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The Tendon That Slides Off the Knuckle
A sagittal band rupture (boxer’s knuckle) is a tear in the small sling that keeps the extensor tendon centered on top of your knuckle. Caught within about 3 weeks, most are treated with a small yoke splint worn full time for 6 to 8 weeks, while you keep using the hand. Caught after 2 to 3 months, it usually needs surgery to rebuild the sling, and a 3 to 6 month recovery.
When it tears, the tendon slips sideways, usually into the valley between the knuckles, and the finger struggles to straighten from a bent position, though it stays straight once you get it there.
Caught inside three weeks, this is a splint problem with a good ending. Caught at three months, it is usually a surgical one.
The middle finger is the one that does it most.
If you’re reading this, the knuckle is probably swollen, someone told you it’s bruised, and the finger keeps flicking or lagging when you open your fist. The X-ray was normal. Here’s the catch with “normal”: every week the sling heals in the wrong position, there’s less a splint can do about it.
What is the sagittal band?
The tendon that straightens your finger runs over the top of the knuckle (the MCP joint). It doesn’t sit there by luck. A thin sheet of tissue on each side, the sagittal bands, wraps from the tendon down around the joint, like the straps of a sling, and keeps the tendon on the crest of the knuckle as you make a fist.
Tear one strap and the tendon slides off the crest toward the intact side every time the knuckle bends.
The radial band (the thumb side of the knuckle) is the one that usually goes, which is why the tendon almost always slips towards the little finger side. If a tendon has slipped the other way, look harder at the diagnosis.
How does a sagittal band rupture happen?
Two very different routes to the same injury.
Traumatic. A punch, hence the nickname, boxer’s knuckle. Or a forceful flick of a resisted finger. Something sudden, and people usually remember it. (Rocky punching frozen sides of beef makes great cinema and terrible knuckle care.)
Attritional. In rheumatoid arthritis, chronic synovitis stretches the band from underneath until it gives. No single moment, no story, just a finger that started drifting.
Sagittal band rupture symptoms
- A painful, swollen knuckle, at the MCP level, not the middle joint.
- A visible or palpable snapping as the finger bends and straightens.
- The tendon sitting off to one side when the finger is bent.
- Inability to START straightening the finger from a fist.
- Once passively straightened, the finger HOLDS straight. This is the key sign.
Can’t start it, but can hold it? That’s a subluxed tendon. Can’t hold it at all? That’s a ruptured one, and it’s a different problem.
That single test separates sagittal band injury from extensor tendon rupture, and it takes ten seconds. If the finger can’t hold straight at all, see extensor tendon repair.
How is a sagittal band injury diagnosed?
- The ten-second test above, plus watching the tendon as the fist closes. It visibly drifts into the valley.
- X-ray, to rule out a boxer’s fracture or a chip off the joint. In a sagittal band injury, it’s normal.
- Ultrasound, done while you bend and straighten the finger, shows the tendon slipping and the torn band.
- MRI, occasionally, when the picture is unclear.
Why does it get missed?
X-rays are normal. Swelling around a knuckle after a punch looks exactly like a bruise or a boxer’s fracture that wasn’t. Ultrasound shows it well, but only if someone thinks to ask for it.
So people are told it’s soft tissue, rest it, and the window closes quietly over the next six weeks.
Which window are you in?
- OPEN: under 3 weeks. Splint, high success, no surgery.
- CLOSING: 3 to 6 weeks. Splinting still worth attempting, success drops. Start this week, not next month.
- SHUT for splinting: beyond 2 to 3 months. The band has scarred short in the wrong position. Splinting rarely recenters a tendon that has healed off-center. The goal changes to surgical reconstruction, which still works well.
It is a small strip of tissue, a few millimeters wide, and it decides whether your finger straightens on command. Not every load-bearing thing is large.
How long does a sagittal band injury take to heal?
Acute, under 2 to 3 weeks. A relative motion extension splint, a small yoke holding the injured finger about 15 to 20 degrees more extended at the MCP than its neighbors. Details on fit in relative motion (yoke) splints.
- First 6 to 8 weeks: full time. Worn day and night, including sleep. You keep using the hand throughout. That is the whole advantage of the design.
- The next 2 to 4 weeks: wean. Off for light tasks, on for heavier work, sport and sleep.
- Around 3 months: full load. Hard gripping, push-ups, and punching last.
Success rates reported in the region of 70 to 80 percent when started early.
Some centers still use a static MCP extension splint. It works too, but it costs you hand function for two months.
Exercises for a sagittal band injury
Check with your hand therapist first. Slow, within comfort, and the tendon should stay centered.
- In the yoke: full fist and full straighten. The splint lets the finger bend while it keeps the tendon centered. 10 slow repetitions, 4 to 5 times a day.
- In the yoke: tendon glides. Straight fingers, hook fist, full fist, flat fist. Hold each 3 seconds, 10 rounds, 3 times a day.
- After weaning: finger lifts. Palm flat on a table, lift each finger in turn, hold 3 seconds, 10 each.
- From about week 10: grip. Soft putty, 3 sets of 10, once a day, building weekly if the knuckle stays quiet.
Surgery for a chronic sagittal band rupture
Chronic, or failed splinting. Surgical reconstruction of the band, usually using a slip of the extensor tendon itself to build a new sling.
- Weeks 0 to 4: splinted, protected extension.
- Weeks 4 to 6: active motion begins.
- Weeks 6 to 12: strengthening, grip returning.
- Months 3 to 6: full loading, punching-level load last.
Can I work, drive and train with boxer’s knuckle?
- Desk work: usually yes, in the yoke. Typing and mousing are fine for most people.
- Manual work: light duties are often possible in the yoke. Heavy gripping and hammering wait until the weaning phase.
- Driving: usually fine in a yoke if you can grip the wheel firmly and control the car in an emergency. After surgery, not in the splint, and usually not before about 4 to 6 weeks.
- Boxing and martial arts: bag work and sparring around 3 months after a splinted injury, 4 to 6 months after reconstruction. Wrap well and use gloves with good knuckle padding. More on returning to work after a hand injury.
What a hand therapist does differently
- Spots it at the first visit with the start-versus-hold test, before the window closes.
- Molds the yoke to the right angle for your finger, and changes it as swelling drops.
- Checks the tendon stays centered at each review, and knows when splinting isn’t winning.
- Builds you back to load, from typing to the heavy bag.
Pain elsewhere on the back of the hand? Pain on top of the hand sorts it by spot.
Go to the ER today if
- You punched someone in the mouth and there’s ANY break in the skin over the knuckle, however small. Teeth carry bacteria straight into the joint. See fight bites.
- The knuckle is getting hotter, redder and more swollen, red streaks track up the hand, or you have a fever.
- The finger is numb, pale or cold.
Get it checked if
- The finger can’t hold straight even when you put it there. That points to a tendon rupture and needs seeing within days.
- The knuckle looks sunken or the finger rotates when you make a fist. Think fracture.
- The tendon still snaps off the knuckle after 3 weeks in a splint.
- You have rheumatoid arthritis and a finger has started drifting or flicking at the knuckle.
Three weeks gets you a splint. Three months gets you an operation.
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 knuckle that snaps after a punch deserves a hand clinician this week, not after the swelling goes down.
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.