Medical therapy devices need to interact with the patient to achieve a desired clinical result. Therefore, the patient is part of the final therapy system. But during the development and validation phase of medical devices the patient can not be involved in the design process. A replacement for the patient, or rather the body part which interacts with the therapy device, is needed. Simulators can be used to solve this problem. This contribution introduces the concept of a Hardware-In-The-Loop (HIL) simulator which is designed to be usable in a wide range of applications in the field of medical technology. HIL simulators consist of a software model of the simulated process and a physical interface to the therapy device. Actuators are used to influence the states of the physical interface and sensors measure the interaction between the HIL simulator and the therapy device. As a first application, the process of a minimally invasive surgery (MIS) was chosen. Insight to the process of the MIS was elaborated at the example of an arthroscopy. A software model of the knee, representing the operation area, was developed. A prototype was designed and different types of actuators were implemented. An over-actuated design was chosen to ensure a high flexibility. Control oriented models of the used actuators were derived. Finally, different actuator control strategies were developed, implemented and validated successfully.