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One Digit. Roughly 40% of Everything Your Hand Does.
Your thumb has 3 bones, 3 joints and 9 muscles. Two short finger bones (phalanges), one long bone in the palm (the first metacarpal), and three joints: the IP joint near the tip, the MCP joint at the knuckle, and the CMC joint at the base, where the thumb sits on a wrist bone called the trapezium.
Impairment guides used by surgeons and insurers rate the thumb at roughly 40% of hand function on its own. Four fingers share the other 60%.
That’s not a quirk of measurement. The thumb is the only digit that can rotate to face the others, and every precise grip you have depends on that one capability.
Which is why a sore thumb base, a wobbly thumb knuckle or a flattening thumb pad costs you so much more than its size suggests. Keys, buttons, pens, jars, phones…all thumb jobs.
And two thumb problems run on a clock. A torn ligament at the knuckle that can’t heal back on its own, and a nerve squeezed long enough to waste the muscle pad. Call either one “just a sprain” or “just a bit of numbness” and the fix gets bigger, or stops being on offer.
Below: the bones, joints, muscles, nerves and ligaments in plain English, what goes wrong in each part, and which thumb injuries can’t wait.
How many bones and joints are in the thumb?
Three bones, one fewer than each finger. The thumb’s long palm bone moves so freely at its base that it behaves like an extra finger bone, which is the whole trick.
- Distal phalanx. The tip bone, carrying the nail and the pad you pinch with.
- Proximal phalanx. The middle section, between the tip joint and the knuckle.
- First metacarpal. The long bone inside the fleshy part of your palm, running down to the wrist.
- Sesamoids. Two pea-sized bones buried in the tendons on the palm side of the knuckle. Normal on X-ray. Nobody chipped anything.
And the three joints:
- IP joint (interphalangeal). Bends the tip only, in one plane. Can’t bend the tip after a deep cut to the thumb crease? That’s the long flexor tendon until proven otherwise. See flexor tendon injuries.
- MCP joint (metacarpophalangeal). The thumb knuckle. Its range varies hugely between people, some bend a long way, some barely move. Always compare with your other thumb before deciding yours is stiff. The finger version of these joints is covered in finger joint anatomy.
- CMC joint (carpometacarpal). The base joint, where the first metacarpal sits on the trapezium. Saddle-shaped, mobile in every direction, and the one that wears out. See the carpal bones for where the trapezium sits.
Why is the thumb 40% of hand function?
Your fingers bend and straighten in one plane, with a little spread. Useful, but limited.
The thumb swings out of the plane of the palm and ROTATES, so its pad can meet the pad of any finger. That’s opposition, and it’s the move that separates a hand from a paw.
Without it you can still hook, hold and press. You cannot pinch a key, hold a pen, do a button, use a phone or pick up a coin. (Even Thanos needed a thumb to snap.)
Fingers give you power. The thumb gives you precision. Precision is what people miss.
What movements can the thumb make?
- Flexion and extension. Bending the thumb across the palm, and straightening it back out to the side.
- Palmar abduction. Lifting the thumb up and away from the palm, like wrapping around a can.
- Adduction. Squeezing the thumb against the side of the index finger. This is key pinch, the one you use to turn a key.
- Opposition. Rotating the thumb so its pad meets each fingertip. A blend of all of the above, done by the CMC joint.
Each movement has its own muscle team on its own nerve. Which is why one nerve problem can take out a single movement and leave the rest looking normal.
Why does the thumb base joint wear out?
That rotation comes from the saddle-shaped CMC joint at the base of the thumb, between the first metacarpal and the trapezium.
A saddle joint allows movement in every direction, which is exactly what makes it unstable. Stability is sacrificed for range.
Then the loading: a pinch at the tip generates something in the order of ten times that force at the base joint. Pinch 2 kg, load the base with roughly 20. Do that ten thousand times a week for forty years.
That is why thumb base arthritis is one of the commonest hand conditions there is, and why it disables people so disproportionately. When splints and exercise stop being enough, the trapezium is the bone removed in a trapeziectomy.
What muscles move the thumb?
Nine muscles for one digit. Four sit in the forearm and pull on long tendons; five sit within the hand itself, most of them forming the muscle pad below the thumb (the thenar eminence).
The four long ones, from the forearm:
- Flexor pollicis longus (FPL). Bends the tip. The only muscle that can.
- Extensor pollicis longus (EPL). Lifts the thumb back off a flat table. It wraps around a bony knob on the wrist, and can fray through after a wrist fracture. See EPL rupture.
- Extensor pollicis brevis and abductor pollicis longus. The pair that share one tight tunnel at the wrist. Inflamed together, that’s de Quervain’s.
The five short ones, inside the hand:
- Abductor pollicis brevis, flexor pollicis brevis, opponens pollicis. The thenar pad. They lift the thumb off the palm and rotate it into opposition.
- Adductor pollicis. Deep in the web space. The power behind key pinch.
- First dorsal interosseous. The muscle that bulges between thumb and index on the back of the hand when you pinch. It helps steady the thumb base, which is why hand therapists strengthen it in thumb arthritis.
For how these tendons run across the whole hand, see hand tendon anatomy.
Which nerves supply the thumb?
All three hand nerves have a share, and each share tells you something. The full map is in which nerve supplies which finger.
- Median nerve. Feeling on the palm side of the thumb, and most of the thenar pad muscles. So a long-standing carpal tunnel flattens that pad, and a flattened pad means the nerve has been compressed long enough to lose muscle. That’s a deadline sign, not a warning sign.
- Ulnar nerve. Adductor pollicis, part of the short flexor and the first dorsal interosseous. Strong pinch is mostly ulnar-nerve-driven. Which is why cubital tunnel costs you pinch strength rather than sensation in the thumb.
- Radial nerve. Feeling on the back of the thumb and the web, and every muscle that straightens or lifts the thumb out. A tight watch strap can numb the back of the thumb: that’s Wartenberg’s syndrome.
Thumb ligaments: the two that matter most
The ulnar collateral ligament (UCL) sits on the index-finger side of the thumb knuckle. It’s what stops the thumb folding sideways when you pinch or grip. Fall on a ski pole, catch a ball badly, and it tears. That’s skier’s thumb.
Here’s the catch. In a complete tear, the torn end can flip outside a sheet of tendon lying over it (a Stener lesion). Once it’s trapped on the wrong side, it can’t heal back to the bone however long you splint it.
This is a Closing Window injury. A fresh complete tear is usually repaired within the first few weeks, while the ligament end is still good enough to reattach. Leave it for months and the job becomes a reconstruction with a tendon graft, or a thumb knuckle that gives way every time you pinch. See thumb UCL repair recovery.
The beak ligament (anterior oblique ligament) holds the base of the first metacarpal snug on the trapezium. As it loosens with age and load, the metacarpal drifts outward at the base, the joint surfaces stop meeting evenly, and arthritis follows.
What goes wrong where: the thumb by location
Point to where it hurts and the list gets short. For the symptom-first version, see thumb pain causes by location.
Tip and IP joint
- Mallet thumb. The tip droops and won’t straighten on its own after a jam or cut. Splinted straight, full-time, for weeks. See the thumb IP splint.
- Can’t bend the tip. After a cut to the crease, the FPL tendon may be divided. That needs a surgeon within days.
The knuckle (MCP joint)
- UCL tear (skier’s thumb). Pain and swelling on the index-finger side of the knuckle, weak pinch, sometimes a lump there.
- Trigger thumb. The tendon catches in its pulley at the knuckle crease, so the thumb clicks or locks bent. See trigger thumb.
- Overuse ache. Hours of one-thumbed scrolling load the knuckle and the tendons around it. See phone thumb.
The base and the wrist side
- Thumb base arthritis. Deep ache at the base with pinching, opening jars and turning keys. Often a bump at the base later on.
- Bennett’s and Rolando fractures. Breaks through the saddle joint itself. See Bennett’s and Rolando fractures.
- De Quervain’s. Sharp pain just past the base, on the wrist, with lifting and thumb use.
- Scaphoid fracture. Pain in the hollow at the base of the thumb after a fall on an outstretched hand. See scaphoid fracture.
The thenar pad and the whole thumb
- Carpal tunnel. Numb thumb, index and middle at night, and later a flatter pad.
- Loss of the thumb. An amputated thumb is a strong candidate for replantation.
All of them cost more function than their size suggests, for the same underlying reason.
Why do surgeons fight so hard to save a thumb?
The 40% figure explains decisions that otherwise look inconsistent.
- Replantation. An amputated thumb is usually worth reattaching; a single index finger often isn’t.
- Reconstruction. Surgeons will move a toe, or the index finger, onto the thumb position rather than accept the loss.
- Stiffness. A stiff thumb gets treated more aggressively than a stiff little finger, and rightly.
- Compensation. Insurance and compensation schedules weight thumb loss far above any single finger.
When patients ask why so much fuss over one digit, I ask them to tape their thumb to their palm for an hour. Nobody lasts the hour.
How are thumb problems diagnosed?
- Pointing. Tip, knuckle, base or wrist. Where it hurts cuts the list to two or three structures before anyone touches you.
- Grind test. Pressing the thumb’s long bone into the base joint and rotating it. Pain and grinding point to CMC arthritis.
- UCL stress test. Gently opening the knuckle sideways, straight and slightly bent, compared with the other thumb. A joint that opens much further, roughly 15 degrees more than your good side or with no firm end point, suggests a complete tear.
- Froment’s sign. Hold a sheet of paper between the thumb and the side of the index finger while someone pulls. If the tip of your thumb bends to hold on, the ulnar-driven pinch muscles are weak.
- Looking at the thenar pad. Both hands side by side, palms up, from the side. A flatter pad on one side is a finding, not a cosmetic quirk.
- Pinch strength. Measured on a pinch gauge and compared side to side. A number, not a guess.
- Imaging. X-rays for fractures and arthritis, ultrasound or MRI for a torn UCL or a suspected Stener lesion, and nerve conduction tests when the thumb pad is numb or wasting.
How a hand therapist treats thumb problems
Most thumbs need a splint, the right exercises and load changes, not an operation. See what a hand therapist does.
- Base arthritis. A short hand-based thumb splint worn for pinch-heavy tasks and often at night, plus fatter pens, lever taps and jar openers.
- Ligament sprains, fractures and de Quervain’s. A thumb spica splint, hand-based or forearm-based depending on what’s injured. A partial UCL tear is typically protected for about 4 to 6 weeks.
- A tight web space. After injury or a long time in a cast, the gap between thumb and index shrinks. A C-bar splint holds it open.
Three exercises I give most often, once your surgeon or therapist has cleared you to move (more in thumb arthritis exercises):
- Opposition walk. Thumb tip to each fingertip in turn, making a round O each time, not a flat pinch. 10 rounds, 3 times a day.
- Web space stretch. Gently ease the thumb away from the index finger with your other hand. Hold 20 to 30 seconds, 3 times, twice a day. A stretch, never sharp pain.
- First dorsal interosseous press. Hand flat on a table, push the index finger sideways toward the thumb against your other hand’s resistance. Hold 5 seconds, 10 reps, once or twice a day. This is the muscle that helps steady the base.
How long do thumb injuries take to heal?
Typical ranges. Your surgeon’s protocol wins if it’s different.
- Partial UCL tear: about 4 to 6 weeks in a splint, then gradual pinch strengthening. Contact sport and skiing often back in a protective splint after that.
- Complete UCL tear, repaired: around 6 weeks protected, with full pinch strength often taking about 3 months.
- Broken thumb (including Bennett’s): 6 to 12 weeks for the bone to heal, with at least 6 to 8 weeks in a cast or splint.
- Trigger thumb release: light use within days, back to full gripping in about 6 to 8 weeks.
- Driving and work: drive once you can grip the wheel firmly enough to control the car in an emergency. See driving after a hand injury and returning to work.
Get it checked today if
- The thumb turns pale, cold or blue after an injury.
- It’s hot, red, swollen and throbbing, or you have a fever. See hand infections.
- A cut on the thumb has left the tip numb, or you can’t bend the tip.
- After a fall or a jam, the thumb knuckle wobbles sideways, you can’t pinch a key, or there’s a lump on the index-finger side of the knuckle.
- The thumb looks bent out of line, or the base is badly swollen after a punch or fall.
- Weeks after a wrist fracture, you suddenly can’t lift the thumb off a flat table.
Get it checked if
- An ache at the base of the thumb with pinching has lasted more than 6 weeks.
- The pad below the thumb looks flatter than the other side, or the thumb, index and middle fingers go numb at night.
- The thumb catches, clicks or locks at the knuckle crease.
- Pinch is weaker and the thumb tip bends when you grip a sheet of paper.
- A thumb sprain is still sore and loose after 2 weeks.
Protect the thumb first. It isn’t one of five digits. It’s the one the other four are working with.
Written by Nigel Chua, hand occupational therapist in practice since 2005. Thousands of hands, wrists and elbows assessed and treated. My credentials are here. This article explains; it doesn’t examine. A thumb that wobbles, wastes or won’t pinch deserves a proper assessment this week.
Every article on this site is written using the Closing Window Method: name the window your condition runs on, find where you are in it, and act while it’s still open.