An experimental study of self-expanding ureteric metallic stents: macroscopic and microscopic changes in the canine ureter.
OBJECTIVE: To evaluate the efficacy and safety of self-expanding metallic stents after insertion into the canine ureter. MATERIALS AND METHODS: Actively expanding metallic (Nitinol) stents (diameter 8-10 mm, length 4-6 cm) were placed in eight mongrel dogs (18-34 kg). Under general anaesthesia, a midline abdominal incision was made and a stent inserted directly into the ureter through a small incision; each animal was stented on one side. The dogs were assessed after 6 or 19 days or 2, 5 (two), 6 (two) and 7 months. Ureteric specimens were evaluated both macroscopically and microscopically. RESULTS: Macroscopically there was no leakage of urine in any of the animals. On day 6 the inserted stent was patent despite blood clots in the mesh. The lumen was smooth and shiny in three dogs examined at 5, 6 and 7 months. One of the two dogs assessed at both 5 and 6 months had hydronephrosis associated with papillary hyper-epithelialization inside the stent, while the other animals had no dilatation of the renal pelvis. Microscopically there was loss of epithelium and bleeding of the ureteric wall at 6 days. Epithelial regeneration started after 19 days and was complete in the lumen at 2 months, with the stent incorporated into the wall of the ureter. Epithelialization was maintained up to 7 months, after which the problem of urothelial hyperplasia persisted, but an adequate lumen was maintained by epithelial regeneration. This epithelium was smooth and shiny, and composed of thin layers of cells. CONCLUSION: Self-expanding metallic stents may be effective if hyperplastic epithelialization does not occur, as the stent was epithelialized within 2 months and the regenerated epithelium maintained for up to 7 months. Metallic stents may be useful for treating patients with renal failure caused by ureteric stenosis from malignant tumours.