Poster Q11 - Connection between phasic creep and water flow threshold of the porcine lumbar disc

Introduction: The global impact of low back pain (LBP) is considerable. Remarkably, ~80% of LBP cases lack a definitive pathology, leading to non-specific diagnoses. Disc injury prior to identifiable injury, such as endplate fracture, may lead to pain. Fatigue testing of lumbar spinal units shows increased lumbar injury when flexion is superimposed on axial loading compared to axial loading alone. The lumbar spine exhibits biphasic mechanical response with behavior arising from both structural and fluid components. Characterization of the creep behavior of the spine has been conducted under various conditions. Quantitative MRI methods have been used to visualize the fluid flow from the nucleus pulposus (NP) to the annulus fibrosus (AF), but with limited ability to differentiate between the intervertebral disc (IVD) microstructure. Evidence on the relationship between phases of creep response and the shift in microstructural components of the spine remains sparse. Thus, our study aims to bridge this gap by correlating changes in microstructural IVD components observed through microCT with the phasic creep response of the lumbar IVD.

Materials and

Methods: Seven porcine lumbar functional spine units (FSUs) were tested in a biaxial apparatus under combined flexion-compression loading, with one additional FSU as a control. FSUs, consisting of two vertebral bodies and an intervertebral disc (IVD), were placed in an environmental chamber to simulate in vivo conditions. Tests applied a peak oscillatory compressive stress of 2.75 MPa with a flexion angle varied between 0-6 degrees at 1 Hz. A range of loading exposures was used to provide final endplate-to-endplate strain levels between 0% and 43%, and all tests were halted before endplate fracture. MicroCT imaging before and after testing was used to measure endplate distance to calculate creep strain. Strain-time histories were linearized using the Hill equation. Nonlinearity in the linearized data was divided into linear and nonlinear phases, identified by maximum curvature points per Dimbath et al. (2024, in review). Due to shorter test durations, no final maximum curvature peak was observed. Data from previous tests with the same loading conditions, showing both curvature peaks, were used to estimate the end of the linear phase. Linear phase durations from eight prior tests at 2.88 MPa (n=2), 3.88 MPa (n=2), and 4.15 MPa (n=4) were averaged and then used to estimate the end of the linear phase for the current study. Results were compared to previous analysis of annulus fibrosus area changes in specimens from this series (Sirhan et al., 2024).

Results, Conclusions, and Discussions: When data were linearized, the resulting behavior was not linear, and instead, a consistent phasic response was observed across all tests with an initial phase of nonlinearity followed by a linear phase. Comparison in linear phase from previous tests showed similar phase length between stress levels. The average duration of the linear phase from previous tests was about 20 ± 4 minutes. Based on the initial peak in curvature and the length of the linear phase, only two tests from this series were run until the end of the linear phase. From these two tests the creep strain at the estimated end time of the initial linear phase was found to be 30%. When compared to results in Sirhan et al., linear phase endpoint aligns with the threshold at which the percent area of the AF begins to increase (Figure 1).
This preliminary work is limited by small sample size and reliance on previous data to estimate creep phase endpoints. The variable and short test durations limited the full analysis of the creep phases due to dependency on later time points for defining phases. Only two tests reached the end of the initial linear creep phase, determining the average phase duration. Although previous tests used for estimation showed consistent linear phase lengths, they were conducted at higher stress values than those in the present study.
These findings present promising insights into the interplay between IVD components in phasic creep behavior. Specifically, the alignment between the end of the linear creep phase and the identified threshold value suggests a transition wherein mechanical behavior becomes dependent on water leaving the disc after a strain of approximately 25-30%. Prior to reaching this threshold, the creep response likely stems from the structural components of the disc. This study serves as an initial exploration into the relationship between phasic creep response and fluid flow within the lumbar IVD. A deeper understanding of this relationship will offer valuable insights into how IVD components shift in response to combined loads and shed light on the development of discogenic back pain.

Acknowledgements (Optional): The authors gratefully acknowledge funding from the Office of Naval Research and US Naval Air Systems Command – Patuxent River through MTEC (Southwest Research Institute-PI)