A new in vitro measurement technique for small three-dimensional joint motion and its application to the sacroiliac...

A new experimental technique for the measurement of three-dimensional (3-D), six degrees of freedom motion of a human joint is described. The apparatus consists of a solid cube and an orthogonal frame. Six spring-loaded linear variable differential transformers (LVDTs) are held by the frame in contact with three adjacent orthogonal surfaces of the cube. Three of the six LVDTs are positioned non-colinearly in contact with the first surface of the cube, two with the second surface, and one with the third surface. With the cube mounted on one of the moving bodies of a joint and the frame on the other one relative 3-D motion of the joint can be obtained from the six LVDT measurements based on the rigid body kinematics. An algorithm was developed to compute three Euler rotation angles and the 3-D translational components of a reference point on the body segment. The system was calibrated and applied to an in vitro study of sacroiliac joint motion. It has a rotational accuracy of 0.04 degree (with a S.D. of 0.07 degree) and a translational accuracy of 0.01 mm (with a S.D. of 0.01 mm). The possibility of adding a redundant seventh LVDT has also been explored and improvements in accuracy are reported. The main advantages of this technique are its compact size, accuracy, and low overall cost.