Synostosis involving the 3rd metacarpal Synostosis involving the 4th metacarpal 3-4 metacarpal synostosis 2021-09-08T21:33:50Z Fused third and fourth metacarpals Middle and ring finger metacarpal synostosis Metacarpal synostosis is best described as a failure of differentiation causing a union between 2 adjacent metacarpals. Synostosis of third and the fourth metacarpal bones A congenital hand malformation characterized by the coalescence of the coalescence of third and fourth metacarpals (i.e., those corresponding to the middle and ring fingers).