Definition
Patellofemoral pain (PFP) is defined as pain around or behind the patella (knee cap), which is aggravated by at least one activity that loads the patellofemoral joint (PFJ) during weight bearing on a flexed (bent) knee (e.g. squatting, stairs, running).
Anatomy & Biomechanics
The PFJ is a compartment of the knee that is formed by the patella and the groove (femoral trochlea) at the distal end of the thigh bone (femur). The primary role of the patella is to act as a biomechanical pulley to improve the mechanical advantage of the quadriceps muscle as the main extensor and decelerator of the knee.
The contact area between the patella and trochlea is lowest when the knee is fully straight (extended) but increases steadily as the patella descends into the trochlea with knee flexion (image 1). The PFJ contact force is related to the size of the external moment arm, which is the perpendicular distance from the line of force to the joint axis (images 2-4, red line). During open-chain knee extension (figure 2), PFJ contact force increases as the knee extends, which, with a decreasing contact area, exerts greater stress on the PFJ, especially during the last 20° of movement. In contrast, during closed-chain activities such as leg press or squatting (figures 3-4), the PFJ contact force rises steadily for the first 45° of knee flexion and is distributed over the concurrently increasing contact area. As the knee flexes past 45°, the contact force increases at a greater rate than the contact area, therefore PFJ stress rises suddenly past this point.
Image 1: PFJ contact (from NKI website)
Image 2: knee extension external moment arm
Images 3-4: external moment arm during closed chain exercises
Causes
The exact cause of PFP is not fully understood but mechanical and non-mechanical models have been proposed, as described below. It is likely that elements of both models co-exist in patients with PFP rather than being mutually exclusive.
Mechanical loading is essential for joint health but abnormal loading of the joint may lead to potential or actual tissue injury; the mechanical model implies that PFP occurs when the load applied to the joint exceeds the amount it has been conditioned to tolerate (load capacity). Once a specific mechanical threshold has been reached within a PFJ structure, nerve endings (nociceptors) trigger signals that alert the brain to potential or actual tissue damage (nociception). Numerous PFJ structures can generate nociceptive signals (e.g. infrapatellar fat pad, PFJ ligaments) but articular cartilage lacks a nerve supply and is therefore incapable of this function directly. However, excessive loading of the cartilage can increase metabolic activity and water content in the underlying (subchondral) bone, stimulating pressure sensitive receptors in the patella. The brain processes these signals and, if deemed necessary, evokes a pain response that serves to protect the individual from injury.
As described earlier, PFJ stress is related to the contact force and the area over which this force is applied.
PFJ contact force:
Larger forces (peak loads and rates of loading) are exerted on the PFJ during certain activities (image 5) and may be amplified by other factors within these activities. For example, individuals that run with greater knee flexion will exert greater forces on the PFJ than those that do not. Reduced ankle dorsiflexion (upwards movement), weak calf and/or gluteal muscles, decreased quadriceps and/or hamstrings flexibility may also increase force on the PFJ. Patients with PFP may have an increased body mass index (BMI) but there is no clear evidence that BMI is a risk factor for developing PFP.
Image 5: PFJ loads with activity, from Running Physio (Tom Goom)
Contact area:
PFJ contact area decreases with knee extension but individuals with a high-riding patella (patella alta) have lower contact areas for any given knee flexion angle. Altered alignment of the patella decreases the PFJ contact area and can occur when the patella translates or tilts outwards (laterally), during adduction or internal rotation of the femur (images 6-9).
Images 6-9: normal patellofemoral alignment, lateral translation, lateral tilt and femoral internal rotation.
The non-mechanical model infers that non-physical, psychological (anxiety, depression, catastrophising, fear of movement) or social factors (family, friends, work, education, income) are important factors in PFP. Following injury, the mechanical receptor’s threshold for generating nociceptive signals may be lowered to sensitise the nociceptive system and protect the individual from further injury. Once the problem has resolved, thresholds should return to normal but if the peripheral and/or central nervous system remain sensitised (due to psychosocial factors) the nociceptive system may generate or process signals abnormally. Abnormal nociceptive processing can produce a pain response to stimuli that are not normally painful and may explain why some patients have persistent or worse PFP despite biomechanically based treatments.
Psychosocial factors are common in patients with PFP and correlate with higher levels of pain or reduced physical function. Although not necessarily causative, psychosocial factors should therefore be considered as part of the clinical picture in patients with PFP.
Presentation
PFP can affect anyone but it is particularly common amongst adolescents and those with high levels of activity, such as athletes and military personnel. Females are twice as likely to develop PFP compared with males, while PFP is also common after knee surgery, with 25-50% of patients experiencing symptoms 1-2 years following anterior cruciate ligament reconstruction.
Individuals describe a gradual onset of pain around or behind the patella that is aggravated by PFJ loading activities (e.g. squatting, stairs, running). PFP may be experienced with prolonged sitting (theatre sign or movie goer’s knee) or when straightening the knee after sitting. There may be tenderness on palpation of the patellar facets and, seldomly, a small amount of swelling may be present within the knee joint (effusion). A grinding sensation (crepitus) originating from the PFJ may be evident during knee flexion but this is also common in asymptomatic individuals and therefore not a defining feature of PFP.
Diagnosis
Anterior knee pain during squatting has been shown to have the greatest diagnostic accuracy of the available tests, but no individual tests can be recommended to diagnose or exclude PFP. For details on the diagnostic accuracy of individual tests for PFP, please visit the statistics section.
PFP is currently considered a diagnosis of exclusion, using the patient’s history, clinical presentation and imaging (if required) to differentiate other potential sources of anterior knee pain. The conditions that should be excluded when considering a diagnosis of PFP include quadriceps and patellar tendinopathy, knee osteoarthritis, iliotibial band pain, infrapatellar fat pad impingement, plica syndrome, Sinding-Larsen and Johansson syndrome and serious pathology (tumour, fracture, tendon rupture, systemic inflammatory disorders).
Treatment
Numerous modalities have been evaluated in the treatment of PFP with various explanations for their mechanism of effect. The ultimate goal of treatment is to improve the individual’s capacity to tolerate load by implementing a physiotherapist-led, appropriate loading programme. It is important to note that these interventions can influence both mechanical and non-mechanical elements of PFP. Treatment should be individualised for each patient; every patient does not require every intervention.
Education:
Exploring the individual’s beliefs can highlight any fears or misconceptions they may have about the condition, which can then be addressed with appropriate education. Active management should be encouraged over passive interventions and the individual should be reassured that the PFJ is robust and resilient but can become sensitive and painful. Appropriate loading is key to reduce sensitivity, optimise PFJ load capacity and allow the individual to return to their desired activities. Realistic timeframes and expectations for recovery should be set at the start of treatment to promote compliance and adherence, which are important for a successful outcome.
PFP may initially require a period of relative rest from activities that place high levels of stress on the PFJ, or by modifying these activities, to avoid aggravating symptoms. For example, patients may be able to continue running by avoiding slopes/stairs or using run-walk intervals. According to a recent high-quality study, the duration (volume), speed (intensity) or number of weekly training sessions (frequency) can be self-modified using the following guidelines;
- pain should be no more than 2/10 during activity
- if present, pain should return to pre-running levels within 60 minutes after cessation of activity
- there should be no increase in symptoms the following morning.
Depending on the individual’s response, running load can then be progressed by gradually increasing distance before adding speed and hills.
If the individual is not able to tolerate activities that involve high PFJ stress, lower demand activities can be performed to maintain a positive load through the joint and progressed as able to prepare the joint for higher demand activities. The following recommendations are based on the 2016 and 2018 consensus statements on exercise therapy and physical interventions for PFP.
Images 10-11: knee extension with weight attached to the ankle or a cable machine.
Image 12: PFJ stress during knee extension and leg press exercises (from Powers et al, 2014)
Performing knee extension with weight applied at the ankle through a restricted range (90-45°) (video 1) may therefore be necessary in the early stages of PFP rehabilitation to minimise PFJ stress and avoid aggravating symptoms. As able, weight and range of motion should be increased and, in later stages of rehabilitation, a cable machine could be used to ensure the joint is stressed through larger ranges of movement (video 2).
Videos 1-2: open chain knee extension between 90-45° of knee flexion
Hip-focused exercise:
Open chain exercises of the hip aim to influence hip and pelvis mechanics. Patients with PFP often demonstrate weakness of the hip abductors and external rotators, but this is thought to be a result of PFP rather than the cause. Patients who are particularly symptomatic may benefit from isolated hip exercises prior to combining them with knee-focused exercises. Videos 3-5 demonstrate hip-focused exercises to strengthen the muscles involved in hip abduction, lateral rotation and extension.
Videos 3-5: examples of hip focused exercises for abduction, lateral rotation and extension.
Combined hip and knee focused exercises:
The hip and knee-focused exercises described above can be used in combination to ensure both groups of muscles are targeted. Alternatively, closed chain exercises can target the hip and knee muscles simultaneously, whilst also challenging the trunk/pelvis, ankle/foot and proprioceptive system (videos 6-7). In contrast to open chain exercises, PFJ stress is low when performing closed chain exercises in low knee flexion angles but stress increases as the knee flexes, especially past 45° (image 12, green line). During the early stages of PFP rehabilitation, closed chain exercises may therefore need to be restricted to 0-45° with an emphasis placed on using high levels of resistance; range of motion can then be manipulated as symptoms allow.
Weight-bearing, closed chain exercises may more closely replicate functional PFJ loading positions (e.g. squatting, stairs) in preparation for a return to full activities. If reduced proximal control (femoral adduction and/or internal rotation, contralateral pelvic drop) is considered a contributing factor to the individual’s symptoms, closed chain exercises can be used to retrain more desirable movement patterns. Mirrors or video recordings may be useful for patient feedback to reinforce a correct technique. Video 7 demonstrates examples of progressively more challenging closed chain exercises.
Videos 6-7: examples of closed chain, knee and hip focused exercises for PFP.
Foot orthoses (insoles)
Prefabricated (off the shelf) foot orthoses are recommended to reduce PFP in the short term. Clinical features that can be used to predict a successful outcome include greater mid-foot mobility or reduced ankle dorsiflexion. However, the simplest test to determine whether an orthoses may be helpful is a patient-reported, immediate improvement in PFP when performing a single-leg squat whilst using the orthoses.
Taping and bracing:
Patellar taping and bracing may provide short-term improvements in PFP, and may therefore have a role in the early stages of management (≤12 weeks) when used in combination with other treatments. However, the role of taping used in isolation is yet to be fully determined.
Combined interventions
Combining interventions (exercise therapy, patellofemoral taping, foot orthoses and patellar mobilisation) are recommended to reduce pain in adults with PFP in the short and medium term.
Gait retraining:
Emerging evidence suggests gait retraining may be effective in reducing PFP, but this modality is currently considered an adjunct treatment until more rigorous research has been conducted on the subject. Increasing running cadence (total number of steps per minute) reduces average PFJ loads by decreasing knee flexion angle, vertical ground reaction force and peak hip adduction angles during the stance phase of running. Although a 5-10% increase in running cadence can produce a 20% reduction in PFJ contact forces, a recent high-quality study showed that gait-retraining with patient education did not provide superior improvements in PFP or function compared with education alone.
Transitioning from a rearfoot to a more forefoot striking pattern (including minimalist shoes) can induce similar changes to increasing cadence but this is likely to increase demand on the calf complex and achilles; the long term effects of this are currently unknown. The response to minimalist footwear can vary between individuals and is difficult to predict; while it may reduce PFJ loading and symptoms in some, transition to minimalism has been associated with injuries to the foot, ankle and calf. Reducing hip adduction through gait retraining cues (with verbal and/or visual feedback) may reduce provocative load and has been found to be effective in two small studies on female runners.
Psychological evaluation:
Given that central pain mechanisms may be altered in PFP, psychological evaluation may be required for some patients to determine the influence of non-mechanical factors on symptoms and function. Generic measures, such as the Orebro Musculoskeletal Pain Questionnaire, may be used to identify individuals with specific psychological constructs (mental health, cognitive, behavioural and other psychological factors) that may benefit from targeted intervention.
Others:
The hip, knee and ankle should be assessed for any deficits in range of motion (images 13-15), endurance and strength. Gluteal and calf strengthening exercises are generally recommended in lower limb rehabilitation as these muscles play an important role in load-sharing through the hip, knee and ankle (the kinetic chain) during high-load activities. Weight loss may also be considered if the patient is overweight or obese.
Images 13-15: assessment of knee and ankle range of motion using a digital inclinometer.
Interventions that are not recommended:
Patellofemoral, knee and lumbar mobilisations. Electrophysical agents (e.g. therapeutic ultrasound, pulsed shortwave diathermy) are not recommended.
Prognosis
While traditionally viewed as a self-limiting condition, recent evidence suggests that PFP can persist in over 50% of patients, can be present for up to 20 years, and may cause a decline in sports participation. Longer duration of symptoms (>12 months) or worse anterior knee pain scores at baseline are poor prognostic factors for patients with PFP.
Acknowledgements
Written by: Richard Norris, The Knee Resource
Reviewed by:
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