Biomechanical, Structural, and Composition Characterization of the Vertebra-Disc Interface

The spinal endplate region consists of the cartilage endplate (CEP), bony endplate (BEP), and subchondral trabecular bone (STB). Although this region is often involved in vertebral fractures and disrupted with disc degeneration, the biomechanics of these tissues are incompletely understood. The goal of this study was to characterize the mechanical, structural, and compositional properties of the spinal endplate region across aging and disc degeneration.

Cadaveric human lumbar spine segments (n = 45) were imaged with magnetic resonance imaging to assess disc degeneration. BEP and STB microstructure were analyzed with micro-computed tomography. BEP specimens were subjected to four-point-bend testing, and CEP specimens to confined compression testing. BEP and STB composition was measured with ash analysis.

Several BEP mechanical properties, including flexural modulus, were associated with structural measures such as bone volume fraction, but showed few correlations to composition. Measures of structure and composition were generally different but correlated between the BEP and STB. Superior and inferior endplates differed from each other in structure but not composition. Apart from the STB microstructure, which became sparser with age in superior endplates, none of the endplate region properties correlated with age or disc degeneration, despite large ranges in values observed for many of these properties, particularly, BEP fracture strain (1%-52% strain).

Structural measures are more reliable indicators of endplate region properties than are compositional measures, age, or disc degeneration. The BEP may play an important, independent role in spine function, due to its range in ductility and limited correspondence in properties to those of neighboring tissues.