Biomedical Engineering Reference
In-Depth Information
dependent upon the urine pH. In vitro experiments with artificial urine showed that the
stents were stable at urine pH less than 7.0 for at least 30 days, while they dissolved
completely within 48 hours at pH greater or equal to 7.0. The principle behind this stent is
that it would remain stable at physiological urine pH of 5-6, but can be triggered to dissolve
by medically altering urinary pH. Although very promising, this technology remains at an
experimental stage and awaits animal trials. An additional factor that may need to be taken
into consideration is the influence of encrustation protein deposition as it can form a
platform for bacterial adherence and infection. The influence of encrustation and protein
deposition must also be considered as it can form a platform for bacterial adherence, all of
which may influence urine pH. Uropathogens in general are known to increase urine pH,
and may have an effect
in vivo
. In addition to this, medically increasing the urinary pH may
introduce an additional risk for infection as more alkaline pH favours bacterial survival and
some increased stone formation (calcium phosphate and struvite stones). In addition,
encrustation of the stent may also prevent May also prevent its exposure to the altered pH
environment, environment and the altered pH, thus limiting its rate of decomposition.
A spiral stent (Spirastent
®
, Urosurge Medical, Coralville, IA) is a polyurethane stent with
metal helical ridges designed to prevent kinking and compression in chronically obstructed
patients. In
in vitro
studies, this stent increased flow and theoretically increased the space
between stent and ureter to facilitate passage of stone fragments.
(Stoller, Schwartz et al. 2000)
The spiral design has been incorporated into biodegradable materials for urethral
stents.(Isotalo, Talja et al. 2002; Laaksovirta, Isotalo et al. 2002) Laaksovirta et al used a self-
reinforced poly-L-lactic and poly-L-glycolic acid (SR-PLGA) copolymer spiral urethral stent
(SpiroFlow stent, Bionx Implants Ltd, Tampere, Finland) following prostatic laser
coagulation.(Laaksovirta, Isotalo et al. 2002) This stent degraded in 6-8 weeks and resisted
encrustation at 4 weeks in artificial urine. After 8 weeks, the SpiroFlow
®
stent was
significantly less encrusted than the metal urethral stents Prostakath
®
(Engineers and
Doctors A/S, Copenhagen, Denmark) and Memokath
®
028 (Engineers and Doctors A/S).
SR-PLGA is the most commonly utilized material for prostatic stents, but it has also been
developed as a ureteral stent and may be incorporated into new degradable, encrustation-
resistant ureteral stents in the future.(Olweny, Landman et al. 2002)
An alginate based ureteral stent was designed to stay in place for at least 48 hours after
uncomplicated ureteroscopy.(Lingeman, Preminger et al. 2003; Lingeman, Schulsinger et al.
2003) Of 87 patients, 80.5% of patients retained their stents greater than 48 hours. Seventeen
patients had early stent passage (earlier than 48 hours), but did not require any
supplemental procedures to insert another stent. Although these results were promising,
several patients (14) still had stent fragments remaining at 30 days, while 3 patients had stent
fragments remaining after 90 days. All three of these patients underwent shockwave lithotripsy
and two went on to have endoscopic ureteroscopy to remove the fragments from the
kidney. Because of the lack of all stents to degrade by 3 months, this stent is no longer
commercially available. (Lingeman, Preminger et al. 2003)
The authors are currently involved in developing a new biodegradable stent (Poly-Med Inc.,
Anderson, SC) that dissolves within 1 to 4 weeks in a porcine model. The animals stented
with degradable stents displayed less histologic inflammation than animals stented with
control polyurethane biostable Double J stents (Chew, Lange et al. 2010). Weekly
intravenous pyelograms displayed less hydronephrosis in the degradable stent group. All
stents degraded by 4 weeks and degradation began after 1 week in a very controlled fashion
