Guws Medical Skull

Introduction

The skull develops from mesenchyme around the developing brain. It consists of two parts—the neurocranium (the protection for the brain) and the viscerocranium (the main skeleton of the jaws). The neurocra-nium is further divided into cartilaginous and membranous portion. The cartilaginous neurocranium (or chondrocranium) consists initially of the cartilaginous base of the developing skull which forms by fusion of several cartilages. Later endochondral ossification of the chondrocranium forms the bones of the base of the skull. Intramembranous ossification occurs in the mesenchyme at the sides and top of the brain, forming the cranial vault or calvaria. During fetal life the flat bones of the vault are separated by dense connective tissue membranes called sutures. The seven large fibrous areas where several sutures meet are called fontanelles. The cartilaginous viscerocranium consists of the cartilaginous skeleton of the first two pairs of branchial arches which will ultimately form the middle ear ossicles, part of the hyoid bone, and the styloid process of the temporal bone. Intramembranous ossification occurs within the maxillary and mandibular prominences of the first branchial arch and subsequently forms the maxillary, zygomatic, and squamous temporal bones and the mandible.

Postnatal growth of the skull occurs because the fibrous sutures of the newborn calvaria permit the skull to enlarge during infancy and childhood. The increase in size is greatest during the first two years, the period of most rapid postnatal brain growth. The calvaria normally increases in capacity until 15 or 16 years of age. After this, a slight increase in size for three to four years is due to thickening of the bones.

The overall head shape is closely related to the bony structures of the skull and to the shape of the underlying brain. It is well established that alterations in head shape can be the result of unusual brain growth, but they may also reflect a number of other factors such as premature synostosis of cranial sutures or unusual intrauterine mechanical forces. Abnormal planes of muscle pull, as in torticollis, can cause asymmetric skull growth. The presentation of the individual during delivery also contributes to the shape of the head. Dolichocephaly or scaphocephaly (long narrow head) is a postural deformation of the head associated with intrauterine breech position or prematurity. It generally resolves during infancy with no apparent residual impairment.

Five major sutures are present in the calvaria (Fig. 7.2). Three—the coronal, lambdoidal, and squamosal—are paired, and two—the sagittal and metopic—are single. Cranial growth normally proceeds in a direction perpendicular to each of the major sutures. Increased length of the skull in comparison to width (dolichocephaly or scaphocephaly) and the converse (brachycephaly) can be normal variants. However, both can also occur because of premature synostosis of cranial sutures, where skull growth at right angles to the fused suture is inhibited with compensatory expansion at other patent sutural sites (Fig. 7.3). Head shape depends on which sutures are prematurely synostosed, the order in which they fuse, and the time at which they synostose. The earlier the synostosis occurs, the more dramatic the effect on subsequent cranial growth and development.

Figure 7.2 Normal fontanelle and suture landmarks. Adapted from Pruzansky (1973).

Fontanelles

Sutures

Figure 7.2 Normal fontanelle and suture landmarks. Adapted from Pruzansky (1973).

Fontanelles

Sutures

Squamosal

Sagittal

Posterolateral

Lambdoidal

Posterolateral

Lambdoidal

Squamosal

Sagittal

Head Bangings Early Skull Closure
Figure 7.3 Abnormal patterns of suture fusion. Adapted from Cohen and MacLean (2000).

Dolichocephaly can occur with early closure of the sagittal suture, producing a long, narrow cranium. When both sides of the coronal suture are prematurely fused, the head is brachycephalic (Fig. 7.3). Unilateral synostosis of the coronal suture results in asymmetry of head shape or plagiocephaly (Fig. 7.3). The frontal eminence on the fused side is flattened and the glabella region is underdeveloped. The eyebrows and orbit on the affected side appear elevated. Premature closure of one lambdoid suture can similarly result in plagiocephaly. In trigonocephaly, premature synostosis of the metopic suture results in a triangular prominence of the frontal bone, usually in association with ocular hypotelorism (Fig. 7.3.). Metopic ridging may occur.

When the major determinants of anteroposterior and lateral growth are impeded by coronal and sagittal synostosis, respectively, the cranial vault grows vertically rather than in its normal longitudinal and horizontal directions, resulting in acrocephaly. Acrocephaly is a tall or high skull (vertical index above 77), and the top of the head may be pointed, peaked, or conical in shape. It is also referred to as oxycephaly, turricephaly, steeplehead, or tower skull. A shortened length of the skull compared to its width is referred to as brachycephaly (cephalic index above 81.0); this is typically caused by premature fusion of both coronal sutures (bicoronal synostosis). An elongation of the skull with narrowing from side to side (cephalic index less than 76) is called dolichocephaly or scaphocephaly, and typically caused by premature fusion of the sagittal suture (see Fig. 7.3).

Premature closure of the cranial sutures can occur in isolation or as part of a syndrome, in association with other clinical anomalies.

In addition to describing the skull shape in terms of its length and width, we can also comment on the prominence of various parts of the skull. Bathrocephaly is a condition characterized by a step-like posterior projection of the skull, caused by external bulging of the squamous portion of the occipital bone. Various craniosynostoses can reduce the depth of the bony orbit, producing prominence of the globe, or proptosis.

Few individuals, if any, have true symmetry of the face. Differences in length and width of the palpebral fissures are common but are rarely significant. Frank asymmetry of any part of the face is an important observation and often will provide a clue to the underlying developmental anomaly.

Cure Tennis Elbow Without Surgery

Cure Tennis Elbow Without Surgery

Everything you wanted to know about. How To Cure Tennis Elbow. Are you an athlete who suffers from tennis elbow? Contrary to popular opinion, most people who suffer from tennis elbow do not even play tennis. They get this condition, which is a torn tendon in the elbow, from the strain of using the same motions with the arm, repeatedly. If you have tennis elbow, you understand how the pain can disrupt your day.

Get My Free Ebook


Post a comment