A disorder (OMIM:615298) characterised by hereditary absence of the proximal, less often distal, interphalangeal joints. Metacarpophalangeal joints are rarely affected, but carpal bone malformation and fusion are common. In the legs, tarsal bone coalition is common. The typical conductive hearing loss is due to fusion of the stapes to the petrous part of the temporal bone.
Molecular pathology Defects of GDF5, which encodes a protein thought to be involved in bone and cartilage formation, cause symphalangism—proximal 1B.