Poor results from the isoelastic total hip replacement

Background To overcome the mismatch between a stiff stem and the more elastic bone, the concept of isoelasticity was introduced in the 1970s. This concept was based on the assumption that the implant and the bone should deform as one unit to avoid stress shielding. The Robert Mathys (RM) cementless total hip replacement (THR) was one of the earliest isoelastic designs.

Patients and methods From 1984 to 1987, we performed 149 total hip replacements in 135 patients (92 women, mean age 47 (21-72) years) using third-gen-eration Mathys isoelastic polyacetal stem with stain-less-steel heads and polyethylene cementless acetabular cups. 11 patients died before revision or before the examination for this study, and 14 were lost to follow-up. Average follow-up time for the remaining 110 patients was 15 (14-17) years.

Results To date, 69 hips (64 patients) have been revised, 5 due to infection. The 10-year survival rate for any reason was 70% (66-74). In the 46 remaining patients (53 hips), the average HHS was 80 (39-100) points. 13 of these were regarded as radiographic failures, with an average HHS of 75 points.

Interpretation The performance of this prosthesis was unacceptably poor. Higher debris production and poor primary fixation are believed to be the main reason for the high failure rate.