(D-130) Progressive Changes and Recovery of Intervertebral Disc Geometry, Opening Pressure, and T2 Relaxation Time

The intervertebral disc supports load transfer and enables while enabling flexibility in the spine, allowing for motion and daily activities. Many studies have evaluated healthy and pathological disc structure, composition, and mechanical behavior using cadaver mechanical testing. It is essential that these evaluations best represent the physiological in vivo condition, as the study outcomes may influence clinical practice.

Cadaver study outcomes are frequently interpreted in the context of in vivo disc for research related to back pain, degeneration, surgery, implants, engineered discs, and mechanobiology. However, there are substantial differences in the physical constraints and boundary conditions between the in vivo spine and ex vivo motion segments (bone-disc-bone), which can alter disc size, shape, hydration, pressure, residual stress, and mechanical behaviors. To mitigate the inherent changes from the in vivo to ex vivo conditions, studies often impose an axial preload in an attempt to mimic the physiological in vivo disc state; however, the preload and associated nucleus pressurization imposed across studies are highly variable.

The differences between the in vivo and ex vivo conditions have not been sufficiently quantified, limiting our ability to mimic them experimentally, and complicating our ability to interpret cadaveric study outcomes in the context of the in vivo disc. The purpose of this work was to evaluate the progressive changes in geometry, opening pressure, and T₂ time in the disc from an in vivo, physiologic condition to excised and preloaded segments.

Repeat 3T MRI (FLASH and T2-times [1-3]) were acquired on the lumbar spines from Yucatan minipigs at seven conditions (Table 1). Animals were first imaged as a Cadaver (within 2 hours of sacrifice). Our prior work confirmed live, anesthetized animals were not different from fresh cadavers [2]), therefore the cadaver represents the physiological reference condition. The full spine was dissected out with surrounding muscle for the Spine and then further dissected into vertebrae-disc-vertebrae motion Segments with facets intact and muscle removed. The segments were Potted in bone cement to enable fixture attachment and were placed in a phosphate buffered saline (PBS) bath with a low axial load (0.1-0.3 MPa) for the Loaded, Hydrated condition. The load was removed and segments were allowed to free swell in the PBS bath for the Unloaded, Hydrated condition, and finally the loading was repeated for another Loaded, Hydrated (2) condition.

From the MRI, geometry was assessed to determine the mid-sagittal area, anterior-posterior width, and disc height (calculated as the area/width) [1]. The T₂ relaxation times, which are positively correlated with hydration, were evaluated in the nucleus pulposus (NP) [3].

The opening pressure (OP) was assessed on a separate set of 4 lumbar spines by injecting contrast and taking continuous radiographs until the contrast entered the disc, indicating it overcame the internal NP pressure [4-5]. To investigate the impact of applied axial load on the NP pressure, a range of stresses (0-0.7 MPa) were used. Pilot studies confirmed outcomes were unaffected by repeated injections.

Throughout dissection, from the Cadaver to Spine to Segment and Potted conditions, the discs experienced an increase in height, decrease in width, and no significant changes in sagittal disc area (Fig1, Fig2). The Loaded segment had a reduced height and increased width compared with the unpotted Segment. The first Loaded and second Loaded (2) conditions were not different from each other. The applied nominal stress (0.1-0.3 MPa) for the Loaded condition was similar to the axial stress experienced in the physiologic Cadaver (0.2-0.4 MPa).  All geometric changes followed the expected response to changes in axial load during the progressive dissection (reduced load) then loading.

The OP was impacted by spine condition and imposed load. From the Cadaver to Spine to Potted segment conditions, the OP progressively decreased (Fig3A). The relationship between imposed stress and OP was linearly correlated, such that it is possible to predictably recover physiological OP with an applied load during hydration (Fig3B). With loading of 0.1-0.4 MPa, the Loaded segments recovered the Cadaver state OP, which represents the physiological in vivo state (Fig3C).

Throughout the conditions, from the Cadaver down to Potted conditions, the NP T₂ relaxation time consistently decreased (Fig4), following a monotonic trend similar to the height increases during this sequential dissection. Surprisingly, the T₂ time was unchanged between Loaded and Unloaded conditions. The T₂ time, which is related to hydration, was expected to increase from the Loaded to Unloaded conditions, as it was given the opportunity to free swell in a PBS bath; these T₂ time outcomes did not follow same trends as the geometry and OP changes. The mechanisms for the T₂ time changes are not fully understood at this time; however, they could be related to matrix and fluid rearrangement.

This study provided valuable quantification of paired in vivo and experimental conditions for disc structure, mechanics, and composition.  We identified that 0.1-0.4MPa applied load restores geometry and OP to the physiological state. It is important that an experiment mimic physiological in vivo conditions to provide relevant outcomes for translational applications.

[1] Meadows+ JOR Spine 2023

[2] Newman+ SB3C 2022

[3] Meadows+ JOR Spine 2020

[4] Elliott+ SPINE 2008

[5] Borthakur+ SPINE 2011