Purpose: In a retrospective study, the radiographs of 39 patients with Applegate-Kennedy Class I or II in the posterior mandible who had been treated with screw-anchored fixed partial dentures supported by IMZ implants and natural teeth were examined for the presence of radiologically detectable peri-implant bone loss. Furthermore, the results were correlated with a mathematical model.
Materials and methods: The radiographs of the implants were digitized, and the areas of bone atrophy mesial and distal to the implants were determined semi-automatically. The data obtained were correlated with the distance between the implant and the abutment tooth. The connection between the tooth-supported crown and the implant-supported denture was made with a vertical screw-lock precision attachment. In a mathematical analysis it was assumed that the fixed partial prosthesis was a rigid beam with 3 elastically embedded supports.
Results: The mean distance between the tooth and the first implant was 11.02 mm (SD: 4.24), and between the tooth and the second implant was 20.25 mm (SD: 5.16). Peri-implant bone loss significantly followed a rational function (mesial implant: P = .03, distal implant: P = .02), meaning that, as the tooth-implant distance increased, the area of atrophy became rapidly larger and then diminished gradually. Distances of 8 to 14 mm between the tooth and the first implant and of 17 to 21 mm between the tooth and the second implant were associated with a more pronounced bone loss. These results were also confirmed mathematically.
Conclusion: A tooth-implant distance of 8 to 14 mm for the first implant and 17 to 21 mm for the second implant should be avoided for implant placement if prosthetic rehabilitation is planned using a fixed partial denture supported by a premolar and 2 IMZ implants in the mandible. Although this investigation was done on IMZ implants only, the results were confirmed by a mathematical model, which indicated that the observed bone loss may be the same in other types of implants placed in the same positions.