The Madelung deformity is a growth disorder of the forearm that is associated with a characteristic deformity. It is caused by reduced growth of the distal radial epiphysis with a mismatch to the relatively longer healthy grown ulna. In other words, a part of the radius stops growing early. The ulna keeps growing and can dislocate, forming a bump.

In the case of Madelung deformity, the mobility of the wrist is limited. Dorsiflexion and ulnar abduction are restricted. There is usually no pain.