This paper deals with the optimization of piezoelectric patches positioning on a beam attached to an inaccessible system. Based on the electro-mechanical coupling coefficients, which are calculated from the mode shapes curvatures, the problem is to provide an optimal positioning of the piezoelectric patches in order to target the modes sensitive to beam boundary conditions. Following the theoretical description of a beam instrumented with collocated piezoelectric patches, a placement optimization strategy is proposed. This strategy lies on the definition of utility functions, based on modal weights and modal sensitivities, calculated from the coupling coefficient. An experimental validation of the method is performed on a concrete case study corresponding to real-time implant stability monitoring. A beam is temporarily rigidly fixed to the implant during its insertion into the bone cavity to allow impaction by the surgeon. The positions of the sensors on the beam are optimized to focus on the beam's modes carrying information on the bone-implant interface, the biomechanical issue being the real-time maximization of implant stability. The originality of the approach lies on the sensor placement optimization on the beam connected to the implant to maximize or minimize simultaneously the amplitude of several modes on the frequency response function at resonances, depending on their sensitivity to the bone-implant interface. The results show very good performance of the piezoelectric placement strategy proposed in this paper, paving the way for new applications of piezoelectric patches design and placement on structures.