Patients who suffer from Duchenne Muscular Dystrophy, have to live with hand impairments for over 15 years. This significantly influences their ability to live independently. Currently, static splinting interspersed with stretching of the hand is the course of treatment for this patient group and developed active hand orthoses focus mainly on stroke and SCI patients. The Symbihand was developed by R. Bos and K. Nizamis in 2019. The active hand orthosis is especially designed to assist DMD patients in ADL. Control is intuitive, the self-aligning mechanism prevents inaccurate joint alignment during flexion and the hydraulic finger modules are forceful and can achieve proper ROM. One of the limitations of the device is that the thumb is restricted to an awkward position in order to facilitate grasping abilities. That while, the thumb is of substantial influence on the versatility of the hand and actuation of this digit could be of additional value for functionality by adding the possibility of the key pinch . This thesis describes the design and evaluation of a thumb module for the Symbihand with the aim to release the awkward static position of the splinted thumb. In order to add a thumb module to the system, preservation of the hydraulic actuation and the self-aligning mechanism is imperative. The design scope of the thumb module becomes as follows: "The design of a hydraulic thumb module for the Symbihand, with the focus on functional assistance of DMD patients who have limited hand function and are wheelchair confined." This can be achieved by assisting the hand in the ROM and force needed for the precision pinch, lateral pinch and cylindrical pinch. It should be taken into account that the addition of a thumb module might complicate control, so added function and diminished ease of control must be in balance. Included in this thesis is a design paper about the thumb module (Section II), a literature study on the fixture design of hand orthoses for DMD patients (Section III) and additional explanations of design choices and requirements in the Appendix.