Margin of error in alignment: a study undertaken when converting from conventional to computer-assisted total knee arthroplasty.

We analyzed alignment of 300 knees, divided equally into groups according to total knee arthroplasty technique: Conventional, CAS-Early (computer-assisted surgery during the surgeon's learning curve), and CAS-Late (after experience with CAS). With the margin of error for alignment of the mechanical axis and femoral and tibial components each within ±3°, the Conventional group's success rate was only 66%; but the rate for the CAS-Early group was 94%. The CAS-Late group's rate was 100%, with no outliers; moreover, the margin of error was reduced to within ±2° in 92% of patients and within ±1° in 52%. For the Conventional group's success rate to be 100%, the margin of error would have to be statistically relaxed to an unacceptable ±8°. Computed-assisted surgery improves alignment even early in the learning curve.