Mechanical Effects of Mesiodistal Tilting of a Fixture on the Strain Distribution in an Implant Structure and Surrounding Bone

The osseointegrated implant is recognized as one of the most reliable means of therapy for prosthetic treatment of missing teeth. However, bone resorption sometimes occurs by concentrating stress at the interface between the fixture surface and the surrounding bone, due to lack of the buffer system provided by the periodontal membrane of the natural tooth. The resulting discordance between the functional long axis and occlusal force of the fixture is the main factor responsible for the loss of osseointegration. In many cases, bone resorption is seen over a wide area, especially on the side of inclination of a tilted fixture.<br/> The present experiment using the strain gauge method was therefore conducted in order to clarify the mechanical effects of the mesiodistal angle of tilt of a fixture on the strain distribution in an implant structure and surrounding bone.<br/> The second, third, and fourth mandibular premolars were extracted from beagle dogs, and 4 months after tooth extraction, two fixtures were installed in the premolar area on each side. At this time, by changing the mesiodistal installation tilt angles of the two fixtures, six placement modality patterns were set up. Four months after fixture installation, at the osseointegration stage, the dogs were sacrificed and their mandibles were removed.Test specimens were prepared by attaching the abutment and superstructure to the fixture. Strain gauges were bonded to the abutment surface, the bone around the connection between the abutment and the fixture, and the buccolingual alveolar bone. A load of 10 kgf was applied vertically to the horizontal plane of the superstructure, and the strain distribution was analyzed chronologically.<br/> The experimental results and conclusions were as follows:<br/> 1. The best dynamic stability was achieved when the two fixtures were set up parallel to the functional axis of the occlusal force.<br/> 2. When the two fixtures were installed with a tilt in the same direction, there appeared to be a risk of loading in a single direction, thus inviting microfracture of the surrounding bone.<br/> 3. When one of the two fixtures was installed in a tilted position, it appeared possible to disperse the stress by installing the other fixture with a tilt in the opposite direction.