The Injury That Looks Fine on Day One and Bends Three Weeks Later
Boutonnière deformity is a finger that ends up stuck bent at the middle joint, with the fingertip pulled backwards. Bent in the middle, cocked up at the end.
The name comes from the French for buttonhole, because the knuckle pokes through the tendon like a button through cloth.
And here is the thing that makes this injury genuinely dangerous, which almost nothing else on this site does:
On the day it happens, the finger usually looks normal. The deformity develops over the following one to three weeks, quietly, while everyone involved is treating it as a jammed finger.
What’s actually happening in there
The tendon that straightens your finger doesn’t arrive at the middle joint as one cord. It splits into three straps.
The middle one, the central slip, attaches to the top of the middle bone and is what actually straightens the middle joint. The two outer ones, the lateral bands, run down either side and past the middle joint to straighten the fingertip.
Boutonnière is the central slip tearing off.
Straight after that happens, the finger still works reasonably well, because the two side straps are intact and can still partly do the job. Sore, swollen, a bit weak…reads exactly like a bad jam.
But those side straps were being held up on top of the joint by the central slip. With it gone, they gradually slide down the sides of the finger, and once they slip below the axis of the joint, their pull reverses.
They stop straightening the middle joint and start bending it. And because they’re now pulled tight, they yank the fingertip backwards at the same time.
(That’s the deformity. It isn’t a new injury at week 3. It’s the same injury finishing its slow collapse.)
How it happens
Four routes, and they’re worth knowing because two of them get missed constantly.
- A forced bend of a straightened finger. Ball to the fingertip, finger caught and jammed downwards.
- A cut or laceration over the top of the middle knuckle. Obvious at the time, and still frequently underestimated.
- A dislocation where the joint went forwards rather than backwards. This is the one I flagged in the dislocation article: it damages the central slip, and if it gets splinted the standard way it becomes a boutonnière over the next few weeks.
- Inflammatory arthritis. In rheumatoid arthritis the same slip gets eroded rather than torn, and the same collapse follows slowly over years.
The test, before the deformity arrives
Since waiting for the finger to bend means waiting too long, there’s a specific test for catching it early.
Hang the injured finger over the edge of a table so the middle joint is bent to a right angle. Now try to straighten that middle joint against gentle resistance, while somebody watches the fingertip.
If the central slip is intact, the fingertip stays floppy and relaxed.
If it’s torn, the side straps have to do all the work, and the fingertip goes rigid and kicks backwards.
The other early signs: tenderness pinpointed on top of the middle knuckle rather than the sides, and difficulty fully straightening that joint by about 20 degrees or so in the first week.
By the time it looks like a boutonnière, the easy treatment window has already closed.
What treatment actually looks like
Caught early, this is a splinting problem, and the splinting is beautifully specific.
The middle joint gets held dead straight, continuously, for around 6 weeks. Same discipline as mallet finger: continuously means continuously, and letting it bend restarts the clock.
But the fingertip is deliberately left OUT of the splint, and you’re given exercises to bend that fingertip actively, many times a day.
That’s not a comfort measure. Bending the fingertip pulls the side straps back up over the top of the joint where they belong, and stops them tightening in their collapsed position. The splint protects the healing slip; the fingertip exercises stop the collapse happening around it. Both halves, or it doesn’t work.
Caught late, once the joint has stiffened into the bent position, it’s a much looooooonger road: serial casting or static progressive splinting to regain the straightening first, over weeks to months, and only then any question of surgery. Reconstructive surgery for an established boutonnière has a mixed reputation among hand surgeons for a reason. Results are modest.
What recovery looks like
- Weeks 0 to 6: middle joint splinted straight full time, fingertip bending exercises daily. Swelling control alongside.
- Weeks 6 to 8: splint weaned gradually, often kept for nights and risky activities. Gentle active bending of the middle joint begins.
- Weeks 8 to 12: range and strength rebuild. A small residual loss of the last few degrees of straightening is common and usually doesn’t matter functionally.
- Months 3 to 12: swelling around that knuckle keeps receding slowly, same long tail as any middle-joint injury.
The short version
A jammed or dislocated middle knuckle that’s tender on TOP and can’t quite straighten is a central slip injury until someone proves otherwise, and it will look almost normal for two more weeks while it quietly collapses.
Six weeks of a splint now beats months of casting later, and beats an operation with modest odds after that.
Written by Nigel Chua, hand occupational therapist in practice since 2005. Tens of thousands of hands, wrists and elbows assessed and treated. My credentials are here. This article explains; it doesn’t examine. A finger that can’t fully straighten at the middle joint after an injury needs looking at this week, not when it starts to bend.