Aim Transanal flap is an attractive technique for complex fistulas. The aim of this study was to identify factors associated with poor outcome, recurrence and incontinence. Method All patients who underwent an advancement flap for the treatment of a complex anal fistula between 1995 and 2019 were prospectively enrolled. The patient data obtained included age, sex, body mass index, comorbid conditions and smoking history. The fistula characteristics analysed included previous anal surgery, anatomy of the fistula according to Park's classification, horseshoe or supralevator extension and preoperative seton drainage. The postoperative data registered included complications, postoperative stay, recurrence and incontinence. Predictive factors for recurrence and incontinence were identified using univariate and multivariate analysis. Results One hundred and ninety patients were included; 134 (70.5%) were men and the mean patient age was 50 years. The median length of follow-up was 44.6 months. The fistula recurred in 14 patients (7.3%). Before surgery, 20 patients (10.5%) reported incontinence symptoms. In the continent patients a Wexner postoperative score of 0 was identified in 79.4%. Wexner scores between 1 and 3 were observed in 17% and scores of 4 or more in five patients (3%). Factors associated with recurrence included age < 50 years (OR = 4.8, P = 0.02, 95% CI 1.2-19), smoking (OR = 4.1, P = 0.03, 95% CI 1-16.5) and suprasphincteric fistula (OR = 0.5, P = 0.01, 95% CI 0.2-0.8) in multivariate log regression analysis. Major incontinence was influenced by female sex and previous anal surgery (OR = 7.5, P = 0.003, 95% CI 1.6-34 and OR = 0.1, P = 0.007, 95% CI 0.1-0.7, respectively). Conclusion Full-thickness transanal advancement flap is a good treatment for complex anal fistula repair. This study provides relevant information on risk factors for failure of therapy and incontinence, which can help in advising patients before surgery and planning a good treatment strategy.

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