Uneven pressure application by the artificial urinary sphincter: an explanation for tissue ischaemia?

To determine, in an in vitro study, the pressure inside the AMS 800 (American Medical Systems, USA) sphincter placed around porcine intestine and to correlate this with the pressure retention for liquids, in an attempt to explain the increased sensitivity of bowel to ischaemic injury when the artificial urinary sphincter is used around bowel neo-urethral segments.

Segments of porcine intestine were placed in a specially designed mechanical jig and an AMS 800 sphincter placed around the segment. The device was inflated by injecting water and the cuff pressure measured. Water was infused into the bowel and the pressure at which leakage occurred through the sphincter recorded. At each inflation pressure, a catheter-tipped microtransducer was used to measure the pressure inside the cuff-fold. It was then placed in the centre of the cuff lumen, rotated by 360 degrees and the pressure continuously recorded.

The cuff occluded the lumen by creating a 'triple-cushion' effect. The fluid retention pressure was 49% of the cuff pressure. There were directional differences in the pressure measured inside the bowel lumen. The pressure was also disproportionately higher inside the cuff folds than in the centre of the sphincter, and increased rapidly with inflation of the cuff.

The thin wall of the bowel may 'crenate' inside the high-pressure areas in the cuff folds and this may explain the increased sensitivity of bowel to ischaemic injury in such cases.