Objective: Periimplantitis generally generates a circular, cupping defect surrounding the implant. In cases of uneven thickness of the implant-bearing bone a dehiscency may become apparent under surgical inspection of the site. With common radiological techniques it is impossible to visualize such a lesion. Computer tomography may reveal dehiscencies but radiation exposure and the effort necessary usually do not justify such measures. In this in-vitro experiment we looked at how Resonance Frequency Analysis (RFA, Osstell™ instrument) and Damping Capacity Assessment (PT, Periotest™ instrument) would react to non-circular peri-implant defects. Material & methods: Screw type oral implants were polymerized into acrylic blocks. Advancing localized peri-implant bone dehiscencies were simulated by successively removing the acrylic on one side of the implant in millimeter increments in an apical direction. Eight measurement series were performed for each bone situation by loosening the RFA transducer after each measurement and reattaching it after rotation by 45° to the next measurement position, until the implant had been measured by 360° in eight different positions. For PT measurements the tapping direction of the instrument's pistil was likewise altered. Results: In cases of an uneven thickness of the implant-bearing material both devices clearly demonstrated a decreased measurement value for implant stability if the measurement direction was in 0° and 180° of the defect location. A mesio-distal defect width of less than three millimeters did not result in any noticeable change of the measured implant stability on simulated peri-implant bone loss, even if it advanced up to ten millimeters in depth. Defect widths extending three millimeters, however, showed lower values for implant stability on increasing attachment loss. Conclusion: With both, the RFA and the PT, it seems to be possible to detect and follow-up non-circular peri-implant bone defects. Invasive diagnostic procedures thus may be replaced by these in the future.

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