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Anatomy of the Sinuses
Surgical Anatomy of the Sinuses With Diagrams

Question: Why is the sinus anatomy important?
Each patient's sinus anatomy has a unique configuration, and it is vital that this is well delineated for surgery to be successful.

Question: Tell me a little about the general anatomy.
The four paired paranasal sinuses are the ethmoid, maxillary, frontal, and sphenoid sinuses. These are named after the cranial bones in which they are located. The sinuses normally contain air and are lined with ciliated pseudostratified columnar epithelium with interspersed mucus secreting cells.

Question: Tell me more details about the anatomy.
This detailed information is provided for those who are interested. This information is complex and may be best understood if you refer to the accompanying diagrams.

Question: Tell me about the ethmoid sinuses.
The ethmoid sinuses are the sinuses "between the eyes." They are typically fully developed by age 12. They are like a honeycomb, consisting of multiple small sinus cells that together form the ethmoid sinus. The ethmoid sinus lies near the brain and near the eyes. For this reason, untreated severe sinusitis can lead to brain and eye complications.

The ethmoid sinuses begin development at the third fetal month. They evaginate from the lateral nasal wall. At birth, usually three or four ethmoid cells are present. However, they are difficult to recognize on x-rays until the infant reaches about six months of age. Only the ethmoid and maxillary sinuses are significantly developed enough at birth to be clinically significant. (Significant growth of the sphenoid and frontal sinuses are rarely seen until three years of age.)

The ethmoid sinuses reach nearly adult size by age 12. They do not develop as single cavities but rather as multiple cells, usually 12-15 on each side that are separated by thin bony septa. The lateral or sidewall of the ethmoid sinus is called the "lamina papyracea," or "paper thin bone." The lamina papyracea also forms the medial (middle) wall of the orbit.

The upper border of the ethmoid sinus is called the fovea ethmoidalis, or "ethmoid roof." The fovea ethmoidalis is a part of the skull base and separates the ethmoid sinus from the brain. Medially along the roof of the ethmoid sinus is the thinner bone of the cribriform plate that separates the nasal cavity from the brain. The cribriform plate tends to lie 2-3mm lower than the fovea ethmoidalis. These anatomic relationships of the ethmoid sinuses to the eyes and the brain are the reasons that untreated severe sinusitis can lead to eye and brain complications.

The ethmoid sinus can be thought of as having two distinct groups of sinus cells. The anterior or front group drains into a space called the middle meatus, and the posterior group drains into a space called the superior meatus. The anterior and posterior ethmoid cells are divided by a plate of bone called the "basal lamella" or "ground lamella."

The anterior ethmoid cells can be further sub-divided into frontal recess cells, infundibular cells, agger nasi cells, bulla cells, and conchal cells. The frontal recess is the most anterior and superior (upper) area within the anterior ethmoid compartment -- the frontal recess communicates with the frontal sinus. Agger nasi refers to a mound immediately in front (anterior) and above (superior) the insertion of the middle turbinates. When the agger nasi is aerated, its increased size may cause structural blockage. The ethmoid bulla is the largest and most constant air cell of the anterior ethmoid complex. Concha bullosa refers to pneumatization of the middle turbinate.

The ethmoid infundibulum represents a cleft or space through which a number of the sinuses (the frontal, maxillary, and anterior ethmoid) drain. Therefore, it is an important "major intersection." The boundaries of this space are formed in the front (anteriorly) by the uncinate process, on the side (laterally) by the lamina papyracea, and in the back (posteriorly) by the ethmoid bulla. The ethmoid infundibulum opens into a space in the middle, called the middle meatus, through an imaginary plane called the hiatus semilunaris.

Question: Tell me about the anatomy of the maxillary sinuses.
The maxillary sinuses are the first of the sinuses to begin in fetal development. While the maxillary sinus is pea sized and fluid filled at birth, it undergoes two rapid growth spurts. The first is between birth and age three years and the second is between age seven and 18. The sinus becomes adult size by adolescence.

The drainage pathway or ostium of the maxillary sinus is located in the upper and front part of the middle wall of the maxillary sinus. Through this ostium the maxillary sinus communicates with the nose. Specifically, mucus drains from the maxillary sinus into the infundibulum in the middle meatus and out into the nose.

If the main drainage pathway becomes closed by inflammation or infection, accessory or secondary openings can be formed. These are essentially holes in the thin bone making up the middle wall of the sinus that are caused when pressure buildup pushes through. If your doctor finds an accessory opening on examination of the inside of your nose, that may be "evidence" that you have had previous problems with sinus blockage.

The maxillary sinus is beneath the eye, so the roof of the maxillary sinus is also the floor of the orbit. The floor of the maxillary sinus is formed by the maxillary bone; specifically, a part called the "alveolar process." It is important to know that the roots of the first and second molar teeth and the second bicuspid teeth often project through the floor of the maxillary sinus, where they are covered only by the thin mucus membrane within the sinus. Infection around these tooth roots may cause inflammation of the sinus mucous membranes and sinus pain. At times, removal of these teeth can cause a fistula (opening between the mouth and sinus) which can lead to repeated sinus infections.

The anterior or front wall of the maxillary sinus separates the sinus from the cheek skin, while the posterior or back wall separates the sinus from a space filled with blood vessels and nerves. The upper part of this space is called the infratemporal fossa, and the lower part is called the pterygomaxillary space.

Question: Tell me a little about the frontal sinuses.
Although the frontal sinuses begin developing during the fourth month of gestation, they are not clinically perceptible at birth. The frontal sinuses can rarely be demonstrated on plain x-ray before two years of age. Growth of the frontal sinuses is typically complete by about age 20. About 5% of the population fails to develop one or both frontal sinuses.

The frontal sinus has a front (anterior) and and back (posterior) bony wall. The front wall separates the sinus from the forehead soft tissue and skin. The back wall separates the sinus from the brain. The front wall is approximately twice as thick as the back wall.

The frontal sinus is usually divided into two sides by a very thin bony divider. The mucus from the frontal sinus drains out a small channel called the frontal recess, into the infundibulum, then into the upper part of the middle meatus, and out into the nose. Sometimes, the frontal recess bypasses the infundibulum and drains directly into the middle meatus.

Question: Tell me about the sphenoid sinus.
The sphenoid sinus begins developing during the third month of gestation but is not perceptible at birth. Further significant growth does not begin until a child is three years of age. Pneumatization and growth usually becomes rapid after seven years and reaches adult size by 12-15 years.

The left and right sphenoid sinuses are separated by a thin bony partition. The sphenoid sinuses are usually asymmetric. The sphenoid sinus drains into the nose through the sphenoid ostium into a space called the sphenoethmoid recess, and out into the nose.

Several important structures occupy positions in relationship to the sphenoid sinus. Importantly, the optic nerve and pituitary gland are above the sphenoid, and a part of the brain called the pons is just behind the sphenoid. The internal carotid arteries are just to the side of the sphenoid sinus. A part of the brain called the cavernous sinus is also found to the side of the sphenoid sinus.

In half of the population, the internal carotid artery forms a visible indentation in the side wall of the sphenoid sinus. The bony wall is not always complete, so great care must be taken when performing sphenoid surgery.

Question: What are the turbinates?
The turbinates are scrolls of bones covered by mucous membrane that project from the side wall of the nose. The shape of the turbinates increases the surface area of the mucus lining within the nose, allowing greater filtration of particulate matter and greater efficiency in warming and humidifying inspired air.

The inferior turbinate usually is the most prominent and obvious projection from the sidewall of the nose on examination. There are usually three turbinates on each side the inferior, middle, and superior. However, at times a small fourth turbinate, the supreme turbinate, can be seen.

Question: What is the agger nasi?
Agger nasi refers to a small prominence seen in front of (anterior to) the middle turbinate. This is an aeration in the bone that overlies the lacrimal sac. At times this aerated agger nasi can be large and contribute to nasal and sinus blockage.

A number of doctors believe that the agger nasi represents the remnant of another turbinate found in animals.

Question: What are the meatuses?
The meatuses or meati are spaces created by the turbinate. The inferior meatus is a space between the inferior turbinate and the lateral nasal wall. The nasolacrimal duct drains into the inferior meatus, at the front.

The middle meatus is the space between the middle turbinate and the lateral nasal wall. A number of sinuses drain into the middle meatus.

Likewise, the superior meatus is located between the superior turbinate and the lateral nasal wall.

The frontal sinus, maxillary sinus, and anterior ethmoid sinus drain into a common channel called the infundibulum. This infundibulum then drains into the middle meatus and out into the nose.

Question: Delineate some more anatomic definitions.
The borders of the infundibulum are the uncinate process anteriorly, the ethmoid bulla posteriorly, the lamina propria laterally, and the hiatus semilunaris medially. The uncinate process is a thin bone attached anteriorly to the lacrimal bone and inferiorly to the superior aspect of the inferior turbinate.

The ethmoid bulla is the most anterior and most prominent ethmoid cell. The lamina papyracea is a paper-thin bone that separates the orbit from the ethmoid sinus. The hiatus semilunaris is the medial opening by which the secretions from the infundibulum are brought through the middle meatus into the nasal cavity.

The basal lamella or ground lamella is an important bony landmark that separates the anterior and posterior drainage systems. In the ethmoid sinus, the basal lamella separates the anterior ethmoid from the posterior ethmoid cavity. The anterior ethmoids, as mentioned, drain into the middle meatus while the posterior ethmoid sinus drains into the superior meatus. Finally, the sphenoid sinus drains posteriorly to the sphenoid ostium, from there into the sphenoethmoid recess into the back of the nasal cavity.

Question: What is the osteomeatal complex?
The osteomeatal complex is the key anatomic area addressed by endoscopic sinus surgeons. Most authorities agree that blockage of the osteomeatal complex prevents effective mucociliary clearance, thus leading to a stagnation of secretions and therefore leading to recurrent or chronic sinusitis.

The OMC is bounded medially by the middle turbinate, posteriorly and superiorly by the basal lamella, and laterally by the lamina papyracea. Inferiorly and anteriorly the OMC is open. This anatomic region therefore includes the anterior ethmoid sinus, ethmoid bulla, frontal recess, uncinate process, infundibulum, hiatus semilunaris, and middle meatus.

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This site is supported by Becker Nose & Sinus Center: Samuel S. Becker, MD and Daniel G. Becker, MD, FACS. Copyright© 2009. For more information visit the Becker Nose & Sinus Center and
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