[Micro-constructive studies of human digital flexor tendon and tendon sheath--observation by scanning electron...

This report concerns a micro-construction of the human digital flexor tendon and tendon sheath. Using fresh human tendons and tendon sheaths, the specimens were prepared as continuous as possible, and observed under a scanning electron microscope. The results obtained from these observations were compared with the histological and histochemical findings and some interesting conclusions were obtained. The tendon and tendon sheath are organized to transmit the muscle contraction most effectively to the finger as a reasonable finger motion. The tendon consists of the bundles of dense collagen fibers, the endotenon which divides each tendon bundles, the epitenon which surrounds all the tendon bundles as a whole and the visceral synovial layer which bundles the top of the tendon. The fiber of the epitenon originates from the paratenon at the proximal reflexion of the synovial layer, and remain around the tendon bundle in the tendon sheath cavity. The epitenon thickens itself at the dorsal side of the tendon, and the longitudinal dorsal tendon vessels are included in the thickened epitenon. Both the epitenon and the endotenon consist of loose connective fibers and these fibers are in continuation from one to the other. The endotenon divides a tendon into bundles and includes the intertendinous longitudinal vessels inside it. It also relieves the strain of tendon aroused in the process of finger motion. The synovial tendon sheath cavity is considered as a bursa formed in the paratenon, and the volar sides of the parietal layer of the synovial tendon sheath act chiefly as the pulley in co-operation with the ligamentous tendon sheath. The dorsal sides of the parietal layer, especially in the area near the volar plate, the tendon anchoring area, the reflexion area of the synovial layer and the origin of the tendon vinculum, form the areolar-type of synovial layer, and are concerned in production of the synovial fluid. Some of the synovial cells in the synovial layer are stained metachromatically with the histochemical stains, and they are considered to produce the synovial fluid mainly composing of the hyaluronic acid. The synovial fluid gives a biological activity and a biomechanical lubrication to the tendon function. The paratenon, the mesotenon and the vinculum are all paratendinous tissues, which function as the tendon anchor and provide the paths for vessels to the tendon. These structures have been modified into the individual forms through the adaptation to the needs of function and to the environments surrounding them. The paratenon locates in the area where the tendon runs straight in the shortest excursion, and the mesotenon in the area of the curved long excursion as in the case for carpal tunnel or tarsal tunnel where is covered by the tendon sheath or the retinaculum. The vinculum exists in a highly differentiated narrow space such as the no-man's land of finger, and has remained as the minimum unit to maintain the blood supply to the tendon.