The second annular pulley: a histologic examination.

The second annular pulley and underlying flexor tendons from 15 fresh frozen fingers were prepared, sectioned transversely and longitudinally and then stained with hematoxylin-eosin, Verhoeff-Van Gieson (Elastin), Masson's trichrome and for the presence of hyaluronic acid. Three discrete layers of the second annular pulley were identified (1) an outer layer, continuous with the membranous sheath, that was richly vascularized; (2) a middle layer, characterized by radially oriented collagen fibrils and significant elastin fibrils; (3) an inner gliding surface, characterized by longitudinally oriented collagen fibrils and modified fibroblasts secreting hyaluronic acid. The three layers of the pulley seem specialized and have developed based on the functional requirements of the pulley apparatus. The outer layer serves as a source of vascular supply and nutrition, the middle layer maintains structural integrity and is organized to efficiently resist flexor tendon bowstringing, and the innermost layer facilitates efficient excursion of the underlying flexor tendons both by the orientation of the collagen fibers and by the secretion of substances that are thought to minimize friction.