A, Plantar surface of first metatarsal head. Entire sesamoid sling with attached intrinsic musculature has been rotated distally off metatarsal head to present schematically relationships of muscle, tendon, capsule, ligaments, and articular configuration of first metatarsophalangeal joint. B, As metatarsal head moves medially, sesamoid sling apparatus becomes valgus deforming force, and metatarsal rotates (pronates) on its longitudinal axis. Intrinsic and extrinsic muscle balance is lost, and deformity increases. (A after Beverly Kessler; courtesy LTI Medica and The Upjohn Company.)