Thicker polyethylene inserts for varus deformity versus thinner inserts for flexion contracture in total knee arthroplasty: a retrospective comparative study

BACKGROUND: The success of total knee arthroplasty (TKA) depends on achieving precise mechanical alignment and stable soft tissue balance, with the thickness of the final polyethylene (PE) insert serving as a critical component. However, the impact of preoperative factors on PE thickness remains unclear. In this study, we investigated the effects of preoperative knee alignment and clinical deformities on PE thickness. MATERIALS AND METHODS: The medical records of patients who underwent primary TKA between August 2024 and July 2025 were retrospectively evaluated. Preoperative radiographic parameters, including the hip-knee-ankle (HKA) angle, medial proximal tibial angle, lateral distal femoral angle (LDFA), joint-line convergence angle, weight-bearing line ratio (WBLR), and posterior tibial slope, were measured, along with the flexion contracture (FC) as a clinical factor. Intra-observer reliability was assessed using intraclass correlation coefficients (ICC). Linear regression analyses were performed to evaluate the effects of these preoperative factors on PE thickness. RESULTS: This study included 233 knees (158 patients). The intra-observer reliability of the radiographic parameters was excellent (ICC > 0.9). Multivariable linear regression revealed that preoperative HKA angle (β = 0.033, p < 0.001), WBLR (β = -0.008, p < 0.001), and LDFA (β = 0.065, p = 0.003) were significantly associated with final PE thickness. Preoperative FC demonstrated a significant negative correlation (β = -0.027, p = 0.011). In a subgroup analysis of knees with minimal FC, the mean PE thickness increased from 10.37 mm in mild varus to 12.75 mm in extreme varus. CONCLUSIONS: Preoperative HKA angle and FC were identified as significant predictors of PE insert thickness in TKA. Specifically, greater varus deformity resulted in a thicker PE insert, whereas more severe FC led to a thinner insert. Clinically, surgeons must anticipate gap expansion after medial release in varus knees, while accommodating the tight extension space in the FC. Level of evidence Level III, retrospective comparative study.