Patellar Dislocation

Aka: Patellar Dislocation, Patellar Subluxation

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II. Definitions

  1. Patellar Subluxation
    1. Excessive Patella laxity in movement with typically lateral displacement
  2. Patellar Dislocation
    1. Severe subluxation with complete displacement of the Patella outside the trochlear groove
    2. Patella does not spontaneously relocate

III. Epidemiology

  1. Most common Knee Injury seen in children
  2. More common in teenage girls and young women
    1. Associated with increased Q-Angle (see below)

IV. Pathophysiology

  1. Mechanism
    1. Forceful deceleration (e.g. planted foot) with flexed knee and concurrent knee rotation or
    2. Valgus force with strong quadriceps contraction
      1. Results in lateral Patella displacement out of trochlear groove
    3. Direct blow to Patella while the knee is flexed
  2. Resulting Effect
    1. Risk of Medial Patellofemoral Ligament (MPFL) Rupture
    2. Lateral Patella displacement out of trochlear groove

V. Risk factors

  1. Miserable Malalignment Syndrome
  2. Tight lateral Retinaculum
  3. Patella Alta
  4. Patella Hypermobility
  5. Vastus lateralis hypertrophy

VI. Symptoms

  1. Anterior knee ripping or tearing Sensation at injury
  2. Knee flexes with dislocation
  3. Patella relocates with knee extension
  4. Subluxation associated with giving way Sensation
  5. Dislocation is associated with severe pain

VII. Exam

  1. See Knee Exam
  2. Red Flag Observation Findings
    1. High Riding Patella (Patella Alta)
      1. May suggest Patellar Tendon Rupture
    2. Large joint effusion with Ecchymosis
      1. Consider hemarthrosis and intraarticular injury (or Knee Dislocation instead of Patella dislocation)
      2. Also seen with Medial Patellofemoral Ligament (MPFL) Rupture
  3. Palpation
    1. Bony landmarks for associated Fracture
    2. Patellar Tendon and Quadriceps Tendon for rupture
  4. Knee Range of Motion (passive and active)
    1. LImited extension is typical in Patella dislocation
    2. Avoid complete extension until performing Patellar reduction
  5. Neurovascular Exam
    1. Evaluate distal pulses, Sensation and motor function

VIII. Signs

  1. Knee held in semi-flexed position
  2. Palpable Patella deviated from normal position
    1. Lateral Patellar Dislocation is more common than medial dislocation
  3. Dislocation
    1. Concurrent osteochondral Fracture in 28-52% patients
    2. Associated with Anterior Cruciate Ligament Tear
  4. Subluxation
    1. Instability and weakness
    2. Reluctant to bear weight
  5. Predisposing factors
    1. Examine for predisposing factors listed above
    2. J-Sign
    3. Quadriceps angle (Q-Angle) >15 degrees

IX. Imaging

  1. Knee XRay
    1. Obtain in all cases after reduction (Fractures are common)
      1. XRays are optional before reduction
    2. Views
      1. Merchant and Infrapatellar views (knee flex 45)
      2. Anteroposterior, Notch, and lateral views
    3. Interpretation
      1. Often normal
      2. Medial Patella avulsion Fracture
      3. Osteochondral Fracture
  2. MRI Knee without Contrast (rarely indicated in Patella dislocation)
    1. Indications
      1. Osseous loose body on XRay
      2. Large joint effusion (hemarthrosis)
    2. Findings
      1. Osteochondral defect
      2. Medial Patellofemoral Ligament (MPFL) Tear

X. Differential Diagnosis

  1. Knee Dislocation
    1. Knee spontaneous reduction prior to presentation may be misdiagnosed as Patella subluxation
    2. Knee Dislocation may occur with low energy mechanism (esp. in Obesity)
    3. Knee Dislocation is a risk for vascular injury, and missed dislocation risks limb loss

XI. Management: Patella Reduction

  1. Indications
    1. Patella still dislocated (often spontaneously reduces)
  2. Contraindications
    1. Associated Fracture, tendon rupture or Knee Dislocation
  3. Imaging
    1. Pre-reduction XRay not needed in isolated Patella dislocation
    2. Always obtain a post-reduction Knee XRay
  4. Administer IV Analgesics or Anesthesia
  5. Maneuver (two providers)
    1. Patient supine with hip in mild flexion (e.g. raise head of bed to 30-45 degrees)
    2. One provider gently extends knee
    3. Second provider applies gentle pressure to relocate the Patella
      1. Medial pressure to relocate a laterally dislocated Patella
      2. Attempt to lift the Patella over the femoral condyle

XII. Management: General

  1. Bracing and taping
  2. Alter aggravating activity
  3. Physical Therapy and Rehabilitation
    1. Soft tissue and Patellar mobilization
    2. Muscle Strength
      1. Vastus medialis oblique
      2. Gluteus
      3. Foot and ankle
  4. Consider immobilization
    1. Indications
      1. First Patella dislocation and
      2. No significant Vastus Medialis disruption
    2. Technique
      1. Immobilize for 6 weeks
      2. Knee in full extension
      3. Non-weight bearing (some guidelines allow weight bearing as tolerated)
      4. Foam pad protects Vastus Medialis
      5. Lateral support holds Patella medially
  5. Maintenance
    1. Patellar stability program after rehabilitation
  6. Surgery Indications
    1. Patella unable to be reduced with closed procedure (urgent consult)
    2. Inadequate improvement in 6 months
    3. Osteochondral Fracture
    4. Medial Patellofemoral Ligament (MPFL) Tear resulting in recurrent subluxation or dislocation

XIII. Complications

  1. Osteochondral Fracture
  2. Medial Patellofemoral Ligament (MPFL) Tear
    1. May result in recurrent Patella subluxation or dislocation

XIV. References

  1. Kirwin, Conroy, McGrath (2021) Crit Dec Emerg Med 35(7): 15-24
  2. Rashidzada (2020) Crit Dec Emerg Med 34(11): 12-3
  3. Warrington (2021) Crit Dec Emerg Med 35(12): 31

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