Fast fix or false hope? Polyethylene exchange in unstable primary unconstrained total knee arthroplasty.

Aims: Instability is a leading cause of revision after primary total knee arthroplasty (TKA). Isolated polyethylene exchange (IPE), typically involving increased insert thickness, is considered a fast and low-risk treatment option. However, evidence supporting its effectiveness remains limited. This study assesses outcomes and risk factors of IPE for secondary instability in unconstrained primary TKAs using data from the German Arthroplasty Registry (EPRD). Methods: Patients with unconstrained primary TKA and subsequent IPE due to presumed instability were included. Polyethylene height increase served as a surrogate for instability severity. Cases with infection were excluded. Secondary patellar resurfacing (SPR) was analyzed as a potential influencing factor. Multivariate Cox regression and time-dependent analyses were used to identify predictors for re-revision, defined as any subsequent revision procedure. Results: The study included 1,890 patients. IPE was performed on at a mean of 639 days (SD 521; 1 to 3343) after primary TKA, with a mean height increase of 3 mm (SD 2.2). The overall re-revision rate was 13.5%; 2.4% were due to periprosthetic joint infection. SPR was associated with lower revision risk (hazard ratio (HR) 0.50; 95% CI 0.37 to 0.68; p < 0.001). IPE one and two years after the index operation was associated with a decreased risk of subsequent revision (HR 0.33, 95% CI 0.20 to 0.55; HR 0.36, 95% CI 0.21 to 0.60). Polyethylene height increase > 3 mm was not associated with worse outcomes (< 3 mm HR 0.87, 95% CI 0.65 to 1.17; p = 0.358). Conclusion: IPE can be a viable treatment for mild secondary instability following primary TKA, particularly with careful patient selection. SPR appears to improve implant survival and should be considered in cases with patellar pathology. Early-onset instability warrants critical evaluation, as it may indicate deeper underlying issues requiring component revision. These findings highlight the importance of an individualized surgical approach and underscore the need for thorough preoperative counselling to align treatment strategy with patient-specific risk factors and expectations.