236 - A Novel Real Time in-vitro 360 Degree Bending Protocol for Multi-level...

#236 A Novel Real Time in-vitro 360 Degree Bending Protocol for Multi-level Lumbar Spinal Segments

Oral Posters: Deformity

Presented by: B. Kelly

Author(s):

C. Bennett (1)
B.P. Kelly (1)

(1) Regional Hospital of Legnano, Legnano, Italy

Abstract

Introduction: Traditional pure bending tests have been confined within anatomical planes often with coupled out of plane rotations constrained or unreported. Such tests provide only a fractional assessment of 360º spinal stability as well as the global influence of pathologies or surgical interventions on stability and 3D kinematics.

Objective: The objective was to develop a novel in-vitro bending protocol that simulated full 360º trunk rotation (starting from and ending in pure flexion) on a full lumbar segment in real time, while maintaining a constant prescribed resultant bending moment in all directions of rotation.

Methods: A custom designed fully controlled 6DOF Cartesian based testing system was used comprised of three linear axes from which a 3-axis rotary gimbal was suspended. Three lumbar spines (L1-S1) were screened, potted and mounted in the frame with distal end fixed and proximal end mounted to the frame gimbal and 6-axis load sensor. All three linear axes and an axial rotation motor were operated under load control to 0N and 0Nm respectively. Specimens were loaded under pure bending from neutral to 8Nm pure flexion followed by continuous real time application of coordinated bending moments that transitioned 360º counter clockwise along a circular profile at 0.4Nm/s from flexion to left lateral bending, extension, right lateral bending and back to flexion. Moments were programmed so as to maintain a targeted constant resultant magnitude of 8Nm regardless of the direction applied.

Results: The protocol produced a full stable 360º counterclockwise rotational sweep of each lumbar segment. Global applied resultant bending moments were maintained at 8Nm with mean and maximum deviations of 0.04 and 0.2Nm respectively (Figure 1). All off axis forces and axial moments were held to a zero value within a tolerance of ±3N, and mean and maximum deviations of 0.025Nm and 0.1Nm respectively. Mean global resultant rotational velocity was 0.9º/s with a maximum of 1.9º/s. Global 3D spinal rotations were recorded from positional feedback of the gimbal axes (Figure 2). Maximum changes in axial rotation averaged 6.3º and were observed to consistently occur when combined flexion and lateral bending (both left and right) rotation values were approximately equal.

Discussion: The protocol developed defines a full 360º envelope of stability for an applied bending moment that dynamically remains constant throughout the full range of motion. This provides for a novel methodology to more comprehensively evaluate global spinal stability and 3D kinematics under all conditions. Future work will consider additional loading profiles and application to single MSU's, and cervical segments.

Figure 1

Figure 2