How Does the Quadriceps Muscle Measure Up in Patellofemoral Pain Syndrome

Athletes aren't the only ones to develop knee pain from a condition called patellofemoral pain syndrome (PFPS). Many people of all ages in the general public develop this problem, too. You may be one of them.

If you have knee pain after sitting with your knee bent for a long time or with any of the following activities (stair climbing, squatting, kneeling, jumping, running), you may have patellofemoral pain syndrome (PFPS). How could you find out for sure?

PFPS is usually diagnosed by the history and an examination of the knee. If your pain is brought on by pressing directly on the patella (kneecap) when the leg is held straight or you experience tenderness underneath along the side edges of the patella, you may have PFPS.

Other telltale signs are pain with resisted knee extension and pain when the patella is compressed against the femur (thigh bone) when you tighten the quadriceps muscle (an isometric contraction) with the knee slightly bent.

What causes this problem? That's been the focus of many, many studies. For sure there is a problem with the way the patella moves up and down over the knee. But what causes this maltracking?

There are many possible factors and it's likely that more than one occur at a time. There could be an odd shape to the patella or an abnormal position of the patella. Or there may be some muscular imbalances -- in particular in the quadriceps muscle. That's the large muscle along the front of your thigh. It's called "quadriceps" because it has four separate parts.

When the quadriceps muscle contracts and pulls evenly on the patella, it moves up and down over the knee joint in the middle where it belongs. But if the lateral quadriceps along the outside edge of the patella pulls more than the medial quadriceps along the inside border, then maltracking and eventually patellofemoral pain syndrome (PFPS) can occur.

To find out more about the role of the vastus medialis obliquus (referred to as the "VMO") a group of researchers from Belgium conducted a new study. They used MRIs to measure the size of the VMO in two groups of people.

Group one were patients diagnosed with PFPS. They ranged in ages from 12 to 40 years old. Group two were considered "normal" controls -- they did not have any knee pain and no sign of PFPS. They were matched by age, similar body type, activity level and sex (male and female).

Special equipment was used to hold the legs still so no muscle contraction would occur during the MRI test. The results were sent to a computer that had a special software program to measure and compare the size of the vastus medialis obliquus (VMO). The measurement was just muscle fibers without any fat, blood vessels, or nerves included.

They found that the cross-sectional area of the VMO was indeed smaller at the patellar level in the patients diagnosed with patellofemoral pain syndrome (PFPS). In fact, the entire quadriceps muscle was smaller in the PFPS group when measured at the midthigh level.

But these findings don't answer the question: which came first -- the PFPS or the change in muscle size? Maybe people born with a smaller vastus medialis obliquus (VMO) are more likely to develop PFPS. Or maybe the pain of PFPS leads to inactivity and the muscle begins to waste away and get smaller.

Understanding the cause and effect of VMO size and PFPS will be the focus of a future study. The authors also suggest looking at muscle strength as it relates to the size of the VMO in different people.

For now we know that there is atrophy of the VMO in patients with patellofemoral pain syndrome. This is new information that hasn't been published before. And since we know the VMO is important in stabilizing the patella during knee motion, it seems logical that the smaller size of this portion of the quadriceps could be a key to directing prevention and treatment.

Reference: Els Pattyn, PT, et al. Vastus Medialis Obliquus Atrophy. In The American Journal of Sports Medicine. July 2011. Vol. 39. No. 7. Pp. 1450-1455.

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