Aims: To develop and test in the surgical theatre an intra-operative device, which could help the surgeon assessing the primary stability of cementless hip stem.

Methods: The device is basically a measurement instrument. It hosts two sterilization-proof sensors, giving separate signals of two magnitudes: a torque and an angle. The prototype has been designed to allow safe handling during surgical manoeuvres and easy insertion/ extraction in the stem under test. A series of leds gives the surgeon information concerning the load applied and the extent of implant stability achieved. A micro-motion threshold was set at 100 microns to discriminate between stable and unstable implants. The device was validated and calibrated in vitro on 5 cadaveric and 2 composite femurs (so as to cover a wide range of bone quality & size) hosting the same kind and size of stem, with increasing press-fit (Cristofolini, 2002). The prototype stability measurement was compared with that obtained with a previously validated in-vitro protocol (Monti et al.,1999), based on an additional high accuracy LVDT. A clinical trial is now in process.

Results: The validation yielded satisfactory results in terms of noise and drift (for both sensors < 0,05% of the range).The final prototype was then calibrated, comparing the device readout with the reference data from the additional LVDT. Good linear correlation was found (R2B30,98). The overall accuracy of the prototype was estimated analysing the data of all the femurs. The final prototype can predict implant stability with an error of 10% on the displacement limit (100 micron). After the first test in the operating room, the surgeons confirmed that the device was easy to handle and to use, and that the parameters extracted were clinically significant. Micromotions were recorded in implants considered sufficiently stable by the surgeons (confirming the stability threshold of 100 micron).

Conclusions: The prototype was successfully designed, validated and calibrated. The overall accuracy was deemed sufficient to discriminate between stable and unstable implants. The clinical trial highlighted the great advantage that this device can give to the surgeon in deciding if press-fit is sufficient.