The paper presents an innovative process of designing elastic braces to protect injured joints. Based on the included box of 3D scanned data from the injured joint, a smallish tentative elastic brace is designed. The strain energy is derived from the strain energy density function, which is the function of the dimension of the brace, elastic material’s parameters and the motion of the joint. The stiffness is then confirmed by the derivative of the strain energy. Compared to the stiffness prescribed by the physician, the modification of these parameters is done until it meets the requirement. For effective calculation, mesh simplification and parameterization techniques are applied, which have been widely investigated in computer graphics field. The proposed approach to strain energy and stiffness calculation makes it available to design a custom-made elastic brace for injured joints. The simulation results prove it work quite well for designing medical assistive devices. The research is useful for developing medical devices and intelligent robots.