Linear influence of distal femur osteotomy on the Q-angle: one degree of varization alters the Q-angle by one degree

Purpose The effect of a distal femur varization osteotomy on patellofemoral biomechanics in genu valgum is unknown. The purpose of this study was to quantify the influence of frontal leg axis correction on the Q-angle with a novel three-dimensional (3-D) measurement method. Methods 3-D surface models of ten lower extremities were generated using patient computed tomography (CT) data. The preoperative 3-D Q-angle was measured using a novel defined and validated 3-D measurement method. Biplanar supracondylar osteotomies were simulated with different degrees of varus correction (from 1° to 15°) in one-degree steps beginning from the preoperative valgus deformity, resulting in a total of 150 simulations. Additionally, mechanical leg axis and 3-D Q-angle measurements were performed on 3-D surface models of the postoperative CT scans of the same individuals. Further, pre- and postoperative TT–TG distance was measured. Results Mean preoperative Q-angle was 15.8 ± 3.9° (range 10°–21.4°) with a mean preoperative mechanical leg axis of 6.5° ± 2.4 valgus (range 3.8°–11.6° valgus). The Q-angle changed linearly 0.9 ± 0° per 1° of varization. No difference was detected between simulated 3-D Q-angles and effectively corrected postoperative values (n.s.). TT–TG distance changed irregularly and minimally, and with no correlation to the degree of varization. Conclusion Distal femur varization osteotomy has a linear effect on the Q-angle with a change of 1° per 1° of varization. The difference in TT–TG distance was mainly due to an unintentional rotational component implemented during surgery.