INTRODUCTION — The clinical manifestations, diagnosis, and management of lateral patellar dislocations are reviewed here.
Knee (tibiofemoral) dislocations and other knee injuries in young athletes are discussed separately. (See "Knee (tibiofemoral) dislocation and reduction" and "Approach to acute knee pain and injury in children and skeletally immature adolescents" and "Approach to chronic knee pain or injury in children or skeletally immature adolescents".)
TERMINOLOGY — Patellar dislocation is distinct from patellar subluxation or knee dislocation as follows:
●Patellar dislocation – Patellar dislocations refer to displacement of the patella from the trochlear groove. They are most commonly are lateral, although medial or superior dislocations are described. Intra-articular dislocations with lodging of the patella within the joint space may be superior, inferior, or vertical. (See 'Mechanisms of injury' below.)
●Patellar subluxation – Patellar subluxation describes excessive lateral movement of the patella and may occur as a result of trauma or in patients with laxity. The patella still tracks within the trochlear groove but may cause discomfort and a feeling of instability. (See 'Anatomy' below.)
●Knee dislocation – As opposed to patellar dislocations, knee (tibiofemoral) dislocations are true surgical emergencies caused by high energy trauma (eg, motor vehicle collision) and involve injuries to multiple ligaments. (See "Knee (tibiofemoral) dislocation and reduction", section on 'Anatomy' and "Knee (tibiofemoral) dislocation and reduction", section on 'Clinical presentation and mechanism of injury'.)
ANATOMY — The anatomy of the knee is discussed in detail separately. (See "Approach to acute knee pain and injury in children and skeletally immature adolescents", section on 'Anatomic considerations'.)
The patella is a sesamoid bone embedded in the quadriceps tendon that articulates posteriorly with the trochlear groove of the femur (figure 1). Its function is to increase the mechanical advantage of the quadriceps during extension . The quadriceps muscle tendon transitions to become the patellar tendon which inserts at the tibial tubercle (figure 2). Medial and lateral patellar retinacula arise from the vastus medialis and vastus lateralis muscles, respectively, and comprise the joint capsule on either side of the patella.
During leg extension, the knee extensors (quadriceps femoris, vastus medialis, vastus intermedius, and vastus lateralis) naturally place an oblique force across the patella that promotes its lateral displacement . This lateral displacing force is markedly increased during flexion. The medial patellofemoral ligament which connects the medial femoral epicondyle and medial patella along with the vastus medialis obliquus provide the primary opposition to this force (figure 3), and also help maintain proper tracking of the patella within the trochlear groove of the femur [3-5]. Abnormalities in several of these structures may predispose to patellar dislocation. (See 'Risk for recurrence' below.)
MECHANISMS OF INJURY — Because of the natural anatomic tendency for lateral displacement of the patella during quadriceps contraction, lateral patellar dislocation is, by far, most common . (See 'Anatomy' above and 'Epidemiology' below.)
The mechanism of injury varies according to the type of patellar dislocation:
●Lateral – Lateral patellar dislocation, the most common type, is usually associated with disruption of the medial patellofemoral ligament and medial retinaculum . It typically occurs when the foot is planted and an internal rotatory force is applied to the flexed knee in valgus (eg, spinning or twirling maneuver in dance or gymnastics, swinging a baseball or softball bat, or quick lateral change of direction while running or ice skating) [7,8]. Less commonly, direct trauma to the medial knee may result in lateral dislocation. Lateral dislocations can usually be reduced by knowledgeable clinicians without orthopedic consultation. (See 'Reduction of lateral dislocations' below.)
●Superior – Superior patellar dislocation is rare. Mechanisms of injury include hyperextension of the knee or a blow to the knee with the leg in extension. Superior patellar dislocations are most commonly reported in older adults with osteoarthritis and a large osteophyte located in the proximal trochlear groove (image 1) [9,10]. (See 'Superior patellar dislocation' below.)
●Medial – Medial patellar dislocation is also rare. It has primarily been described as a post-operative complication of a lateral retinacular release procedure. However, lateral trauma has resulted in medial patellar dislocation in one case [11,12].
●Intra-articular patellar dislocation – Intra-articular patellar dislocations are rare injuries that are typically associated with injury to the quadriceps insertion on the proximal pole of the patella. These injuries warrant prompt orthopedic consultation for reduction. (See 'Indications for orthopedic referral or consultation' below.)
•Inferior – Inferior intra-articular patellar dislocation arises from direct trauma to the proximal portion of the patella of a flexed knee sufficient to cause avulsion of the deep fibers of the quadriceps tendon and permit horizontal dislocation of the superior portion of the patella inferiorly into the joint (picture 1 and image 2) [13,14]. These dislocations are most frequently described in adolescent males and older patients. Interposition of the quadriceps tendon, avulsion fractures, or sleeve fractures of the patella can prevent closed reduction .
•Superior – Superior horizontal intra-articular patellar dislocations follow trauma to the inferior pole of the patella during knee flexion in elderly patients with arthritis .
•Vertical – Vertical dislocation is the rarest form of patellar dislocation and follows a medial blow to the patella that causes it to flip and wedge between the femoral epicondyles [14,16].
EPIDEMIOLOGY — Lateral patellar dislocations account for most patellar dislocations and approximately 3 percent of all knee injuries [5,17-19]. First-time lateral patellar dislocation occurs with an incidence of approximately 23 per 100,000 person-years . Superior and medial dislocations and intra-articular dislocations are rare. (See 'Mechanisms of injury' above.)
Lateral patellar dislocations occur frequently in teenage athletes and young adults [17,20-23]. Patients younger than 20 years of age are at highest risk, and 75 percent of all first-time patellar dislocations occur in people less than 25 years of age [20,23,24]. Adolescent females appear to be predisposed [17,21-23] as are taller, overweight males .
Sports and activities frequently associated with lateral patellar dislocation include gymnastics, soccer, ice hockey, dance, weight lifting, and military training [23,25,26]. As an example, in one series of 73 men with lateral patellar dislocations, 93 percent of injuries occurred without a history of direct trauma . (See 'Mechanisms of injury' above.)
CLINICAL FEATURES AND DIAGNOSIS — Acute lateral dislocation of the patella is usually obvious on clinical evaluation, unless the patella has reduced on its own with knee extension .
●History – Patients with patella dislocation often describe the knee as “giving way” followed by severe pain. They may also report hearing a pop or tear at the time of dislocation .
Additional findings related to tearing of the restraining medial retinacular tissue and the medial patellofemoral ligament may include [8,29]:
•An acutely swollen knee
•Tenderness to palpation along the medial edge of the patella
•Tenderness just proximal to the medial femoral epicondyle
●Imaging – Plain radiographs are not necessary before dislocation reduction unless the diagnosis is in question. In one observational study of 80 patients with lateral patella dislocation, radiographs prior to reduction found fractures in 10 percent of patients (image 3) . However, reduction was successful in all patients regardless of radiographic findings.
Thus, pre-reduction radiographs do not change management and may prolong the time to reduction.
Radiographs should be obtained after reduction or in patients in whom reduction is not successful. Acute patellar dislocation is frequently associated with osteochondral fractures, including capsular avulsions of the medial patellar margin and loose intra-articular fragments detached from the patella and/or lateral femoral condyle . (See 'Differential diagnosis' below and 'Reduction of lateral dislocations' below.)
Spontaneously reduced patellar dislocation — Lateral patellar dislocation is usually obvious based upon clinical examination. Patients with patellar subluxation rather than dislocation may have findings that are difficult to distinguish from a spontaneously reduced patellar dislocation.
Patellar subluxation — Patellar subluxation refers to excessive lateral movement of the patella and may occur as a result of trauma or in patients with laxity. These patients often complain of knee pain and popping or clicking, and occasionally knee swelling, stiffness, or catching. Patellar subluxation, rather than dislocation, may have occurred if the history is consistent with dislocation, but the pain and abnormal examination findings have resolved .
On physical examination, patients with patellar subluxation may have patella alta, synovitis, and/or effusion. Other findings of patellar subluxation include:
●Laterally displaced patella
●Tenderness localized to the adductor tubercle (figure 3) and patellar facets (underside of the patella), especially the lateral facets
●Abnormal patellar tracking including a positive jump or “J”-sign (the seated patient straightens the knee, and the patella moves laterally at terminal knee extension)
●Positive patellar apprehension test as determined by one of two versions of this test:
•Traditional patellar apprehension test – With the patient’s knee in 30 degrees of flexion and quadriceps relaxed, both thumbs are placed along the medial border of the patella and pushed laterally to displace the patella. The test is positive if the patient becomes uncomfortable and apprehensive as the patella reaches the point of maximum passive displacement or if the patient begins to resist and straightens the knee to pull the patella back into normal alignment. However, the sensitivity for this test is low (approximately 40 percent) .
•Moving patellar apprehension test – With the knee in full extension, the clinician pushes the patella laterally while the knee is flexed to 90 degrees and then returned to full extension . The clinician then pushes the patella medially while the knee is again flexed to 90 degrees and then fully extended. Apprehension or attempts to resist flexion of the knee while the patella is pushed laterally but not associated with any discomfort while the patella is pushed medially constitutes a positive test. In an observational study of 51 patients (25 with lateral patellar instability), this test had a sensitivity of 100 percent, specificity of 89 percent, and negative predictive value of 100 percent when compared with the ability to dislocate the patella under anesthesia .
Patients with patellar subluxation should have knee radiographs to evaluate for an associated avulsion fracture of the patella or femoral condyle and patellar/femoral trochlear dysplasia .
Treatment of patellar subluxation is best guided by a sports medicine specialist or orthopedic surgeon. Non-operative management is aimed at strengthening the quadriceps, principally the vastus medialis obliquus, and minimizing knee valgus by strengthening the hip abductors. In addition, stretching of the iliotibial band is typically encouraged to decrease lateral pull on the patella through its distal attachment. Patellar taping and patellar stabilizing orthoses may also be helpful [34,35]. Surgical procedures are also available to either release the tension from the lateral patellar retinaculum or reconstruct the medial patellofemoral ligament when conservative treatment has failed.
Swollen knee in a fixed position — Similar to a lateral patellar dislocation, other types of patellar dislocation, patellar tendon tear, quadriceps tendon tear, and patellar fracture may also present with a knee that is swollen and held in a fixed position.
Superior patellar dislocation — Superior dislocations of the patella typically occur in patients over 40 years of age when the knee traumatically hyperextends, or the patella is directly struck on the inferior pole with the leg straight. Physical examination demonstrates tenting of the suprapatellar region with a dimple below. The patellar tendon is intact by palpation. Both the patient age and appearance help distinguish a superior patellar dislocation from a lateral patellar dislocation [9,10]. Antero-posterior and lateral radiographs are diagnostic and often show interlocking of inferior patellar and anterior femoral osteophytes (image 1).
After administration of parenteral analgesia (eg, morphine or fentanyl), reduction is performed by gently moving the patella side to side . Post-reduction films, including AP, lateral, and patellar ("sunrise" or "tunnel" view) knee radiographs should be performed. If reduction is unsuccessful, prompt orthopedics consultation or referral is warranted. (See 'Indications for orthopedic referral or consultation' below.)
Intra-articular patellar dislocation — Patients with inferior intra-articular patellar dislocations tend to be elderly. They usually present with the knee fixed in 90 degrees of flexion, tenting of the skin in the infrapatellar region and a corresponding sunken region superior to the patella that may give the appearance of ruptured quadriceps tendon (picture 1 and image 2).
Patients with superior intra-articular dislocations hold the knee in extension with tenting of the skin in the suprapatellar region. Both appearance and radiographs help distinguish intra-articular patellar dislocations from lateral patellar dislocations. These patients warrant prompt consultation or referral to an orthopedic surgeon. (See 'Mechanisms of injury' above and 'Indications for orthopedic referral or consultation' below.)
Patellar fracture — Most fractures of the patella result from direct forces, such as a fall onto a flexed knee or striking the dashboard during an automobile accident. Fractures can also result from an indirect force applied to the patella through sudden, forceful contraction of the quadriceps, such as landing on one's feet after jumping from a moderate height or coming to a sudden stop from a full sprint. Examination typically reveals a joint effusion and focal tenderness of the patella. The patella is not displaced laterally. The patient may not be able to extend the knee against gravity, and a gap in the extensor mechanism may be palpable. A hemarthrosis is noted if the joint is aspirated to relieve the pain from a tense effusion. (See "Patella fractures", section on 'Mechanism of injury and fracture pattern' and "Patella fractures", section on 'Clinical presentation and examination'.)
A patella fracture is diagnosed by radiograph. Standard radiographic evaluation includes anteroposterior (AP), lateral, and sunrise views (image 4 and image 5). The lateral view is generally most helpful for assessing displacement (image 6). A tangential view can be helpful in identifying osteochondral fragments and in assessing vertical fractures (image 7). Contralateral views can confirm bipartite patella. (See "Patella fractures", section on 'Diagnostic imaging'.)
Quadriceps tendon rupture — Quadriceps tendon rupture is primarily seen in older adults with degenerative changes caused by rheumatoid arthritis, diabetes mellitus, obesity, chronic kidney disease, gout, hyperparathyroidism, or steroid use. This injury may occur with minor or no identified trauma. Plain radiographs show an inferior position of the patella often with effusion of the prepatellar bursa (image 8). The diagnosis is typically confirmed by magnetic resonance imaging. (See "Quadriceps muscle and tendon injuries", section on 'Risk factors for quadriceps injury' and "Quadriceps muscle and tendon injuries", section on 'Diagnosis'.)
Patellar tendon rupture — Complete tear, or rupture, of the patellar tendon is rare in active adults and occurs more commonly in patients under age 40. The injury typically occurs when a patient’s knee is flexed 60 degrees or more and sustains a force that suddenly increases the degree of flexion. In such circumstances, the patellar tendon avulses from the distal patellar pole. If the rupture occurs within the substance of the tendon, the clinician should consider secondary causes, such as anabolic steroid abuse or recent treatment with quinolone antibiotics. The patient with a patellar tendon rupture often has significant knee swelling and pain and cannot fully extend the knee. Rarely, the injury may be associated with ACL tears or other injuries, such as superior patellar dislocation (image 9). (See "Quadriceps muscle and tendon injuries", section on 'Quadriceps and patellar tendon tears'.)
INDICATIONS FOR ORTHOPEDIC REFERRAL OR CONSULTATION — Patients with the following patellar dislocations warrant prompt orthopedic consultation :
●Lateral dislocations that cannot be reduced
●Superior dislocations that cannot be reduced (see 'Superior patellar dislocation' above)
●Intra-articular dislocations (see 'Intra-articular patellar dislocation' above)
Follow up with an orthopedic surgeon after reduction is suggested for patients with reduced lateral patellar dislocations that are recurrent, are associated with a fracture, or associated with a medial collateral or anterior cruciate ligamentous injury.
Orthopedic or sports medicine referral is also warranted in patients with contralateral patellofemoral instability (positive apprehension test) or a family history of patellar dislocation [17,36]. Because of the high risk of recurrence, children younger than 15 years of age also merit referral if the primary care physician lacks experience with prescribing and monitoring rehabilitation .
REDUCTION OF LATERAL DISLOCATIONS — Reduction is necessary if the dislocated patella fails to reduce spontaneously. Immediate reduction may be attempted if the dislocation is recent (ie, on the playing field) or witnessed. Patients who present to the emergency department may require pain control (eg, parenteral morphine or fentanyl) and sedation before reduction [21,38]. (See "Procedural sedation in children: Approach" and "Procedural sedation in children: Selection of medications", section on 'Moderately or severely painful procedures' and "Procedural sedation in adults in the emergency department: General considerations, preparation, monitoring, and mitigating complications".)
Once adequate analgesia is achieved, the patient is placed in the supine position with the hips flexed to relax the quadriceps and hamstring muscles. The clinician slowly extends the knee while applying gentle and gradual medial pressure to the lateral aspect of the dislocated patella (figure 4). Successful reduction is indicated by the return of the patella to the tibiofemoral tract and normal flexion and extension of the knee [28,39]. Knee effusion may occur after the reduction or may remain if it was present before the procedure. (See 'Additional resources' below.)
AP, lateral, and patellar ("sunrise" or "Merchant" view) knee radiographs should be obtained after the reduction of an acute patellar dislocation to assess for fracture or avulsion [18,27]. Associated osteochondral fractures occur in up to 40 percent of patients with patellar dislocation . In addition, the patient should be examined for signs of ligamentous injury. (See "Anterior cruciate ligament injury", section on 'Physical examination' and "Medial (tibial) collateral ligament injury of the knee", section on 'Physical examination' and "Approach to acute knee pain and injury in children and skeletally immature adolescents", section on 'Patellar dislocation' and "Approach to acute knee pain and injury in children and skeletally immature adolescents", section on 'Ligamentous injury'.)
POST-REDUCTION MANAGEMENT — After reduction and complete evaluation, the knee should be placed in a patellar-stabilizing brace or in a knee immobilizer until a brace may be obtained. In addition, the patient should use crutches to assist with ambulation. Non-steroidal anti-inflammatory medications (eg, ibuprofen) may be used as needed for pain control but should not be used continuously for more than a few days. Rest, ice, compression, and elevation are recommended as follows [18,38,39,41]:
●Limit walking, standing, impact, and repetitive bending, particularly if these activities cause pain.
●Apply ice and elevate the leg for 10 to 15 minutes four times daily.
●Use a patellar restraining brace with fabric stays during the day.
All patients with patellar dislocation should have follow-up with their primary care physician or an orthopedic surgeon two to three days after the initial injury to start the rehabilitation process. Indications for referral to an orthopedic or sports medicine specialist are discussed separately. (See 'Indications for orthopedic referral or consultation' above.)
REHABILITATION — The post-reduction treatment for patients who have a first-time lateral dislocation with a normal radiograph and normal patellofemoral alignment is aimed at regaining quadriceps strength and protecting against subsequent dislocations . In early rehabilitation, the emphasis should be on improving range of motion, especially knee flexion, and decreasing edema and swelling.
Athletes with acute patellar dislocation can expect to return to play within four to six weeks with appropriate strengthening exercises and the use of a knee brace .
OPERATIVE TREATMENT — There are limited and conflicting data regarding the benefit of operative repair in children and young adults with lateral patellar dislocation. In a meta-analysis of 10 studies (8 randomized trials and 2 quasi-randomized trials with a total of 519 adolescents and young adults with patellar dislocation), there was high uncertainty that surgery, compared with non-operative treatment, decreased the risk of recurrent dislocations or improved knee function . In two studies not included in this meta-analysis, early operative treatment of first-time patellar dislocation followed by rehabilitation in children <16 years of age did not appear to reduce recurrence compared with rehabilitation alone [37,45]. As an example, in a trial of 62 children <16 years of age with 64 lateral patellar dislocations, early surgical repair of the medial patella-femoral ligaments was associated with recurrence in 67 percent of patients compared with 71 percent of patients managed non-operatively . Larger, high-quality trials are needed to determine whether operative versus non-operative treatment of lateral patellar dislocation provides the best long-term outcomes.
RISK FOR RECURRENCE — Children younger than 16 years of age with lateral patellar dislocations have a high rate of recurrence (up to 70 percent in some studies) [17,36,37]. Certain conditions may also predispose to patellar instability and recurrent dislocation, as follows [17,46-50]:
●Generalized joint laxity
●Lower extremity malalignment
●Passive lateral hypermobility of the patella
●Iliotibial band tightness
●Patella alta (high-riding patella)
●Below the knee amputation
●Weakness of the vastus medialis obliquus muscle
●History of contralateral dislocation
Individuals with these conditions have an increased incidence of recurrent and bilateral patellar dislocation, a decreased incidence of associated fractures, and occurrence of patellar dislocation with less significant trauma .
Patellar dislocation also occurs as a familial trait with dominant inheritance [51-53]. In addition, it is frequently seen in patients with Rubinstein Taybi and nail-patella syndromes . (See "Nail-patella syndrome", section on 'Clinical manifestations'.)
Although an increased angle between the force vector of the quadriceps and the patellar tendon, also known as the Q angle, is also mentioned as a risk factor for lateral patellar dislocation, this angle varies based upon the degree of quadriceps muscle contraction or knee extension and has been shown to not reliably identify patients with patellar instability relative to other validated standards (eg, the tibial tuberosity-trochlear groove [TT-TG] distance) [55,56].
ADDITIONAL RESOURCES — A demonstration video that shows a successful patellar dislocation reduction is available as supporting information for the online version of the reference .
SUMMARY AND RECOMMENDATIONS
●Terminology – Patellar dislocation describes patellar displacement from the trochlear groove. Intra-articular dislocations with lodging of the patella within the joint space may be superior, inferior, or vertical. Because of the natural anatomic tendency for lateral displacement of the patella during quadriceps contraction, lateral patellar dislocation is, by far, most common. (See 'Terminology' above.)
Patellar subluxations refer to excessive lateral movement of the patella, which may occur as a result of trauma or in patients with laxity.
By contrast, knee (tibiofemoral) dislocations are true surgical emergencies caused by high energy trauma (eg, motor vehicle collision) and involve injuries to multiple ligaments.
●Clinical features and diagnosis – Acute lateral dislocation of the patella is usually obvious on clinical evaluation. The knee typically is held in 20 to 30 degrees of flexion and the patella is palpable laterally (picture 2). Additional findings may include (see 'Clinical features and diagnosis' above):
•An acutely swollen knee
•Tenderness to palpation along the medial edge of the patella
•Tenderness just proximal to the medial femoral epicondyle
●Differential diagnosis – Patients with patellar subluxation rather than dislocation may have findings that are difficult to distinguish from a spontaneously reduced patellar dislocation. Other types of patellar dislocation and patellar fracture may present with a knee that is swollen and held in a fixed position. When the diagnosis of lateral patellar dislocation is not clear on physical examination, the clinician should obtain plain radiographs of the knee consisting of AP, lateral, and patellar ("sunrise" or "Merchant" view) views. (See 'Differential diagnosis' above.)
●Specialty referral – Lateral or superior patellar dislocations that cannot be reduced and all intra-articular dislocations (picture 1) warrant prompt referral to an orthopedic surgeon. (See 'Indications for orthopedic referral or consultation' above and 'Mechanisms of injury' above.)
●Reduction (lateral dislocations) – Immediate reduction may be attempted if the lateral patellar dislocation is recent (ie, on the playing field) or witnessed. Patients who present to the emergency department may require pain control and sedation before reduction. Plain radiographs are not necessary before dislocation reduction unless the diagnosis is in question. (See 'Reduction of lateral dislocations' above.)
Once adequate analgesia is achieved, the patient is placed in the supine position with the hips flexed to relax the quadriceps and hamstring muscles. The knee is slowly extended while gentle medial pressure is applied to the lateral aspect of the dislocated patella (figure 4). (See 'Reduction of lateral dislocations' above and 'Additional resources' above.)
Successful reduction is indicated by the return of the patella to the tibiofemoral tract and normal flexion and extension of the knee. (See 'Reduction of lateral dislocations' above and 'Additional resources' above.)
●Post-reduction imaging and management (lateral dislocations) – Knee radiographs as previously described should be obtained after the reduction to assess for fracture or avulsion. (See 'Reduction of lateral dislocations' above.)
Post-reduction management consists of (see 'Post-reduction management' above):
•Splinting with a patellar-stabilizing brace or knee immobilizer and crutches to assist with ambulation
•Non-steroidal anti-inflammatory medications (eg, ibuprofen) as needed for pain control
•Rest, ice, and elevation of the extremity
•In patients with normal radiographs and normal patellofemoral alignment, rehabilitation initiated soon after reduction (see 'Rehabilitation' above)
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