Biocompatibility of nitinol and stainless steel in the bladder: An experimental study

Purpose: We tested the biocompatibility of nitinol, a nickel titanium alloy, and stainless steel (SS) as bladder implant materials. Materials and Methods: Rats received a nitinol implant, an SS implant or were sham controls. Two, 3, 6 and 8 weeks following implantation 24-hour voiding behavior studies were per-formed to investigate bladder irritation. All animals were sacrificed 8 weeks after implantation and a sample of urine was aspirated for culture. The bladders were examined by light microscopy and scanning electron microscopy (SEM). Results: No visible encrustations or infections were noted in urine. Voiding frequency in the light period 6 weeks after implantation was significantly decreased in the 2 implant groups compared with sham controls. There were no other significant differences in frequency or mean volume per void in the light or dark periods at any time point. Light microscopy demonstrated similar implant tissue effects in all groups with little or no inflammation or fibrosis. Under SEM all implants showed a brittle, amorphous coating devoid of cells. The transition between the urothelium, mucosa and the rod was smoother for SS than for nitinol, suggesting an affinity of SS for mucosa. In all nitinol rods discontinuity was present between the mucosa and rod. Conclusions: Nitinol and SS do not cause more irritation than the effects of surgery alone and the 2 materials seem to be biocompatible in the bladder. Nitinol may be more inert than SS based on SEM results.