Abstract
Introduction
Mobile-bearing TKRs allow some axial rotation and may provide a more natural patellar movement. The aim was to compare patellar kinematics among the normal knee, fixed-bearing and mobile-bearing TKR.
Methods
Optical computer navigation (Brainlab) was used to track the position of the femur, tibia and patella in 9 whole lower extremities (5 fresh cadavers) in the natural knee, in the same knee with the trial components of a posterior stabilised fixed-bearing TKR (FB) (Sigma PFC, Depuy) and a posterior stabilised mobile-bearing TKR (MB) (Sigma RP Stabilised). The patellae were not resurfaced. Values: mean+/−one standard deviation. Statistical analysis: two tailed paired Student's T-test.
Results
M/L shift: There was a tendency for the patella to track 2mm more laterally with a FB or MB TKR compared to the natural knee, but this did not reach significance.
Tilt: The patella in the natural knee tilted progressively laterally from extension to flexion, plateauing at 50° of flexion (20°: 1.9+/−2.7°, 40°: 5.6+/−5.4°, 60°: 6.2+/−6.4°, 80°:6.5+/−7.3°, 90°: 6.4+/−7.7°).
With a FB or MB TKR the patellae also tilted laterally up to 50 degree of flexion, but then started to tilt back medially, reaching the neutral position at 90° again. There was no difference between the FB and MB TKRs. (Fixed bearing: 20°: 2.5+/−7.2° p=0.30, 40°: 3.7°+/−6.5° p=0.15, 60°: 3.1+/−5.8° p=0.02, 80°:1.2+/−6.5° p=0.001, 90°: 0.3+/−7.2° p=0.001, Mobile bearing: 20°: 0.3+/−5.5° p=0.27, 40°: 3.6+/−5.2° p=0.08, 60°: 2.1°+/−5.8 p=0.01, 80°: 0.2+/−6.8 p=0.003, 90°: -0.6+/−7.3 p=0.002; vs. natural)
Trochlea position: The centre of the patellar groove of the femur component was more lateral than the trochlea by 2-5mm, it also extended 10mm further proximally.
Conclusion
There are kinematic differences in patellar tracking between the natural knee and a FB/MB TKR. Compared to the FB TKR the patellar kinematics of the MB TKR is not more natural.
