(D-155) Role of cyclic loading in porcine lumbar intervertebral disc behavior: a preliminary study

Lower back pain (LBP) is a main contributor to disability [1] which is especially problematic in military populations, where LBP is the most common type of chronic pain, accounting for up to 82% of all injuries [2]. Excessive loading of the spine due to occupational exposure is a main contributor to LBP [3]. Exposure to whole-body vibration and spinal posture during exposure are attributed to increased prevalence of LBP [4] [5].

Injury behavior of the spine due to whole body vibration, or cyclic loading, is not well understood. Additionally, it is not known whether static loading of the spine is better, worse, or different than cyclic loading. Pure axial compression studies show stiffness coefficients differ between static and cyclic loading [6]. Thus, oscillations likely play a role in tissue behavior. However, the spine is not typically loaded in compression alone with flexion being a common component. Flexion has been shown to influence tissue behavior and contribute to injury [7] [8]. Furthermore, combined flexion-compression loading adversely affects the mechanical integrity of intervertebral disc structures [9]. Studying the mechanical response of the lumbar spine under various loading conditions, particularly combined loading, may improve understanding of the role of repetitive cyclic loading in LBP.

Currently, it is unknown if and how the cyclic mechanism plays an important role in response and injury. This study compares the creep response of porcine lumbar spinal units under cyclic and quasistatic loading to understand the role of repeated loading in combined flexion-compression loading scenarios.

Six porcine functional spinal units (FSUs) from two pig specimens were tested at two different stress and duration levels. For specimen consistency, one FSU from each pig underwent flexion-compression with cyclic loading while the other two FSUs were held in quasistatic flexion-compression. Superior and inferior ends of the FSUs were fixed in pots before being fixed in a biaxial test apparatus. An environmental chamber surrounding the test apparatus simulated in vivo temperature and humidity conditions.

The cyclic loading condition, modeled from loading patterns experienced by military highspeed boat operators [10], applied sinusoidal axial compression and an offset cyclic flexion ramp, both at 1 Hz. Flexion oscillated from 0-6° and maximum axial compressive force applied either 2.05 MPa or 4.15 MPa of stress. For the quasistatic loading condition, constant axial compression was applied at the equivalent stress as the paired cyclic test. Constant flexion was applied at 5°, which was considered to be the most representative quasistatic angle in regard to primary creep response [11]. Short duration tests under 2.05 MPa of stress were run for about 25 minutes and longer duration tests under 4.15 MPa of stress were run for about 2.5 hours. The endplate-to-endplate distance adjacent to the intervertebral disc (IVD) was used to calculate engineering strain for each FSU. Strain time histories were used to compare the creep profiles between cyclic and quasistatic tests. An exponential function was used to model the experimental creep response and the percent difference was calculated to quantitatively compare the curves.

The short duration tests at 2.05 MPa of stress resulted in similar creep response between loading regimes (Fig 1). Table 1 shows the exponential rate variable from the exponential fit for each curve and the percent difference between cyclic and quasistatic functions. The percentage difference between loading regimes ranged from less than 1% to 13%. When the stress and duration of the tests were increased to 4.15 MPa and 2.5 hours respectively, the creep response showed distinct differences (Fig 2). At 40-50% strain, the quasistatic creep behavior begins to taper off while the cyclic creep continues the increasing exponential trend. Subsequently, the percent difference between the cyclic and quasistatic exponential fit variables is greater, with values ranging from 71.5% to 85% difference. Because increasing strain values increase the risk of injury [12], these creep profiles suggest cyclic loading could lead to injury quicker. However, when comparing the creep behavior only up to the short duration of 1000 seconds, under 4.15 MPa, the cyclic and quasistatic behavior is quite similar with the percent difference decreasing to 13% and 22% (Table 3). These findings are consistent with a prior study, which reported comparable overall deformation in caprine lumbar IVDs under static and dynamic loading conditions yet revealed distinct mechanical responses throughout the loading duration [13]. Similarity is also seen in results from Gooyers et al. [8] in that cyclic and static loading profiles showed differences in rate of FSU height loss over the test duration. Nevertheless, Gooyers et al. [8] did not compare creep response, therefore a direct comparison is not appropriate between studies.

The results of this study demonstrate that the presence of cyclic loading influences the creep response of lumbar IVDs. Creep response of IVDs is often attributed to water flow [14] [15], thus these results suggest water flow may be altered by cyclic loading. Additionally, the lumbar spine may be better suited for sustaining quasistatic loads rather than repeated loads for combined flexion-compression configurations, especially in the long term.