In a patient with an acute knee injury and suspected medial patellar fragment after a recent patellar dislocation, unable to perform a straight‑leg raise and showing signs of patellar instability, should a CT scan be expedited?

CT Scan Should NOT Be Expedited in This Case

In a patient with acute patellar dislocation, inability to perform straight-leg raise, and suspected medial patellar fragment, plain radiographs are the appropriate initial imaging study, not CT. CT is not routinely used as the initial imaging modality for acute knee trauma and should only be considered after radiographs if there is suspicion for radiographically occult fractures 1.

Recommended Imaging Algorithm

Initial Imaging: Plain Radiographs First

• Obtain standard knee radiographs immediately (anteroposterior, lateral, and patellofemoral views) 1
• The patellofemoral view is specifically indicated for suspected patellar fractures and/or subluxation or dislocation 1
• This patient meets Ottawa Knee Rule criteria (inability to bear weight/perform straight-leg raise, focal patellar tenderness, effusion), making radiographs the mandatory first step 1

When to Consider Advanced Imaging

CT may be appropriate as a next study only if:

• Radiographs are negative but clinical suspicion remains high for occult fracture 1
• CT shows 100% sensitivity for detecting tibial plateau fractures and 80% sensitivity for bony avulsion fractures 1
• However, CT has low sensitivity for soft-tissue injuries (meniscal, ligamentous) 1

MRI is superior to CT and should be considered when:

• Radiographs show a fracture and you need to evaluate for associated soft-tissue injuries 1
• There are osteochondral lesions requiring surgical planning 2
• MRI has 95% sensitivity for confirming acute patellar instability and identifies MPFL tears, chondral lesions, and bone contusions 2, 3

Critical Clinical Considerations

Why This Matters for Morbidity and Mortality

The inability to perform a straight-leg raise suggests either:

1. Extensor mechanism disruption (patellar tendon or quadriceps rupture)
2. Large displaced patellar fracture
3. Severe pain/effusion limiting function

These require urgent diagnosis but radiographs will identify all clinically significant fractures that need immediate surgical intervention 1.

Surgical Indications That Require Prompt (Not Emergent) Diagnosis

• Osteochondral fractures ≥1 cm² in the patellofemoral contact area warrant surgical repair 4, 5
• Displaced patellar fractures with extensor mechanism disruption require operative fixation 6
• Large chondral fragments should undergo fragment refixation rather than removal 4

However, these decisions are made based on radiographic findings first, with MRI for surgical planning if needed—not CT 2, 7.

Common Pitfalls to Avoid

1. Do not bypass radiographs to get CT - This violates established imaging guidelines and exposes patients to unnecessary radiation 1

2. Do not confuse "expedited" with "appropriate sequence" - While imaging should be obtained promptly, the correct sequence is radiographs → MRI (if needed) → CT only for specific indications 1

3. Recognize that 70% of acute patellar dislocations have osteochondral injury on MRI, but most are bone contusions rather than displaced fragments requiring immediate surgery 3

4. MPFL injury occurs in 94-100% of first-time patellar dislocations 6, but this is a soft-tissue injury best evaluated by MRI, not CT 1, 2

Bottom Line

Order standard knee radiographs now, not CT. If radiographs show a displaced osteochondral fragment or patellar fracture requiring surgical planning, obtain MRI for comprehensive evaluation of cartilage and soft-tissue injuries 2, 4. CT has no role in the initial evaluation of acute patellar dislocation and should only be considered if radiographs are negative but occult fracture is strongly suspected 1.