In this situation the two adjacent fingers are joined together since birth.
The fingers are surgically separated and made independent.
My preference is to operate them early in the first year of life. The logic is, the growth of two fingers is not equal and delay in surgery causes bony deformity and deviation of fingers.
There are multiple levels of severity of the deficiency. Based on what is absent, reconstruction is carried out to offer maximum benefit. A few examples are illustrated.
The thenar muscles are absent.
Absent Thumb. In this not so uncommon situation, the best treatment is to transfer the index finger to create a new thumb. This provides excellent hand function and look as well. This procedure called POLLICISATION ,is best done when the child is less than 1 year old. Delaying is not good.