Patients with Frozen Shoulder suffer from a decreased mobility and pain. Exercise-based physiotherapy is a common treatment and patients mostly perform the exercises at home. Correct exercise performance and compliance are the main issues in home-based therapy of Frozen Shoulder patients. To support patients diagnosed with Frozen Shoulder, a multimodal 3D smartphone app was designed, developed and evaluated. Additional to ten potential users, one physician, five physiotherapists, three computer scientists, two 3D artists and one HCI specialist were involved in the co-creation process. The app was evaluated by five patients during a three-week pilot study, which showed the feasibility of our approach. Exercise correctness, usage of multimodal instructions and user satisfaction were analyzed. Exercise correctness was nearly perfect and the interactive 3D animation was used for exercise instructions. Satisfaction using the app was rated very high according to SUS score. The results confirm that the co-creation process led to an effective, highly satisfactory and actually used system.