When would I order imaging in a suspected thumb sprain?

When to Order Imaging in Suspected Thumb Sprain

Order plain radiographs (3-view series minimum) immediately for any suspected thumb sprain to rule out fracture, and proceed to ultrasound or MRI if clinical examination suggests ulnar collateral ligament (UCL) laxity to differentiate complete displaced tears requiring surgery from nondisplaced injuries amenable to conservative treatment.

Initial Imaging: Plain Radiographs

Always obtain radiographs first to exclude fracture or avulsion injury before assuming a pure ligamentous injury 1, 2:

• Minimum 3-view series required: posteroanterior (PA), lateral, and oblique views of the thumb 2
• Some centers include a PA view of the entire hand while others focus on the injured digit 2
• Two views are inadequate and will miss important pathology 2
• Radiographs identify fractures, avulsion injuries at ligament insertion sites, and joint alignment abnormalities 1, 3

Common Pitfall

Relying on only 2 radiographic views is a critical error that leads to missed fractures, particularly at the base of the proximal phalanx where the UCL inserts 2.

Advanced Imaging: When Initial Radiographs Are Normal

If radiographs are negative but clinical examination reveals laxity of the metacarpophalangeal (MCP) joint on stress testing, proceed to advanced imaging 1:

Ultrasound as First-Line Advanced Imaging

Ultrasound should be strongly considered as the next imaging modality when UCL injury is suspected 4:

• 100% accuracy in identifying complete displaced UCL tears that require surgery 4
• Cost-effective alternative to MRI for differentiating surgical versus nonsurgical injuries 4
• Approximately 90% accuracy overall in depicting UCL tear position 5

However, be aware of ultrasound pitfalls 5:

• Dislocation of palmar joint capsule to the ulnar joint space can mimic nondisplaced tears
• Scalloping of the adductor aponeurosis from displaced UCL creates confusing images
• Scar tissue from prior injury can obscure findings
• Operator-dependent technique

MRI: The Reference Standard

Use MRI when 5, 6:

• Ultrasound suggests a nondisplaced tear and you're considering conservative therapy (to confirm absence of Stener lesion)
• Ultrasound findings are equivocal
• Clinical suspicion remains high despite negative ultrasound
• You need to definitively rule out a Stener lesion (displaced UCL superficial to adductor aponeurosis)

MRI without IV contrast is the reference standard for characterizing thumb UCL injuries and detecting the Stener effect 1, 6, 4.

Clinical Decision Algorithm

1. Obtain 3-view radiographs on all suspected thumb sprains 1, 2

2. If radiographs show fracture: Manage fracture appropriately; may still need MRI or ultrasound to assess associated ligamentous injury 1

3. If radiographs are normal:

   • No laxity on examination: Immobilize with thumb spica and treat conservatively 7, 6
   • Laxity present on examination: Obtain ultrasound or MRI to detect complete displaced tears (Stener lesion) requiring surgery 5, 6, 4

4. If ultrasound suggests nondisplaced tear and conservative treatment is planned: Confirm with MRI to avoid missing a Stener lesion that would lead to chronic instability 5

Timing Considerations

• Stress examination (to assess laxity) is optimally performed 4-5 days post-injury when sensitivity and specificity are maximized, similar to ankle ligament assessment 1
• If immediate assessment is needed, consider performing stress radiographs under local anesthesia 8
• Repeat radiographs in 10-14 days are an alternative if initial films are negative but clinical suspicion persists 1, 2

Key Clinical Point

The critical distinction is between complete displaced UCL tears (Stener lesions) requiring surgery versus nondisplaced tears that can be treated conservatively 5, 6. Missing a Stener lesion leads to chronic instability and poor outcomes, making accurate imaging essential when laxity is present on examination 5, 6.