sphenoid; their lateral parts usually appear in the middle cranial fossa between the greater and lesser wings of the sphenoid.

The frontal sinuses are two irregular cavities that ascend posterolaterally for a variable distance between the frontal laminae. They are separated by a thin septum and usually deflected from the median plane, which means that they are rarely symmetrical. The sinuses are variable in size and usually larger in males. Their openings lie anterior to the ethmoidal notch and lateral to the nasal spine, and each communicates with the middle meatus in the ipsilateral nasal cavity by a frontonasal canal.

The frontal sinuses are rudimentary at birth and can barely be distinguished. They show a primary expansion with eruption of the first deciduous molars at about 18 months, and again when the permanent molars begin to appear in the sixth year. Growth is slow in the early years but it can be detected radiographically by 6 years. They reach full size after puberty, although with advancing age osseous absorption may lead to further enlargement. Their degree of development appears to be linked to the prominence of the superciliary arches, which is thought to be a response to masticatory stresses. The frontal sinuses are described in Chapter 32.

ETHMOID BONE

The ethmoid bone is cuboidal and fragile (Fig. 29.3B, Fig. 29.4, Fig. 29.5, Fig. 29.6). It lies anteriorly in the cranial base and contributes to the medial walls of the orbit, the nasal septum and the roof and lateral walls of the nasal cavity. It has a horizontal perforated cribriform plate, a median perpendicular plate, and two lateral labyrinths that contain the ethmoidal air cells.

Ethmoidal labyrinths

The ethmoidal labyrinths consist of thin-walled ethmoidal air cells between two vertical plates. The lateral surface (orbital plate) of the labyrinth is part of the medial orbital wall. The air cells are arranged in anterior, middle and posterior groups. On average there are 11 anterior ethmoidal air cells, three middle, and six posterior. In the disarticulated bone, many air cells are open, but in life, and in the articulated skull, they are closed by proximity to adjoining bones, except where they open into the nasal cavity. The superior surface is crossed by two grooves that are converted into the anterior and posterior ethmoidal canals by the frontal bone; it shows open-air cells that are covered by the edges of the ethmoidal notch of the frontal bone. On the posterior surface open air cells are covered by the sphenoidal conchae and the orbital process of the palatine bone. The middle and posterior ethmoidal air cells are covered by a thin, smooth, oblong orbital plate that articulates superiorly with the orbital plate of the frontal bone, inferiorly with the maxilla and orbital process of the palatine bone, anteriorly with the lacrimal bone and posteriorly with the sphenoid bone. The walls of the air cells lying anterior to the orbital plate are completed by the lacrimal bone and frontal process of the maxilla.

A thin, curved uncinate process, variable in size, projects posteroinferiorly from the labyrinth. The upper edge of this process is a medial boundary of the hiatus semilunaris in the middle meatus. The uncinate process appears in the medial wall of the maxillary sinus as it crosses the ostium of the maxillary sinus to join the ethmoidal process of the inferior nasal concha.

The medial surface of the labyrinth forms part of the lateral nasal wall. It appears as a thin lamella that descends from the inferior surface of the cribriform plate and ends as the convoluted middle nasal concha. Superiorly the surface contains numerous vertical grooves that transmit bundles of olfactory nerves. Posteriorly it is divided by the narrow, oblique superior meatus, bounded above by the thin, curved superior nasal concha. Posterior ethmoidal air cells open into the superior meatus. The convex surface of the middle nasal concha extends along the entire medial surface of the labyrinth, anteroinferior to the superior meatus. Its lower edge is thick and its lateral surface is concave and forms part of the middle meatus. Middle ethmoidal air cells produce a swelling, the bulla ethmoidalis, on the lateral wall of the middle meatus, and open into the meatus, either on the bulla or above it. A curved infundibulum extends up and forwards from the middle meatus and communicates with the anterior ethmoidal sinuses. In more than 50% of crania it continues up as the frontonasal duct to include the drainage point for the frontal sinus. (The ethmoidal air cells are described further in Chapter 32.)

INFERIOR NASAL CONCHA

The inferior nasal conchae are curved horizontal laminae in the lateral nasal walls (Fig. 29.5) (see also Ch. 32). Each has two surfaces (medial and lateral), two borders (superior and inferior) and two ends (anterior and posterior). The medial surface is convex, much perforated, and longitudinally grooved by vessels. The lateral surface is concave and part of the inferior meatus. The superior border, thin and irregular, may be divided into three regions: an anterior region articulating with the conchal crest of the maxilla; a posterior region articulating with the conchal crest of the palatine bone; and a middle region with three processes, which are variable in size and form. The lacrimal process is small and pointed and lies towards the front. It articulates apically with a descending process from the lacrimal bone, and at its margins with the edges of the nasolacrimal groove on the medial surface of the maxilla, thereby helping to complete the nasolacrimal canal. Most posteriorly, a thin ethmoidal process ascends to meet the uncinate process of the ethmoid bone. An intermediate thin maxillary process curves inferolaterally to articulate with the medial surface of the maxilla at the opening of the maxillary sinus. The inferior border is thick and spongiose, especially in its midpart. Both the anterior and posterior ends of the inferior nasal concha are more or less tapered, the posterior more than the anterior.

LACRIMAL BONE

The lacrimal bones are the smallest and most fragile of the cranial bones and lie anteriorly in the medial walls of the orbits (Fig. 29.5B). Each has two surfaces (medial and lateral) and four borders (anterior, posterior, superior and inferior). The lateral (orbital) surface is divided by a vertical posterior lacrimal crest. Anterior to the crest is a vertical groove whose anterior edge meets the posterior border of the frontal process of the maxilla to complete the fossa that houses the lacrimal sac. The medial wall of the groove is prolonged by a descending process that contributes to the formation of the nasolacrimal canal by joining the lips of the nasolacrimal groove of the maxilla and the lacrimal process of the inferior nasal concha. A smooth part of the medial orbital wall lies behind the posterior lacrimal crest: the lacrimal part of orbicularis oculi is attached to this surface and crest. The surface ends below in the lacrimal hamulus which, together with the maxilla, completes the upper opening of the nasolacrimal canal. The hamulus may appear as a separate lesser lacrimal bone. The anteroinferior region of the medial (nasal) surface is part of the middle meatus. Its posterosuperior part meets the ethmoid to complete some of the anterior ethmoidal air cells. The anterior border of the lacrimal bone articulates with the frontal process of the maxilla, the posterior border with the orbital plate of the ethmoid bone, the superior border with the frontal bone, and the inferior border with the orbital surface of the maxilla.

NASAL BONE

The nasal bones are small, oblong, variable in size and form, and placed side by side between the frontal processes of the maxillae (Fig. 29.4, Fig. 29.5, Fig. 29.6B). They jointly form the nasal bridge. Each nasal bone has two surfaces (external and internal) and four borders (superior, inferior, lateral and mesial). The external surface has a descending concavo-convex profile and is transversely convex. It is covered by procerus and nasalis and perforated centrally by a small foramen that transmits a vein. The internal surface, transversely concave, bears a longitudinal groove that houses the anterior ethmoidal nerve. The superior border, thick and serrated, articulates with the nasal part of the frontal bone. The inferior border, thin and notched, is continuous with the lateral nasal cartilage. The lateral border articulates with the frontal process of the maxilla. The medial border, thicker above, articulates with its fellow and projects behind as a vertical crest, thereby forming a small part of the nasal septum. It articulates from above with the nasal spine of the frontal bone, the perpendicular plate of the ethmoid bone, and the nasal septal cartilage.

VOMER

The vomer is thin, flat, and almost trapezoid (Fig. 29.4). It forms the posteroinferior part of the nasal septum and presents two surfaces and four borders. Both surfaces are marked by grooves for nerves and vessels. A prominent groove for the nasopalatine nerve and vessels lies obliquely in an anteroinferior plane. The superior border is thickest, and possesses a deep furrow between projecting alae which fits the rostrum of the body of the sphenoid bone. The alae articulate with the sphenoidal conchae, the vaginal processes of the medial pterygoid plates of the sphenoid bone, and the sphenoidal processes of the palatine bones. Where each ala lies between the body of the sphenoid and the vaginal process, its inferior surface helps to form the vomerovaginal canal. The inferior border articulates with the median nasal crests of the maxilla and palatine bones. The anterior border is the longest, and articulates in its upper half with the perpendicular plate of the ethmoid bone. Its lower half is cleft to receive the inferior margin of the nasal septal cartilage (see Ch. 32). The concave posterior border is thick and bifid above and thin below: it separates the posterior nasal apertures. The anterior extremity of the vomer articulates with the posterior margin of the maxillary incisor crest and descends between the incisive canals.

ZYGOMATIC BONE

Each zygomatic bone forms the prominence of a cheek, contributes to the floor and lateral wall of the orbit and the walls of the temporal and infratemporal fossae, and completes the zygomatic arch. Each is roughly quadrangular and is described as having three surfaces, five borders and two processes (Fig. 29.7).

The lateral (facial) surface is convex and is pierced near its orbital border by the zygomaticofacial foramen, which is often double and occasionally absent, and transmits the zygomaticofacial nerve and vessels. This surface gives attachment to zygomaticus major posteriorly and zygomaticus minor anteriorly. The posteromedial (temporal) surface has a rough anterior area for articulation with the zygomatic process of the maxilla, and a smooth, concave posterior area that extends up posteriorly on its frontal process as the anterior aspect of the temporal fossa. It also extends back on the medial aspect of the temporal process as an incomplete lateral wall for the infratemporal fossa. The zygomaticotemporal foramen pierces this surface near the base of the frontal process. The smooth and concave orbital surface forms the anterolateral part of the floor and adjoining lateral wall of the orbit, and extends up on the medial aspect of its frontal process. It usually bears zygomatico-orbital foramina which represent the openings of canals leading to the zygomaticofacial and zygomaticotemporal foramina.

The smoothly concave anterosuperior (orbital) border forms the inferolateral circumference of the orbital opening, and separates the orbital and lateral surfaces of the bone. The anteroinferior (maxillary) border articulates with the maxilla. Its medial end tapers to a point above the infraorbital foramen. A part of levator labii superioris is attached at this surface. The posterosuperior (temporal) border is sinuous, convex above and concave below, and is continuous with the posterior border of the frontal process and upper border of the zygomatic arch. The temporal fascia is attached to this border. There is often a small, easily palpable, marginal tubercle below the frontozygomatic suture. The posteroinferior border is roughened for the attachment of masseter. The serrated posteromedial border articulates with the greater wing of the sphenoid bone above, and the orbital surface of the maxilla below. Between these serrated regions a short, concave, non-articular part usually forms the lateral edge of the inferior orbital fissure. Occasionally absent, the fissure is then completed by the articulation of the maxilla and sphenoid (or with a small sutural bone between them).

The frontal process, thick and serrated, articulates above with the zygomatic process of the frontal bone and behind with the greater wing of the sphenoid bone. A tubercle of varying size and form, Whitnall’s tubercle, is usually present on its orbital aspect, within the orbital opening and about 1 cm below the frontozygomatic suture. This tubercle provides attachment for the lateral palpebral ligament, the suspensory ligament of the eye, and part of the aponeurosis of levator palpebrae superioris. The temporal process, directed backwards, has an oblique, serrated end that articulates with the zygomatic process of the temporal bone to complete the zygomatic arch.

MAXILLA

The maxillae are the largest of the facial bones, other than the mandible, and jointly form the whole of the upper jaw. Each bone forms the greater part of the floor and lateral wall of the nasal cavity, and of the floor of the orbit, contributes to the infratemporal and pterygopalatine fossae, and bounds the inferior orbital and pterygomaxillary fissures. Each maxilla has a body and four processes, namely the zygomatic, frontal, alveolar and palatine processes (Fig. 29.6, Fig. 29.8).

Body

The body of the maxilla is roughly pyramidal, and has anterior, infratemporal (posterior), orbital and nasal surfaces that enclose the maxillary sinus.

Orbital surface

This surface is smooth and triangular, and forms most of the floor of the orbit. Anteriorly its medial border bears a lacrimal notch, behind which it articulates with the lacrimal bone, the orbital plate of the ethmoid and, posteriorly, with the orbital process of the palatine bone. Its posterior border is smoothly rounded, and forms most of the anterior edge of the inferior orbital fissure. The infraorbital groove lies centrally. The anterior border is part of the orbital margin, and is continuous medially with the lacrimal crest of the frontal process of the maxilla. The infraorbital groove transmits the infraorbital vessels and nerve, and begins midway on the posterior border, where it is continuous with a groove on the posterior surface. It passes forwards into the infraorbital canal which opens on the anterior surface below the infraorbital margin. Near its midpoint, the infraorbital canal gives off a small lateral branch, the canalis sinuosus, that transmits the anterior superior alveolar nerve and vessels. The canalis sinuosus descends in the orbital floor lateral to the infraorbital canal, curves medially in the anterior wall of the maxillary sinus, and then passes below the infraorbital foramen to the margin of the anterior nasal aperture in front of the anterior end of the inferior concha. It follows the lower margin of the aperture and opens near the nasal septum in front of the incisive canal. The site of the attachment of inferior oblique may be indicated by a small depression in the bone at the anteromedial corner of the orbital surface, lateral to the lacrimal groove.

Palatine process

The palatine process, thick and horizontal, projects medially from the lowest part of the medial aspect of the maxilla. It forms a large part of the nasal floor and hard palate and is much thicker in front. Its inferior surface is concave and uneven, and with its contralateral fellow it forms the anterior three-fourths of the osseous (hard) palate. The palatine process displays numerous vascular foramina and depressions for palatine glands and, posterolaterally, two grooves that transmit the greater palatine vessels and nerves. The infundibular incisive fossa is placed between the two maxillae, behind the incisor teeth. The median intermaxillary palatal suture runs posterior to the fossa, and although a little uneven, is usually relatively flat on its oral aspect. Its bony margins are sometimes raised into a prominent longitudinal palatine torus. Two lateral incisive canals, each ascending into its half of the nasal cavity, open in the incisive fossa: they transmit the terminations of the greater palatine artery and nasopalatine nerve. Two additional median openings, anterior and posterior incisive foramina, are occasionally present: they transmit the nasopalatine nerves, the left usually passing through the anterior, and the right through the posterior foramen. On the inferior palatine surface a fine groove, sometimes termed the incisive suture, and prominent in young skulls, may be observed in adults. It extends anterolaterally from the incisive fossa to the interval between the lateral incisor and canine teeth. The superior surface of the palatine process is smooth, concave transversely, and forms most of the nasal floor. The incisive canal lies anteriorly, near its median margin. The lateral border is continuous with the body of the maxilla. The medial border, thicker in front, is raised into a nasal crest that, with its contralateral fellow, forms a groove for the vomer. The front of this ridge rises higher as an incisor crest, prolonged forwards into a sharp process which, with its fellow, forms an anterior nasal spine. The posterior border is serrated for articulation with the horizontal plate of the palatine bone.

Ossification

The maxilla ossifies from a single centre in a sheet of mesenchyme that appears above the canine fossa at about the sixth week in utero and spreads into the rest of the maxilla and its processes. The pattern of spread of ossification may initially leave an unmineralized zone roughly corresponding to a site where a premaxillary suture may occur. However, this deficiency is soon ossified; there is no evidence of a separate centre of ossification for the incisor-bearing portion of the maxilla (i.e. premaxilla).

The maxillary sinus appears as a shallow groove on the nasal aspect at about the fourth month in utero. Though small at birth, the sinus is identifiable radiologically. After birth it enlarges with the growing maxilla, though it is only fully developed following the eruption of the permanent dentition. The infraorbital vessels and nerve are for a time in an open groove in the orbital floor; the anterior part of the groove is subsequently converted into a canal by a lamina that grows in from the lateral side.

At birth the transverse and sagittal maxillary dimensions are greater than the vertical. The frontal process is prominent, but the body is little more than an alveolar process, because the alveoli reach almost to the orbital floor. In adults the vertical dimension is the greatest, reflecting the development of the alveolar process and enlargement of the sinus. When teeth are lost, the bone reverts towards its infantile shape: its height diminishes, the alveolar process is absorbed, and the lower parts of the bone contract and become reduced in thickness at the expense of the labial wall.