Polyethylene wear simulation models applied to a prosthetic hip joint based on unidirectional articulations.

Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as soft-bearing material in total joint replacements. However, the release of polymeric wear debris is still related to complications leading to aseptic loosening. Recently, a novel hip prosthesis showing reduced wear was developed by the authors of this study, consisting of unidirectional cylindrical articulations instead of the conventional multidirectional ball-and-socket design. This study evaluates four different theoretical wear models applied to this new design. The calculated volumetric wear was compared to experimental results. Although all models provided a good indication of the wear rates for the ball-and-socket prosthesis, they exhibited high discrepancies when predicting the amount of wear of the new unidirectional design. It was observed that the closest agreement with experimental results was obtained by the models that consider the friction-induced molecular orientation phenomenon exhibited by UHMWPE.

A novel hip joint prosthesis with uni-directional articulations for reduced wear.

A novel polymer-on-metal hip joint prosthesis design that makes use of uni-directional articulations was developed and tested in this work. The new implant was tested using two polymer variants, virgin ultra-high molecular weight polyethylene (UHMWPE), and Vitamin E-infused highly crosslinked polyethylene (VEHXPE). The degrees of freedom of the ball-and-socket are reproduced by three cylindrical orthogonally-aligned articulations. This unconventional design leverages on the molecular orientation hardening mechanisms of the polyethylene and increased contact area to minimize wear. An experimental hip joint simulator was used to compare the gravimetric wear of the conventional ball-on-socket and the new implant. The new prosthesis including UHMWPE components produced a 78% reduction in wear, whereas the new prosthesis with VEHXPE components produced a 100% reduction in wear, as no measurable wear was detected. Machining marks on the acetabular cups of the new prosthesis were retained for both polyethylene variants, further demonstrating the low levels of wear exhibited by the new implants. Both polyethylene materials produced particles in the range of 0.1-1.0 µm, which are the most biologically active. Nonetheless, the extremely low wear rates are likely to induce minimal osteolysis effects. Furthermore, the novel design also offers an increase of more than 24% in the range of motion in flexion/extension when compared to a dual-mobility hip implant. A prototype of the prosthesis was implanted into a Thiel-embalmed human cadaver during a mock-surgery, which demonstrated high resistance to dislocation and the possibility of performing a figure of four position.