Extensor tendons run along the BACK of your finger, and they are the opposite of their flexor cousins in almost every way: thin, flat, sitting just under the skin, and usually treated with a splint rather than an operation.
That makes them sound minor – they aren’t.
Most extensor injuries have a deadline measured in days to a couple of weeks, and the finger that ends up permanently bent is almost always one that was called a sprain.
The tendon isn’t a cord, it’s a hood
On the palm side you have two round cords per finger. On the back you have something more like a flat sheet wrapping over the knuckle, splitting into three slips over the middle joint, then rejoining to reach the fingertip.
That’s why extensor problems show up as a finger sitting in a strange POSITION rather than a finger that can’t move at all.
Cut one part of the hood and the rest keeps pulling… just in the wrong direction. Over weeks, that imbalance is what builds a deformity. (The layout is spelled out in hand tendon anatomy.)
Flexor injuries announce themselves. Extensor injuries deform slowly, which is why they’re caught late.
The four that matter, by where they sit
Over the fingertip (Zone 1). The tip droops and won’t lift. That’s mallet finger, and the treatment is a splint holding the tip dead straight for 6 to 8 weeks, CONTINUOUSLY. Let it bend once at week four and the clock restarts from zero.
Over the middle joint (Zone 3). A jammed or cut finger that seems fine, then two weeks later starts bending at the middle joint and hyperextending at the tip. The central slip has gone, and what you’re watching is a boutonnière deformity forming. This is the single most-missed hand injury I see. Splinted in the first fortnight it usually does well. Found at three months, it’s a reconstruction.
Over the knuckle (Zone 5). The tendon is held centred on the knuckle by a thin sling. Tear that and the tendon slides into the valley between the knuckles, so the finger drifts sideways and won’t fully straighten from a bent fist. That’s a sagittal band rupture, and splinting works well inside a few weeks and poorly after.
Back of the hand and wrist (Zones 6 to 8). Here the tendons are separate and a cut usually means the finger simply won’t lift at the knuckle. These get repaired surgically, and the recovery is in extensor tendon repair.
The knuckle wound that is not a cut
A small wound over a knuckle, acquired in a fight, from a mouth, is not a laceration. It is an inoculation.
The fist punches the tooth, the tooth pierces the extensor hood and the joint, the hand relaxes, and the puncture in the tendon slides back under intact skin where nobody can see it. Hours later the joint is infected.
Any wound over a knuckle from a tooth needs to be seen today, not tomorrow. The full version is in bites to the hand, and I will keep repeating it because the people it happens to are, understandably, not keen to explain how they got it.
The test: can you hold it, not just lift it
Lifting the finger proves less than people think, because neighbouring tendon connections can cheat.
- Fingertip — rest the hand flat, lift just the tip off the table. Can’t lift, or it lags behind the others? Zone 1.
- Middle joint — bend the finger over the edge of a table at the middle joint, then try to straighten it against a light push. Weak or painful? Suspect the central slip.
- Knuckle — make a loose fist, then open the hand. If the finger has to be flicked or pushed to start straightening, suspect the sagittal band.
- Hold, don’t just reach — the useful version of every test is holding the position against gentle resistance, not reaching it once.
In clinic I always test the same finger on the other hand first. Half of what looks abnormal is just how that person is built.
Two things people get wrong
“It’s only a small droop.” A ten-degree lag at the fingertip is cosmetic. A thirty-degree lag catches on pockets, bags and keyboards for the rest of your life. And an untreated mallet finger is one of the routes to a swan neck deformity further down the finger, because the forces have to go somewhere.
“The splint hurts, I’ll take it off at night.” Night is when the finger bends unsupervised. Splint compliance IS the treatment here, in a way it simply isn’t for most other injuries. If the splint is causing skin problems, that’s a reason to have it refitted, not a reason to stop.
Get it checked if
- A finger joint droops or lags behind its neighbours after any knock or cut
- A jammed finger is still swollen at the middle joint after 10 days
- A finger has started drifting sideways or needs a flick to straighten
- There is any wound over a knuckle, however small
- A thumb has stopped lifting off a flat table weeks after a wrist fracture — that’s a ruptured EPL, not stiffness
On the back of the finger, the injury is rarely dramatic and the deadline is rarely obvious. Those two facts are the same problem.
If a finger isn’t straightening the way its neighbour does, get it assessed inside the first two weeks. That fortnight is the difference between a splint and an operation.
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.