The discrepancy of continuously decreasing clinical training opportunities and increasing complexity of interventions in surgery has led to the development of different training options like anatomical models, computer-based simulators or cadaver trainings. However, trainees, following this training and ultimately performing patient treatment, still face a steep learning curve. To address this problem for C-arm based surgery, we introduce a realistic radiation-free simulation system that combines patient-based 3D printed anatomy and simulated X-ray imaging using a physical C-arm. This mixed reality simulation system facilitates a transition to C-arm based surgery and has the potential to complement or even replace large parts of cadaver training and to reduce the risk for errors when proceeding to patient treatment. In a technical evaluation, we show that our system simulates X-ray images accurately with an RMSE of 1.85 mm compared to real X-ray imaging. To explore the fidelity and usefulness of the proposed mixed reality system for training and assessment, we conducted a user study. Six surgical experts performed a facet joint injection on the simulator and rated aspects of the system on a 5-point Likert scale. They expressed agreement with the overall realism of the simulation and strong agreement with the usefulness of such a mixed reality system for training of novices and experts.