There are several methods for evaluating stability of the joint during total knee replacement (TKR). Activities of daily living demand mechanical loading to the knee joint, not only in full extension, but also in mid-flexion. The purpose of this study was to compare the varus-valgus stability throughout flexion in knees treated with either cruciate-retaining or posterior-stabilised TKR, using an intra-operative navigation technique. A total of 34 knees underwent TKR with computer navigation, during which the investigator applied a maximum varus-valgus stress to the knee while steadily moving the leg from full extension to flexion both before and after prosthetic implantation. The femorotibial angle was measured simultaneously by the navigation system at every 10° throughout the range of movement. It was found that posterior-stabilised knees had more varus-valgus laxity than cruciate-retaining knees at all angles examined, and the differences were statistically significant at 10° (p = 0.0093), 20° (p = 0.0098) and 30° of flexion (p = 0.0252).

In total knee arthroplasty (TKA), the patella is significantly associated with range of motion and gait performance. Currently, no highly accurate methods are available that can measure the 3D in vivo behavior of the TKA patellar component, as the component is made of x-ray-permeable ultra-high molecular weight polyethylene. Previously, we developed a computer simulation that matches CT scan and unidirectional radiographic images using image correlations, and applied it to kinematic studies of natural and TKA knees. The examination of the measurement accuracy for the patellar bone of a fresh-frozen pig knee joint yielded a root mean square error of 0.2 mm in translation and 0.2˚in rotation. In this study, we recruited four patients who had a TKA and investigated 3D movements of the patellar component during squatting. We could visualize the patellar component using the position of the holes drilled for the component peg, and estimated and visualized the contact points between the patellar and femoral components. The principles and the utility of the simulation method are reported. This analytical method is useful for evaluating the pathologies and post-surgical conditions of the knee and other joints. Total knee arthroplasty (TKA) is a reliable surgical procedure for reconstructing severely impaired knee joints. TKA can improve pain relief, functional mobility, weight-bearing capability, and patient quality of life.1 The widespread use of TKA promoted studies on TKA kinematics, particularly of the femorotibial joint.2,3 TKA knee kinematics, including range of motion (ROM) and physical performance, are also influenced by the biomechanical properties of the patella. Surgeons sometimes report post-TKA complications involving the patellofemoral articulation, for example, anterior knee pain, patellofemoral impingement and instability.4,5 Therefore, it is important to examine whether the implanted total knee prosthesis is functioning as designed. However, conventional imaging techniques cannot analyze the biomechanical characteristics of the patellofemoral joint accurately enough because of the small volume of patella bone and the overlapping of patellar and femoral silhouettes on scan images. In TKA, the patella is commonly resurfaced, which further reduces its volume, and the radiopaque metal femoral component is often superimposed on the patellar component on 2D images. In addition, the patellar component is made of radiolucent ultra-high molecular weight polyethylene. Therefore, it is impossible to determine radiographically the external contour of the patellar component precisely. Because of these technical limitations, no methods have been established to track the dynamic in vivo trajectory of the patella.Previously, we developed a unique computerassisted image matching procedure to analyze the kinematics of natural and TKA knees by applying an image window-based analytical method to serial unidirectional x-ray scans. [6][7][8][9][10] In this study, we aimed to visualize the patellar component and iden...

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