**3. Frontal bone anatomy**

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

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

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

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

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

cases, the frontal sinuses are absent or hypoplastic. [4]

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and remodeling become increasingly important.

all absent in the neonatal skull. [4]

sinus development (Figure 1).[7]

birth. By the age of 2 years, all the fontanels have become ossified. [5]

The frontal bone forms the forehead and the anterior part of the top of the skull, the anterior cranial fossa and the roofs of the orbits. It consists of two parts, vertical called the squamous part and horizontal called the orbital part. From the nasion FB extends approximately 12.5 cm superiorly, 8.0 cm laterally, and 5.5 cm posteriorly. [8]

**The squamous part** has a convex outer surface which forms the main substance of the forehead and the anterior part of the vault of the skull. The squamous part of FB has the nasal notch which articulates with the nasal bone on either side of the middle line and more laterally with the frontal process of maxilla and with the lacrimal bone. The squamous part of the frontal bone consists of two layers of compact bone separated by a layer of cancellous bone (the diploë) which contains red bone marrow and a number of diploic veins.

**Its outer surface** shows the following features:

**Frontal eminences** are the most prominent parts of FB.

**Superciliary arches**, thick curved ridges lie little above the medial portions of the supraorbital margins. They are well developed in males and less marked or even totally absent in females.

**Supraorbital margins**, which form the upper boundaries of the orbits, end laterally at each side in the zygomatic processes of the FB. They have the supraorbital notches at the junctions of the middle and intermediate thirds. In some cases there may be foramina instead of notches. Supratrochlear foramina are located medially to the supraorbital foramina or notches and laterally to the nasal bones. The smooth area of the frontal bone just above the root of the nose is called the glabella. Temporal line, a well-marked ridge, runs from the zygomatic process of FB upward and backward (Figure 2).

**The inner surface** of the squamous part is concave and forms the anterior cranial fossa.

**The sagittal groove** lies in the upper part of the middle line. The two edges of this groove unite below to form a ridge - the frontal crest. The sagittal groove accommodates the anterior part of the venous superior sagittal sinus.

**The frontal crest** gives attachment to the falx cerebri, a fold of dura matter. The frontal crest ends below in a small hole called the foramen caecum between the frontal and the ethmoid bone. The foramen caecum does not usually transmit any structure but may transmit a vein from the nose to the superior sagittal sinus. [5, 8, 9]

**The orbital parts** of the FB extend laterally from the nasal notch, become concave and form the orbital roofs. A spine or concavity exists along the medial anterior orbital roof, where the trochlea of the superior oblique muscle is attached. The arched roofs of the orbits are separated from one another by a median gap called the ethmoid notch. In the intact skull the ethmoid notch is filled by the cribriform plate of ethmoid bone. The margins of the ethmoid notch of the frontal bone contain many half cells which unite with corresponding half cells on the upper surface of the ethmoid bone to form together the ethmoid air cells (Figure 2).

The frontal bone articulates with 12 other cranial bones: two parietals, two nasals, two maxillae, two lacrimal, two zygomatic, the sphenoid and the ethmoid. The bones are separated by sutures which hold the bones firmly together in the mature skull. Occasionally the squamous part of FB may be separated into two halves by a midline metopic suture persistent from early childhood. Normally, two halves of the frontal bone unite completely by the 8th year.

**The arterial blood supply** to the frontal bone is by the supraorbital, anterior superficial temporal, anterior cerebral and middle meningeal arteries. The venous drainage is transoss‐ eous through the anastomosis of vessels of the subcutaneous, orbital, and intracranial struc‐ tures. The primary venous drainage is through the supratrochlear, supraorbital, superficial temporal, frontal diploic (veins of Breschet), superior ophthalmic, and superior sagittal sinuses. [4, 10, 11]

**The frontal sinus** may consist of one or more compartments, depending on the source of pneumatization. The inter-sinus septum, which separates the left and right cavities of the sinus, is continuous with the crista galli and cribriform plate inferiorly. The septum is usually deviating from the midline sagittal plane. FSs vary in size in different people. The average height of the sinuses is 32 mm, and their average width is 26 mm. The surface area is approx‐ imately 720 mm2 . [6-8, 12] The FS is in critical approximation to anatomical structures, which underscores the importance of its management in injury. Posteriorly, the cribriform plate, dura mater, and frontal lobes of brain are in close apposition to one another and to the posterior wall of the sinus. The dura is densely adherent to the deep surface of the posterior table and becomes more adherent and thinner along the caudal edge, where it turns to cover the fovea ethmoidalis. [13] The lateral floor of the FS is the roof of the orbit, whereas the medial floor of the frontal sinus contains the opening of the nasofrontal duct. Each sinus opens into the anterior part of the corresponding nasal middle meatus by the ethmoidal infundibulum or nasofrontal duct (NFD), traversing the anterior part of the ethmoid labyrinth. Anatomically significant variations exist in the width, length, and shape of the NFD. The duct opening usually lies in the posteromedial floor of the sinus. It is a funnel shaped constriction that passes between the Contemporary Management of Frontal Sinus Injuries and Frontal Bone Fractures http://dx.doi.org/10.5772/59096 439

**The sagittal groove** lies in the upper part of the middle line. The two edges of this groove unite below to form a ridge - the frontal crest. The sagittal groove accommodates the anterior part

**The frontal crest** gives attachment to the falx cerebri, a fold of dura matter. The frontal crest ends below in a small hole called the foramen caecum between the frontal and the ethmoid bone. The foramen caecum does not usually transmit any structure but may transmit a vein

**The orbital parts** of the FB extend laterally from the nasal notch, become concave and form the orbital roofs. A spine or concavity exists along the medial anterior orbital roof, where the trochlea of the superior oblique muscle is attached. The arched roofs of the orbits are separated from one another by a median gap called the ethmoid notch. In the intact skull the ethmoid notch is filled by the cribriform plate of ethmoid bone. The margins of the ethmoid notch of the frontal bone contain many half cells which unite with corresponding half cells on the upper

The frontal bone articulates with 12 other cranial bones: two parietals, two nasals, two maxillae, two lacrimal, two zygomatic, the sphenoid and the ethmoid. The bones are separated by sutures which hold the bones firmly together in the mature skull. Occasionally the squamous part of FB may be separated into two halves by a midline metopic suture persistent from early

**The arterial blood supply** to the frontal bone is by the supraorbital, anterior superficial temporal, anterior cerebral and middle meningeal arteries. The venous drainage is transoss‐ eous through the anastomosis of vessels of the subcutaneous, orbital, and intracranial struc‐ tures. The primary venous drainage is through the supratrochlear, supraorbital, superficial temporal, frontal diploic (veins of Breschet), superior ophthalmic, and superior sagittal

**The frontal sinus** may consist of one or more compartments, depending on the source of pneumatization. The inter-sinus septum, which separates the left and right cavities of the sinus, is continuous with the crista galli and cribriform plate inferiorly. The septum is usually deviating from the midline sagittal plane. FSs vary in size in different people. The average height of the sinuses is 32 mm, and their average width is 26 mm. The surface area is approx‐

underscores the importance of its management in injury. Posteriorly, the cribriform plate, dura mater, and frontal lobes of brain are in close apposition to one another and to the posterior wall of the sinus. The dura is densely adherent to the deep surface of the posterior table and becomes more adherent and thinner along the caudal edge, where it turns to cover the fovea ethmoidalis. [13] The lateral floor of the FS is the roof of the orbit, whereas the medial floor of the frontal sinus contains the opening of the nasofrontal duct. Each sinus opens into the anterior part of the corresponding nasal middle meatus by the ethmoidal infundibulum or nasofrontal duct (NFD), traversing the anterior part of the ethmoid labyrinth. Anatomically significant variations exist in the width, length, and shape of the NFD. The duct opening usually lies in the posteromedial floor of the sinus. It is a funnel shaped constriction that passes between the

. [6-8, 12] The FS is in critical approximation to anatomical structures, which

childhood. Normally, two halves of the frontal bone unite completely by the 8th year.

surface of the ethmoid bone to form together the ethmoid air cells (Figure 2).

of the venous superior sagittal sinus.

438 A Textbook of Advanced Oral and Maxillofacial Surgery Volume 2

sinuses. [4, 10, 11]

imately 720 mm2

from the nose to the superior sagittal sinus. [5, 8, 9]

cancellous part of the anterior wall underlying the glabella and the anterior ethmoidal cells. Its course is highly variable, running caudally from a few millimeters to up to 2 cm. The NFD terminates at the uncinate process in the nasal cavity, which is a thin bone plate that is covered on either side by mucosa. When the uncinate process is attached to the lamina papyracea, the drainage is medial to the uncinate process through the middle meatus. This type of drainage pattern is seen in 66-88 % of cases. When the uncinate process attaches superiorly to more medial structures (middle turbinate, cribriform, or skull base), the drainage of the sinus is lateral to the uncinate process. This type of drainage pattern is seen in 12-34% of cases. A true identifiable duct may be absent in up to 85% of FSs. In this situation, the FS drains indirectly through ethmoid air cells to the middle meatus. Therefore, some investigators chose the term *nasofrontal outflow tract* (NFOT) or *frontal sinus outflow tract* (FSOT) for the drainage path of the FS (Figure 3). [7, 12-18]

**Figure 3.** Opposite extremes of frontal sinus development; aplasia (left) versus hypertrophy (right).
