Dynamic Loading and Tendon Healing Affect Multiscale Tendon Properties and ECM Stress Transmission

Abstract

The extracellular matrix (ECM) is the primary biomechanical environment that interacts with tendon cells (tenocytes). Stresses applied via muscle contraction during skeletal movement transfer across structural hierarchies to the tenocyte nucleus in native uninjured tendons. Alterations to ECM structural and mechanical properties due to mechanical loading and tissue healing may affect this multiscale strain transfer and stress transmission through the ECM. This study explores the interface between dynamic loading and tendon healing across multiple length scales using living tendon explants. Results show that macroscale mechanical and structural properties are inferior following high magnitude dynamic loading (fatigue) in uninjured living tendon and that these effects propagate to the microscale. Although similar macroscale mechanical effects of dynamic loading are present in healing tendon compared to uninjured tendon, the microscale properties differed greatly during early healing. Regression analysis identified several variables (collagen and nuclear disorganization, cellularity, and F-actin) that directly predict nuclear deformation under loading. Finite element modeling predicted deficits in ECM stress transmission following fatigue loading and during healing. Together, this work identifies the multiscale response of tendon to dynamic loading and healing, and provides new insight into microenvironmental features that tenocytes may experience following injury and after cell delivery therapies.

Tendon Biomechanics and Crimp Properties Following Fatigue Loading Are Influenced by Tendon Type and Age in Mice

“…Previously, we demonstrated that fatigue loading increased the crimp amplitude across the tendon width and length, and these structural alterations were shown to be both region and load dependent . Later work discovered that fatigue loading negatively altered tendon macroscale mechanical and structural properties . At the microscale, fatigue loading abrogated collagen and nuclear reorganization with applied strain, leading to reduced nuclear strain transfer and deficits in ECM stress transmission .…”

“…Tendons are known to become more prone to injury and rupture during aging, yet, the biomechanical underpinning of this clinical presentation remains poorly understood . The ability of tendon to maintain homeostasis following fatigue loading may be affected by restoration of native multiscale strain transfer mechanisms . The differential mechanical and structural response between tendon type and age to fatigue loading may therefore have important implications for multiscale strain transfer and ECM stress transmission.…”

“…Later work discovered that fatigue loading negatively altered tendon macroscale mechanical and structural properties . At the microscale, fatigue loading abrogated collagen and nuclear reorganization with applied strain, leading to reduced nuclear strain transfer and deficits in ECM stress transmission . Recent work investigating aging in craniofacial tendons in zebrafish showed that the overall decline in the tendon modulus of aged tendons is attributable primarily to the loss of non‐linearity in the stress–strain curve of aged tendons compared with their young counterparts .…”

“…Recent work has taken advantage of techniques in which the whole tissue can be analyzed using polarized‐light imaging integrated into the mechanical testing apparatus . In this and our previous work, we advance the use of this non‐destructive, ex‐vivo imaging method for analyzing fascicle‐level crimp during fatigue testing, which has been shown to be a sensitive metric to assess a tendon's response to cyclic loading …”

“…Tendinopathy affects many tendon types, with magnetic resonance imaging‐defined patellar tendinopathy in one study having a prevalence of 28.3% . Although the underlying pathophysiology of tendinopathy is not completely defined, the various hierarchical structures of tendon may be prone to damage from the repetitive application of load, and as such contribute in this degenerative process . The ultrastructural characteristic waviness of collagen, termed crimp, is one of the emergent properties of tendon that may be important in the development of fatigue‐induced sub‐rupture damage .…”