Orthotopic bladder substitution can be realized from ileum only or from colonic segments either alone (1,2) or in combination with small bowel segments as a composite reservoir (3,4,5,6,7,8). The rationale of using large bowel for urinary diversion is based on anatomic and functional considerations. Surgical creation of a urinary reservoir from bowel segments generally means transformation of a cylinder into a sphere (9). The length of bowel to be excluded from the intestinal tract for formation of a reservoir of a given capacity depends on the diameter of bowel used to create the reservoir. In the volume formula of a cylinder (V = τr² · l), the radius determines the volume by its second power; hence, the larger the bowel diameter, the less proportional bowel length is required. Thus, large bowel with its greater diameter can contribute significant capacity to a continent reservoir despite only a shorter segment having to be excluded from bowel continuity.

Physiologically, fat-soluble vitamins (A, D, E, K) and watersoluble vitamins B12 and folic acid as well as biliary acids are all absorbed from the entire ileum but not from colon. Consequently, possible malabsorption syndromes are related only to resection of ileum but not to colon resection. The critical length of ileum resection is about 60 cm above, which the risk of secondary malabsorption syndromes starts to increase. These facts should influence choice of bowel segments for reservoir formation in favor of reducing the length of ileum segments for a composite reservoir or of avoiding ileum entirely in an all-colon reservoir (10,11,12).

However, the large bowel is less distensible than small bowel (13). Although at first, this may be regarded as a disadvantage, it may be advantageous, specifically for spontaneous voiding from an orthotopic bladder substitute in the long run. Intestinal urinary reservoirs start their life at a surgically determined volume that increases over time by gradual distention. During this process, large bowel segments have the advantage that the longitudinal arrangement of taeniae prevents the development of a decompensated substitute megacystis as has been described for ileum reservoirs requiring in up to 60% of females emptying of the orthotopic bladder substitute by intermittent self-catheterization. In addition, large bowel segments, especially cecum and ascending colon, offer several alternatives for safe and effective antirefluxive ureteral implantation.

The downsides of using large bowel for creation of an orthotopic reservoir have to be weighed against the advantages of large capacity, less malabsorption, less overdistention, and reliable antireflux techniques.

First, many urologic surgeons are not familiar with large bowel surgery and feel uncomfortable with it, specifically when performing the bowel anastomosis after resection of the segments for the urinary reservoir. Second, generous mobilization and rotation of segments is required to allow a tensionfree transposition as an orthotopic reservoir into the true pelvis. Surgical techniques using large bowel may also be more time-consuming because adhesions with the greater omentum have to be freed.

The different techniques of constructing an orthotopic reservoir, either from large bowel only or in combination with small bowel segments as a composite reservoir, refer to the very same principles of continent urinary diversion: detubularization and spherical reconfiguration (14). Thus, the success of orthotopic bladder substitution is not confined to selection
of small bowel segments; good results have been reported with the ileocecal segment, the right colon, and the sigmoid colon. However, the physiology of large bowel segments changes from proximally to distally with decreasing capacity and compliance and increasing wall tension and intraluminal pressures. This translates into higher capacity and lower pressure of the cecum and ascending colon as compared to the descending and sigmoid colon. The surgical technique of the Mainz ileocecal pouch as described in the following in detail (8) may in this context serve as a general example of construction of an ileocolonic composite pouch.

However, one single technique of continent urinary diversion does not fit all patients and all pathologies. Thus, modern concepts of intestinal urinary diversion require surgical versatility for individualization of the surgical technique according to the requirements of the underlying pathology and the wishes of the patient.