We developed a quantitative 3-dimensional (3D) technique for the assessment of changes in the tibial joint line as well as distal and posterior femoral joint lines using a 3D image-matching procedure. In this study, we performed 3D joint line assessment of 20 knees that had undergone total knee arthroplasty. most of the knees exhibited proximal change in the tibial joint line, distal change in the distal femoral joint line, and anterior change in the posterior femoral joint line. There were clear differences in changes in the joint lines between the medial and lateral condyles. The findings indicate that changes in the joint line should be individually assessed for each condyle and that the assessment should include not only the tibial condyle but also the distal and posterior femoral condyles.