A mechanism for carpal tunnel syndrome

Posted on December 18, 2013

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From an upcoming, much longer, post detailing my visit to Stanford University’s Clinical Anatomy Lab, where I worked with cadavers:

One area a cadaver made things easier was understanding the carpal tunnel. From the top there is a bunch of stuff covering it.

Palm of the hand.

Palm of the hand.

Gray's anatomy flexor retinaculum with carpal tunnel

Once you get down to it though, it’s really a simple tunnel formed by the bones.

Carpal tunnel top view

We can get a little fancier by looking at things through a cross section:

Carpal bones cross section

Adding other structures:

Cross section carpal bones and ligament

Cross section carpal tunnel everything

We talked a bit about carpal tunnel surgery. If you don’t know, it’s pretty simple. The idea behind it is the transverse carpal ligament becomes thickened; putting pressure on the median nerve, so it’s cut to relieve this pressure:

Transverse carpal ligament thickened.

Transverse carpal ligament thickened.

Transverse carpal ligament cut.

Transverse carpal ligament cut.

I started thinking what would cause this pressure on the nerve. Why would the ligament become thickened? Is that always the case, that it’s always the ligament? I don’t know much about carpal tunnel syndrome; it’s not something I see in my clients, so I looked around a bit. Pretty much all I found was “It’s likely congenital,” “May be associated with previous trauma,” “Nobody really knows why it happens.”

A couple places reference “women get it more than men and this is likely because their tunnel is smaller.” These places also reference carpal tunnel syndrome is only in adults; never children. Don’t children have the smallest tunnels of all? These explanations didn’t sit well with me.

I went cruising through the second edition of Movement System Impairment Syndromeswhich deals with the hand, thinking there has to be some possible movement issue(s) with carpal tunnel too. I found something that made a lot of sense.

“CTS may be aggravated during gripping or prehension when using the finger flexors with the wrist flexed because this movement pattern results in anterior movement of the flexor tendons.”

Let’s go back to our drawing to put this in easier to understand terms.

Cross section carpal tunnel everything

This explanation is saying when you bend your wrist and grab something at the same time, the flexor tendons move forward, possibly putting pressure on the median nerve.

Flexor tendons moving forward.

Flexor tendons moving forward.

You can easily see this connection with your own hand. Bend your wrist, flex your fingers, and watch how the tendons in front of your wrist bulge out:

Carpal tunnel top view GIF

Different angle:

Carpal tunnel side view GIF

Now notice the difference if the wrist is stationary and only the fingers flex:

Carpal tunnel better GIF

The tendons don’t glide forward.

This seems to coincide with a much greater amount of people who grab a lot, such as assembly workers, getting carpal tunnel issues compared to desk workers. Think grabbing a box: Wrist and fingers both flex.

If you’re someone with carpal tunnel symptoms, who grabs a lot, you may very well need to change how you’re grabbing things i.e. keep your wrist straighter. This meshes with how many people seem to get relief from a wrist splint -it keeps the wrist straight, preventing anterior glide of the tendons.

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Posted in: Pain, Wrist Pain