Accurate knowledge of tibiofemoral articular contact kinematics of the knee after total knee arthroplasty (TKA) is important for understanding the intrinsic knee biomechanics and improving the longevity of the components. The objective of this study was to compare the in vivo articular contact kinematics of the knees with end-stage medial osteoarthritis (OA) during a weightbearing, single leg lunge activity before and after a posterior cruciate retaining TKA (CR-TKA) using a dual fluoroscopic imaging technique. We found that the CR-TKA resulted in more posterior contact positions on the tibial surface and a reduced range of motion in the medial and lateral compartments. The distances between medial and lateral contact locations in the CR-TKA knees were statistically larger than the OA knees. The articular contact centers have shifted from medial side of the tibial plateau pre-operatively to the lateral side after operation. This study indicated that the CR-TKA resulted in significant changes in contact kinematics of the knees in both anteroposterior and mediolateral directions. Keywords: TKA; knee osteoarthritis; fluoroscopy; in vivo kinematics; contact kinematics Contemporary total knee arthroplasty (TKA) aims to reduce pain and restore function of the knee with severe osteoarthritis (OA) and other end-stage knee joint diseases.1,2 Recently, many innovative component designs have been introduced to enhance knee joint flexion and improve longevity of the prosthesis. [3][4][5][6][7] Although satisfactory clinical follow up data has been reported, 1,8 there are also data showing persistent difficulties or pain during motion in patients postoperatively. 1,9 Polyethylene wear related problems have been reported to be a leading cause for re-operation. 1,8,[10][11][12] Various factors, such as component design and surgical implantation, have been attributed to post-operative complications. 10,11,13 Abnormal knee kinematics following TKA has been thought to alter intrinsic knee joint biomechanics and lead to suboptimal patient satisfaction. 10 Numerous studies have been conducted to investigate the kinematics of the knee after TKA.3,14-18 Early studies performed investigation of TKA kinematics using motion analysis system to quantify the 6DOF kinematics of the TKA knees and compared the data to normal knees. 19,20 Many studies also used cadaveric specimens to compare the TKA kinematics with that of native knees under controlled conditions. 21,22 In-vivo articular contact forces have been measured using implanted force sensors. 23,24 Recently, fluoroscopic imaging technique has been widely used to investigate medial and lateral condyle motions of the knee after TKAs.18,25-27 While these studies greatly improved our knowledge on TKA biomechanics, the changes on articular contact locations after TKA compared to those measured on the knee before operation have not been reported. This information could provide further insight on how intrinsic biomechanics of the knee is altered after the surgery.Therefore, the obj...