Anal fistula is a sequela of the abscess ulceration or incision drainage that occurs around the anus and rectum, which is manifested as the formation of abnormal channels connecting the anal canal and rectum with the skin around the anus. There are 20,000 to 25,000 newly confirmed cases in the USA each year. A statistical analysis based on a large population database in the UK showed that the incidence of anal fistula is 1.69 cases per 10,000 individuals. This was also evidenced by other relevant studies. Patients with anal fistula are mainly adults between 30 and 40 years old, and the incidence rate of this condition in men is higher than that in women. In addition to severely affecting the quality of life of patients, anal fistula has also a negative impact on the psychological state of patients who often suffer from depression or anxiety symptoms. In general, anal fistula cannot be cured without therapeutic intervention. Surgical therapy is the main method used to treat anal fistula. The best treatment criterion is to eradicate the infected lesion, ensure sufficient drainage, and promote the closure of the fistula, while minimizing damage to the anal sphincter. The integrity of the internal anal sphincter (IAS) and external anal sphincter (EAS) is the most important guarantee for keeping normal anal function of patients.
Anal fistula can be divided into simple and complex types according to the degree of lesions. According to the classification standards of the American Society of Colon and Rectal Surgeons (ASCRS), the former includes low transphincteric, and intersphincteric fistulas, which account for lesser than 30% of the sphincter complex. Regarding simple anal fistula, especially distal cases, fistulotomy can be used to obtain ideal treatment results. However, complex anal fistula is one of the refractory diseases encountered in colorectal surgery; it is transphincteric fistula that account for more than 30% of sphincter complex, and includes anal fistulas related to malignancy, inflammatory bowel disease, radiation, chronic diarrhea, or preexisting fecal incontinence. Due to the diverse causes and forms of complex anal fistula, its treatments are often accompanied by a high risk of recurrence and potential incontinence disorders, and there still is a lack of clinical consensus on the best surgical approach. Cutting Seton is a preliminary exploration of sphincter-sparing technology. It works on the principle that gradual detachment of muscles will lead to fibrosis and necrosis, which can maintain the integrity of the sphincter complex with minimal damage to the cutting end. Nevertheless, studies have shown that cutting Seton does not sufficiently protect the anal sphincter, and the postoperative anal incontinence rate was even as high as 63%. The slow section of the sphincters by cutting Seton produces a sphincter injury, with outcomes perhaps even less controllable than a simple lay-open fistulotomy. Total sphincter preservation surgery has gradually become the first choice for anal fistula treatment. In the past few decades, several sphincter-sparing techniques have been established, including endorectal advancement flap (ERAF), ligation of the intersphincteric fistula tract (LIFT), fibrin glue, anal fistula plug, fistula laser closure, video-assisted anal fistula treatment (VAAFT), and adipose-derived stem cells; the last review of these studies was published in 2015. Based on these independent sphincter-sparing techniques, to further diminish the recurrence rate, protect the anal sphincter and obtain better postoperative outcomes, some innovative, combined, and modified new therapies have been proposed and applied in clinical studies in recent years. However, due to the diversity of treatment methods and the inevitable heterogeneity of clinical trials, their variable outcomes are prone to generate confusion and misunderstanding.