This happens when the base of the finger and the outermost joint bend, while the middle joint straightens. Over time, this imbalance of the finger joints can result in the crooked “swan-neck” position. Rheumatoid arthritis can cause it.
A swan-neck deformity can make it almost impossible to bend the affected finger normally. It can make it hard to button shirts, grip a glass, or use your fingers to pinch things.