Development and Implementation of Mathematical Modeling, Vibration and Acoustic Emission Technique to Correlate In Vivo Kinematics, Kinetics and Sound in Total Hip Arthroplasty with Different Bearing Surfaces

The evaluation of Total Hip Arthroplasty (THA) outcome is difficult and invasive methods are often applied. Fluoroscopy has been used as an in vivo diagnostic technique to determine separation which may lead to vibration propagation and audible interactions. The objective of this study was to develop a new, non-invasive technique of digitally capturing vibration and sound emissions at the hip joint interface and to correlate those with the hip kinematics derived from fluoroscopy. Additionally, an examination of the role of hip mechanics on walking performance in THA subjects of various bearings surfaces was performed.

In vivo kinematics, kinetics, corresponding vibration and sound measurements of THA were analyzed post-operatively using video-fluoroscopy, mathematical modeling, sound sensors and accelerometers during gait on a treadmill. Twenty-seven subjects (31 hips) with a metal-on-metal, metal-on-polyethylene, ceramic-on-ceramic, ceramicon- polyethylene or metal-on-metal polyethylene-sandwich THA were analyzed. A data acquisition system was used to amplify the signal and filter out associated frequencies attributed to noise. The sound measurements were correlated to in vivo kinematics. A mathematical model of the human extremity was derived to determine in vivo bearing and soft-tissue forces.

For all bearings a distinct correlation of a high frequency sound occurring at the time when the femoral head slides back into the acetabular component was observed. Subjects having a hard-on-hard bearing seemed to attenuate a squeaking and/or impacting sound, while those having polyethylene liner only revealed a knocking sound attributed to impact loading conditions.

For the first time, audible effects can be derived in vivo and the examined correlation brings valuable insight into the hip joint performance in an inexpensive and non-invasive manner. This research may allow for a further correlation to be derived between sound and different types of failure mechanisms. Results from this study will give surgeons and engineers a better understanding of in vivo mechanics of the hip joint and this way improve the quality of life of THA patients. In addition, the developed technique builds the first milestone in the design and implementation of a cost effective, non-invasive diagnostic technique which has the potential to become a routine diagnosis of joint conditions.