[Early failure of the polyethylene liner in the ABG I total hip prosthesis]

The polyethylene liner has appeared to be a weak point in total hip arthroplasty. An early wear-through or a fracture of the polyethylene liner have been referred to as a catastrophic failure. The aim of the present study is to report our experience with this rare complication in ABG I hip prostheses.

Between September 1994 and January 2000 a total of 506 ABG I prostheses were implanted at our clinic. Due to wear-related complications, revision arthroplasty had to be performed in 90 hips in 23 male and 63 female patients by January 2002. Extensive titanium metallosis associated with polyethylene failure was found in 11 hips of 11 female patients. The remaining 56 hips of 52 female patients served as a control group.

The characteristics recorded in every patient's included age, type of patient, sex, height, weight, ESR, CRP, diagnosis, Harris hip score before revision, cup size, abduction angle of the cup, time between the primary and revision surgery and implant stability. Student's t-test, the Mann-Whitney U-test and Pearson's chi 2 test were used to evaluate the results.

The complete failure of ABG I prostheses occurred in 2.17% of the replaced hips (11/506). A fracture or complete wear-through were the causes of failure in three (3/11, 27%) and eight (8/11, 73%) hips, respectively. The average cup size in hips without metallosis was 48.9 mm (range, 44-58; SD, 3.1) and that in hips with metallosis was 46.5 mm (range, 46-48; SD, 0.9). This difference was highly significant (t-test, p = 0.00002; U-test, p = 0.014). In the hips with metallosis, the average thickness of the polyethylene liner was 5.17 mm (range, 4.9-5.9; SD, 0.45) and the Harris hip scores before revision were significantly lower than in the hips without metallosis (59 versus 70.3 points; t-test, p = 0.023; U-test, p = 0.044). There were no other significant differences between the hips with early catastrophic failure of the polyethylene liner and those without it.

Several studies have shown an indirect relationship between polyethylene thickness and its wear. Berry et al. reported catastrophic failure in hip prostheses with a polyethylene thickness of less than 5 mm; in our study, the three failed polyethylene liners were 5.9 mm in thickness.

This is the first report on a wear-through of the polyethylene liner in ABG I cups. It is suggested that a thin polyethylene liner, design characteristics and probably also poor polyethylene quality are crucial factors responsible for rapid wear in ABG I cup. The highest risk is associated with the combination of a 46 mm cup and a 28 mm head. Patients who have prostheses of this size should be checked up more frequently.