Do You Know How To Use Your Thumbs (So They Don’t Wear Out)?
⌚️ read time: 5 minutes
We are all destined for thumb arthritis. The key is delaying that eventuality as long as possible. And today we’ll talk about how.
I could write for pages about the thumb (and I will), but let’s try to keep this to the bite-sized content you all appreciate.
What I want to do today is describe to you the structure of the human thumb, the forces it endures during a normal day, and how that translates to actionable tips you can start using today to preserve the longevity of your thumb.
Evolution
Believe it or not, the human thumb is still evolving. Of all the primate thumbs, the human thumb is the longest…and the most mobile. This allows our thumb to swivel nearly 180° (look at your thumb in the ‘high-five’ position and again in the “OK sign” position) as well as reach across to touch each of our other fingers (a movement called opposition).
Maybe you’ve heard someone say ‘our opposable thumb’? That means a thumb that can perform opposition.
Just look at how short a chimpanzee’s thumb is by comparison (see figure).
Look at that tiny little thumb!
The Physics Of The Thumb
So what’s my point?
The point is, our thumb is long, and we use it constantly. If you remember any of your high school physics, you may recall that increased length results in increased torque. This is why a longer wrench is easier to use than a shorter one.
Well. All that force has to go somewhere. And in our thumb, it goes to your thumb CMC (carpometacarpal) joint. This is where your thumb and your wrist meet (see picture below).
In fact, any time you pinch or grip between your thumb and index finger, 13-15 times the force of your pinch gets transmitted down through your thumb CMC joint!
Thumb Position Matters…A Lot!
Now, here’s where things get actionable. You can’t do anything to change the 15x force you grind through your thumb CMC joint. But you can change the direction of the force and how destructive it is. You just have to be mindful about how you use your thumb.
Let’s start with the bad. I want you to imagine the way you would grab a key to unlock a door. Or the way you would grab your socks to pull them up. Do you see how narrow the space is between your thumb and palm/index finger? We call this ‘narrow’ or ‘key’ pinch.
Below I’ve drawn a schematic of the bones in the thumb and index finger. Here’s the important part. Notice during narrow pinch how the thumb metacarpal comes inwards toward the palm (teal arrow).
This inward tilt of the metacarpal causes the base of the metacarpal to slide out from the CMC joint (red arrow). This is called subluxation.
Now imagine a pinch and a load of force directed down into the joint (teal arrows below). That metacarpal is going to edge-load and shear across the joint (joint contact forces in red) in a very unstable and uneven pattern! OUCH!
What we don’t see in this picture is the 360° of ligaments encasing the joint and attaching the metacarpal to the wrist bone below (the trapeizum bone). Over time, as you pinch and grip and twist your way through life (imagine the way you hold your phone to text all day!), you are constantly edge loading and stretching those ligaments out.
More stretched out ligaments leads to even less stability and even more edge loading. Eventually, this uneven load will grind down the cartilage and you will get…arthritis! If you need a refresher on arthritis, please find my previous article here.
When I operate on thumb arthritis, I typically see about half the cartilage worn away on the outside edge, with the inner cartilage preserved. Classic edge-loading.
How To Maximize Your Thumb’s Longevity
Ok, so what do we do about it? Make an “OK” and you’ll be … OK!
See below.
In the images above, notice how if you make an “OK sign” shape to grab between the thumb and index finger (some call this a “C” shape or a wide grip), the thumb metacarpal bone is now maintained in a wider position away from the palm. What this does is center the metacarpal within the CMC joint.
Now when you grip and transmit 15x the force into the CMC joint (teal arrows), it will be directed centrally (balanced joint contact forces in red), no edge loading, no ligament stretching, no pain!
This is why tools with wide handles naturally feel better to use than those with narrow handles.
This all may seem like a small difference. And over the course of any one day, week, or even year, it probably is.
But over 60 years? 80? 100?
We grip and manipulate objects with our thumbs thousands of times every day. That seemingly small difference all of a sudden becomes the difference between getting early painful arthritis and….not.
So, if you take one thing away from today, let it be this. Make the conscious effort to widen the distance between your thumb and index finger into that “OK sign” shape any time you grab an object. You won’t be able to do it with everything (sometimes you just have to pull up your socks), but every change you make matters.
Don’t Believe Me?
My wonderful wife kindly told me a couple of months ago… “Yeah that’s great and all, but I never pinch things.”
Ok, maybe pinch is the wrong word. Grip? Grab? Do anything at all ever with your thumb? Oh, how I begged to differ with her. She’s almost never wrong. But not this time.
I followed her around for about 20 minutes pointing out her thumb ‘pinch.’
Here’s a brief list of all the daily thumb activities I saw her do before I lost count:
Picking up your phone/holding it to scroll or text
Picking up your laptop to move it to the office
Pulling the sheets when making your bed
Wrestling a toddler into their pajamas
Gripping the handle of a coffee cup
Gripping a knife to cut food
Tearing open a food wrapper
Pulling up your pants/socks
Opening caps or jar lids
Peeling a banana
Zipping a jacket
Turning a key
Writing
You get the point…
Takeaways:
Our primate thumb continues to evolve — and our body’s ability to resist arthritis has not kept up with the increased utility of the opposable thumb
This means we are all destined for thumb arthritis. Delaying it as long as possible should be a priority for everyone.
How you use your thumb thousands of times a day is one of the primary determinants of how early or late in life you will develop thumb arthritis. Make some changes today!
As you can tell, this is a common and important problem for our daily hand function. So we will discuss much more in the future. But let this be your early lesson in preventative care for the longevity of your most important digit!
