Surface Damage Is Not Reduced With Highly Crosslinked Polyethylene Tibial Inserts at Short-term.

BACKGROUND

Highly crosslinked ultrahigh-molecular-weight polyethylene (XLPE) has been shown to reduce wear in hip arthroplasty, but the advantages over conventional polyethylene (PE) in total knee arthroplasty (TKA), if any, remain unclear.

QUESTIONS/PURPOSES: Do differences exist in (1) surface damage as measured by damage score and percent area affected; and (2) extent and location of dimensional changes between XLPE and conventional PE observed on retrieved TKA tibial inserts?

METHODS

In this study of components retrieved at the time of revision surgery, we matched 44 XLPE to 44 conventional PE inserts from four manufacturers; the matching approach considered implant design (exact match), insert size (exact match), and length of implantation (matched ± 6 months). Surface damage on the articular surfaces was subjectively graded and digitally mapped to determine the percent damaged area of each damage mode. Three-dimensional changes that had occurred as a result of implantation were determined by comparing laser scans of the retrieved inserts with size-matched pristine inserts.

RESULTS

The differences of damage scores and percent damaged areas between the matched XLPE and conventional PE inserts were not large enough to be clinically significant with low corresponding levels of statistical significance (scores: 42 ± 13; 95% confidence interval [CI], 38-46 versus 45 ± 13; 95% CI, 41-49; p = 0.4; percent areas: 54% ± 38%; 95% CI, 44%-64% versus 54% ± 32%; 95% CI, 42%-65%; p = 0.9). However, XLPE inserts showed greater articular surface dimensional changes with high significance (root mean square of the distance: 0.16 ± 0.06 mm; 95% CI, 0.13-0.18 mm versus 0.14 ± 0.05 mm; 95% CI, 0.11-0.16 mm; p = 0.03). Within the same design, deviation patterns were consistent between the two materials; however, as expected, the location of the dimensional changes differed among designs: the negative deviations on the plateaus were centrally located in Zimmer PS inserts, were located on the perimeter in Smith & Nephew PS inserts, and were across the entire surface in DePuy PS inserts.

CONCLUSIONS

We found no difference in surface damage between matched XLPE and conventional PE inserts of the same designs. However, increased dimensional changes in TKAs with XLPE may reflect larger contact areas and potentially explain improved performance of XLPE in published simulator studies.

CLINICAL RELEVANCE: The lack of meaningful differences between the two polyethylene materials suggests caution in adopting a new, more expensive bearing material over another material that has a long track record of excellent behavior. A possible advantage is the greater dimensional changes, which could be the result of the lower creep resistance of XLPE, but this advantage awaits long-term results.