How should I diagnose a patellar dislocation?

Diagnosing Patellar Dislocation

Obtain standard radiographs (anteroposterior, lateral, and patellofemoral/axial views) as the initial imaging study to assess for patellar fracture, osteochondral injury, and anatomic risk factors. 1

Initial Clinical Assessment

Key History Elements

• Mechanism of injury: Non-contact knee sprain in flexion and valgus accounts for 93% of cases, typically occurring during physical or sports activity 2
• Age: Two-thirds of acute patellar dislocations occur in patients under 20 years old 2
• Prior instability: Determine if this is a first-time or recurrent dislocation 3

Physical Examination Findings

• Lateral patellar glide test: Assess with the knee in extension or 20° flexion; displacement greater than 50% of patellar width is abnormal and may induce apprehension 2
• Patellar position: Check for anterolateral positioning and internal rotation of the patella from the coronal plane, which suggests irreducible dislocation 4
• Knee flexion angle: Irreducible dislocations typically present with less knee flexion than typical lateral dislocations 4
• Joint effusion/hemarthrosis: Presence indicates need for advanced imaging 5

Imaging Algorithm

Step 1: Plain Radiographs (Mandatory)

• Views required: Anteroposterior, lateral, and patellofemoral (axial) views 1
• Weight-bearing axial radiographs are preferred to evaluate patellofemoral kinematics and demonstrate the degree of patellar tilt or subluxation 6
• What to assess:
  • Patellar fracture 1
  • Osteochondral fracture fragments 2, 5
  • Small osseous fragments along the medial patellar margin (suggests prior dislocation) 7
  • Bony risk factors: trochlear dysplasia 2
  • Internal rotation of patella about vertical axis ("flipped patella" sign = pathognomonic for irreducible dislocation) 4

Step 2: CT Scan (Indicated for Surgical Planning)

• Use CT without IV contrast when detailed assessment of bony anatomy is needed 1
• Specific indications:
  • Measuring tibial tubercle-trochlear groove distance (TT-TG) 2
  • Assessing axial malrotation 1, 6
  • Confirming diagnosis when closed reduction fails 4
  • Evaluating trochlear dysplasia severity 2

Step 3: MRI (Selective Use)

• Obtain MRI without IV contrast if:

  • Hemarthrosis is present 5
  • Displaced osteochondral fragment is suspected 3, 5
  • Radiographs show small osseous fragment along medial patellar margin 7
  • Planning treatment and need to assess soft tissue injury 2

• MRI provides critical information on:

  • Medial patellofemoral ligament (MPFL) integrity—almost always injured at femoral attachment 2
  • Medial patellar retinaculum status 7
  • Cartilage injury extent (70% of first-time dislocations have patellofemoral cartilage injury) 8
  • Free bodies or osteochondral fractures requiring surgical treatment 2

Common Pitfalls to Avoid

• Missing osteochondral fractures: Always obtain MRI if hemarthrosis is present, as displaced fragments require surgical intervention 5
• Overlooking irreducible dislocation: Look for the "flipped patella" sign on radiographs and less knee flexion than typical—these require open reduction 4
• Inadequate radiographic views: Axial/patellofemoral views are essential and often missed; they demonstrate patellar tilt and subluxation 7, 6
• Assuming all first-time dislocations are benign: Even single dislocations can lead to progressive cartilage deterioration, so thorough initial assessment is critical 8

Decision Points for Advanced Imaging

Proceed directly to MRI if any of the following are present:

• Hemarthrosis 5
• Suspected osteochondral fracture on radiographs 3
• Substantial disruption of medial patellar stabilizers on examination 3
• Laterally subluxated patella with normal contralateral knee alignment 3
• Failed closed reduction attempt 4

Use CT for:

• Surgical planning when operative intervention is considered 1
• Detailed measurement of TT-TG distance and rotational abnormalities 2