Kinematic differences between optical motion capture and biplanar videoradiography during a jump-cut maneuver

Abstract

Jumping and cutting activities are investigated in many laboratories attempting to better understand the biomechanics associated with non-contact ACL injury. Optical motion capture is widely used; however, it is subject to soft tissue artifact (STA). Biplanar videoradiography offers a unique approach to collecting skeletal motion without STA. The goal of this study was to compare how STA affects the six-degrees-of-freedom motion of the femur and tibia during a jump-cut maneuver associated with non-contact ACL injury. Ten volunteers performed a jump-cut maneuver while their landing leg was imaged using optical motion capture (OMC) and biplanar videoradiography. The within-bone motion differences were compared using anatomical coordinate systems for the femur and tibia, respectively. The knee joint kinematic measurements were compared during two periods: before and after ground contact. Over the entire activity, the within-bone motion differences between the two motion capture techniques were significantly lower for the tibia than the femur for two of the rotational axes (flexion/extension, internal/external) and the origin. The OMC and biplanar videoradiography knee joint kinematics were in best agreement before landing. Kinematic deviations between the two techniques increased significantly after contact. This study provides information on the kinematic discrepancies between OMC and biplanar videoradiography that can be used to optimize methods employing both technologies for studying dynamic in vivo knee kinematics and kinetics during a jump-cut maneuver.

Figure 1

A, experimental set-up including image intensifiers and x-ray sources. The optical motion capture cameras are not shown. The subject is performing the jump-cut maneuver. In this example they were cued to cut to their left upon landing on the force plate. The ‘X’ marks the landing location and the arrows represent the left (L) and right (R) cut directions. B, the OMC (dotted red) and biplanar videoradiography (solid green) knee flexion/extension and GRF (solid blue) for the entire jump-cut activity including the flight phase, landing, rotation, cut, and toe-off. The field of view for the biplanar videoradiography limits its ability to collect kinematic data for the entire jump-cut activity. However, it can be tailored to measure motion for specific periods of an activity where OMC is more sensitive to soft tissue artifact.

Figure 2

Panels A and B represent a single frame of the biplanar videoradiography data for source 1 and 2, respectively. Panels C and D represent the same frame of the biplanar videoradiography data (blue) after image processing. Contrast and edge detection is used to enhance the images. Additionally, the digitally reconstructed radiographs generated from the CT volume are displayed in tan and are superimposed on the blue and black biplanar videoradiography data. The images represent the outcome of the Autoscoper software after bone tracking is completed for the current frame. The 3-D models of the tibia and femur driven by optical motion capture (tan) and biplanar motion capture (blue) are shown in panels E and F. All four independently tracked anatomical coordinate systems are also shown. The short and lighter coordinate systems are being driven by OMC and the long and darker coordinate systems are being driven by biplanar videoradiography. The external markers for the thigh and shank are also shown in tan. Panel E represents the initial frame, where OMC and biplanar videoradiography are perfectly aligned. Panel F represents a frame where soft tissue artifact is affecting the OMC driven bones and coordinate systems.

Figure 3

Top, the difference between OMC and biplanar videoradiography for the three rotational axes (Rx, Ry, and Rz) and origin (T) of the independently driven femoral and tibial ACSs. Middle, the rotation difference (FL\EX, AB\AD, IN\EX) between OMC and biplanar videoradiography. Bottom, the translational difference (ME\LA, AN\PO, CO\DI) between OMC and biplanar videoradiography. For all knee joint rotational and translational differences, the data is displayed for the period before contact (A) and the period after contact (B). For each graph, the data is summarized using means plus standard deviations and the brackets represent significant differences (p ≤ 0.05) between groups.

Figure 4

Example OMC (dotted red) and biplanar videoradiography (solid green) knee flexion angle and ground reaction force (solid blue) data versus time. The dotted vertical lines represent the contact event. The period before contact is period A and the period after contact is period B. Period A began and period B ended when the femur and tibia entered and exited the FOV of the biplanar videoradiography system. Thus, the arrows shown above the ME/LA translation graph denote the time point where the knee entered and exited the field of view of the biplanar videoradiography system, respectively.