Reaction Forces and Flexion–Extension Moments Imposed on Functional Spinal Units With Constrained and Unconstrained In Vitro Testing Systems

Abstract

A mechanical goal of in vitro testing systems is to minimize differences between applied and actual forces and moments experienced by spinal units. This study quantified the joint reaction forces and reaction flexion-extension moments during dynamic compression loading imposed throughout the physiological flexion-extension range-of-motion. Constrained (fixed base) and unconstrained (floating base) testing systems were compared. Sixteen porcine spinal units were assigned to both testing groups. Following conditioning tests, specimens were dynamically loaded for 1 cycle with a 1 Hz compression waveform to a peak load of 1 kN and 2 kN while positioned in five different postures (neutral, 100% and 300% of the flexion and extension neutral zone), totalling ten trials per FSU. A six degree-of-freedom force and torque sensor was used to measure peak reaction forces and moments for each trial. Shear reaction forces were significantly greater (25.5 N - 85.7 N) when the testing system was constrained compared to unconstrained (p < 0.029). The reaction moment was influenced by posture (p = 0.037), particularly in C5C6 spinal units. In 300% extension (C5C6), the reaction moment was, on average, 9.9 Nm greater than the applied moment in both testing systems and differed from all other postures (p < 0.001). The reaction moment error was, on average, 0.45 Nm at all other postures. In conclusion, these findings demonstrate that comparable reaction moments can be achieved with unconstrained systems, but without inducing appreciable shear reaction forces.

Incidence of Compression-Induced Microinjuries in the Cartilage Endplate of the Spine

“…The superior aluminum cup was bolted to the flexion-extension rotation carriage that was actuated by a torque motor (T120-106-1K; Sensor Data Technologies, Sterling Heights, MI) and positioned in series to the compression actuator of a material testing system (Model 8872, Instron, Toronto, ON, Canada). The inferior aluminum cup was secured to an aluminum base that overlaid an oiled bearing covered surface 24. To prevent dehydration, specimens were wrapped with saline-soaked plastic-backed gauze and received additional superficial moistening every 20 minutes.…”

“…The inferior aluminum cup was secured to an aluminum base that overlaid an oiled bearing covered surface. 24 To prevent dehydration, specimens were wrapped with saline-soaked plastic-backed gauze and received additional superficial moistening every 20 minutes.…”