Long Flexor Tendons and Tendon Sheaths in Hand

The tendons of the flexor digitorum superficialis and flexor digitorum profundus enter the common flexor sheath (ulnar bursa) deep to the flexor retinaculum. The tendons enter the central compartment of the hand and fan out to enter their respective digital synovial sheaths. The flexor and digital sheaths enable the tendons to slide freely over each other during movements of the fingers. Near the base of the proximal phalanx, the tendon of flexor digitorum superficialis splits to permit passage of the tendon of flexor digitorum profundus; the crossing of the tendons makes up a tendinous chiasm. The halves of the flexor digitorum superficialis tendon are attached to the margins of the anterior aspect of the base of the middle phalanx. Distal to the tendinous chiasm, the flexor digitorum profundus tendon attaches to the anterior aspect of the base of the distal phalanx.

The fibrous digital sheaths are the strong ligamentous tunnels containing the flexor tendons and their synovial sheaths. The sheaths extend from the heads of the metacarpals to the bases of the distal phalanges. These sheaths prevent the tendons from pulling away from the digits (bowstringing). The fibrous digital sheaths combine with the bones to form osseofibrous tunnels through which the tendons pass to reach the digits. The anular part and cruciform part (often referred to clinically as “pulleys”) are thickened reinforcements of the fibrous digital sheaths.

The long flexor tendons are supplied by small blood vessels that pass within synovial folds (vincula) from the periosteum of the phalanges. The tendon of the flexor pollicis longus passes deep to the flexor retinaculum to the thumb within its own synovial sheath. At the head of the metacarpal, the tendon runs between two sesamoid bones, one in the combined tendon of the flexor pollicis brevis and abductor pollicis brevis and the other in the tendon of the adductor pollicis.