What is the difference between boutonnière deformity and swan neck deformity?

What is the difference between boutonnière deformity and swan neck deformity?

Swan neck deformity and boutonnière deformity are similar in some ways. They can both happen at birth, or as a result of injury or arthritis. However, different joints are involved in these conditions. In swan neck deformity, the PIP joint is hyperextended and the distal interphalangeal (DIP) joint is bent. In boutonnière deformity, the PIP joint can’t be straightened.