Development and modeling of an *impact test to determine the bone -implant interface properties of osseointegrated implants

The ongoing need for a clinically effective, non-invasive technique for monitoring implant stability has led to a number of testing methods based on the concept of resonant frequency. Resonant frequency measurements provide an indirect measure of the bone-implant interface. To date, these measurements do not provide specific detail of the changes occurring due to modeling/remodeling at the interface.

In this study an impact testing method based on the Periotest ^® handpiece has been developed and with the use of a four degree of freedom analytical model has led to a method of directly estimating the bone-implant interface properties. The technique is applied in a variety of in vitro tests to gain an understanding of the parameters influencing the measurements and to provide a measurement protocol. The impact technique, protocol and model are then used in vivo to evaluate the interface properties of Bone Anchored Hearing Aid (BAHA^®) implants in twelve patients at installation, l, 2, 3, 6 and 12 month intervals.

Analytical model estimates of the interface stiffness in vitro found the interface stiffness of FRB-10 modeling material to be 7.5--7.7 GPa which is comparable to the 9.3 GPa modulus of elasticity. In vivo, the average interface stiffness for the patients at implant placement was 5.0 GPa which increased to 7.9 GPa by the twelfth month. Individual interface stiffness values showed a great deal of variation within the first three months, with changes up to 7 GPa occurring.

The developed impact testing technique is shown to be accurate and sensitive in the measurement of interface properties both in vitro and in vivo. The analytical model provides a method to extract basic mechanical properties of the interface from the impact measurements. Ultimately, the testing method is shown to have clinical significance as it provides a means to track the mechanical properties of the bone-implant interface in a non-invasive way in patients.