Effect of foot rotation on the mechanical axis and correlation between knee and whole leg radiographs

The purposes of this study were (1) to evaluate the foot rotational effects on local and whole leg alignment and (2) to confirm the correlation between local and whole leg alignment. The hypotheses of this study were that (1) the alignment would become varus if the rotation of the foot changes from internal to external rotation, and (2) there would be some correlation between local and whole leg radiographs, and local knee radiographs could then be used indirectly for the assessment of whole leg alignment in patients with bilateral medial compartment knee osteoarthritis. A total of 80 lower limbs with genu varum of patients who complained of medial knee pain were examined. The standing anterior-posterior view of whole leg radiographs was taken in the four foot positions, and a custom-made foot plate was used for the attainment of accurate radiographs: feet straight ahead with foot contact at the medial side (R: routine), feet straight ahead at shoulder width (N: neutral), 30A degrees external rotated (ER) and 15A degrees internal rotated (IR) position. In order to obtain a local radiograph of the knee, we took only whole leg radiographs and selected the area of interest on the whole leg radiograph. We evaluated the total width of the tibia plateau (Total), the length of the weight-bearing line, the ratio of weight-bearing line/Total and femorotibial angle (FTA). The absolute value of weight-bearing line was shifted laterally in the 30A degrees ER position and shifted medially in the 15A degrees IR position compared to the neutral position (1.8 mm lateral and 0.2 mm medial in the WLR; 3.5 mm lateral and 3 mm medial in the local radiograph). Significant statistical differences were observed in the local knee weight-bearing line; however, no significant statistical differences were observed in the weight-bearing line of the whole leg radiograph (n.s.). Results of the % (weight-bearing line/Total) were similar to those of weight-bearing line. The FTA of the local radiograph showed statistical differences, and it showed more valgus in the 30A degrees ER position. In the correlation analysis between whole leg radiograph and local knee radiograph, moderate correlation (correlation coefficient = 0.67) was observed; however, significant statistical differences were observed in the comparison of weight-bearing line and % weight-bearing line/Total (p < 0.01 and < 0.01, respectively) between local knee and whole leg radiograph. Foot position of ER could show less varus alignment and the reverse could occur in the IR position, compared to the neutral foot position. The severity of varus alignment could be underestimated in the local radiograph, compared with that of whole leg radiograph. Cohort study (diagnosis), Level II.