Boutonniere Deformity (BD) is an acquired finger injury that severely impairs the ability to straighten the digit. It is characterized by a distinctive position where the finger’s middle joint, the proximal interphalangeal (PIP) joint, is permanently bent downward. Simultaneously, the joint closest to the fingertip, the distal interphalangeal (DIP) joint, is hyperextended, or bent backward. This unnatural positioning is named for its resemblance to a buttonhole.
Defining the Deformity and Its Mechanism
The cause of a Boutonniere Deformity is a disruption in the balance of the finger’s extensor tendon apparatus, the system that allows the finger to straighten. This mechanism operates along the back of the finger and depends on the integrity of its central component, known as the central slip. The central slip attaches to the middle bone and is responsible for extending the PIP joint. Damage to the central slip, through tearing or severing, causes the PIP joint to flex. This failure allows the two side bands, called the lateral bands, to shift from their normal position down to the sides (volarly). Once displaced, the lateral bands act as flexors, worsening the bend at the PIP joint. The displaced bands also increase tension on the remaining tendon structures that extend the fingertip. This compensatory imbalance forces the DIP joint into an hyperextended position, resulting in the classic Boutonniere shape.
Acute Injuries and Direct Trauma
The Boutonniere Deformity most commonly arises from a sudden, forceful injury that directly damages the central slip tendon. This acute trauma is frequent in sports, such as football, basketball, and wrestling, where fingers are prone to blunt impacts. The mechanism often involves a direct blow to the back of the middle joint while the joint is flexed. This blunt force can cause a closed injury, where the central slip tendon ruptures without breaking the skin. The forceful impact can also pull a small piece of bone away from the middle phalanx where the tendon attaches, known as an avulsion fracture. Prompt medical attention is necessary because the full deformity may develop gradually over several weeks. Alternatively, the central slip can be severed in an open injury, such as a deep laceration over the PIP joint. Severe joint dislocations can also stretch or tear the central slip and surrounding ligamentous structures.
Chronic Disease and Non-Traumatic Factors
While trauma is the most common cause, the Boutonniere Deformity can develop slowly due to underlying systemic conditions that weaken the tendon structure. The primary non-traumatic trigger is inflammatory arthritis, specifically Rheumatoid Arthritis (RA). RA causes chronic inflammation of the joint lining, known as synovitis. The inflamed tissue (pannus) erodes the surrounding cartilage, ligaments, and tendons over time. This degenerative process weakens the central slip where it attaches to the bone, causing it to stretch or rupture. This inflammatory destruction leads to the same mechanical failure seen in acute trauma, but the onset is gradual. Other non-traumatic causes include severe thermal injuries, such as full-thickness burns to the hand. The resulting scar tissue contracture puts excessive tension on the extensor mechanism, leading to tendon rupture or displacement. Chronic infection within the tendon sheath can also degrade the tissue, causing the central slip to fail.