EFFECT OF CUP POSITION AND ORIENTATION ON LONG-TERM LINEAR AND VOLUMETRIC WEAR AFTER HIP REPLACEMENT

Abstract

Introduction: The biological activity of PE-particles released due to wear is an established risk-factor for osteolysis and loosening after Total Hip Arthroplasty (THA). Cup position and orientation might have an effect on the risk of impingement and wear, thus contribute to the risk of aseptic loosening in the long-term what should be studied.

Methods: Between 1984 and 1987 a total of 149 cemented total hips (Müller all-poly cup, Müller straight stem, 32 mm head) have been implanted. All implants had a standardised clinical and radiological follow-up. The pre- and postoperative centre of rotation of the hip and the orientation of the cup were determined. Migration, linear wear and direction of wear were measured twice with standard Methods: and the digital EBRA method. Wear-volume was calculated, taking direction of wear and cup orientation into account. Radiographs were analysed for progressive osteolysis and loosening.

Results: 1 patient was lost to follow-up, 47 had died, 7 had been revised before 10 years follow-up. 18 patients had a missing or poor final radiograph, leaving 75 hips for long-term analysis. 41 were in male patients, mean age was 66.2 (+/− 11.0) years, mean follow-up 15.4 (+/−4.1) years. Mean inclination was 40.7° (+/− 7.1), mean anteversion was 14.8° (+/−8.4) And the mean cup positioning was 3.8 mm (+/− 4.3) medial and 5.3 mm (+/− 3.5) cranial.

Osteolysis was found in 36 cups, 18 of them have been revised.

The average linear wear was 1.1 (+/− 0.9) mm, the average wear volume 798.7 (+/−622.3) mm3, the linear wear rate 0.07 (+/−0.06) mm/year and the volumetric wear rate 54.5 (+/− 43.2) mm3/year.

Younger patients had increased linear wear rates (p=0.035). Osteolysis of the cup, cup migration and cup revision were correlated with linear and volumetric wear (all p=< 0.001).

There was no correlation of the cup position, inclination and anteversion with osteolysis, loosening and any of the wear parameters. Volume calculation did not provide further information.

Conclusion: We found a strong correlation between wear and loosening of PE cups, but cup position and orientation did not affect osteolysis and loosening. Thus in contrast to hard-hard bearings polyethylene is a forgiving bearing surface and improvement of the cup orientation (e.g. due to navigation) will not result in increased cup survival, as long as extreme positioning errors are avoided.