• The cranial base
• The nasomaxillary complex • The mandible
INTRODUCTION
For the basis of simplicity, the growth of the craniofacial complex can be divided into four areas that grow rather differently:
a. The cranial vault the bone that covers the upper and outer surface of the brain.
b. The cranial base the bony floor under the brain, which is also a dividing line between the cranium and the face.
c. The nasomaxillary complex made up of the nose,
maxilla, and the associated structures. d. The mandible.
THE CRANIAL VAULT
The growth in the cranial vault is because of the enlarging brain (Fig. 4.1).
Sagittal suture Skin Calvaria Arachnoid granulalions Pericranium Lateral (venous) lacuna Muscle I I
Diploe Epidermis Dura mater
/
Fig. 4.1: The cut section of the developing skull
The rate of bone growth is more during infancy and by the fifth year of life more than 90 percent of the growth of cranial vault is achieved (Table 4.1). It is made up of a number of flat bones that are formed directly by intramembranous ossification, without cartilaginous precursors. From the time that ossification begins at a number of centers that foreshadow the eventual anatomic bony units, the growth process is entirely the result of periosteal activity at the surfaces of the bones. Some selective resorption occurs early in postnatal life on the inner surfaces of the cranial bones to help flatten them out as they expand. Apposition can be seen on both the internal and external tables of the cranial bones as they become thicker. This increase in thickness which permits the development of the diploe is not uniform. Sicher attributes this to the fact that the inner cranial table is primarily under the influence of the growth of the brain-the brain capsule while the outer plate has certain mechanical influences operating upon it. These mechanical influences contribute to the growth of cranial superstructures. Of particular significance are the supraorbital, otic, and mastoid regions. These structures are usually more marked in the males than females.
Remodeling and growth occur primarily at the periosteum lined contact areas between adjacent skull bone, called the skeletal sutures. At birth, the flat bones of the skull are rather widely separated by relatively loose connective tissues. These open spaces, the fontanelles (Figs 4.2 A to C), allow a considerable
amount of deformation of the skull at birth-a fact which is important in allowing the relatively large head to pass through the birth canal.
After birth, apposition of bone along the edges of the fontanelles eliminates these open spaces fairly quickly, but the bones remain separated by a thin periosteum lined suture for many years, eventually fusing in adult life.
The newborn not only has his frontal bone sepa- rated by the soon to close metopic suture, but also has no frontal sinuses. Both the inner and outer surfaces are quite parallel and quite close to each other. With the general growth and thickening of the cranial vault there is an increase in the distance between the intema I Fig.4.2A: The locationof the variousfontanelles(lateralview) and external plates in the supraorbital region. This may
be seen on the external surface as a ridge. The spongy bone between the external plates is gradually replaced by the developing frontal sinus .
.The cranial vault increases in width primarily through 'fill in' ossification of the proliferating connec- tive tissue in the coronal, lambdoidal, interparietal, parietosphenoidal and parietotemporal sutures. It should be realized that there is actual translation as well as remodeling of the individual bones, with the structures being moved outward by the growing brain. Despite early accomplishment of the pattern, the parietal bones do not close until the middle of third decade of life.
Increase in length of the brain case may be primarily due to the growth of the cranial base with Fig. 4.28: Locationof the various fontanelles (superiorview) active response at the coronal suture.
Height of the brain case is due to the activity of the parietal sutures along with the occipital, temporal, and sphenoidal contiguous osseous structures.
Fig. 4.2C: Locationof the various fontanelles (posteriorview)
Tabfe 4.1: Percentage for growthin lengthof the cranial vault at various ages (Davenport)
Birth 06 months 1 year 2 years 3 years 5years 10 years 15 years 63 percent 76 percent 82 percent 87percent 89 percent 91 percent 95percent 98 percent .
THE CRANIAL BASE
The cranial base, unlike cranial vault, is not completely dependent on brain growth and may have some intrinsic genetic guidance and a pattern that is, similar in some dimensions, to that of the facial skeleton.
In contrast to the cranial vault, the bones of the cranial base are formed initially in the cartilage and are later transformed by endochondral ossification into bone. This is particularly true of the midline structures. As one moves laterally, growth at sutures becomes more important, but the cranial base is essentially a midline structure.
Centers of ossification appear early in embryonic life in the chondrocranium, indicating the eventual location of the basioccipital, sphenoid and ethmoid bones that form the cranial base (Fig. 4.3).
Ethmoid
Frontal bone
Fig. 4.3: The bones that formthe base of the skuil The cranial base grows primarily by cartilage growth in the sphenoethmoidal, intersphenoidal, spheno-occipital and intraoccipital synchondroses, mostly following the neural growth curve (Fig. 4.4).
Activity at the intersphenoidal synchondrosis disappears at birth. The intraoccipital synchondrosis closes in the 3rd to 5th years of life. The spheno- occipital synchondrosis is a major contributor as the ossification here extends till the 20th year of life.