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Three Joints, Three Personalities. Know Which One You Hurt.
Each finger has three joints. From the hand outward: the MCP, the big knuckle where the finger meets the hand; the PIP, the middle joint; and the DIP, the joint nearest the nail. The thumb has two: its own MCP knuckle and one IP joint near the nail, plus the CMC joint at its base, down by the wrist.
Here’s the part that matters. They are built to fail in completely different ways.
The knuckle tolerates stiffness well. The fingertip joint can lose half its movement and you’d barely notice. The middle joint is the opposite. It’s the joint that ruins hands, cos it stiffens fastest, hates being held still, and is one of the hardest joints in the body to get movement back from once it’s lost.
Most people who jam a finger tape it, shrug, and wait. Get the joint wrong and you wait on the one injury that doesn’t wait for you, then spend months trying to straighten a finger that set itself in a bend.
Knowing which joint you’ve injured tells you how worried to be. So let’s name them properly.
What are the finger joints called?
The names come from the bones on either side. Your hand has five long bones inside the palm (the metacarpals), and each finger has three short bones (the phalanges: proximal, middle and distal). The thumb has only two phalanges.
| Joint | Plain name | Where to find it | Common problems |
|---|---|---|---|
| MCP (metacarpophalangeal) | The knuckle, first knuckle | Where the finger meets the hand; the bumps you see when you make a fist | Boxer’s fracture nearby, sagittal band rupture, trigger finger (palm side), rheumatoid arthritis |
| PIP (proximal interphalangeal) | The middle joint, second knuckle | Middle of the finger | Jammed finger, volar plate injury, dislocation, collateral sprain, boutonniere, swan neck |
| DIP (distal interphalangeal) | The end joint, fingertip joint | Just behind the nail | Mallet finger, jersey finger, Heberden’s nodes, mucous cyst |
| Thumb IP (interphalangeal) | The thumb’s end joint | Just behind the thumbnail | Mallet thumb, trigger thumb, arthritis |
| Thumb CMC (carpometacarpal) | The thumb base | Down at the wrist, at the root of the thumb | Thumb base arthritis |
Every joint is held together by the same kit:
- Collateral ligaments. One on each side of the joint. They stop it bending sideways.
- Volar plate. A thick pad of tissue on the palm side. It stops the joint bending backward. This is what tears when a ball bends your finger back.
- Tendons. Flexors on the palm side bend the joints, extensors on the back straighten them. Each joint has its own attachment point, which is why one tendon tear produces one very specific droop. See how the tendons in your hand work.
MCP joint: the knuckle
A ball-and-socket-ish joint, so it bends, straightens AND spreads the fingers sideways. It does most of the work when you grip something big, like a bottle or a steering wheel.
The key fact: its side ligaments are SLACK when the finger is straight and TIGHT when the knuckle is bent to about 90 degrees.
Which means a knuckle held straight for weeks lets those ligaments shorten, and the finger then won’t bend into a fist. This is why hand splints hold the knuckles bent, the “safe position”. A hand left to rest flat and swollen drifts the other way, knuckles straight and fingers curled, which is the start of a claw. See stiff hand after a cast or surgery.
Knuckle bent, middle joint straight. That’s the safe position, and it’s the opposite of what feels comfortable.
What usually goes wrong at the knuckle:
- Punching something hard. The bone just below the knuckle breaks, and the knuckle looks flattened. See boxer’s fracture.
- The tendon slipping off the top. A painful knuckle where the tendon flicks sideways into the valley as you make a fist. See sagittal band rupture.
- Clicking or locking on the palm side. The A1 pulley sits right under the knuckle, which is why trigger finger is felt at the base of the finger, not the middle. See trigger finger.
- Swollen knuckles on both hands, stiff in the morning. That pattern points to inflammatory arthritis rather than wear and tear. See rheumatoid arthritis in the hands.
PIP joint: the middle joint, and the problem child
A pure hinge. A tight capsule, a volar plate at the front, a collateral ligament on each side, and a complicated tendon hood over the back called the extensor mechanism. Its job is to curl the finger around things. Lose it and every grip you own gets weaker.
It’s the least forgiving joint in the hand:
- It stiffens fast. Within days of being held still.
- It stays swollen. A thick, sausage-shaped middle joint for 6 to 12 months after a simple sprain is common, and some thickening can be permanent.
- It loses straightening first. It settles into a bend far more easily than it loses bending.
- Lost range is hard to win back. Even with surgery.
What usually goes wrong at the middle joint:
- A ball hits the tip and the finger “jams”. Swelling at the middle joint, pain on bending. Usually a sprain, sometimes more. See jammed finger: sprain, or something worse?
- The finger bent backward, or popped out and back in. The volar plate has torn, or the joint dislocated. See PIP sprains, volar plate injuries and dislocations.
- A chip of bone came off with it. That’s a fracture-dislocation, and it needs an X-ray and a plan. See PIP fracture-dislocation.
- Bent sideways. Pain on one side of the joint, wobbly when stressed. See finger collateral ligament sprain.
- Middle joint bent, tip pushed back. The central tendon slip over the joint has failed. See boutonniere deformity.
- Middle joint bent BACK, tip curled down. The opposite shape, often from a lax volar plate, an old mallet finger or rheumatoid arthritis. See swan neck deformity.
A stiff PIP joint costs more hand function than a fused DIP or a stiff knuckle. That’s why a “boring” jammed finger gets taken seriously here and everywhere else on this site.
This is a closing window. A middle joint that won’t straighten is far easier to straighten in the first 3 months than after 6, usually with a splint worn for hours a day. See Capener splint for a bent middle joint.
DIP joint: the fingertip joint
A small hinge with limited range. It does the fine work: pinching a coin, picking at a sticker, pressing a guitar string. Its contribution to grip is modest, which is why fusing a DIP joint costs you surprisingly little.
Where it gets into trouble is the tendons and the arthritis.
- Fingertip droops and won’t straighten on its own. The thin extensor tendon slip has torn or pulled off a flake of bone. Usually a splint worn day and night, without a break, for about 6 to 8 weeks. See mallet finger.
- Can’t bend the fingertip after grabbing a jersey or a strap. The deep flexor tendon has pulled off the bone, most often in the ring finger. This one usually needs a surgeon within days. See jersey finger.
- Hard, bony knobs either side of the joint. Osteoarthritis. See Heberden’s nodes.
- A soft dome behind the nail and a groove down the nail. See mucous cyst.
What about the thumb joints?
The thumb skips the middle joint. It has a CMC joint at its base, which lets it swing across the palm, an MCP knuckle, and one IP joint. The two you’ll hear about most: the ligament on the index-finger side of the thumb knuckle tearing in a fall or a ski-pole injury, and arthritis at the base. See thumb anatomy and skier’s thumb.
Why your fingers curl toward one spot
Make a fist slowly and watch: your fingers don’t close in parallel. They converge, all pointing roughly toward the scaphoid at the base of your thumb.
That cascade is why rotation is the alignment that matters in a finger fracture. A few degrees of twist sends the finger crossing over its neighbor in a fist, and a finger cannot work around that. Angulation on an X-ray can often be tolerated. Rotation can’t.
Try it now on your good hand. Then, if you’ve hurt a finger, try it on that one. Any crossing over is worth a same-day check. See broken finger recovery.
How to check each finger joint yourself
These are the same moves I use in the clinic to see which joint is moving and which one is cheating. If you’re under a surgeon or therapist for an injury, follow their protocol first.
- Middle joint (PIP) bend. Hold the finger just below the middle joint with your other hand so the knuckle can’t help. Bend only the middle joint, then straighten. 10 slow reps.
- Fingertip (DIP) bend. Hold the middle segment still. Bend just the tip. 10 slow reps.
- Knuckle (MCP) bend. Keep the finger straight and bend only at the knuckle, like a tabletop. 10 reps.
- The flat-hand test. Lay your hand palm down on a table. Every joint should lie flat. A middle joint that won’t touch the table is losing extension, and that’s the one to act on.
As exercises after a minor sprain, 3 to 4 times a day is a sensible start. (In clinic, the finger that “can’t” bend at the tip often can, once you stop the knuckle doing all the work.) More in hand exercises after injury.
The practical summary
- MCP injury: splint bent, not straight. Generally forgiving.
- PIP injury: get it moving early, expect long-lasting swelling, and treat lost straightening as urgent.
- DIP injury: usually about a tendon rather than the joint, and usually splinted for weeks.
- Any finger fracture: check rotation in a fist, not just the X-ray.
Not sure which joint your pain is coming from? See finger joint pain: what the location tells you.
Get it checked today if
- A joint looks crooked or out of place after an injury, or it popped out and you can’t fully straighten or bend it now.
- The finger crosses over its neighbor when you make a fist.
- You can’t bend the fingertip at all after a grab or a pull.
- A joint becomes hot, red, rapidly swollen and very painful, especially with a fever, a small cut or a bite nearby. A cut over the knuckle from hitting someone’s teeth is an emergency. Go to the ER. See cellulitis or septic arthritis in a finger.
- Numbness in the finger after a cut, or a finger that’s pale, cold or blue.
Get it checked if
- The fingertip droops and won’t straighten on its own. Same week, not same month.
- The middle joint won’t fully straighten 2 to 3 weeks after a jam.
- A joint feels wobbly sideways, or still hurts to pinch after 6 weeks.
- Several knuckles on both hands are swollen with morning stiffness lasting over an hour.
Three joints, three personalities. The middle one is the one to protect from stiffness, and it’s the one that gets taped and forgotten.
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 finger joint that won’t straighten after an injury deserves a look 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.