Current challenges and future directions in Achilles tendon rupture repair: A biomechanical perspective

The Achilles tendon (AT) fulfills critical physiological functions, such as transmitting plantar flexor forces, maintaining ankle stability, and buffering movement-related loads. Its exceptional mechanical performance stems from a multiscale hierarchical system, comprising macroscopic twisting tripartite subtendons, microscopic hierarchical fibers, and nanoscopic quarter-staggered and cross-linked collagen. Structural complexity leads to poor vascularity and limited self-repair capacity after Achilles tendon rupture (ATR), resulting in high re-rupture rates and inadequate mechanical restoration with conventional treatment. Current tissue engineering (TE) scaffolds offer promising alternatives, but face challenges such as unclear correlations between multiscale structures (twisting subtendons, fiber arrangement, collagen cross-linking) and mechanical behaviors (nonlinearity, negative Poisson's ratio) and inadequate cross-scale structure-mechanics characterizations and analyses, hindering precise scaffold design and recapitulation of the structural and mechanical integrity of native AT. This review systematically summarizes AT's macro-micro-nano structural characteristics, mechanical behaviors, and relevant characterizations, focuses on biomechanics-based TE scaffold strategies (structural design, biomaterial optimization, fabrication), analyzes existing limitations (insufficient quantification of cross-scale correlations, difficulty in balancing multiscale topology, and mechanical compatibility), and proposes future research directions centered on refining cross-scale quantitative laws and developing multiscale synergistic biomimetic scaffolds. This review provides the theoretical and technical references for innovative ATR repair research by integrating interdisciplinary advances in biomechanics, materials science, and TE.