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Restoration of patellar alignment in patients with patellar lateral subluxation

Anterior knee pain also known as patellofemoral pain syndrome (PFPS), is mainly caused by patellar lateral subluxation (PLS). Muscle tone imbalance of Vastus medialis obliquus (VMO) and Vastus lateralis (VL) had been well established in patient with PLS. Isokinetic dynamometry with surface electromyography was used to measure maximal muscle activity in terms of vastus medialis oblique/vastus lateralis ratio in the following studies. In Tang’s study the open kinetic chain exercise was performed with an isokinetic dynamometer while the close kinetic chain exercise was performed with squat-to-stand and stand -to -squat exercise. The results revealed that VMO/VL ratio measuring by surface EMG is lower for PFPS patients than for asymptomatic subjects. Close kinetic chain exercise from 0 to 60 degree of knee flexion can induce maximal VMO firing.
We further studied the EMG ratio of muscle activation and bony structure in OA knee patients with and without patellar malalignment. The results that the VMO/VL ratio in patellar malalignment group was lower than the normal alignment group. Malalignment group showed larger sulcus angles, lateral patellar tilt and displacement on Merchant’s view. The peak muscle torques of malalignment group had smaller than normal alignment group.
To correct the patellar malalignment, intramuscular injection of Botulinum toxin type A (BTA) to VL was applied. Remarkable improvement after receiving BTA injection was obtained not only in the questionnaire of WOMAC but also in knee flexion torque. No significant change of knee extension torque was noted.
On the other hand, EMG ratio between VMO and VL before and after BTA injection was also assessed. The EMG ratio between VMO and VL in patient with PLS showed 0.46±0.12 in angular velocity of 60 rad/sec, 0.46±0.15 in angular velocity of 120 rad/sec and 0.48±0.15 in angular velocity of 180 rad/sec. After BTA injection, the EMG ratio between VMO and VL reached a peak improvement significantly at 4 weeks post injection and reported 1.00±0.55, 0.87±0.35 and 0.99±0.59 at angular velocity of 60, 120 and 180 rad/sec respectively. BTA injection could restore the EMG ratio between VMO and VL, and further lead to a patellar alignment restoration in patients with PLS.

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