Abstract
First, we need to define “contemporary UHMWPE”. Then we can discuss whether or not this is “the ultimate bearing partner”. The essential criterion for contemporary UHMWPE is intentional crosslinking. There are a number of such acetabular bearing products in the worldwide marketplace. They can differ in several ways including the base resin, the method of consolidation, the method of crosslinking, remelting v. annealing, the packaging and method of sterilisation, and the incorporation of any antioxidant. Thus “contemporary UHMWPE” is not one material, but a family of materials that may have some practical (clinical) differences. There is one essential similarity: substantially reduced wear and osteolysis compared to UHMWPE that is not intentionally crosslinked.
In one literature review, Kurtz et al. reported a weighted-average femoral head penetration rate (wear) for crosslinked acetabular bearings of 0.042 mm/year based on 28 studies (n = 1,503 hips) and 0.137 mm/year for non-crosslinked bearing based on 18 studies (n = 695 hips). The pooled odds ratio for the risk of osteolysis in crosslinked versus conventional liners was 0.13 (95% CI, 0.06–0.27) among studies with minimum 5-year follow-up. Reduction in femoral head penetration or osteolysis risk was not established for large-diameter (>32 mm) femoral heads. In another minimum 5 year follow-up study, Lachiewicz et al. reported that there was no hip with pelvic or femoral osteolysis. They found no association between femoral head size and the linear wear rate, but observed an association between larger (36- and 40-mm) head size with higher volumetric wear rate and higher total volumetric wear.
Leung et al. compared wear, osteolysis incidence, location, and volume on CT scans between 40 hips with non-crosslinked UHMWPE and 36 hips with crosslinked UHMWPE, at a minimum of 5 years. The incidence of osteolysis was statistically greater for patients with non-crosslinked UHMWPE (11/40, 28%) compared to patients with crosslinked UHMWPE (3/36, 8%; P = 0.04). The average lesion volume for hips with non-crosslinked liners (7.5 ± 6.7 cm3) was significantly greater than the average lesion volume for hips implanted with crosslinked liners (1.2 ± 0.1 cm3, P = 0.01).
Bragdon et al. combined a single-centre and two multicentre studies to include 768 primary patients (head size 26–36 mm) with a minimum of 7 years follow-up. Serial plain radiographs showed no osteolysis. The average femoral head penetration rates did not correlate with time in vivo for patients with standard femoral head sizes. There was an indication of higher wear in patients with 36-mm diameter bearings; it was below the threshold for producing osteolysis.
Battenberg et al. quantified the activity of 14 healthy patients with a well-functioning THA at two time periods: early (within 3.5 years of implantation) and late (10–13 postoperative years). Wear was measured on serial radiographs using edge detection-based software. Mean activity decreased by 16% from the early to the late period. Mean gait speed decreased by 9%. Gait speed was 26% slower for patients ≥65 years than for patients <65 years. The mean linear penetration rate decreased by 42% from the first 5 years (early wear rate) to the next 8 years (late wear rate, 5–13 years): 0.043 mm/year to 0.025 mm/year. The greatest patient activity and wear occur during the first 5 years. Walking speed and gait cycles both decreased with aging, resulting in deceasing wear and risk of osteolysis over time.
Crosslinked UHMWPE has consistently demonstrated decreased wear and osteolysis with up to 13 years follow-up. Volumetric wear is increased with larger diameter bearings but appears to be below the osteolysis threshold for most hips, especially considering that patient activity decreases with their aging.
