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technique.The subjects were 16 knees of 8 consecutive patients (all women, average age 75.9), whounderwent primary mobile-bearing PS TKA (P.F.C. sigma RP-F: Depuy Orthopedics Inc.,Warsaw, IN, USA) between February 2007 and May 2008. All cases were osteoarthritis withvarus deformity. Postoperative radiographs were taken at the position of extension, halfsquattingand deep flexion kneeling 3 month after the surgery, and the degrees of internalrotation of the tibial component was measured by 2D-3D image matching technique. Pre- andpost-operative ROM was recorded.Then, we compared the absolute value and relativemovement of tibial internal rotation between extension, half-squatting and deep flexionkneeling, and evaluated the correlation of the ROM and the internal rotation.The mean preoperative ROM was from -12 to 118 degrees. After the surgery, ROM was from -2to 123 degrees. Clinical scores of all cases were significantly improved after surgery. Internalrotation of tibial component was -6.8 to 9.7 (mean, 1.7) degrees at half-squatting position, and -7.2 to 13.6 (mean; 1.9) degrees at kneeling position. There was no correlation betweenmaximum flexion angle and tibial rotation during flexion. There was significant negativecorrelation between tibial internal rotation angle in extension and tibial rotational movementfrom half-squatting to deep flexion kneeling (R; -0.824, P