This study was approved by the Research Ethics Committee of the East Hospital of Shijiazhuang Traditional Chinese Medicine Hospital. Written informed consent was acquired from all patients prior to the utilization of their clinical data.

Patients hospitalized in the Anorectal Department of the East Hospital of Shijiazhuang Traditional Chinese Medicine Hospital from April 2021 to March 2023 were recruited, and 100 patients who met the diagnostic criteria for high simple anal fistulas were included in the study (a simplified CONSORT flow diagram is shown in Figure 1).

The diagnostic criteria for high simple anal fistulas were based on the “Guidelines for Clinical Diagnosis and Treatment of Anal Fistula” (2006 edition), jointly developed by the Anorectal Society of the China Association of Chinese Medicine, the Colorectal and Anal Surgery Group of the Surgical Branch of the Chinese Medical Association, and the Colorectal and Anal Diseases Specialized Committee of the Chinese Society of Integrative Medicine.

A high simple anal fistula is defined as a fistula with a single tract originating from an internal opening in the anal crypt and extending above the deep external sphincter, involving structures above the puborectalis and levator ani muscles (Figure 2 shows a representative imaging example).

Inclusion criteria: the inclusion criteria included patients diagnosed with high simple anal fistula through preoperative pelvic MRI or double-plane anal canal ultrasound and digital rectal examination; patients aged between 18 and 65 years, regardless of sex; those with normal anal morphology and function before surgery; those whose internal orifice was clarified by preoperative specialist examination and three-dimensional rectal color Doppler ultrasound; and those who voluntarily agreed to take part in the study and provided signed informed consent.

Exclusion criteria: the elusion criteria included patients with trauma-induced anal fistula; specific anal fistula caused by Crohn’s disease, ulcerative colitis, tuberculosis, or actinomycosis; patients with diabetes mellitus, vitamin C deficiency, or other metabolic disorders affecting wound healing; patients with a history of previous anal fistula surgery; patients with major comorbidities involving the cardiovascular, cerebrovascular, hepatic, renal, or hematopoietic systems; patients with allergy, malignant neoplasms, psychiatric disorders, or inflammatory bowel diseases; and pregnant or breastfeeding women.

One hundred eligible patients were divided into four groups according to the order of their admission date, with each group containing 25 patients, as determined by a computer-generated randomization scheme. The 1/5 group: during the operation, the rubber band was tightened by 1/5 of the circumference of the encircled muscle bundle using a scaled rubber band. Subsequent tightenings were performed every 7–8 days, each time reducing the circumference by an additional 1/5. In the 1/4, 1/3, and 1/2 groups, during the operation, the cutting force of the rubber band was tightened by 1/4, 1/3, and 1/2 of the circumference of the encircled muscle bundle, respectively. Subsequent tightenings followed the same proportional reductions (1/4, 1/3, or 1/2) at intervals of 7–8 days. A single-blind method was used, in which the outcome assessor was blinded to group allocation (Figure 3 shows the technical route flowchart).

Preoperative preparation: all patients underwent comprehensive preoperative examinations, including routine blood and urine tests, liver function, renal function, electrolytes, coagulation profile, transfusion screening, blood typing, blood sedimentation, blood glucose, tuberculosis antibody test, electrocardiogram, chest radiographs, abdominal ultrasound, electronic colonoscopy, three-dimensional rectal ultrasound, and anorectal manometry. These assessments aimed to delineate the direction of the fistula tract, identify the internal opening, and evaluate its relationship with the sphincter.

Patients fasted for 6 h before the operation. A sodium phosphate solution enema was administered the night before surgery, and the operative area was thoroughly cleaned and prepared.

Surgical procedures: all operations were performed by the same team of experienced colorectal surgeons. Patients in all groups underwent a standardized low-level incision and high-level thread-drawing procedure. Under sacral anesthesia and sterile conditions, a probe was used to trace the fistula tract from the external opening to the internal orifice. The fistula tract below the anal canal sphincter was first incised up to the dentate line. Necrotic tissue was excised, the tract was curetted, and adequate drainage was ensured. For the portion of the fistula above the anorectal ring, a scaled rubber band was passed through the tract and internal orifice using a probe, exiting from the incision site below the anorectal ring. The rubber band was then tightened and tied, ensuring that the seton reached the uppermost point of the fistulous cavity. The seton typically involved the muscle layer of the anorectal ring, mucosa, or a combination thereof, using a standardized double-stranded rubber band. Wound closure involved the insertion of Vaseline gauze into the anus for compression, followed by sterile gauze dressing with moderate pressure to ensure fixation and stability (Figure 4 shows a visual representation of the seton placement in the 1/3 group).

Postoperative care: patients resumed a liquid diet for 6 h after the operation and were allowed bowel movements on the second day after the operation. Oral or topical medications were provided as needed to facilitate defecation. All patients received identical postoperative care protocols. After defecation, all patients were given a sitz bath with Nuo Erkang lotion (produced by Shijiazhuang Traditional Chinese Medicine Hospital, Hebei; Registration No. Z20051133; it consists of Dioscorea nipponica, Sophora flavescens, Radix et Rhizoma Rhei, Mirabilitum, alum, Gentiana macrophylla, Saposhnikoviae Radix, Zanthoxylum bungeanum, Pollen Typhae, Galla chinensis, Flos Carthami, and borneolum). The sitz bath and dressing were applied twice daily, in the morning and evening. All patients were given Zigui Jiedu Ointment (produced by Shijiazhuang Traditional Chinese Medicine Hospital, Hebei; Registration No. Z20051128; it contains Arnebia euchroma, Angelica sinensis, and Angelica dahurica) until the wounds were healed. In the early postoperative period, wounds were irrigated with 0.5% metronidazole solution to promote fresh granulation. In the middle and late periods, the wound was irrigated with saline. Broad-spectrum antibiotics were used continuously for 3 days to prevent infection. If patients experienced severe pain, diclofenac potassium sustained-release tablets were administered orally for analgesia following the pain assessment.

Evaluation criteria for overall clinical efficacy: the clinical efficacy standard of postoperative anal fistula in this study was formulated with reference to the “Diagnostic Efficacy Standard of Chinese Medicine Illnesses (1994 version)” issued by the State Administration of Traditional Chinese Medicine. The criteria were as follows: cured: complete disappearance of symptoms and signs and full wound healing; significantly effective: disappearance of symptoms, improvement of signs, with ≤25% of the wound unhealed, but fresh granulation tissue present; effective: disappearance of symptoms, improvement of signs, with ≤25% of the wound unhealed, but fresh granulation tissue present; ineffective: no improvement in symptoms and signs or persistent exudation and poor wound healing with >50% of the wound unhealed and only sparse granulation tissue present. Overall effective rate = (cured + significant effective + effective cases)/total number of cases × 100%.

General postoperative conditions: these included wound healing time (days from surgery to complete wound healing), length of hospitalization (days from admission to discharge), the recurrence rate, and postoperative complications. The criteria for determining the recurrence rate and postoperative complications were as follows:

Recurrence rate: patients were followed up for 6 months after the operation through regular hospital visits or remote consultations via telephone or WeChat. Postoperative recurrence was defined as the occurrence of any of the following conditions during the follow-up period, confirmed by both clinical examination and auxiliary imaging (three-dimensional rectal ultrasound): (1) incomplete healing of the surgical incision and/or external fistula opening 6 months after surgery; (2) emergence of a new external fistula opening; and (3) local signs of recurrence such as redness, swelling, pain, rupture, or purulent discharge. The recurrence rate in each group was recorded. Additionally, postoperative complications were monitored and documented, including urinary retention on the day of surgery, wound bleeding during hospitalization, and any evidence of anal deformity detected during outpatient follow-up after discharge.

Postoperative wound symptom scoring: symptom evaluation was based on the four-tier weighted scoring method for clinical diagnosis and efficacy outlined in the Guidelines for Clinical Research of New Chinese Medicines (2002 edition). Pain, exudation, edema, itching, and other wound-related symptoms were assessed on postoperative days 1, 7, 14, and 21.

Scoring of postoperative pain symptoms: pain was assessed using the visual analog scale (VAS), where 0 points indicated no pain; 1 point indicated mild pain (VAS 1–3) without the need for medication; 2 points indicated moderate pain (VAS 4–6) requiring general analgesics or rectal suppositories; and 3 points indicated severe pain (VAS 7–10) necessitating morphine analgesia. Exudation was scored as follows: 0 points for localized wetting of two or fewer dressings (6 × 6 cm, four-ply) within 12 h; 1 point for wetting of three dressings; 2 points for wetting of four or more dressings; and 3 points for profuse exudation requiring the use of cotton pads (10 × 10 cm, 3 cm thick). Edema of the wound margin was scored as follows: 0 points for no edema; 1 point for mild swelling with visible dermatoglyphics; 2 points for moderate swelling with faint dermatoglyphics; and 3 points for pronounced swelling with loss of dermatoglyphics and a shiny skin appearance. Perianal itching was scored as follows: 0 points for no itching; 1 point for occasional itching; 2 points for intermittent itching; and 3 points for persistent itching.

Changes in anorectal pressure: anorectal manometry was employed as an objective measure of anal function. Anal resting pressure (ARP) and anal maximal contraction pressure (AMCP) were selected as the primary indices for functional assessment. Measurements were performed preoperatively and at 3 months postoperatively using the Polygram 98 multi-channel gastrointestinal manometry system (Medtronic, USA) with an eight-channel water-perfused catheter.

Wexner score: the Wexner incontinence score, a subjective index of anal continence, was assessed preoperatively and at 1, 3, and 6 months postoperatively. Patients self-reported their ability to control solid stool, liquid stool, and gas; their need for protective liners; the impact on their lifestyle; and the frequency of these occurrences. Scoring criteria were as follows: 0 for never; 1 for rarely (less than once per month); 2 for sometimes (more than once per month but less than once per week); 3 for often (more than once per week but less than once per day); and 4 for always (more than once per day). The total score ranged from 0 to 20, with lower scores indicating better anal function.

All data were processed using SPSS 26.0 (IBM SPSS Statistics, Chicago, IL, United States) and GraphPad Prism 8.0 (GraphPad software, Inc., La Jolla, CA, United States). Measurement data were presented as the mean ± standard deviation. Measurement data were compared using paired t-tests for within-group comparisons, one-way ANOVA and Tukey’s test were performed for multiple comparisons if they met normal distribution and homogeneity of variance, the Kruskal–Wallis H-test was performed for between-group comparisons, and Wilcoxon’s rank-sum test was performed for within-group comparisons if they did not meet normal distribution and homogeneity of variance. Categorical data were expressed as n (%), and groups were compared using the χ² test. P < 0.05 was considered statistically significant.

Comparison of general information such as gender, age, course of disease, history of smoking, history of drinking, and history of diabetes among patients in the 1/5, 1/4, 1/3, and 1/2 groups showed no significant differences, indicating comparability among the groups (p > 0.05) (Table 1).

The total effective rates of the 1/5 group, 1/4 group, 1/3 group, and 1/2 group were 100%, 100%, 96.00%, and 92.00%, respectively. No notable difference was witnessed when comparing these rates among the four groups (p > 0.05) (Table 2).

There was no significant difference among the four groups in terms of hospitalization time and recurrence rates (p > 0.05). Postoperatively, no complications occurred in the 1/5 group. One case of wound bleeding occurred in the 1/4 group, one case of urinary retention occurred in the 1/3 group, and two cases of wound bleeding and one case of urinary retention occurred in the 1/2 group. However, there was no difference in the occurrence of complications among the four groups (p > 0.05). No cases of anal deformity were observed in any group. Wound healing time among the four groups was compared, and it presented a notable difference (p < 0.05), in which the healing time of the 1/5 group was the longest (Table 3).

Postoperative wound symptom scores were compared among the four groups. The results revealed that on the first postoperative day, the differences in the scores of postoperative pain symptoms, wound exudation, edema of wound margin, and perianal itching were compared among the four groups of patients, and the differences were not statistically significant (p > 0.05). The scores of postoperative pain symptoms, wound exudation, edema of wound margin, and perianal itching decreased in all four groups on the 7th, 14th, and 21st postoperative days compared to those on the 1st postoperative day; on the seventh postoperative day, the difference between the wound symptom scores of the 1/5 group, 1/4, and 1/3 groups was statistically significant compared with that of the 1/2 group (p < 0.05), but no difference was witnessed between the 1/5, 1/4, and 1/3 groups (> 0.05). On the 14th and 21st postoperative days, no differences were observed in wound symptom scores among the four groups of patients (p > 0.05) (Table 4).

Before surgery, the differences in the levels of ARP and AMCP among the four groups of patients were not statistically significant (p > 0.05), and 3 months after surgery, the levels of ARP and AMCP in each group decreased significantly compared with those before surgery (p < 0.05). However, ARP and AMCP levels of patients in the 1/5 group were notably higher those that of the other three groups, and those of the 1/4 and 1/3 groups were higher than that of the 1/2 group (p < 0.05). The difference between the 1/4 and 1/3 groups was not significant (p > 0.05) (Table 5).

The highest preoperative and postoperative Wexner scores in the four groups of patients were 1 point, reflecting occasional gas incontinence or infrequent use of sanitary pads in a small number of patients. No difference was found in preoperative Wexner scores of the anal function of the patients in each group (p > 0.05). At 1 and 3 months postoperatively, the Wexner scores of patients in all groups were higher than those before surgery (p < 0.05), with the lowest score in the 1/5 group and the highest in the 1/2 group, and the scores in the 1/5, 1/4, and 1/3 groups were lower than that in the 1/2 group (p < 0.05). No differences were observed in pairwise comparisons between the 1/5, 1/4, and 1/3 groups (p > 0.05). At 6 months post-operation, there was no difference in Wexner scores across all groups (p > 0.05) (Table 6).