Posterior Tibial Slope Measurements Using the Anatomic Axis Are Significantly Increased Compared With Those That Use the Mechanical Axis

Purpose: To compare posterior tibial slope (PTS) measurements from standard lateral knee radiographs with measurements from full-length lateral tibia radiographs. Methods: We performed a multicenter, prospective study. Lateral knee and full-length lateral tibia radiographs were obtained for each patient, and PTS was measured. Slope measurements were obtained by measuring the angle between an average of the medial and lateral tibial plateaus and a representative tibial diaphysis line. The proximal anatomic axis was measured on lateral knee radiographs, and both the mechanical axis and anatomic axis were measured on full-length lateral tibia radiographs. The mechanical axis was defined as the center of the plateau to the center of the plafond, and the anatomic axis was defined as the center of the tibial diaphysis. The minimal clinically significant difference was defined a priori as 2 degrees of PTS or greater. Results: A total of 140 patients met the inclusion criteria. The average PTS using the proximal anatomic axis was 11.6 degrees +/- 3.2 degrees on lateral knee radiographs; the PTS measured on full-length lateral tibia radiographs was 9.5 degrees +/- 3.4 degrees using the mechanical axis and 11.8 degrees +/- 3.1 degrees using the anatomic axis. There was a significant difference between the measurements with the mechanical axis and both anatomic axis measurements (P < .01) but no significant difference between the 2 anatomic axis measurement techniques (P = .574). In total, 55% of patients (n = 77) had a 2 degrees or greater difference between the proximal anatomic axis and mechanical axis PTS measurement techniques. Conclusions: There was no significant difference between PTS measurements that used the proximal anatomic axis from lateral knee radiographs and those that used the anatomic axis from full-length lateral tibia radiographs. Thus, lateral knee radiographs are adequate to accurately obtain tibial slope measurements. However, there was a significant difference between PTS measurements that used the anatomic axis and those that used the mechanical axis of the tibia.