A shoulder orthosis for long term support of a patient's arm in postoperative shoulder immobilization posture for rotator cuff tears, comprising a waist belt, an arm splint, and a piston and cylinder assembly interconnecting the waist belt to the splint and biasing the splint to an abduction upper limit position. The orthosis provides stable support of the forearm about a horizontal plane, while enabling both pivotal movement of the arm about a vertical axis intersecting the patient's elbow and towards or away from the patient's torso within this horizontal plane. This orthosis also enables cyclical up and down motion of the splint, the downward motion being healthy adductor-assisted against the bias of the piston and cylinder assembly. A lock system releasably locks the piston and cylinder at a selected retracted position of the piston rod.