Primary stability of cementless prostheses is critical for the long term outcome of the operation. Cementless implants are mechanically stabilized during surgery through a press-fitting procedure. To achieve a good initial stability, it is important that the surgeon performs an optimal press-fitting, avoiding both problems of stem loosening, and micro-cracking of the host bone. A possible approach to solve this problem and assist the surgeon in achieving the optimal compromise, involves the use of the vibration analysis. This technique was used in the presented study, which was aimed to design and test a prototype device able to evaluate the primary stability of a cementless prosthesis, at the femoral level. In particular, the goal was to discriminate between stable and quasi-stable implants; thus the stem-bone system was assumed linear in both cases. For that reason, it was decided to study the frequency responses of the system, instead of the harmonic distortion. The prototype was developed. It is mainly composed by a piezoelectric exciter connected to the stem and an accelerometer attached to the femur. Preliminary tests were performed on a composite femur implanted with a conventional stem. The results showed that the input signal is repeatable and the output can be accurately recorded. The parameters that seem to be more sensitive to stability are the resonance frequency and the amplitude at the resonance frequency.

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