Implications of Implant Framework Misfit: An Animal Study on an Ovine Model
Although clinicians routinely aim to provide a prosthesis with an accurate fit on implants, a degree of prosthesis misfit is inevitable. This exploratory pilot animal study evaluated the effects of framework vertical misfit and the timing of implant loading on implant position and screw loosening. Four implants were placed in healed ridges of each side of mandibles of 3 sheep. On the right side, 2 immediate frameworks were placed after 2 days. One framework was fitting, and the other one had a vertical gap of 0.5 mm on the distal implant. After 8 weeks (first review), the left side received 2 conventional frameworks with similar fit conditions to the right side. All animals were euthanized after 8 weeks (second review). At the first and second reviews, implant-level impressions were taken to measure the vertical displacement of distal implants, and the loosening torque values of the retaining screws were measured. The loosening torque values for the immediate fitting frameworks were considerably greater than the immediate misfitting frameworks. This was noticeable at the first review. At the second review, the loosening torque values were comparable to the immediate fitting and misfitting frameworks. Vertical implant displacement was observed for all misfitting frameworks. However, much greater implant displacement occurred under the immediate frameworks. Therefore, implant frameworks with vertical misfit in the present study were associated with less screw stability and more implant displacement. Retightening the retaining screws during the maturation of bone seemed to maintain the torque values.
Figure 1. Surgical placement of the implants. Figure 2. Implant level impressions were taken immediately after the placement. Figure 3. The fitting surface of a framework showing a nonengaging interface. Figure 4. An example of a fitting framework. Figure 5. An example of a misfitting framework, in which a 0.5-mm vertical gap was added on between the fitting surface and the distal implant. Figure 6. An image of the 2 frameworks immediately after fitting.
Figure 7. An example of the heat map of a superimposed pair of implants. The mesial implant (left) was used as a reference, which was confirmed by the homogenous green color. As a result, the discrepancies were restricted on the distal implant (right). The vertical gap between the distal implants was measured as an indication of implant displacement. Figure 8–11. The torque loss values of the different frameworks. Figure 8. Immediate fitting frameworks. Figure 9. Immediate misfitting frameworks. Figure 10. Conventional fitting frameworks. Figure 11. Conventional misfitting frameworks. For the immediate frameworks, data from the first and second reviews are available. For the conventional frameworks, only the data from the second review are available. Figures 12–15. The vertical displacement of the distal implants at the first and second reviews of the different frameworks. Figure 12. Immediate fitting frameworks. Figure 13. Immediate misfitting frameworks. Figure 14. Conventional fitting frameworks. Figure 15. Conventional misfitting frameworks.
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