Size and Shape

During infancy the neurocranium is larger relative to the face than at any other time during normal growth. Ratios of the respective areas of the neurocranium in lateral projection are roughly 3 : 1 to 4 : 1 at birth, and they decrease to 2 : 1 to 2.5 : 1 by age 6 years. The bones of the calvarium lie in their incompletely mineralized membranous capsule; they are separated by broad strips of connective tissue that form the sutures and by patches of connective tissue, the fontanelles. The six constant, or major, fontanelles are located at the four corners of the parietal bones—two in the midline of the skull and two pairs on each side (Fig. 18-1). Accessory fontanelles may occur in several parts of the cranium but usually are located in the sagittal suture. The sutures and the synchondroses in the base are prominent in newborns but diminish in width during the first 2 to 3 months. Obliteration of the sutures does not begin until the second to third decades. Figures 18-1 through 18-4 and e-Figure 18-5 illustrate sutures, fontanelles, and synchondroses.

The sphenoid bone at birth consists of a single central mass composed of the body and the lesser wings and two symmetric lateral osseous masses, each of which is made up of a greater wing and a pterygoid process. The pituitary fossa in the body of the sphenoid bone tends to be round with smooth margins; the dorsum sella is short and blunt, and the clinoid processes are rudimentary. The angle between the body of the neonatal mandible and the ascending ramus in lateral projection is about 160 degrees; the relatively large bodies are separated in the midline by a prominent cartilaginous symphysis mentalis (see Fig. 18-3). Early calcification of teeth is seen in the fifth fetal month.¹

Components of the individual bones that are not united in infancy may lead to confusion unless they are correctly recognized. The frontal bone is divided in half laterally by the metopic suture (see Figs. 18-1 and 18-3). Apparent discontinuity of the sphenoid bone with the frontal bone superiorly and the occipital bone posteriorly indicates the sites of the sphenoid bone’s synchondroses with these two bones (see Fig. 18-2). The four major components of the occipital bone (e-Fig. 18-5 and Fig. 18-2) likewise may simulate discontinuities of structure.

Normal Variations

Intrasutural, or wormian, bones occur most frequently along the lambdoid sutures (Fig. 18-6 and e-Fig. 18-7; Box 18-2). They occur much less frequently in the fontanelles (see e-Fig. 18-7). The interparietal or Inca bone (Fig. 18-8) results from division of the supraoccipital portion of the occipital bone into two parts by the mendosal suture, with the superior part arising from membranous bone and the inferior part arising from cartilage continuous with that of the supracondylar portions and the basiocciput. A rare synchondrosis or suture line runs vertically through the squamous portion of the occipital bone (Fig. 18-9); persisting superior and inferior portions of the line are known as the superior longitudinal fissure or bi-interparietal suture and the cerebellar synchondrosis or median cerebellar suture. Where the supraoccipital portion of the occipital bone forms the posterior border of the foramen magnum, accessory supraoccipital bones occasionally are found (e-Fig. 18-10). The configuration caused by an outward bulge of the occipital squamosa just above the torcular Herophili in a newborn (Fig. 18-11) is called bathrocephaly. Rarely, a horizontal interparietal suture divides the parietal bones into superior and inferior moieties (e-Fig. 18-12).

Compression of the fetal skull and its molding during passage through the maternal pelvis produce significant radiographic findings that persist after birth (Fig. 18-13).² During the first weeks and months of life, widths of sutures vary so much that caution is required in their evaluation for the diagnosis of increased intracranial pressure, particularly because positioning is difficult and partial superimposition of bilateral sutures can produce spurious widening (Fig. 18-14).

In children older than 2 years, the sutures extend through both tables and the diploic space. The outer table portion of the suture may be deeply serrated when the inner table portion is practically a straight line (e-Fig. 18-15) and may be interpreted erroneously as a “fracture through a suture.” Persistence of the metopic suture may simulate a vertical fracture in the occipital bone in anteroposterior, caudally angulated exposures if extension of the superimposed radiolucent line into the area of the foramen magnum is invisible or if the inferior portion of the suture has been obliterated. The frontal crest on the internal surface of the frontal squamosa in the midsagittal plane may be sufficiently prominent to simulate calcification of the falx cerebri that attaches to it (e-Fig. 18-16).