A 45-year-old man visits the outpatient clinic after a digit of his left hand was injured when a door was slammed on his hand. A superficial cut on his middle finger has been sutured, but functional deficits are observed in the finger: The proximal interphalangeal joint is pulled into constant flexion, whereas the distal interphalangeal joint is held in a position of hyperextension. What is the most likely diagnosis?
High-Yield Explanation
In boutonniere deformity, the central portion of the extensor tendon expansion is torn over the PIP joint, allowing the tendon to move palmarward, causing the tendon to act as a flexor of the PIP joint. This causes the DIP joint to be hyperextended. The tear in the extensor tendon is said to resemble a buttonhole ("boutonniere" in French), and the head of the proximal phalanx may stick through the hole.