Different mechanisms activated by mild versus strong loading in rat Achilles tendon healing

Abstract

BackgroundMechanical loading stimulates Achilles tendon healing. However, various degrees of loading appear to have different effects on the mechanical properties of the healing tendon, and strong loading might create microdamage in the tissue. This suggests that different mechanisms might be activated depending on the magnitude of loading. The aim of this study was to investigate these mechanisms further.MethodsFemale rats had their right Achilles tendon cut transversely and divided into three groups: 1) unloading (calf muscle paralysis by Botox injections, combined with joint fixation by a steel-orthosis), 2) mild loading (Botox only), 3) strong loading (free cage activity). Gene expression was analyzed by PCR, 5 days post-injury, and mechanical testing 8 days post-injury. The occurrence of microdamage was analyzed 3, 5, or 14 days post-injury, by measuring leakage of injected fluorescence-labelled albumin in the healing tendon tissue.ResultsPeak force, peak stress, and elastic modulus of the healing tendons gradually improved with increased loading as well as the expression of extracellular matrix genes. In contrast, only strong loading increased transverse area and affected inflammation genes. Strong loading led to higher fluorescence (as a sign of microdamage) compared to mild loading at 3 and 5 days post-injury, but not at 14 days.DiscussionOur results show that strong loading improves both the quality and quantity of the healing tendon, while mild loading only improves the quality. Strong loading also induces microdamage and alters the inflammatory response. This suggests that mild loading exert its effect via mechanotransduction mechanisms, while strong loading exert its effect both via mechanotransduction and the creation of microdamage.ConclusionIn conclusion, mild loading is enough to increase the quality of the healing tendon without inducing microdamage and alter the inflammation in the tissue. This supports the general conception that early mobilization of a ruptured tendon in patients is advantageous.

Early Growth Response Genes Increases Rapidly After Mechanical Overloading and Unloading in Tendon Constructs

“…Furthermore, induced tissue damage by needling can also lead to increased levels of EGR1 and EGR2 . Thereby, the increased expression of EGR1 and EGR2 after loading in rats might be due to induced microtrauma and small tissue damage and not due to mechanotransduction.…”

“…Treadmill exercise of rats with healing Achilles tendons have been shown to induce an increased expression of both EGR1 and EGR2 . The tissue, during early tendon healing is weak as why unrestricted walking and full loading can induce micro damages and small bleedings in the tissue . Skin wounding in mice can induce an increased expression of EGR1 and EGR2 , which are believed to work as kick‐start genes for the healing response .…”

Response to mechanical loading in rat Achilles tendon healing is influenced by the microbiome

“…We have previously used 8 days of healing for several studies on unloaded and loaded healing tendons [6,7]. However, 8 days is only a few days after the inflammatory phase of healing, which last approximately 3-5 days in rats [11].…”

“…In a rat Achilles tendon healing model, complete load protection can reduce the strength of the healing tendon by a factor of 5 [6]. The magnitude of loading appears to activate different mechanisms which will ultimately lead to different effects on the mechanical properties [7]. The Achilles tendon heals through callus formation and the initial tissue contains weak matrix with primarily leukocytes infiltrating this matrix [8].…”

“…The Achilles tendon heals through callus formation and the initial tissue contains weak matrix with primarily leukocytes infiltrating this matrix [8]. Full loading, when applied to this matrix, has been shown to induce microdamage and alter the immune cell composition and ultimately promote a pro-inflammatory response [7,9]. Hence, in part, the response to mechanical loading seems to be closely connected to immune cell reactions and thereby possibly to changes in the microbiome [8][9][10].…”