What Is A Knee Cap?

(Originally posted on bcotoronto.com on Mar 21st, 2013)

Our knee cap (“Patella”) is a funny little bone. It is what is called a “Sesamoid” bone which means that it is a bone suspended within a tendon of a muscle. These sesamoid bones develop in areas where tendons rub over bones as they are stretched by the movement of a joint. They take the pressure and friction off of the tendon and for a joint with the bone they are against instead. This is what the patella does with the Quadriceps muscle and the Femur. The back side of the patella is covered in cartilage just like the any other joint and glides through a depression in the Femur called the “Patella Groove”.

Anterior Knee Bony AnatomyLateral Knee Bony Anatomy

Just like any other point, the Patellar-Femoral joint is susceptible outside pressures and because it is not held securely by ligaments these pressures can lead to significant dysfunction. This can lead to a condition called Patellar Tracking Syndrome (PTS). This is most commonly caused by an imbalance in the quadriceps muscles which pull the patella up through its groove as the knee straightens. The muscles of the quadriceps attach to the patella from three different angles and so if one side pulls harder than the other the Patella will be guided out of the groove.

Anterior Knee - The Quadriceps Muscles

Sit in a chair and keep an eye on your knee cap as you slowly straighten your knee. If the patella moves to one side of the other this indicates an imbalance in the strength of the quadriceps. If left unchecked this deviation in movement can cause extra wear and tear on the cartilage underneath the patella and eventually lead to arthritis and pain.

Before reaching the point of full-blown arthritis the cartilage may become inflamed to the point of causing pain and swelling beneath the patella. This is called Subpatellar Chondritis and can cause a significant amount of grief and get in the way of our activities.

So what can we do about this? Well, the most common muscular imbalance in the quadriceps is a weakness in the Vastus Medialis Obliqus (VMO) which causes a lateral (towards to outer side of our leg) shift in the patella out of its groove. The VMO is important in the last little bit of knee extension and so to strengthen it we take it through this activity.

Sit on a flat raised surface like a chair or bed and straighten your knee as much as you can (if you can bend your knee backwards, only extend the knee to be straight and not more). Bend your knee approximately 5o then slowly extend your knee to be straight again. Repeat this 5-10 times at least once a day. This will help to strengthen the VMO and balance out the side-to-side pull on the patella.

It is possible to have an imbalance in muscular strength in the quadriceps due to other reasons like injuries to nerves and scar formation, in which case significantly more therapy and care is needed and exactly how much should be determined by your healthcare professional. We can also injure our patella directly in other ways, but why not do a little work now to prevent the injuries we can?

Thanks for reading,
Dr. Ben


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