Analysis of Relative Motion between Femoral Head and Acetabular Cup and Advances in Computation of the Wear Factor for the Prosthetic Hip Joint

Abstract

The amount and type of wear produced in the prosthetic hip joint depends on the type of relative motion between the femoral head and the acetabular cup. Wear particles removed from the bearing surfaces of the joint can cause adverse tissue reactions resulting in osteolysis and ultimately in loosening of the fixation of the implant. When designing a simulator for evaluation of prospective materials for artificial hip joints it is important to verify that the type of relative motion at the articulation is similar to that produced in walking, involving continually changing direction of sliding. This paper is an overview of recent research done at Helsinki University of Technology on the analysis of the relationship between relative motion and wear in the prosthetic hip joint.To analyze the relative motion, software for computing tracks, referred to as slide tracks, drawn on the counterface by marker points on the bearing surface was developed and experimentally verified. The overall relative motion of the joint was illustrated by a slide track pattern, produced by many points. The patterns resulting from walking motion and from motion produced in ten contemporary hip simulator types were compared. The slide track computations were not limited to illustrational purposes but offered a basis for computing variations of sliding distances, sliding speeds and direction of sliding during a cycle. This was done for the slide track termed the force track, drawn by the resultant contact force. In addition, the product of the instantaneous load and increment of sliding distance was numerically integrated over a cycle. This track integral of load had so far not been determined for the majority of contemporary hip simulators. The track integral can be used in determining the wear factor, making it possible to compare clinical wear rates with those produced by hip simulators. The computation of the wear factor was subsequently improved by replacing the track integral of the resultant contact force with a surface integral computed as the sum of track integrals of a large number of smaller normal forces obtained by discretizing the contact pressure distribution. The slide track software could also be utilized in the conceptual design of new simulators because it was possible to rapidly investigate the effect of changes to the motion waveform amplitudes or phases, or of omitting certain waveforms to simplify the design of a simulator.The slide track analysis showed that walking motion produced mainly open tracks on the center of contact, implying continually changing direction of sliding. This phenomenon, which is crucial for obtaining the correct wear mechanisms for acetabular cups made of polyethylene, was reproduced by simulators having abduction-adduction motion in addition to flexion-extension motion. In the force track computations involving contemporary simulators with the common femoral head size of 28 mm, the sliding distance per cycle and the force track integral per cycle ranged from 19.7 to 34.3 mm and from 17.4 to 43.5 N m, respectively. The average sliding speed ranged from 19.7 to 49.0 mm/s. The sum of track integrals computed with forces obtained by discretizing the contact pressure distribution reached a substantially higher value than the track integral obtained with the resultant contact force only. This suggests that the wear factor is actually overestimated when computed in the conventional way by dividing the wear rate with the force track integral.