Springs, soft tissue simulation

Another fundamental criticism goes to the heart of the concept of force-control and soft-tissue simulation. In order to specify the correct spring stiffness (for soft tissue simulation), early critics argued that some estimate of... 

The stiffness of the springs used and their detailed configuration and settings (i.e., precompression or gap and gap distance) all affect the restraint and thus affect the TKR specimen kinematics [55], which in turn influence the wear. As will be seen later, the non-precompressed springs (i.e., with gaps) would also more realistically simulate the physiological soft tissue situation. 

When these restraint settings are generalized (for any simulator) into one linear net restraint (and not two parallel springs), and when the leverage issue is corrected for, the desired net stiffness values recommended for soft tissue restraint are as detailed in Table 26.1. These generalized settings have been proposed as part of a major revision of the force-control ISO standard 14243-1 [38]. 

This concept takes advantage of measured real-time feedback of AP and IE rotation from sensors on board the knee simulator. Based on those instantaneous AP and IE implant positions/motions, a real-time (or strictly, semi-real-time) computation is made of the influence/contribution of the desired soft tissue springs or any virtual mathematical restraint models. The computed AP force and IE torque contributions are added (vectorially, mathematically) to the electronic command signals for the externally actuated AP and IE torques of the simulator. 

Haider H, Walker PS. Analysis and recommendations for the optimum spring configurations for soft tissue restraint in force-control knee simulator testing. Trans Orthop Res Society, 48th Annual Meeting-, 2002. 

Haider H, Sekundiak TD, Garvin KL. Simulation of the spring-based soft tissue restraint in testing knee wear under force control. Proceedings of the First International Conference on Mechanics of Biomaterials Tissues. Hawaii, USA 2005 December. 

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