Patellofemoral Syndrome

Etiology

The etiology of patellofemoral syndrome is without a clear consensus; however, it is likely multifactorial and secondary to training practices. It is thought to involve 6 anatomic areas, including subchondral bone, synovium, retinaculum, skin, nerve, and muscle.[1] Studies point to four major contributing factors: malalignment of the lower extremity and/or patella, muscular imbalance of the lower extremity, overactivity/overload, and trauma.[2] Of the four contributing factors, overuse appears to be the most important. Also, early sport specialization practices have been shown to increase the relative risk of PFS 1.5 fold when compared to multisport athletes.[3]

Malalignment and Muscular Imbalance

The patellofemoral joint function relies on a complex interaction between static and dynamic structures involving the entire lower extremity as the patella tracks in the trochlea. Static components include leg length discrepancies, abnormal foot morphology, hamstring and hip musculature tightness, angular or rotational deformities, and trochlea morphologies. Dynamic components include muscle weakness, ground reaction forces, and insufficient or excessive foot pronation. Studies involving malalignment that potentially contribute to PFS are conflicting with no clear consensus, likely secondary to its multifactorial nature. Multiple studies have concluded that hip abductor weakness may play a major role.[4] Another study looking at female runners pointed to hip biomechanics as a cause, finding that greater hip adduction angles were associated with an increased risk for the development of PFS.[5] Though multiple studies have shown an association between hip abductor weakness, others have not been able to show a relationship and, in other cases, have shown an increased hip abduction strength to be the cause.[6]

Overactivity and Overload

Many patients with PFS do not demonstrate any signs of malalignment. Instead, during careful interviewing, overload of the patellofemoral joint is often described, which can lead to the development of PFS.[7] Studies have been performed showing that an increased workload on the joint such as miles ran/volume of work, correlates to the development of PFS, and patients usually state that pain began during a period of increased activity.[8][9] Risk factors that can lead to overload, thus increasing the risk of PFS, include prior fitness level, prior exercise regimen, and BMI >25.[10]

Trauma

Direct or indirect injuries to the patellar area can damage structures leading to PFS. 

Though studies have pointed to the above causes or risks of developing patellofemoral syndrome, most will agree its development is rarely secondary to a single component.

Epidemiology

Patellofemoral syndrome is one of the most common knee conditions seen by clinicians. In active individuals, it may account for 25% to 40% of all knee problems seen in a sports medicine clinic, although the true incidence is unknown.[4] PFS affects women more so than men at a ratio of close to 2:1, according to studies.[11][12] Age of occurrence is typically seen in adolescents and adults in the second and third decades of life.[11] Its prevalence in adolescence was found to be over 20%.[13]

History and Physical

The diagnosis of patellofemoral syndrome is heavily reliant on a detailed and accurate history and physical examination. Symptoms can be unilateral or bilateral and can be gradual or acute. Patients will also describe a worsening of symptoms with squatting, running, prolonged sitting, or using stairs.[14] The pain will usually be poorly localized.  It can be described as behind or around the patella and is usually achy, but it can also be described as sharp. PFS is considered a diagnosis of exclusion; therefore, one must rule out other conditions that it may mimic. Some patients may describe a giving way or a catching sensation in the knee. Both of which could be signs of ligamentous or intraarticular pathology. When taking a patient's history, it is most important to ask the patient about trauma to the knee, including previous surgeries and activities of overuse.

On physical exam, start with the general overview and observation of the patient and the affected joint. Are they obese? What is their age? Are there any muscular abnormalities such as vastus medialis atrophy? Is there erythema on the joint suspicious of infection? Palpation can be useful to determine if the quadriceps or patellar tendons are tender while also checking for effusion or warmth. Simple muscle strength testing can be beneficial, looking for weakness in hip abductors or quadriceps. Make a note of differences between affected and non-affected sides as PFS can lead to weakness. Also, assess the range of motion of the affected knee. Lastly, examination of the ipsilateral hip should also be performed as the pain could be referred.

Multiple special tests can be performed; however, many are nonspecific for PFS. One study comparing the validity of clinical features found the sensitivity of the patellar tilt, active instability, patella alta, and apprehension tests was low at around 50%; specificity ranged between 72% and 100%.[15] In the same study, patellofemoral pain syndrome presented with increased quadriceps angle, lateral and medial retinacular tenderness, patellofemoral crepitation, squinting patella, and reduced mobility of the patella. Measurements of the popliteal angle have also been shown to be closely associated with the development of patellofemoral syndrome. Popliteal angle measurements are used to measure the flexibility of the hamstring musculature. Tight hamstrings add compressive forces across the patellofemoral joint, increasing the likelihood of developing PFS.[16]

Evaluation

The diagnosis of patellofemoral pain syndrome is typically a clinical one. Further investigation with a plain radiograph is usually not performed until symptoms fail to improve after one to two months of conservative management. Radiographs usually do not correlate well with the complaint, and the affected side vs. the unaffected side is usually difficult to differentiate.[15] If the patient fails to respond to therapy, imaging is used to rule out other causes of similar pain, such as bipartite patella, osteoarthritis, loose bodies, and occult fracture. Advanced imaging such as MRI, musculoskeletal US, and CT are usually not indicated and typically used again to evaluate other pathology.

Treatment / Management

Patellofemoral syndrome treatment is usually conservative and targeted at pain reduction, improved patellar tracking, and return to their previous level of function. The treatments of the patellofemoral syndrome are divided into 2 main sections, the acute and the recovery phase. The acute phase involves activity modification, NSAID use, and other conservative modalities such as ice.  NSAIDs, specifically naproxen, have been shown to decreased overall pain when compared to aspirin and placebo; however, they are usually not recommended as a long-term treatment.[17] Other modalities such as the therapeutic US and electrical stimulation have not been shown to improve symptoms.[18][19] After the acute phase of treatment, the patient enters the recovery phase, which tries to correct the issue that most likely led to the development of the condition. The combination of knee and hip exercises to increase lower extremity strength, mobility, and function is the most effective intervention.[20] If the patient experiences pain with exercises, then adjunct therapy can be used. This includes patellar taping. Patellar taping has been shown to decrease overall pain when used in conjunction with physical therapy when compared to physical therapy alone.[21] However, in patients with a larger BMI, taping is less effective.[22] Therapy should be patient-specific and tailored to correct the dysfunction present. Referral to orthopedic surgery is not recommended and is considered a treatment of last resort.[23] Non-operative therapy should be pursued for 24 months before operative interventions are considered.[24](A1)

Differential Diagnosis

The differential diagnosis for a patient with PFS is quite broad and can be divided into 6 anatomic areas, as discussed earlier. These include patellofemoral OA, Osgood Schlatter's disease, plica, bursitis (prepatellar or Hoffa's), Saphenous neuritis, quadriceps tendinopathy, patellar tendinopathy, or referred pain from hip or back. Due to the broad differential, the clinician must perform a thorough history and physical exam to recognize certain risk factors and effectively treat the patient.

Prognosis

The prognosis of patellofemoral syndrome is good; however, around 40% of patients diagnosed with PFS will have continued symptoms one year following typical treatment.[25] At 7 years, one study has shown nearly 85% of patients treated with a home exercise regimen with self-reported successful outcomes. Predictors of poor long-term prognosis include a hypermobile patella, older age, and bilateral symptoms.[26]

Complications

One complication from PFS includes the development of patellofemoral osteoarthritis secondary to the inadequate tracking of the patella, which may lead to chronic pain. Another complication is that some patients must stop activities that they previously enjoyed due to that activity causing them pain.