Int J Colorectal Dis (2011) 26:729–735 regions were taken at a magnification of ×200 using cross polarization light microscopy (Leica DM5000B, Leica Microsystems, Heerbrugg, Switzerland). The collagen content in the anastomotic and perianastomotic region was quantified as a percentage of total tissue surface and expressed as relative collagen area using the Qwin morphometry system (Leica QWin V3.5.1, Leica Microsystems). Furthermore, maturity of collagen was estimated by calculating the red versus green ratio, where red indicates thick, more mature collagen fibers and green indicates the thinner, immature collagen fibers. All histological assessments were performed by an experienced pathologist, blinded to the study groups. Statistical analysis All data are expressed as means ± standard error of means (SEM). Loss of body weight, bursting pressures, and collagen ratios were compared using one-way ANOVA and paired samples t test. Mortality and bowel obstructions were analyzed using Chi square and Fisher's exact tests. A Bonferroni corrected P value <0.050 was considered statistically significant. All analyses were performed using SPSS version 15.0 (SPSS, Chicago, Illinois, USA). Results Rats lost an average of 8.8 ± 0.8% of initial body weight during the first 4 days after surgery (P= 0.107 between groups), regaining weight from day 5. Three rats in the control group (15%; one on day 2, two on day 3), three in the fibrin glue group (15%; on days 1, 3, and 5), and one in the group with a collagen fleece (4.8%; on day 3) died before the intended time of sacrifice (P= 0.497). All but two of these rats showed signs of fecal peritonitis with dehiscence of the anastomosis at autopsy. The other two (one with fibrin glue and one with a collagen fleece) showed pus on the anastomosis accompanied with a mild peritonitis. Minimal to moderate adhesions between the anastomotic region and peritesticular fat or seminal vesicles were found in most rats. Implanted collagen fleeces were still in place at 3 days and partly absorbed at 7 days follow-up. Bursting pressure Bursting pressures were not significantly different among control, fibrin glue, and collagen fleece groups at 3 days follow-up (60.5 ± 11.3, 67.9 ± 12.6, and 50.7 ± 15.5 mmHg, respectively, P= 0.659), nor at 7 days follow-up (119.9 ± 27.9, 167.7 ± 32.0, and 114.8 ± 27.5 mmHg, respectively, P= 0.427) (Fig. 1). In general, a significant increase in 731 Fig. 1 Bursting pressures were measured by intraluminally increasing the air pressure while the anastomotic area was submerged. The maximum pressure before bubbles appeared was regarded the bursting pressure. Both at 3 and 7 days follow-up, there were no significant differences between groups. All means (SEM) bursting pressures occurred between 3 and 7 days follow-up (59.7±41.1 vs. 130.4±86.4 mmHg, P < 0.001). Bowel obstruction Rats with a collagen fleece showed a significantly higher incidence of bowel obstructions than the control group (14/21 vs. 3/20, P= 0.003). Anastomoses covered only with fibrin glue showed intermediate proportions of bowel obstructions, not significantly different from the other groups (Fig. 2). In case of a collagen fleece, the lumen at the anastomosis appeared stiff and smaller than the normal colonic lumen. However, lumena in all groups were open. Fig. 2 Bowel obstructions were defined as a colon distended more than two times its normal diameter proximal to the anastomosis and tapering to a normal diameter distal to the anastomosis. Rats with a collagen fleece showed a significantly higher incidence of bowel obstructions than the control group (14/21 vs. 3/20, P=0.003)