Thicker polyethylene inserts (≥ 13 mm) increase the risk for early failure after primary cruciate-retaining total knee arthroplasty (TKA): a single-centre study of 7643 TKAs

The most important finding of the present study was the markedly increased revision rate in primary TKAs with thicker polyethylene inserts. Sensitivity analyses showed that the difference in revision rates was most remarkable in favour of standard PE inserts with Triathlon TKAs. This difference had already been identified in short-term follow-up, and the divergent trend between the survival curves continued throughout the whole follow-up period. The Triathlon TKA was used at our institution until 2013, and the reason for this finding is unclear, as similar surgical techniques were used to balance the knee intraoperatively, irrespective of the TKA design used. The Triathlon TKA has a single-radius design which differs from the PFC Sigma and NexGen designs. Therefore, it may be more sensitive for revision with thicker inserts. There was also a higher revision rate identified in the PFC Sigma subgroup very short term. In the NexGen and the other PFC Sigma time-period subgroups, there was a slight trend for higher revision rates in knees with thicker PE inserts. In the Cox regression model, the 2-year analysis of the PFC Sigma subgroup was divided into time periods of 0 to 0.7 and 0.7 to 2 years because of the non-proportionality of Schoenfeld residuals.

To date, there are only three previous studies that have investigated the effect of PE insert thickness on implant survival. Berend et al. [2] found that thicker PE (16 mm or more) inserts were associated with higher failure rates in TKA. In their study, they included 5997 AGC (Biomet) CR TKAs and used similar inclusion criteria as in the present study. A total of 53 TKAs underwent revision due to mechanical issues during a mean follow-up time of 6.8 years. For thicker inserts, the likelihood ratio for failure was 3.2 times higher and survivorship was 4% lower [2].

Namba et al. [8] investigated the effect of PE insert thickness (15 mm or more) on the outcomes of primary TKA in high-flexion and non-high-flexion TKA designs from different implant manufacturers. They included 64 000 TKAs with both CR- and posterior-stabilized (PS) designs. The mean follow-up was 3.3 years. In their study, the highest revision risk was discovered with the NexGen high-flexion fixed CR design with HR of 9.08 (95% CI 1.58 to 52.32). However, there were only two revisions in the thicker PE insert group and the sample sizes remained relatively small in each subgroup. The non-high-flexion NexGen components did not demonstrate an increased risk for revision with thicker tibial inserts [8].

In contrast, Greco et al. [4] studied clinical outcomes, revision rates and overall implant survival rates for 6698 Vanguard TKAs, where the PE insert was defined as thick when it was 15 mm or more. Consequently, 3.5% of the inserts were considered thick. In their study, the type of PE insert varied, as the study included both standard CR PE inserts, posterior lip inserts and highly conforming anterior-stabilised inserts with significant posterior build-ups. Mean follow-up was 5.6 years. Revision and implant survival rates were similar between groups, and no failures for aseptic loosening or instability were detected. However, there were four revisions in the thick PE group and two of these were due to prosthetic joint infection [4].

It must be noted that in these earlier studies a thick PE insert was defined as between 15 and 16 mm or more, whereas in the present study, a thick PE insert was defined as 13 mm or more. Our rationale for choosing 13 mm as the borderline thickness in this study derives directly from clinical practice. Regardless of the TKA design used, the aim is a polyethylene thickness of between 9 and 10 mm with the bone cuts. In our experience, most surgeons who perform primary TKAs with contemporary designs follow a rather similar surgical strategy. Thus, if one aims at a 9–10 mm polyethylene insert with the tibial cut, and ends up using a 11–12 mm insert, it is still a very small difference clinically. With similar bony cuts, one knee may require a 10 mm PE insert, whereas the other knee may require a 12 mm PE insert to achieve correct ligamentous stability. This is because there are individual differences in the soft tissue tension around the knee. Moreover, there is also variability in the amount of ligamentous stability that different surgeons are willing to accept intraoperatively. Thus, a 2 mm difference in the planned vs chosen polyethylene thickness seldom indicates postoperative problems. However, from a clinical point of view, if one aims at a polyethylene insert thickness of 9–10 mm, but ends up using 13–14 mm or even more, there is usually something abnormal in the knee. This abnormality may be caused by inherited ligamentous laxity, deep tibial resection, gap imbalance or iatrogenic collateral ligament injury [2].

Clinical problems caused using a thicker PE insert are, however, rare. Indeed, as reported in the current study, knees with a thicker PE insert still showed survival rates of over 90% at long-term follow-up, regardless of the TKA design used. Still, the revision risk markedly increased with the use of thicker PE inserts. This finding may indicate that in a small proportion of these knees the surgeon has tried to solve an intraoperative problem using a thick PE insert. Unfortunately, in some cases, increasing the thickness of the PE insert is not the right solution and the intraoperative change to a more constrained TKA design would most probably have been warranted. If a thick PE insert is required to satisfactorily stabilize the knee after the standard bony cuts and ligamentous balancing, one should carefully assess the reasons for this and consider using a more constrained TKA design if there is still something abnormal in ligamentous stability, knee range of motion or gap balance in the knee. Using a constrained TKA design, the stability of the TKA is achieved, and it no longer relies solely on the balance of the ligaments. As a result, the outcome is expected to be better than using a CR component.

We acknowledge a few limitations in the present study. First, we applied different cut points for the “standard” and the “thick” groups compared to previous studies, and this complicated the direct comparison of our results to the earlier literature. However, when the difference in revision rates between the PE insert thickness groups is observed with a lower cut point, it supports the hypothesis that thicker PE inserts lead to a greater risk for revisions. Second, the sample sizes in our TKA-specific subgroups were relatively small, and there were only 35 revisions in the thicker PE insert group. We did not compare the different implant types, for example, CR vs more constrained designs. This might have provided more information on whether the more constrained designs would have had lower revision rates. Furthermore, the reasons for the revisions could have been assessed in more detail.

We also feel the current study has some strengths that serve to increase the generalisability of the results. First, we had a large patient cohort with complete follow-up data from a single high-volume joint replacement centre. Second, our data contain different implant designs and an individual analysis for each of these designs was performed. Third, we only included the non-constrained type of CR designs to minimise residual confounding. In the study by Greco et al. [4], variable PE insert bearing types were included. However, when the bearing types are unevenly distributed in the study groups, it may be the bearing type rather than the thickness of the PE insert that explains the possible differences observed in revision rates.

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