Design and validation of a novel Cartesian biomechanical testing system with coordinated 6DOF real-time load control: application to the lumbar spine (L1-S, L4-L5).

Robotic methods applied to in-vitro biomechanical testing potentially offer more comprehensive evaluations however, standard position control algorithms make real-time load control problematic. This paper describes and evaluates a novel custom developed Cartesian force controlled biomechanical testing system with coordinated 6 degree of freedom (DOF) real-time load control. A custom developed 6-DOF serial manipulator with cascaded force over position control algorithms was designed, assembled, and programmed. Dial gauge tests assessed accuracy of custom linear axes. Standard test input and tuning procedures refined control performance. Two single motion segment units (L4-L5) and lumbar (L1-S) spine segments were tested under continuous pure moment application in flexion-extension, left-right lateral bending and axial rotation to 8Nm under full 6-DOF load control. Mean load control tracking errors between commanded and experimental loads were computed. Global spinal ranges of motion were compared to previously published values for standard non-robotic protocols. Individual linear and rotational axis position control accuracies were equal to or less than 6.35μm and 0.0167° respectively. Pilot pure bending tests demonstrated stable load control performance, as well as load rates, rotational velocities, and ranges of motion comparable to those for standard non-robotic in-vitro tests. Tracking errors for zero commanded forces and all moment controlled axes were less than 0.81±0.68N and 0.18±0.19Nm over all tests, respectively. The Cartesian based system simplified control application and demonstrated robust position and load control that was not limited to single axis or zero commanded loads. In addition to emulating standard biomechanical tests, the novel Cartesian force controlled testing system developed is a promising tool for biomechanical assessments with coordinated dynamic load application and coupled motion response in 6DOF.