Tracking the Causes of Kneecap PainPatellofemoral pain syndrome (PFPS) is basically pain in front of or behind the kneecap (the patella) that doesn't always have a clear cause. PFPS is a common knee condition, especially among athletes. The pain is often felt after sitting for a long time, going up and down stairs, and when squatting or kneeling. There are many causes of PFPS. Knowing these causes could help health care providers choose the best treatments.
Medical researchers have long believed that abnormal tracking of the patella may contribute to problems of PFPS. The patella normally runs (tracks) in a groove on the front of the thigh bone (the femur). Two muscles of the thigh--the vastus medialis obliquus (VMO) and the vastus lateralis (VL)--attach to the patella and help control its position in the groove as the leg straightens. The VMO runs along the inside of the thigh, and the VL lies along the outside of the thigh. If the timing between these two muscles is off, the patella may be pulled off track.
The theory sounds nice, but does it really happen that way? These researchers tested how the VMO and VL work during stair climbing in 33 people with PFPS and 33 people with no knee pain (the control group). Electrodes were placed over the two muscles to get an electromyograph (EMG) reading of the activity in the muscles. The subjects then went up and down two stairs at a normal pace. As expected, the group with PFPS felt pain during the test, and the control group felt no pain.
Researchers found a significant difference between the way the muscles worked in the two groups. The EMG signals happened at the same time in most of the control group. In the group with PFPS, the VL muscle kicked in well before the VMO muscle, both going up and going down the stairs.
This research supports the theory that patellar tracking is partly to blame for PFPS. However, the results also raise many questions. There was a wide variation in onset of EMG signals within both groups of subjects. So how much difference in timing is too much? Not everyone with PFPS has problems in the timing between the two thigh muscles. Why not? Does their PFPS have a different cause? Also, it is impossible to tell from this study whether the differences in muscle timing happened before problems with PFPS developed, or if the differences were actually the result of a painful knee from PFPS. More research is needed, but the current trends in rehabilitating PFPS are on track with the findings of this study.
Sallie M. Cowan et al. Delayed Onset of Electromyographic Activity of Vastus Medialis Obliquus Relative to Vastus Lateralis in Subjects With Patellofemoral Pain Syndrome. In Archives of Physical Medicine and Rehabilitation. February 2001. Vol. 82. No. 2. Pp. 183-189.
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