Comparison of 3D alignment of knee between normal and osteoarthritis subjects by measuring the femoral tibial angle (FTA) using, 3D bone model image fitting technique.

In the field of orthopedic surgery, lower extremity alignment is generally assessed 2D (two dimension) on plain radiographs. However 2D radiographic measurements have some limitations. Therefore in this study we used 3D (three dimension) bone model image fitting technique, to assess and compare the 3D alignment of lower extremity of normal and osteoarthritis subjects by measuring femoral tibial angle (FTA). Frontal AP (antero posterior) and 600 oblique CR (computed radiography) images of 20 subject’s lower limb were taken. After obtaining bi-planner radiographic images, several bony reference points were digitized as follows, 3 points on the counter of the femoral head, 3 points on the counter of the medial and lateral femoral condyle, medial and lateral most points on the proximal joint surface and medial and lateral points on the top of the taler dome. Using above digitized reference points, anatomical coordinate systems of the tibia and femur were established on each of CR images and femoral and tibial X,Y,Z axis were defined. 3D digital models of reference bones already were created earlier by the CT (computed radiography) scans of dried femur and tibia. Then the images of the 3D digital models were projected onto CR images of the subject’s tibia and femur using the projection matrix. Then projected images of reference bones were mathematically superimposed and deformed by an image fitting technique. Thereafter using the inverse projection matrix, 3D digital model of the subject’s lower limb was created. By using this technique we could able to measure the FTA of each subject. Results indicates significant difference (P=0.002) in FTA value between normal and osteoarthritis subjects. We assume the 3D bone model image fitting technique can be used to determine and evaluating treatment for knee osteoarthritis, with more developments in future.