Swan-neck deformity. A, Terminal tendon rupture may be associated with synovitis of distal interphalangeal joint, leading to distal interphalangeal joint flexion and subsequent proximal interphalangeal joint hyperextension. Rupture of flexor digitorum superficialis tendon can be caused by infiltrative synovitis, which can lead to decreased volar support of proximal interphalangeal joint and subsequent hyperextension deformity. B, Lateral-band subluxation dorsal to axis of rotation of proximal interphalangeal joint. Contraction of triangular ligament and attenuation of transverse retinacular ligament are depicted.