Metacarpal 4-5 fusion (MF4) is a rare congenital malformation of the hands characterised by the partial or complete fusion of the fourth and fifth metacarpal bones. The anomaly manifests clinically as ulnar deviation of the fifth fingers, clinodactyly, reduced mobility, and shortening of the fifth metacarpals. MF4 can occur in either an isolated manner or with other features, as part of a syndromal diagnosis, for example, in Kallmann syndrome. In most of the reported families, isolated MF4 was inherited in an X-linked recessive manner, although pedigrees typical of autosomal dominant inheritance have been described. Several reports provide evidence that mutations in FGF16 are associated with isolated X-linked recessive MF4. Syndactyly type V (SD5) is an isolated limb malformation showing partial clinical overlap with MF4, in which metacarpal but additionally metatarsal synostosis coexists with other hand and foot abnormalities clearly distinguishing the two conditions.
