A Boutonnière deformity is when the finger or thumb is bent down at the middle joint and bent backwards at the end joint (see photo above). This deformity can happen for a couple of different reasons, including:
- A cut tendon on the back of the finger or thumb
- Tearing or weakening of the tendon from a disease such as rheumatoid arthritis
These two reasons are what can cause the middle joint to bend down. The backwards bending of the end joint is caused after the middle joint bends because there is more pull on the end joint of the finger.