The finding of our cohort study showed that in a difficult-to-treat patient population with complex perianal fistula, a 53.3% healing rate was achieved using Obsidian RFT®. Additionally, our data confirmed the safety of this therapy with no observed serious adverse events in the perioperative period.

Currently, no standard treatment exists in dealing with complex anal fistula. Several surgical techniques have been introduced in recent years, aiming to be effective, minimally invasive and sphincter preserving. Previous more invasive techniques, such as endoanal advancement flap repair, are still commonly conducted but do not guarantee healing in all patients. Additionally, impairment of anal incontinence could be observed in 10–35% of operated patients [2, 18].

Therefore, it is critical in the treatment of complicated perianal fistulas to develop innovative procedures and improve existing methods to optimize outcomes and minimize the impact on anal continence [19, 20].

The application of fibrin sealant for the management of postoperative enterocutaneous fistulas was initially documented in the 1980s [21]. Subsequently, fibrin glue was also implemented in the surgical management of anal fistula because of its easy use and the potential advantage of preservation of incontinence. Successful closure of the fistula showed a wide range between 14 and 86% [22,23,24,25]. Concurrently, platelet-rich fibrin, owing to its high concentration of growth factors and its capacity to orchestrate crucial processes in tissue regeneration, such as cell proliferation, chemotaxis (directed cell migration), cell differentiation and extracellular matrix synthesis, has gained further attention as a regenerative tissue modality with potentially better results [8,9,10,11,12,13].

In a prospective double-blind randomized study, De La Portilla et al. demonstrated that the therapeutic efficacies of two procedures, autologous platelet-rich plasma and autologous fibrin, were equivalent, with no observed adverse events. However, both procedures revealed recurrence rates of 33.3% and 31.3%, respectively [26].

Pérez Lara et al. reported on their prospective longitudinal multicentre study of anal fistula therapy with platelet-rich fibrin in 60 patients. A success rate of 66.6% was achieved after a 24-month follow-up [5]. In contrast to our study, patients with Crohn's disease were excluded from the study population.

Another study with only a small number (n = 10) of enrolled patients showed favourable results for anal fistula treatment with autologous platelet-rich plasma and platelet-rich fibrin glue [27]. Abdollahhi et al., who used autologous platelet-rich plasma (PRP) as well as platelet-rich fibrin glue (PRFG), showed a healing rate in 60% of patients. In contrast to other studies, 2 ml of PRP was injected into the tissue around the fistula (the depth of penetration at injection was 5–6 mm), and 4 ml of PRFG was mixed with 1 ml of thrombin and injected into the tract [27].

Van der Hagen et al. presented the therapeutic concept of injecting autologous platelet-rich plasma as an adjunct to a mucosal advancement flap in 10 patients. They observed a recurrence in only one patient [28].

Unlike the aforementioned studies, Perez et al. introduced a treatment approach involving the administration of platelet-rich fibrin into the fistula, omitting the closure of the internal orifice. This method was performed exclusively on an outpatient basis using anesthesia [29]. They reported a healing rate solely by in fistula application of 52.86% non-Crohn patients with an average of 1.92 sealant procedures.

Closure of the internal fistula opening is considered a logical healing approach to keep the fistula track clean in terms of preventing bacterial contamination. Controlled randomized clinical studies in the placebo group showed that the closure of the internal anal fistula opening alone can produce a healing value of approximately 36% in Crohn's patients [30].

In our results, a longer operation time and a prolonged hospital stay were associated with significantly less favourable outcomes, which may indicate that a shorter operation time reflects an easier procedure. Parameters indicating a more simple operation could not be determined as no differences in the complexity of the fistulas, experience of the surgeons or other factors were detected.

The analysis of our results also showed that patients < 42.7 years old had a better outcome in contrast to patients with of older age. It could be speculated that certain factors contribute to this finding. The number and activity of stem cells in the skin decrease with age, which can significantly hamper the skin's ability to regenerate [31]. Additionally, aging leads to a decline in the production of essential growth factors, contributing to slower wound healing processes. The structural integrity of the skin deteriorates, with the epidermal and dermal layers becoming thinner and less elastic, making the skin more susceptible to injury and slowing recovery [31]. Furthermore, chronic diseases such as diabetes and vascular disorders, which are more prevalent in older populations, impair blood flow and nutrient delivery to wound sites, exacerbating healing difficulties.

When interpreting the results of the present study several limitations need to be considered. The sample size of included patients is small. and due its retrospective study design, selection bias cannot be ruled out.

Although the inclusion of Crohn's patients leads to a higher heterogeneity of the treated population, it shows even more that a good cure rate is associated with Obsidian RFT despite more difficult systematic circumstances.

However, considering the lack of clinical therapeutic options for sphincter-preserving methods, we still believe that we can further provide important results to more clearly define the role of Osidian RFT® in anal fistula treatment.