Overview of Pathologies

Patellofemoral pain is a common musculoskeletal ailment in active adolescents and adults. The incidence may be as high as 50% in some populations. Patellofemoral pain, which is also known as anterior knee pain, is attributable to multiple etiologies, and despite being commonplace, it often results in consternation for clinicians as they attempt to accurately diagnose and efficiently treat it. Whereas most chapters in this book address specific disease processes, the topic of “patellofemoral pain” reflects the broad application of this term as a generic descriptor for various causes of pain in a particular location. The reason for this generic description is multifactorial but includes the similarity in clinical presentation among different etiologies, the elusiveness in defining the exact focus of pain based on simple diagnostic studies, and the similarity in treatment protocols, which often emphasize early nonsurgical management.

A subset of patients with patellofemoral pain have no definable pathology. This process has been described by different names, including patellofemoral pain syndrome and idiopathic anterior knee pain. It is important to note that these terms to some extent are diagnoses of exclusion and should not be used to mislabel a definable disease process. It is also a misnomer to attribute all patellofemoral pain to “chondromalacia” because in many cases no chondral pathology exists.

Anatomic Considerations

The anterior aspect of the knee has several unique properties that serve as a predisposition for pain. First, the patellofemoral joint experiences joint reaction forces that become higher as the knee flexion angle increases ( Fig. 105-1 ). Squatting and jumping have been reported to create forces 7.6 times and 12 times body weight, respectively. The anterior aspect of the knee also provides significant sensory feedback. Dye underwent arthroscopy without anesthesia and reported the highest subjective pain sensation in the anterior synovium, fat pad, and joint capsule. Interestingly, structures that commonly require intervention (e.g., the menisci, articular cartilage, and ligaments) were much less sensitive. Dye and colleagues also reported accurate spatial localization to palpation in the anterior structures ( Fig. 105-2 ), which correlates with previous findings about the relatively high concentration of type IVa afferent nerve fibers in the patellar ligament and retinaculum.

The patellofemoral joint reaction force ( PFJRF ) becomes higher as the knee flexion angle increases. In complete extension, M1 and M2 are in opposite directions, but in the same plane; the resultant PFJRF is almost zero. As flexion increases, M1 and M2 converge, and the vector PFJRF increases.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Posterior anatomy of the patella with adjacent capsular thickenings and fat pads with the medial and lateral meniscopatellar ligaments.

(From Noyes FR, editor: Noyes’ Knee Disorders: Surgery, Rehabilitation, Clinical Outcomes, Philadelphia, 2010, Elsevier.)

Entheses are defined as the insertion sites between tendon and bone. A recent study noted adipose tissue at multiple entheses and stressed that this common occurrence is not a pathologic process. The investigators noted a rich innervation of this adipose tissue and suggested a role in mechanosensory feedback. Although it is commonly implicated in pain syndromes, this innervation may serve an important function in normal knee physiology. In certain pathologic conditions, a proliferation of nociceptive axons and an increase in neural growth factor may be found. The subchondral bone is also richly innervated, and thus any pathology involving trauma or edema to this area would likely prove noxious.

Malalignment is a commonly proposed mechanism for patellofemoral pain. A significant amount of literature questions a link between malalignment and patellofemoral pain, however. The common occurrence of pain at rest is an argument against malalignment as a major component of most anterior knee pain. Dye proposed that pain is from a loss of tissue homeostasis. The loads encountered may reach the limitation of some of the soft tissue structures. Increased intraosseous pressure in the patella with resultant tissue ischemia has also been proposed. Some studies support altered vascular flow (arterial and venous) and concomitant degenerative conditions. However, it is difficult to assess if these changes are primary or secondary in nature.

History

Patients with patellofemoral conditions should be stratified into those experiencing pain and those experiencing instability. Admittedly some overlap occurs, but patients commonly endorse either a chief complaint of instability with reasonable pain resolution after the acute incident or significant pain with no symptoms of instability. This determination is a useful initial stratification tool. The temporal relationship of the patient’s symptoms also helps aid in diagnosis. Much of the acute pathology (e.g., extensor mechanism disruption, patellar fractures, and instability) is addressed in other chapters. Activity-related chronic pain is a common clinical scenario that encompasses multiple disorders and has a broad differential diagnosis ( Table 105-1 ). This chapter focuses predominantly on these processes, and the clinical parameters specific to certain etiologies are reviewed in greater detail.

For patients with activity-related chronic pain (without acute trauma), the discomfort commonly occurs while ascending or descending stairs, with deep squats, or while sitting for prolonged periods. It is common to have the symptoms bilaterally (in one study of adolescents with anterior knee pain, bilateral symptoms were described by approximately two thirds of subjects). Patellofemoral pain is also common during periods of increased activity, such as during high-intensity training with military recruits.

Physical Examination

In all patients presenting with knee pain (particularly patellofemoral pain), the physical examination should include an assessment for hip pathology. This assessment is particularly important for adolescents with anterior knee and distal thigh pain. Conditions such as slipped capital femoral epiphysis often present with knee pain as the chief complaint. One study demonstrated a 52% missed diagnosis rate and a referral delay of 76 days after primary care visitation for patients with slipped capital femoral epiphysis. A simple logroll examination should not provoke pain in the patient with patellofemoral pathology. An alternative examination can be performed with the patient seated at the edge of the examination table and the knee flexed to approximately 90 degrees. The examiner can rotate the hip into internal and external rotation by moving the foot and ankle in a pendulum motion (abduction/adduction). If pain is felt at the extremes of motion either in the groin, thigh, or knee, then a strong consideration should be given to pathology of the hip joint.

It is important to note that many variations exist in patellofemoral anatomy. These variations are evident during examination and may predispose a patient to patellofemoral pain; however, it is important to note that few, if any, examination findings are highly specific for a diagnosis in the differential diagnosis for patellofemoral pain. Malalignment is not pathognomonic for anterior knee pain. The examination findings are a portion of the complex clinical picture commonly encountered in the patient with anterior knee pain.

Global

The association of chronic pain and pain syndromes with the patellofemoral region requires an awareness of psychological and social factors. A component of anxiety or an exaggerated emotional response should be considered when evaluating treatment plans and determining prognosis. J.T. Andrish (personal communication, Cleveland Clinic, Cleveland, Ohio, 2011) noted a somatization of pain as a result of sexual or physical abuse in female adolescents with patellofemoral pain. This consideration is important in patients without significant structural pathology or who have failed to respond to multiple interventions.

Inspection

Gross inspection identifies limb alignment, prior incisions, gross atrophy, and large effusions and may demonstrate vasomotor changes associated with complex regional pain syndrome. Standing examination allows for evaluation of limb alignment and rotation profile. Patients with increased femoral anteversion and genu valgum may be at an increased risk for patellofemoral pain. A seated examination with the legs hanging off the end of the examination table provides a rough assessment of the Q angle. The Q angle is a measurement from the anterior superior iliac spine to the center of the patella and then from the center of the patella to the tibial tuberosity ( Fig. 105-3 ). It is important to appreciate that the Q angle may be affected by the relationship of the proximal femur and the amount of knee flexion ( Figs. 105-4 and 105-5 ). An elevated Q angle may predispose the patient to anterior knee pain, although significant variability exists. A flexion/extension cycle helps assess patellar tracking. A J sign is noted when the patella deviates laterally when the knee reaches terminal extension, and thus the Q angle may be less accurate in full extension in a patient with a positive J sign. Patients with external tibial torsion of foot pronation may experience increased patellofemoral pain.

Q angle.

(Modified from Livingston LA, Mandigo JL: Bilateral Q angle asymmetry and anterior knee pain syndrome, Clin Biomech [Bristol, Avon] 14[1]:7–13, 1999.)

High femoral neck retroversion rotates the distal end of the femur externally. In combination with internal tibial torsion, the Q angle is decreased. Patellar tracking is improved, and patellofemoral sulcus alignment is normal. High femoral neck anteversion rotates the distal end of the femur internally. In combination with external tibial torsion, the Q angle is increased. Patellar tracking is compromised, and the patella tends to track laterally.

(From Tria AJ, Jr, Klein KS: An Illustrated Guide to the Knee, New York, 1992, Churchill Livingstone.)

Flexion of the knee decreases the Q angle as the result of internal tibial rotation.

(From Tria AJ Jr, Klein KS: An Illustrated Guide to the Knee, New York, 1992, Churchill Livingstone.)

Gait monitoring may help with patellar tracking and determining excessive femoral anteversion, as well as an awareness of global deficiencies in the kinetic chain. These factors may provide information regarding patellar instability, which is addressed in greater depth in Chapter 104 . It is important to include an assessment for instability in the examination of patients with patellofemoral pain to determine any causal relationship between the two.

Palpation

Many of the involved structures (e.g., the fat pad, quadriceps and patellar tendon attachments to the patella, retinaculum/patellofemoral ligaments, and tibial tubercle) can be palpated with reasonable ease and can aid in localizing the focus of pain. In one study, 98% of adolescent patients with patellofemoral pain experienced pain with palpation over the medial patellofemoral ligament. An appreciation of the different diagnoses helps the examiner with this portion of the examination by identifying which locations are commonly painful (e.g., the patellar/quadriceps tendons, iliotibial [IT] band, tibial tuberosity, infrapatellar fat pad, menisci, and retinaculum).

Range of Motion

Fulkerson advises a prone examination to compare knee flexion, and any discrepancy would be attributable to quadriceps tightness. The motion should be symmetric and approach the gluteal crease. Decreased extension can be determined in the supine, prone, or standing positions by requesting that the patient straighten his or her legs completely and assessing them from a lateral position. If genu recurvatum is possible, this characteristic can be determined in the standing or supine position by attempting to hyperextend the knee while supporting the thigh just cephalad. Crepitus is common during range of motion testing in both symptomatic and asymptomatic patients.

Provocative Tests

Pressure applied in various manners to the patella has different names, including patellar grind, patellar compression, or the Clarke sign. The first mention of patellar grind was by Owre in 1936:

Pressure-pain over the patella is tested by clasping the patella with the thumb and index finger of each hand with the remaining fingers resting against the thigh and leg. While the patient lies with the leg relaxed and extended, the patella is pressed against the medial and lateral femoral condyles. By moving the patella in an upward and down-ward direction the greater part of the surface cartilage may be examined in this manner. In some cases, pain is elicited on the slightest pressure of the patella against the condyle, at other times considerable pressure must be exerted to obtain a positive response of an unpleasant sensation.

This test was described as positive in patients with retropatellar chondral pathology. Other variations of the grind test or Clarke sign (or test) involve application of pressure on the superior border of the patella (with the web space of the thumb) while asking the patient to contract the quadriceps muscles. In a study that examined the Clarke sign in patients undergoing arthroscopy and in which direct visualization was used as the gold standard for chondral pathology, the authors noted very poor diagnostic value (sensitivity 0.39, specificity 0.67, and positive predictive value 0.25) and further indicated that a literature search identified multiple issues pertaining to this test, and thus they recommend that it not be a part of a routine examination.

A more gently applied posterior force to the patella during flexion and extension may elicit crepitus and pain in patients with chondral pathology ( Fig. 105-6 ). This test may help identify the location of pathology, because a more distal lesion will be painful in early flexion as the patella engages the trochlea, whereas a more proximal lesion will be painful at greater degrees of knee flexion ( Fig. 105-7 ).

The patellar compression test. With the knee flexed slightly to engage the patella in the femoral trochlea, direct compression is applied to the patella.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

The patellofemoral joint contact areas according to the knee flexion angle.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Patellar instability testing is discussed in greater detail in Chapter 104 . The patellar apprehension test is performed by application of a laterally directed pressure to the medial aspect of the patella with both thumbs in approximately 30 degrees of flexion. The test is positive if the patient experiences apprehension that the patella may dislocate laterally. The sensitivity of this test was noted to be 0.39 in one study. However, in a study evaluating patients with patellofemoral pain, apprehension testing elicited pain in 89% of patients at the focus of pressure, and thus it is important to appreciate that although patients with patellofemoral pain may experience pain during apprehension testing, this pain does not represent a positive test for instability. Manipulation of the patella in the transverse plane may reveal a tight retinaculum, as evidenced by a lack of motion ( Fig. 105-8 ). The patellar tilt test should also be performed as an assessment for a tight retinaculum and may have some utility in determining possible interventions.

The passive patellar tilt test. In full extension, the transverse axis of a normal patella tilts beyond the horizontal. The inability to perform this maneuver may indicate an excessively tight lateral retinaculum.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Diagnostic Studies

Standard radiographs help detect pathologic processes such as degenerative changes, certain osteochondroses, calcified loose bodies, osteochondritis dissecans (OCD), and some neoplasms. They also provide information regarding alignment and subtle anatomic variations. Standard views include weight-bearing anteroposterior and lateral views, as well as an axial view. The lateral radiograph is useful for determining patellar height ( Fig. 105-9 ). Patients with patella alta may have a predisposition to instability, whereas patients with patella baja may encounter increased patellar load and the subsequent risk of degenerative conditions.

Lateral radiograph of the knee.

An axial radiograph commonly obtained is the Merchant view ( Fig. 105-10 ). It is taken with the knee flexed to 45 degrees and the x-ray beam projected 30 degrees caudad from the long axis of the femur (see Fig. 105-10 ). It is important to note that the patella engages the trochlea in flexion greater than approximately 20 degrees and that this view is not an accurate representation of the relationship between the patella and the femur in full extension or slight flexion. Murray et al. demonstrated that the lateral and axial views are the most useful views for evaluating instability and noted a high sensitivity of the lateral view for detecting prior dislocation; thus a normal lateral radiograph can help rule out instability as a component of anterior knee pain in some patients. The axial view is also useful for evaluating patellar tilt and subluxation. Different parameters have been defined, and the clinical utility of these parameters remains uncertain ( Fig. 105-11 ). When evaluating axial radiographs, it is important to note that the sulcus of the trochlea lies lateral to the midline of the femoral condyles and is not completely centered as previously thought. Magnetic resonance arthrograms further confirmed that articular congruity may exist even when osseous incongruity appears on plain radiographs.

Merchant view. A, Technique. B, A normal Merchant view. Patellofemoral alignment is normal bilaterally, and the osseous structures and articular cortices are normal.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Axial views performed at 30, 60, and 90 degrees of knee flexion. Note how the trochlear shape changes as flexion increases and the medial facet seems bigger. It is not necessary to routinely perform 60- and 90-degree flexion radiographs in common patellofemoral disorders.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Computed tomography (CT) has utility in evaluating instability by helping determine the relationship of the patella and the trochlear groove, as well as identifying trochlear dysplasia. It is used to measure the tibial tuberosity–trochlear groove distance (the discussion of these topics is addressed in separate chapters). Indications for CT scans in patients without instability are limited. Magnetic resonance imaging (MRI) is the modality of choice for evaluating chondral pathology because plain radiographs have limited utility ( Fig. 105-12 ). MRI is also commonly used to diagnose or rule out intraarticular pathology. One study demonstrated that edema in the superolateral fat pad on MRI correlates with other anatomic parameters (e.g., trochlear morphology and patellar alignment) suggestive of patellar maltracking or impingement in the young, symptomatic patient. Radionuclide scanning can be used to identify occult fractures or insertional tendinopathies. Diffuse uptake can also be seen in persons with complex regional pain syndrome.

Tunnel view. A, A normal tunnel view demonstrates the posterior aspect of the femoral condyles, the tibial spines, the articular surfaces of the tibial plateau, and the intercondylar notch. B, A tunnel view from a different patient demonstrating an ovoid area of lucency at the inner margin of the medial femoral condyle (arrow) that is suspicious for osteochondritis desiccans. Coronal ( C) and sagittal proton density (D) magnetic resonance imaging confirms the large osteochondral defect (arrow) with a completely displaced osteochondral fragment located in the suprapatellar joint recess (arrow).

(Case courtesy the Hospital for Special Surgery, New York, NY. From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

Specific Conditions

Synovial Impingement Syndromes

Impingement is a common cause of patellofemoral pain. The offending structures are plicae or the infrapatellar fat pad. The plicae are embryologic remnants that are normal anatomic structures. The determination of pathologic versus normal plica is difficult. The medial plica is most commonly symptomatic ( Fig. 105-13 ). This plica was shown to be present in approximately 80% of the population in one cohort. Patients describe activity-related pain that worsens with flexion and is relieved with extension. With a hypertrophic plica, a thick palpable cord may be present. Both flexion and extension tests have been described to aid in the diagnosis of pathologic plicae, but they are of limited utility. MRI may detect a prominent plica, but no study has demonstrated a role for determining pathologic versus normal plicae. Chondral pathology of the medial femoral condyle has been noted in cases of severe plicae, and this pathology may be appreciated on MRI.

An axial magnetic resonance image with a low-signal, thick medial patellar plica (arrow) that is highlighted by the large effusion.

(From Scott WN, editor: Insall & Scott Surgery of the Knee, ed 5, Philadelphia, 2012, Elsevier.)

The treatment of symptomatic plicae should begin with conservative modalities, including activity modification, use of antiinflammatory medications, physiotherapy (focused primarily on stretching), and corticosteroid injections. This treatment is reported to succeed in fewer than half of the patients affected. However, significant variability exists in the diagnostic criteria, and thus results should be interpreted cautiously. For patients who do not respond to nonoperative treatment, arthroscopy with plica excision is a straightforward option with low associated morbidity. In patients with associated cartilage degeneration, midterm results support plica excision. The procedure should include a thorough evaluation for other pathology.

The infrapatellar fat pad was implicated in anterior knee pain by Hoffa in 1904. As previously mentioned, this area has a rich innervation and thus can result in significant pain. This area may become swollen and tender to palpation. The Hoffa maneuver involves applying pressure just medial and lateral to the patellar tendon (separately) and extending the knee, which may elicit either pain or apprehension. Plain radiographs are of limited utility because this diagnosis is predominantly clinical. Some evidence indicates that MRI may aid in the diagnosis, and use of MRI may become more relevant with the advent of more powerful MRI machines.

The treatment of fat pad impingement is similar to the initial treatment for symptomatic plica. The modalities include activity avoidance, physiotherapy, cryotherapy, and transcutaneous electrical nerve stimulation. For patients who do not respond to conservative modalities, surgical excision has been described. We recommend this treatment only when the patient has exhausted all nonoperative modalities.