Which projection of the skull projects the petrous bones in the lower third of the orbits?

Lorisidae

The skulls of Lorisidae, compared with those of other prosimians, give the impression of being dorsoventrally flat, especially in species of genera Perodicticus and Nycticebus. The interorbital distance is generally smaller in Lorisidae than in Malagasy Lemuridae or Indriidae. This narrowness is also seen in lorisids in the lessened postorbital breadth of the skull, or “postorbital constriction.” Moreover, among lorisids the snout does not taper toward the front as much as in Lemuridae and thus gives the impression of being less long and pointed comparatively. Among lorisines, the nasal bones are flatter than in Lemuridae. A characteristic elongation of the snout beyond the front end of the tooth row is found in species of the two lorisid genera Arctocebus and Loris. This phenomenon is brought about as a result of their comparatively large premaxillae, the upper margins of which project forward. The nasal bones also enter this projection, thus forming a pipelike nasal opening. In addition, the snout is narrow in Arctocebus and Loris. In Nycticebus, the occipital is flattened and faces backward. The foramen magnum opens most directly backward in Nycticebus of all Lorisidae. Skulls of galagids resemble those of lorisids, but slight differences can be detected. For example, with galagids the cranial vault is slightly more rounded than in Lorisidae, and the interorbital distance is somewhat wider. The postorbital constriction, however, is much more marked in Galagidae than in Lemuridae. The small lacrimal bone at the lower inside corner of the orbit extends considerably forward onto the outside of the orbital wall, and the lacrimal canal (tear duct) is positioned externally. Figure 5.16 shows the skull base of a loris genus Perodicticus.

4.2 Elements of the Skull

The term “skull” is often misused in common speech. Terms such as this have very specific meanings to anatomists and osteologists. It is worthwhile to review the proper use of terminology.

The skull is the entire bony framework of the head, including the lower jaw.

The mandible is the lower jaw.

The cranium is the skull without the mandible.

The calvaria (or calvarium) is the cranium without the face.

The calotte is the calvaria without the base.

The splanchnocranium is the facial skeleton.

The neurocranium is the braincase.

The three basic divisions of the endocranial surface at the base of the neurocranium correspond to the topography of the base of the brain. These anterior, middle, and posterior cranial fossae are respectively occupied by the frontal lobes, temporal lobes, and cerebellum of the brain.

When the ear ossicles (three pairs of tiny bones associated with hearing) are included and the hyoid excluded, there are usually 28 bones in the adult human skull. Distinguishing these bones is occasionally made difficult because some of them fuse together during adult life. For this reason, it is advisable to begin study with young adult specimens, in which the bones are most readily recognizable. In addition to the 28 normal skull bones, there are often sutural bones (also called Wormian bones, or extrasutural bones), which are irregular ossicles that occur along some sutures. A large, triangular inca bone is occasionally found at the rear of human crania.

7.3 Growth and Architecture, Sutures and Sinuses

At birth the skull is made up of forty-five separate elements and is large relative to other parts of the body. The facial part of the newborn skull, however, is relatively small, reflecting the dominance of brain development at this stage of maturation. The face “catches up” to the neurocranium as development, particularly in the mandible and maxilla, proceeds. Important stages in the development of the skull include emergence of the first set of teeth (between the ages of 6 and 24 months), the emergence of the permanent teeth (beginning at about 6 years), and puberty. Figure 7.7 illustrates growth of the skull.

At birth the skull contains intervals of dense connective tissue between plates of bone. These “soft spots,” or fontanelles, are cartilaginous membranes that eventually harden and turn to bone. In the adult the skull bones contact along joints with interlocking, sawtooth, or zipper-like articulations called sutures. Cranial articulations in the adult human skull are summarized in Figure 7.8. Many of these sutures derive their name directly from the two bones that contact across them. For example, zygomaticomaxillary sutures are sutures between the zygomatics and maxillae, and frontonasal sutures are short sutures between the frontal and nasals. Some sutures have special names. The sagittal suture passes down the midline between the parietal bones. The metopic suture passes between unfused frontal halves and only rarely persists into adulthood. The coronal suture lies between the frontal and parietals. The lambdoidal suture passes between the two parietals and the occipital. Squamosal sutures are unusual, scale-like, beveled sutures between temporal and parietal bones. The sphenooccipital, or basilar suture (actually a synchondrosis) lies between the sphenoid and the occipital. Parietomastoid sutures pass between the parietals and the temporals, constituting posterior extensions of the squamosal suture. Occipitomastoid sutures pass between the occipital and temporals on either side of the vault.

Sinuses are void chambers in the cranial bones that enlarge with the growth of the face. There are four basic sets of sinuses, one each in the maxillae, frontal, ethmoid, and sphenoid. These sinuses are linked to the nasal cavity and, in life, irritation of their mucous membranes may cause swelling, draining, and headache-related discomfort.

Abstract

The skull base is made of flat bones that separate the intracranial compartment from the extracranial head and neck. Owing to this unique location, it can be affected both by intrinsic lesions originating from the bony elements of the skull base and by lesions originating outside the skull base proper or from trapped embryonic remnants. This chapter specifically focuses on the imaging techniques, developmental anomalies, skull base lesions originating from embryonic remnants, skull base infection, and other diffuse skull base lesions. Diagnostic imaging may require the use of different techniques, most often CT, MRI, bone scintigraphy, and PET-CT and is crucial in patient’s management, as the skull base is hidden from clinical inspection. Awareness of anatomic variants is crucial in surgical planning and to avoid potential confusion with skull base lesions. Moreover, radiologists should also be familiar with embryonic abnormalities affecting the skull base. Often, their particular location and other associated imaging features can be diagnostic. Proximity with sinus cavities (sinonasal and otomastoid) and with the nasopharynx makes the skull base prone to spread of infectious processes when appropriate treatment is delayed. Skull base osteomyelitis from bacterial or fungal agents remains a diagnostic challenge, particularly in the absence of relevant clinical data, and requires a high degree of suspicion. Diffuse bone lesions affecting the skull base result from defects of endochondral ossification, which may also affect the remaining skeleton. Overall, diagnostic imaging of these conditions is similar to that of bone lesions outside the skull base and the same rules for differential diagnosis apply.

Radiography

Plain skull radiographs are of limited value in the diagnosis of a primary brain tumor; however, they may be helpful in detecting neoplasms of the skull or nasal cavity that involve the brain by local extension. Occasionally, lysis or hyperostosis of the skull may accompany a primary brain tumor (e.g., meningioma of cats), or there may be radiographically visible mineralization within a neoplasm (Figure 29-2).1,74 General anesthesia is required for precise positioning of the skull for radiographs, and various projections have been recommended to identify abnormalities.1

Anatomy

The skull base forms the floor of the cranium, and serves as a passageway for the spinal cord, cranial nerves, and cerebral vasculature. The ethmoid, sphenoid, occipital, paired frontal, and temporal bones constitute the skull base, which is subdivided into the anterior, middle, and posterior cranial fossa (Policeni and Smoker, 2015). An indepth knowledge of the nerves and surrounding structures is essential to understand the clinical syndromes associated with lesions in this location. The frequency of SBM in the various anatomic regions of the skull base is presented in Table 14.4.

Table 14.4. Anatomic location of cranial base metastases referred for surgery: the MD Anderson experience

Adapted from Chamoun RB, Suki D, DeMonte F (2012) Surgical management of cranial base metastases. Neurosurgery 70: 802–809; discussion 809–810.

Abstract

Skull base imaging requires a thorough knowledge of the complex anatomy of this region, including the numerous fissures and foramina and the major neurovascular structures that traverse them. Computed tomography (CT) and magnetic resonance imaging (MRI) play complementary roles in imaging of the skull base. MR is the preferred modality for evaluation of the soft tissues, the cranial nerves, and the medullary spaces of bone, while CT is preferred for demonstrating thin cortical bone structure. The anatomic location and origin of a lesion as well as the specific CT and MR findings can often narrow the differential diagnosis to a short list of possibilities. However, the primary role of the imaging specialist in evaluating the skull base is usually to define the extent of the lesion and determine its relationship to vital neurovascular structures. Technologic advances in imaging and radiation therapy, as well as surgical technique, have allowed for more aggressive approaches and improved outcomes, further emphasizing the importance of precise preoperative mapping of skull base lesions via imaging. Tumors arising from and affecting the cranial nerves at the skull base are considered here.

Conclusions

Skull base trauma implies transfer of significant forces to the cranium. Fractures of the skull base are often remote from the site of impact. The presence of a skull base fracture should heighten awareness for the possibility of associated cervical spine instability, cranial nerve deficits, vascular injury, and CSF fistula. Because of the location and extent of damage, operative repair of skull base injury requires subspecialized approaches and skills. Surgical repair is complex and often carries significant risk of hemorrhage from major arteries or venous sinuses. Advanced TBI monitoring and medical management in the ICU, together with expertise in a diverse repertoire of surgical approaches and techniques, are critically important to enable optimal recovery for patients with skull base trauma.

Skull mechanical properties

The pediatric skull differs from the adult skull in many ways. It is unique because of a combination of higher plasticity and deformity, and consequently, forces are absorbed in a very different way compared to adults. Open sutures may function as “joints,” allowing for a certain degree of movement in response to a mechanical stress (Ghajar and Hariri, 1992). Open sutures also prevent early and rapid rise of the intracranial pressure related to mild brain swelling and space-occupying lesions. This feature can prevent or limit secondary brain injuries due to the various types of brain herniation. The pediatric skull base is also different compared to the adult skull base. The pediatric petrous bone is compact at an early age, which results in a high mechanical stress between the dense petrous bone and the soft/cartilaginous skull base and skull when exposed to traumatic forces. Absorption of forces is different in adults, in whom the difference in densities between the petrous bone and the skull base/skull is less prominent.

Skull X-ray

The skull X-ray is of very limited use in modern psychiatric practice. Nonetheless, it allows detection of intracranial calcifications, especially hypophyseal calcification which, if present, enables radiologists to see any shift of midline structures caused by space-occupying lesions. Erosion of bone, for example of the sella turcica by pituitary tumours or the cranial vault by other brain tumours, can also be visualized on skull X-ray, as can thickening of the skull in Paget disease. However, psychiatrists had recognized the limitations of skull X-ray even prior to the introduction of modern neuroimaging techniques. One study found no abnormality in 53 ‘routine’ skull X-ray examinations and only one abnormality – a skull fracture – in 30 clinically indicated skull X-ray examinations (Larkin 1985). It is of interest to note that Jakobi and Winkler in 1927 described ventricular enlargement in patients with schizophrenia examined with pneumoencephalography, a technique dependent on X-ray and the introduction of (radiolucent) air into the subarachnoid space via lumbar puncture.