Effect of Off-Axis Fluoroscopy Imaging on Two-Dimensional Kinematics in the Lumbar Spine: A Dynamic In Vitro Validation Study

Abstract

Spine intersegmental motion parameters and the resultant regional patterns may be useful for biomechanical classification of low back pain (LBP) as well as assessing the appropriate intervention strategy. Because of its availability and reasonable cost, two-dimensional (2D) fluoroscopy has great potential as a diagnostic and evaluative tool. However, the technique of quantifying intervertebral motion in the lumbar spine must be validated, and the sensitivity assessed. The purpose of this investigation was to (1) compare synchronous fluoroscopic and optoelectronic measures of intervertebral rotations during dynamic flexion-extension movements in vitro and (2) assess the effect of C-arm rotation to simulate off-axis patient alignment on intervertebral kinematics measures. Six cadaveric lumbar-sacrum specimens were dissected, and active marker optoelectronic sensors were rigidly attached to the bodies of L2-S1. Fluoroscopic sequences and optoelectronic kinematic data (0.15-mm linear, 0.17-0.20 deg rotational, accuracy) were obtained simultaneously. After images were obtained in a true sagittal plane, the image receptor was rotated in 5 deg increments (posterior oblique angulations) from 5 deg to 15 deg. Quantitative motion analysis (qma) software was used to determine the intersegmental rotations from the fluoroscopic images. The mean absolute rotation differences between optoelectronic values and dynamic fluoroscopic values were less than 0.5 deg for all the motion segments at each off-axis fluoroscopic rotation and were not significantly different (P > 0.05) for any of the off-axis rotations of the fluoroscope. Small misalignments of the lumbar spine relative to the fluoroscope did not introduce measurement variation in relative segmental rotations greater than that observed when the spine and fluoroscope were perpendicular to each other, suggesting that fluoroscopic measures of relative segmental rotation during flexion-extension are likely robust, even when patient alignment is not perfect.

Fig. 1

Lumbar spine specimen instrumented with active marker optoelectronic sensors and mounted in a custom spine simulator within the free space of the fluoroscope

Fig. 2

Representative cumulative intervertebral data for all motion segments in the aligned condition (fluoro = solid lines and Optotrak = dashed lines)

Fig. 3

Comparison of mean (SD) rotational ROM assessed by fluoroscopy and optoelectronics

Fig. 4

Comparison of mean absolute differences for fluoroscopy versus optoelectronic-assessed spinal motion. The mean absolute difference was calculated for each individual specimen; the graph shows the mean (SD) of the mean absolute differences for all the six specimens. The only motion segments showing significantly different mean absolute differences were L2–L3 and L3–L4.

Fig. 5

Bland and Altman plot showing fluoroscopy–Optotrak differences versus the mean values for all the specimens in the aligned condition