Diagnosis and Next Step
The diagnosis is a complete ulnar collateral ligament (UCL) tear of the thumb MCP joint (gamekeeper's thumb), and the next step is MRI of the hand to evaluate for a Stener lesion, which will determine whether surgical repair is required.
Clinical Diagnosis
The presentation of 30° greater laxity compared to the contralateral thumb with no fixed endpoint on stress testing is pathognomonic for a complete UCL rupture 1, 2, 3. The lack of a firm endpoint is actually more reliable than absolute degree measurements, as normal individuals can have up to 15° difference between thumbs 1. The mechanism (fall) and functional impairment (pain with gripping steering wheel) are classic for this injury.
Diagnostic Criteria for Complete UCL Tear:
- >30° of laxity in extension, OR
- >15° more laxity than contralateral thumb, OR
- Absence of a firm endpoint (most reliable sign) 2, 3
Your patient meets all three criteria, confirming complete rupture.
Imaging Algorithm
Step 1: Initial Plain Radiographs
Obtain standard thumb radiographs to exclude avulsion fractures. If negative for fracture (as implied by your clinical scenario):
Step 2: MRI Hand (Recommended Next Step)
MRI is the gold standard for evaluating UCL tears and is specifically indicated to detect Stener lesions 4. The ACR Appropriateness Criteria (2019) explicitly state that MRI is ideal for surgical planning and has 100% sensitivity and specificity for assessing thumb UCL tears 4.
Why MRI is Critical:
A Stener lesion occurs when the ruptured UCL retracts proximally and becomes trapped superficial to the adductor pollicis aponeurosis, preventing healing 3, 5. This occurs in the majority of complete UCL tears and requires surgical repair because the interposed aponeurosis prevents ligament-to-bone healing 3.
Alternative: Ultrasound
Dynamic ultrasound can identify Stener lesions by showing absence of the UCL and a hypoechoic mass proximal to the metacarpal tubercle 4. However, MRI provides more comprehensive visualization for surgical planning 4.
Treatment Decision Tree
If MRI shows Stener lesion present:
- Surgical repair is mandatory 3, 5
- Primary repair with suture anchors within 3 weeks yields best outcomes 2
If MRI shows complete tear WITHOUT Stener lesion:
- Treatment remains controversial 5
- Most experts recommend surgery for complete tears to prevent chronic instability 2, 3
- Conservative treatment risks chronic laxity, osteoarthritis, and functional impairment 6
If partial tear (unlikely given your exam):
- Thumb spica splint for 4-6 weeks 2
Critical Pitfalls to Avoid
Do not rely solely on comparison to contralateral thumb - 34% of normal individuals have ≥10° difference between sides 1
Do not miss the Stener lesion - Without surgery, these will not heal and lead to chronic instability 3, 5
Do not delay imaging - Early diagnosis within 3 weeks allows primary repair; chronic injuries require reconstruction with tendon grafts 2
Do not treat conservatively without imaging - Your clinical exam indicates complete tear, which typically requires surgery regardless of Stener lesion presence 2, 3