Can axial loading restore in vivo disc geometry, opening pressure, and T2 relaxation time?

Background: Cadaveric intervertebral discs are often studied for a variety of research questions, and outcomes are interpreted in the in vivo context. Unfortunately, the cadaveric disc does not inherently represent the LIVE condition, such that the disc structure (geometry), composition (T2 relaxation time), and mechanical function (opening pressure, OP) measured in the cadaver do not necessarily represent the in vivo disc. Methods: We conducted serial evaluations in the Yucatan minipig of disc geometry, T2 relaxation time, and OP to quantify the changes that occur with progressive dissection and used axial loading to restore the in vivo condition. Results: We found no difference in any parameter from LIVE to TORSO; thus, within 2 h of sacrifice, the TORSO disc can represent the LIVE condition. With serial dissection and sample preparation the disc height increased (SEGMENT height 18% higher than TORSO), OP decreased (POTTED was 67% lower than TORSO), and T2 time was unchanged. With axial loading, an imposed stress of 0.20-0.33 MPa returned the disc to in vivo, LIVE disc geometry and OP, although T2 time was decreased. There was a linear correlation between applied stress and OP, and this was conserved across multiple studies and species. Conclusion: To restore the LIVE disc state in human studies or other animal models, we recommend measuring the OP/stress relationship and using this relationship to select the applied stress necessary to recover the in vivo condition.

Geometric determinants of the mechanical behavior of image-based finite element models of the intervertebral disc.

The intervertebral disc is an important structure for load transfer through the spine. Its injury and degeneration have been linked to pain and spinal fractures. Disc injury and spine fractures are associated with high stresses; however, these stresses cannot be measured, necessitating the use of finite element (FE) models. These models should include the disc's complex structure, as changes in disc geometry have been linked to altered mechanical behavior. However, image-based models using disc-specific structures have yet to be established. This study describes a multiphasic FE modeling approach for noninvasive estimates of subject-specific intervertebral disc mechanical behavior based on medical imaging. The models (n = 22) were used to study the influence of disc geometry on the predicted global mechanical response (moments and forces), internal local disc stresses, and tractions at the interface between the disc and the bone. Disc geometry was found to have a strong influence on the predicted moments and forces on the disc (R2 = 0.69-0.93), while assumptions regarding the side curvature (bulge) of the disc had only a minor effect. Strong variability in the predicted internal disc stresses and tractions was observed between the models (mean absolute differences of 5.1%-27.7%). Disc height had a systematic influence on the internal disc stresses and tractions at the disc-to-bone interface. The influence of disc geometry on mechanics highlights the importance of disc-specific modeling to estimate disc injury risk, loading on the adjacent vertebral bodies, and the mechanical environment present in disc tissues.

MRI-based measurement of in vivo disc mechanics in a young population due to flexion, extension, and diurnal loading.

Background: Intervertebral disc degeneration is often implicated in low back pain; however, discs with structural degeneration often do not cause pain. It may be that disc mechanics can provide better diagnosis and identification of the pain source. In cadaveric testing, the degenerated disc has altered mechanics, but in vivo, disc mechanics remain unknown. To measure in vivo disc mechanics, noninvasive methods must be developed to apply and measure physiological deformations. Aim: Thus, this study aimed to develop methods to measure disc mechanical function via noninvasive MRI during flexion and extension and after diurnal loading in a young population. This data will serve as baseline disc mechanics to later compare across ages and in patients. Materials & Methods: To accomplish this, subjects were imaged in the morning in a reference supine position, in flexion, in extension, and at the end of the day in a supine position. Disc deformations and vertebral motions were used to quantify disc axial strain, changes in wedge angle, and anterior-posterior (A-P) shear displacement. T2 weighted MRI was also used to evaluate disc degeneration via Pfirrmann grading and T2 time. All measures were then tested for effect of sex and disc level. Results: We found that flexion and extension caused level-dependent strains in the anterior and posterior of the disc, changes in wedge angle, and A-P shear displacements. Flexion had higher magnitude changes overall. Diurnal loading did not cause level-dependent strains but did cause small level-dependent changes in wedge angle and A-P shear displacements. Discussion: Correlations between disc degeneration and mechanics were largest in flexion, likely due to the smaller contribution of the facet joints in this condition. Conclusion: In summary, this study established methods to measure in vivo disc mechanical function via noninvasive MRI and established a baseline in a young population that may be compared to older subjects and clinical disorders in the future.

Multiaxial validation of a finite element model of the intervertebral disc with multigenerational fibers to establish residual strain.

Finite element models of the intervertebral disc are used to address research questions that cannot be tested through typical experimentation. A disc model requires complex geometry and tissue properties to be accurately defined to mimic the physiological disc. The physiological disc possesses residual strain in the annulus fibrosus (AF) due to osmotic swelling and due to inherently pre-strained fibers. We developed a disc model with residual contributions due to swelling-only, and a multigeneration model with residual contributions due to both swelling and AF fiber pre-strain and validated it against organ-scale uniaxial, quasi-static and multiaxial, dynamic mechanical tests. In addition, we demonstrated the models' ability to mimic the opening angle observed following radial incision of bovine discs. Both models were validated against organ-scale experimental data. While the swelling only model responses were within the experimental 95% confidence interval, the multigeneration model offered outcomes closer to the experimental mean and had a bovine model opening angle within one SD of the experimental mean. The better outcomes for the multigeneration model, which allowed for the inclusion of inherently pre-strained fibers in AF, is likely due to its uniform fiber contribution throughout the AF. We conclude that the residual contribution of pre-strained fibers in the AF should be included to best simulate the physiological disc and its behaviors.

Viscoelastic heating of insulated bovine intervertebral disc.

Back pain is the leading cause of disability globally and the second most common cause of doctors' visits. Despite extensive research efforts, the underlying mechanism of back pain has not been fully elucidated. The intervertebral disc (IVD) is a viscoelastic tissue that provides flexibility to the spinal column and acts as a shock absorber in the spine. When viscoelastic materials like the IVD are cyclically loaded, they dissipate energy as heat. Thus, diurnal, regular movements of the vertebral column that deform the IVD could increase disc temperature through viscoelastic heating. This temperature rise has the potential to influence cell function, drive cell death and induce nociception in innervating nociceptive neurons within the IVD. The present study was conducted to investigate the capacity of IVD to increase in temperature due to viscoelastic heating. Insulated caudal bovine IVD were subjected to physiological cyclic uniaxial compression over a range of frequencies (0.1-15 Hz) and loading durations (1-10 min) ex vivo, and the temperature rise in the tissue was recorded. According to our findings, the IVD can experience a temperature rise of up to 2.5°C under cyclic loading. Furthermore, under similar conditions, the inner nucleus pulposus exhibits more viscoelastic heating than the outer annulus fibrosis, likely due to its more viscous composition. The measured temperature rise of the disc has physiological relevance as degenerative IVD tissue has been shown to produce a sensitization of nociceptive neurons that spontaneously fire at 37°C, with a T50 response at 37.3°C and a maximum response at 38°C. Our results suggest that viscoelastic heating of IVD could interact with sensitized nociceptive neurons in the degenerative IVD to play a role in back pain.