Background <p>Chronic anal fissure is a painful condition that can greatly affect quality of life. When conservative treatment fails, lateral internal anal sphincterotomy is performed to promote healing. The procedure may involve partial or complete sphincter division, which can influence the risk of fecal incontinence. This study aimed to compare the outcomes of tailoring the length of the lateral internal anal sphincter with either the whole length or the fissure apex and their effect on fecal continence in the treatment of chronic anal fissure.</p> Methods <p>This comparative, prospective, observational study included 40 adults with chronic anal fissures that did not improve with medical treatment. Patients were divided into two equal groups on the basis of the type of sphincterotomy performed: full-length sphincterotomy or sphincterotomy limited to the fissure apex. Patient details and postoperative outcomes were closely monitored, with follow-up visits conducted over 1 month to assess recovery and complications.</p> Results <p>The present study revealed significant differences between the studied groups in terms of postoperative pain and bleeding occurrence (<i>P</i> &lt; 0.001). Postoperative pain was significantly higher among patients who underwent complete sphincterotomy (70%) compared with those who underwent partial sphincterotomy (20%). Similarly, bleeding was significantly more common in the complete sphincterotomy group (55%) than in the partial sphincterotomy group (25%).</p> Conclusions <p>Partial lateral internal anal sphincterotomy appears to offer a balance between fissure healing and continence preservation, making it a preferable option for patients at higher risk of incontinence.</p> Trial registration <p>Not applicable.</p>