Contact area and pressure changes of patellofemoral joint during stair ascent and stair descent

PurposeTo investigate the differences of patellofemoral joint pressure and contact area between the process of stair ascent and stair descent.MethodsThe finite element models of 9 volunteers without disorders of knee (9 males) to estimate patellar cartilage pressure during the stair ascent and the stair descent. Simulations took into account cartilage morphology from magnetic resonance imaging, joint posture from weight-bearing magnetic resonance imaging, and ligament model. The three-dimension models of the patella, femur and tibia were developed with the medical image processing software, Mimics 11.1. The ligament was established by truss element of the non-linear FE solver. The equivalent gravity direction (-z direction) load was applied to the whole end of femur (femoral head) according to the body weight of the volunteers, and the force of patella was observed. A paired-samples t-test or Wilcoxon rank sum test to make comparisons between stair ascent and stair descent. Statistical analyses were performed using SPSS 22.0 using a P value of 0.05 to indicate significance.ResultsDuring the stair descent (knee flexion at 30 degrees), the contact pressure of the patella was 2.59 +/- 0.06Mpa. The contact pressure of femoral trochlea cartilage was 2.57 +/- 0.06Mpa. During the stair ascent (knee flexion at 60 degrees), the contact pressure with patellar cartilage was 2.82 +/- 0.08Mpa. The contact pressure of the femoral trochlea cartilage was 3.03 +/- 0.11Mpa. The contact area between patellar cartilage and femoral trochlea cartilage was 249.27 +/- 1.35mm2 during the stair descent, which was less than 434.32 +/- 1.70mm2 during the stair ascent. The area of high pressure was located in the lateral area of patella during stair descent and the area of high pressure was scattered during stair ascent.ConclusionThere are small change in the cartilage contact pressure between stair ascent and stair descent, indicating that the joint adjusts the contact pressure by increasing the contact area.