What are the diagnostic criteria and management options for patellar subluxation?

Diagnostic Criteria and Management of Patellar Subluxation

Radiographic imaging with patellofemoral views is the primary diagnostic method for patellar subluxation, with axial radiographs demonstrating the degree of patellar tilt or subluxation. 1

Diagnostic Criteria

Clinical Assessment

• History findings suggestive of patellar subluxation:

  • Anterior knee pain
  • Pain during activities requiring knee flexion
  • Sensation of instability or "giving way"
  • Pain at rest (may indicate more advanced pathology)

• Physical examination findings:

  • Patellar apprehension test (positive when lateral pressure on patella causes apprehension)
  • Lateral patellar tracking with knee flexion/extension
  • Tenderness along medial patellofemoral structures
  • Assessment of lower extremity alignment (Q-angle)
  • Evaluation of femoral and tibial component rotation if post-TKA

Imaging Studies

Initial Imaging

• Radiographs are the first-line diagnostic tool 1
  • Minimum of two views (anteroposterior and lateral) 1
  • Patellofemoral view is specifically indicated for suspected patellar subluxation or dislocation 1
  • Weight-bearing axial radiograph provides better assessment of patellofemoral kinematics 1
  • Lateral view with knee at 25-30 degrees flexion to demonstrate patella in profile 1

Advanced Imaging

• CT is the modality of choice for measuring axial malrotation and patellar alignment 1

  • Particularly useful for evaluating component rotation in post-TKA patients
  • Shows higher sensitivity (0.96) and specificity (0.90) than conventional radiographs for diagnosing patellar subluxation 2

• MRI is valuable for evaluating patellar articular cartilage damage 3, 4

  • Can detect moderate to advanced cartilage injury with high sensitivity
  • Combination of SE (TR 400ms/TE 19ms) and FLASH sequences are effective for detecting cartilage abnormalities 3
  • MRI grading correlates with arthroscopic findings:
    • Grade 2 (surface irregularity): 85% sensitivity, 94.7% specificity
    • Grade 3 (cartilage loss): 100% sensitivity, 100% specificity 4

Management Options

Non-operative Management

• First-line treatment for most cases of patellar subluxation:
  • Physical therapy focusing on quadriceps strengthening
  • Patellar taping or bracing
  • Activity modification
  • NSAIDs for pain management

Surgical Management

• Indicated for patients with refractory symptoms despite conservative management:

1. Lateral Release

   • For patients with tight lateral retinaculum causing lateral patellar tilt
   • May need to be combined with medial repair/reconstruction in some cases

2. Medial Patellofemoral Ligament (MPFL) Reconstruction

   • For recurrent subluxation with incompetent medial restraints
   • Minimally invasive "crescentic" imbrication technique shows good outcomes 5

3. Tibial Tubercle Transfer

   • For patients with abnormal Q-angle or patellar tracking
   • May be combined with lateral release and MPFL reconstruction for complex cases 6

4. Component Revision (for post-TKA patients)

   • When patellar subluxation is due to component malrotation
   • Excessive combined internal rotation of tibial and femoral components is directly proportional to severity of patellofemoral complications 1

Pitfalls and Caveats

• Early cartilage changes may be underestimated on MRI compared to arthroscopic findings 4
• Medial patellar subluxation is rare but can be overlooked; requires high index of suspicion 6
• In post-TKA patients, patellofemoral instability occurs in 1-12% of cases, most often due to internal malrotation of components 1
• CT scan-based 3D models may underestimate knee joint deformity compared to weight-bearing radiographs 1
• Patellar fractures can occur with subluxation and may be asymptomatic; radiographic evaluation is important for identification 1