Introduction
Patellofemoral pain is one of the most common sports-related knee conditions. , Patellofemoral pain is characterized by pain around or behind the patella during activities that load the patellofemoral joint, such as squatting, stair ambulation, running, jumping, and prolonged sitting with knees in a flexed position. Patellofemoral pain is particularly prevalent in younger, active populations, , , with females twice as likely to develop patellofemoral pain as males (29.2% vs. 15.5%, respectively). ,
As the pathogenesis of patellofemoral pain remains largely unknown, it continues to be a complex and challenging problem to treat. While once thought as a benign, self-limiting condition, research has shown that patellofemoral pain can be recurrent and can persist for years . Patellofemoral pain can negatively affect sports participation in adolescent female athletes. Notably, single-sport specialization was found to increase the risk of patellofemoral pain incidence by 1.5-fold compared to multiple-sport adolescent female athletes. Patellofemoral pain may also be a contributing factor to the development of patellofemoral osteoarthritis. In addition, young females with patellofemoral pain may have risk factors that increase the risk of future ACL injury as they mature. Emerging research is allowing for better understanding of gender-specific risk factors that will lead to better treatment and, more importantly, prevention strategies. Here, patellofemoral anatomy, patellofemoral pain causes, risk factors, diagnoses, and both nonsurgical and surgical treatments will be reviewed.
Anatomy
Bony Anatomy
The patella is the largest sesamoid bone in the body, coupling the quadriceps tendon to the patellar tendon and acting to optimize the extensor mechanism’s mechanical advantage by increasing its moment arm. The patella has seven articular facets, but it can largely be referred to by its medial and lateral facets with interposing central ridge. Articular cartilage is present on the superior two-thirds of the patella, with the inferior pole remaining as an extra-articular attachment point for the patellar tendon. The size of the patella relative to the trochlea has been tied to patellofemoral pain, with increased patellar to trochlear width and volume ratios observed in adolescent females with patellofemoral pain.
The femoral trochlea is complementary to the patella, composed of medial and lateral facets of the femoral sulcus, allowing a congruent surface for patellar tracking throughout knee flexion, with increasing engagement due to a deeper groove more distally. In cases of trochlear dysplasia, the trochlea is shallow to flattened or even convex with increasing degrees of medial facet hypoplasia and supratrochlear spur formation. Trochlear dysplasia represents one of the most impactful anatomic variants in cases of patellar instability, but it should not be overlooked in patients with patellofemoral pain without instability. Dejour et al. described Potential Patellar Instability as occurring in patients with anatomic instability factors but without a history of patellar dislocations. Within this group, there is a high incidence of patellofemoral cartilage damage. Particularly in females with trochlear dysplasia, the incidence of chondromalacia patella and structural cartilage damage is elevated. In patients with end-stage patellofemoral osteoarthritis at the time of patellofemoral arthroplasty, 78% of patients had high-grade trochlear dysplasia, whereas only 33% had a history of patellar instability. These findings highlight the role that trochlear dysplasia plays not only in patellar instability but also in patellofemoral pain and arthrosis.
Limb Alignment
Frontal plane limb alignment plays an essential role in patellofemoral mechanics. The quadriceps angle (Q-angle) is commonly used as a means of quantifying the quadriceps’ summative force vector on the patellofemoral joint. This is assessed by measuring the angle formed by lines drawn (1) from the anterior superior iliac spine to the center of the patella and (2) from the tibial tubercle to the center of the patella. The mean Q-angle is 15 degrees, but on average the Q-angle is 3–6 degrees larger in females than males, attributable largely to females’ wider pelvis. , Q-angle greater than 20 degrees is generally considered abnormal. While elevated Q-angle theoretically increases lateral displacement forces across the patellofemoral joint, current literature does not support static Q-angle as a risk factor for patellofemoral pain, but rather emphasizes the impact of alterations in dynamic quadriceps force vector.
Axial alignment, namely, lateral patellar tilt, is another important consideration in evaluating the female athlete with patellofemoral pain. Excessive lateral patellar tilt increases contact stresses on the lateral patellar facet and lateral trochlea, with resultant pain and chondral wear. Contributors to lateral patellar tilt include tight lateral retinaculum, weak vastus medialis, medial ligamentous laxity, and bony alignment. While more commonly pathologic in those with patellofemoral instability, relative lateralization of the tibial tubercle is also seen with increased frequency in adolescent females with patellofemoral pain without instability. Excessive femoral anteversion and tibial external torsion each function to increase lateralization of the quadriceps vector and likely play a role in some cases of patellofemoral pain when significantly altered; however, their exact role in patellofemoral pain is not yet fully understood.
Patellar height is also implicated in patellofemoral pain. The inferior patellar position in patella baja allows contact with the trochlea earlier in a range of flexion and has been shown to result in lower patellofemoral contact stresses. , Patella baja can be secondary to severe quadriceps inhibition, postoperative arthrofibrosis, patellar tendon scarring, or congenital growth disorders. Thus anterior knee pain associated with patella baja may be attributable to poor parapatellar soft tissue mobility rather than alterations in joint loading patterns. Patella alta, on the other hand, allows a greater arc of flexion prior to patellar engagement in the trochlear groove, which can result in instability as well as smaller patellar contact area. Patella alta has been shown to correlate with significantly increased contact force and contact pressure , and can yield inferior-predominant patellar chondrosis and pain.
There is some evidence that an overpronated foot posture, measured using navicular drop, may represent a minor component of the multifactorial cause for patellofemoral pain development. Conflicting studies debate the magnitude of this impact, which does not appear to be gender-specific. , Despite inconclusive evidence to support foot alignment as the risk factor for patellofemoral pain development, there is likely a subset of persons in which this is a contributory factor.
Pathophysiology
Pain Pathogenesis
The mechanisms that cause patellofemoral pain remain poorly understood, and the prevailing theory is that patellofemoral pain is the result of excessive joint loading and elevated joint stress. , Although the local tissue structures and specific pain pathways involved with the pain associated with patellofemoral pain are unknown, , it is thought that the abnormal loading causes strain to innervated structures within the patellofemoral joint. These structures include the infrapatellar fat pad, medial and lateral retinacula, synovium, and subchondral bone. , , Furthermore, those with persistent patellofemoral pain can exhibit altered nociceptive processing such as generalized hyperalgesia and increased perceived pain through sensitization of both peripheral and central pain mechanisms. , , Generalized hyperalgesia has been reported more frequently in females with patellofemoral pain, with evidence from a meta-analysis suggesting the presence of altered pain processing and sensitization in individuals with patellofemoral pain, particularly in females. Pain sensitization and other nonmechanical influences on symptoms, such as psychologic factors, may play roles in patellofemoral pain pathogenesis and are current research focus areas.
Altered Lower Extremity Joint Kinematics
The theorized biomechanical risk factors for patellofemoral pain include lower extremity structural malalignment as discussed earlier in this chapter as well as altered lower extremity joint kinematics, decreased muscle strength, altered neuromuscular recruitment, and muscle tightness. , In particular, altered hip and knee , frontal plane dynamic alignment have been reported as potential risk factors for patellofemoral pain development in females. Excessive activity leading to overuse or overload is also considered a potential contributor. , With its multifactorial cause, patellofemoral pain appears to be the result of multiple interactive underlying conditions and/or impairments that challenge the load-bearing capacity of the joint, leading to symptom onset and contributing to its persistence. ,
Evidence suggests that under weight-bearing conditions, patellofemoral maltracking is the result of femoral internal rotation generating a relative lateral displacement of the patella with respect to the femur. This lateralized patellar tracking reduces patellofemoral contact area, which increases focal joint stress and can ultimately contribute to the development of patellofemoral pain . This suggests that the control of femoral rotation, especially during weight-bearing activities, may be important for optimal patellofemoral joint function, highlighting the influence that proximal factors have on knee biomechanics. Elevated dynamic Q-angle describes altered lower extremity kinematics, which increases laterally directed forces on the patellofemoral joint and may contribute to patellofemoral pain development, especially in females . The altered kinematic factors considered to have greatest influence on the dynamic Q-angle, or dynamic functional valgus, include increased hip adduction affecting knee frontal plane motion through excessive knee valgus and increased hip internal rotation affecting transverse plane knee motion. Even 5 degrees of excessive femoral internal rotation has been shown to increase patella cartilage stress during a squatting task in females.
It should be noted that the available data is not consistent regarding an association between altered lower extremity kinematics and patellofemoral pain development. However, evidence is mounting that, especially in females, impaired control of the hip can adversely impact patellofemoral mechanics. Specifically, increased hip adduction, hip internal rotation, contralateral hip drop, and reduced peak hip flexion are seen in both female and mixed-gender patellofemoral pain populations. , These unfavorable proximal kinematics are most influential during running, jump landing, and single-leg squat activities. In particular, increased hip adduction angles are seen in female but not male runners with patellofemoral pain as well as decreased peak hip flexion angles. Increased hip internal rotation during a single-leg squat has also been reported in females with patellofemoral pain. In regard to jump landing, increased dynamic knee valgus as well as hip adduction and knee internal rotation have significant correlation with the development of patellofemoral pain in females. , , In one study, two-dimensional measures of knee valgus displacement ≥10.6 degrees during a jump landing task predicted patellofemoral pain in adolescent females, with a sensitivity of 0.75 and specificity of 0.85, and appeared to represent a manifestation of altered motions at the hip. As knee valgus displacement is also considered a risk factor associated with ACL injury in females, Myer et al. concluded that young girls with patellofemoral pain may have risk factors that increase their odds of future ACL injury as they mature. In summary, for active females, altered hip kinematics significantly affect patellofemoral biomechanics, causing abnormal joint stress and likely represent a contributing factor to patellofemoral pain development.
Muscle Strength, Neuromuscular Recruitment, and Muscle Tightness
While retrospective studies have found that females with patellofemoral pain have decreased hip abduction, external rotation, and extension strength on the affected side as compared with asymptomatic controls, these studies cannot determine whether muscle weakness is a cause or an effect of patellofemoral pain. , To this point, prospective studies have not found an association between isometric hip strength and the risk of developing patellofemoral pain. , Reduced hip strength is more likely a result of patellofemoral pain, not the cause. , Of note, in adults with patellofemoral pain, larger hip strength deficits are found in females than in males and mixed-gender populations, indicating the importance of hip strengthening in female adults and possibly less so in male adults and adolescents of either sex. Conversely, one large prospective study found that female adolescent basketball athletes with greater hip abduction strength were at an increased risk for patellofemoral pain development. The authors theorized that this could be a result of increased eccentric loading of the hip abductors due to higher peak hip adduction during landing to correct for dynamic valgus biomechanics. This highlights the need for different treatment strategies between female and male adolescent and adult populations.
Individuals with patellofemoral pain often exhibit altered trunk kinematics in the sagittal and frontal planes, which can contribute to added strain to the patellofemoral joint. Specifically, those with patellofemoral pain more often exhibit an ipsilateral trunk lean during a jump landing task and single-leg squat, which can increase the potential for a knee valgus moment by shifting the center of mass of the body toward the stance limb. Abnormal motions of the trunk, including ipsilateral trunk lean, may be a compensatory strategy for weak hip strength, especially of the hip abductors. , ,
Altered neuromuscular function of the knee extensor and gluteal muscles has been investigated as a potential contributor to patellofemoral pain. Increased neural drive to the vastus lateralis as compared to the vastus medialis is seen in females with patellofemoral pain and may represent a risk factor in a subgroup of individuals with patellofemoral pain, but the larger role of this altered activation and its effect on patellar function remains uncertain. In regard to gluteal muscle activity and patellofemoral pain, studies have found delayed and shorter duration of gluteus medius activity in those with patellofemoral pain during climbing stairs, in females while performing single-leg squats, and in female runners.
Decreased flexibility of the quadriceps, hamstrings, gastrocnemius, and soleus has been implicated as risk factors for patellofemoral pain theorized to increase patellar compression. , , Those with patellofemoral pain can also exhibit a tighter and thicker iliotibial band, which can affect patellar alignment and increase lateral patellar forces. However, there are inconsistent findings among prospective studies linking muscle tightness to patellofemoral pain development, so no direct relationship is currently delineated.
Several systematic reviews from the past decade have consistently found only decreased knee extension strength as a consistent risk factor for patellofemoral pain development in both adult female and male populations. , , Unique to adolescent populations, quadriceps weakness was not found to be a risk factor for patellofemoral pain development, but stronger hip abductors were. Despite the many variables that have been investigated in relation to the development of patellofemoral pain, clearly identified risk factors are not evident. This is in comparison to the structural, biomechanical, pain-processing, and certain psychologic factors found in those with patellofemoral pain. As patellofemoral pain pathogenesis is believed to be complex and multifaceted, future studies investigating interactions of multiple variables as predictive of patellofemoral pain are warranted.
Overuse/Overload
As with other overuse injuries, training errors and overactivity can contribute to patellofemoral pain. Any increase in frequency, duration, or intensity without proper rest and adaptation can push the knee outside its “load acceptance capacity” through loss of tissue homeostasis that can ultimately lead to pain and injury. For some active individuals with patellofemoral pain, there is lack of significant findings on physical examination, suggesting a role of overuse and training errors. This is highlighted by the fact that the most common overuse running injury is patellofemoral pain. Education on load management is thus recommended as an important treatment strategy. , ,
Imaging
Assessment of static anatomic factors contributing to patellofemoral pain is best achieved through a combination of physical examination with critical analysis of orthogonal radiographs and, when indicated, magnetic resonance imaging (MRI) and/or computed tomographic (CT) scan. Standard knee radiographs including weight-bearing anteroposterior (AP) and true lateral views, as well as an axial view of the patellofemoral compartment should be obtained in all patients with anterior knee pain. A summary of measurements and findings on orthogonal radiographic views pertinent to patellofemoral pain is provided in Table 7.1 .
| Anteroposterior View |
| Bipartite patella |
| Tibiofemoral degenerative changes |
| True lateral View |
| Caton-Deschamps index Normal: 0.6–1.2 |
| Trochlear dysplasia findings (supratrochlear spur, crossing sign, double contour) |
| Axial (Merchant or Laurin) View |
| Sulcus angle Normal: 138 ± 6 degrees |
| Congruence angle Normal: −6 ± 6 degrees |
| Lateral patellofemoral angle Normal: ≥8 degrees |
| Patellofemoral index Normal: 1.6 |
| Signs of lateral overload (subchondral sclerosis, lateral patellar spur) |
| Standing Long-Leg Alignment View |
| Limb mechanical axis |
| Frontal plane malalignment |
| Signs of malrotation (hidden lesser trochanters, squinting patellae) |
The AP view is useful for identifying tibiofemoral degenerative changes and can also demonstrate bipartite patella if present.
Lateral radiographs allow assessment of patellar height and classification of trochlear morphology. Patellar height can be measured by several ratios including the Insall-Salvati ratio, Caton-Deschamps index (CDI), and Blackburne-Peel index. Among patellofemoral surgeons, the CDI is generally preferred because of its independence from both degree of knee flexion and tibial tubercle position, maintaining the index’s reliability after tibial tubercle osteotomy. The CDI is measured as the ratio between the patellar articular surface length and the distance from the inferior articular surface to the anterosuperior tibial border ( Fig. 7.1 ). Normal CDI is defined as 0.6 to 1.2, with values less than 0.6 denoting patella baja and greater than 1.2 denoting patella alta. The lateral view is also useful in identifying findings of trochlear dysplasia, including the crossing sign, a supratrochlear spur, and the double contour sign. Radiographic and MRI findings consistent with trochlear dysplasia are discussed in greater depth in Chapter 8 .
Axial views in low degrees of flexion, 30 degrees or less, provide an optimal view of patellofemoral articular congruence, lateral tilt, subluxation, and trochlear dysplasia. Axial radiographs in greater flexion allow for increased trochlear capture and can obscure patellar subluxation present at lesser degrees of flexion. The sulcus angle ( Fig. 7.2 ), discussed in further detail in Chapter 8 , is increased in the setting of trochlear dysplasia. The congruence angle is measured as the angle between a line bisecting the sulcus angle and a line from the deepest point of the sulcus to the retropatellar apex ( Fig. 7.3 ). A patellar apex line located medial to the bisector line is considered a negative value, with normal congruence angles averaging −6 ± 6 degrees. The lateral patellofemoral angle is formed between a line connecting the anterior medial to lateral condyle and a line along the lateral patellar facet ( Fig. 7.4 ). In normal knees, this opens laterally (≥8 degrees) with smaller angles signifying increased lateral tilt. , Patellofemoral index is a ratio between the smallest distance from the median ridge to medial femoral condyle and the smallest distance between the lateral facet and lateral condyle ( Fig. 7.5 ). Normal patellofemoral index is 1.6 or less, with smaller values seen in the setting of increased lateral tilt. Subtle signs of lateral overload can also be visualized on axial views even in the absence of any grossly abnormal values as earlier, with lateral facet subchondral sclerosis or cyst formation, lateralization of the patellar trabecular pattern, and lateral patellar facet traction spur formation ( Fig. 7.6 ).
A scanogram, or bilateral standing long-leg lower extremity alignment radiograph, allows the identification of the lower extremity mechanical axis and quantification of frontal plane malalignment. Significant valgus alignment is associated with greater incidence of patellofemoral pain and instability. Scanogram can also demonstrate qualitative indicators of femoral malversion, with decreased visualization of the lesser trochanters and squinting patellae suggesting increased femoral anteversion. Although the sensitivity for malrotation identification on scanogram is poor, these findings can hint at a need for formal analysis of limb rotation.
MRI has become a common adjunct to traditional radiographs in the workup of patients with patellofemoral pain. MRI allows for visualization of cartilaginous and bony surfaces as well as the soft tissues intimately involved with the patellofemoral joint. This can reveal parapatellar causes of pain, such as quadriceps or patellar tendinopathy, fat pad hypertrophy, edema, or scarring, and lateral retinacular thickening. MRI is limited in its provision of a static view of the joint in knee extension and thus should not be regarded as a substitution for appropriate orthogonal radiographs and physical examination. MRI should be considered in cases of significant effusion, mechanical symptoms, or failure of nonoperative care, but it should not represent the first-line imaging study in a majority of patients with patellofemoral pain. Outerbridge grade 3–4 chondral defects can be seen in the patellofemoral joint in up to 45% of knee arthroscopies, representing the most frequently compromised compartment of the knee when considering high-grade cartilage injury. MRI is highly sensitive and specific in identifying patellofemoral cartilage defects, but it is important to recognize that these lesions are often asymptomatic incidental findings and structural abnormalities are seen on MRI imaging of the patellofemoral compartment with similar frequency in patients with and without patellofemoral pain. Lateral patellofemoral chondral lesions are more often symptomatic than those located medially, and defects with subjacent bone marrow edema either medially or laterally are frequently symptomatic.
Lateral patellofemoral overload is commonly involved in cases of patellofemoral pain and has several indicators on MRI. Generally, this involves lateral chondral lesions and subchondral marrow edema, thickening of the lateral retinaculum, and lateral tilt or translation of the patella. Lateral patellar tilt can be measured on MRI axial series as the angle between a lie drawn along the transverse axis of the patella (on the cut with the greatest patellar width) and a line tangent to the posterior femoral condyles on axial MRI ( Fig. 7.7 ). Normal patellar tilt is on average 2–5 degrees, but is most predictive of full-thickness chondral defect when in excess of 15 degrees. , Bisect offset ( Fig. 7.8 ) is another indicator of lateral patellar malalignment, measured as the percentage of the patella lateral to the midline of the trochlea (with the trochlear midline transposed from the axial slice with the widest posterior condylar line to the slice with the widest patella). Bisect offset greater than 61.5% is strongly predictive of full-thickness lateral patellofemoral cartilage defect and pain with stairs. Elevated tibial tubercle to trochlear groove (TT-TG) distance has been found to correlate with Q-angle and is linked to patellar instability, chondrosis, effusion, and pain. , , TT-TG distance is the horizontal distance between the tibial tubercle and the trochlear groove, parallel to the posterior intercondylar line, on superimposed axial MRI images ( Fig. 7.9 ). TT-TG distance greater than 15–20 mm is considered abnormal.
Patellar height can also be assessed on sagittal MRI images. The patellar articular overlap provides a means of assessing functional patellar engagement on MRI and has been found to correlate well with radiologic indices such as CDI. This is assessed by looking at the patellar articular length, which aligns with the trochlear cartilage on a sagittal view as a percentage of total patellar articular surface length ( Fig. 7.10 ).
In cases of suspected malrotation, a CT scanogram can help quantify femoral version and tibial torsion. Cuts should be taken through the femoral neck, the mid-trochlear region of the knee, the proximal tibia, and the distal tibiofibular joint to provide a complete limb rotational profile. CT scan can also be used to make axial-cut measurements such as TT-TG distance, sulcus angle, patellar offset, and trochlear inclination, as on MRI.
Patient Presentation
Patients with patellofemoral pain classically present with anterior knee pain. The differential diagnosis of patients complaining of anterior knee pain is extensive ( Table 7.2 ), and identification of those with patellofemoral pain central to their symptomatology depends on careful consideration of the history, physical examination, and imaging findings. Patients should be asked about the onset of pain, noting the presence of any antecedent trauma or change in training activities. While patellofemoral pain is classically a problem of overuse, direct trauma to the anterior knee can cause crush-type injuries to the articular cartilage and subchondral bone. Patients should be asked to point with one finger to the location of their pain, as this can help discern anterior knee pain located within the patellofemoral joint (deep to the patella) versus along the patellar or quadriceps tendons. Symptoms of patellofemoral pain are often worst with activities requiring prolonged or repetitive knee flexion, such as squats, stair climbing, and prolonged sitting. Depending upon the underlying pathophysiology, patients may also complain of swelling, crepitus, locking, and weakness or giving way in the affected limb. The position of mechanical locking can be helpful in determining the underlying cause. A knee locked in full extension with hesitancy to flex is due to avoidance of engaging the patella within the trochlear groove and thus typically relates to the patellofemoral joint. Loose body or meniscal pathology conversely locks the knee in some degree of flexion.
| Muscle/Tendon | Soft Tissue |
| Quadriceps tendinopathy | Patellofemoral instability/subluxation |
| Patellar tendinopathy | Lateral patellofemoral overload |
| Quadriceps atrophy/deconditioning | Potential patellofemoral instability |
| Hamstring contracture | Fat pad impingement |
| Bone | Prepatellar bursitis |
| Patellar stress fracture | Pes anserine bursitis |
| Symptomatic bipartite patella | Iliotibial band syndrome |
| Loose body | Synovial |
| Osteochondritis dissecans | Symptomatic synovial plica |
| Bone tumor | Pigmented villonodular synovitis |
| Frontal plane malalignment (genu valgum) | Inflammatory |
| Rotational malalignment (femoral or tibial) | Rheumatologic/autoimmune arthropathy |
| Apophyseal | Lyme disease |
| Sinding-Larsen-Johansson syndrome | Septic joint |
| Osgood-Schlatter disease | Neurologic |
| Cartilage | Lumbar radiculopathy |
| Articular cartilage injury | Saphenous neuritis |
| Chondromalacia | Neuroma |
| Osteoarthritis | Complex regional pain syndrome |
| Referred | |
| Hip pathology |
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