The Tendon That Slides Off the Knuckle
A sagittal band rupture is a tear in the small sling that holds the extensor tendon centered on top of your knuckle.
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.
How it happens
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.
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.
The radial band 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.
What it looks like
- A painful, swollen knuckle — 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.
Why it gets 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.
Treatment
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.
- Worn full-time for 6 to 8 weeks
- You keep using the hand throughout — this is the whole advantage of the design
- Success rates reported in the region of 70 to 80 percent when started early
- Weaned over a further 2 to 4 weeks
Some centers still use a static MCP extension splint. It works too, but it costs you hand function for two months.
Chronic, or failed splinting. Surgical reconstruction of the band, usually using a slip of the extensor tendon itself to build a new sling.
- Week 0 to 4: splinted, protected extension
- Week 4 to 6: active motion begins
- Week 6 to 12: strengthening, grip returning
- Month 3 to 6: full loading, punching-level load last
The window
Under 3 weeks: splint, high success, no surgery.
3 to 6 weeks: splinting still worth attempting, success drops.
Beyond 2 to 3 months: the band has scarred short in the wrong position. Splinting no longer recentres a tendon that has healed off-center.
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.
Related reading
- Boxer’s Fracture: Treatment & Recovery Timeline
- Extensor Tendon Repair: Recovery & the Extensor Lag
- Rheumatoid Arthritis in the Hands: Signs & the Window
- Bites to the Hand: Why the Small Ones Are the Dangerous Ones
For the splint used, see relative motion (yoke) splints.
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.