Written by Eliaz Kaufman, DMD, MS Saturday, 31 August 2002 19:00
The restoration of the edentulous posterior maxilla using dental implants presents distinctive challenges. The height of the residual alveolus is often limited and bone density is often poor, leading to a diminished implant success rate. The proximity of the maxillary sinus to the alveolar crest as a result of the position of the sinus and resorption of the alveolar ridge because of tooth extraction, trauma, or pathology may prevent the placement of implants of adequate length and position.1,2 A minimum of 10 mm of vertical bone height is usually required for predictable implant success.3 To address this problem, maxillary sinus elevation surgery was developed to increase the height of bone available for implant placement in the posterior maxilla.
Sinus elevation surgery for implant placement was initially described by Boyne and James and by Tatum.4,5 In these reports the sinus was exposed by a modified Caldwell-Luc approach. A window was created in the lateral maxillary wall, the sinus membrane was carefully elevated, and grafting material (autogenous bone) was placed prior to implant placement. Since these early reports, several modifications have been made to the surgical technique and the materials used. Furthermore, the efficacy and predictability of this procedure has been determined in a number of studies.
ANATOMY OF THE MAXILLARY SINUS
|Figure 1. The maxillary sinuses are seen in an axial CT cut at the level of the zygoma. The opening of the sinus to the nasal cavity (the ostium) is observed on the medial wall of each of the sinuses that is also the lateral wall of the nasal cavity. On the anterior wall of the right maxillary sinus a thickening of the sinus membrane can be seen.||Figure 2. This is a coronal CT cut of the maxillary sinus. The location of the ostium is in the superior part of the sinus and therefore placement of grafting material in the inferior part of the sinus does not interfere with normal sinus drainage.|
The maxillary sinus is pyramidal in shape. The base of the pyramid is the medial wall of the sinus, which is also the lateral wall of the nasal cavity, and its apex is pointed towards the zygomatic bone (Figure 1). The maxillary sinus begins to develop in early childhood and continues to increase in size. The average volume of a fully developed sinus is 15 mL. The maxillary sinus is lined with respiratory epithelium (pseudostratified columnar epithelium) that covers a loose and highly vascular connective tissue. Underneath this loose connective tissue and immediately next to the bony walls of the sinus is the periosteum. These structures (the sinus epithelium, connective tissue, and periosteum) are collectively referred to as the Schneiderian membrane. Drainage of the maxillary sinus is through the ostium, which is positioned in the superior medial aspect of the sinus and opens into the nasal cavity between the middle and lower nasal conchas (Figure 2). The fact that the sinus opening to the nasal cavity is not in the lower part of the sinus (where the graft is placed) is important and provides an anatomical rationale to the sinus elevation technique, as the grafting procedure does not interfere with normal sinus function. The blood supply to the maxillary sinus is derived primarily from the maxillary artery and to a lesser degree from the anterior ethmoidal and superior labial arteries. The sensory innervation is derived from the maxillary division of the trigeminal nerve by means of the anterior, middle, and posterior superior alveolar nerves. The functions of the maxillary sinus are to warm the aspirated air, reduce the weight of the craniofacial complex, and provide resonance to the voice.6,7
PREOPERATIVE PATIENT EVALUATION
Prior to sinus elevation surgery, a thorough preoperative evaluation should be performed. Contraindications to this surgery include all medical conditions and medications that would preclude implant placement. In addition, the patient should be questioned regarding sinus problems and previous sinus surgery that could complicate or contraindicate the procedure. The presence of sinus pathology including acute sinusitis, antral polyps, cysts, or tumors could certainly compromise the success of the procedure. A consultation with an ENT specialist prior to surgery is essential in these cases. Other contraindications to this procedure are the use of inhaled steroids and cocaine dependency.
Clinically, the inter-arch distance (the distance between the mandibular occlusal plane and the maxillary ridge) should be measured. If this distance is too small (less than 5 mm) it could compromise the prosthetic treatment because there is not enough room for the necessary prosthetic components. More often, an increased inter-arch distance is observed as a result of resorption of the alveolar ridge. This could lead to the fabrication of long abutments and crowns that result in compromised aesthetics, unfavorable crown-to-root ratio, and increased mechanical forces on the implants. The maxillo-mandibular ridge relationship should also be determined in a bucco-lingual dimension. Normally, the buccal cusps of maxillary posterior teeth are positioned lateral to the buccal cusps of the mandibular teeth. Bucco-palatal resorption of the maxillary alveolar ridge could result in palatal positioning of the fixtures and creation of a buccal cantilever that can exert unfavorable loading on the implants. The best way to determine the inter-arch relationships and the future occlusal scheme is to fabricate a diagnostic wax-up. The information obtained from the clinical examination and the diagnostic wax-up is combined with a two-dimensional or three-dimensional radiographic evaluation. The two-dimensional radiographic evaluation is useful to determine the height of the residual alveolar ridge, the location of the maxillary sinus floor, the height of the maxillary sinus, and the presence of sinus pathology or septa. The most frequently used two-dimensional radiograph is the panoramic film, although this radiograph can cause enlargement of up to 25%. Three-dimensional radiography (computed tomography or CT) is more accurate than conventional two-dimensional radiography and can also provide information about the width of the alveolar ridge and the maxillary sinus, the thickness of the lateral wall of the sinus, and can accurately identify the presence of septa and sinus pathology. The diagnostic wax-up can be used to fabricate a radiographically identifiable template that can be worn by the patient at the time of the CT examination. This template is used to demonstrate the position of the future prosthetic restoration in relation to the alveolar ridge and sinus.
|Figure 3. An oval osteotomy in the lateral wall of the maxillary sinus. The sinus membrane has a bluish-purple color that can be seen when the osteotomy is almost complete.||Figure 4. After completion of the osteotomy the bony window is removed and the intact sinus membrane can be visualized.|
|Figure 5. Here the bony wall is not removed but instead tapped into the sinus hinging on its superior margin while still being attached to the sinus membrane. In this case, the bony wall will ultimately serve as the new sinus floor and the roof of the chamber containing the bone graft.||Figure 6. The bone graft material (in this case DFDBA) is placed into the sinus.|
Sinus elevation surgery can usually be accomplished under local anesthesia. However, when the patient is apprehensive or when a second surgical site is used for harvesting of an autogenous bone graft, oral sedation, IV sedation, or even general anesthesia is sometimes employed. After administration of local anesthesia, the lateral wall of the maxillary sinus is exposed by a full thickness mucoperiosteal flap. The first incision is usually crestal and should be longer in length than the future osteotomy in the anterior-posterior dimension. To facilitate visibility, a mesial (and often a distal) releasing incision is performed. All incisions should be removed from the area of the anticipated osteotomy to facilitate primary closure. Following flap reflection the dimensions of the osteotomy are determined based on the clinical and radiographic examinations. The lower border of the osteotomy should be approximately 3 mm above the sinus floor. The osteotomy should be oval or rectangular in form, and corners and sharp edges should be avoided to minimize the risk of tearing the sinus membrane. The osteotomy should be performed utilizing a round diamond (such as No. 8 bur) or carbide bur at low speed with copious saline irrigation. When the osteotomy is almost complete the sinus membrane, which is bluish-purple in color, can sometimes be observed (Figure 3). After completion of the osteotomy the bony wall should be mobile and attached only to the underlying sinus membrane. The bony wall can now be carefully removed, and retained for later incorporation into the graft material (Figure 4) or tapped into the sinus hinging on its superior margin while still attached to the sinus membrane (Figure 5). If the sinus wall is tapped into the sinus, it will ultimately serve as the new sinus floor and the roof of the chamber containing the bone graft material. Tapping the wall into the sinus or removing it is a matter of clinical preference. Some clinicians prefer to remove this bony wall in order to improve access and visibility. In this case, the bony wall should be gently released from the underlying sinus membrane and carefully removed to prevent tearing of the membrane. After the osteotomy is completed, the sinus membrane is gently reflected and elevated using special curets to create room for the graft material. Sinus membrane reflection should be to the medial wall of the sinus and superior enough to prevent pressure on the graft and prevent membrane tearing during graft placement. The graft material of choice is then placed and packed (Figure 6). A synthetic membrane is sometimes used to cover the window on the lateral wall of the sinus and the graft. The placement of a membrane was found to increase bone formation in the grafted sinus.8 Finally, the mucoper-iosteal flap is repositioned and sutured. Primary soft tissue closure is very important, and with appropriate flap design should be achieved without difficulty.
|Figure 7. In an axial CT cut septa can be seen in the right (one septum) and the left (two septa) maxillary sinuses. Thickening of the sinus membrane can be seen bilaterally.|
The presence of septa in the maxillary sinus can complicate the procedure. Septa are present in approximately 31% of patients and are most common in the area between the second premolar and first molar. Septa are more frequently found in edentulous atrophic maxillae than dentate maxillae.9,10 Preoperatively, the best way to locate septa is by CT (Figure 7). When a septum is identified the osteotomy should be designed to avoid it if possible. In some cases two osteotomies, one anterior and one posterior to the septum can be performed.
Postsurgical instructions to the patient include rest, pressure on the surgical site, and application of ice packs. The patient should be instructed to avoid blowing the nose. Coughing and sneezing should be done with an open mouth. Antibiotic coverage with a broad spectrum bactericidal antibiotic (such as amoxicillin or amoxicillin and clavulanate [Augmentin]) should start the day of the surgery and be continued for 7 to 10 days postoperatively. Appropriate analgesics (preferably one that is also anti-inflammatory) and a chlorhexidine mouthwash should be prescribed. Administration of a short-term tapering dose of glucocorticoids (such as dexamethasone) is sometimes used to reduce postoperative swelling, trismus, and pain.11 Systemic and topical nasal decongestants can also be used to improve ostial drainage.
|Figure 8. Sinus membrane perforation is a minor complication of sinus elevation surgery. In this picture a small (1- to 2-mm) perforation can be observed.|
As in any surgical procedure, complications can occur at the time of surgery or postoperatively. The most common intraoperative complications are sinus membrane perforation and hemorrhage (Figure 8). Small membrane perforations can be managed by folding the sinus membrane on itself in the process of sinus membrane elevation, by suturing the torn membrane using absorbable sutures, or by placement of an absorbable collagen membrane under the perforated sinus membrane. In a case of very large perforations the procedure must be aborted. Intraoperative bleeding can originate from blood vessels in the mucoperiosteal flap or in the sinus membrane or bone. Bleeding from soft tissue can be managed by pressure, administration of anesthetic with a vasoconstrictor, cauterization, or the use of Gelfoam or Surgicel. Bone wax is usually effective for bleeding originating from bone.
Postoperative complications include infection of the sinus and graft, soft tissue perforation, and development of an oral antral fistula. Infection can occur by contamination of the site by oral or sinus bacteria. The normal sinus flora includes Bacteroides species, Gram-positive cocci, Fusobacterium species, and Hemophilus influenzae. The pathogens predominantly involved in acute maxillary sinusitis are Streptococcus pneumoniae and H influenzae.12 The antibiotic selected should be bactericidal and suppress both oral and sinus pathogens. Therefore, amoxicillin or Augmentin are usually the antibiotics of choice. Cefaclor or clindamycin are appropriate alternatives for patients allergic to the penicillins.13 In the case of a postoperative infection that does not respond to antibiotics, removal of the graft may be indicated. In this case the procedure can be repeated after complete healing has occurred.
Various graft materials can be used for sinus elevation. The graft materials can be categorized into four groups: autogenous bone14,15; allografts, which are usually harvested from human cadavers16,17; alloplasts (synthetic materials)18,19; and xenografts, which are grafts from a nonhuman species such as bovine-derived bone.20,21 Autogenous bone grafts are preferred in almost any grafting procedure. Autogenous grafts provide an excellent source of osteogenic cells with no risk of antigenicity or cross infection. The disadvantages of autogenous bone grafts include the morbidity and complications that are associated with a second surgical site. Various donor sites have been described in the literature, and these donor sites can be classified as intraoral and extraoral. The most commonly used extraoral donor site for sinus elevation is the iliac crest. Intraoral sites include the maxillary tuberosity,22 mandibular symphysis,23 coronoid process,24 mandibular ramus,25 and mandibular tori.26
The high osteogenic potential of autogenous bone is particularly important when the sinus elevation surgery is performed in a large sinus. Generally, in sinus elevation surgery the bone graft placed is resorbed and replaced by cells originating from the periphery of the graft, ie, the bony walls of the sinus that are in contact with the graft. The volume of graft placed is usually proportional to the size of the sinus. The time required for graft resorption and replacement by new bone is longer in larger sinuses. Thus, the presence of living cells within the graft (as is the case when autogenous bone is used) can reduce the healing time. Because intraoral donor sites generally provide only a limited amount of graft, and because harvesting bone from extraoral sites is not always possible, intraorally harvested autogenous bone is often mixed with other grafting materials (allograft, xenograft, or alloplast).
Bone allografts can be cortical or trabecular and are harvested from cadavers. They are available in freeze-dried form, and the risk of cross infection is eliminated by the method of processing. Allografts can be mineralized or demineralized. Both forms have osteoconductive properties and can serve as a scaffold that promotes the growth of new bone. In addition to being osteoconductive, demineralized freeze-dried bone allograft (DFDBA) was believed to have osteoinductive properties (the ability to induce bone growth). The process of demineralization exposes the organic components of bone and enhances release of bone morphogenetic proteins (BMP). However, the amount and activity of the BMP in DFDBA is uncertain, and thus the actual osteoinductive ability of DFDBA is questionable.27,28
Other graft materials that are osteoconductive are alloplasts and xenografts. Alloplasts such as calcium sulfate can be successfully used for this procedure.18 Xenografts, particularly bovine-derived bone, are utilized for sinus elevation although they have a relatively long resorption time of up to 4 years.21
A report analyzing the data collected by 38 clinicians who collectively performed 1,007 sinus elevation surgical procedures that involved the placement of 2,997 implants over a 10-year period was published in 1998 by the Academy of Osseointegration.29 The database was extremely variable and did not allow definitive conclusions to be drawn. It was, however, evident that various graft materials (autogenous bone, allografts, alloplasts, and xenografts) used alone or in combination with each other were effective as grafting materials in sinus elevation surgery.
|Figure 9. Immediate implant placement. After the sinus membrane is elevated, a direction indicator can be seen protruding into the maxillary sinus.|
Implant placement can be performed simultaneously with the sinus elevation procedure (immediate approach, Figure 9) or following a healing period that is usually 4 to 9 months (delayed approach). The advantages of the immediate approach are reduced overall healing time between the sinus elevation surgery and implant uncovering, and elimination of a surgical procedure. However, in the immediate approach failure of the graft is also likely to result in implant failure. Conversely, in the delayed approach unsatisfactory results of the sinus elevation surgery would be evident prior to implant placement and could be improved in a second procedure. The most important indication for the immediate approach is the presence of sufficient bone quantity and quality to provide for primary stabilization of the fixtures. Five mm of vertical bone height is considered a minimal amount for immediate implant placement, although in some reports implants were successfully placed in 2 to 3 mm of bone.30,31
THE OSTEOTOME TECHNIQUE
|Figure 10. The osteotome technique is a more conservative approach to the sinus elevation surgery. The osteotomy on the lateral wall of the sinus is avoided, and instead the sinus floor is elevated by a crestal approach using osteotomes in increasing diameters.|
A more conservative and less invasive approach to the conventional sinus elevation surgery is the osteotome technique (Figure 10). In this technique the osteotomy on the lateral sinus wall is avoided and the sinus is elevated by a crestal approach. Osteotomes in increasing diameters (usually 1, 2, 3, and 4 mm) are used to create a site for implant placement and to simultaneously elevate the sinus floor. Because in this technique minimal drilling is performed, the bone is pushed to the periphery of the site by the osteotomes, and thus bone density is increased. In addition, small increments of bone graft can be placed in the site and condensed by the osteotomes to further elevate the floor of the sinus. Following site preparation by the osteotomes and elevation of the sinus floor, implants can be placed immediately in the same surgical site.32 This procedure is useful because it not only elevates the floor of the maxillary sinus but also increases bone density at the implant site and improves the density of the maxillary bone. The success rate of implants placed in this way may depend on the preexisting bone height between the sinus floor and the alveolar crest. Survival rate diminishes when 4 mm or less of bone is present.33
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20. Zitzmann NU, Scharer P. Sinus elevation procedures in the resorbed posterior maxilla. Comparison of the crestal and lateral approaches. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 1998;85:8-17.
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33. Rosen PS, Summers R, Mellado JR, et al. Int J Oral Maxillofac Implants. 1999;14:853-858.
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