Abstract
Work carried out by Bennett [1], identified a link between patient gait pattern and total hip prothesis wear rate. This study found that the shape of the patient gait pattern (as quantified by aspect ratio) and sliding distance of the movement loci were found to have an improved positive correlation with wear rate compared to the factors of activity and patient weight. The distribution of theoretical shear stresses at selected points on the acetabular cup suggests that orientation of the polymer chains may occur. Wang et al, 1997 [2] has shown that failure of the UHMWPE wear surfaces occurs in the form of transverse rupture between oriented molecules.
This work investigates the hypothesis that the gait pattern of pre-revision THR patients has an effect on the wear, surface characteristics and material properties of the artificial hip joint, in particular the degradation of chemical and mechanical properties of the UHMWPE acetabular socket. Gait analysis is performed on patients prior to revision of a primary THR, with the retrieved socket used for subsequent analysis.
Chemical and mechanical analysis of a large number of retrieved UHMWPE acetabular sockets has shown clear structural changes, which are dependent on the length on time in-vivo. Increasing the length of time in-vivo between 2 and 20 years results in an increase in the percentage crystallinity of the UHMWPE of 12.7 %. A positive linear correlation (R2 = 0.765) between percentage crystallinity and number of years in-vivo is shown. This suggests recrystallisation of the polymer at a constant rate over time. This partial recrystallisation of the amorphous region correlates with degradation in the mechanical properties of the material. This pilot study aims to assess the effect of patient gait pattern on the chemical and mechanical degradation of UHMWPE, which will ultimately affect the clinical performance of the prothesis.
Theses abstracts were prepared by Mr Peter Kay. Correspondence should be address to him at The Hip Centre, Wrightington Hospital, Appley Bridge, Wigan, Lancashire WN6 9EP.
References:
1 Bennett D. B., Orr J. F., Beverland D. E., Baker R. The influence of shape and sliding distance of femoral head movement loci on the wear of acetabular cups in total hip arthroplasty. Proceedings of the Institute of Mechanical Engineering, Part H, Journal of Engineering in Medicine (in press). Google Scholar
2 Wang A., Sun D. C., Yau S.-S., Edwards B., Sokol M., Essner A., Polineni V. K., Stark C., Dumbleton J. H. Orientation softening in the deformation and wear of the ultra-high molecular weight polyethylene. Wear1997b; 203–204: 230–241. Google Scholar
