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A study of vastus medialis obliquus tension on patellofemoral joint pressure in a simulated weight bearing position G Ng*1,
A Tang2, A Mak2 & K Chan3 To study the patellofemoral (P-F) joint pressure under different tensions of vastus medialis obliquus (VMO) in a simulated weight bearing position, five human knee specimens were tested. Each was prepared by removing the skin and separating the different heads of quadriceps muscle group, with the vastus medialis (VM) further dissected into a longus (VML) and an obliquus (VMO) component. The P-F joint was opened superiorly to allow access to the joint. The specimen was mounted on a specially designed jig, such that the leg was in a close kinetic chain, and the weight of the jig simulated the body weight acting on the leg. The knee was positioned at 30o flexion. Each component of the quadriceps was attached to a strain gauge, which was positioned along the line of action of each muscle. The tension of each muscle was adjusted in the ratio of 2.5 (VL): 2 (RF & VI): 1.5 (VMO): 1 (VM). A super-low grade Fuji Prescale Film (pressure range: 0.5-2.5 MPa) was inserted in the P-F joint to register the pressure. Afterwards, the tension of VMO was reduced to 65%, 50% and 35% respectively of its original value, and the P-F joint pressure was measured in each condition with the Fuji Film. After the above procedures, the knee was positioned at 90o flexion and the tests were repeated with 100%, 80%, 50% and 35% of VMO tensions. After these tests, the Fuji Films were analyzed with a Leica Q500CM image analysis system, which measured the color density to determine the relative pressure magnitude and area of contact. Results showed a general decrease in pressure area of the medial P-F joint and an increase in the lateral area with decrease VMO tension at both flexion angles. The change in pressure magnitude was inconsistent and no general trend was found (see table). The change in contact area indicates VMO may affect patellar tracking. However, the inconsistent effect of VMO tension on the magnitude of P-F joint pressure reveals that the change in P-F joint kinematics may not be parallel with the change of contact pressure of the joint.
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