Effect of instrument diameter on the depth of penetration of fiberoptic sigmoidoscopes.

Newer fiberoptic sigmoidoscopes tend to have smaller diameter insertion shafts than older models. The extent to which this diameter affects how deeply an instrument can be inserted into the colon and the frequency with which sigmoidoscopes of different diameters could reach more proximal areas of the colon was evaluated. Instrument diameter did not influence the length of the sigmoidoscope shaft that could be introduced into the patient. However, when fully inserted to 60 cm, the 12-mm diameter sigmoidoscopes viewed significantly less of the sigmoid and descending colon than did the 16-mm diameter sigmoidoscopes. This lesser degree of anatomic depth of penetration has direct implications for the diagnostic capabilities of smaller diameter instruments.