Embryology, Anatomy, Normal Findings, and Imaging Techniques

Size and Shape

During infancy, the neurocranium is larger relative to the face. Neurocranium to face ratios in the lateral projection are roughly 3 : 1 to 4 : 1 at birth, and decrease to 2 : 1 to 2.5 : 1 by age 6 years. The calvarium is an incompletely mineralized membranous capsule with individual bones divided from one another by connective tissue sutures and fontanelles. The six major fontanelles are located at the four corners of the parietal bones—two in the skull midline and two pairs on each side ( Fig. 18.1 ). Accessory fontanelles may occur in several locations but usually are in the sagittal suture. The sutures and the skull base synchondroses 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. Figs. 18.1 through 18.4 and e-Fig. 18.5 illustrate sutures, fontanelles, and synchondroses.

The sphenoid bone at birth consists of a single central mass composed of the body, lesser wings, and two symmetric lateral masses, each of which includes a greater wing and a pterygoid process. The pituitary fossa is 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 ramus in lateral projection is about 160 degrees; the bodies are separated by a midline cartilaginous symphysis mentalis (see Fig. 18.3 ). Early tooth calcification is seen in the fifth fetal month.

Components of the individual ununited bones in infancy may cause confusion if they are not correctly recognized. The frontal bone is divided in half 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 sites of the sphenoid synchondroses (see Fig. 18.2 ). The four components of the occipital bone (see Fig. 18.2 and e-Fig. 18.5 ) likewise may simulate discontinuities.

Normal Variations

Intrasutural, or wormian, bones occur most frequently along the lambdoid sutures ( Fig. 18.6 and e-Fig. 18.7 ; Box 18.2 ), and 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 from cartilage continuous with the supracondylar basiocciput. A rare synchondrosis runs vertically through the squamous occipital bone ( e-Fig. 18.9 ); persisting superior and inferior portions of the line are known as the superior longitudinal fissure (bi-interparietal suture) and the cerebellar synchondrosis (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 ). 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 ).