Boutonniere and Swan Neck Deformity
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Direct answer
A boutonniere deformity is flexion of the proximal interphalangeal (PIP) joint with hyperextension of the distal interphalangeal (DIP) joint, caused by rupture of the central slip of the extensor tendon. A swan neck deformity is the reverse, with PIP hyperextension and DIP flexion, usually from volar plate laxity or intrinsic imbalance. Both are classic finger deformities of rheumatoid arthritis.
What you must remember
- Boutonniere mechanism: central slip rupture lets the lateral bands slip volarly, so the PIP flexes while the DIP is pulled into hyperextension.
- Elson test: the PIP is flexed to 90 degrees over a table edge and the patient extends it against resistance; an intact central slip extends the PIP with the DIP relaxed, whereas a rigid or hyperextended DIP with weak PIP extension points to central slip rupture.
- Swan neck mechanism: PIP hyperextension with DIP flexion, from volar plate laxity, attenuation of the central slip, or intrinsic tightness.
- Rheumatoid pattern: synovitis erodes the central slip insertion or the volar plate, and both deformities are classic in rheumatoid hands.
- Early boutonniere: PIP extension splinting for about six weeks protects the central slip while it heals.
- Flexible versus fixed swan neck: a flexible deformity is corrected passively and treated by splinting, while a fixed one needs surgery such as volar plate arthroplasty or flexor digitorum superficialis tenodesis.
Comparing the two deformities
Boutonniere and swan neck are mirror-image deformities, and the examiner usually tests whether the candidate can tell them apart from the joint positions alone. The table sets out the features that matter most.
| Feature | Boutonniere | Swan neck |
|---|---|---|
| PIP joint | Flexed | Hyperextended |
| DIP joint | Hyperextended | Flexed |
| Core lesion | Central slip rupture | Volar plate laxity or intrinsic tightness |
| Classic test | Elson test | Passive correction on examination |
| Early treatment | PIP extension splint | Splint in PIP flexion if flexible |
Reading the joint positions correctly is the fastest route to the answer, since the deformity is named by the PIP posture. The mechanism also explains the treatment, because a torn central slip needs the PIP held in extension while a lax volar plate needs the PIP protected from hyperextension.
In rheumatoid arthritis the two deformities evolve from synovitis, which weakens the extensor mechanism or the volar plate over time. Early synovitis control and splinting may slow progression, but fixed deformities need reconstruction.
Where the exam tries to catch you
Finger deformity questions are usually decided by the position of the PIP joint and the test that localises the lesion.
- Trap: calling a PIP flexion deformity a swan neck. Correct: PIP flexion with DIP hyperextension is a boutonniere, while swan neck is PIP hyperextension with DIP flexion.
- Trap: using active flexion exercises for an early boutonniere. Correct: the central slip must heal, so PIP extension splinting is the early treatment.
- Trap: assuming every swan neck needs surgery. Correct: a flexible swan neck is treated by splinting, and surgery is reserved for fixed deformities.
Frequently asked questions
What is the Elson test used for?
The Elson test is used to diagnose central slip rupture in a boutonniere-pattern injury. The PIP is flexed to 90 degrees over a table edge and the patient extends it against resistance. An intact central slip allows the PIP to extend with the DIP relaxed.
Which tendon structure is injured in a boutonniere deformity?
A boutonniere deformity results from rupture of the central slip of the extensor tendon at its insertion on the middle phalanx. The lateral bands then slip volarly to the PIP axis. This causes PIP flexion and DIP hyperextension.
How does rheumatoid arthritis produce a swan neck deformity?
Rheumatoid synovitis weakens the volar plate of the PIP joint and alters the balance of the intrinsic and extrinsic tendons. This allows the PIP to hyperextend while the DIP flexes. Flexible swan neck deformities are splinted, and fixed deformities need surgery.
How is a flexible swan neck deformity treated?
A flexible swan neck deformity is treated by splinting the PIP in slight flexion, which holds the joint out of hyperextension. The deformity is corrected passively on examination. Surgery such as volar plate arthroplasty or flexor digitorum superficialis tenodesis is reserved for fixed deformities.