The use of Computer Assisted Surgery (CAS) systems is becoming very common in the clinical practice, therefore the evaluation of such systems in terms of clinical outcomes and ergonomic features is more and more relevant. This paper goals has been to define some domain specific guidelines for the design of Human Computer Interfaces (HCI) for surgical application and to provide an evaluation protocol of existing CAS systems. The demonstration application has been a planning system developed for the Total Knee Replacement (TKR), a high skill demanding procedure, where the planning phase is crucial for the success of the intervention. The results we have obtained can be extended also to surgical training systems and surgical navigation platforms.