Lower limb alignment in the frontal plane: analysis from long standing radiographs and computer tomography scout views: an experimental study

A profound knowledge of physiologic lower limb alignment is essential to understand deformities and to plan surgical correction. The gold standard in radiographic assessment is the long standing radiograph with a forward directed patella. The advantage of computed tomography (CT) is that its cutting-edge image technique can visualize the femur condyles. Study purpose was to determine if the CT-scout view has the potential to replace the standing radiograph. We compared the geometric data obtained from long standing radiograph and CT-scout views both with patella forward position. Furthermore, we developed a method of positioning the lower extremity stable on the CT table, where the femoral condyles became the new orientation criterion. Finally, we evaluated differences in the data ascertainment between the long standing radiograph with patella facing forward and the CT-scout view with the posterior edge of femoral condyles orientated parallel to the radiographic cassette. The geometric data of long standing radiograph and CT-scout views are comparable if the leg is in the same rotational position. We developed a CT positioning jig to adjust the femur condyles parallel to the radiographic cassette. In 80 % of the cases, the deviation was 5A degrees or less. These scout views showed statistically significant differences when compared with data from standing radiograph with a forward centered patella. No evidence was found clearly excluding the possibility of an exclusive use of the CT-scout view for the analysis of the leg geometry. However, advantages of the long standing radiograph became obvious.