The tendon of flexor digitorum superficialis enters the fibrous flexor sheath on the palmar surface of the tendon of flexor digitorum profundus. It divides into two halves, which flatten a little and spiral around the profundus tendon and meet on its deep surface in a chiasma (a partial decussation). This forms a tendinous bed in which lies the profundus tendon. Distal to the chiasma the superficialis tendon is attached to the sides of the whole shaft of the middle phalanx.
The profundus tendon enters the fibrous sheath deep to the superficialis tendon, then lies superficial to the partial decussation of the latter, before passing distally to reach the base of the terminal phalanx. In the flexor sheath both tendons are invested by a common synovial sheath that possesses parietal and visceral layers. Each tendon receives blood vessels from the palmar surface of the phalanges. The vessels are invested in synovial membrane. These vascular synovial folds are the vincula, and each tendon possesses two, the short and long (Fig. 2.54). The profundus tendon has its short vinculum in the angle close to its insertion. Its long vinculum passes from the tendon between the two halves of the superficialis tendon (proximal to the chiasma) to the palmar surface of the proximal phalanx. The superficialis tendon has a short vinculum near its attachment into the sides of the middle phalanx. The long vinculum of the superficialis tendon is double, each half of the tendon possessing a vinculum just distal to its first division, passing down to the palmar surface of the proximal phalanx. The short vincula, attached to the capsules of the interphalangeal joints, serve to pull these out of harm’s way during flexion. Only the long vinculum is vascular.
The extensor tendons to the four fingers have a characteristic insertion. Passing across the metacarpophalangeal joint, the tendon is partly adherent to the articular margins, and indeed in this situation its deepest fibers form the posterior capsule of the joint. The bulk of the tendon, however, passes freely across the joint, and broadens out on the dorsal surface of the proximal phalanx. This flat part now divides into three slips; the central slip passes on to the base of the middle phalanx. The two lateral or marginal slips diverge around the central slip. Each receives a strong attachment from the tendons of the interosseous and lumbrical muscles, forming a broad extensor expansion. The palmar border of the extensor expansion is free and lies virtually in the long axis of the finger. More proximally the fibers from the interossei and lumbricals radiate across the dorsum of the proximal phalanx to complete the extensor expansion, whose most proximal fibers pass transversely across the base of the phalanx. The fused interosseous tendons and marginal slips of the expansion, passing distally across the middle phalanx, converge to be inserted together into the base of the distal phalanx.
Certain fibrous bands are attached to the extensor apparatus; they are tightened by appropriate positions of the phalanges. The oblique retinacular ligaments are bilateral strong narrow bands attached near the head of the proximal phalanx, straddling the fibrous flexor sheath. Each passes palmar to the proximal interphalangeal joint and joins the marginal slip of the extensor tendon. Extension of the proximal joint draws them tight and limits flexion of the distal joint. Flexion of the proximal joint slackens them and permits full flexion of the distal joint. The two joints thus passively tend to assume similar angulations.
Long tendons of the thumb
On the flexor aspect there is only one tendon that of flexor pollicis longus invested by its synovial sheath as it passes to the distal phalanx. On the extensor surface the tendons of extensor pollicis brevis and longus are each inserted separately (into the proximal and distal phalanx). There is no extensor hood as in the four fingers, but the extensor pollicis longus tendon receives a fibrous expansion from both abductor pollicis brevis and adductor pollicis. These expansions serve to hold the long extensor tendon in place on the dorsum of the thumb.