Boutonniere deformity. A, Primary synovitis of proximal interphalangeal (PIP) joint can lead to attenuation of overlying central slip and dorsal capsule and increased flexion at PIP joint. Lateral band subluxation volar to axis of rotation of PIP joint can lead in time to hyperextension. Contraction of oblique retinacular ligament, which originates from flexor sheath and inserts into dorsal base of distal phalanx, can lead to extension contracture of distal interphalangeal joint. B and C, Clinical photographs illustrate flexion posture of PIP joint and hyperextension posture of distal interphalangeal joint in boutonniere deformity.