Digital Nerve Compression

Digital Nerve Anatomy

• Position: Volar to the digital artery.
• Thumb: The Ulnar Digital Nerve (UDN) is most prominent at the MP joint level.
• Blood Supply: Vasa nervorum. Check for digital ischemia (e.g. Hypothenar Hammer).
• Origin: The UDN of the thumb arises from the median nerve (via branches from the palmar digital nerve).
• Course: Runs along the ulnar border of the thumb from the MCP joint to the tip.

The nerve is tethered by Cleland's and Grayson's ligaments, preventing it from escaping pressure.

The Lesion

• Stage 1: Neurapraxia (Edema).
• Stage 2: Fibrosis (Thickening of epineurium).
• Stage 3: Neuroma-in-continuity (palpable mass, scarred fascicles).
• Microscopy: Perineural fibrosis, axon dropout, mucinous degeneration.
• Histology: Schwann cell proliferation, collagen deposition, myxoid degeneration.

The mass is NOT a neuroma in the sense of a cut nerve, but a fibrosis in continuity.

Mechanism of Injury

• Bowling: The edge of the thumb hole creates a pressure point at the ulnar side.
• Grip Force: High grip force (14-15kg at release) concentrates stress.
• Repetition: Thousands of deliveries per year in competitive bowlers.
• Friction: The release motion drags the thumb edge across the nerve.
• Vascular: Compression of vasa nervorum leads to ischemia.

Clinical Grading (Dobyns)

• Mild: Paresthesia with activity only. No palpable mass.
• Moderate: Persistent paresthesia. Small palpable mass. Tinel's positive.
• Severe: Constant pain at rest. Large mass. Measurable sensory deficit.

Progression depends on continued exposure. Early recognition allows conservative management.

Etiological Types

• Sports: Bowler's Thumb, Baseball Finger (Index), Gymnast's Wrist.
• Occupational: Scissors use (Barber), Flute/Harp playing, Typing.
• Tools: Use of pliers/wrenches (often affects Index finger).
• Musical: String instruments (violin, guitar), wind instruments.

Vibration white finger (HAVS) is a separate vascular entity but may coexist.

Anatomic Classification

• Thumb: Ulnar Digital Nerve (Bowler's Thumb) - most common.
• Index: Radial Digital Nerve (Tool use, Harp).
• Middle: Either digital nerve (Typing, musical instruments).
• Ring/Small: Less common, usually occupational.

Location determines mechanism and treatment approach.

Physical Examination

• Inspection: Callus may be present overlying the nerve. Look for skin changes.
• Palpation: Firm, rubbery, tender fusiform mass (2-3mm to 1cm).
• Tinel's: Strongly positive. "Zing" or electric sensation to the tip.
• Sensation: 2PD may be normal or reduced (greater than 6mm) in severe cases.
• Motor: Normal (Digital nerves are purely sensory).
• Provocative Tests: Pressure on mass reproduces symptoms.
• Allen's Test: Rule out vascular contribution (Hypothenar Hammer).

Always compare with the contralateral thumb for baseline.

Red Flags to Exclude

• Rapid Growth: Consider malignancy (Schwannoma, Neurofibroma).
• Multiple Masses: Neurofibromatosis type 1.
• Motor Weakness: More proximal lesion (Median/Ulnar nerve trunk).
• Night Pain: Infection, tumor, inflammatory arthritis.
• Weight Loss: Systemic disease.
• Fixed Deformity: Rheumatoid nodule, GCTTS.

A detailed history and thorough examination can exclude most serious pathology.

Ultrasound

• Finding: Hypoechoic swelling of the nerve. Loss of fascicular pattern.
• Comparison: Compare diameter with contralateral digit.
• Doppler: Hypervascularity suggests active inflammation.
• Cross-sectional area: Increased CSA at the site of compression.
• Mobility: Reduced nerve gliding on dynamic assessment.

Essential to distinguish from Ganglion. US is the first-line imaging modality.

MRI

• Role: Problem solving and differential diagnosis.
• Findings: Fusiform enlargement, high T2 signal.
• Differential: Excludes Giant Cell Tumor of Tendon Sheath (GCTTS).
• Protocol: High-resolution extremity coil, T1 and T2 sequences.
• Contrast: Enhancement may indicate active inflammation.

MRI is expensive and often unnecessary if Tinel's is classic.

Nerve Conduction Studies

• Limited Utility: Distal digital nerves are hard to test reliably.
• SNAP: May show reduced amplitude or conduction block across the lesion.
• Comparison: Always compare with contralateral digit.
• Needle EMG: Not useful for sensory-only nerves.

Usually a clinical diagnosis. MRI and US confirm it.

The "Splint and Spare" Approach

• Modification: Change the grip. Increase bevel of the hole. Move trigger finger.
• Protection: Custom molded thermoplastic thumb shell ("Thimble").
• Rest: 3-6 months off bowling.
• Success: High (if compliant). Mass may persist but become painless.

Changing the mechanics of the grip is the most sustainable solution.

Operative Options

• Neurolysis: Releasing the scar. Often fails due to recurrence.
• Transposition: Moving the nerve dorsal to the Adductor Pollicis aponeurosis (protection).
• Neurectomy: Cutting the nerve. Definitive but leaves a numb thumb.

Surgery is for those who cannot play despite protection or have localized pain at rest.

Neurolysis & Transposition

• Incision: Mid-lateral or zig-zag over the mass.
• Dissection: Identify nerve proximal and distal to mass.
• Release: Carefully dissect scar from epineurium (Magnification!).
• Transposition: Create a bed dorsal to the Adductor mechanism.
• Fat Graft: Consider wrapping with vein or fat to prevent adhesion.

This is technically demanding in the small space of the thumb.

Neurectomy

• Indication: Intractable pain, failure of neurolysis.
• Technique: Section nerve proximal to the lesion (in healthy tissue).
• Handling: Bury proximal end deep in muscle or bone (Adductor pollicis).
• Result: Permanent numbness. (Test with lidocaine block pre-op to ensure patient accepts).

"Better a numb thumb than a painful thumb."

Stump management is critical to prevent recurrence.