Permanent Temporary and Biodegradable Stents

Stenting is a technique used to open closed ducts in the body. The urethra is the duct between the bladder and external environment. Expandable materials, or stents, are inserted into the urethra to aid in the flow of urine by opening the channel in the case of urethral stricture or detrusor sphincter dyssynergia. In men, the urethra passes through the prostate gland. If the gland swells, as in benign prostatic hyperplasia, a stent can be placed into that portion of the urethra (prostate stent) to retain urine flow until treatment to reduce prostate size achieves its end. Stent placement is a minimally invasive alternative to surgery and an option when surgery is not tolerable.

The materials used in stenting have evolved over time and differ based on whether placement is permanent or temporary. Temporary stents may be biodegradable or non-biodegradable. All stents must meet certain criteria: Ease of insertion and removal, biocompatibility, rigid enough to avoid obstruction, resistant to infection, resistant to migration, and radio-opaque for confirmation of placement via radiography. Metal alloys and polymers are both used. Prior to the introduction of biodegradable stents in 1993, polyurethane and other plastics were primarily used, but both plastics and metal have disadvantages compared to newer materials in regard to migration and biocompatibility. Self-expandable, spiral stents are now often used.

Permanent Stents

Permanent urethral stents are generally used in the anterior urethra, though they have found use in the prostate urethra in recent years. These stents tend to be made of metal, specifically self-expandable nickel alloy wire mesh or stainless steel, and designed to be integrated into the urethral wall. Until integration, the expansion force of the stent holds it in place.

Temporary Stents

Temporary urethral stents and prostate stents are made from materials that do not get incorporated into the wall of the urethra, or if they do, that biodegrade once covered by epithelium. Available stents are made of stainless steel (including gold-plated stainless steel to reduce encrustation), nickel titanium alloy (nitinol), polyurethane, biodegradable polymers (e.g., hydroxyl acids and poly-D/L-lactic acid). The specific stents on the market and their properties were outlined by Duvdevani, Chew, and Denstedt of the Division of Urology at University of Western Ontario, Canada in 2010 (Urethral stents: Review of technology and clinical applications).

Biodegradable Stents

Biodegradable stents in the prostatic urethra generally break down and are washed out with the urine, whereas those in the anterior urethra become covered by the epithelium and degrade within the tissue. The most common materials used in biodegradable stents are poly-lactic acid and polyglycolic acid. Both component materials are common in the body and absorbed as the polymers break down. An overview of the technology used in the manufacturing and improvement of biodegradable stents was written by Tammela and Tulja at the Department of Urology of Tampere University Hospital and Medical School, Finland, which was published by the respected medical journal BJU International in 2003 (Biodegradable urethral stents).