Abstract
Introduction
Wear of the ultra-high molecular weight polyethylene (UHWMPE) component and the subsequent aseptic loosening remains a primary reason for late revision of total knee replacements (TKRs).[1] While improved measurement techniques have provided more quantitative information on the wear of surgically retrieved inserts, it is not well understood how observed damage patterns translate to volume loss of polyethylene in vivo. The overall purpose of this study is to investigate the relationship of damage patterns and volume loss at the articular surface of total knee replacements. We hypothesize that damage patterns are reliable predictors of volume loss.
Methods
Two different investigators independently analyzed damage patterns and volume loss on 43 revision- and 21 postmortem-retrieved MG II (Zimmer Inc.) tibial UHMWPE components. Areas of damage patterns on the articular surfaces were outlined with a video microscope (SmartScope, OGP) and were separated into four spatially exclusive categories (Fig. 1): delamination, pitting, striations and polishing. Articular surfaces were digitized with a low-incidence laser coordinate measuring machine (SmartScope, OGP). Autonomous reconstruction, a previously described and validated method,[2] calculated volume loss on the medial and lateral sides of each component. To investigate the predictability of volume loss using observed patterns, stepwise linear regression models were rendered in PASW Statistics 18 (SPSS Inc).
Results
Components with total volume loss higher than 300 mm3 exhibited the jagged and flaky surface topography characteristic of severe delamination. Excluding delaminated components, the population's total wear rate (±95% CI) was 12.9 ± 5.97 mm3/year (Pearson's r = 0.527, p < 0.001), which accounted for volume loss due to creep (Figure 2). The wear rates on the medial side (6.56 ± 3.91 mm3/year, Pearson's r = 0.434, p = 0.002) and on the lateral side (6.32 ± 3.44 mm3/year, Pearson'sr = 0.467, p < 0.001) were not significantly different (p = 0.926). Linear regression models that included delaminated components revealed that delaminated and striated areas were significant predictors of volume (Figure 3 - Table 1). Pitted area was also a significant factor on the total part, but fell out of the medial and lateral models. When delaminated components were excluded, volume loss was similarly explained by striated and pitted areas on the medial side and total part. However, the adjusted R2 of these models was low − 0.177 and 0.166, respectively.
Discussion
We found that damage patterns were not reliable surrogates for material volume loss. Other than delamination, area of striated patterns best predicted volume loss. However, the large reduction in adjusted R2 of the linear regression model indicates that delaminated areas were essential to explaining variations in the volume loss. While other groups have reported striations on retrievals [3], this damage pattern remains widely unrecognized in retrieval and knee simulator studies, with the mechanism poorly understood.
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