Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

Featured on Channel NewsAsia

Patellar Dislocation

Basics
Description
  • Patellar dislocation usually refers to lateral displacement of the patella out of its normal alignment in the trochlear groove of the femur.
  • The patella is held stable by ligamentous forces, muscular forces, and bony anatomy.
    • Disruption of any of these 3 components can lead to recurrent patellar instability.
  • Classification:
    • Subluxation: Patella sits on the edge of the femoral groove, but not out of the track.
    • Dislocation: Patella is completely displaced out of the patellofemoral groove, usually laterally.
Epidemiology
  • Seen primarily in young patients (10-17 years old)
  • Occurs more often in females than in males
Incidence
  • The incidence is difficult to quantitate because many knees relocate spontaneously and are misdiagnosed.
  • A 9% incidence of a positive family history  is noted.
Risk Factors
  • Positive family history
  • Participation in football, basketball, baseball, gymnastics, or dancing
  • Age 10-17 years
  • High level of activity or competition in a youth
  • Mechanism other than a direct blow
  • Hypermobility of the patella
  • Previous dislocations (49% of patients with a dislocation have a history of a previous dislocation)  
  • Patella alta (50% of patients)  
  • Shallow patellofemoral groove
  • Excessive Q angle
  • Ligamentous laxity
  • Excessive femoral anteversion
  • Vastus medialis dysplasia
  • Excessive genu valgus
Genetics
A congenital predisposition to knee malalignment and propensity to patellar dislocation is thought to exist.
 
Pathophysiology
  • Patellar dislocation can disrupt the medial patellofemoral ligament.
  • Chondral injury or fracture can occur from the impact of the patella on the trochlea .
Etiology
  • A direct blow to the medial aspect of the patella
  • Severe valgus injury to the knee
  • Twisting injury or other minor trauma, usually associated with congenital deficiencies
Associated Conditions
  • Connective tissue disease with ligamentous laxity, such as Ehlers-Danlos and Marfan syndromes
  • Femoral anteversion and pes planus
Diagnosis
Signs and Symptoms
  • Patients with acute dislocation may present with the knee held in a flexed position as a result of hamstring spasms.
  • The femoral condyles may be prominent medially.
  • Often, the patella has spontaneously reduced, with the following findings:
    • Diffuse parapatellar tenderness
    • Positive apprehension test
    • Palpable defect at the insertion of the vastus medialis muscle
    • Hemarthrosis
History
  • Direct blow to the knee
  • Twisting injury to the leg
Physical Exam
  • After the acute symptoms subside, examine the knee for the following:
    • Effusion
    • Apprehension, with patellar translation both medially and laterally
    • Lateral tracking of the patella (in the shape of a J) with the knee extended from a flexed position
    • Injury to the medial, collateral, or cruciate knee ligaments
    • Lateral tilt
Tests
Imaging
  • Radiography:
    • Postreduction plain radiographs are obtained for evidence of osteochondral fragments.
    • Axial views of the bilateral patella may show substantial lateral tracking.
  • MRI or CT can reveal osteochondral injury and rupture of the medial patellofemoral ligament.
Pathological Findings
  • Abnormalities in the patellofemoral articulation allow the pull of the vastus lateralis and lateral retinaculum muscles to overcome that of the vastus medialis, even during minor trauma.
  • This unbalanced pulling often tears the medial retinaculum and vastus medialis insertion.
Differential Diagnosis
  • Cruciate ligament injury
  • Patellar fracture
  • Patellofemoral pain syndrome
  • Osteochondral fracture
Treatment
General Measures
  • Reduce acute dislocations and then immobilize the knee.
    • Reduce acute dislocation by gentle, steady extension of the knee, facilitated with the patient prone and the patient’s hip extended to relax the hamstrings.
    • Avoid forceful manipulation.
    • Once the knee is reduced, immobilize it in extension with a compression dressing.
    • Evaluate the medial retinacular structure for tenderness every 2 weeks for up to 6 weeks.
    • When the patient is comfortable, apply a Neoprene sleeve with a laterally based felt pad.
  • Surgical stabilization is recommended for the following:
    • Recurrent dislocations
    • Dislocations in carefully selected, highly active, competitive athletes
    • Acute dislocations with avulsive detachment of the vastus medialis muscle by bony fragment, seen on radiographs
Activity
  • Patients may bear weight after relocation.
  • Crutches should be supplied to facilitate weightbearing as tolerated.
  • Twisting motions should be avoided.
Special Therapy
Physical Therapy
  • The main goal of therapy is to strengthen the injured extensor mechanism and to improve patellofemoral tracking.
    • Straight-leg raises may begin immediately with appropriate support.
  • Patients with recurrent dislocations often advance more quickly with physical therapy for quadriceps strengthening.
Medication
Advise the patient that analgesics may be taken in the acute phase.

Surgery
  • Several procedures have been attempted for treatment of patellar dislocation .
  • Patients with acute dislocations and disruption of the medial patellofemoral ligament have been treated with acute repair .
  • Lateral release for patellar instability has poor results.
  • Patients with excessive Q angles (>150°) and recurrent dislocation may benefit from distal realignment via the Elmslie-Trillat or Fulkerson procedure.
  • To prevent distal migration of the tibial tubercle, procedures that involve the tibial physis should be avoided in children with open physeal plates.
  • Arthroscopy does not have a place in the treatment of patellar dislocation other than for intra-articular assessment of patellofemoral tracking or the treatment of associated injuries.
Follow-up
Prognosis
  • Overall, 75% of patients are treated successfully with nonoperative means.
  • The key is to identify patients at risk for recurrence and to treat them more aggressively early in the course.
  • Long-term results after surgical treatment do not seem to be better than those after nonoperative treatment.
  • Long-term results of the Elmslie-Trillat osteotomy show improvement of instability, but results deteriorate over time.
Complications
  • Recurrent dislocations
  • Chronic anterior knee pain
  • Reflex sympathetic dystrophy
  • Hemarthrosis with a lateral release
  • Patellofemoral arthritis
  • Osteochondral fractures
Patient Monitoring
Patients are followed at 2-4-week intervals to monitor the progress of quadriceps rehabilitation.
 
Miscellaneous
Codes
ICD9-CM
836.3 Patellar dislocation
 
Patient Teaching
Discuss the risk factors for recurrent dislocation with a possible recommendation that the patient avoid high-risk activities or sports.
 
Prevention
Patients who are at high risk or have had previous instability should keep their quadriceps strong and consider bracing or taping.

FAQ
Q: What is the treatment for a 1st-time patellar dislocation?
A: Treatment is nonsurgical with relocation, quadriceps strengthening, and gentle return to activity. Surgical intervention should be considered with care and only if specific indications are present.

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