The posterior region of the maxilla is a challenging zone for implants. Not only is the density of the bone in this region often compromised, but the amount of available bone may also be limited. This is largely due to the pneumatisation (formation of air cells) of the maxillary sinus located in this region, a process that follows the extraction of teeth from the posterior maxillary alveolus. The maxillary sinus expands in size as a result of this process and often does at the expense of local alveolar bone that is resorbed. Eventually, the process leads to a situation where the bone in the posterior maxilla is limited for implant placement.
The maxillary sinus augmentation procedure is a procedure that is aimed at regaining maxillary bone height. The technique basically involves the elevation of the sinus membrane by different approaches into the posterior maxilla and subsequently grafting of the regained space to create bone that is suitable for the placement of implants. The choice of approach is largely dictated by the quantum of residual alveolar bone that is available below the sinus membrane.
Surgical Technique for Maxillary Sinus Lift Implants
The Subantral (Internal) Sinus Lift
The implant osteotomy (bone drill) site is first prepared to a depth that is 1-2 mm short of the sinus floor. A sinus osteotome (drilling instrument) of appropriate diameter is then inserted into the osteotomy site and gently manipulated by tapping to cause a greenstick-type fracture in the antral floor followed by elevation of the bone and sinus membrane. Generally, the osteotome is tapped in for a distance of 2 mm thus barely reaching the membrane. A bone graft of choice is then carefully condensed into the oseteotomy site.
The bone graft mass is used to gradually elevate the membrane and create an area of grafted bone around the apical region of the implant to be placed. The graft tends to condense around the apical region as a result of the containing effect of the raised sinus membrane. Enough bone graft must be condensed into the osteotomy site in order to ensure sufficient elevation of the membrane to accommodate the length of the implant being placed.
Radiological verification of sufficient elevation is also a useful technique to employ. Indeed, sequential radiographs are useful to verify the correct preparation of the osteotomy site and to verify the proper use of osteotome as well. Nevertheless, the success of the intact sinus membrane lift cannot be confirmed before or at the time of implant placement and the procedure does require surgical experience and dexterity. Subsequently, the implant is inserted into the osteotomy site and a typical radiograph taken at this stage reveals a mass of condensed graft material around the apical portion of the implant.
As the implant is sufficiently anchored in coronal residual subantral bone, there is sufficient primary stability during healing. The bone graft undergoes gradual conversion to natural bone during the healing process resulting in osseointegration along the entire length of the implant. Following a 4-6 month healing period that varies depending on the type of the graft employed, the implant is loaded by following routine prosthodontic protocols.
The Lateral Window Technique for Implants
This technique is typically indicated when there is less than 5 mm of subantral residual alveolar bone available. The lack of subantral bone is contraindicated for the internal sinus lift technique described above. This is primarily because of the lack of sufficient amount of coronal alveolar bone required for stabilization of the implant in the primary stage.
The internal sinus lift is also limited in its ability to gain bone height, and the amount of elevation of the sinus membrane that can be achieved by the technique is limited.
The technique typically employs a high-speed rotary drill with a small round bur to create a rectangular window through the cortical bone. Paint brush strokes are used to score the bone until a bluish hue representing the sinus membrane is observed below the alveolar bone.
The size of the cortical window is primarily determined by the length of the edentulous span and the height of the implant to be placed. The superior border must be approximately equal to the length of the implant planned and the width of the window must provide sufficient access to graft the edentulous span in which implants are planned.
Once the entire window has been completed and the sinus membrane is visible throughout the periphery of the window, the operator may consider beginning the elevation procedure.
The sinus membrane is carefully teased from the sinus floor & lateral wall while the bony osteotomy window is gently pushed medially at the inferior border. Lack of care at this stage may result in membrane perforation and much attention must be paid to careful membrane elevation. Small perforations in the sinus membrane may be covered by placing a resorbable collagen wound dressing below the perforation. Larger perforations may require the procedure to be aborted and postponed to a later date after healing has taken place.
The space created by elevation of the sinus is now available for grafting. A suitably chosen graft material is carried into the sinus with the help of a sinus grafting syringe or carrier and the material is placed gently into the cavity vacated by the elevated membrane. A graft packing instrument may then be used to carefully condense the graft material into the space. Once the graft has been packed to a moderate density, a collagen membranes may be placed at the sinus window and the flap may be sutured into place.
Implants may be placed at the time of the sinus elevation procedure if there is enough subantral bone to provide primary stabilization. In the absence of such bone, it is important to place implants only after full healing of the grafted sinus. This process takes anywhere between 4-9 months depending on the chosen graft material and healing at the operation site.