Knee Cap Pain

November 5, 2013

Knee Cap Pain

Knee pain is a common complaint in patients presenting to the orthopedist. The kneecap is often the culprit. To understand why this bone plays such a prominent role in knee pain, a review of anatomy is in order.

The knee cap, known as the patella, is a fig newton size bone, triangular in cross section and located at the end of the quadriceps, the powerful muscle-tendon structure that runs down the front of the thigh and is responsible for extending the lower leg.

When our knee bends, the patella slides into a groove in the front of the femur (the   thigh bone) called the patello-femoral groove. This V shaped groove perfectly matches the two sides of the triangular (V-shaped) patella. When the quadriceps contracts, pulling the lower leg to the straight (i.e. extended) position, the undersurface of the kneecap is subjected to compression stresses (pressure):  the stronger the contracture (intensified in running or stair climbing, for example), the greater the stress.

The knee joint, like all joints in the body, is lined with a smooth, slippery white material called cartilage.  The cartilage on both sides of the patella-femoral joint can tolerate the high pressures of intense quadriceps activity if the patella glides in the groove perfectly centered, thereby absorbing equally the compression stresses on both of its V-shaped sides (facets).  And, “therein lies the rub”.

For the patella to stay perfectly centered in the groove during activity, many things have to be in place: the pull of the quadriceps has to be equal (symmetrical) on both sides of the patella; the rotation of the thigh must be aligned with that of the lower leg; the muscles around the knee, foot and ankle must be balanced and strong enough to perform their function; and the individual muscles in the entire leg must be of the right tension, not too tight and not too loose.

If the forces around the patella and the whole limb are not balanced, the patella glides asymmetrically in the groove (i.e. slips out of place) during quadriceps contracture predominantly loading one (usually the outside i.e. lateral) facet over the other. This concentrated pressure over just half the usual surface area causes the cartilage to fail resulting in pain, buckling and stiffness. If this continues unchecked, arthritis occurs.

Often a person has a slipping patella, but doesn’t know it. The knee has no symptoms because the person’s lifestyle does not include activities with intense demands on the quadriceps.  The knee remains symptom free as long as the activities of daily living (ADL) are not stressful enough to cause symptoms.

All is well until the person embarks on a new activity (running, biking, hiking, lifting, etc.) that markedly increases the load on the knee.  This new load is concentrated on the slipping patella and is more than a single lateral facet can bear.  Pain occurs in the front of the knee and occasionally behind the knee. Simple every day activities like squatting, climbing stairs and getting up from a chair now are painful.  Swelling, cracking and stiffness become daily occurrences. It is at this point the patient seeks medical help.

Orthopaedic treatment must be tailored to the specific mechanical abnormality, with the goal being to balance the forces acting on the patello-femoral joint so that the patella remains centered in the femoral groove during quadriceps contracture. Conservative therapy involving physical therapy, orthotics, and bracing (individually or in combination) is successful in most cases.  When rehabilitation efforts fail, surgery is used to balance the forces around the kneecap so that the patella stays in its groove.

When a patient`s chief complaint is knee pain, orthopedists must first find the site of the pain and then the reason it exists. When the pain site is the knee cap, we must identify the mechanical abnormalities, educate the patient as to why they exist and then correct them with rehabilitation and, if necessary, surgery. This, plus a lifelong commitment by the patient to a maintenance exercise program gives us our best chance at a successful and lasting outcome.