Assessment of relative rotational alignment in total knee arthroplasty: usefulness of the modified Eckhoff method

In 1995, Eckhoff and colleagues demonstrated a new method for assessing relative rotational alignment of the femoral and tibial components in total knee arthroplasty (TKA). We studied the usefulness of a modified Eckhoff method. Five knee prostheses (Natural Knee, Deltafit, and three different sizes of Press-Fit Condylar) were used in an in-vitro study. A series of lateral radiographs were taken with the femoral component rotated in 1 degrees increments from 15 degrees internal rotation to 15 degrees external rotation. The rotation of the implant was calculated based on the geometric relationship of pegs that were present symmetrically on the medial and lateral sides of the femoral components in all prostheses. In a study of the clinical applications, two independent observers measured the relative rotation of 18 knees in 16 patients after TKA, using this method. In the in-vitro study, the measurement error of rotation averaged 0.27 degrees, and the maximum error was 0.96 degrees. Clinically, the mean difference between the measurements of the two observers in each knee was 0.64 degrees; there was a high correlation between the values measured by the two observers (r = 0.89; P<0.0001). This is a useful method for determining the relative rotation in TKA accurately and reproducibly.