Cartilage thickness at the apex of femoral resections in kinematically aligned total knee arthroplasty is close to 2.5 millimeters

Abstract Purpose Determining the correct resection thickness on worn femoral surfaces in kinematically aligned (KA) total knee arthroplasty (TKA) is uncertain since cartilage thickness is unknown. In the past, a 2 mm fixed adjustment has been used, but this may not reflect cartilage thickness at the apices of resections. Our objectives were to determine apex thickness for each resection based on a new method involving MR images, compare this thickness to direct measurements using calibrated photographs, and determine whether an increase in the 2 mm fixed adjustment is a refinement of interest to an already highly successful surgical procedure. Methods MR images from 100 knees without evidence of OA were accessed in the Osteoarthritis Initiative (OAI) database. After aligning images in kinematic planes, the apices of the distal and posterior femoral resections were identified, and the apex cartilage thickness was computed. Results From MR images and calibrated photographs, respectively, apex cartilage thickness was 2.3 mm ± 0.5 mm vs. 2.6 mm ± 0.7 mm for the distal medial resection, 2.3 mm ± 0.4 mm vs. 2.7 mm ± 0.6 mm for the distal lateral resection, 2.4 mm ± 0.5 mm vs. 2.7 mm ± 0.6 mm for the posterior medial resection, and 2.4 mm ± 0.6 mm vs 2.5 mm ± 0.5 mm for the posterior lateral resection. Mean differences were statistically significant ( p ≤ 0.0035) except for the posterior lateral cartilage thickness ( p = 0.1498). Conclusions Apex cartilage thickness for MR image measurements was closer to 2.5 mm than 2 mm for all four resections. Apex cartilage thickness for calibrated photographs was greater than 2.5 mm and was comparable for all four resections. Although unlikely to affect patient‐reported outcome measures, a fixed adjustment for worn cartilage on the femur in KA TKA of 2.5 mm is a refinement of interest to bring more patients closer to the ideal alignment. Level of Evidence N/A.