Development and evaluation of a noninvasive vibration modeling technique for primary stability measurement in artificial cortical bone blocks

Noninvasive methods of quantifying stability are important for monitoring implant health. Current techniques are unable to provide a universal measurement of stability. The Advanced System for Implant Stability Testing (ASIST) is a device that noninvasively measures implant stability by analyzing the vibration response of the implant system to estimate the interfacial stiffness, reported as the ASIST Stability Coefficient (ASC). The aim of this in vitro study was to determine whether implant stability can be measured as the interfacial stiffness with a refined analytical model that accounts for cortical and cancellous layers. Bone-level tapered implants were placed in polyurethane foam comprised of 2 layers mimicking cortical and cancellous bone. Cancellous bone was simulated with 320 kg/m3 foam, while cortical bone was simulated with 3 densities (480, 640, 800 kg/m3) and 3 thicknesses (1, 2, 3 mm) for a total of 9 experimental groups. Each group contained 10 specimens. The stability of each implant was measured with the ASC, Implant Stability Quotient (ISQ), insertion torque (IT), and pullout test. Two-way ANOVA was used to compare stability values across groups of the same cortical density and thickness (α=.05). The Dunnett test was also used to compare experimental groups with a control group (n=10) of purely cancellous foam (320 kg/m3). Correlations between outcome measurements were described with the Pearson correlation coefficient. ASC values followed similar trends with IT and pullout force (PF) measurements where varying the cortical density had the largest effect on stability values at the highest cortical thickness and vice versa. The control group showed values similar to those of the groups with a 1-mm cortical thickness. However, ISQ values significantly increased with cortical thickness (P<.001) but did not change significantly with cortical density (P=.265). Strong correlations were observed between the IT and pullout force (r=0.946), ASC and IT (r=0.872), and ASC and pullout force (r=0.917). Weaker correlations were observed between the ISQ and IT (r=0.495), and ISQ and pullout force (r=0.552). With the refined analytical model, ASC values were comparable with trends in IT and PF, with stronger correlations compared with the ISQ. Within the confines of this controlled in vitro study, the results suggest that the ASIST may provide an improved estimate of an implant's mechanical stability. However, further work with real bone, other implant systems, and human participants is warranted before its clinical feasibility can be assessed.