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. 2021 Nov 26;7(48):eabi5584. doi: 10.1126/sciadv.abi5584

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Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

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Fig. 5. — (A) Ten-week-old mice were subjected to two degeneration models: Underuse degeneration was induced via muscle paralysis, and overuse degeneration was achieved through downhill treadmill running for 4 weeks. (B) Postfailure contrast-enhanced microCT imaging revealed that pathological entheses exhibited exclusively avulsion-type failures under tensile mechanical testing. Scale bars, 500 μm. (C to J), Physiological in vivo degeneration models reduced the ability of the enthesis to protect against failure. (D) Failure area, (E) avulsed fragment quantity, and (F) failure interfaces were affected by enthesis pathology. Underuse degeneration led to (G) lower strength (P < 0.01) and (H) lower stiffness (P < 0.05) and (I) trended towards decreased toughness (P = 0.075) compared to that of control. Overuse degeneration decreased (J) tendon cross-sectional area (P < 0.01), (H) stiffened the enthesis (P < 0.01), and (I) significantly reduced toughness compared to control (P < 0.05). (K and L) Bone morphometric analysis revealed that underuse led to (K) reduced bone volume (BV/TV) (P < 0.0001) and (L) reduced bone mineral density (BMD) in the bone underlying the attachment (P < 0.0001). (M) The volume of load-bearing trabecular plates (pBV/TV) increased significantly (P < 0.0001) because of overuse and decreased significantly (P < 0.0001) because of underuse, with significant changes in their (N) orientations (P < 0.01, two-way ANOVA followed by Dunnet’s multiple comparison test). (O) Enthesis strength correlated with BMD (R = 0.60, P < 0.001), cortical thickness (R = 0.69, P < 0.001), and trabecular plate thickness (R = 0.44, P < 0.001). Enthesis toughness correlated with tendon cross-sectional area (R = 0.44, P < 0.01, Pearson correlation). (*P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001, ANOVA followed by the Dunnett’s multiple comparison test unless otherwise reported).