1. Anorectal Physiology Laboratory , Department of Surgery, University of Groningen, , Groningen, 9700 RB , the Netherlands
2. University Medical Center Groningen , Department of Surgery, University of Groningen, , Groningen, 9700 RB , the Netherlands
3. Department of Obstetrics and Gynecology , University Medical Center Groningen, , Groningen, 9700 RB , the Netherlands
4. University of Groningen , University Medical Center Groningen, , Groningen, 9700 RB , the Netherlands
5. Department of Geriatric Medicine , University of Groningen, , Groningen, 9700 RB , the Netherlands
6. University Medical Center Groningen , University of Groningen, , Groningen, 9700 RB , the Netherlands
7. Department of Obstetrics and Gynecology, Isala Hospital , Zwolle, 8000 GK , the Netherlands
8. Division of Pediatric Surgery , Department of Surgery, University of Groningen, , Groningen, 9700 RB , the Netherlands