Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

INTRODUCTION: Almost one in five people will suffer from osteoporotic vertebral fracture. Accurate fracture risk prediction is clinically important to facilitate decision-making in preventative measures and treatments. Bone mineral density (BMD) is the most commonly used surrogate of strength, however, trabecular strain has recently been revealed as a more accurate metric [1]. Although it has been shown that trabecular strains may be influenced by the presence of intervertebral discs (IVDs) [2,3], the effect of disc degeneration on these strains is unknown. Before trabecular strain can be considered a useful metric for predicting osteoporotic vertebral fracture, a deeper understanding of typical magnitudes and distributions, and how they are influenced by disc degeneration, is required.

AIMS: The aim of this study was to assess the influence of disc degeneration on strain and stress distributions in human lumbar vertebrae.

METHODS: “Degenerated” (average Pfirrmann grade ≥ 3) and “Non-degenerated” (average Pfirrmann grade ≤ 2) bisegment specimens (N = 3 per group) were axially loaded to 1000N (Figure 1A). Digital volume correlation (DVC) combined with µCT was used to quantify axial, principal, shear, and Von Mises trabecular strains of the middle vertebra. Quantitative CT was acquired to extract volumetric BMD, from which Young’s modulus was calculated and then registered with the Von Mises strain field to calculate Von Mises stress. Regional differences were assessed between the anterior, posterior, lateral, core, superior, and inferior regions of the vertebra. The stresses in the superior and inferior endplate regions adjacent to degenerated (N = 5) and non-degenerated (N = 7) IVDs were additionally compared.

RESULTS: Degenerated specimens displayed significantly lower BMD (-61.8±9.5 mg/cm³) relative to Non-degenerated, and exhibited significantly higher peak axial strains (4270±1460 vs 2670±1040 µS, p<0.01) and minimum principal strains (1550±350 vs 1020±100 µS, p<0.01) (Figure 1B,C). Degenerated specimens also showed significantly greater strain heterogeneity, with 1.4x higher anterior strains relative to posterior, and 1.7x greater endplate-region strains relative to the core. However, Degenerated specimens displayed significantly lower Von Mises stress (0.44±0.39 vs 1.00±0.73 MPa, p<0.01). Stresses in the endplate region adjacent to degenerated IVDs tended to be more homogenous, with peaks in the anterior peripheral regions (Figure 1D), whilst stresses peaked at the core of endplate regions adjacent to Non-degenerated IVDs (Figure 1E).

DISCUSSION: This study demonstrates the potential of using DVC and BMD measures to quantify internal trabecular stresses in vitro for the first time. The low Von Mises stresses seen in Degenerated specimens support the idea that coupling osteoporosis and non-degenerated IVDs is the “worst case scenario” for fracture risk [4], perhaps arising from the high hydrostatic pressure of the nucleus pulposus. However, increased anterior strain in Degenerated specimens may be explained by anterior-region bone resorption, following from reduced disc height and increased facet load-bearing [5]. These findings demonstrate that both vertebral strains and stresses may be useful metrics to supplement vertebral fracture risk prediction in osteoporotic patients, and confirms that disc degeneration influences how stresses and strains are distributed within vertebrae.