Anatomy

What structures are most commonly injured?

The hand begins at the wrist as a collection of small bones called the carpal bones. The carpal bones are arranged in two rows. The proximal row includes the scaphoid, lunate and triquetrum. These three bones form the wrist joint with the forearm. The second row includes the trapezium, trapezoid, capitate, hamate and pisiform. The bones in the second row sit between the proximal row and the metacarpal bones. Each of the five metacarpal bones fans out to form a joint with the four fingers and the thumb. The bones of the fingers and thumb are called phalanges. There are three phalanges in each finger and two in the thumb.

The thumb is much more mobile than the other digits and has a fleshy mass of muscle over the metacarpal. There are actually quite a number of small muscles in the palm of the hand. These muscles are called intrinsic muscles they are involved in fine manipulation rather than power gripping.

The main muscles that bend and straighten the fingers are actually in the forearm - not the hand. These muscles attach to the bone just above the elbow. The main parts of the muscles, sometimes referred to as the muscle belly, form the flesh of the forearm. These muscles form into tendons in the wrist that enter the hand and extend to their point of attachment into the bone in the digits, carpal bones and metacarpals bones.

The anatomy of the finger flexor tendons is quite complicated. These tendons must travel through tunnels on the palm side of the fingers. These tunnels are made up of bands of connective tissue that form an arch over the tendon. Each of these bands is referred to as a pulley named so because of it functions. The pulleys hold the tendons against the bone, similar to the guides on a fishing rod. Otherwise the tendons would bowstring away from the bone when the finger is curled up. The entire tunnel is lined with a slippery membrane called tenosynovium. Tenosynovium helps the tendon slide through the tunnel easily so that the fingers move smoothly.

If the finger bones are broken or if surgery needs to be done on the hand, the flexor tendon sheaths may be damaged and need to be reconstructed to permit normal finger function. Because injury usually results in scar tissue and scar tissue binds everything together, early restoration of gliding movement of the tendons in their tendon sheaths is a goal of treatment. Early motion reduces or eliminates the risk of stiff fingers after the fracture has healed.

The flexor tendons all enter the palm through the carpal tunnel that lies in front of the wrist. The tendons are accompanied by the median nerve that supplies most of the intrinsic muscles and carries sensation from the thumb side of the hand. Accidents that cause fractures and dislocations of the carpal bones may very well injure the tendons and nerves in the carpal tunnel.

The shape of the metacarpophalangeal joints is important to understand when a cast or splint is used as part of the treatment of any hand injury. From the side, the metacarpal head is cam shaped, deeper than it is long. The end of the proximal phalanx that forms the other part of the joint is flat or slightly dished. The ligaments that keep the joint stable are attached at the center of rotation of the cam. This means that when the joint is flexed the ligaments are tight; when the joint is straight they are slack.

You can check this. With the finger straight it can be moved from side-to-side because the ligaments are slack. When the joint is flexed there is no side-to-side movement because the ligaments are tight. If the joint is held in extension during treatment, the ligaments may contract and prevent the joint from being fully flexed. It is very important to splint the metacarpophalangeal joints in flexion otherwise the joint may not function normally even if it is uninjured.