Mechanical cues affect tendon healing, homeostasis, and development in a variety of settings. Alterations in the mechanical environment are known to result in changes in the expression of extracellular matrix proteins, growth factors, transcription factors, and cytokines that can alter tendon structure and cell viability. Loss of muscle force in utero or in the immediate postnatal period delays tendon and enthesis development. The response of healing tendons to mechanical load varies depending on anatomic location. Flexor tendons require motion to prevent adhesion formation, yet excessive force results in gap formation and subsequent weakening of the repair. Excessive motion in the setting of anterior cruciate ligament reconstruction causes accumulation of macrophages, which are detrimental to tendon graft healing. Complete removal of load is detrimental to rotator cuff healing, yet large forces are also harmful. Controlled loading can enhance healing in most settings; however, a fine balance must be reached between loads that are too low (leading to a catabolic state) and too high (leading to micro-damage). This review will summarize existing knowledge of the mechanobiology of tendon development, homeostasis, and healing.

The attachment between tendon and bone occurs across a complex transitional tissue that minimizes stress concentrations and allows for load transfer between muscles and skeleton. This unique tissue cannot be reconstructed following injury, leading to high incidence of recurrent failure and stressing the need for new clinical approaches. This review describes the current understanding of the development and function of the attachment site between tendon and bone. The embryonic attachment unit, namely, the tip of the tendon and the bone eminence into which it is inserted, was recently shown to develop modularly from a unique population of Sox9- and Scx-positive cells, which are distinct from tendon fibroblasts and chondrocytes. The fate and differentiation of these cells is regulated by transforming growth factor beta and bone morphogenetic protein signaling, respectively. Muscle loads are then necessary for the tissue to mature and mineralize. Mineralization of the attachment unit, which occurs postnatally at most sites, is largely controlled by an Indian hedgehog/parathyroid hormone-related protein feedback loop. A number of fundamental questions regarding the development of this remarkable attachment system require further study. These relate to the signaling mechanism that facilitates the formation of an interface with a gradient of cellular and extracellular phenotypes, as well as to the interactions between tendon and bone at the point of attachment.

The attachment of dissimilar materials is a major engineering challenge, yet this challenge is seemingly overcome in biology. This study aimed to determine how the transcription factor Scleraxis (Scx) influences the development and maturation of the tendon-to-bone attachment (enthesis ] littermates were killed at postnatal days 7-56 (P7-P56). Enthesis morphometry, histology, and collagen alignment were investigated throughout postnatal growth. Enthesis tensile mechanical properties were also assessed. Laser microdissection of distinct musculoskeletal tissues was performed at P7 for WT, cKO, and muscle-unloaded (botulinum toxin A treated) attachments for quantitative PCR. cKO mice were smaller, with altered bone shape and impaired enthesis morphology, morphometry, and organization. Structural alterations led to altered mechanical properties; cKO entheses demonstrated reduced strength and stiffness. In P7 attachments, cKO mice had reduced expression of transforming growth factor (TGF) superfamily genes in fibrocartilage compared with WT mice. In conclusion, deletion of Scx led to impairments in enthesis structure, which translated into impaired functional (i.e., mechanical) outcomes. These changes may be driven by transient signaling cues from mechanical loading and growth factors.-Killian, M. L., Thomopoulos, S. Scleraxis is required for the development of a functional tendon enthesis. FASEB J. 30, 301-311 (2016). www.fasebj.org

Background Chronic rotator cuff tears present a clinical challenge, often with poor outcomes after surgical repair. Degenerative changes to the muscle, tendon, and bone are thought to hinder healing after surgical repair; additionally, the ability to overcome degenerative changes after surgical repair remains unclear. Purpose/Hypothesis The purpose of this study was to evaluate healing outcomes of muscle, tendon, and bone after tendon repair in a model of chronic rotator cuff disease and to compare these outcomes to those of acute rotator cuff injuries and repair. The hypothesis was that degenerative rotator cuff changes associated with chronic multitendon tears and muscle unloading would lead to poor structural and mechanical outcomes after repair compared with acute injuries and repair. Study Design Controlled laboratory study. Methods Chronic rotator cuff injuries, induced via detachment of the supraspinatus (SS) and infraspinatus (IS) tendons and injection of botulinum toxin A into the SS and IS muscle bellies, were created in the shoulders of rats. After 8 weeks of injury, tendons were surgically reattached to the humeral head, and an acute, dual-tendon injury and repair was performed on the contralateral side. After 8 weeks of healing, muscles were examined histologically, and tendon-to-bone samples were examined microscopically, histologically, and biomechanically and via micro–computed tomography. Results All repairs were intact at the time of dissection, with no evidence of gapping or ruptures. Tendon-to-bone healing after repair in our chronic injury model led to reduced bone quality and morphological disorganization at the repair site compared with acute injuries and repair. SS and IS muscles were atrophic at 8 weeks after repair of chronic injuries, indicating incomplete recovery after repair, whereas SS and IS muscles exhibited less atrophy and degeneration in the acute injury group at 8 weeks after repair. After chronic injuries and repair, humeral heads had decreased total mineral density and an altered trabecular structure, and the repair had decreased strength, stiffness, and toughness, compared with the acute injury and repair group. Conclusion Chronic degenerative changes in rotator cuff muscles, tendons, and bone led to inferior healing characteristics after repair compared with acute injuries and repair. The changes were not reversible after repair in the time course studied, consistent with clinical impressions. Clinical Relevance High retear rates after rotator cuff repair are associated with tear size and chronicity. Understanding the mechanisms behind this association may allow for targeted tissue therapy for tissue degeneration that occurs in the setting of chronic tears.

Rotator cuff tears can cause irreversible changes (e.g., fibrosis) to the structure and function of the injured muscle(s). Fibrosis leads to increased muscle stiffness resulting in increased tension at the rotator cuff repair site. This tension influences repairability and healing potential in the clinical setting. However, the micro- and meso-scale structural and molecular sources of these whole-muscle mechanical changes are poorly understood. Here, single muscle fiber and fiber bundle passive mechanical testing was performed on rat supraspinatus and infraspinatus muscles with experimentally induced massive rotator cuff tears (Tenotomy) as well as massive tears with chemical denervation (Tenotomy+BTX) at 8 and 16 weeks post-injury. Titin molecular weight, collagen content, and myosin heavy chain profiles were measured and correlated with mechanical variables. Single fiber stiffness was not different between controls and experimental groups. However, fiber bundle stiffness was significantly increased at 8 weeks in the Tenotomy+BTX group compared to Tenotomy or control groups. Many of the changes were resolved by 16 weeks. Only fiber bundle passive mechanics was weakly correlated with collagen content. These data suggest that tendon injury with concomitant neuromuscular compromise results in extracellular matrix production and increases in stiffness of the muscle, potentially complicating subsequent attempts for surgical repair.