This study showed patients with unilateral PFP showed significantly greater knee valgus angles on unilateral loading tasks then either asymptomatic controls or their asymptomatic limb.
The study undertaken demonstrated that patients with PFP had significantly greater FPPA during single leg squats (SLS) and single leg landings (SLL) than controls or their contralateral asymptomatic limb. Further, it also showed FPPA to increase between tasks. Few previous studies have investigated the effect of different tasks on an individual PFP patient's performance. Willson and Davis [4] used 3D motion analysis to assess changes in knee motion in PFP patients across the tasks of SLS, running and single leg hopping. They found the PFP group to have significant greater motion, but the magnitude of that motion did not change across the tasks. This project demonstrated significant increases in FPPA between SLS and SLL, with the angle increasing with the increased load of the SLL task. The differences between this study and that of Willson and Davis [4] might be explained to a degree by the extent and differences in load in the respective tasks. Willson and Davis [4] had subjects hopping on the spot to an average height of 9.2 cm this study had subjects landing from a 30 cm step. McNair and Marshall [16] reported vertical ground reaction (VGRF) forces to be approximately 3.8 times body weight when landing onto a single leg, from a 30 cm step, whereas Podraza & White [18] calculated VGRF to be approximately 1.3 times body weight when landing from a 10 cm step. It would appear that when the load is increased between tasks (SLS to SLL) the performance, in terms of maintaining knee alignment of both controls and PFP patient's, deteriorates. This deterioration in performance would potentially increase patellofemoral joint stress with the increased load being focused on a smaller contact area [7]. This is reflected by the significant increase in pain reported by the PFP patients (Table 2) and reflects the work of Salsich et al. [15] who found that deliberate exaggeration of these movement patterns that is increasing knee valgus during single leg squat significantly increased pain.
There are potential limitations with using a 2D analysis of limb motion; McLean et al. [19] though reported that 2-D video analysis was suitable to screen individuals with excessive knee valgus. Both McLean et al. [19] and Willson and Davis [4] stated that this conclusion should be viewed with caution as the 2D method lacks sensitivity to measure small changes in angles. This study has shown significantly larger differences between symptomatic and asymptomatic knees than the previously reported standard error of measurement [14] demonstrating the differences to be real and beyond measurement error. McLean et al. [19] found an average peak 2-D knee valgus angle accounted for 58% to 64% of the variance in average peak 3-D knee valgus angle between subjects during side-step and side-jump activities, demonstrating a degree of validity to the measure. Willson and Davis [4] found 2-D knee valgus reflected 23% to 30% of the variance of 3-D measurements during single leg squat, but interestingly found knee valgus to be significantly correlated with knee external rotation (r = 0.54, p = 0.001) and hip adduction (r = 0.32, p = 0.04), which are major components of the “medial collapse” 2-D knee valgus angle which it aims to represent.
This study is cross sectional in nature and so its nature prevents making direct causal links between the altered kinematics seen in the patient group and pain. It cannot be concluded that the knee valgus position caused the pain or the pain created the altered mechanics. What is significant is that without the correction of these mechanical issues, the pain is likely to continue and potentially worsen, because of the altered stresses placed on the PFJ.