Mesoaxial synostotic syndactyly (MSSD) with phalangeal reduction is a novel and distinct form of non-syndromic syndactyly including complete syndactyly of the 3rd and 4th fingers with synostoses of the corresponding metacarpals and associated single phalanges, syndactyly of the 2nd and 3rd toes and 5th finger clinodactyly.

Fusion involving two or more metacarpal bones (A synostosis of the first metacarpal and the proximal phalanx of the thumb can also be observed, note that the first metacarpal bone corresponds to a proximal phalanx).