The moment patients hear their dentist or surgical specialists mention "bone grafts", often you see the backs of patients as they rapidly head for the door. Often times patients are never truly educated on why bone grafts are needed. Not every dental implant case requires bone grafting, but a fair number of them do. Patients must understand that bone provides the foundation for the support of the implant. The bone, depending on the type of restoration desired, must have adequate height, width and positioning for dental implant placement. Additionally, the bone normally has to be at or near the same level as the adjacent bone.
Imagine the bone being the foundation for the construction of a house. It must be solid and level. It isn't that different in the mouth. After you have an extraction or have a tooth missing for some time, the bone deteriorates (atrophies). The alveolar bone (the bone that houses teeth and their roots) atrophies typically in width greater that in height, but both components are involved. If the bone is too thin, an implant cannot be placed because the body of the implant will not be covered by bone circumferentially. If the bone is not high enough, the implant could be too close to adjacent anatomic structures. Moreover, even if an implant could be placed, but the bone is not at the same level as the adjacent bone, the implant may not be hygienic, it may be very unaesthetic and/or create a periodontal issue for the patient. A general rule of thumb for implants surgeons, is to reconstruct the foundation for the implant back to ideal prior to placing an implant or implants.
MEMBRANE
There are many types of bone grafts. Normally, when a tooth is removed, banked bone (called an allograft) or a xenograft (bone from another species, typically bovine or cow) is placed into the socket. Additionally a resorbable collagen membrane is placed over the bone to prevent the gum tissue from invading the socket site. Occasionally, in an extraction site without grafting, the gum tissue invades into the socket before bone can heal and some loss of width more so than height occurs. The bone graft to preserve the socket is called an alveolar preservation procedure. Normally after three to four months, the implant can then be placed.
If the bone is too thin and/or too short, autogenous bone grafting is usually needed. Autogenous bone grafting is typically taking bone from one part of the body and transferring to another. For most situations in the mouth, bone can be taken from non-tooth bearing areas (at or above the wisdom tooth site called the ramus), from the front part of the chin, the site where the upper wisdom tooth once was (tuberosity), the malar buttress (where the bottom of the cheek bone meets the upper jaw), or from tori. Tori are naturally occurring bone outcroppings of the upper and/or lower jaws. This anomaly is seen 5 to 10% of the population. The site where the bone is taken is called the harvest site. The donor site, where the bone is to be placed, is prepared to accept the block of bone or particulated bone. Particulated or ground up or scraped bone is placed into a defect or into a titanium mesh or titanium reinforced Gore-Tex (PTFE-Polytetrafloroethylene). If a block of bone is taken, once the donor site is prepared, the block is secured to the site using titanium or stainless steel bone screws. After a period of healing, typically 5-6 months, the mesh, Gore_tex or bone screws are removed and the implant(s) are placed.
Bone of the upper back jaw often does not atrophy horizontally significantly. However, vertical atrophy causes the alveolar bone to shrink upwards and approaches the bottom portion of the maxillary sinus. Then a decision has to be made whether to add bone vertically to the upper jaw (maxilla) or elevate the sinus. The sinus is a hollow cavity of the skull lined by a membrane (Schneiderian membrane). The membrane consists of respiratory epithelium or ciliated columnar epithelium. The cilia are little hairs that beat and clear the sinus of fluid and mucus. When there isn't enough bone present, the sinus can be elevated and bone placed under the membrane. The procedure consists of an approach to the sinus from either the alveolar ridge (where the tooth was) or from the side (cheek side of the jaw). Access is made into the sinus without tearing the membrane and elevating the membrane off of the bone. The mobilized membrane creates the matrix to contain the bone graft. The bone graft can be an autogenous, an allograft, and/or a xenograph. Depending on the amount of bone present at the time of surgery, the implant can be placed at the same time or in a secondary procedure 5-6 months later.
Often times patients are more concerned with the harvest site or the taking of the bone graft rather than the placement of the graft. Are there other options besides using the patient's own bone? Yes, there are other alternatives to consider. One option is an allograft block. It is a block of bone taken from a human cadaver and treated to remove all disease and protein that cause rejection. However in most cases, the amount of resorption is unpredictable. What that means, is it is hard to determine how much of the bone graft will actually stay behind. Additionally, some times the bone can incorporate but never get fully turned over by your body. Typically when allografts are placed, they are resorbed by your body and replaced by your natural bone within the matrix of the graft placed. Your skeleton is not static and constantly rids itself of old bone and turns over new bone. This process happens to about 0.7% of your skeleton everyday. The area that has the most turnover is the mouth where the teeth and periodontal ligament meet the bone. With these allograft blocks and with xenografts, some of the graft material occasionally never gets turned over and can have a poor blood supply. Implants placed into this bone can suffer bone loss and failure. The other option is human recombinant bone morphogenic protein. Commonly called BMP, this protein actually signals the body to put bone where the protein is placed. For sinus lifts, a collagen membrane is soaked in BMP and placed into the sinus. After 6 months or so, implants can then be placed. Success rates are relative on par with autogenous bone grafts. Patients often elect this procedure when they wish to avoid bone harvesting. The only negative is the cost of the protein which can be a few thousand dollars by itself.
When there isn't enough bone that can be obtained from the mouth, the bone must be harvested from elsewhere. Typically for dental implant procedures, bone can be obtained from the anterior (front part of the hip), the tibia (big bone of the lower leg), or the skull. The hip and the tibia are typically used. Some of these procedures can be done in the office, but some require hospitalization. Other options to bone grafting can be distraction osteogenesis. The is where a cut in the bone is made and freed up from the mandible or maxilla but still left attached to the tissue one side. Therefore the freed up piece of bone still has a blood supply. The freed up part of the bone, called the transport bone, is attached to a device with screws and the other end of the device is attached to part of the bone where the freed piece came from. Slowly over time, the device is activated and slowly spreads apart. If done properly, as the bone segments are moved apart, bone fills in gap and "new" bone is grown. The difficulties with the procedure is controlling the direction of the transported bone segment, the patient tolerating the device for several weeks and the transported bone is occasionally too thin for implants and requires further grafting.
In the lower jaw, if there is not enough height, one other option beside bone grafting is nerve lateralization. If the bone is wide enough, what typically limits vertical placement of implants is the position of the inferior alveolar nerve canal. This is an intrabony canal that houses the nerve that supplies feeling to the lower teeth and to the lip and chin. It is the nerve that makes your lip and chin feel fat after the dentist numbs your lower arch for treatment. To gain height for implants, the nerve canal can be unroofed from the side and moved away, the implants placed and then the nerve redraped. Obviously there is some risk of nerve damage in this procedure and is usually a secondary consideration to bone grafting.
When patients understand why bone grafts are needed, the case acceptance rates improves dramatically. Patients must have a firm understanding of the procedure and reasoning behind procedures to reduce their reluctance to proceed. Understanding that creating the ideal foundation for implants improves dental implant success, longevity, function and greatly reduces post-implant complications, motivates patients not to compromise their dental implant treatment plan. Therefore, dentist and specialist must take their time to explain not only the procedure but the reasoning behind bone grafting for dental implants.
Role of Bone Grafting in Dental Implants MEMBRANE
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