A novel transducer for the measurement of cement-prosthesis interface forces in cemented orthopaedic devices

When a cemented orthopaedic device is being investigated there is a need to estimate the forces at the cement-prosthesis interface. For this reason a miniature transducer was developed that could be included inside the surface of most prostheses. A load cell (based on a piezoelectric sensor) and the required accessories and amplification were custom designed and built. The present work describes the validation that was performed on the piezo sensors alone, when mounted on a simplified structure, and when applied to a hip stem. Linearity, repeatability, reproducibility, and sensibility to shear and axial eccentric loads were tested, yielding satisfactory results. The repeatability on the same sensor was found to be good while reproducibility between sensors was lower. Thus, each sensor was calibrated separately with a second order relationship. Sensitivity to shear and eccentric loads was very low. The overall accuracy of the load cell (including non-linearity, and signal drift) was of the order of about 1%. A hip stem instrumented with four such sensors was successfully implanted in a composite femur, yielding meaningful readouts. Thus, this type of sensor can readily be used to assess the cement-prosthesis interface forces in cemented devices.