INVESTIGATIVE UROLOGY

Expansion and fixation properties of a new braided biodegradable urethral stent: An experimental study in the rabbit
Vaajanen A, Nuutinen J-P, Isotalo T, Törmälä P, Tammela TLJ, Talja M
From the Department of Urology, Tampere University Hospital, and Medical School, University of Tampere and Institute of Biomaterials, Tampere University of Technology, Tampere and Department of Surgery, Päijät-Häme Central Hospital, Lahti, Finland
J Urol. 2003; 169:1171-4

• Purpose: Biodegradable spiral urethral stents have been used with favorable results combined with thermal treatments of the prostate and for recurrent urethral strictures but the configuration of the helical spiral is not ideal. We developed a new tubular mesh configuration for the biodegradable urethral stent and evaluated its expansion and locking properties in the rabbit urethra.
• Materials and Methods: The stents were made of self-reinforced polylactic acid polymer (Bionx Implants, Ltd., Tampere, Finland) blended with BaSO4 (Alfa Chem, New York, New York) to achieve radiopacity. Two braiding patterns, that is 1 over 1 and 2 over 2 + 1, were used to produce a tubular mesh structure. Stainless steel stents (pattern 1 over 1) served as controls. The stents were inserted into the posterior urethra of 27 male rabbits. The animals were sacrified after 1 week, 1 and 6 months, respectively. X-rays were done immediately after stent insertion and at sacrifice. Longitudinal movement and expansion were assessed on the x-rays.
• Results: All stents maintained position in the urethra without fixation. Macroscopic disorientation of the structure of the 2 over 2 + 1 braided self-reinforced polylactic acid polymer stents began before 1 month, while 1 over 1 braided stents retained their construction. At 6 months 3 of 6 biodegradable stents were degraded. Average longitudinal movement was 2 mm. (range 1 to 3) in the 1 over 1 self-reinforced polylactic acid polymer group, 2 mm. (range 0 to 7) in the 2 over 2 + 1 polylactic acid group and 3 mm. (range 3 to 3) in controls at 1 month.
• Conclusions: Biodegradable polymers are suitable materials for braided urethral stents. The expansion properties of the 2 braiding models tested in this study sufficed to fix the stents in situ in the prostatic urethra. However, the 1 over 1 braiding pattern was superior to the 2 over 2 + 1 pattern, in that it retained its macroscopic construction until the degradation of single self-reinforced polylactic acid polymer fibers.
  
  
• Editorial Comment
  The development and use of biodegradable devices for urological application has increased in the last 10 years. This is an elegant study performed by the group that developed some years ago the first biodegradable spiral stents for urology (1,2). In the present work, the authors produced from biodegradable polymers, a self-expanding, self-reinforced braided biodegradable stent for urological use. The stents were tested after insertion into the prostatic urethra of 27 male rabbits. After stent release into the urethra the device expanded to its original shape due to the viscoelastic memory of biodegradable polymers. The authors found that its expansion property was good and fixed the stent in situ as firmly as metallic devices. Although there are some differences between the 1 over 1 pattern and the 2 over 2 + 1 pattern, in thesis, the developed stents are promising and may be suitable for clinical use in the near future. Nevertheless, clinical trials will be necessary to demonstrate the usefulness and the cost-effectiveness of biodegradable stents in urological practice.

References
1. Kemppainen E, Talja M, Riihela M, Pohjonen T, Tormala P, Alfthan O: A bioabsorbable urethral stent. An experimental study. Urol Res. 1993; 21:235
2. Talja M, Tammela T, Petas A, Välimaa T, Taari K, Viherkoski E, et al: Biodegradable self-reinforced polyclycolic acid spiral stent in prevention of postoperative urinary retention after visual laser ablation of the prostate-laser prostatectomy. J Urol. 1995; 154: 2089

Dr. Francisco J.B. Sampaio
Chairman, Urogenital Research Unit
State University of Rio de Janeiro
Rio de Janeiro, Brazil