Optical motion capture accuracy is task-dependent in assessing wrist motion

Abstract

Optical motion capture (OMC) systems are commonly used to capture in-vivo three-dimensional joint kinematics. However, the skin-based markers may not reflect the underlying bone movement, a source of error known as soft tissue artifact (STA). This study examined STA during wrist motion by evaluating the agreement between OMC and biplanar videoradiography (BVR). Nine subjects completed 7 different wrist motion tasks: doorknob rotation to capture supination and pronation, radial-ulnar deviation, flexion-extension, circumduction, hammering, and pitcher pouring. BVR and OMC captured the motion simultaneously. Wrist kinematics were quantified using helical motion parameters of rotation and translation, and Bland-Altman analysis quantified the mean difference (bias) and 95% limit of agreement (LOA). The rotational bias of doorknob pronation, a median bias of -4.9°, was significantly larger than the flexion-extension (0.7°, p < 0.05) and radial-ulnar deviation (1.8°, p < 0.01) tasks. The rotational LOA range was significantly smaller in the flexion-extension task (5.9°) compared to pitcher (11.6°, p < 0.05) and doorknob pronation (17.9°, p < 0.05) tasks. The translation bias did not differ between tasks. The translation LOA range was significantly larger in circumduction (9.8°) compared to the radial-ulnar deviation (6.3°, p < 0.05) and pitcher (3.4°, p < 0.05) tasks. While OMC technology has a wide-range of successful applications, we demonstrated it has relatively poor agreement with BVR in tracking wrist motion, and that the agreement depends on the nature and direction of wrist motion.

Keywords: Accuracy; Kinematics; Optical motion capture; Soft tissue artifact; Wrist.

Figure 1.

Marker Placement. A cluster (n=4, 6.4mm dia.) of retroreflective markers placed on the dorsal side of the third metacarpal (MC3) and a cluster (n=4, 6.4mm dia.) of retroreflective markers placed on the dorsal side of the radius was used for optical motion capture (OMC) data collection.

Figure 2.

Tasks. All subjects performed seven motion tasks (doorknob rotations were captured separately) as part of the experimental procedure.

Figure 3.

Coordinate System Definition. The radius anatomical coordinate system (RCS) and third metacarpal (MC3) coordinate system (MCS) were constructed from anatomical landmarks during biplanar videoradiography (BVR) data processing. A transformation matrix describing the position of the markers as a rigid body represented the coordinate system of the markers (OMC). The rotation about the x-axis represents supination (−) pronation (+) plane, the rotation about the y-axis represents the flexion (+) extension (−) plane, and the rotation about the z-axis represents the radial (−) ulnar (+) plane.

Figure 4.

Rotational Agreement Between Biplanar Videoradiography (BVR) and Optical Motion Capture (OMC). (a) Mean bias (BVR – OMC) across 9 subjects of the rotation about the screw axis (ϕ) by task, referencing biplanar videoradiography as the gold standard. (b) Range of the limit of agreement (LOA) across subjects of the rotation about the screw axis (ϕ) by task, referencing BVR as the gold standard.

Figure 5.

Translational Agreement Between Biplanar Videoradiography (BVR) and Optical Motion Capture (OMC). (a) Mean bias (BVR – OMC) across 9 subjects of the translation along the screw axis by task, referencing biplanar videoradiography as the gold standard. (b) Range of the limit of agreement (LOA) across subjects of the translation along the screw axis by task, referencing BVR as the gold standard.