What is the term for the distal third of the thumb angled outward due to arthritis?

Boutonnière Deformity: The Term for Distal Third of Thumb Angled Outward Due to Arthritis

The term for the distal third of the thumb angled outward due to arthritis is "boutonnière deformity" of the thumb, which is characterized by hyperextension of the metacarpophalangeal (MCP) joint and flexion of the interphalangeal (IP) joint, creating a zigzag appearance.

Pathophysiology and Presentation

• Boutonnière deformity in the thumb commonly develops as a consequence of advanced thumb base (carpometacarpal/CMC) osteoarthritis, creating a characteristic zigzag collapse pattern 1, 2
• The deformity occurs when the CMC joint becomes unstable due to ligamentous laxity and arthritic changes, leading to compensatory hyperextension at the MCP joint 3, 2
• This compensatory mechanism creates an outward angling of the distal third of the thumb, with the metacarpal adducting and the proximal phalanx hyperextending 4, 2

Clinical Significance and Impact

• The deformity significantly impacts hand function, with functional impairment potentially as severe as that seen in rheumatoid arthritis 1
• Pain is typically localized to the thumb base but can also occur at the MCP joint due to the abnormal mechanical stresses 5, 2
• The severity of the deformity correlates with disease progression, with more pronounced deformities seen in advanced stages of thumb CMC arthritis 2

Associated Conditions and Risk Factors

• Thumb CMC osteoarthritis is the most common site of surgical reconstruction for osteoarthritis in the upper extremity 5
• Higher prevalence in women (40%) compared to men (25%) in those over age 75 5
• Hypermobility has been reported as a risk factor for thumb base OA 1
• The deformity is more common in erosive hand OA, which shows worse clinical and structural outcomes than non-erosive forms 1

Diagnostic Approach

• Clinical diagnosis is based on the characteristic appearance of the thumb with outward angling of the distal portion 1
• Radiographic assessment with at least 2 views (PA and lateral) is essential to evaluate the extent of arthritic changes and joint displacement 6
• The Eaton classification system provides a reproducible radiographic staging of disease severity in thumb CMC arthritis 5
• Plain radiographs remain the gold standard for morphological assessment, showing joint space narrowing, osteophytes, and subchondral bone sclerosis 1

Management Options

• Orthoses (splints) should be considered as first-line treatment for symptom relief, with long-term use advocated 1
• Custom-made thermoplast or neoprene orthoses have shown beneficial effects on pain and function when used for at least 3 months 1
• Hand exercises to improve joint mobility, muscle strength, and thumb base stability should be considered for every patient 1
• Topical NSAIDs are recommended as first-line pharmacological treatment due to their favorable safety profile 1

Surgical Considerations

• Surgical intervention should be considered for severe thumb base OA when conservative treatments have failed 6
• Options include ligament reconstruction, metacarpal extension osteotomy, arthroscopic partial trapeziectomy, implant arthroplasty, and trapeziectomy with or without ligament reconstruction and tendon interposition 5
• For severe hyperextension deformities (>40° of passive flexion), external fixators may be required in addition to CMC arthroplasty 2
• CMC joint denervation is a relatively novel method that can provide effective relief of arthritis pain that was durable at 5+ years after surgery in more than half of patients 7

Common Pitfalls and Caveats

• Treating only the CMC joint without addressing the MCP hyperextension deformity may lead to recurrence of the deformity and persistent functional limitations 2
• Orthoses that are not well-fitted or custom-made may lead to poor patient compliance and reduced effectiveness 1
• Exercises alone without addressing joint stability through orthoses may not provide sustainable improvement 1
• The long-term functional outcome for erosive OA appears worse than for nodal OA, requiring more aggressive management 1