BACKGROUND: External anal sphincter contractility significantly contributes to control the stools passage. An artificial anal sphincter placed into the intersphincteric space is a safe and effective procedure to treat fecal incontinence, even if its mechanism of action has not been fully elucidated.OBJECTIVE: The aim of this study was to evaluate external anal sphincter contractility changes after a self-expandable hyexpan prostheses was implanted into the intersphincteric space of the anal canal and clinical outcomes compared.DESIGN: Prospective clinical study.SETTINGS: The study was conducted at a University teaching hospital.PATIENTS: Consecutive patients affected by fecal incontinence for at least 6 months, after failure of conservative treatment.INTERVENTIONS: All patients underwent 10 prostheses implantation, and were examined pre- and postoperatively by endoanal ultrasound and anorectal manometry.MAIN OUTCOME MEASURES: Fecal incontinence symptoms were assessed by severity scores. The external anal sphincter muscle-tension was calculated using a specific equation.RESULTS: Thirty-nine patients (34 females; median age 68 years) were included in the study; no morbidity was registered. After a median follow-up period of 14 months, both the median maximum voluntary squeeze pressure and the median inner radius of the external anal sphincter significantly increased. A statistically significant increase of external anal sphincter muscle tension was detected. A decrease of any fecal incontinence symptom and an improvement in severity scores were observed at the last follow-up examination. The external anal sphincter contractility was significantly higher in patients reducing incontinence episodes to solid stool by more than 50% and improving ability to defer defecation for more than 15 minutes.LIMITATIONS: Single-center experience; relatively small and heterogeneous sample size; patients with a potentially more severe disease because our institution is a referral center; absence of quality of life evaluation.CONCLUSIONS: Artificial anal sphincter implantation improved the external anal sphincter muscle-tension; there was a positive correlation between its increase and the clinical outcome. See Video Abstract at http://links.lww.com/DCR/B468 .