A technique to measure three-dimensional in vivo rotation of fused and adjacent lumbar vertebrae

Abstract 

Background context

Previous attempts to measure vertebral motion in vivo have been either static measure, imprecise, two-dimensional, or overly invasive to be applied to serial studies.

Purpose

This study evaluated the efficacy of a unique high-speed biplane X-ray system for tracking lumbar vertebrae in vivo during dynamic motion. Additional goals were to determine parameters for future studies using this tool and to obtain preliminary data on the effects of lumbar fusion on vertebral kinematics.

Study design/setting

A high-speed biplane radiographic X-ray system was used to measure the three-dimensional (3D) relative rotation between fused and adjacent vertebrae in vivo during muscle driven movement. Subjects were tested 2, 3, and 6 months after fusion procedures to assess vertebral motion of fused and adjacent vertebrae.

Patient sample

Five subjects received lumbar fusion surgery.

Outcome measures

Physiologic measures included 3D vertebral rotation of fused and adjacent vertebrae.

Methods

Tantalum beads were implanted into lumbar vertebrae during fusion operations. Radiographic data was collected continuously at 50 frames per second during flexion-extension, lateral bending, and axial twist movements serially, at 2, 3, and 6 months after fusion surgery.

Results

Implanted beads were tracked with an accuracy of 0.18mm during dynamic motion. Vertebral rotation was not necessarily linearly related to trunk rotation, supporting the use of continuous data collection during movement; collecting only movement start and end points may not be sufficient. Some movements indicated fusion was complete, whereas others indicated incomplete fusion. This suggests patients be tested performing a variety of movements to test for complete fusion. The fusion site often acted as a pivot point for vertebral rotation, with vertebrae superior to the fusion rotating in the direction of the trunk and vertebrae inferior rotating opposite trunk rotation.

Conclusions

This technique is sufficiently accurate for in vivo serial studies of vertebral motion during muscle driven movements. A variety of movements should be performed to assess surgical results, and the data should be collected continuously through the entire range of motion, not just at the movement endpoints. However, care must be exercised in subject selection, in camera location, and in the placement of tracking beads in relation to implanted instrumentation.

Keywords: Kinematics, Dynamic X-ray, In vivo, Biomechanics, Lumbar fusion