Cause of Trigger Thumb
Trigger thumb is primarily caused by a mechanical size mismatch between the flexor tendon and the A1 pulley, resulting in stenosing tenosynovitis—a condition where the tendon cannot glide smoothly through the pulley system. 1, 2
Primary Pathomechanism
The fundamental problem is stenosis (narrowing) at the A1 pulley level, which creates mechanical obstruction to normal tendon gliding. 2 This size discrepancy between the flexor tendon and its fibrous sheath leads to the characteristic catching or locking sensation. 1
Etiological Categories
Idiopathic (Primary) Stenosing Tenosynovitis
- Most cases are idiopathic with no identifiable precipitating cause, occurring more frequently in middle-aged women than men. 3, 1
- The thumb is the most commonly affected digit overall. 3
Secondary Stenosing Tenosynovitis
Several systemic conditions predispose to trigger thumb development: 1
- Diabetes mellitus is a major risk factor and these patients respond less favorably to conservative treatment. 3, 2
- Rheumatoid arthritis causes connective tissue disorders that promote tenosynovitis (though these patients require tenosynovectomy rather than simple A1 pulley release). 1, 2
- Gout and other connective tissue disorders can precipitate triggering. 1
Mechanical/Traumatic Factors
- Repetitive finger movements and occupational overuse contribute to development. 3
- Direct hyperextension injury can initiate stenosis in the tendon sheath, with trauma potentially serving as the sole initiating factor in some adult cases. 4
- Carpal tunnel syndrome is associated with trigger finger development. 3
Important Clinical Distinctions
The diagnosis is primarily clinical, based on examination findings of intermittent mechanical obstruction and tenderness at the palmar aspect of the metacarpophalangeal joint. 1, 5
Key Pitfall to Avoid
Always perform detailed physical examination and obtain radiographs when there is persistent metacarpophalangeal joint tenderness, even with obvious triggering present, as conditions like sesamoid arthritis can coexist and mimic trigger thumb. 5 Overlooking concurrent pathology around the metacarpophalangeal joint is a common error. 5
Classification Significance
Classification by type of tenosynovitis and duration from symptom onset may be prognostically significant and affect treatment outcomes, though the underlying cause remains the mechanical stenosis at the A1 pulley. 1