It is the deformity that the middle joint of the finger can not be flatten. It is usually arisen after a trauma. Flattening the finger is provided with a very complex mechanism. While a tendon which goes to the middle line on the finger flattens lower and middle knots (proximal and middle phalanx), tendons which come from both side of the finger flatten the end knot (distal phalanx). In button hole deformity, the middle tendon is injured on proximal phalanx where near to the middle joint.  

Diagnosis is very easy with typical view. It may be seen in rheumatic diseases besides traumas. 
Professional help is necessary. The first approach in closed injury should be physiotherapy. Requirement of surgery may be disappeared with suitable splinting by a control of a physiotherapist.  (Figure 1)

Surgical treatment comes to agenda in cases that only physiotherapy is insufficient.  Generally to sew the ruptured tendon on end to end is not possible. There are various repair techniques by using stable side tendons after the middle finer joint of the patient is softened enough with physiotherapy. But long term physiotherapy is sine qua non even tough surgical treatment is applied.