CHAPTER 29 Face and scalp
SKIN
The scalp and buccolabial tissues are described here. The structure of the eyelids is described in Chapter 39.
BUCCOLABIAL TISSUE
Lips
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.
Fig. 29.1 A, Distribution of relaxed skin tension lines (Kraissl’s lines) lateral view. B, Anterior view.
SOFT TISSUE
FASCIAL LAYERS
Fascial layers and tissue planes in the face
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.
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 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 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
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 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.)
Ossification
The ethmoid bone ossifies in the cartilaginous nasal capsule from three centres, one in the perpendicular plate, and one in each labyrinth. The latter two appear in the orbital plates between the fourth and fifth months in utero, and extend into the ethmoid conchae. At birth, the labyrinths, although ill-developed, are partially ossified, and the remainder are cartilaginous. The perpendicular plate begins to ossify from the median centre during the first year, and fuses with the labyrinths early in the second year. The cribriform plate is ossified partly from the perpendicular plate, and partly from the labyrinths. The crista galli ossifies during the second year. The parts of the ethmoid bone unite to form a single bone at around 3 years of age. Ethmoidal air cells begin to develop at about 3 months in utero, and are therefore present at birth, however, they are difficult to visualize radiographically until the end of the first year. They grow slowly and have almost reached adult size by the age of 12 years.
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).
Fig. 29.7 Zygomatic bone. A, Anterolateral aspect. B, Posterolateral aspect. Muscle attachments shown in A.
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
Frontal process
The frontal process projects posterosuperiorly between the nasal and lacrimal bones. Its lateral surface is divided by a vertical anterior lacrimal crest which gives attachment to the medial palpebral ligament and is continuous below with the infraorbital margin. A small palpable tubercle at the junction of the crest and orbital surface is a guide to the lacrimal sac. The smooth area anterior to the lacrimal crest merges below with the anterior surface of the body of the maxilla. Parts of orbicularis oculi and levator labii superioris alaeque nasi are attached here. Behind the crest, a vertical groove combines with a groove on the lacrimal bone to complete the lacrimal fossa. The medial surface is part of the lateral nasal wall. A rough subapical area articulates with the ethmoid, and closes anterior ethmoidal air cells. Below this an oblique ethmoidal crest articulates posteriorly with the middle nasal concha, and anteriorly underlies the agger nasi, a ridge anterior to the concha on the lateral nasal wall. The ethmoidal crest forms the upper limit of the atrium of the middle meatus. The frontal process articulates above with the nasal part of the frontal bone. Its anterior border articulates with the nasal bone and its posterior border articulates with the lacrimal bone.
PALATINE BONE
The palatine bones are posteriorly placed in the nasal cavity, between the maxillae and the pterygoid processes of the sphenoid bones. They contribute to the floor and lateral walls of the nose, to the floor of the orbit and the hard palate, to the pterygopalatine and pterygoid fossae, and to the inferior orbital fissures. Each has two plates (horizontal and perpendicular) arranged as an L-shape, and three processes (pyramidal, orbital and sphenoidal) (Fig. 29.9).
