Polyethylene thickness is a risk factor for wear necessitating insert exchange

The results indicate that PE thickness is indeed associated with the risk of insert exchange. In addition to PE thickness diagnosis, BMI and weight also increase the risk of insert exchange. Apart from one case of instability with loosening of the femoral component, there were no re-revisions for the patients treated with insert exchange at a mean 4.8 years follow-up (range 0.1–7.4 years).

Insert exchange is the ultimate consequence of (linear) PE wear, which can be monitored on X-rays. With decreasing in vivo PE thickness the contact stress on the PE increases, which may lead to fatigue and subsequent fracture of the PE [5]. For this reason the risk of revision for wear could be higher in TKA with thin inserts. This effect may be amplified in modular TKA because modularity offers the advantage of exchanging only the PE insert and thus preserving the tibial and femoral component. If the ideal window for PE insert exchange is missed the tibial and femoral components could be damaged necessitating a full revision [16].

Although thickness of PE as a risk factor for revision for wear has been identified before [5‐7], our study is the first to determine the magnitude of the clinical effect while correcting for known confounders (age, gender, weight, BMI, diagnosis, KSS score and alignment). Diagnosis, BMI and weight are also associated with an increased risk for insert exchange. OA patients have an almost five-fold higher risk for insert revision compared to RA patients. However, we should consider the fact that the RA patients, who are referred to our academic centre, are severe cases with involvement of multiple joints restricting their mobility. The decreased risk of insert exchange for RA patients may therefore be a result of decreased mobility rather than the result of RA alone [17]. Furthermore, the RA patients in this study are from an era in which aggressive treatment strategies with Disease-modifying Antirheumatic Drugs and biological response modifiers were being developed, so the results may not be fully translated to present day RA patients. In fact, the Australian National Joint Replacement Registry shows a smaller reduction in revision rates for RA [18] (0.55 revisions per 100 observed component years ) compared to OA (0.77 revision per 100 observed component years ); OA: RA = 0.77:0.55 = 1.4 [95%CI 1.14–1.70]. Therefore present day OA patients have a higher risk of insert exchange compared to RA which is likely to be in the magnitude of 1.4 rather than 5.

For every point increase in BMI the risk for insert revision increases 1.40 times and for every kilogram gain in weight the risk increases 1.14 times. Whereas these risk factors have been described before, the magnitude of their clinical effect had not been previously determined [3].

Several authors have identified postoperative alignment in addition to difference in tibial and femoral component size to be risk factors for PE wear and subsequent insert revision [7, 12, 19]. Although we found indications for these risk factors—difference in component size HR 2.34 and FTA angle HR 1.30 (see Table 3)—there was no statistical significance. It is possible, however, that a cohort of 84 TKA is not large enough to detect them.

The results of the insert exchange cases are comparable to recently published larger patient series [20, 21]. Apart from one case of instability with loosening of the femoral component, there were no re-revisions for the patients treated with a insert exchange at a mean 4.8 years follow-up (range 0.1–7.4 years). The high revision rate for PE failure in the Interax TKA, used in our study, has been previously described by Sugimoto et al [12]. Sterilisation method (gamma radiation in air) in addition to implant design features are related to this high failure rate [12, 19]. The large proportion of worn lateral inserts may be due to a combination of implant design features and the relatively large proportion of pre-operative valgus deformities in RA patients in both knee and hindfoot of those patients [12]. One case was lost to follow-up. When assuming that this patient had undergone a revision for PE wear in a different hospital—a worst case analysis—the increased risk of revision for the thin inserts compared to the thick inserts would even have been more substantial: 13 times increased versus 11 times increased. Therefore this case of lost to follow-up does not alter the conclusions.

An issue with PE thickness is the difference in actual minimal PE thickness and the size reported by the manufacturer. The real minimal PE thickness can be up to 4 mm thinner than the thickness reported by the manufacturer [22]. For this reason orthopaedic surgeons could still be using inserts of 6 mm or less without being aware of it. Regarding the inserts used in our study, we verified that minimal PE thickness of the Interax corresponds with the size reported by the manufacturer by measuring the minimal PE thickness with a calliper in a small sample of inserts.

A recent study has shown the catastrophic consequences of increased PE wear, leading to a high failure rate in a contemporary TKA design [23, 24]. Therefore, the problems of excessive wear are not only a problem of the past [25, 26]. Knowledge of risk factors for wear and the magnitude of their clinical effect is thus imperative. The recent GRADE guidelines are developed for grading the strength of evidence [13]. According to those guidelines a very strong association is defined as a relative risk of 5 or more. There should also be evidence of a dose response gradient and the association should remain after correction for confounders [13]. According to the GRADE guidelines, the evidence for PE thickness as a risk factor for wear and liner exchange may be classified as very strong, because the relative risk is greater than 5 (HR = 11), there is evidence of a dose response gradient and the association remains after correcting for confounders [13]. Choosing the right minimal insert thickness is an elegant and cost-effective intraoperative measure to achieve a ten-fold reduction in the amount of revision operations for wear, which outweighs the required additional bone resection. In conclusion we therefore advise against the use of thin PE inserts in modular TKA and recommend PE inserts with a minimal 8-mm thickness.