Now let’s look at the structures in the fingers, and in the thumb, which hold the flexor and extensor tendons in place, allow them to move, and guide their direction of pull.
In each finger this structure, the flexor tendon sheath, provides the two flexor tendons with a smoothly lined, tightly enclosing tunnel to run in. The sheath starts just proximal to the MP joint, and extends all the way to the distal phalanx. To see the sheath better, we’ll divide it.
Parts of the sheath are thick and fibrous, and parts of it are thin and collapsible. On this finger we’ll remove the thin parts of the sheath and just leave the thick parts. These act as pulleys for the flexor tendons, as we’ll see. At each joint the sheath is attached to the edge of the palmar plate. Between the joints the sheath is attached along each phalanx.
The floor of the tunnel for the flexor tendons is formed by the palmar plates, and by the smooth flattened surfaces of the phalanges. The thumb has a similar flexor tendon sheath for its one long flexor tendon.
The arrangement for the extensor tendon is entirely different, and quite complex. On each finger the extensor tendon, and the tendons of three intrinsic muscles, come together to form a structure called the extensor mechanism. Let’s take a look at it. We’ll look at the muscles themselves a little later. So that we can see the extensor mechanism from all sides, well look at one finger in isolation.
Here’s the extensor tendon, approaching the back of the MP joint. Here, both on the radial side, and on the ulnar side, is the tendon of one of the interosseous muscles. In addition here, on the radial side only, is the tendon of a lumbrical muscle. On each side a triangular sheet of tendinous tissue fans out, and connects the extensor tendon to the interosseous tendon. This triangular sheet is called the extensor hood.
The big extensor tendon divides into three slips over the proximal phalanx. The central slip crosses the proximal IP joint and inserts here, on the base of the middle phalanx.
The slips on each side fuse with the interosseous tendon to form the two lateral bands. The lateral bands join together over the middle phalanx and insert here, on the base of the distal phalanx.
The thumb doesn’t have such a complex extensor mechanism. The insertion of its two extensor tendons is relatively simple, as we’ll see.
One last structure to look at before we move on to muscles is the palmar fascia, or palmar aponeurosis.
It’s a dense triangular sheet of fibrous tissue which covers the middle part of the palm of the hand. Proximally it’s continuous with the flexor retinaculum and with the tendon of palmaris longus. Distally it separates into slips, which insert into the edges of the palmar plates of the MP joints. The palmar fascia protects the underlying nerves, tendons and vessels from harm. The skin of the palm of the hand is firmly attached to it.