For the surgical treatment of lumbar pathologies, there are essentially two medical devices that are implanted. Spinal fusion by means of a screwed plate generates a loss of intervertebral mobility, whereas a spinal dynamic implant (SPD) allows relative displacements and thus preserves the adjacent segments and discs. The main goal of this study is to structure the design and to optimize such SPDs. An analytic model based on geometrical and mechanical relations is set up. The characteristics derive from data taken from bibliographical studies. Optimization of the implant characteristics is carried out (materials used, component stiffness, orientation associated with bone tissue degeneration, etc.). To do this, the optimization problem is structured to combine design variables, the mechanical behavior of the medical device (MD) and functional requirements into a single design model allowing the optimization of the MD behavior through a global index.