Assessment of implant stability of cementless hip prostheses through the frequency response function of the stem-bone system

Cementless joint prostheses require adequate initial press-fitting to achieve sufficient primary stability, which is necessary for bone ingrowth and implant success. A device was developed that measured intra-operatively the stability of a hip stem in the host bone. It included an excitatory piezoelectric system based on a ceramic multilayer bender, which delivered a controlled excitation in the range 1200–2000 Hz to the prosthesis. An accelerometer mounted on the host bone measured the transmitted vibration so as to identify the resonance frequency. Resonance frequency (and its associated shift) was measured immediately after implant press-fitting, and while a torque was applied to the implant. The proposed method was validated in vitro on 5 femurs covering a wide range of bone quality. Each bone was tested with different degrees of implant press-fitting. Implant stability estimated with the vibration method was compared against implant–bone micromotions that were measured simultaneously by a displacement transducer during this validation session. A strong correlation was found between the shift of the resonance frequency caused by load application, and implant stability. A quantitative threshold was identified that enabled consistently discriminating stable implants form quasi-stable ones: when the resonance frequency shifted less than 5Hz during torque application, the residual micromotion after load removal was always less than 150micron.