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Mechanical and structural changes to the annulus fibrosus in response to Sub-failure cyclic loading
Jack Seifert1, Lance L Frazer2, Dennis Maiman3
1Marquette University, Milwaukee, WI, USA; Medical College of Wisconsin, Milwaukee, WI, USA; Zablocki Veterans Affairs Medical Center, Milwaukee, WI, USA.
Acta Biomaterialia
|August 29, 2025
Summary
Repetitive tensile loading significantly reduced annulus fibrosus (AF) mechanical properties, but did not alter ultimate tissue strength. Structural analysis revealed elastic fiber disruption without collagen fractures, offering insights into AF degeneration.
Area of Science:
- Biomechanical Engineering
- Spine Biomechanics
- Tissue Mechanics
Background:
- The annulus fibrosus (AF) is crucial for spinal stability, yet its response to repetitive tensile loading is not fully understood.
- Degeneration and injury of the AF contribute to debilitating spinal conditions.
Purpose of the Study:
- To quantify the dose-dependent mechanical and structural changes in the annulus fibrosus (AF) under repetitive tensile loading.
- To investigate the effects of varying strain magnitudes and cycle counts on AF properties.
Main Methods:
- A three-step protocol was used: pre-damage characterization, damage induction via cyclic loading (400-12,800 cycles; 11-44% strain), and post-damage characterization.
- Mechanical properties (dynamic, viscoelastic, quasi-static) and structural integrity (histology, F-CHP staining) were assessed.
Main Results:
- Cyclic loading induced dose-dependent decreases in AF elastic and viscoelastic properties, approaching 100% reduction.
- Transition strain magnitude was affected, but ultimate mechanical properties remained unchanged.
- Structural analysis showed clefts and collagen fiber uncrimping, but no significant collagen fiber denaturation.
Conclusions:
- Repetitive tensile loading significantly impairs the dynamic and viscoelastic functions of the AF.
- Structural damage involves elastic fiber disruption, not collagen fracture, under sub-failure cyclic loading.
- Findings provide foundational data for understanding AF degeneration and injury mechanisms.
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