Magnetic resonance imaging of in vivo patellofemoral kinematics after total knee arthroplasty.

Simulated partial weight bearing during magnetic resonance imaging of the knee was used to measure patellar tilt, medial-lateral patellar shift, and patellofemoral contact area in three groups of subjects; patients with posterior cruciate retaining (PCR) TKA, patients with bicruciate substituting (BCS) TKA, and healthy controls. Contact stress was also calculated based on the contact area and body weight-based estimates of contact force. Contact stress was significantly (p<0.05) higher in PCR knees (2.5+/-3.0 MPa) than in BCS knees (0.2+/-0.1 MPa) when knees were fully extended, but this difference was not significant (3.7+/-3.5 MPa for PCR knees vs. 1.4+/-1.9 MPa for BCS knees; p>0.05) in early flexion. The results also indicate that patellar tilt (normal=2.4 degrees +/-4.8 degrees, BCS=5.5 degrees +/-5.5 degrees, PCR=-3.0 degrees +/-6.9 degrees change in lateral tilt when moving from full extension to early flexion) and contact area (full extension: normal=267+/-111 mm(2), BCS=344+/-201 mm(2), PCR=83+/-80 mm(2); early flexion: normal=723+/-306 mm(2), BCS=417+/-290 mm(2), PCR=246+/-108 mm(2)) in BCS TKA mimic those in the normal knees more closely than PCR knees do. These results suggest that the patellar component in BCS TKA may be expected to experience less wear than the patellar component in PCR TKA over time.