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Anatomy Of Paranasal Sinuses

Clinically, paranasal sinuses have been divided into two groups:
  1. Anterior group. This includes maxillary, frontal and anterior ethmoidal. They all open in the middle meatus
  2. Posterior groups. This includes posterior ethmoidal sinuses which open in the superior meatus, and the sphe­noid sinus which open in sphenoethmoidal recess.
  1. Maxillary Sinus (Antrum of Highmore)
    It is the largest of paranasal sinuses and occupies the body of maxilla. It is pyramidal in shape with base towards lat­eral wall of nose and apex directed laterally into the zygo­matic process. On an average, maxillary sinus has a capacity of 15 (30) ml in an adult. Is the earliest sinus to start development in human embryology.
  2. Relations
    1. Anterior wall is formed by facial surface of maxilla and is related to the soft tissues of cheek. Posterior wall is related to infratemporal and pterygopalatine fossae. Medial wall is related to the middle and inferior meatuses. At places, this wall is thin and membranous.
    2. Floor is formed by alveolar and palatine processes of the maxilla and is situated about 1 cm below the level of floor of nose.
    3. Depending on the age and pneumatisation of the sinus, the roots of all the molars, sometimes the premolars and canine, are in close relation to the floor of maxillary sinus separated from it by a thin lamina of bone or even no bone at all. Oroantral fistulae can result from extraction of any of these teeth.
    4. Dental infection is also an important cause of maxillary sinusitis.
    5. Ostium of the maxillary sinus is situated high up in medial wall and opens in the posterior part of ethmoidal infundibulum into the middle meatus.
    6. It is unfavourably situated for natural drainage.
    7. Roof of the maxillary sinus is formed by the floor of the orbit. it is traversed by infraorbital nerve and vessels.

Fig.: Coronal section showing relationship of maxillary and ethmoidal sinuses to orbit and nasal cavity.
  1. Frontal Sinus
    1. Each frontal sinus is situated between the inner and outer tables of frontal bone, above and deep to the supraorbital margin.
    2. It varies in shape and size and is often loculated. The loculations are also called scallops.
    3. The two frontal sinuses are often asymmetric and the intervening bony septum is thin and often obliquely placed or may even be deficient. Frontal sinus may be absent on one or both sides or it may be very large extend­ing into orbital plate in the roof of the orbit.
    4. Anterior wall of the sinus is related to the skin over the forehead; inferior wall, to the orbit and its contents; and posterior wall to the meninges and frontal lobe of the brain.
    5. Opening of frontal sinus is situated in its floor and leads into the middle meatus directly or through a canal called frontonasal duct. In the middle meatus, frontal sinus drains into frontal recess.(55%), above but not into the infundibulum (30%), into the infundibulum (15%) and above the bulla ethmoidalis (1 %).
  2. Ethmoidal Sinuses (Ethmoid Air Cells)
    Ethmoidal sinuses are thin-walled air cavities in the lat­eral masses of ethmoid bone. Their number varies from 3 to 18. They occupy the space between upper third of lat­eral nasal wall and the medial wall of orbit.
    1. Clinically, ethmoidal cells are divided into anterior ethmoid group which opens into the middle meatus, and posterior ethmoid group which opens into the superior meatus and sphe­noethmoidal recess.
  3. Relations of Ethmoidal Sinuses
    1. Roof is formed by anterior cranial fossa, lateral to the cribri­form plate. Meninges of brain form important relations here.
    2. Lateral wall is related to the orbit. The thin paper­ like lamina of bone (lamina papyracea) separating air cells from the orbit can be easily destroyed leading to spread of ethmoidal infections into the orbit.
      Optic nerve forms close relationship with the posterior ethmoidal cells and is at risk during ethmoid surgery.
  4. Sphenoid Sinus
    1. It occupies the body of sphenoid. The two, right and left sinuses, are rarely symmetrical and are separated by a thin bony septum which is often obliquely placed and may even be deficient (compare frontal sinus).
    2. Ostium of the sphenoid sinus is situated in the upper part of its anterior wall and drains into sphenoethmoidal recess.
    3. Relations of the sphenoid sinus are important to the surgeon while doing transsphenoidal hypophysectomy. The relations of the sinus differ in the anterior and posterior parts.
    4. In the anterior part, roof is related to the olfactory tract, optic chiasma and frontal lobe while the lateral wall is related to the optic nerve, internal carotid artery and maxillary nerve and these structures may stand in relief in the sinus cavity.
    5. In the posterior part, roof is related to pituitary gland in the sella turcica while each lateral wall is related to cavernous sinus, internal carotid artery and CN III, IV, VI and all the divisions of VDegree of pneumatisation of sphenoid sinus varies and so do the extent of sinus and the structures related to it. Sinus cavity may be large and extend into the wings of sphenoid and even pterygoid plates.

Fig.: Coronal section of sphenoid sinuses. Note the reliefs made by various structures in the cavity of sphenoid sinus. Optic nerve forms the superolateral ridge.


Fig.: Relations of sphenoid sinus

Table: Development and growth of paranasal sinuses
Status at birth                                  Growth First radiologic evidence
Present at birth Maxillary Rapid growth from birth to 3 years and from 7-12 years.                                          Adult size - 15 years 4-5 months after birth
Present at birth ethmoids   
Not present frontal                                 
Reach adult size by 12 years
Invades frontal bone at the age of                                                                                 6 years
4 years. Size increases until teens   
1 year
6 years
Not present sphenoids               Reaches sella turcica by the age                         of 7 years, dorsum sellae by late teens and basisphenoid by adult age. Reaches full size between 15 years to adult age                                               4 years


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