Patellofemoral pain syndrome

Patellofemoral pain syndrome, now more often referred to as patellofemoral pain (PFP), is pain located around or behind the kneecap, usually triggered by activities that increase the stresses on the patellofemoral joint: running, going down and up stairs, squats, lunges, jumping, prolonged sitting with the knee bent, or cycling in certain situations.

The pain is usually located around the kneecap, behind it or along its edges; it is sometimes harder to pinpoint precisely.

It is important to understand that patellofemoral pain syndrome is not simply a problem of a “badly positioned” kneecap. It is a multifactorial condition involving the load applied to the knee, muscle capacity, quadriceps strength, the hip muscles, lower limb biomechanics, movement technique, activity level, as well as psychological and contextual factors. Recent international guidelines therefore favour an individualised approach rather than the systematic correction of a supposed biomechanical abnormality.

The patellofemoral joint is the joint between the kneecap (patella) and the femoral trochlea. The patella is a sesamoid bone located within the quadriceps tendon; it improves the quadriceps’ lever arm, transmits forces between the quadriceps and the patellar tendon, protects the front of the knee and helps guide the flexion-extension movement.

The back of the patella is covered with articular cartilage, which is particularly thick because patellofemoral stresses can become very high during squats, on stairs, when running or when jumping.

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Patellofemoral biomechanics

As the knee bends, the patella gradually engages in the trochlea. The force exerted by the quadriceps increases the compressive stresses between the patella and the femoral trochlea: the greater the load, the greater the patellofemoral force. This is why certain activities are particularly symptomatic: deep squats, lunges, going down stairs, downhill running, jumping and landing.

But pain is not simply proportional to the pressure exerted on the joint. A knee can withstand considerable stress without pain, and conversely a lightly loaded knee can become painful. Pain also depends on the tissues’ capacity to tolerate load, recent exposure to exercise, recovery, nervous system sensitivity, the psychological context and previous experience of pain.

Patellofemoral pain is one of the most common causes of anterior knee pain. It particularly affects adolescents, young adults, athletes, runners and people who play sports involving jumping and changes of direction. It is especially common in runners.

In some patients, it can become chronic, with a prolonged course despite initial management. This is why it is important not to limit treatment to short-term pain relief, but also to gradually restore the knee’s capacity to tolerate load.

There is no single factor responsible for patellofemoral pain: it is a multifactorial condition combining elements related to load, muscle capacity, biomechanics and the patient’s individual profile.

The role of the quadriceps

The quadriceps is essential to patellofemoral function: it extends the knee, absorbs loads, controls knee flexion during running and dynamically stabilises the lower limb. Weakness or loss of capacity in the quadriceps can reduce the knee’s tolerance to stress. However, a classic mistake must be avoided: patellofemoral pain is not simply due to a “too weak” quadriceps. Strengthening it is extremely important, but it must be part of a comprehensive management plan.

The role of the hip

The hip muscles — gluteus medius, gluteus maximus, external rotators, abductors — help control the lower limb, particularly hip adduction, internal rotation and dynamic knee alignment. Poor control of the lower limb during single-leg stance can increase certain patellofemoral stresses. But here again, increased knee adduction or rotation is not on its own the cause of the pain: it is one factor among others.

Load-related factors

Increased mileage, returning to running, a sudden increase in intensity or elevation gain, introducing interval training, returning to a sport after a break, an increase in the number of sessions.

Muscular factors

Quadriceps strength deficit, hip strength deficit, overall muscle capacity deficit, muscle fatigue.

Biomechanical factors

Insufficient control of the lower limb, changes in cadence, running technique, reduced ankle or hip mobility.

Individual factors

History of patellofemoral pain, activity level, sleep, recovery, stress, fear of movement.

The main symptom is anterior knee pain, described as pain around or behind the kneecap, pressure, discomfort, burning or a feeling of an “irritated” knee. It may be present during or after running, when going down stairs, during squats or lunges, or after prolonged sitting.

The well-known “movie sign” (or “cinema sign”) refers to pain that appears after sitting for a long time with the knees bent. The patient typically explains: “After 30 minutes sitting down, I need to straighten my leg because my knee starts to hurt.” This position increases the duration of patellofemoral compression, and this sign is frequently found in patients with PFP.

Clinical examination

The diagnosis is essentially clinical. History-taking looks at the exact location of the pain, how it started, how it has progressed, aggravating and relieving factors, any history of trauma, and any episodes of instability, swelling or locking.

The functional examination then relies on several tests that reproduce the pain: the squat (pain, depth, knee control, pelvic stability, movement quality), the single-leg squat (lower limb control, hip adduction, pelvic stability), the step-down (stepping down from a step, knee and pelvic control) and the lunge (load tolerance, lower limb control, quadriceps and hip strength). An assessment of the strength of the quadriceps, hamstrings, gluteus medius and maximus and calf completes the examination; ideally, this strength can be measured objectively using a dynamometer, isokinetic tests, force platforms or standardised functional tests, in order to track progress rather than relying solely on the patient’s own perception.

In classic patellofemoral pain, surgery is exceptional. It should be reserved for situations in which a clearly identified structural abnormality is responsible for the symptoms and appropriate conservative treatments have failed.

In particular, a distinction must be made between classic patellofemoral pain, whose treatment is mainly rehabilitative, recurrent patellar instability, a different situation that may sometimes require surgery, and cartilage lesions or major anatomical abnormalities, which call for specialist discussion. Surgery should therefore not be offered simply because an MRI shows an abnormality of the patella or cartilage.

Improvement can be achieved within a few weeks, but full recovery of muscle capacity and a return to the previous sporting level may take several months. The duration depends in particular on how long the symptoms have been present, their irritability, sporting level, training load and consistency with rehabilitation.

It is possible to reduce the risk of recurrence, although it cannot be completely ruled out. The most important elements are a reasonable progression in training volume, keeping up muscle strengthening, sufficient recovery, varying intensities, adjusting loads when returning to training and maintaining good muscle capacity.

For runners, the best prevention is often a knee that is strong enough to tolerate the desired level of running.