Biomedical Engineering Reference
In-Depth Information
removal involves multiple surgical procedures (Rana and Sabooh 2007) and may result in
loss of the renal unit or potentially even death (Singh, Srinivastava et al. 2005).
3. Current stent biomaterials
The synthetic polymer, polyethylene, was previously used in stent construction, but was
abandoned due to its stiffness, brittleness, and tendency to fragment. Blends of polyethylene
and other polymers, such as polyurethane, have been shown to resist encrustation (Gorman,
Tunney et al. 1998; Gomha, Sheir et al. 2004). Silicone is currently the most biocompatible
stent material as it is the most resistant to biofilm formation, infection and encrustation
(Watterson, Cadieux et al. 2003), and is one of the most lubricious materials available (Jones,
Garvin et al. 2004); however, its softness and elasticity make it difficult to handle,
particularly through tortuous or tight ureters. In addition, the low tensile strength of silicone
makes it susceptible to extrinsic compression. The development of new stent materials
aimed to meld the flexible and elastic properties of silicone with the rigidity of polyethylene
which resulted in the development of polyurethane, the most common class of polymer
currently used in stents. Polyurethane, however, is a stiff material that causes patient
discomfort and significant ureteral ulceration and erosion have been reported in an animal
model (Marx, Bettmann et al. 1988). New proprietary materials and combinations are softer,
more comfortable, and easier to maneuver within the urinary tract. Examples of commonly
used materials in stents include Percuflex
®
(Boston Scientific Corporation, Natick, MA),
Silitek
®
(Surgitek, Medical Engineering Corporation, Racine, WI, USA), C-Flex
®
(Consolidated Polymer Technologies, Clearwater, FL, USA), Tecoflex
®
(Thermedics,
Wilmington, MA, USA), and ethylene-vinyl-acetate (from the polyefin family of which
polyethylene is a member). They have been designed to provide rigidity to facilitate
handling by the surgeon and to provide adequate drainage while being soft enough to limit
patient discomfort.
4. New materials
New materials include metal stents that are designed to keep the ureter open despite
extrinsic ureteral compression secondary to lymphadenopathy due to malignancy. Ureteric
obstruction may result in decreased renal function, pain, or infection requiring urinary
diversion (Chitale, Scott-Barrett et al. 2002; Allen, Longhorn et al. 2010). As these stents must
remain in place for long periods of time, they require frequent exchanges because they are
susceptible to infection and encrustation with increased indwelling time. The goal in this
patient population is to develop a stent that maintains ureteral patency during extrinsic
compression, is soft to minimize discomfort, and is resistant to encrustation and infection.
5. Metal ureteral stents
Metal ureteral stents were introduced by Pauer in 1992 (Pauer and Lugmayr 1992) and have
been utilized in the treatment of malignant ureteric obstruction (Kulkarni and Bellamy 2001;
Liatsikos, Karnabatidis et al. 2009; Masood, Papatsoris et al. 2010; Papatsoris and Buchholz
2010; Sountoulides, Kaplan et al. 2010), ureteral strictures (Daskalopoulos, Hatzidakis et al.
2001; Papatsoris and Buchholz 2010), and ureteropelvic junction obstruction (Barbalias,
Liatsikos et al. 2002; Masood, Papatsoris et al. 2010; Benson, Taylor et al. 2011). Current
