Increased lumbar spinal column laxity due to low-angle, low-load cyclic flexion may predispose to acute injury

Complex In Vivo Motion of the Bovine Tail Provides Unique Insights Into Intervertebral Disc Adaptation

Introduction

The intervertebral disc (IVD) of the bovine tail is a commonly used research analogue for the human disc at the organ, tissue, and cellular levels. While these tails are subjected to thousands of dynamic motion events daily, little is known about how these motions might induce tissue remodeling, particularly in the outer annulus fibrosus (AF) of IVDs connecting adjacent vertebrae. This study hypothesized that despite the similarities in geometry and biochemical composition of IVDs in the bovine tail, level-wise variations in repetitive in-vivo motion would be associated with tissue level adaptations.

Methods

In-vivo active range of motion (RoM) was measured by placing inertial measurement unit sensors on the tails of adult cows and using a multi-segment rigid body model to calculate level-wise flexion-extension and lateral bending angles. Level-wise passive RoM was measured from cadaveric adult bovine tails in flexion, extension, and lateral bending with skin and muscles removed. IVDs were extracted for measurement of height, diameters, AF radial thicknesses, and AF fiber crimp periods.

Results

In-vivo joint RoM was found to vary drastically by level, largely due to a prominent second order mode with inflection point at the fourth joint. Joint levels near this inflection point were found to have the highest passive RoMs. In the proximal tail, decreased RoM was associated with an increased fiber crimp period in the outer AF, while in the distal tail it was associated with increased AF thickness.

Discussion

Taken together, these findings suggest that IVDs in the bovine tail respond to repeated complex dynamic motions through a process of adaptation at the mesoscale (AF thickening during growth) and microscale (residual strain accumulation in the mature state). The bovine tail thus provides a powerful tool for modeling how the human lumbar intervertebral disc may remodel in response to changes in exposure to repetitive motions.

Prediction of Temperature and Loading History Dependent Lumbar Spine Biomechanics Under Cyclic Loading Using Recurrent Neural Networks

Extended-duration cyclic loading of the spine is known to be correlated to lower back pain (LBP). Therefore, it is important to understand how the loading history affects the entire structural behavior of the spine, including the viscoelastic effects. Six human spinal segments (L4L5) were loaded with pure moments up to 7.5 Nm cyclically for half an hour, kept unloaded for 15 min, and loaded with three cycles. This procedure was performed in flexion-extension (FE), axial rotation (AR), and lateral bending (LB) and repeated six times per direction for a total of 18 h of testing per segment. A Long Short-Term Memory (LSTM) Recurrent Neural Network (RNN) was trained to predict the change in the biomechanical response under cyclic loading. A strong positive correlation between the total testing time and the ratio of the third cycle to the last cycle of the loading sequence was found (BT: [Formula: see text] =  0.3469, p = 0.0003, RT: [Formula: see text] =0.1988, p  =   0.0377). The moment-range of motion (RoM) curves could be very well predicted with an RNN ([Formula: see text]=0.988), including the correlation between testing time and testing temperature as inputs. This study shows successfully the feasibility of using RNNs to predict changing moment-RoM curves under cyclic moment loading.

Establishing a through-puncture model for assessing post-injection leakage in the intervertebral disc

PURPOSE

Needle injection through the outer annulus fibrosus of the intervertebral disc (IVD) is the most practical approach for delivery of therapeutic agents, which have been shown to potentially leak following needle retraction. The goal of this work was to establish a protocol for quantifying post-injection leakage and test its sensitivity to factors believed to affect needle track geometry.

METHODS

A through-puncture defect procedure, followed by controlled injection, was performed on bovine caudal IVDs. Sensitivity to needle size was tested by injection of saline into unconstrained discs with either a 30G, 26G, or 21G hypodermic needle. Sensitivity to axial load was tested by repeated injection via a 26G needle with either no constraint, fixed height, or 10% axial compressive strain. Sensitivity to flexion was tested by applying combined 0.2 MPa compression and 15° of flexion following injection of 5% of disc volume.

RESULTS

Needle diameter significantly affected maximum volume prior to leakage, ranging from 34.6 ± 31.9 µL when using 21G to 115.6 ± 23.6 µL when using 30G. While all unloaded discs leaked, axial compression decreased the incidence of leakage events by 50-100% depending on load history. Forward flexion resulted in a 22% incidence of leakage.

CONCLUSION

Fluid injected into IVDs is at significant risk of leakage following needle retraction. This risk depends on factors which alter the geometry of the needle track, including needle size, pinching due to axial compression, and stretching as a result of forward flexion.