How do Boutonnière and Swan neck deformities differ clinically?

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The key idea is how the extensor mechanism is disrupted at the finger joints, producing distinct postures at the PIP and DIP joints. In Boutonnière deformity, the central slip across the PIP is injured, so the PIP cannot actively extend and rests in flexion. The lateral bands then pull the distal joint into an extended position, commonly causing the DIP to be hyperextended. In Swan neck deformity, there is laxity or imbalance around the PIP (often a stretched volar plate), allowing the PIP to hyperextend while the DIP flexes under the influence of the flexor mechanism. So the classic pattern is PIP flexion with DIP hyperextension for Boutonnière, and PIP hyperextension with DIP flexion for Swan neck.