Recutting the distal femur to increase maximal knee extension during TKA causes coronal plane laxity in mid-flexion.

BACKGROUND: The aim of this study was to quantify the effects of distal femoral cut height on maximal knee extension and coronal plane knee laxity.

METHODS: Seven fresh-frozen cadaver legs from hip-to-toe underwent a posterior stabilized TKA using a measured resection technique with a computer navigation system equipped with a robotic cutting guide. After the initial femoral resections were performed, the posterior joint capsule was sutured until a 10° flexion contracture was obtained with the trial components in place. Two distal femoral recuts of +2mm each were then subsequently made and the trials were reinserted. The navigation system was used to measure the maximum extension angle achieved and overall coronal plane laxity [in degrees] at maximum extension, 30°, 60° and 90° of flexion, when applying a standardized varus/valgus load of 9.8 [Nm] across the knee.

RESULTS: For a 10 degree flexion contracture, performing the first distal recut of +2mm increased overall coronal plane laxity by approximately 4.0° at 30° of flexion (p=0.002) and 1.9° at 60° of flexion (p=0.126). Performing the second +2mm recut of the distal femur increased mid-flexion laxity by 6.4° (p<0.0001) at 30° and 4.0° at 60° of flexion (p=0.01), compared to the 9 mm baseline resection (control). Maximum knee extension increased from 10° of flexion to 6.4° (± 2.5° SD, p<0.005) and to 1.4° (± 1.8° SD, p<0.001) of flexion with each 2mm recut of the distal femur.

CONCLUSIONS: Recutting the distal femur not only increases the maximum knee extension achieved but also increases coronal plane laxity in midflexion.