Needles used in medical percutaneous procedures are brought to deform because of its interactions with inhomogeneous and anisotropic tissues. In this paper, the premises of the development of a new generation of tools for assistance in the realization of gestures taking into account these deformation are presented. We provide a new approach for determining, in real time and in 3D, the shape of an instrumented needle inserted into a complex tissue by using strain gauges. The knowledge of the real time local deformation from these strain gauges would improve the current navigation systems by considering not only the rigid needles but also the flexible needles. Our aim is to reconstruct in real time the instrumented needle shape in order to track and to steer it during a medical intervention.