Effects of Needle Injuries and Simulated Repetitive Lifting on Lumbar Spinal Segments Mechanics and Annulus Fibrosus Structure

Abstract

Despite the intervertebral disc's ability to withstand considerable loading, its resilience is critical in the context of widespread back disorders, such as low back pain (LBP), which impose significant global health and economic challenges. These challenges are exacerbated by factors like repetitive lifting, further complicating the disc's vulnerability not only to occurrences like annular tears but also to artificial interventions, including clinical needle punctures used in diagnosing and treating spinal issues. Although prior in vitro studies have examined the mechanical impact of needle injuries on intervertebral discs, they have not considered simulated in vivo mechanical loading conditions reflective of repetitive lifting, which is closely associated with these injuries. Moreover, there has been a lack of investigation into the development and potential morphological changes in the annulus fibrosus due to needle-induced rupture under conditions that mimic repetitive lifting. Bridging this gap requires an examination of the individual mechanical effects of repetitive lifting on functional spinal units (FSUs), incorporating more realistic conditions that include inter-day disc recovery. Consequently, this thesis aims to assess the mechanical influence of simulated repetitive lifting on ovine FSUs immediately after lifting and following a recovery period, both independently and in conjunction with disc needle injuries. It also seeks to quantify the morphological changes in needle-induced ruptures within the annulus fibrosus after exposure to simulated repetitive lifting.

To fulfill the aims of the research, the sole mechanical effects of repetitive lifting were first evaluated. Twenty ovine FSUs underwent six degrees of freedom (6DOF) testing at 0.1 Hz for five cycles, followed by simulated repetitive lifting to replicate a day of lifting. This simulation involved 1000 cycles combining compression (1.1 MPa)—equivalent to lifting an intermediate weight of 20 kg, within safe manual handling limits—and flexion (13°). Subsequently, two additional 6DOF tests were conducted: one immediately after the lifting session and the other after a recovery period, allowing the discs to reach fluid equilibrium similar to that during sleep. Once these FSUs were adapted to the recovery state, further investigation was performed on the combined effects of needle injuries and simulated repetitive lifting. The FSUs were divided into control and injury groups, each comprising 10 FSUs. The injury group received 25G needle injuries in the posterolateral regions of the discs before repeating the simulated repetitive lifting and testing protocol on both groups. Microtome sections were successfully collected from some of these injuries, perpendicular to the injury axis, where microscope images were obtained, and the morphologies of these injuries were quantified.

The sole mechanical effects of repetitive lifting were found to cause significant biomechanical changes in flexion, a primary direction applied to the load. The changes appeared as a decrease in stiffness (77.2%, P<0.001) and an increase in the phase angle (89%, P<0.001) immediately following the simulated repetitive lifting. After a period applied for disc recovery to allow for fluid equilibrium, the observed changes in flexion persisted. Specifically, there was a continued reduction in stiffness (71.2%, P<0.001) and an increased phase angle (63.8%, P<0.001). These findings suggested that the recovery period was insufficient to fully moderate the biomechanical damage induced by repetitive lifting, likely in the disc’s microstructure.

Assessments of FSU mechanics by combined needle injuries and repetitive lifting demonstrated an increase in stiffness in right lateral bending (27.27%, P=0.01) immediately following the lifting, suggesting a compensatory mechanism for a compromised left posterolateral side due to a needle injury. This increase in stiffness response might relate to a permanent reduction in flexion stiffness caused by the previous repetitive lifting prior to needle injury, considering that an intrinsic aspect of forward bending relates to lateral bending. Furthermore, the vulnerability of the left side annulus to needle injuries could be due to disruption likely in the inner annulus as a consequence of cumulative damage from repetitive lifting. The right side's increased stiffness during lateral bending, previously not present, suggested a distinctive biomechanical response of the disc to counteract the reduced flexion stiffness, thereby maintaining equilibrium in bending movements and potentially preventing further injury or stress to the injured left side. The increase in stiffness was temporary since it diminished after a recovery period.

Exploration of how repetitive lifting impacts the morphology of needle injuries in the annulus was insufficient to draw definitive conclusions due to encountered variability in the measurements. This variability might be linked to the different forms of needle injury inherent when inflicted into the annulus. The present research emphasised this variability further by identifying, for the first time, a hybrid injury form—a combination of the known parallel and cross forms. These forms naturally occur and intrinsically manifest, aligning with or intersecting the oblique fibres of the annulus, respectively. Preliminary analysis, conducted by taking morphological measurements on limited data with consistent injury forms, may encourage future studies to investigate the potential for differential responses to repetitive lifting among needle injuries based on their form.

The findings of this thesis on the biomechanical effects of repetitive lifting can contribute to the development of safety guidelines for workers engaged in repetitive lifting tasks. Furthermore, investigating the interplay between repetitive lifting and disc needle injuries sets a foundation for future research to improve diagnosis and treatment strategies for disc issues.