A Direct Comparison of Biplanar Videoradiography and Optical Motion Capture for Foot and Ankle Kinematics

Abstract

Measuring motion of the human foot presents a unique challenge due to the large number of closely packed bones with congruent articulating surfaces. Optical motion capture (OMC) and multi-segment models can be used to infer foot motion, but might be affected by soft tissue artifact (STA). Biplanar videoradiography (BVR) is a relatively new tool that allows direct, non-invasive measurement of bone motion using high-speed, dynamic x-ray images to track individual bones. It is unknown whether OMC and BVR can be used interchangeably to analyse multi-segment foot motion. Therefore, the aim of this study was to determine the agreement in kinematic measures of dynamic activities. Nine healthy participants performed three walking and three running trials while BVR was recorded with synchronous OMC. Bone position and orientation was determined through manual scientific-rotoscoping. The OMC and BVR kinematics were co-registered to the same coordinate system, and BVR tracking was used to create virtual markers for comparison to OMC during dynamic trials. Root mean square (RMS) differences in marker positions and joint angles as well as a linear fit method (LFM) was used to compare the outputs of both methods. When comparing BVR and OMC, sagittal plane angles were in good agreement (ankle: R² = 0.947, 0.939; Medial Longitudinal Arch (MLA) Angle: R² = 0.713, 0.703, walking and running, respectively). When examining the ankle, there was a moderate agreement between the systems in the frontal plane (R² = 0.322, 0.452, walking and running, respectively), with a weak to moderate correlation for the transverse plane (R² = 0.178, 0.326, walking and running, respectively). However, root mean squared error (RMSE) showed angular errors ranging from 1.06 to 8.31° across the planes (frontal: 3.57°, 3.67°, transverse: 4.28°, 4.70°, sagittal: 2.45°, 2.67°, walking and running, respectively). Root mean square (RMS) differences between OMC and BVR marker trajectories were task dependent with the largest differences in the shank (6.0 ± 2.01 mm) for running, and metatarsals (3.97 ± 0.81 mm) for walking. Based on the results, we suggest BVR and OMC provide comparable solutions to foot motion in the sagittal plane, however, interpretations of out-of-plane movement should be made carefully.

Keywords: ankle; biplanar videoradiography; foot; kinematics; motion analysis.

Figure 1

Schematic depiction of the laboratory set up. The two x-ray sources were set at a 130° angle ipsilateral to each other. The intensifiers were stationed as close to the translucent Dragon Plate (AllRed and Associates, Elbridge, USA) as possible while maintaining visibility of the foot. The entire platform was evenly raised 0.60 m from the ground. Eight OMC cameras were stationed around the collection space to optimize viewing of the foot markers.

Figure 2

Optical motion capture (OMC) marker placement was performed in accordance with the Rizzoli model (Leardini et al., 2007). Markers are listed in order from proximal to distal, and numbered, to assist with identification. Tibial Markers (Magenta): (1) Lateral Shank (LtS), (2) Medial Shank (MdS), (3) Distal Shank (Shk). Calcaneal Markers (Green): (4) Superior Calcaneal Ridge, (5) Inferior Calcaneal Ridge (ICR), (6) Sustentaculum Tali (ST), (7) Peroneal Tubercle. Navicular Marker (Blue): (8) Navicular (TN). Metatarsal Markers (Purple): (9) First Metatarsal Base (FMB), (10) Second Metatarsal Base (SMB), (11) Fifth Metatarsal Base (VMB), (12) First Metatarsal Head (FMH), (13) Second Metatarsal Head (SMH), (14) Fifth Metatarsal Head (VMH).

Figure 3

Scientific Rotoscoping: Depiction of the manual alignment of the calcaneus just prior to toe off. Each bone is aligned with the bony landmarks visible on the x-ray images in both cameras (camera 1, left and camera 2, right). Once the 3D bone is simultaneously aligned within both camera 1 and camera 2, it is considered “tracked” for that frame.

Figure 4

Ankle Angle Group Means: The group mean ankle angle across stance with the first column showing walking data, and the second column representing running data. The y-axes represent the three rotations (Inversion/Eversion, Abduction/Adduction, and Plantarflexion/Dorsiflexion), and the x-axes represent percent of stance. The optical motion capture (OMC) ankle angle ± 1 SD is represented in teal while the biplanar videoradiography (BVR) ankle angle ± 1 SD is displayed in light purple.

Figure 5

Group mean (± 1 SD) medial longitudinal arch (MLA) angle during stance phase for walking and running: optical motion capture (OMC) MLA angle (teal) and biplanar videoradiography (BVR) MLA angle (light purple). The y-axis represents MLA normalized to MLA at static. The x-axis represents percent of stance. General patterns of motion appear similar despite an offset in initial angle.