Anatomy & Physiology of
Balasubramanian M.S. D.L.O.
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.
Fig showing the
sizes of sinuses in various age
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
showing canine fossa
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
Roof: is formed by the frontal bone anteriorly
and by the face of sphenoid and orbital process of palatine bone
The ethmoidal cells increase in size
from above downwards, and from before
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
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
representation of paranasal sinuses
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
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
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