A fixed 2 mm adjustment for worn distal femoral cartilage when the cartilage of the unworn compartment is ≥3 mm does not negatively affect outcome scores after kinematically aligned total knee arthroplasty.

Abstract

A goal of kinematic alignment (KA) total knee arthroplasty (TKA) is to align the femoral component to restore the distal and posterior femoral joint lines to prearthritic. To restore the joint line on the worn distal condyle, the resection thickness is adjusted by 2 mm for cartilage loss based on the reported average cartilage thickness on magnetic resonance imaging. It remains unclear whether a 2 mm fixed adjustment is ideal for all patients, especially those with thicker cartilage. This study examined whether a 2 mm fixed adjustment in patients with cartilage thickness ≥3.0 mm negatively affects the Forgotten Joint Score (FJS), Oxford Knee Score (OKS) and Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS JR) compared to those with cartilage thickness <3.0 mm. In 96 KA TKAs with preoperative varus (85%) and valgus (15%) deformities, cartilage thickness was measured using a calibrated photographic method on a medial-lateral bisection of the unworn distal femoral condyle. At 1 year after surgery, equivalence was assessed in FJS, OKS and KOOS JR between patients with cartilage thickness ≥3.0 mm and those with <3.0 mm. The overall average cartilage thickness was 2.8 ± 0.6 mm (range, 1.6-5.0 mm), with 41% having a cartilage thickness ≥3.0 mm. The mean FJS was 78 versus 74 points for those with <3.0 and ≥3.0 mm cartilage, and these results were statistically equivalent (p = 0.0150). Similarly, mean OKS scores of 42 versus 41 points (p = 0.0071), and KOOS JR scores of 80 versus 81 points, were statistically equivalent (p < 0.0001). Although a large percentage of patients had unworn cartilage thickness ≥3 mm, outcome scores were comparble to patients with thickness <3 mm. Hence, surgeons can use a 2 mm fixed adjustment for cartilage loss, even when the cartilage thickness exceeds 3 mm, with a range up to 5.0 mm, without negatively affecting the FJS, OKS or KOOS JR. Level III.