Digital imaging in lower limb bone deformities-standards and new perspectives

The gold standard for radiographic assessment and correction planning of lower limb deformities is the long standing radiograph (LSR). Additional computer tomography (CT) can measure exact length and torsion. For correct positioning in LSR the patella should be centered on the femoral condyles (knee forward position-KFP). The flexion extension axis of the knee (KFEA) is mainly defined by the shape of the femoral condyles represented by their dorsal tangent. In 30 CT scans the position of the patella and its relation to the dorsal tangent to the femoral condyles were retrospectively analyzed. With a custom-made positioning apparatus to control the leg rotation and load exactly for each limb separately we analyzed 38 CT scans. 20 digital LRS were retrospectively compared with the correlating CT-data. In CT scans with KFP the KFEA differed 11° from the frontal plane (p < 0.05). With the special positioning apparatus it was possible to control the leg rotation and align the KFEA in the frontal plane with an accuracy of 82%. There was no significant difference of axis and joint angles between the LSR and CT scanogram (p < 0.05). The aberration of the KFEA in KFP has to be to taken into consideration for analysis and correction planning. The alignment of the KFEA parallel to the frontal plane can be achieved by optimized CT scans and a special positioning fixture. In certain cases the LRS can be replaced by a CT scanogram.