Boutonniere deformity is a finger condition where the middle joint bends downward and the fingertip angles upward, creating a distinctive zigzag shape. It happens when a key tendon on top of the finger is torn or weakened, disrupting the normal balance of forces that keep your finger straight. The name comes from the French word for “buttonhole,” because the bone of the finger pokes through the torn tendon like a button through a hole.
What Happens Inside the Finger
Each finger has an extensor tendon that runs along its top surface, responsible for straightening the finger. At the middle joint, this tendon splits into three parts: a central slip that attaches directly over the joint, and two side bands that continue down to the fingertip. A small ligament called the triangular ligament holds those side bands in place on top of the finger.
When the central slip tears, you lose the ability to straighten the middle joint. But the damage doesn’t stop there. Without the central slip anchoring the system, the triangular ligament also fails, and the two side bands slip downward to the sides of the finger. Once they’ve shifted out of position, those bands start pulling the fingertip into hyperextension, bending it backward. The result is the characteristic posture: middle joint stuck in a bent position, fingertip pointing up.
Over time, the situation gets progressively worse. A ligament along the side of the finger gradually tightens and contracts, locking the deformity in place. This is why early treatment matters so much. What starts as a correctable tendon injury can become a fixed, rigid deformity if left alone for weeks or months.
Common Causes
Most cases result from a direct injury to the finger. A jammed finger during sports is one of the most common triggers, where the force tears the central slip tendon and sometimes the surrounding ligaments as well. A broken finger can also cause it if the bone fragment stretches or tears the tendon. Finger dislocations, cuts, and blunt trauma to the top of the finger are other frequent culprits.
Trauma isn’t the only path to boutonniere deformity, though. Rheumatoid arthritis is a significant cause. Chronic inflammation and swelling in the finger joints gradually weaken the central slip tendon until it gives way. Burns can damage or rupture the tendon as well. Less commonly, certain connective tissue disorders that weaken tendons and ligaments can predispose someone to the condition, and in rare cases it’s present from birth as a congenital hand deformity.
How It Looks and Feels
The hallmark sign is a finger that bends at the middle joint and won’t fully straighten, while the fingertip hyperextends (angles backward). In the early stages after an injury, the middle joint is often swollen and painful, and you may still have some ability to straighten it, though with difficulty. Many people initially mistake it for a simple jammed finger and don’t seek treatment right away.
As the side bands continue to shift out of position over the following one to three weeks, the deformity becomes more obvious. You’ll notice you can’t straighten the middle joint at all, and the fingertip starts locking into that upward position. Gripping objects, typing, and fine motor tasks become increasingly difficult. The longer it goes untreated, the stiffer the joints become as the surrounding ligaments contract and tighten permanently.
How It’s Diagnosed
A doctor or hand specialist can usually identify boutonniere deformity through a physical examination, looking at the finger’s resting position and testing your ability to straighten each joint. One key test involves bending the middle joint over the edge of a table and asking you to push upward against resistance. In a healthy finger, the force goes through the central slip and straightens the middle joint. When the central slip is torn, the effort is rerouted through the side bands, causing the fingertip to go rigid instead. That abnormal pattern confirms the diagnosis.
X-rays are typically taken to check whether the tendon pulled a small chip of bone away from its attachment point (an avulsion fracture) or whether there’s a joint dislocation or fracture involved. The tendon itself doesn’t show up on X-rays, so if the diagnosis is uncertain, imaging like ultrasound or MRI may be used to visualize the soft tissue damage directly.
Treatment: Splinting and Rehabilitation
For acute injuries caught early, the standard treatment is splinting the middle joint in a fully straight position for about 6 to 8 weeks without interruption. The splint holds the torn central slip in the right position so it can heal and reattach. During this period, you wear the splint continuously, day and night. Removing it, even briefly, can set back the healing process. Meanwhile, you’ll do gentle exercises of the fingertip joint every hour while keeping the splint on, which helps prevent stiffness in the rest of the finger and encourages the side bands to slide back into their correct position on top of the finger.
If the fingertip is significantly hyperextended (more than about 15 degrees), a second small splint may be used on the fingertip joint, holding it in slight flexion to take tension off the displaced side bands and support healing.
After the initial 6 to 8 weeks of full-time splinting, you’ll begin removing the splint during the day for gentle bending and straightening exercises. The splint stays on at night and during activities that could stress the finger, like housework or sports, for another 2 to 4 weeks. Full recovery is gradual. Regaining complete range of motion can take several months of consistent exercise, and some people notice mild stiffness for longer than that.
When Surgery Is Needed
Surgery is generally reserved for cases where splinting hasn’t worked, where the deformity has become fixed and rigid, or where there’s a large bone fragment that needs to be reattached. It’s also considered when the cause is rheumatoid arthritis and the joint itself is damaged beyond what splinting can address.
Surgical options depend on the severity. In milder cases, the procedure involves repairing or reconstructing the torn central slip and repositioning the side bands. When the joints have become stiff and contracted, the surgeon may need to release tightened ligaments before the tendon repair. In severe cases with significant joint destruction, particularly from arthritis, joint fusion or replacement may be the most practical option. Recovery after surgery still involves weeks of splinting followed by a structured hand therapy program, and outcomes are generally better when the deformity is treated before the joints become permanently stiff.
Why Early Treatment Matters
Boutonniere deformity is one of those conditions where timing makes an enormous difference. When treated within the first few weeks, splinting alone often produces good results. But the ligament that runs along the side of the finger progressively contracts over time, and once that tightening becomes permanent, the deformity is much harder to correct, even with surgery. A finger that could have been straightened with a simple splint at two weeks may require a complex surgical reconstruction at two months. If your finger is swollen at the middle joint after an injury and you’re having trouble straightening it, getting it evaluated promptly gives you the best chance of a full recovery.