The purpose of this research was to compare the intra-operative and post-operative measured angle in total knee arthroplasty (TKA) and to evaluate the postoperative improvement in the mechanical axis.

Forty TKAs were performed using CAOS between February and May 2004. We measured the mechanical axis in the preoperative orthograms. After inserting the implant, we measured the femoral component angle (FCA), tibial component angle (TCA) and tibial component slope (TCS) using the verification tool in CAOS, on the post-operative orthogram and X-ray of the lateral tibial view, 2 observers measured the FCA, TCA, TCS and mechanical axis.

The mean FCA, TCA and TCS using the verification tool in CAOS, were valgus 0.03±0.85°, valgus 0.49±1.02°, nd 4.26±1.64°. The mean FCA, TCA and TCS by observer 1 were valgus 0.67±1.24°, varus 0.54±1.54°, and 4.57±1.74° and those by observer 2 were valgus 0.48±1.17°, varus 0.52±1.44°, and 4.24±2.83°. There was a positive correlation between the measured angle by respective observers and the angle measured by the CAOS (p<0.05). The mechanical axis by observers 1 and 2 improved from varus 13.96° to valgus 0.04° and from varus 14.12° to valgus 0.08° respectively.

CAOS affords the possibility of precisely placing the femoral and tibial components. The angles measured by the respective observers and in the CAOS were significantly correlated.