Observations in proximal interphalangeal flexion of the finger

Abstract

The extensor assembly of the finger, consisting of tendon fibers from extrinsic and intrinsic finger muscles, rearranged into one medial bundle and two lateral bundles, undergoes distal displacement during proximal interphalangeal flexion. The lateral bundles, conjoining distally into one terminal extensor tendon for the distal phalanx, lie slack alongside the flexed proximal interphalangeal joint (Landsmeer, 1976). The distal phalanx becomes “loose”, thus allowing simultaneous distal interphalangeal flexion. During unresisted flexion of the proximal interphalangeal joint of the finger, without simultaneous distal interphalangeal flexion, the phenomenon of the “floating distal phalanx” can be observed (Tubiana et al., 1996). To further investigate this phenomenon, morphological features of the extensor assembly in relation to the proximal interphalangeal joint were observed by means of microdissection and high resolution imaging techniques, and simulated by mathematical modelling. In the extended proximal interphalangeal joint, the position of the ligamenta collateralia propria coincides with an almost frontal plane. They thereby support the lateral bundles of the extensor assembly to maintain dorsal positions. After proximal interphalangeal flexion, these collateral ligaments show transversal positions with respect to the head of the proximal phalanx, enabling the lateral bundles of the extensor assembly to shift in palmar and distal directions during subsequent distal interphalangeal flexion. Then the lateral bundles of the extensor assembly assume more sagittal positions.