Implant stability measurements using resonance frequency analysis: biological and biomechanical aspects and clinical implications

Osseointegrated implants for prosthetic rehabilitation of the edentulous patient show high success rates if certain preconditions are fulfilled. Implant stability plays a critical role for a successful critical outcome since short implants and implants placed in soft bone are more prone to failure (13, 49). In the original protocols for implant placement, primary implant stability was ensured by new bone formation and remodelling, termed osseointegration, which was accomplished during an initial healing period in which implants remained nonloaded to secure undisturbed bone formation onto the implant surface. The process of osseointegration increases the stiffness of the bone around the implant, and the bony interlock with the implant surface prevents micro-movement and the formation of fibrous scar tissue at the time of implant loading. However, the development of new implant surfaces and clinical techniques has enabled a marked reduction of the initial healing period, even to the point of an immediate ⁄ early loading of implants that show high primary stability (7, 37). Thus, the success of immediate ⁄ early loading implant techniques is dependent on the ability of the clinician to determine the degree of primary implant stability and changes in stability along with new bone formation and remodelling. The clinical perception of primary implant stability is frequently based on the cutting resistance of the implant during its insertion. The feeling of good stability may be accentuated if there is the sense of an abrupt stop at the seating of the implant. Root forms of tapered implants often have a geometry that will provide a firm stop and perhaps a false perception of high stability. A percussion test has also been used to assess implant stability. The percussion test may involve the tapping of a mirror handle against the implant carrier and is designed to elicit a ringing sound from the implant as an indication of good stability or osseointegration. Percussion tests probably provide more information about the tapping instrument, and will at best only yield poor qualitative information. Insertion torque measurements are sometimes used to determine primary implant stability (6). Application of a reverse or unscrewing torque has also been proposed for the assessment of implant stability at the time of abutment connection (53). Implants that rotate under the applied torque are considered failures and are then removed. However, an implant surface in the process of osseointegrating, albeit slowly, may fracture under the applied torque stress. Moreover, as animal experiments have demonstrated the re-integration of loosened and rotationally mobile implants (26), the reverse torque testing has fallen into disrepute. Other techniques, such as the Periotest and resonance frequency analysis, aim to provide an objective measure of implant stability and osseointegration that is noninvasive and does not damage the implant–tissue interface (6, 28). The resonance frequency analysis technique has been extensively used in experimental and clinical research for the last 10 years. The purpose of this