An investigation into the origins of time-dependent variation in penetration rates with Charnley acetabular cups--wear, creep or degradation?

The total penetration of femoral heads into acetabular cups is achieved by a combination of long-term wear and early non-recoverable deformation or creep. The former is important in determining the total rate of production of polyethylene wear debris, recently implicated in the development of osteolysis and loosening, while the latter contributes to the overall penetration and the possibility of neck impingement in some designs of implants. Attention is drawn to the need to evaluate and to separate out these two physical processes in the assessment of clinical penetration rates. This is particularly important with more recent designs and combinations of materials, which are capable of operating with much reduced wear rates. Measurements of the penetrations of metallic femoral heads into polyethylene acetabular cups in 87 explanted Charnley hip arthroplasties from 85 patients are reported and assessed. The vast majority of the acetabular cups had been sterilized by gamma irradiation and the established shadowgraph technique was used to determine the penetrations. The average implantation time was 8.75 years, with a range from 0.2-18.6 years and the average age was 55 years, covering the span 19-73 years. The influence of sterilization procedure, implantation time and patient age are considered. Assessments of the relative magnitudes of wear and non-recoverable deformation are made by different statistical techniques and compared with previously reported data from clinical and laboratory studies.