Frontal bone & sinus
(Butler p.3)
- The frontal bone forms in two halves, which normally fuse at five years. The intervening suture is known as the metopic suture
- Occasionally, the halves remain separate and the suture may persist wholly or in part into adult life in 5-10% of individuals
- The orbital plates of the frontal bone contribute most of the anterior fossa floor with a cribriform plate of the ethmoid bone interposed between in the midline
The frontal sinus and frontal recess (Butler p. 68)
- The frontal sinuses are asymmetrical extensions from the anterior ethmoidal air cells between the tables of the frontal bones. They are the last paranasal sinus to aerate and are not fully developed until just after puberty. Aplasia or lack of any extension into the frontal bone is present between 5 and 8% of people and hypoplasia in 4%
- The frontal sinus drainage pathway is via the frontal recess, which is the shape of an inverted funnel, measures approximately 13mm long and is formed by the walls of the adjacent air cells, hence the term ‘recess’ rather ‘duct’
- The frontal recess has an oblique course 50° to the orbito-meatal plane and is therefore best demonstrated on the sagittal reconstructed images.
- The usual boundaries of the frontal recess are posteriorly the ethmoidal bulla, anterior and inferiorly the agger nasi air cell, medially the olfactory fossa and midline turbinate, laterally the lamina papyracea and superiorly the roof of the anterior ethmoidal air cells (fovea ethomoidalis)
- The frontal recess anatomy, however, is complex due to the variable accessory air cells that may form part of its boundaries, and drains either into the ethmoidal infundibulum or middle meatus depending on the superior attachment of the uncinate process
- These frontal region accessory air cells are as follows:
- The agger nasi air cell (ANC) is the most anterior of the ethmoidal air cells and may vary in size. If large it can displace the frontal recess posteriorly and narrow the ostium
- The frontoethmoidal air cells are variably classified depending on their number and extent. They are located superior to the ANC and extend into the frontal sinus.
- The frontal bulla is an extension of the ethmoidal bulla into the frontal region. The suprabulla air cell is an air cell just superior and anterior to the ethmoidal bulla and the supraorbital air cell usually arises from the anterior ethmoidal air cell and extends into the orbital plate of the frontal bone.
- When assessing the frontal sinus and recess region the priority must be first to identify the frontal drainage pathway and to then clearly describe the site of origin, size and relationship of the adjacent air cells forming the frontal recess
- These frontal region accessory air cells are as follows:
(Ryan p. 1, 3)
- The frontal bone forms the front of the skull vault. It is formed by two frontal bones that unite at the metopic suture. The frontal bones join the parietal bones at the coronal suture
- The orbital plates of the frontal bones are thin and irregular and separate the anterior cranial fossa from the orbital cavity
The frontal sinuses (Ryan p.14)
- These lies between the inner and outer tables of the frontal bone above the nose and medial part of the orbits; they vary greatly in size and are often asymmetrical. They may extend into the orbital plate of the frontal bone
(Ryan p.16)
- The frontal sinuses are not visible on the skull radiograph until the age of 2 years and achieve adult proportions by the age of 14. Asymmetry is common, and one or both may fail to develop. Absence of both may be associated with persistence of the metopic suture between the two halves of the frontal bone. Development of the ethmoids occurs at a rate similar to that of the frontal sinuses