finite element analysis

Predictors of patellofemoral joint stress: an examination of patellofemoral joint morphology

INTRODUCTION: Patellofemoral pain (PFP) is a common condition seen in orthopedic practice, accounting for approximately 25-40% of all knee injuries [1]. A commonly cited hypothesis as to the cause of PFP is elevated patellofemoral joint (PFJ) stress [2] secondary to abnormal PFJ structure. Previous studies have shown that persons with PFP exhibit altered patella position [3], abnormal femoral morphology [4], and decreased patella cartilage thickness [5] when compared to healthy individuals. However, the influence of the abnormal morphology on PFJ stress is unknown. METHODS: Nineteen subjects (10 PFP and 9 pain-free controls) were recruited for this study. Each subject completed 2 phases of data collection: magnetic resonance imaging (MRI) assessment and biomechanical testing. The measurement of morphological variables (patella height (Insall-Salvati ratio or ISR), lateral trochlear inclination angle (LTI), and patella cartilage thickness). For the biomechanical testing, kinematic, kinetic, and electromyographic were obtained. RESULTS AND DISCUSSION: Pearson correlation coefficients revealed that only patella height (r=0.48, p=0.018) and patella cartilage thickness (r=-0.58, p=0.005) were significantly correlated with peak hydrostatic pressure (Table 1). Results of the stepwise regression analysis revealed that patella cartilage thickness was the single best predictor of peak hydrostatic pressure, followed by patella height. Together, these 2 variables explained 50% of the variance in peak PFJ stress. The results of the current study support the premise that PFJ stress is associated with PFJ morphology. Patella height was the best predictor of PFJ stress with greater degrees of patella height being correlated with greater stress. This is logical given that a higher positioned patella articulates with the more shallow portion of the trochlear groove, thus decreasing PFJ contact area [6]. The finding that patella cartilage thickness was negatively correlated with PFJ stress is in agreement with the results of Li et al. [7], who demonstrated that a reduction of cartilage thickness causes increase cartilage stress. Furthermore, our findings revealed that 50% of the variance in PFJ stress could be explained by morphological factors. CONCLUSIONS: Identifying the underlying factors that contribute to elevated PFJ stress is an important step in developing effective interventions for persons with PFP. Although abnormal structure may not be correctable through conservative measures, it is important to recognize abnormal structure may play a role in contributing to pain and pathology.
Listed In: Biomechanics, Sports Science


Influence of femur rotation and knee valgus on patellofemoral stress

Background: Patellofemoral pain (PFP) is a common condition seen in orthopedic practice. A commonly cited hypothesis as to the cause of PFP is increased patellofemoral joint (PFJ) stress secondary to abnormal lower extremity kinematics (ie. excessive hip internal rotation and knee valgus). However, the influence of these motions on PFJ contact mechanics is unknown. Purpose: To assess the influence of hip rotation and knee valgus on PFJ stress using finite element (FE) analysis. Methods: Patella cartilage stress profiles for a healthy participant were quantified utilizing a subject-specific FE model. Input parameters included: joint geometry, quadriceps muscle forces, and weight-bearing PFJ kinematics. Using a nonlinear FE solver, quasi-static loading simulations were performed to quantify patella cartilage stress during a static squatting maneuver (45° knee flexion). To simulate hip rotation (0-8°) and knee valgus (0-12°), the femur and tibia were rotated in the transverse and frontal plane respectively in 2° increments. Results: Increasing hip rotation resulted in a linear increase in patella cartilage stress. In contrast, increasing knee valgus resulted in a decrease in patella cartilage stress. The combination of hip rotation and knee valgus did not result in higher PFJ cartilage stress compared to isolated hip rotation. Conclusions: Patella cartilage stress appears to be influenced to a greater degree by hip internal rotation as opposed to knee valgus. Surprisingly, higher degrees of knee valgus resulted in decreased cartilage stress (in the absence of hip rotation). Our finding supports the premise that persons exhibiting excessive hip internal rotation may be pre-disposed to elevated patella cartilage stress.
Listed In: Biomechanics, Gait, Physical Therapy