What red-flag findings after a patellar dislocation indicate the need for MRI?

Red Flags for MRI After Patellar Dislocation

Any patient with a suspected or confirmed first-time patellar dislocation should undergo MRI, as this has become the standard of care given the extremely high rate of associated injuries that require surgical intervention 1, 2, 3.

Key Red Flags Mandating MRI

Clinical Red Flags

• Suspected osteochondral injury - Present in 50% of first-time dislocations, with loose bodies found in 34% of cases 1
• Persistent effusion or hemarthrosis - Suggests significant intra-articular pathology
• Inability to bear weight after reduction - May indicate substantial cartilage or ligamentous damage
• Recurrent instability symptoms - Even subtle sensations of instability warrant imaging 3
• "Swollen knee" without clear history - In 31 of 39 cases presenting with just a swollen knee, MRI confirmed patellar instability and associated injuries 3

Radiographic Red Flags

• Visible osteochondral fragments on plain films - Only identifies 20.5% of actual osteochondral lesions; MRI detects far more 3
• Trochlear dysplasia - Present in 97.5% of cases and increases recurrence risk 3
• Normal radiographs do not exclude significant injury - CT identified free bodies in 19/26 patients versus only 5 on radiographs 4

Why MRI Should Be Standard of Care

The evidence overwhelmingly supports routine MRI for first-time patellar dislocations:

MPFL injuries are nearly universal:

• Complete MPFL tears occur in 47% of cases
• Partial tears in 41% of cases
• Combined, 95% have MPFL injury requiring surgical consideration 1, 3

Osteochondral injuries are extremely common but frequently missed:

• 50% have osteochondral pathology on MRI 1
• 34% have osteochondral fractures requiring surgical intervention 1
• 27.9% have loose bodies 2
• Medial patellar facet lesions occur in 66% of cases 1

Associated ligamentous injuries occur frequently:

• MCL injuries in 18% of cases 2
• ACL injuries in 1.6% of cases 2
• VMO edema/tear in 56% of cases 5

Surgical decision-making requires MRI:

• When MRI and CT were combined, 21 of 26 patients (81%) had imaging indications for surgery versus only 13 patients based on MRI alone 4
• The presence of bony contusions on MRI predicts recurrent instability (86% of reinjured patients had contusions) 6

Practical Algorithm

1. Initial presentation: Obtain standard knee radiographs (AP, lateral, patellofemoral views) to rule out gross fractures 7

2. Order MRI within 15 days for any confirmed or suspected patellar dislocation, regardless of whether the patella is currently reduced 3

3. Consider urgent MRI if:

   • Large effusion suggesting significant osteochondral injury
   • Palpable loose bodies
   • Locked knee
   • Young active patients where early surgical intervention may be beneficial

4. Add CT if MRI shows potential surgical indication - CT better identifies free bodies and exact fracture patterns for surgical planning 4

Critical Pitfalls to Avoid

Do not rely on clinical examination alone - In patients presenting with just a "swollen knee" and vague symptoms, 95% had confirmed patellar instability on MRI with associated injuries 3

Do not assume spontaneous reduction means minor injury - 78.4% of first-time dislocations have MPFL injury, and one-third have osteochondral fractures 5

Do not use plain radiographs to exclude osteochondral injury - Radiographs miss the majority of chondral lesions that are clearly visible on MRI 4, 3

Pediatric patients require the same imaging approach - The injury patterns are similar with 95% MPFL injury rate and 34% osteochondral fracture rate 5

The ACR Appropriateness Criteria state that MRI is not routinely used for initial acute knee trauma 7, but this applies to general knee trauma, not specifically to patellar dislocations where the injury pattern and surgical implications are distinctly different and well-characterized.