Relationship between interfacial micromotion and bone density around cementless hip implant

The hip is one of the most injured joint in the body with ageing. Osteoarthritis and degenerative diseases often lead to the necessity of a total hip arthroplasty in order to reduce pain and increase mobility. The advantage of cementless implants resides in the natural stem anchorage by osseointegration instead of the cement use. Nevertheless, the related stability is highly dependent on the micromotion between the implant and the bone. As more and more arthroplasties are performed each year, it is important to control the outcome and to reduce the risks linked with implant loosening due to excessive micromotion. The goal of this study is to improve a micromotion measurement method based on compression and torsion tests on cadaveric femurs. Moreover, in order to help the surgeon to position the implant to limit high micromotion, density around the implant is studied in order to compare with micromovements distribution. Markers made of stainless steel are put on the internal bone surface. The compression and torsion tests are performed under micro-computed tomography environment. The locations of markers are visible on scans performed before, during and after the tests. Post-processing based on micro-CT scans allows the extraction of micromotion, subsidence and gap values between loading and rest states. Contrary to other studies, which use LVDT to measure micromotion on the outer bone surface, this method allows local measurement. Thanks to micromotion values projection on the implant, it is possible to see the local distribution around the stem. The amplitude of measured micromotion remains under the limit value of 150 [µm] that is supposed to lead to the formation of brous tissue, except when bone quality is damaged. The improved method is accurate and fast. Nevertheless, it is highly dependent on the image acquisition quality. However, no general statistical correlation has been found between density and micromotion. Nonetheless, based on the rendering of the postoperative planning software HipPlan (Symbios, Switzerland), it seems that few support zones present similarities between low micromotion and high bone density.