Femoral cartilage variability affects the accuracy of kinematic alignment and imageless navigation in total knee arthroplasty: A prospective study from the FP-UCBM Knee Study Group

Purpose: Kinematic alignment (KA) and imageless computer-assisted surgery in total knee arthroplasty (TKA) typically compensate for cartilage wear by applying a standard 2-mm adjustment. However, recent concerns have emerged regarding the accuracy of this assumption. This study aimed to assess, for the first time in vivo, the femoral cartilage thickness on the unworn side in a consecutive series of TKA patients. The hypothesis was that significant inter-individual variability exists, thereby challenging the fundamentals of the calipered KA technique and imageless computer-assisted TKA. Methods: This prospective study analysed intraoperative femoral cartilage thickness in 182 patients (189 knees) with end-stage knee osteoarthritis undergoing TKA. Measurements were taken from the unworn distal and posterior femoral condyles. Correlations with demographic factors and coronal plane angular measurements of the lower limb were assessed, and a significance level of p < 0.05 was used for all tests. Results: The mean cartilage thickness was 2.6 ± 0.7 mm, with significant variability among individuals (range: 1.5-5 mm). Intra-individual cartilage thickness within the unworn compartment remained consistent (93% of cases differed by ≤0.5 mm). Men exhibited a thicker mean cartilage (2.8 ± 0.7 mm) compared to women (2.5 ± 0.6 mm) (p = 0.041). Weak correlations were found between cartilage thickness and height, weight, and age (p < 0.05). The posterior lateral condyle in valgus knees was more frequently worn than the posterior medial condyle in varus knees (65.8 vs. 40.3%; p = 0.043). Varus osteoarthritis patterns significantly influenced cartilage wear at the posterior medial femoral condyle, with wear being more prevalent in postero-medial osteoarthritis than in antero-medial or centro-medial osteoarthritis (p = 0.029). Conclusion: Femoral cartilage thickness varies significantly across patients, while remaining nearly uniform within the unworn compartment of the same patient. Relying on the 'one-cartilage-fits-all' paradigm may compromise the accurate restoration of individual anatomy in calipered KA total knee arthroplasty (TKA) and surgical planning in imageless computer-assisted TKA. Level of evidence: Level IV, case series.