The present invention provides a new and improved orthosis for use in effecting relative movement between bones in an arm of a patient. The apparatus includes a lower cuff gripping distal bone, such as a wrist, and an upper cuff gripping a proximal bone, such as an upper arm. The lower cuff is secured in a rotatable drive assembly substantially coincident to a longitudinal axis of a medial bone, such as the forearm, during rotational distal adjustment. The angle between the forearm and the upper arm is adjustable and can be securely fixed at a desired angle. The rotation of the rotatable drive assembly effectuates the pronation and supination of the hand and wrist relative to the patient's forearm. The orthosis of the present invention can be disassembled with interchanging parts substituted depending on the patient's needs.