Impact of Septa During Bone Augmentation in the Maxillary Sinus

In the maxillary sinus, fin-like, sharp bony anatomical structures often protrude from sinus walls and are referred to as septa (Figure 1).¹ The term septum means a dividing partition between two tissues or cavities² The plural of septum is septa or septums. In the maxillary sinus, septa can be complete (e.g., extending from the sinus wall to an opposing wall) or incomplete (e.g., protruding from a bony wall, but not compartmentalizing the sinus cavity).^3,4 Typically, they do not traverse the entire sinus cavity vertically or horizontally.³ Septa appear as white lines on periapical and panoramic films, and are more defined in cross-sectional, sagittal and axial views of a computed tomography (CT) or cone beam computed tomography (CBCT) scan.^3,5–8 It is beneficial to be aware of septa prior to attempting a lateral wall sinus lift or transcrestal sinus floor elevation because these structures can complicate maxillary sinus bone augmentation procedures.

This article addresses a variety of issues related to the prevalence, morphology, orientation and management of maxillary sinus septa. In developing this paper, the literature was searched with PubMed using the following terms: maxillary sinus septa, maxillary septum, sinus floor elevation, sinus anatomy, sinus surgery, lateral wall sinus lift, and the bibliographies of article references were searched for pertinent studies.

BACKGROUND AND TERMINOLOGY

In 1910, the anatomist Arthur Underwood described septa in the maxillary sinus.⁹ Subsequently, these structures are often referred to as Underwood’s septa. The maxillary sinus is also called the antrum of Highmore.⁹ An antrum is a natural space or a cavity in bone¹⁰ and it was English physician Nathan Highmore who first described this maxillary structure in 1642.⁹

Two sinus procedures routinely used to augment bone in an atrophied posterior maxilla are a lateral window sinus lift and transcrestal sinus floor elevation. A sinus lift procedure was first reported in the literature by Boyne and James in 1980;¹¹ however, Tatum developed this procedure in the 1970s. Subsequently, Summers¹² described a transcrestal sinus floor elevation in which an osteotome is used to up-fracture the sinus floor to facilitate bone grafting into the sinus via an osteotomy. Both methods can be affected by septa; thus, it is helpful to note their presence when planning treatment.

When addressing the percentage of sinuses that have septa, it should be referred to as the prevalence, not the incidence, of septa. The term incidence refers to the number of findings per population in a given time period, whereas prevalence denotes the total number of disease cases or structures in a population.¹³ The term recess refers to an empty space or a cavity.¹⁴ Underwood⁹ denoted different regions in the maxillary sinus, including alveolar, zygomatic and infraorbital recesses. The alveolar recess points toward the alveolar bone; the zygomatic cavity is directed laterally and is bounded by the zygomatic arch; and the infraorbital recess aims superiorly to the inferior orbital surface of the maxilla. In addition, the alveolar region has compartments referred to as anterior, middle and posterior recesses. These correspond, respectively, to areas apical to the second premolar, first molar and second molar sites. Researchers often report the occurrence of sinus floor septa with respect to various alveolar recesses.^4,7,15

ALVEOLAR VERSUS BASAL BONE

Basal bone is the osseous tissue of the mandible or maxilla except for the alveolar processes. Teeth are encased in alveolar bone, which rests upon basal bone — the native bone that is present before alveolar bone forms.¹⁶ Histologically, basal and alveolar bone cannot be differentiated. However, Park et al¹⁶ conducted a study of bone density and found that basal bone is denser than alveolar bone (835 to 1113 versus 810 to 940 Hounsfield units, respectively). Congenital septa consist of basal bone, whereas septa that form due to resorption of alveolar bone can consist of basal and/or alveolar bone.

IDENTIFYING SEPTA

Different criteria have been used to define the sizes of bony protuberances before they are characterized as septa: 2 mm,¹⁷ > 2 mm,¹⁸ > 2.5 mm,^6,19–22    3 mm^15,23 and 4 mm.²⁴ These criteria were proposed to differentiate septa from bone irregularities that are caused by uneven bone resorption. When Pommer et al³ compared septa prevalence using the criteria < 2.5 mm versus > 2.5 mm, they reported no statistically significant difference in the prevalence of sinus septa (27.9% versus 27.1% of sinuses). In addition, it should be noted that an investigation might report a high prevalence of septa if more than a typical amount of septa are present in a small study population.²⁵

Septa can be identified using various methodologies, such as an ortho­pantomograph (panoramic),^5,6 three-dimensional (3D) CBCT,^4,17,26 during surgery,²⁷ and in cadaveric dissection.^15,28,29 Studies have consistently demon­strated that 3D CBCT is more accurate than panoramic films in detecting septa. In general, panoramic films have a higher false positive and false negative finding than 3D CBCT scans.^3,5–8

Septa also need to be differentiated from an antrolith, which are rare, calcified masses in the sinus that form around a foreign body.³⁰ These masses may not be attached to a wall of the sinus; in addition, their shape is usually irregular and they have a homogeneous density. Antroliths may be solitary or multiple and require no therapy unless they are symptomatic.

FUNCTION AND FORMATION OF SEPTA

Neivert³¹ suggested the origin of septa is due to fingerlike projections produced by the embryological outpouching of the ethmoid infundibulum where the contiguous walls do not resorb. Septa function as force-carrying struts, according to van den Bergh et al.³² Septa are divided into two types, depending on their origin. Congenital or primary septa arise during development of the maxilla, whereas secondary or acquired septa develop after tooth loss and bone resorption^8,33 or sinus pneumatization.⁸ Several authors indicated that more secondary septa are seen in edentate than dentate individuals.^3,8,22,26 Sharan and Majdar³⁴ observed that when a single posterior tooth is removed, patients may experience 0.54 mm of pneumatization (internal bone loss) within six months. Furthermore, if multiple teeth are extracted, a mean bone loss of 2.22 mm occurs due to sinus pneumatization. This uneven bone resorption can result in formation of secondary septa.⁸

While it is not possible to denote a septum as primary or secondary without a radiographic history, it can be deduced that septa situated apical to the dentate regions are primary (developmental), and those found apical to the edentulous maxilla are either primary or secondary.³⁵

SEPTA SHAPE

Septa typically manifest several contour characteristics. They are usually thin, but can be wide.⁹ Their height is usually greater than their width, and septa are often found against the medial wall.^3,7,8 Furthermore, they are often larger as they course latero-medially and manifest a shape similar to an inverted Gothic arch.^7,8 Septa are usually composed of cortical bone,²⁵ however, if there is a wide septum, cancellous bone may be present.⁹

If a complete or almost-complete septum extends from one end of the sinus to the other either vertically or horizontally, it divides the sinus into two or more distinct compartments. A complete vertical septum (lateral view) divides the sinus into anterior and posterior compartments (Figure 2 and Figure 3), whereas a complete or almost-complete horizontal septum (lateral view or cross-sectional view) divides a sinus into superior and inferior chambers (Figure 2 and Figure 4).³⁶ If there is a complete vertical septum in a cross-sectional view, it divides the sinus into lateral and medial compartments (Figure 5).

ORIENTATION, PREVALENCE, LOCATION and size of septa

While the prevalence of septa in the maxillary sinus varies in different studies, it ranges from 9.5% to 65.5% (Table 1).^{1,4–7,9,15,17,19–23,26–29,37–45} Sometimes, data is reported as the percentage of patients who have septa, not the percentage of sinuses affected. In this regard, the percentage of patients manifesting septa may be higher than the percentage of sinuses with septa (because patients may not have septa in both sinuses). For example, Qian et al²⁶ reported the percentage of patients versus the percentage of sinuses with septa was 48.2% and 32.6%, respectively.

In a systematic review, Pommer et al³ reported the following findings. Septa were present in 28.4% of sinuses (8923 sinuses assessed) and the range between studies was 9.5% to 55.5%. The 95% confidence limit ranged from 24.3% to 32.5%. One quarter of the patients manifested septa in one sinus and 17.2% had them bilaterally, therefore, 42.2% of patients had a septum. There were more septa in edentate than dentate maxillae due to formation of secondary septa.^21,46 Pommer et al³ also noted the following distribution of septa with respect to orientation: buccopalatal 87.6%, mesiodistal 11.1%, and horizontal (parallel to the sinus floor) 1.3%. Complete septa dividing the sinus into compartments occurred in 0.3% of the sinuses, and multiple septa occurred in 4.2%. While septa can be found on the floor or walls of the sinus, they most often arise from the floor (58.6% of septa).⁴ Table 2 delineates the percentage of sinus floor septa found in the anterior, middle or posterior recesses, with most sinus floor septa arising in the middle recess.^{3,4,7,15,17,20,22,23,41,44}

The systematic review by Pommer et al³ reported a mean septal height of 7.5 mm, but the range varies extensively in different studies (Tab­le 3).^{6,9,15,19,28,42,45} Thus, a mean statistic provides limited information with regard to clinical management of septa.

SCHNEIDERIAN MEMBRANE thickness

Although the thickness of the Schneiderian membrane is normally approximately 1 mm, it varies in different regions of the sinus.⁴⁷ Rancitelli et al⁴² found that the mean Schneiderian membrane thickness close to a septum was 1.8 mm, and far from it, the membrane was 0.85-mm thick. In contrast, Cakur et al⁴⁸ reported a negative correlation between septa proximity and membrane thickness (r = -0.168). In other words, the membrane was thinnest in the vicinity of the septum. It was suggested that septa may be a causation for membrane thinness. These latter data underscore the concept that elevation of the Schneiderian membrane over septa should be performed with caution. However, there is too little data at this time to reach a definitive conclusion regarding the impact of septa on membrane thickness.

ISSUES ASSOCIATED WITH SEPTA

There are several potential problems associated with septa during lateral window sinus augmentation or transcrestal sinus floor elevation procedures. Besides increasing the likelihood of membrane perforation,^49–51 the presence of septa can impede moving the lateral window medially³² or prevent the up-fracturing of the sinus floor.⁵² Furthermore, vertical or horizontal septa may divide the sinus into multiple compartments that re­quire different treatment ap­proaches.^4,36,53,54 Therefore, clinicians must be prepared to modify the treatment method to accommodate and manage septa.

Schneiderian Membrane Tears: Anatomic variations in the sinus, such as septa, increase the risk of membrane perforation during elevation procedures.^{8,21,32,35,53} For example, Schwarz et al⁵⁵ reported a perforation rate of 8.6% upon membrane elevation when septa were absent, and 45.5% when septa were present. Other researchers noted the Schneiderian membrane tear rate is inversely related to its thickness,⁵⁶ with perforations occurring more often when the membrane is < 1 mm⁵⁷ or < 1.5mm⁵⁶ thick. With respect to membrane elevation over partial septa, the Schneiderian membrane should be released latero-medially, because its elevation antero-posteriorly over a sharp ledge leaves it prone to perforation.⁵⁸

Lateral Window Sinus Augmentation Modifications Due to Maxillary Septa: Wen et al⁵⁴ classified septa management into easy, moderate or difficult categories, based on the location, number, orientation and size of septa.

Easy septa are located anterior to the zygomatic arch and course mediolaterally. When a septum is ≤ 6 mm in height, Wen et al⁵⁴ suggested removal of the lateral bony window to complete a lateral sinus lift. However, if a septum is > 6 mm, their recommendation was to create two buccal windows — one on each side of the septum.

Moderate septa are found posterior to the zygomatic arch and oriented mediolaterally. If a septum is ≤ 6 mm, Wen et al⁵⁴ advised removing the bony window to facilitate performing a sinus lift. But if a septum is > 6 mm, they suggested removing the bony window and septum through a single window.

Difficult septa include situations in which they occur anterior or posterior to the zygomatic process; in this case, treatment scenarios are subclassified as A, B and C, based on orientation and number of septa. For these subclassifications, Wen et al⁵⁴ recommended the following treatments:

• Subclass A: If there is one anterior-posterior septum measuring ≤ 6 mm, use a single window and a lateral approach
• Subclass B: In the presence of one anterior-posterior septum that is > 6 mm, use a single window with a crestal approach and remove the bony window
• Subclass C: If there are two or more mediolateral septa, use multiple windows, combined with removing the bony window

With respect to septa removal, Boyne and James¹¹ had another point of view. They recommended that septa be routinely removed with a chisel and mallet so a bone graft can be placed over the entire floor without interruption. However, the authors of this article do not think this is routinely necessary, unless the septa impede visualization of the sinus or inhibit membrane elevation.

When a lateral window sinus lift is performed, the osteotomy is often created so it facilitates horizontal rotation of a trap door (buccal plate) into the sinus cavity; subsequently, this island of bone becomes part of the roof of a sinus lift. However, a small septum situated medial to the trap door could impede rotation of the lateral bony window into the sinus. Thus, when there is a small septum on the sinus floor (seen on a CBCT scan), clinicians are advised to create a W-shaped lateral window (with the center of the window more apical) so it will pass over the septum when the trap door is rotated³² — alternately, it may be easier to remove the entire lateral bony window.

If a vertical septum (lateral view) divides the sinuses into anterior and posterior compartments (Figure 2 and Figure 3), clinicians can manage this in two ways: Separate windows can be created (one on each side of the septum),^53,54,59,60 or one large window can be developed and the entire bony window over the two sections removed. In this case, each side of the septum (i.e., the anterior and posterior compartments) is treated independently. The need to do the posterior compartment is dictated by the amount of bone (mesio-distally) required to accommodate future implants.

An uncommon finding is a horizontal septum that compartmentalizes the sinus into superior and inferior cavities (Figure 2 and Figure 4).³⁶ In this case, clinicians must determine how much bone is needed to accommodate implants. It may only be necessary to treat the inferior cavity adjacent to the alveolar ridge to encompass the length of the planned implants. Otherwise, dual compartments must be managed with either two separate windows or one large window with the buccal bony plate removed.

Another septal variation that is rarely encountered is a vertical septum that divides the sinus into lateral and medial compartments (Figure 5).⁴ In this scenario, the location of the sections (with respect to the ridge) needs to be assessed. If the lateral compartment is under the ridge, only that section needs to be managed with a sinus lift. However, if both compartments require treatment, clinicians must gain access to the lateral section to elevate the membrane, and proceed through the vertical septum to treat the medial compartment.

Mailleux et al⁶¹ reported a rare occurrence of the inferior orbital nerve within a horizontal septum in the superior aspect of the maxillary sinus. They cautioned that disruption of these septa could result in paresthesia of the maxillary region and noted these unusual anatomic findings need to be detected during a preoperative CT work-up.

Transcrestal Sinus Lift Modification Due to Septa: Prior to performing a transcrestal sinus floor elevation, it is prudent to verify that a septum is not superior to a site where the sinus floor will be up-fractured.⁵² When a septum is present, it will impede elevating the sinus floor; moreover, if significant force is used during up-fracturing, it is possible to splinter the maxillary bone in an uncontrolled manner. Thus, if a septum is present over a site to be up-fractured, clinicians should choose a different location or perform a lateral wall sinus lift.

IMPLANT PLACEMENT INTO A SEPTUM

Placing an implant into a maxillary septum is conceptually possible (Figure 6). Although this idea was initially addressed by Fortin et al,⁶² Dragan et al⁶³ subsequently suggested that if a septum was thick enough, an implant could be inserted into it. However, this is usually impractical because septa are infrequently wide enough to encase an implant. Furthermore, the study by Dragan et al⁶³ was not performed on live patients; rather, it was a simulation investigation using 3D reconstructions.

CONCLUSION

Septa in the maxillary sinus are a common finding and their presence may require modification of a sinus augmentation procedure. With respect to identifying septa, the data indicate that CT or CBCT scans provide less false positive and false negative findings than panoramic imaging.^3,5–8 Thus, prior to performing a sinus bone grafting procedure, it is strongly recommended that a CBCT scan be obtained to facilitate and enhance treatment planning.

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From Decisions in Dentistry. May 2017;3(5):46—51.