Abstract
Background:
Individuals with patellofemoral instability (PFI) often demonstrate altered knee morphology, such as trochlear dysplasia, patella alta, patellar tilt, and genu valgum [1]. The vastus medialis obliquus (VMO) plays an important role in patellar stabilization as a dynamic medial force vector on the patella [2]. Arthrogenic muscle inhibition is a mechanism that can occur following injury or surgical reconstruction of the knee and is especially impactful on the VMO [3].
Hypothesis:
It was hypothesized that limb-to-limb asymmetries would be present preoperatively and at return-to-sport clearance. Specifically, peak VMO, rectus femoris, and hamstring muscle activity were hypothesized to be significantly less in the treated limb.
Methods:
25 adolescents with unilaterally treated PFI (14F, 14.1±2.1 years, 164.6±12.7 cm, and 58.5±14.0 kg) were evaluated prior to arthroscopic assisted allograft medial patellofemoral ligament (MPFL) reconstruction surgery. Preoperative testing was performed 58.4±67.9 days after the most recent instability event. A separate group of 20 PFI patients (11F, 14.8±1.1 years, 166.2±9.4 cm, and 63.1±13.5 kg) were tested at return-to-sport clearance (27.3±7.4 weeks from surgery to return-to-sport clearance, 28.6±7.0 weeks from surgery to testing). All participants completed an unweighted, overhead squat task in a motion capture lab. Surface electromyography sensors were placed on the rectus femoris, VMO, hamstring, tibialis anterior, and gastrocnemius. Electromyography data were filtered, rectified, and a linear envelope was created. Peak activity of each muscle was analyzed. Wilcoxon signed-rank tests were used to compare muscle activity between limbs (α=0.05).
Results:
Preoperatively, peak rectus femoris activity of the treated limb was 40% less than the contralateral limb (p=0.006), however no difference in VMO activity was observed (Table 1). At return-to-sport clearance, peak VMO activity was 61% less in the treated limb compared to the contralateral side during the squat movement (p=0.002), with no difference in rectus femoris activity. No significant limb-to-limb differences in muscle activity were observed preoperatively or at return-to-sport for the hamstring, tibialis anterior, or gastrocnemius (p≥0.098).
Conclusion:
A significant reduction in treated limb rectus femoris activity was observed preoperatively, indicating that there may be compensations in movement patterns among those with PFI. The VMO demonstrated clear differences in activity at return-to-sport clearance following MPFL reconstruction with a significant difference between limbs, suggesting that there may be muscle weakness following surgery and ongoing through physical therapy. A greater emphasis on VMO training may prove beneficial during rehabilitative processes and help prevent the reduction in VMO activity observed during squat movements at return-to-sport clearance.
