PURPOSE:The significance of anal sphincter injury from transanal inserted staplers was studied. A randomized, controlled comparison was made of anorectal manometry and clinical function after sigmoid colectomy (avoiding nerve injury from rectal mobilization), anastomosed by either transanal inserted stapler or biofragmentable anastomotic ring (avoiding anal manipulation).METHOD:Fifty‐eight consecutive patients with sigmoid adenocarcinoma were randomly assigned to transanal inserted stapler or biofragmentable anastomotic ring groups. Anorectal manometry and clinical bowel function assessment were performed by an independent blinded observer before surgery and six weeks and six months after surgery.RESULTS:At six weeks after surgery, there was significant impairment of mean anal resting pressures (mean impairment, 23 percent;P<0.001) and physiologic anal length (mean impairment, 31 percent;P<0.01) in the transanal inserted stapler group (27 completed the trial), but not in the biofragmentable anastomotic ring group (18 completed the trial). Pressures remained impaired at six months. When changes in the anal pressures were compared between groups, the mean anal resting pressure (P<0.001) and maximum squeeze pressure (P<0.01) at six weeks and mean anal resting pressure at six months (P<0.01) were significantly more impaired in the transanal inserted stapler group. Postoperative bowel function was not different between the two groups. Postoperative complications were similar. In the transanal inserted stapler group one patient died of anastomotic leak sepsis and one had wound infection; in the biofragmentable anastomotic ring group one patient died of myocardial infarct and one had wound infection.CONCLUSION:Direct injuries to the internal anal sphincter occurred after transanal inserted stapler but not biofragmentable anastomotic ring anastomoses. Clinical function was not correspondingly affected, probably because of the adequate residual rectal reservoir after sigmoid colectomy.