Anatomy of Paranasal sinuses
Definition: Paranasal sinuses are air filled sacs found in the skull bone. These sacs infact surround the nasal cavity. There are 4 paired sinuses. They are:
1. Maxillary sinuses
2. Frontal sinuses
3. Ethmoidal sinuses
4. Sphenoidal sinuses
Maxillary sinus (Antrum of Highmore): These paired sinuses lie under the cheek. It is the largest of the group of para nasal sinuses. The capacity of the maxillary sinus is roughly 1 fluid ounze (30ml). It is more or less shaped like a pyramid.
Base (medial wall): The base of the pyramid corresponds to the lateral nasal wall. This wall has its convexity facing the sinus. The central portion of the base is very thin, and in some areas could even be membranous. The natural ostium of this sinus is present in this wall. It is present more towards the roof of the sinus cavity than its base.
Anterior: Wall corresponds to the facial surface of the superior maxilla. Over the canine fossa it is only 2mm in thickness. It is though this canine fossa area that maxillay antrum is entered during Caldwell Luc surgery.
Boundaries of Canine fossa:
Inferior: Bounded by the alveolar ridge
Laterally: Bounded by the canine eminence which is caused by the canine tooth.
Superior: Infra orbital foramen
Medial: Pyriform aperture
Posterior wall: of maxillary sinus is also known as temporal surface. It is very thick and is formed by the body of the superior portion of the maxilla.
Roof: of the sinus is formed by its thin orbital wall which is traversed by the infra orbital foramen containing the infra orbital vessels and nerves. This wall is very fragile and any disease process involving the maxilla is likely to affect the orbit through this wall. This wall is further thinned out where the infra orbital canal is present.
Floor: is formed alveolar process of the maxilla and the hard palate. The roots of the first and second molar reach up to the floor of the maxillary sinus. In children the floor lies at the same level as that of the nasal cavity. In adults it lies 5 - 10 mm below the nasal cavity. It is just separated from the floor of the sinus by a thin lamella of bone. This lamella may be dehiscent commonly. Dental infections involving the 1st and 2nd molars may involve the maxillary sinus through this thin lamella of bone.
Image of skull showing the canine fossa
Figure showing the sizes of paranasal sinuses in various age groups
The maxillay sinus has biphasic growth. The first phase of growth occur during the first 3 years of life while the second phase occur between 7 - 18 years.
Is situated in the anterior skull base. It is made up of complex bony labyrinth of thin walled cells. A few ethmoid cells may be present at birth. The common infections affecting the pediatric age group occur in this sinuses. In adults 6 - 10 ethmoid cells may be present.
Lateral wall: is formed by the orbital plate of the ethmoid otherwise known as the lamina papyracea. This is a thin lamina of bone separating the orbit from the ethmoidal air cells. This wall could be dehiscent (normal variant). Infections involving the ethmoid air cells may spread to the orbit through this wall.
Roof: is formed by the frontal bone anteriorly and by the face of sphenoid and orbital process of palatine bone posteriorly.
The ethmoidal cells increase in size from above downwards, and from before backwards.
The cells of the ethmoid sinus are divided into two groups, the anterior and posterior group. The anterior ethmoidal cells drain into the infundibulum of the middle meatus while the posterior ethmoid cells drain into the superior meatus. The anatomy of the ethmoidal cells are highly variable, sometimes the middle turbinate may contain an air cell known as the concha bullosa. An enlarged concha bullosa may impede drainage from the middle meatus. Another common anatomical variation is the presence of agger nasi air cell. This is a large ethmoidal air cell present just anterior to the antero superior attachment of the middle turbinate. Since these cells lie in close proximity to the frontal recess area, they could impede ventilation and drainage of the frontal sinus. These agger nasi cells are commonly involved in the pathogenesis of the formation of frontal mucocele.
Obstruction to the frontal sinus drainage by the presence of a lage agger nasi cell may cause secretions within the frontal sinus to be dammed inside. Accumulation of mucoid secretions cause enlargement of frontal sinus. At first the frontal sinus enlarges in size by expansion of its bony walls. At a later stage bone erosion can also occur. commonly the posterior table of the frontal sinus is eroded. The anterior table also can be eroded in rare cases.
Haller cells are ethmoidal air cells belonging to the anterior ethmoidal group. These cells are also known as the infra orbital cells. Enlargement of these cells can impede the maxillary sinus drainage. Another variation is the extension of the posterior group of ethmoidal air cells supero lateral to the sphenoid sinus. These cells are known as onodi cells. These cells lie periliously close to the optic nerve making them at risk during fess surgeries.
Diagram of maxillary and ethmoid sinsues
Frontal sinus: Among the para nasal sinuses this sinus shows the maximum variations. Infact variations are so immense that it can safely be stated that frontal sinuses are unique in each and every individual. It may be absent in 5 % of individuals. It is more or less shaped like a L.
Posterior wall: corresponds to the anterior wall of the anterior cranial fossa.
Floor: is formed by the upper part of the orbits.
Frontal sinus appear very late in life. Infact they are not seen in skull films before the age of 6.
The sinus drains into the anterior part of the middle meatus through the fronto nasal duct.
Is located in the skull base at the junction of the anterior and middle cranial fossa. Pnematisation of sphenoid starts during the 4th year of childhood and gets completed by the 17th year. The sphnoid sinuses vary in size and may be asymmetric.
They drain through the superior meatus via a small ostium about 4mm in diameter located disadvantageously 20mm above the sinus floor.
This sinus is related to several important vital structures. They are:
1. Pituitary gland lies above the sphenoid sinus.
2. Optic nerve and internal carotid arteries traverse its lateral wall.
3. The nerve of pterygoid canal lie in the floor of the sinus.
Hence infections of sphenoid sinus may involve the optic nerve if the canal of the optic nerve is dehiscent.
Functions of para nasal sinuses:
1. They lighten the skull.
2. They add resonance to speech
3. They play a role in conditioning the inspired air.