Boutonniere deformity describes an abnormal positioning of a finger joint that limits its ability to move and function. The name, French for “buttonhole,” refers to the characteristic appearance of the affected digit. This condition involves a paradoxical bend in two joints of the finger.
The most noticeable feature is the middle knuckle, known as the proximal interphalangeal (PIP) joint, becoming fixed in a bent or flexed position toward the palm. Simultaneously, the outermost joint, the distal interphalangeal (DIP) joint, snaps backward into hyperextension. This combination creates the distinctive zigzag shape that impairs the finger’s ability to straighten and grasp objects.
The Anatomical Mechanism of Deformity
The underlying cause of the deformity is the disruption of the finger’s extensor tendon apparatus, which straightens the joints. This system includes the central slip, which attaches to the middle bone of the finger. When the central slip is torn or significantly damaged, its ability to extend the PIP joint is lost, causing the joint to collapse into flexion.
The failure of the central slip has a cascading effect on the other tendons, known as the lateral bands, which run along the sides of the finger. Normally, these bands are held in a balanced position by surrounding ligaments. With the central slip compromised, they shift position toward the palm side of the finger, a migration called volar subluxation. This relocation changes their mechanical function.
Once positioned on the palm side of the PIP joint’s axis of rotation, the lateral bands no longer contribute to straightening the middle knuckle. Instead, they begin to act as flexors, pulling the PIP joint further into its bent position. This new mechanical tension is transferred to the terminal tendon, which connects to the DIP joint, causing an exaggerated pull that completes the classic boutonniere pattern.
Causes Originating from Acute Trauma
The most common cause for the sudden onset of this condition is an acute, forceful injury that directly damages the extensor mechanism. A frequent scenario involves a blunt force impact, such as a finger being “jammed” by a ball or striking an object. This trauma can cause a closed rupture of the central slip, tearing the tendon off its attachment point on the bone.
Another immediate cause is a deep laceration or penetrating injury to the back of the finger, which can partially or completely sever the central slip tendon. Because the extensor apparatus is located just beneath the skin, it is vulnerable to cuts from sharp objects. A severe joint dislocation or an avulsion fracture, where the tendon pulls a small fragment of bone away from the middle phalanx, can also instantly disrupt the mechanical balance.
These acute events create an immediate mechanical failure, initiating the anatomical cascade that leads to the deformity. The resulting pain and swelling at the PIP joint may initially mask the underlying tendon damage. The characteristic zigzag shape may take a week or two to fully develop after the initial traumatic event.
Systemic Disease as a Root Cause
Beyond acute injury, the boutonniere deformity can develop gradually due to chronic degradation from systemic diseases. Rheumatoid Arthritis (RA) is the most prominent non-traumatic cause, affecting up to half of all patients with RA. This autoimmune disease triggers persistent inflammation, known as synovitis, within the finger joints.
The chronic inflammation slowly weakens and stretches the ligamentous structures and tendons, including the central slip, leading to its eventual failure. The persistent swelling and tissue destruction cause the supporting ligaments to lose their integrity, allowing the lateral bands to drift into the palm-side position. This process mirrors the mechanical failure seen in acute trauma but unfolds over months or years.
Less common inflammatory conditions, such as Systemic Lupus Erythematosus (Lupus), can also contribute to the weakening of the soft tissues surrounding the joint. Even advanced, chronic Osteoarthritis that severely deforms the joint can occasionally lead to the breakdown of the extensor mechanism. In all these systemic cases, the underlying disease compromises the mechanical stability of the joint, resulting in the same final deformed posture.