Aging Biology of Bone-to-Tendon Healing and the Epigenetic Clock: A Biological-Age Readout of Rotator Cuff Healing Capacity

Vollständiger Abstract

Worum geht es in dieser Arbeit?

The “unexplained failure” of rotator cuff repair is multifactorial, but its structural endpoint is anatomically consistent, i.e., the failure of the tendon-to-bone interface (enthesis) to heal. The native enthesis is a four-zone fibrocartilaginous gradient that does not regenerate but heals as a mechanically inferior fibrovascular scar, so the outcome of repair hinges on the interface’s healing capacity—which chronological age predicts poorly. This review organizes the aging biology governing bone-to-tendon healing capacity into eight domains: progenitor competence, cellular senescence and the SASP, immune aging, extracellular-matrix and collagen aging via advanced glycation end-product cross-linking, footprint angiogenesis, morphogen signaling, mechanotransduction, and bone quality. We then precisely define the DNA-methylation epigenetic clock—a continuous value produced by weighted CpG methylation, with defined units, reproducibility, and effect sizes—and propose it as a candidate quantitative readout of these domains; whether or not it truly integrates them into a single biologically meaningful measure at the enthesis is a hypothesis of this review, not an established mechanism. In 1087 twins, epigenetic age acceleration predicted fracture and osteoporosis risk, with hazard ratios of 1.29–3.17 per standard deviation; moreover, aging is tissue-specific, so the enthesis may run ahead of blood. Critically, the clock provides a single axis on which current regenerative-medicine strategies—stem cells, exosomes, immunomodulation, biomimetic gradient scaffolds, growth factors, senolytics, and epigenetic reprogramming—can be systematically categorized by how far each shifts biological age toward a healing-competent state; partial reprogramming, which rewinds the clock directly, shows that the clock is simultaneously the readout and the therapeutic target. We integrate this into a “hidden biological age of bone-to-tendon healing”, explicitly stating that this remains an unvalidated hypothesis requiring prospective validation.