PATELLOFEMORAL PAIN (PFP)

The generic term “patellofemoral pain syndrome” is frequently refer to as a specific diagnosis, but it has never been specifically defined. In fact, it is not a diagnosis per se. To facilitate the arrival at a specific diagnosis, we categorize anterior knee pain into:

  1. Conditions pertaining to the patella proper
  2. Conditions pertaining to the peripatellar soft tissues
Most common type of knee pain?

The most common form of knee pain is known as patellofemoral pain syndrome (PFPS). PFPS is an umbrella term also commonly known as anterior knee pain which includes various subcategories of knee pain. Patellofemoral pain occurs across age groups, from young active children to older sedentary individuals.

What is the incidence and prevalence of PFPS?

Prevalence ranges from 3% to 85% for idiopathic anterior knee pain (AKP) or PFP in different parts of the world, and its associated diagnoses with a prevalence of 25%. The recurrence of PFP is alarmingly high, with reports of 70% to 90% having recurrent symptoms.

Anatomy of knee joint:
The patellofemoral joint (PFJ) comprises of the articulation between the patella and the trochlear groove of the femur. The patella is a large sesamoid bone embedded in the quadriceps extensor mechanism. The roles of the patella are to increase the moment arm of the quadriceps muscles, provide bony protection to the distal joint surfaces of the femoral condyles when the knee is flexed, and prevent damaging compressive forces on the quadriceps tendon with resisted knee extension.
PFJ KINEMATICS IN WEIGHT BEARING AND NON- WEIGHTBEARING

According to recent evidence PFJ kinematics may be different during non- weight bearing and weight-bearing tasks. During the non–weight-bearing condition, the patella was observed to tilt and displace laterally relative to the fixed femur. In contrast, the primary contributor to lateral patella tilt and displacement during the weight-bearing condition was internal rotation of the femur underneath a stable patella as the patella is attached to the tibia via the quadriceps tendon. The quadriceps contraction during weight bearing anchors the patella to the comparatively stable tibia, allowing the femur to move underneath the extensor mechanism. Conversely, movement of the tibia during non–weight-bearing knee extension allows the patella to move relative to the fixed femur.

One of the guiding principles of PFPS is improper alignment of the patella in the trochlear groove, increasing the stress within the PFJ, mainly on the outside or lateral part of the joint. This is known as "patellar mal-tracking"

Structural irritation due to mal-tracking:
  • Synovial plicae
  • Infrapatellar fat pad
  • Retinaculae (medial or lateral)
  • Joint capsule
  • Patellofemoral ligaments
  • Subchondral bone
  • Increased cartilage degeneration

How will PFP Present?

Clinical Presentation

PFP is a condition that is characterized by insidious onset of poorly defined pain quality localized to the anterior retropatellar and/or peripatellar region of the knee. The onset of symptoms can be slow or acute, with a worsening of pain with lower-limb loading (e.g., squatting, prolonged sitting, ascending /descending stairs, jumping, or running, especially with hills).Diagnosis is based on a cluster of signs and symptoms, after ruling out other patho-anatomic diagnoses (e.g. trapped meniscus) .Because there is typically a progressive, insidious onset of symptoms, diagnosis is often delayed, and describing the typical clinical course is difficult.

Pain

PFP can be reproduced by functional tasks such as squatting, stair climbing, and sitting with flexed knees. Often, patients report pain with palpation of the distal pole or medial aspect of the patella, the medial plica, and the medial femoral condyle. Pain is also often present with grinding or compressing of the patella.

Anthropometrics

Patient characteristics, anthropometrics, and patellofemoral alignment are often postulated as important factors in the development of PFP. However, a recent systematic review of observational studies concluded that age, body mass, height, and body mass index (BMI), the quadriceps angle (Q angle, assessed either in weight bearing or non–weight bearing) and excessive foot pronation does not appear to be a feature across studies examining individuals with PFP. However, we cannot rule out the impact these characteristics have on loading patterns of the knee joint.

Decreased Thigh Force Production

Compared with healthy, matched controls, people who develop PFP have weaker quadriceps, as measured by a dynamometer. Impaired lower extremity muscle function is common in individuals with PFP. Quadriceps atrophy is also a common finding in individuals with PFP, but only when it is evaluated by imaging, not by girth or visual assessment. Quadriceps atrophy is consistent across the vastii musculature (i.e., not isolated to the vastus medialis oblique musculature!).

Quadriceps inhibition (inability of the central nervous system to fully activate the quadriceps) of roughly 18% when assessed with surface electromyography (EMG), suggesting that at least some of the loss of quadriceps strength is due to inhibited central neural drive. This may be partly due to the pain generated from the PFJ through arthrogenic inhibition.

Decreased Hip Force Production

People with PFP have weakness of the hip abductors, extensors, and external rotators. Rate of force development of the hip abductors and extensors is also reduced There is a strong possibility that hip weakness is a result of PFP, and not a direct cause of PFP.

IMPACT OF DYNAMIC ACTIVITIES ON PFJ

Altered biomechanics are commonly observed during functional movements in people with PFP. People with PFP may walk, run and negotiate stairs with reduced knee flexion compared to healthy controls, which may represent a compensation pattern.

A biomechanical risk factor for PFP is increased hip internal rotation during a jump-landing task, was shown to be a significant predictor of individuals who went on to develop symptoms of PFP.(9) When performing a dynamic task, individuals who have decreased quadriceps strength may display decreased knee flexion angles because the task demands a large amount of eccentric force from the quadriceps musculature and the quadriceps are weak in these individuals. Although, increased or decreased knee extension moment did not predict the development of PFPS, the descriptive analysis showed that individuals who developed PFPS had significantly less knee extension moment during the jump-landing task. Decreased knee extension moment and decreased quadriceps strength may lead to decreased dynamic control of the patella. Additionally, an increase in hip internal rotation possibly due to the increased navicular drop, may lead to a laterally aligned patella.

The combination of increased hip internal rotation angle and decreased knee flexion angle will most likely increase the patellofemoral contact pressures, and over time, repetitive movements in this position may lead to the development of PFP. However, the finding of increased hip internal rotation angle along with the increased hip external rotation strength leads us to believe this may be a neuromuscular control issue, meaning individuals do not know when to recruit the hip external rotators during dynamic tasks, leading to the increased hip internal rotation angle.

Factors affecting PFJ?
How to Diagnose PFP?

Reproduction of retropatellar or peripatellar pain during squatting as a diagnostic test for PFP.

Use performance of other functional activities that load the PFJ in a flexed position, such as stair climbing or descent, as diagnostic tests for PFP.

Criteria for diagnosis of PFP:
  • The presence of retropatellar or peripatellar pain.
  • Reproduction of retropatellar or peripatellar pain with squatting, stair climbing, prolonged sitting, or other functional activities loading the PFJ in a flexed position.
  • Exclusion of all other conditions that may cause AKP, including tibiofemoral pathologies.
  • Clinicians may use the patellar tilt test with the presence of hypomobility to support the diagnosis of PFP.

Classification guidelines for PFP?

1. Support for the “overuse/overload without other impairment” subcategory

One potential factor leading to PFP is performing activities that load the patellofemoral compartment with too much load magnitude, too much load frequency and/or at too great a rate of increase, that is, overuse. When individuals increase the magnitude and/or frequency of PFJ loading during an activity at a rate greater than musculoskeletal tissues can adapt, they move into a zone of supraphysiologic overload and eventual pain.

2. Support for the “muscle performance deficits” subcategory

Females with PFP have hip weakness, particularly with isometric strength testing. Originally thought to be a risk factor for PFP onset, hip weakness, especially of the hip abductors, extensors, and external rotators, has been shown to result from PFP.

The reason for the resulting hip weakness remains unclear; however, evidence supports the importance of resistance exercises targeting the hip muscles as part of the intervention for individuals with PFP.

3. Support for the “movement coordination deficits” subcategory

Increased hip adduction, hip internal rotation, and knee abduction (i.e., knee valgus) during dynamic activities can increase the dynamic Q angle. Knee abduction and external rotation also increase the Q angle by moving the tibial tubercle lateral relative to the patella. Together, these altered movements can impart increased stress to the lateral PFJ.

4. Support for the “mobility impairments” subcategory

A subcategory of individuals with PFP may have impairments related to either hypermobile or hypomobile structures. The diagnosis of PFP with mobility impairments is made with a fair level of certainty when the patient presents with higher than normal foot mobility and/or flexibility deficits of 1 or more of the following structures: hamstrings,quadriceps, gastrocnemius, soleus, lateral retinaculum, or iliotibial band.