A NEW IN-VITRO MEASUREMENT TECHNIQUE FOR SMALL 3-DIMENSIONAL JOINT MOTION AND ITS APPLICATION TO THE SACROILIAC JOINT

A new experimental technique for the measurement of three-dimensional (3-D), six degrees of freedom motion of a human joint is described. Th e apparatus consists of a solid cube and an orthogonal frame. Six spri ng-loaded linear variable differential transformers (LVDTs) are held b y 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 o ne 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 t he rigid body kinematics. An algorithm war developed to compute three Euler rotation angles and the 3-D translational components of a refere nce point on the body segment. The system was calibrated and applied t o an in vitro study of sacroiliac joint motion. It has a rotational ac curacy of 0.04 degrees (with a S.D. of 0.07 degrees) and a translation al accuracy of 0.01 mm (with a S.D. of 0.01 nm). The possibility of ad ding a redundant seventh LVDT has also been explored and improvements in accuracy are reported. The main advantages of this technique are it s compact size accuracy, and low overall cost. Copyright (C) 1996 Else vier Science Ltd for IPEMB.