Polyurethane foam as a model to study primary implant stability. A series of in vitro studies

Primary stability of dental implants is defined as the absence of movement of an implant after surgical insertion. Primary stability is influenced by bone density, bone quality, surgical technique, and implant body geometry. A direct relationship seemed to exist between implant primary stability and bone density. So far, several nondestructive methods have been suggested to evaluate implant stability, such as insertion torque measurement (IT) and resonance frequency analysis (RFA). IT measures the compression induced by implant placement into the surgical site and it determines the primary implant stability, which is considered the most important factor for successful implant treatment. The ISQ values, obtained through the RFA, reflect the micromobility of the implant, which is in turn determined by factors such as bone density, surgical technique, implant design, and healing time. Polyurethane foam has been proposed for in vitro tests to simulate the consistency and the density of the bone. These rigid blocks have been produced as an alternative test medium to human cancellous bone. This material displays mechanical properties in the range of human cancellous bone as described by the ASTM (American Society Testing Materials) F-1839-08 2012 standard. Its features make it an ideal material for comparative testing of different implant materials. The physical features of polyurethane are homogenous throughout their volume, so as to obtain a good standardization of the procedures with the exclusion, then, of factors inherent to anatomical and structural differences of bone. Polyurethane blocks of different densities have been used, e.g., to evaluate the primary stability of different types of implants (cylindrical vs. conical implants), different lengths (short implants vs. standard length implants), and different diameters (narrow, regular, and wide implants).