Causes / typical injury mechanism: Swan-neck deformity can be associated with a number of underlying conditions, most commonly trauma, rheumatoid arthritis, or cerebral palsy.

For boutonnière deformity, the typical mechanism of injury is blunt or penetrating trauma to the dorsal aspect of the proximal interphalangeal (PIP) joint or a volar PIP joint dislocation.

Classic history and presentation: Swan-neck deformity consists of hyperextension of the PIP joint and flexion of the distal interphalangeal (DIP) joint. Boutonnière deformity consists of flexion of the DIP joint and hyperextension of the PIP joint. Onset of either is typically insidious but can be acute in the setting of trauma.

Prevalence: Swan-neck deformity is more common than boutonnière deformity.

Risk factors: Chronic mallet finger injury is the most common cause of swan-neck deformity.

Pathophysiology: Swan-neck deformity can be caused by increased pull of the extensor tendon over the PIP joint, tightness of the intrinsic muscles, damage to the PIP joint volar plate, or disruption of the flexor digitorum superficialis tendon.

In boutonnière deformity, damage to the central slip causes attenuation of the triangular ligament, leading to volar displacement of the conjoined lateral bands. This causes PIP joint flexion during attempted finger extension. The conjoined lateral bands can eventually become contracted, which results in DIP joint hyperextension.