How to Determine a Sprain
Use the Ottawa Ankle Rules immediately to exclude fracture, then perform a delayed physical examination at 4-5 days post-injury to accurately diagnose ligament injury severity.
Initial Assessment: Rule Out Fracture First
The Ottawa Ankle Rules are your primary screening tool with 86-99% sensitivity and 97-99% negative predictive value 1. Order radiographs only if the patient has:
- Pain on palpation at the posterior edge of either malleolus (within 6 cm) 1
- Pain on palpation at the base of the fifth metatarsal 1
- Pain on palpation of the navicular bone 1
- Inability to bear weight immediately after injury and walk at least four steps 1
If none of these criteria are met, no radiograph is needed 1. This approach correctly rules out fracture in 299 out of 300 patients without unnecessary imaging 1.
Critical Timing: Don't Diagnose Ligament Severity Immediately
Physical examination in the first 48 hours cannot reliably distinguish between simple distortion and complete ligament rupture due to excessive swelling and pain 1, 2. This is a common pitfall—attempting immediate definitive diagnosis leads to inaccuracy.
Optimal Diagnostic Window: 4-5 Days Post-Injury
Re-examine the patient at 4-5 days after injury when the anterior drawer test reaches optimal sensitivity (84%) and specificity (96%) 1, 2. At this delayed examination, look for four key findings that indicate lateral ligament rupture:
- Hematoma present 1, 2
- Pain on palpation around the distal fibula 1, 2
- Positive anterior drawer test 1, 2
- Significant swelling 1, 2
When all four findings are present, 96% of patients have lateral ligament rupture; when absent, only 14% have rupture 1.
Performing the Anterior Drawer Test
The anterior drawer test detects excessive anterior displacement of the talus onto the tibia 1. If the anterior talofibular ligament is torn, the talus will subluxate anteriorly compared with the unaffected ankle 1. This test is most accurate at the 4-5 day mark, not acutely 1.
Grading the Sprain
Once ligament injury is confirmed, classify severity:
- Grade I: Mild sprain with minimal ligament damage 1
- Grade II: Moderate sprain with partial ligament tears 1
- Grade III: Severe sprain with complete ligament rupture 1
When to Consider Advanced Imaging
Do not routinely order MRI or ultrasound for uncomplicated ankle sprains 1, 2. Reserve MRI for specific scenarios:
- Suspected high-grade ligament injuries with persistent symptoms 1, 2
- Suspected osteochondral defects 1, 2
- Suspected syndesmotic injuries 1, 2
- Suspected occult fractures 1, 2
- Symptoms persisting beyond expected recovery period 1, 2
MRI has excellent sensitivity (93-96%) and specificity (100%) for these injuries 1, 2, but ultrasonography lacks adequate specificity (64%) despite good sensitivity (92%) 1, 2.
Special Considerations
Check for high ankle (syndesmotic) sprain using the crossed-leg test: pressure applied to the medial side of the knee producing pain in the syndesmosis area indicates this injury 1. These injuries result from dorsiflexion and eversion with internal tibial rotation 1.
Previous ankle sprain is a significant risk factor for current injury and should raise suspicion for chronic instability 1, 2.
Common Diagnostic Pitfalls to Avoid
- Don't attempt definitive ligament diagnosis in the first 48 hours—swelling and pain obscure findings 1, 2
- Don't order radiographs without applying Ottawa Ankle Rules—this leads to unnecessary imaging in 85% of cases 1
- Don't use stress radiographs—they are painful, obsolete, and no better than delayed physical examination 1
- Don't use arthrography—it's invasive with no advantage over delayed examination 1
- Don't order routine MRI for simple sprains—reserve for specific indications only 1, 2