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Evaluation and Management of the Unstable Patella

Andrew J. Cosgarea, MD; James A. Browne;
Tae Kyun Kim, MD, PhD; Edward G. McFarland, MD


In Brief: Patellar dislocation is an uncommon source of anterior knee pain, but it can cause significant morbidity in young, active patients who are particularly prone to recurrent instability. Most first-time dislocations are successfully treated with initial brace immobilization followed by rehabilitation. Surgical stabilization of recurrent instability has a high success rate and is usually required if nonoperative measures fail. By identifying the problem early and prescribing appropriate treatment, physicians may prevent further knee damage and minimize long-term sequelae.

Anterior knee pain is the most frequent complaint of athletes who have knee problems and is also among the more troublesome musculoskeletal complaints that the sports medicine physician encounters.1 In the wide array of conditions that cause knee pain (eg, acute trauma, overuse injuries, osteochondral lesions, patellar tendinosis, quadriceps tendinitis, osteoarthritis, malalignment, and instability), acute and recurrent patellar instability are sometimes overshadowed. The literature commonly focuses on overuse injuries and not on the small but significant group of patients who have acute or recurrent patellar instability.2 Careful history taking, a thorough physical examination, and an appropriate radiological evaluation are critical for diagnosing and successfully treating instability.

Sorting Out Instability

Despite the ubiquity of anterior knee pain among athletic, military, and civilian populations, nomenclature is a tangled mess with little consensus or consistency in application.3 Historically, chondromalacia patellae was the diagnosis given to most patients with anterior knee complaints. The term, however, has fallen into disfavor and should be used solely for describing specific pathologic softening of articular cartilage and not as a vague clinical diagnosis.

Patellofemoral instability refers to both subluxation and dislocation of the patella; the distinction between to the two is based on degree (figure 1). Subluxation refers to the slippage of the patient's patella out of its normal tracking relationship with the trochlea, and dislocation occurs when the patella leaves the trochlear groove entirely (see "Patellofemoral Anatomy and Biomechanics"). Instability is a clinical diagnosis and a dynamic condition; it does not necessarily include every patella that appears subluxated or malaligned on radiographs. Subluxation and dislocation almost always occur laterally, although medial instability can occur, particularly after overaggressive surgical treatment.

Patellar instabilities occur in three distinct forms: acute, recurrent, and habitual. In the acute form, the patient experiences a sudden episode of patellar instability. Episodes of frank instability can cause significant short-term morbidity and chronic problems. Recurrent instability refers to occasional, repeated dislocation or subluxation and is the most common form. Recurrent episodes are unpredictable and may occur at intervals of weeks, months, or years. Finally, habitual instability refers to a patella that permanently remains in a nonanatomic position. This rare condition is usually diagnosed in children and will not be addressed further. The diagnosis of patellofemoral pain syndrome is reserved for idiopathic anterior knee pain and should be based on exclusion of other specific pathologies.

Who's at Risk?

Most studies show a higher incidence of acute instability in active young patients. One study4 reported that most patients who had acute instability were between ages 13 and 20; recurrence was higher in patients whose initial dislocation occurred when they were younger than 15. Acute dislocation is seen predominantly in football and basketball, although injuries have been documented in a variety of sports and other activities.5

Female athletes appear to be at a greater risk for recurrent instability than males,6 particularly during the teenage years, which may reflect anatomic and activity-based changes during puberty.7

Etiology and Mechanism

Crucial anatomic factors contribute to acute and recurrent patellofemoral instability and possible mechanisms of injury. Trauma is often a factor.

Anatomic factors. Our own clinical experiences, and the reports of others,4,8,9 indicate that most patients who have recurrent patellofemoral instability have an underlying anatomic predisposition (table 1). However, not all patients who exhibit instability have malalignment, nor do all patients who have malaligment show signs of patellofemoral instability.

TABLE 1. Anatomic Factors Predisposing
People to Patellar Instability

Excessive femoral anteversion

Genu valgum

External tibial torsion

Pes planus

Hypoplastic lateral trochlear ridge

Patella alta

Weak or hypotrophic vastus medialis obliquus

Hypertrophic vastus lateralis

Proximal causes of malalignment include excessive femoral anteversion, patella alta, and genu valgum. Distal causes include tibial torsion and excessive pronation of the foot. All of these abnormalities can increase lateral force across the patella. Any abnormality in the tilt, rotation, or mediolateral or superioinferior positioning of the patella can also alter patellar tracking and predispose patients to patellofemoral instability. The problem is further exacerbated by a hypoplastic lateral trochlear ridge or deficient vastus medialis obliquus (VMO).

Trauma. Direct or indirect trauma may cause instability of the patella. Direct trauma may cause acute instability. A force or blow to the medial patella causes lateral patellar displacement. The resultant torn medial retinaculum and medial patellofemoral ligament (MPFL) heal with increased length and decreased substance, predisposing the patella to further instability.

Acute and recurrent instability are related disorders. The natural history of acute patellar dislocation is not fully elucidated, but the literature suggests that 14% to 49% of patients who sustain a primary acute dislocation will experience recurrent dislocation.4,10-12 Furthermore, one study13 indicated that only 76% of patients returned to sports following nonoperative treatment, and 63% continued to have episodes of instability.

Concomitant injuries to the knee, such as anterior cruciate and medial collateral ligament tears, can accompany acute dislocation. Fractures of the medial border of the patella are seen in up to half of the cases of acute dislocation.14

Indirect trauma is the most common cause of acute and recurrent dislocation. The indirect mechanism typically occurs with internal rotation of the femur while the leg is in valgus and the foot is firmly planted. Tension in the extensor mechanism creates strong lateral forces that can cause patellar bowstringing and failure of the medial soft-tissue structures. Pivoting and twisting movements may subsequently result in recurrent episodes of instability.

Patients who sustain patellar dislocations have an increased risk of developing osteoarthritis.15 The severity of the initial acute event appears to correlate with poor long-term results, presumably because greater forces cause greater articular cartilage damage. The number and frequency of subsequent recurrent episodes, however, does not seem to worsen the prognosis for degenerative arthritis.

Telltale History

An accurate history is crucial for evaluating patellofemoral instability. Often, the history alone will suggest the diagnosis. The events accompanying the onset of pain or instability will suggest a mechanism of injury. The character of the pain is also important—specifically, whether the pain was sudden or gradual. Bilateral pain of insidious onset usually suggests malalignment and not true instability.8 It is also important to ask the patient about mechanical symptoms like catching or locking that suggest loose bodies.

Acute instability. The patient who has experienced acute instability of the patella will be aware that the patella has "given way" or "slipped out of place." The experience of dislocating a patella is impressive, and the patient will usually be aware of the significance of the injury. The patient may report seeing the patella displaced laterally or even pushing the kneecap back into place. Any patient suspected of sustaining a patellar dislocation should be referred to an orthopedic specialist for evaluation.

Initial examination of the knee can be complicated by effusion or hemarthrosis, which can develop within hours of the injury. Aspiration is usually not necessary because hemarthrosis frequently recurs, and it poses a theoretical risk of introducing bacteria into the joint. Limited range of motion is common because of pain and swelling. Palpation of the patellar and peripatellar regions will reveal tenderness over the medial retinaculum and the medial epicondyle (Bassett's sign) in most patients.

The patient will also typically demonstrate apprehension at any attempt to displace the patella laterally. A complete knee examination should be performed to rule out associated meniscal or ligamentous pathology.

Recurrent instability. The evaluation of recurrent instability is more complex. Patients may have an acutely injured knee, or they may be relatively pain free and seek treatment for a knee that feels unstable. A thorough history of the mechanism of injuries and the type of pain is crucial to a correct diagnosis. Patients with recurrent instability often report aching with intermittent episodes of sharp pain. They may have persistent patellofemoral pain with intermittent instability, particularly when they have chondral damage.

Clinical Examination

A comprehensive physical assessment is important because many recurrent instabilities are associated with malalignment.4,10 With the patient standing and bearing weight, back alignment, hip rotation, tibiofemoral angulation, and overall alignment of the lower extremities are evaluated. The patient's gait should be observed for any femoral anteversion, tibial torsion, or foot malalignment.

Laxity. General ligamentous laxity should be assessed. Hyperextension greater than 10° in the elbow and knee and 90° in the metacarpophalangeal joints suggests hypermobility syndrome. Because of the lower force required to dislocate the patella, hypermobile patients are less likely to sustain osteochondral injuries and often recover quickly. Patients with extreme ligamentous laxity may also be susceptible to medial subluxation.

Q angle. The standard clinical estimate of lateral force is assessed by measuring the quadriceps angle (Q angle) formed by the anterior superior iliac spine, the midpoint of the patella, and the tibial tubercle. Normal values average 10° ± 5° for men and 15° ± 5° for women. A larger Q angle signifies greater lateral force on the patella.

Apprehension. The apprehension sign is an important test for diagnosing recurrent instability. With the patient supine and the knee extended and relaxed, firm pressure is applied to the medial border of the patella, subluxating the patella laterally. The patient's acute apprehension that the patella will dislocate, with or without discomfort, indicates a positive finding. Medial apprehension should also be assessed.

Quadriceps. With the patient lying prone, quadriceps tightness is assessed. Inability to touch the heel to the buttocks and asymmetry with the normal side suggest quadriceps tightness that predisposes the patient to patellofemoral pain. To assess VMO tone, the muscle is palpated as the patient contracts the quadriceps. Thigh circumference can be measured 10 cm proximal to the patella to evaluate quadriceps atrophy and to gauge progress during rehabilitation.

Radiologic Evaluation

Radiologic tools help confirm clinical suspicion and rule out other pathologies; however, patellofemoral instability is a clinical diagnosis, and patients may have normal films despite acute and recurrent episodes. Trauma to the patella may cause articular cartilage injury, and loose osteochondral fragments may be detected on radiographic examination or, less frequently, palpated in the joint space. One study16 reported arthroscopic diagnoses of articular cartilage damage in 71% of adolescents following acute episodes; radiography documented articular injury in only one third of these patients.

Radiography. Patients with acute injuries should undergo a standard x-ray series including anteroposterior, lateral, sunrise, and tunnel views. Radiographs can detect loose bodies as well as osteochondral defects involving the medial patellar facet or lateral trochlear ridge.

Tangential or sunrise views will help assess for subluxation. The most useful of these views is the Merchant view.17 With the patient supine and the knee in 45° flexion, the x-ray beam is projected 30° from horizontal. From this view, patellar subluxation can be determined by calculating the congruence angle of the apex of the patella to the bisected femoral trochlea. One study17 measured the average congruence angle at -6° ± 11°; another study18 reported an upper limit of normal at 4°.

Other imaging modalities. Magnetic resonance imaging is a useful tool in further assessing acute instability and is indicated for patients with suspected chondral, meniscal, or ligamentous pathology. Typical findings show disruption of the medial retinaculum and clearly demonstrate hemarthrosis. The characteristic bone bruise pattern suggesting trauma to the medial facet of the patella and lateral trochlear ridge is pathognomic for a recent lateral patellar dislocation (figure 2A).

Occasionally, computed tomography (CT) imaging is indicated to better evaluate bony anatomy (figure 2B). Unlike plain radiographs, CT permits axial visualization of the knee in extension, revealing patellofemoral articulation before the patella engages in the trochlea.

Nonoperative Management

Reduction. In some patients, the patella may spontaneously reduce with leg extension. If the dislocation remains unreduced, the characteristic deformity of the leg painfully fixed in flexion with a laterally displaced patella will be seen (figure 3). If required, manual closed reduction may be performed by applying gentle medial force to the patella while gradually extending the knee. This is usually best achieved with mild intravenous sedation, though occasionally general anesthesia may be necessary, particularly in prolonged dislocations. The patella rarely requires an open reduction.

Modalities. The traditional cast immobilization approach to acute instability has largely been replaced with symptomatic care. Initial management of acute patellar subluxation or dislocation should center around reducing pain and swelling. Nonsteroidal anti-inflammatory drugs may help. Ice should be applied for 20 minutes every few hours for the first 3 to 4 days following injury and then once daily until pain and swelling diminish.

Acute patellar dislocation initially requires brief immobilization and protection. Depending on the severity of the symptoms, the knee should be braced for comfort for 2 to 4 weeks.

Rehabilitation. The mainstay of nonoperative management for acute and recurrent instability is rehabilitation. Reassurance throughout rehabilitation is important, as many patients will be apprehensive. It is generally preferable to refer the patient for formal physical therapy, particularly when substantial weakness is present or compliance is a concern.

The patient can begin quadriceps-strengthening exercises, such as quadriceps sets and straight-leg raises, while still immobilized. Range-of-motion exercises are started as soon as tolerated, usually 5 to 7 days after injury. Electrical stimulation can help patients who have substantial weakness or muscle atrophy. Any physical therapy program should include open- and closed-chain quadriceps strengthening (with emphasis on VMO strength) and hamstring stretching. Patients are often more comfortable in a neoprene brace with a lateral buttress to prevent lateral patellar displacement. The external patellar supports should be removed as the symptoms improve. The final stage of therapy should include initiation of sport-specific knee-loading exercises.

Strategies to decrease the recurrence of pain and instability should be discussed with the patient. These include activity modification, maintenance of quadriceps strengthening exercises (eg, straight-leg raises and wall sits), and stretching programs. Prophylactic use of a neoprene patellar brace is controversial,19 although it appears that these sleeves offer some subjective improvement in symptoms for many patients. Corrective orthoses may help if pes planus contributes to the patient's symptoms.

Surgical Management

Surgery following acute dislocation is indicated for patients who have significant loose bodies or in rare cases of irreducible patella. More commonly, realignment surgery is performed on patients who have recurrent instability after conservative measures fail. Because no single approach corrects all types of instability, the procedure is individualized.

Acute instability. Acute dislocation usually does not require surgical intervention unless there is a large osteochondral or chondral loose body. Surgery is usually performed arthroscopically, sometimes including a lateral retinacular release. The surgeon should be prepared to repair osteochondral fragments that are often significantly larger than they appeared on radiographs.

Some authors advocate repair of the torn MPFL.20 Most injuries to the MPFL are avulsions of the femur and may be repaired by reanchoring the ligament to the bone. The indications for MPFL repair following acute dislocation remain unclear as there are no long-term outcome studies to date.

Recurrent instability. For skeletally mature patients who have recurrent instability, both proximal soft-tissue and distal bony procedures are routinely performed. The goal of the proximal procedure is to reduce the pull of a tight lateral retinaculum and increase medial pull on the patella by advancing the vastus medialis and VMO distally and laterally over the patella.

Distal osteotomy reorients the patellar tendon and tibial tubercle medially to reduce the Q angle. Distal procedures may be performed along with proximal soft-tissue advancement. Currently, the most popular operations are performed with a flat osteotomy cut that results in a straight medialization of the tibial tubercle (Elmslie-Trillat procedure) or an oblique cut that combines anterior and medial displacement (Fulkerson's procedure). With both procedures, the Q angle is directly decreased by moving the tibial tubercle medially. The Fulkerson osteotomy has the added benefit of decreasing patellofemoral forces by concomitant anteriorization; however, the risk of postoperative fracture is higher,21 necessitating crutch use for 6 or more weeks postoperatively.

We recommend medialization for patients who have isolated instability and anteromedialization for patients who have significant patellofemoral pain or concomitant chondral lesions. Both techniques have proven to be highly successful in preventing future instability,5,13,22 although the results have been less predictable for reducing patellofemoral pain.

Distal soft-tissue procedures are recommended for skeletally immature patients. Tenodesis of the semitendinosus to the patella (Galiazzi procedure) effectively restrains the patella medially and prevents lateral patellar dislocation. The semitendinosus tendon is passed through an obliquely drilled hole in the patella and brought back medially to its insertion. Results have been generally good with only rare recurrences.23

Managing a Spectrum of Presentations

Patellofemoral instability is associated with acute and potentially long-term morbidity. The physician must be able to identify and treat patients with varying presentations. Some patients have only a single instability episode, but many develop recurrent instability that often does not respond to nonoperative measures. If conservative measures fail, a variety of surgical techniques are available to address the spectrum of injuries. In general, recurrent instability is well controlled with realignment surgery, although pain and progressive joint degeneration may persist, especially when patients have concomitant articular cartilage damage.


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  3. The International Patellofemoral Study Group: Patellofemoral semantics: the Tower of Babel. Am J Knee Surg 1997;10(2):92-95
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Patellofemoral Anatomy and Biomechanics

The patella is the largest sesamoid bone in the body and lies within the quadriceps tendon. The posterior surface of the patella is covered by hyaline articular cartilage and has seven facets and two ridges. The patella articulates with the trochlear sulcus of the distal femur.

In leg extension, the patella sits proximal to the trochlea, with the median ridge lateral to the center of the sulcus. The patella enters the trochlea at about 15° to 20° of flexion. When the patella is engaged in the groove, the lateral trochlear ridge functions as a buttress to prevent lateral patellar translation.1 The area of contact between the patella and trochlea increases with continued flexion and moves distal to proximal on the patella. Slight rotation and tilt of the patella is seen during flexion and extension. The patella moves about 7 cm distally from full extension to full flexion.2

In addition to bony anatomy, soft-tissue restraints are critical to joint stability. These include the patellar tendon, quadriceps tendon, and the medial and lateral patellar retinacula. The single most important soft-tissue stabilizer is the medial patellofemoral ligament (MPFL), which provides more than half of the total restraining force that prevents abnormal lateral patellar translation.3 The MPFL is torn from the adductor tubercle in up to 87% of acute dislocations.4 The vastus medialis obliquus (VMO) is the most distal portion of the vastus medialis muscle and is an important dynamic stabilizer. The high angle of insertion of the VMO on the proximal medial side of the patella resists pathologic lateral translation. Excessive weakness or injury of the VMO can lead to lateral patellar instability and may be a risk factor for patellofemoral pain.5

Biomechanically, the patellofemoral joint centralizes the four divergent quadriceps muscle forces and acts as a fulcrum to increase the efficiency of the extensor mechanism. The quadriceps tendon and patellar tendon exert a primarily posterior compressive force that holds the patella in contact with the femur. The resultant patellofemoral joint reactive force depends on the angle of knee flexion and the magnitude of the force in the quadriceps and patellar tendons. Increasing knee flexion or extensor tendon forces (eg, squatting or jumping) will increase the compressive force.


  1. Fulkerson JP, Buuck DA, Post WR: Disorders of the Patellofemoral Joint, ed 3. Baltimore, Williams & Wilkins, 1997
  2. Buckwalter JA, Einhorn TA, Simon SR: Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 730-827
  3. Boden BP, Pearsall AW, Garrett WE, et al: Patellofemoral instability: evaluation and management. J Am Acad Orthop Surg 1997;5(1):47-57
  4. Sallay PI, Poggi J, Speer KP, et al: Acute dislocation of the patella: a correlative pathoanatomic study. Am J Sports Med 1996;24(1):52-60
  5. Witvrouw E, Lysens R, Bellemans J, et al: Intrinsic risk factors for the development of anterior knee pain in the athletic population: a two-year prospective study. Am J Sports Med 2000;28(4):480-489

Dr Cosgarea and Dr McFarland are associate professors in the department of orthopaedic surgery, division of sports medicine; Dr Kim is a research fellow, and Mr Browne is a medical student in the School of Medicine; all are at The Johns Hopkins University in Baltimore. Address correspondence to Andrew J. Cosgarea, MD, 10753 Falls Rd, Suite 215, Lutherville, MD 21093.

Disclosure information: Drs Cosgarea, Kim, and McFarland and Mr Browne disclose no significant relationship with any manufacturer of any commercial product mentioned in this article. No drug is mentioned in this article for an unlabeled use.