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Knee News

Kneecaps under Pressure

Once again magnetic resonance imaging (MRI) has given us a way to look inside the body and measure something we couldn't see before. This time it's how much contact area occurs at the patellofemoral (PF) joint. The PF joint is where the kneecap comes in contact with the lower end of the thighbone (the femur).

In the past researchers used cadavers (human knees saved after death for study) to look at joint stress. But without live tissue there's no way to measure the effect of muscle contraction on joint loading. This is the focus of this study done by physical therapists. They used MRIs to view and measure the contact area in 10 healthy subjects. Is there more contact area on the inside (medial) or the outside (lateral) edge? What does the contact area look like as the knee is flexed? The researchers answered these questions. They also looked at the contact area while the quadriceps muscle was contracting. The quadriceps is the large muscle in front of the thigh. It goes over and around the patella and attaches just below it.

The therapists found a big increase in contact area on both sides of the PF joint as the knee bends. Contracting the quadriceps muscle didn't make any difference. Contact area is the greatest by the time the knee is flexed 40 degrees. More contact occurs on the lateral side. The authors of this study explain these findings based on the anatomy and movement of the PF joint.

It’s good to know where the PF contact points are located and how much force is put on them. This information will help physical therapists prevent and treat problems at this important joint.


Gretchen B. Salsich, PhD, PT, et al. In Vivo Assessment of Patellofemoral Joint Contact Area in Individuals Who Are Pain Free. In Clinical Orthopaedics and Related Research. December 2003. Vol. 417. Pp. 277-284.

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