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Patellofemoral Pain Syndrome (PFPS) also known as runners knee is a generic term used to describe pain at the front of the knee which comes on gradually with symptoms increasing over a period of time.
It is sometimes also called anterior knee pain and occurs when the patella does not move or track correctly when the knee is being bent and straightened. This can lead to damage to the cartilage on the underside of the patella itself resulting in pain and inflammation. Patellofemoral pain is common in people who do a lot of sport, in particular adolescent girls.
Symptoms of patellofemoral pain syndrome are an aching pain in the knee joint, particularly at the front of the knee around and under the patella. There will be tenderness along the inside border of the kneecap and swelling will sometimes occur after exercise.
Patellofemoral pain is often worse when walking up and down hills or sitting for long periods of time. Pain under the kneecap when sitting still for a while is known as the theatre sign or movie-goers knee.
Other signs a sports medical practicioner may pick up include a click or cracking sound when bending the knee, wasting of the quadriceps muscles if the injury is an old one and tight muscles including calf muscles, hamstrings, quadriceps (especially vastus lateralis on the outside) and iliotibial band. With assessment a Q-angle greater than 18 to 20 degrees is often seen. The Q angle is the ankle between the quadriceps and the patella tendon which would indicate mal tracking of the patella.
When bending and straightening the knee, several muscles surrounding the joint act together to cause the patella to run in a straight line within the intercondylar groove, formed by the Femur and Tibia. If any of the structures are particularly tight or weak, this causes an imbalance which can result in the patella mal-tracking.
The most common example of this is when the lateral (outer) structures of the knee including the vastus lateralis, iliotibial band and lateral retinaculum are tight and the vastus medialis oblique muscle on the inside of the knee is weak. This results in the patella moving too far laterally (to the outside) as the tight structures pull it across and the muscles on the inside are not strong enough to control this force.
Patellofemoral pain syndrome can also occur following a knee injury if the muscles of the quadriceps, especially the vastus medialis on the inside become inhibited or considerably weakened. Other factors which can cause patellofemoral pain include:
PFPS is often confused with another condition known as Chondromalacia Patellae (CMP). This is damage to the cartilage which lines the underside of the knee cap. CMP can be a result of PFPS, although it can also occur independently, usually due to damage from an impact.