**2. Development and growth of the frontal bone**

Ossification of the intra-membranous calvarial bones depends on the presence of the brain; in its absence (anencephaly), no bony calvarium forms. A pair of FBs appears from single primary ossification centers, forming in the region of each superciliary arch at the 8th week postconception. Three pairs of secondary centers appear later in the zygomatic processes, nasal spine and trochlear fossae. Fusion between these centers is complete at 6 to 7 months post-

conception. At birth, the frontal bones are separated by the metopic suture. Synostotic fusion of this suture usually starts about the 2nd year and unites the frontal bones into a single bone by 7 years of age. The metopic suture persists into adulthood in 10 to 15% of skulls. In such cases, the frontal sinuses are absent or hypoplastic. [4]

The cranial and facial bones are first made of fibrous connective tissue. In the third month of fetal development, fibroblasts become more specialized and differentiate into osteoblasts, which produce bone matrix. From each center of ossification, bone growth radiates outward as calcium salts are deposited in the collagen model of the bone. This process is not complete at birth; a baby has areas of fibrous connective tissue remaining between the bones of the skull. These are called fontanels, which permit compression of the baby's head during birth without breaking the still thin cranial bones. The fontanels also permit the growth of the brain after birth. By the age of 2 years, all the fontanels have become ossified. [5]

Growth of the calvarial bones is a combination of suture growth, surface apposition and resorption (remodeling), and centrifugal displacement by the expanding brain. The propor‐ tions attributable to the various growth mechanisms vary by age. Accretion to the calvarial bones is predominantly sutural until about the 4th year of life, after which surface apposition and remodeling become increasingly important.

The bones of the newborn calvarium are unilaminar and lack diploë. From about 4 years of age, lamellar compaction of cancellous trabeculae forms the inner and outer tables of the cranial bones. The tables become continuously more distinct into adulthood. This differen‐ tial bone structure creates a high stiffness - to - weight ratio, with no relative increase in the mineral content of cranial bone from birth to adulthood. Whereas the behavior of the inner table is related primarily to the brain and intracranial pressures, the outer table is more responsive to extracranial muscular and buttressing forces. The internal plate becomes stable at 6 to 7 years of age, reflecting the near cessation of cerebral growth. The thicken‐ ing of the frontal bone in the midline at the glabella results from separation of the inner and outer tables with invasion of the FS between the cortical plates. Growth of the external plate during childhood produces the superciliary arches and other bony landmarks that are all absent in the neonatal skull. [4]

FS is a small out-pouching at birth and undergoes almost all of its development thereafter. The FS may develop from one or several different sites (primary pneumatization): as a rudiment of the ethmoid air cells, as a mucosal pocket in or near the frontal recess, as an invagination of the frontal recess, or from the superior middle meatus. The process starts 3 to 4 months post conception, but they do not yet invade the frontal bone. Secondary pneumatization takes place between the ages of 6 months to 2 years postnatally and it develops laterally and vertically. FS itself cannot be identified radiographically until approximately the age of 6 to 8 years, and most pneumatization is completed by the time the child is 12 to 16 years- old, but it continues until the age of 40. [4,6] In 10% of persons, FS develops unilaterally, in 5% it is a rudimentary structure, and in 4% it is absent altogether, so that almost one-fifth of individuals have aberrant sinus development (Figure 1).[7]

**Figure 1.** Postnatal development of the frontal sinus.
