TomoSAXS: fibril-to-fibre mechanics in intact spinal discs

A research team has introduced TomoSAXS, a full-field 3D small-angle X-ray scattering tomography that maps mechanics from the fibril scale to the fibre scale across an intact tissue, according to the paper cited at doi.org (Nature, 2026) doi.org. The method was applied to intact rat intervertebral discs (IVDs) under physiologically relevant loading. It was then extended to intact human IVDs in static conditions doi.org.

The IVD has a central nucleus pulposus, a hydrated, proteoglycan-rich matrix that redistributes mechanical loads. It is surrounded by the annulus fibrosus, a fibrous outer ring that provides structural support doi.org. In the annulus, concentric lamellae tens of microns thick consist of collagen fibres 1–20 μm in diameter, with alternating orientations in adjacent lamellae. This architecture is believed to convert hydrostatic pressure from the nucleus into circumferential tensile stress doi.org.

How TomoSAXS combines two scales

TomoSAXS is a correlative, multiscale approach. It integrates phase-contrast computed tomography (pCT) at the microscale with small-angle X-ray scattering (SAXS) at the nanoscale doi.org. The pCT images allow individual collagen fibres to be traced and coloured by their lateral angle. SAXS patterns recorded along the X-ray beam path through the disc carry nanoscale information on the collagen fibrils doi.org. The D-period relates to the peak position along the radial axis. Fibril diameter is inversely related to the peak width along the circumferential axis. D-period variation is inversely related to the peak width along the radial axis doi.org.

Strain patterns in the rat disc

The authors report that intrafibrillar molecular pre-strain, the D-period stagger, is lamellar textured and tightly correlated with microscale fibre strain doi.org. Pre-strain is inversely related to fibril strain and to its variability, which the authors describe as consistent with load-sharing through molecular unwinding doi.org.

Radial strain bridges and high-curvature zones at the interfaces between the annulus fibrosus and the nucleus pulposus emerge as critical regulators of local mechanics doi.org. According to the authors, these findings reveal concerted fibril–fibre interactions that sustain mechanical equilibrium in the IVD and preserve its elasticity and shape doi.org.

The paper notes that IVD injury and age-related degeneration disrupt the disc's mechanical environment and are a key driver of back pain. It adds that the development of synthetic disc replacements is hindered by an incomplete understanding of structure–function relationships across length scales doi.org. The authors state that TomoSAXS can serve as a platform for visualizing matrix mechanics in biological and synthetic fibrous materials. They list possible applications in ageing and disease, therapeutic evaluation, and the design of bio-based and bio-inspired materials doi.org.

TomoSAXS maps fibril-to-fibre mechanics in intact rat intervertebral discs under load

How TomoSAXS combines two scales

Strain patterns in the rat disc

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