Sunday, May 8, 2011

Biomechanical factors affecting long-term implant success

Bone resorption around dental implants can be caused by premature loading or repeated overloading. Vertical or angular bone loss is usually characteristic of bone resorption caused by occlusal trauma. When preasure from traumatic occlusion is concentrated, bone resorption occurs by osteoclastic activity. 


In the natural dentition, bone reapposition would typically occur once the severe stress concentration is reduced or eliminated. However, in the osseointegrated implant system,after bone resorbs, it will not usually reform. Because dental implants can resist forces directed primarily in the long axis of the implant more effectively than they can resist lateral forces, lateral forces on implants shoud be minimized. Lateral forces in the posterior part of the mouth have higher impact and are more destructive than lateral forces in the anterior part of the mouth. When lateral forces cannot be completely eliminated from the implant prosthesis, efforts shoud be made to equally distibute the lateral forces over as many teeth and implants as possible.

Divergent implant placement can also increase the moment arm through which force is transferred to the bone-implant interface. Such force could potentially axceed the threshold for bone resorption. Inadequate implant distibution, which leads to excessive cantilevers, can also pontentially overload individual fixtures.
Connecting a single osseointegrated implant to one natural tooth with a fixed partial denture may effectively create an excessive cantilever situation. Because of the relative immobility of the osseointegrated implant compared with the functional mobility of a natural tooth, when loads are applied to the bridge, the tooth can move within the limits of its periodontal ligament. This can create stresses at the neck of the implant up to 2 times the applied load on the prosthesis.

Pathogenetic forces can be placed on implants by nonpassively fitting frameworks. If screws are tightened to close gaps between the abutment and the nonpassive framework, compressive force is placed on the interfacial bone. Excessive force of this nature can lead to implant failure.

Because osseointegrated implants feature an immobile interface between the implant and the bone, it may be necessary to incorporate some type of shock-absorbing buffer layer between the occlusal force and the implant. Occlusal collision impact force may exceed the threshold necessary to cause bone resorption. The recommended shock absorber could take the form of a specially designed element such as in the IMZ system or, as recommended in the Branemark system, acrylic resin may be used as the restoration material of choice. These recommendations are based on theoretic calculations rather than clinical data. Therefore the necessity for shock-absorbing elements remains a controversial issue in implant dentistry. Many clinical problems can be avoided by joining implants to each other but not to natural teeth.

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