Significant damage to the VF, including disruption of anatomical structures, can occur in many ways, including (prolonged) intubation or VF surgery. The resulting tissue damage leads to the release of a variety of chemoattractants and factors, such as danger-associated molecular patterns (DAMPs). These play a key role in initiating wound healing. Ultimately, this can lead to loss of physiological organ architecture and tissue fibrosis with the aforementioned reduction in quality of life when the VF is involved [
18,
19]. Improving wound healing and ideally preventing fibrosis is, therefore, of paramount importance. The purpose of this study was to evaluate whether a single topical application of HA (with or without diclofenac) immediately after injury has a beneficial effect on postoperative VF wound healing.
Wound healing
Wound healing can be divided into three partially overlapping phases: inflammatory, proliferative, and tissue remodeling. During the inflammatory phase, immune cells migrate to the site of injury to remove cellular debris and pathogens [
3]. In the proliferative phase, fibroblasts produce new ECM to provide structural support for the developing tissue. Finally, during the remodeling phase, excess ECM components are degraded, refining the tissue and forming a mature scar [
20,
21].
The foundation of wound healing is laid during the inflammatory phase, as dysregulation can lead to chronic inflammation and impaired wound healing [
22]. It is, therefore, particularly attractive to target this phase to improve the functional properties of the remodeled tissue. Indeed, many approaches have been investigated to improve the oscillatory properties of the VF after injury. However, none of these approaches has achieved the desired effect [
4,
23]. As a result, there is currently no agent applied in clinical routine in laryngology.
Inflammatory reaction
The inflammatory phase involves hemostasis and an inflammatory response. Studies have shown that the destruction of keratinocytes leads to the release of IL-1, the body's first signal after tissue injury. This leads to the activation of platelets and the release of other factors, such as growth factors, which are designed to attract more inflammatory cells and thus disarm the infectious agents [
19,
24]
In the present work, a significant upregulation of
Has1,
Ptgs2,
Il1b and
Il10 was seen in all three injury groups compared to the uninjured control group. Similarly, Welham et al. reported a significant upregulation of
Has1,
Ptgs2,
Il1b and
Tnfa after 1 h of injury [
2]. The fact that
Il1b,
Il10 and
Ptgs2 were significantly upregulated, suggests that the VF injury model in the present study was functional. Although the other inflammatory markers measured (
Ptgs1,
Tnfa,
Tgfb1,
Nfkb,
Ifng) are also described as key regulators in the inflammatory phase and would, therefore, be expected to be upregulated, they were not significantly in these experiments [
25]. We attribute this to the short observation period.
Tissue repair and remodeling occurs at later stages of wound healing, which explains why there was no significance in the expression of either
COL or
ELN. The increase of
Has expression was, therefore, interesting, as this enzyme is responsible for the production of HA. However, HA is not only a scaffolding protein but plays an important role in immunomodulation and angiogenesis and is, therefore, also relevant at an early stage [
26].
Effects of topical treatment
From a risk–benefit perspective, topical therapy is the most promising, as no systemic side effects are to be expected. Akdogan et al. investigated the effect of topical retinoic acid on VF wound healing in rabbits. They showed that there was a significant reduction in both collagen and fibroblast deposition in the retinoic acid-treated group [
18]. However, local irritation has been reported in dermatologic studies, where it is used to treat acne vulgaris [
27]. In VF, this side effect can be serious and may lead to significant deterioration of the voice. Therefore, this agent does not appear to be suitable for the treatment of VF scars in humans.
Another study investigated the effect of HA collagen nanofibers on early wound healing. Twelve New Zealand white rabbits underwent unilateral VF injury. The six rabbits in the control group received superficial saline treatment using cotton swabs for three consecutive days, while the other six rabbits were treated with an HA–collagen mixture for three consecutive days. Half of the rabbits in each group (treatment and control) were then sacrificed on day 7, while the other half were euthanized on day 21. Based on a significant reduction in collagen fiber diameter on day 21 in the treatment group, they concluded that topical application of HA–collagen could lead to reduced scar tissue formation [
28]. Although an interesting approach, this study may lack statistical power due to the small number of animals in each group. In addition, the sequential application on three consecutive days as described in this publication may not be feasible in clinical routine as the majority of awake patients would probably not tolerate topical treatment and repeated anesthesia for this purpose would simply not be justifiable. Nevertheless, HA appears to be a promising agent for improving VF wound healing and was used in the present study for the reasons described in more detail in the following paragraphs.
HA is a ubiquitous polysaccharide that plays a key role in wound healing and is widely used in other areas of medicine due to its high biocompatibility and low cost [
29]. HA is known to inhibit the profibrotic effect of TGFβ1 via the Smad signaling pathway [
30]. This may explain the significant downregulation of
Tgfb1 in the present study by the addition of HA to the wound. TGF-β1 plays a critical role in the inflammatory phase of wound healing and excess can lead to hypertrophic scars [
31]. Targeting TGF-β1 is, therefore, a popular way to improve wound healing. Studies have shown that inhibition of TGF-β improves wound closure and reduces fibrosis [
32,
33]. Therefore, the gene downregulation of
Tgfb1 demonstrated in this study may provide a favorable environment for wound healing.
Our results showed that
Eln, which is also partially regulated by Smad [
34], was significantly downregulated by the addition of HA. ELN is urgently needed for tissue repair, but it appears that the composition of the cross-links is more important than the concentrations themselves. [
35] Based on the available results, it is not possible to make a conclusive assessment in this regard and further investigation is required.
In the presence of HA,
Has was downregulated, but only
Has3 was significant. This may suggest a negative feedback mechanism to maintain HA homeostasis. According to Tammi et al. the relationship between HA concentration and
Has expression is poorly understood. This may be due to technical difficulties in reliably detecting HAS at the protein level [
36]. Therefore, this feedback mechanism can currently not be demonstrated in a healthy population.
In the group, where a mixture of HA and diclofenac was applied to the wound, no significant changes were observed compared to the injury group. Diclofenac belongs to the group of NSAIDs and exerts its anti-inflammatory effect through COX inhibition. However, the effect of topical diclofenac on wound healing is controversial. While the anti-inflammatory effect may be beneficial, histomorphometric studies have shown a reduction in fibroblasts, which may be associated with impaired wound healing [
37,
38]. The lack of significance with the addition of diclofenac in the present study could have several reasons. These include the short observation period, the low dose, or an actual lack of effect.
Limitations
There are some limitations to this study. One is that the observation period was very short. To study the effect of the drugs over a longer period of time, the animals would have had to be awakened from anesthesia. This was impossible, because the gel would have blocked their airways and they would have suffocated. Therefore, it is not possible to draw definitive conclusions about the effects of the local therapy used here on the fibrotic processes and thus on the vibrational properties of the VF. One way to overcome these problems in the future would be to use larger animals. Rats were used in this work, because they have a very similar structure to the human VF [
39]. At the cost of greater histological differences, other animal models (e.g., sheep) could more easily be used to study long-term effects. Another limitation is that only changes at the mRNA level were examined. This was also due to the small size of the rat larynx, as only very little tissue could be obtained. This could have been achieved by increasing the number of animals or switching to another animal model. The latter would be preferable, because it would require fewer animals, taking into account the principle of 3Rs (reduce, replace, refine).