Patellofemoral cartilage stresses are most sensitive to variations in vastus medialis muscle forces

Abstract

The purpose of this study was to evaluate the effects of variations in quadriceps muscle forces on patellofemoral stress. We created subject-specific finite element models for 21 individuals with chronic patellofemoral pain and 16 pain-free control subjects. We extracted three-dimensional geometries from high resolution magnetic resonance images and registered the geometries to magnetic resonance images from an upright weight bearing squat with the knees flexed at 60°. We estimated quadriceps muscle forces corresponding to 60° knee flexion during a stair climb task from motion analysis and electromyography-driven musculoskeletal modelling. We applied the quadriceps muscle forces to our finite element models and evaluated patellofemoral cartilage stress. We quantified cartilage stress using an energy-based effective stress, a scalar quantity representing the local stress intensity in the tissue. We used probabilistic methods to evaluate the effects of variations in quadriceps muscle forces from five trials of the stair climb task for each subject. Patellofemoral effective stress was most sensitive to variations in forces in the two branches of the vastus medialis muscle. Femur cartilage effective stress was most sensitive to variations in vastus medialis forces in 29/37 (78%) subjects, and patella cartilage effective stress was most sensitive to variations in vastus medialis forces in 21/37 (57%) subjects. Femur cartilage effective stress was more sensitive to variations in vastus medialis longus forces in subjects classified as maltrackers compared to normal tracking subjects (p = 0.006). This study provides new evidence of the importance of the vastus medialis muscle in the treatment of patellofemoral pain.

Keywords: Patellofemoral pain; finite element modelling; patellofemoral stress; probabilistic modelling; quadriceps force; vastus medialis.

Figure 1.

Subject-specific finite element model simulating weight bearing knee flexion during a stair climb task. Three-dimensional knee geometries were registered to weight bearing magnetic resonance images.

Figure 2.

Mean and 5 and 95 percentile bounds of effective stress in the (A) femur and (B) patella cartilage averaged over all subjects (n = 37).

Figure 3.

Relative sensitivity of effective stress to individual quadriceps muscle forces in the (A) femur and (B) patella cartilage. The sensitivity values represent the projections of a unit vector to a most probable point in a standard normal variate space (Haldar and Mahadevan, 2000). The sensitivity values were averaged over all subjects (n = 37). VML – vastus medialis longus, VMO – vastus medialis oblique, VLL – vastus lateralis longus, VLO – vastus lateralis oblique, VI – vastus intermedius, RF – rectus femoris.

Figure 4.

Percent breakdown of subjects (n = 37) with effective stress most sensitive to individual quadriceps muscle forces in the (A) femur and (B) patella cartilage. There was no subject whose femur cartilage effective stress was most sensitive to variation in rectus femoris or vastus intermedius muscle forces (A), and there was no subject whose patella cartilage effective stress was most sensitive to variation in rectus femoris muscle forces (B).

Figure 5.

Average (+1 SD) sensitivity of femur cartilage effective stress to variation in (A) vastus medialis longus (VML) and (B) vastus medialis oblique (VMO) muscle forces for all subjects classified into maltracking (n = 11) and normal tracking (n = 26) groups.

Figure 6.

Average (+1 SD) sensitivity of femur cartilage effective stress to variation in (A) vastus medialis longus (VML) and (B) vastus medialis oblique (VMO) muscle forces for patellofemoral pain subjects classified into maltracking (n = 7) and normal tracking (n = 14) groups.