The goal of this clinical trial is to study whether the minimally invasive approach in the mucosal advancement flap technique for the treatment of complex perianal fistula improves the healing rate compared to non-minimally invasive surgery.

The main question it aims to answer is:

- Does the healing rate of patients undergoing minimally invasive approach in the mucosal advancement flap technique improve compared to patients undergoing non-minimally invasive surgery?

Researchers will also compare minimally invasive approach in the mucosal advancement flap technique to the non-minimally invasive approach in terms of postoperative pain, fecal incontinence and healing time.

Quality of life and global morbidity will also be analyzed.

Participants will:

• Undergo surgical treatment of complex perianal fistula performed by mucosal advancement flap performed by a minimally invasive technique.
• Attend control visits and fill in symptom forms one week, one month, three months and six months after the surgical procedure.

Inclusion Criteria:

• Age older than 18.
• Present with a complex perianal fistula not surgically treated with curative intention.
• Present only one main fistulous tract.
• Mucosal advancement flap as the chosen technique for the treatment of the fistula.
• Patient has given informed consent to participate in the study.

Exclusion Criteria:

• Non-cryptoglandular internal fistulous orifice (IFO).
• Extrasphincteric fistula.
• Coexistence of anal fissure.
• Coexistence of hemorrhoids tributary to surgery.
• Coexistence of anal or rectal prolapse.
• Coexistence of inflammatory bowel disease.
• Coexistence of anal or colorrectal cancer.