Recently mobile-bearing total knee arthroplasty (TKA) has become more popular. However, the advantages of mobile bearing (MB) PS TKA still remain unclear especially from a kinematic point of view. The objective of this study was to investigate the difference and advantage in kinematics of mobile baring PS TKA compared with fixed bearing (FB) PS TKA. Femorotibial nearest positions for 20 subjects (20 knees), 10 knees implanted with NexGen Legacy flex with mobile bearing PS TKA, and 10 knees implanted with NexGen Legacy flex with fixed bearing PS TKA were analyzed using the sagittal plane fluoroscopic images. All the knees were implanted by a single surgeon. All the subjects performed weight bearing deep knee bending motion. The average range of motion between femoral component and tibial component was 119±18 in MB and 122±10 in FB. The axial rotation of the femoral component was 11.8±6.2 in MB and 11.8±4.9 in FB. There was no significant difference both in range of motion and axial rotation between BM and FB. The kinematic pathway pattern was externally rotated due to a lateral pivot pattern in both MB and FB. In four subjects, more than 12°axial rotation was observed in knees implanted with FB TKA which allows only 12°axial rotation.

The data in this study demonstrates that there was no significant difference in kinematics of weight bearing deep knee bending motion. The advantage of MB is allowance of axial rotation which restricted until 12 °in FB NexGen Legacy flex PS TKA.