When a sternum prosthesis is implemented, it must allow the movement of the rib cage. The sternum prosthesis designs that have been developed are completely rigid, this aspect can present discomfort when breathing, as well as fixation problems between the prosthesis and the bone tissue. The aim of this work is to develop a flexible device that allows the movement of the rib cage during breathing. Compliant mechanism design was developed by using the pseudo-rigid body model (PRBM) starting from a 4-bar rigid mechanism. The flexible model was obtained by developing the methodology of replacement from rigid to flexible body. The results show the obtained parameters of the compliant mechanism and the three-dimensional model for subsequent numerical and experimental analysis.