Patellofemoral kinematics in mobile-bearing and fixed-bearing posterior stabilised total knee replacements: a cadaveric study

The aim was to compare the patellar kinematics in the normal knee, fixed-bearing (FB) and mobile-bearing total knee replacement (MB-TKR). The hypothesis that a mobile-bearing TKR has a more natural patellar movement was tested. Computer navigation was used to track the patella in nine whole lower extremities in the natural knee and in the same knee with a posterior stabilised FB-TKR and a posterior stabilised MB-TKR from 0A degrees to 90A degrees flexion. The form and position of the trochlea in the natural knee and the patellar groove of the TKR femoral component was also analysed. There were no differences between the FB and MB-TKRs. But the patella in the TKRs at flexion angles of more than 50A degrees had a more medial tilt compared to the natural knee. The patella of the natural knee tended to rotate externally with flexion, this was not seen in both TKR types. There were no significant differences in absolute mediolateral translation nor in translation relative to the patellar groove. During flexion, the patella lost contact with its groove earlier in the TKRs. The radius of the patellar groove of the femoral component was larger. The groove extended more superiorly and less far posteriorly, it was also positioned further laterally compared to the natural knee. There are subtle kinematic differences in patellar tracking between the natural knee and a TKR presumably due to differences in the shape and position of the patellar groove. There are no kinematic differences in patellar movement between the FB- and MB-TKR.