Boutonnière deformity describes a posture of the finger in which the middle joint is bent down and the end joint is bent back. The usual cause is a stubbing injury of the finger, but the deformity can also be due to arthritis.
In the normal finger, the straightening forces in the tendons are balanced between the central slip (solid arrow) and the terminal tendon (dotted arrow). If the central slip is torn by injury it can no longer straighten the middle joint, which drops into a bent position. The straightening force is now all transferred to the terminal tendon, which pulls the end joint up and it then becomes difficult to bend the end of the finger fully into the palm.
The same posture can develop if the central slip is injured by a cut on the back of the middle joint.
Pain and swelling of the middle joint of the finger
• The finger lying in a bent position from the middle joint
• Inability to actively straighten the middle joint of the finger.
• Inability to bend the finger fully at the end joint
It can be hard to distinguish this injury from the much more common middle joint sprains, as both injuries cause pain, swelling and stiffness, and the characteristic deformity may not appear for some days or weeks after the injury.
An x-ray should be taken to look for a fracture and check the joint is in place. Occasionally an ultrasound scan may be required.