The results of this study confirmed the initial effect of LED treatment on tendon healing. The histology scores and immunochemistry results exhibited remarkable differences between the LED treatment and control groups.
PBM using low-level laser therapy (LLLT) or LED therapy was first introduced by Mester in the early 1960s. PBM accelerates healing by increasing cell activity by stimulating mitochondrial and cell membrane photoreceptor synthesis of ATP. Such modulations in these cells can promote fibroblast proliferation, growth factor synthesis, collagen production, and angiogenesis. Various laboratory studies and animal experiments have been conducted in the field of orthopedic surgery. Silva et al.19) reported a positive effect of LLLT/LED irradiation on tendon damage. Rosso et al.20) documented that PBM has beneficial effects on the recovery of nerve lesions. In vitro experimental findings suggest that PBM may facilitate tissue homeostasis, thus stimulating articular tissue components and promoting chondroprotective effects.
In our previous study, LED irradiation at 630 nm and 630 nm + 880 nm for 20 min was observed to strikingly affect the proliferation and migration of human tendon fibroblasts. Cell and animal experiments have revealed positive results for PBM in the field of orthopedic surgery, especially in the treatment of ligaments. Future studies are needed to derive clinical results related to this.
Ruptured Achilles tendons have been reported to exhibit marked collagen degeneration, disordered arrangement of collagen fibers, augmented cellularity, and an increase in the number of tenocytes with round nuclei.21) The failure of the intact tendon healing due to fibrotic scar formation after medical and surgical treatment, which can lead to chronic symptoms and reinjury, is a common issue1–4). Histologic analysis showed that PBM therapy improved the tendon healing through inhibition of cell density and nuclear circularity. This outcome accelerated fiber regeneration and alignment, and collagen synthesis. The tissue structure in LED-irradiated injured Achilles tendons exhibited greater parallelism, in addition to having a denser deposition of new collagen fibers than that of LED-non-irradiated injured Achilles tendons. These results indicate that LED can shorten the recovery period of the damaged tendon.
Collagen 1 is the main ECM component of tendons, whereas collagen 3, which is generally associated with scar tissue and injury, accumulated at injured tendons. The increased content of type 3 collagen can cause thinner collagen fibers, decrease the tensile strength. 22) The ratio of collagen 1 and 3 was markedly restored in the LED treated group to a level similar to the normal group. With histologic results, LED irradiation stimulates the organization and arrangement of collagen to promote healing.
Tendon injury is associated with tissue regeneration and fibrosis. TGF-β1 is activated upon tendon injury and is key in tendon healing and fibrosis.23) The expression of TGF-β1 and vimentin, markers of the presence of myofibroblasts, has been implicated in fibrosis.24) TGF-β1 and vimentin levels decreased to the normal range in the LED group at the two-week mark. Although our results do not directly reflect changes in collagen level during the early stages of healing, considering the histological and collagen level results, it is thought that the levels normalized as recovery rapidly progress.
Macrophages key regulators of the healing of injured tendons. For example, an increase in the concentration of Macrophage has been reported to play a key role in regulating the healing process of injured tendons.25) The specific function of macrophages depends on their phenotype. While the M1 phenotype macrophage exhibits a phagocytic and proinflammatory function6–9), the M2 phenotype macrophage is associated with tissue repair and deposition in inflamed tissue6, 7, 10). During tendon healing, increases in the concentration of the M2 macrophage phenotype occur later in the healing process.25)
In this study, the ratio of M2 macrophages to total macrophages increased in the LED irradiation group compared to the injured group. This increase, as driven by LED irradiation, may inhibit abnormal or excessive inflammatory responses. Therefore, LED-facilitated recovery of injured Achilles tendon is controlled through an increased differentiation toward the M2 macrophage phenotype.
Our study had several limitations. First, it was an in vivo study and owing to its limited sample size and brief duration, comprehending the cumulative healing process is deemed challenging. Second, the mechanism underlying each stage of the healing process could not be ascertained. Therefore, it is imperative to conduct further investigation into the mechanisms and impacts of each process through long-term experiments. Another major limitation of our study lies in the fact that we were not able to conduct mechanical testing in animals. In general, mechanical testing should be performed in conjunction with histology to accurately illuminate the healing trajectory (or lack thereof) of a tendon. To address this limitation in future studies, we intend to conduct mechanical testing after PBM irradiation to illuminate more accurate dynamics of tendon healing.