Boutonnière deformity | Radiology Signs

Answer:

A boutonnière deformity is fixed flexion of the proximal interphalangeal joint with compensatory hyperextension of the distal interphalangeal joint. It follows disruption or attenuation of the central slip of the extensor mechanism at its insertion on the middle phalangeal base. Once the central slip fails, the head of the proximal phalanx protrudes between the two lateral bands like a button through a buttonhole, and the bands, no longer held dorsally by the intact triangular ligament, migrate volar to the joint axis. They then act as PIP flexors instead of extensors while continuing to extend the DIP joint, which produces and then fixes the deformity. Causes are traumatic (forced PIP flexion against resistance, laceration, volar dislocation, burns) and inflammatory, above all rheumatoid arthritis, where synovitis erodes the central slip. Diagnosis is clinical; radiographs show associated fractures, subluxation and erosive change, and MRI or ultrasound demonstrates the central slip directly. It is the converse of the swan neck deformity.

Why is it called so?

The proximal phalangeal head buttonholes forward between the lateral bands, as a button passes through its hole (French boutonnière).

Pathophysiology

The extensor mechanism at the PIP joint has a central slip inserting on the middle phalanx and two lateral bands that pass dorsolaterally to converge on the terminal tendon. The triangular ligament holds the lateral bands dorsal to the joint axis. When the central slip is torn or eroded, unopposed flexor digitorum superficialis pull flexes the PIP joint; the proximal phalangeal head then displaces dorsally between the lateral bands, stretching the triangular ligament until the bands slip volar to the axis of rotation. From that position their pull flexes rather than extends the PIP joint, while the terminal tendon they feed becomes relatively tight and hyperextends the DIP joint. Progressive contracture then makes the deformity fixed.

Alternative names: Buttonhole deformity

Other associated named signs: Swan neck deformity, mallet finger, Z-thumb deformity, and ulnar deviation at the metacarpophalangeal joints in rheumatoid arthritis

References

1. Meadows L, Gorbachova T. MR Imaging of Injury of the Extensor Mechanism of the Fingers. Magn Reson Imaging Clin N Am. 2026;34(1):117-128. PMID: 41265971.