The maxillary sinus may present challenges in both the diagnosis and of dental and non-dental related problems in the orofacial region. In order to establish proper diagnoses treatment modalities and methods to prevent and to treat complications in this area, the Anatomy of This Region Must Be Known in Detail.
This article provides details on the anatomy of the maxillary sinus, as well as applications and techniques relevant to performing surgery in and around that structure.
Maxillary sinus anatomy
The maxillary sinus is the largest of the four para-nasal sinuses (frontal, ethmodial, maxillary and sphenoid) and is known as the antrum of Highmore.
Function of the maxillary sinus
Although the function of the maxillary sinus is not clearly understood, it has been reported that it might contribute to the resonance of the voice, have some olfactory function, reduce the weight of the skull, and humidify inspired air.
Shape and walls
The maxillary sinus is a “quadrangular” pyramidal shaped cavity.
The superior wall is the orbital floor, which is typically thin and brittle and often has dehiscence that makes the Schneiderian membrane the only barrier between the sinus and the orbit.
The anterior part is formed by the posterior part of the maxilla and houses the nerves and vessels that supply the teeth. It is typically thinner anteriorly and thickens posterior where it joins with the zygomatic process. The thickness of the bone is variable depending on the duration of Edentulism. The posterior wall is the infra – temporal surface of the maxilla and separates the sinus from the pterygomaxillary fissure and the internal maxillary artery and pterygoid plexus distally. The medial wall of the sinus is the lateral nasal wall and separates the sinus from the nasal cavity. This wall communicates with the nasal cavity via the ostium semilunaris to the hiatus semilunaris (middle meatus). The ostium is oval in shape and is approximately 6mm by alveolar process and the hard palate from the floor of the sinus (antral floor). The floor is thinnest in the molar region
SINUS MEMBERANE (SCHNEIDERIAN MEMBRANE)
The maxillary sinus is lined internally by a thin pseudo stratified ciliated columnar or cuboidal epithelium known as the Schneiderian membrane that is normally 0.13 to 0.5- mm thick and is grayish-blue in color . Goblet cells and glands are present and function to produce mucous. There is a thin endosteal basement membrane with scattered osteoblasts, which may account for sinus pneumatization with loss of teeth. There are also some elastic fibers present, which may facilitate movement and reflection of the membrane. The membrane is continuous with the nasal epithelium but is thinner and less vascular than the nasal mucosa. The epithelium derives from the cranial end of the middle meatus and descends during the 12th week embryo logically.
The average dimensions of the adult maxillary sinus are 25-35mm in width, 36-45mm in height, and 38-45mm in length. The average volume of the sinus is 15ml. The floor of the antrum is approximately 10mm below the nasal floor in adults. Anteriorly the sinus extends to the canine-premolar region. The floor of the sinus is convex and is deepest at the molar region. Roots of the maxillary teeth cause the floor of the sinus to have convolutions. When the posterior maxilla is edentulated, the size of the maxillary sinus will increase further and fill part of the alveolar process.
The presence of septae in the sinus also is highly variable.
Underwood showed, using dried skulls, that septae were present 33% of the time that 77% of septae were in the anterior region, they averaged in height between 6.5-13 mm, and that they occurred more on the left side than the right (3:1) .
Ulm found similar rates of occurrence (31.7%) with separate having a mean height of 7.9 mm. In a study utilizing CT images in Korean patients, Kim found the occurrence of one or more septae per sinus was found to be 26.5%, 31.76%, And 22.61% in the overall study population, the atrophic/edentulous and the nonatrophic/ dentate patients, respectively. Also the septae were located in the anterior region of the sinus 25.4% of the time, in the middle segment 50.8% of the time, and in the posterior region 23.7%of the time. The mean heights also varied between 1.63-5.46mm +2-3mm . Yet again, there is a large variability in not only the population at large as well as within the individual.
The use of modern radiographic techniques does allow for the visualization of these structures prior to any surgical procedures and one should be aware that panorex images have up to 20-25% distortion error.
Periapical films have distortion error of 14% on average and CT images have very little distortion (1.8%).
BLOOD SUPPLY TO THE SINUS
The arterial supply to the maxillary sinus is derived from the
Usually several anastomoses of the posterior superior alveolar artery and the infra – orbital artery can be found inside the bony lateral antral wall, which also supplies the schneiderian membrane as well as the epi – periosteal vestibular tissues.
In a study by solar and coworkers, the endosseous branch of the posterior superior alveolar artery in the lateral wall was found in 100% of specimens. The mean difference between these intra – osseous anastomoses and the alveolar ridge was found to be 18.9mm, which is the area for creation of the periosteum overlying the lateral wall, located an average distance of 22.75mm from the alveolar crest. In a CT study, the intra – osseous artery could be visualized in 52.9% of the scans in the study group and the average height of the artery from the alveolar ridge was 16.4mm. Results also showed that 80% of the arteries were located more than 15mm from the crest.
This indicates that only approximately 20% of the cases present a potential complication if the goal is to place implants 13mm to 15mm in length.
THE LYMPH DRAINAGE
The lymph drainage occurs through the infra – orbial foramen and the osteum semilunaris. The ciliated epithelium transports approximately 2 liters of fluid and mucous per day to the ostium at the cranial side in the superior part of the sinus.
THE INNERVATION OF THE MAXILLARY SINUS
The innervations of the maxillary sinus are derived from branches of the second division of the trigeminal nerve, including the greater palatine nerve, the infraorbital nerve and the posterior superior alveolar nerve.
THE NORMAL FLORA OF THE MAXILLARY SINUS
The normal flora of the maxillary sinus is derived from the normally found flora in the respiratory tract.
Clinical Procedures Involving the Maxillary Sinus
During the extraction of teeth in the posterior maxilla, careful radiographic analysis should be performed before the treatment has commenced. Techniques such as sectioning teeth may benefit in cases of dilacerated roots or severely carious teeth.
When exposure and perforation of the antrum occur during extractions, the least invasive therapy is indicated initially. If the opening is small, establishment of a blood clot in the site with proper stabilization along with proper nasal precautions for the next 10-14 days and systemic decongestants should be sufficient.
If the communications is larger in size, other treatments are required such as bone grafting techniques (GBR) or pedicle flaps to establish primary closure.
During external sinus augmentation procedures, anatomy also plays a critical role in the prevention and management of any adverse events. In modifications to the procedures proposed by Boyne and Tatum, the flap design should have crestal or slightly palatal incision that extends 8-10mm beyond the anterior wall. Anterior and posterior releasing incisions are made at the canine fossa area to the vestibule to the tuberosity area, respectively. The lateral window should be prepared with a diamond bur or piezoelectric unit so as to not perforate the thin scheinderian membrane. The initial Osteotomy is begun 3-4mm above the alveolar ridge and is about 20mm in length. The superior Osteotomy is created 10-15mm above the inferior Osteotomy and should not come within 4-5mm of the superior border of the flap .
In the retraction of the flap, the retractor to be positioned carefully as to not threaten the neurovascular bundle of the infraorbital foramen. When placing graft material, should not fill the sinus beyond 15mm so to avoid potentially blocking the osteum. Septal patterns in the sinus may require the use of their removal by special instrumentation or the modification of the surgical technique (i.e., making more than one lateral window). If imaging shows the lateral or inferior wall having fenestrations, split thickness flaps are required to avoid tearing the membrane of the sinus during flap elevation and the residual tissue must be pushed into the sinus during membrane elevation.
If there is a tear in the schneiderian membrane, treatment options are dependant on the size of the perforation. If small, the membrane may be elevated further away from the tear, which may require an enlargement of the lateral window, to allow the membrane to fold onto itself. If of larger size, the tear may require resorbable collagen membrane to be placed over it. If the tear is very large, the surgery should be aborted and the surgery can be performed after adequate healing in 6-8 weeks. If the membrane is thickened due to pathology or chronic infection, elevation is typically easier due to the thickening of the membrane but an ENT consult should be obtained before surgery. Occurrence rates from 10 to 40% are reported in the literature for membrane perforation.
Hemorrhage during sinus grafting is rare since the main arteries are not within the surgical area. If arteries of smaller size are encountered, the best treatment to arrest the bleeding is direct pressure with moist gauze, ligation of the vessel with resorbable suture material, or burnishing the bone .
If the use of antibiotics is warranted, the choices to those with the spectrum to cover respiratory organisms such as H. influenza. Most infections that occur are usually of the graft itself.
If the infection continues for more than 10 days, surgical invention is required to remove the graft. Typical regiments for antibiotics for the prevention of sequelae include 875mg amoxicillin and clavulanic acid BID for 7-10days, cephalexin 500mg q8h for 3 days, or clarithromycin 500mg BIDfor 3days. Of importance is that clindamycin has been shown ineffective in the prevention and treatment of sinus related infections. If post operative infection occurs, regiments include levofloxacin 500mg given 2 tabs on day 1 and 1 tab QD for 8days, moxifloxacin 400mg QD for 7-10 days, and in the case of severe infection metronidazole 500mg TID for 7-10days.
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