Anatomy for Plastic Surgery of the Face, Head, and Neck (eBook)

Introduction

The skull can be divided into two parts: the neurocranium, which forms a protective case around the brain, and the viscerocranium, which forms the skeleton of the face. This chapter details the viscerocranium and bones of the neurocranium that pertain to the viscerocranium.

Neurocranium

The neurocranium in adults is formed by a series of eight bones: the singular frontal, ethmoid, sphenoid, occipital bones centered on the midline, and the temporal and parietal bones occurring as bilateral pairs.1 The primarily flat frontal, parietal, and occipital bones form the calvaria (skullcap) by intramembranous ossification of head mesenchyme derived from the neural crest. The primarily irregular, yet considerably flat, sphenoid and temporal bones contribute to the cranial base via endochondral ossification of cartilage or from more than one type of ossification. The irregular ethmoid bone slightly contributes to the neurocranium but is primarily part of the viscerocranium. In reality, the flat bones and flat portions of the bones forming the neurocranium consist of convex external and concave internal curved surfaces.1

Fibrous interlocking sutures unite most calvarial bones in adulthood, although during childhood, the sphenoid and occipital bones are unified by synchondroses.2 Some sutures, comprising narrow closures of connective tissue at birth, remain open until adulthood. The sagittal suture is derived from neural crest cells and the coronal suture from paraxial mesoderm.2 The newborn skull contains fontanels, the most prominent being the anterior fontanel, which are widened sutures at points where more than two bones meet. The anterior fontanel, found where the two parietal and frontal bones meet, closes in most cases by 18 months of age, and the posterior fontanel closes by 1 to 2 months of age.2

Two primary centers of ossification traverse the frontal (metopic) suture in the second year, dividing the frontal bone into halves. Usually, the frontal suture disappears by age 6 years, when the halves fuse, but it can persist into adulthood as a metopic suture either totally, running from the midline of the glabella to the bregma, or partially.2 The glabella is a smooth anterior projecting prominence on the frontal bone superior to the root of the nose, and the bregma is the junction of the coronal and sagittal sutures.

The maxillae and mandible provide the sockets and supporting bone for the maxillary and mandibular teeth. The maxillae contribute the greatest part of the upper facial skeleton, forming the skeleton of the upper jaw, which is fixed to the cranial base. (The mandible is detailed in Chapter 19.)

On the lateral aspect of the skull is the thin pterion. The pterion, located two finger breadths superior to the zygomatic arch and a thumb's breadth posterior to the frontal process of the zygomatic bone, is formed by the articulations of the frontal, parietal, sphenoid, and temporal bones.1 The pterion overlies the anterior branch of the middle meningeal artery. Therefore, an injury to this region can damage the vessel, producing an epidural hematoma.1

The air-filled paranasal sinuses, including the maxillary, frontal, and ethmoidal sinuses, are discussed. The sphenoidal sinuses are discussed in Chapters 2 and 16. The bony articulations of the neurocranium and viscerocranium are described in Table 1.1, and the general processes of ossification are displayed in Table 1.2.

Frontal Bone

The frontal bone forms the forehead via its squamous, orbital, and nasal parts and two cavities, the frontal sinuses.

Squamous Part

The flat squamous part is the largest part of the frontal bone forming most of the forehead.3 The supraorbital margin of the frontal bone is the angular boundary between the squamous and the orbital parts (Fig. 1.1).4

On the external surface of the squamous part, about 3 cm above the midpoint of this margin, are the frontal tuberosities.4 These tubercles are more prominent in children and adult women. Ventrally, a shallow groove separates the frontal tuber-osities from the paired and curved superciliary arches.4 These arches extend laterally from the medially located, smooth, and elevated glabella and are more prominent in males. Partly dependent on frontal sinus size, supercilliary arch prominence is occasionally associated with small sinuses.4

The supraorbital notch (or foramen), which transmits the supraorbital vessels and nerve, lies at the junction between the sharp, lateral two-thirds and the rounded medial third of the supraorbital margin.4 The variably occurring frontal notch (or foramen) occurs medial to the supraorbital notch in 50% of skulls.4

Surgical Annotation

Recent interest in the surgical treatment of migraines has led to numerous anatomical studies identifying areas of nerve compression. The supraorbital nerve, as it emerges from the supraorbital foramen or notch, has been identified as a migraine trigger area.5 The supraorbital nerve can have compression from both a foramen as well as a notch as a result of the associated fascial bands. In addition to the soft tissue procedure, a supraorbital foraminotomy or fascial band release has been shown to improve postoperative outcomes.6 A transpalpebral incision can be used to access the supraorbital nerves to perform the decompression. An incision is made in the upper tarsal crease, with subsequent dissection identifying the supraorbital nerve. Muscles such as the corrugator supercilii are resected, and the foraminotomy is performed. Endoscopic techniques have also been described.7 With the use of the endoscopic technique, release of the zygomaticotemporal branch can also be performed.

The supraorbital margin extends laterally, forming the prominent zygomatic process, which articulates with the zygomatic bone. A posterosuperiorly curving line, which continues onto the squamous part of the temporal bone, divides into superior and inferior temporal lines.4 The temporal surface of the frontal bone is inferior and posterior relative to these temporal lines. The anterior surface of the temporal surface forms the anterior part of the temporal fossa. The rough inferior surface of the posterior margin of the squamous part articulates with the greater wing of the sphenoid.4

The nasal part of the frontal bone is discussed in the Nasal Bone: Nasal Bridge and Bony Septum section of this chapter. The interior surface of the frontal bone is detailed in Chapter 2.

Orbital Parts of the Frontal Bone

The two orbital parts of the frontal bone are thin, curved, and triangular laminae, consisting entirely of compact bone (Fig. 1.2).4 Forming the largest part of the orbital roofs, the orbital parts are separated by a wide, quadrilateral ethmoidal notch that is occupied by the cribriform plate of the ethmoid bone.4 The labyrinths of the ethmoid bone, which contain the ethmoidal air cells, articulate with the inferior surface of the lateral margins of the ethmoidal notch. This articulation converts two transverse grooves across each margin into anterior and posterior ethmoidal canals. These canals transmit the anterior and posterior ethmoidal nerves and vessels into the medial orbit.4

The posterolaterally ascending frontal sinuses open anterior to the ethmoidal notch and lateral to the nasal spine (Fig. 1.3). Deflecting from the median plane, these rarely symmetrical sinuses ascend between the frontal laminae and are separated by a thin septum.4 Each sinus communicates with the ipsilateral nasal cavity's middle meatus via the frontonasal canal.4

Table 1.2 Neurocranium and viscerocranium ossification patterns

Source: Data from...