Patellofemoral contact patterns before and after total knee arthroplasty: an in vitro measurement

Background: Patellofemoral complications are one of the main
problems after Total Knee Arthroplasty (TKA). Retropatellar
pressure distribution after TKA can contribute to these symptoms.
Therefore we evaluated retropatellar pressure distribution
subdivided on the ridge, medial and lateral surface on
non-resurfaced patella before and after TKA. Additionally, we
analyzed axial femorotibial rotation and quadriceps load before and
after TKA. Methods: Seven fresh frozen cadaver knees were tested in
a force controlled knee rig before and after TKA (Aesculap,
Tuttlingen, Germany, Columbus CR) while isokinetic flexing the knee
from 20 degrees to 120 degrees under weight bearing. Ridge, medial
and lateral retropatellar surface were defined and pressure
distribution was dynamically measured while quadriceps muscles and
hamstring forces were applied. Aside axial femorotibial rotation
and quadriceps load was recorded. Results: There was a significant
change of patella pressure distribution before and after TKA (p =
0.004). In physiological knees pressure distribution on medial and
lateral retropatellar surface was similar. After TKA the ridge of
the patella was especially in higher flexion grades strongly loaded
(6.09 +/-1.31 MPa) compared to the natural knee (2.92 +/-1.15 MPa,
p < 0.0001). Axial femorotibial rotation showed typical internal
rotation with increasing flexion both before and after TKA, but
postoperatively it was significantly lower. The average amount of
axial rotation was 3.5 degrees before and after TKA 1.3 degrees (p
= 0.001). Mean quadriceps loading after implantation of knee
prosthesis did not change significantly (575 N +/- 60 N in natural
knee and after TKA 607 N +/- 96 N; p = 0.28). Conclusions: The
increased retropatellar pressure especially on the ridge may be one
important reason for anterior knee pain after TKA. The trochlea of
the femoral component might highly influence the pressure
distribution of the non-resurfaced retropatellar surface.
Additionally, lower axial femorotibial rotation after TKA might
lead to patella maltracking. Changing the design of the prosthesis
or a special way of patella shaping might increase the conformity
of the patella to trochlea to maintain natural contact patterns.