Background

Wear particles are considered to be the major culprit for the aseptic loosening. Their characterization is thus crucial for the understanding of their bioreactivity and contribution to the development of aseptic loosening.

Introduction

Alternative bearings – metal-on-metal and ceramic-on-ceramic bearings have been introduced in the last decade with the aim to diminish wear and, subsequently, aseptic loosening and osteolysis. These bearings were aimed for younger, more active patients. Clinical results which would compare the performance of various alternative and traditional bearings are scarce.

We present a retrospective series of 170 cemented titanium straight-stem femoral components combined with two types of femoral head: cobalt-chromium (CoCr) alloy (114 heads) and alumina ceramic (50 heads). Of the study group, 55 patients (55 stems) had died and six (six stems) were lost to follow-up. At a mean of 13.1 years (3 to 15.3) 26 stems had been revised for aseptic loosening. The mean follow-up time for stable stems was 15.1 years (12.1 to 16.6).

Survival of the stem at 15 years was 75.4% (95% confidence interval (CI) 67.3 to 83.5) with aseptic failure (including radiological failure) as the end-point, irrespective of the nature of the head and the quality of the cement mantle. Survival of the stem at 15 years was 79.1% (95% CI 69.8 to 88.4) and 67.1% (95% CI 51.3 to 82.9) with the CoCr alloy and ceramic heads, respectively. The quality of the cement mantle was graded as a function of stem coverage: stems with complete tip coverage (type 1) had an 84.9% (95% CI 77.6 to 92.2) survival at 15 years, compared with those with a poor tip coverage (type 2) which had a survival of only 22.4% (95% CI 2.4 to 42.4). The poor quality of the cement mantle and the implantation of an alumina head substantially lowered the survival of the stem.

In our opinion, further use of the cemented titanium alloy straight-stem femoral components used in our series is undesirable.

Introduction: Stress shielding has been found to cause cortical thinning and loss of trabecular bone in the proximal femur due to stiff steam and elastic bone. To avoid these problems a prosthesis was developed according to the concept of isoelasticity. Its aim was to ensure the implant and the bone to deform as one unit loading the bone in a more physiological way. Methods: From 1984 Ð1987 we performed 149 total hip replacements using third generation Mathys isoelastic polyacetal stem with stainless-steel heads and polyethylene cementless acetabular cups. Average patient age was 47 years (21–82). Eleven patients (12 hips) has died before revision and fourteen (15 hips) were lost to follow up. Results: Hitherto 69 hips were revised, þve due to infection. The 10 years revision rate for any reason was 68.7%. The remaining 48 patients (53 hips) have been clinically and radiologically assessed or reviewed by a questionnaire. In these patients the average HHS was 80 points. Thirteen among them had failed radiologically, many without a poor subjective result. Conclusions: The performance of this prosthesis was unacceptably poor but nevertheless the concept of isoelasticity should not be completely disregarded. Future trends may reconsider and reintroduce some new materials or implant designs which would be able to achieve the aim of isoelasticity without former problems. Long term total hip loosenings are still all but solved.

We analysed revised Mathys isoelastic polyacetal femoral stems with stainless-steel heads and polyethylene acetabular cups from eight patients in order to differentiate various types of particle of wear debris. Loosening of isoelastic femoral stems is associated with the formation of polyacetal wear particles as well as those of polyethylene and metal. All three types of particle were isolated simultaneously by tissue digestion followed by sucrose gradient centrifugation. Polyacetal particles were either elongated, ranging from 10 to 150 μm in size, or shred-like and up to 100 μm in size. Polyethylene particles were elongated or granules, and were typically submicron or micronsized.

Polyacetal and polyethylene polymer particles were differentiated by the presence of BaSO₄, which is added as a radiopaque agent to polyacetal but not to polyethylene. This was easily detectable by back-scattered SEM analysis and verified by energy dispersive x-ray analysis.

Two types of foreign-body giant cell (FBGC) were recognised in the histological specimens. Extremely large FBGCs with irregular polygonal particles showing an uneven, spotty birefringence in polarised light were ascribed to polyacetal debris. Smaller FBGCs with slender elongated particles shining uniformly brightly in polarisation were related to polyethylene. Mononucleated histiocytes containing both types of particle were also present.

Our findings offer a better understanding of the processes involved in the loosening of polyacetal stems and indicate why the idea of ‘isoelasticity’ proved to be unsuccessful in clinical practice.