Structure
Location and Boundaries
The paired frontal sinuses are found within the frontal bone and are the most superior of the paranasal sinuses. They are usually separated by a bony septum and there may be other incomplete septum further dividing them. When viewed anteriorly, they are triangular in shape. The points of this triangle are:
- The top of the nasion
- A point 3 cm above the first
- The junction of the medial third and lateral two-thirds of the supraorbital ridge
The sinus may extend anteriorly within the supraorbital ridge and posteriorly along the orbital part of the frontal bone. The aperture which connects to the nasal cavity usually opens as a frontonasal recess (the middle meatus).
Relations
Medially, with the contralateral frontal sinus.
Superiorly, laterally and posteriorly with the frontal bone and frontal lobe.
Inferiorly with the orbit.
Macroscopic Features
The sinus is lined by pale pink mucosa.
Microscopic Structure
Respiratory epithelium is a pseudostratified columnar type. Cilia are present near the aperture that connects the sinus with the nasal cavity.
Physiological Variations
The sinuses are fixed within the bone and do not move with physiological functions. They may be different sizes or shapes on each side. Hyperplasia of the sinus epithelium may occur with chronic inflammation
The size and shape of the sinuses vary considerably between different people. This variation is even used to identify bodies in forensic cases if radiological imaging of the sinuses is available. The aperture to the nasal cavity is variable in size and positioning.
Men usually have more prominent sinuses which gives their forehead more obliquity than females and children. The frontal sinuses usually begin to form after birth and are well developed by the age of 8.
Neurovascular Supply
Arterial Supply
Blood is delivered to the frontal sinuses via the supraorbital and anterior ethmoidal arteries.
Venous Drainage
Blood drains through an anastamotic vein to the supraorbital notch, where it returns to the heart via supraorbital and superior ophthalmic veins.
Lymphatics
Lymph vessels from the frontal sinus pass to submandibular nodes
Innervation
The sinuses receive general sensory input from branches of the supraorbital nerve (V) and secretomotor input from the pterygopalatine ganglion (via VII).
Routes of Cancer Spread
Sinus tumours are frequently locally aggressive but only uncommonly metastasize to nodal groups and even more rarely to distant sites. The frontal sinus is the site of origin in less than 1% of cases of sinus tumours.
Local Invasion
Bony destruction is common most sinus tumours. In the frontal sinus, this may lead to invasion of the anterior cranial fossa, the orbit, the ethmoid sinuses, or the soft tissues of the face.
Lymphatic Spread
The submandibular nodes are most frequently affected in lymphatic spread from the frontal sinuses.
Haematogenous Spread
Haemotogenous spread is unusual with squamous cell carcinoma (the most common aetiology).
Neurological Spread
Invasion of the cranial nerves may occur. The supraorbital branch of the trigeminal is the usual nerve involved.
