Anatomy Lesson: The Carpal Tunnel

Too many times in a week, or even a day, I hear clients, friends and people in general complaining that they think they have "carpal tunnel". Most people don't realize that the carpal tunnel is an anatomical part of the body. Sometimes I point out that, actually, everyone has two carpal tunnels, and it's compression or inflammation of the median nerve that runs through the carpal tunnel that is referred to as carpal tunnel syndrome.

The carpal tunnel lies at the radiocarparl (wrist) joint. The bones that articulate to make the wrist joint are the radius in the forearm and the proximal carpal bones at the heel of the hand. The radiocarpal joint is an ellipsoid joint. An ellipsoid joint is when an oval-shaped end of one bone articulates with the elliptical basin of another bone. This type of joint allows for flexion, extension, abduction (movement away from the center line of the body) and adduction (movement toward the center line of the body). In your wrist, the carpal tunnel lies on the palmar side.

You have eight carpal bones in the heel of your hand (this is your anatomical wrist). These form the "floor" of the carpal tunnel by creating an arch. You can feel the concave groove of this arch on the palmar side of your hand, at its heel. It's called the sulcus carpi.

Of the eight carpal bones, there are four that are very important to the carpal tunnel: the scaphoid, the trapezium, the pisiform and the hamate. These carpals are the attachment sites for the flexor retinaculum, a band of connective tissue that forms the roof of the carpal tunnel. As illustrated here, the tubercles and other bony landmarks of these four carpals are the exact attachment sites for this "roof".

These attachment sites are easily felt in the heel of your hand as bony protrusions (illustrated further below) and "anchor" the flexor retinaculum, creating, in effect, pillars on each side of your carpal tunnel.

So, to review, your carpal tunnel is a floor of carpal bones, bony landmarks on the sides holding up the connective tissue that forms the roof. Kind of like a tent.

In your forearm, you have two main muscle groups: the extensors and the flexors. The tendons of these flexor muscles (not the muscles themselves) pass through the carpal tunnel:

• flexor digitorum profundus (four tendons)
• flexor digitorum superficialis (four tendons)
• flexor pollicis longus (one tendon)
• flexor carpi radialis (one tendon), considered part of the carpal tunnel, although it is more precise to state that it travels in the flexor retinaculum, which covers the carpal tunnel, rather than running in the tunnel itself.

The median never is the only nerve to pass through the carpal tunnel, between tendons of flexor digitorum profundus and flexor digitorum superficialis.

The carpal tunnel is narrow and when any of the long flexor tendons passing through it swells or degenerates, the narrowing of the canal often results in the median nerve getting entrapped or compressed, and that is carpal tunnel syndrome.

Now you can school all your friends in the anatomy of the carpal tunnel when they claim to "have" it.

These pictures are from Trail Guide to the Body, Second Edition, by Andrew Biel, illustrated by Robin Dorn (both massage therapists) and published by Books of Discovery.