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Background: The immune system reflects the microbiome (microbiota). Modulation of the immune system during early tendon remodeling by dexamethasone treatment can improve rat Achilles tendon healing. The authors tested whether changes in the microbiota could influence the effect of dexamethasone treatment., Hypothesis: A change in microbiome would influence the response to dexamethasone on regenerate remodeling, specifically tendon material properties (peak stress)., Study Design: Controlled laboratory study., Methods: Specific opportunist and pathogen-free female rats were housed separately (n = 41) or together with specific pathogen-free rats carrying opportunistic microbes such as Staphylococcus aureus (n = 41). After 6 weeks, all co-housed rats appeared healthy but now carried S aureus. Changes in the gut bacterial flora were tested by API and RapID biochemical tests. All rats (clean and contaminated) underwent Achilles tendon transection under aseptic conditions. Flow cytometry was performed 8 days postoperatively on tendon tissue. Sixty rats received subcutaneous dexamethasone or saline injections on days 5 through 9 after transection. The tendons were tested mechanically on day 12. The predetermined primary outcome was the interaction between contamination and dexamethasone regarding peak stress, tested by 2-way analysis of variance., Results: Dexamethasone increased peak stress in all groups but more in contaminated rats (105%) than in clean rats (53%) (interaction, P = .018). A similar interaction was found for an estimate of elastic modulus ( P = .021). Furthermore, dexamethasone treatment reduced transverse area but had small effects on peak force and stiffness. In rats treated with saline only, contamination reduced peak stress by 16% ( P = .04) and elastic modulus by 35% ( P = .004). Contamination led to changes in the gut bacterial flora and higher levels of T cells (CD3 + CD4 + ) in the healing tendon ( P < .05)., Conclusion: Changes in the microbiome influence tendon healing and enhance the positive effects of dexamethasone treatment during the early remodeling phase of tendon healing., Clinical Relevance: The positive effect of dexamethasone on early tendon remodeling in rats is strikingly strong. If similar effects could be shown in humans, immune modulation by a few days of systemic corticosteroids, or more specific compounds, could open new approaches to rehabilitation after tendon injury.

Inflammation initiates tendon healing and then normally resolves more or less completely. Unresolved inflammation might disturb the remodeling process. We hypothesized that suppression of inflammation during the early remodeling phase by systemic dexamethasone treatment can improve healing. 36 rats underwent Achilles tendon transection and were randomized to dexamethasone or saline on days 0-4 after surgery (early inflammatory phase), and euthanasia day 7. Another 54 rats received injections days 5-9 (early remodeling phase) and were euthanized day 12 for mechanical, histological and flow cytometric evaluation. Dexamethasone treatment days 0-4 reduced the cross-sectional area, peak force and stiffness by day 7 to less than half (p < 0.001 for all), while material properties (peak stress and elastic modulus) were not significantly affected. In contrast, dexamethasone treatment days 5-9 increased peak force by 39% (p = 0.002) and stiffness by 58% (p < 0.001). The cross-sectional area was reduced by 42% (p < 0.001). Peak stress and elastic modulus were more than doubled (p < 0.001 for both). Semi-quantitative histology at day 12 showed that late dexamethasone treatment improved collagen alignment, and flow cytometry revealed reduced numbers of CD8a + cytotoxic T cells in the tendon callus. These results suggest that downregulation of lingering inflammation during the early remodeling phase can improve healing.

Early tendon healing can be stimulated by mechanical loading and inhibited by cyclooxygenase (COX) inhibitors (nonsteroidal anti-inflammatory drugs). Therefore, we investigated if impairment of tendon healing by a COX-2 inhibitor (parecoxib) is related to loading. Because loading might infer microdamage, which also stimulates healing, we also investigated if this effect is inhibited by parecoxib. The Achilles tendon was transected in 114 rats. Three degrees of loading were used: full loading, partial unloading, and unloading (no unloading, Botox injections in the plantar flexor muscles, or Botox in combination with tail suspension). For each loading condition, the rats received either parecoxib or saline. In a second experiment, rats were unloaded with Botox, and the tendon was subjected to microdamage by needling combined with either saline or parecoxib. Mechanical testing day 7 showed that there was a significant interaction between loading and parecoxib for peak force at failure (P < 0.01). However, logarithmic values showed no significant interaction, meaning that we could not exclude that the inhibitory effect of parecoxib was proportionate to the degree of loading. Microbleeding was common in the healing tissue, suggesting that loading caused microdamage. Needling increased peak force at failure (P < 0.01), and this effect of microdamage was almost abolished by parecoxib (P < 0.01). Taken together, this suggests that COX-2 inhibition impairs the positive effects of mechanical loading during tendon healing, mainly by reducing the response to microdamage. [ABSTRACT FROM AUTHOR]