Esophageal stent migration: Testing few hypothesis with a simplified mathematical model.

Esophageal stent placement has significantly improved the quality of life in patients with malignant as well as benign esophageal obstructing lesions. Despite its early success and rapid adoption, stent migration still occurs in as many as 30% of cases especially with fully covered stents. To date, few models of interaction between the stent and the esophageal wall have been published and these have only focused on the deployment of the stent or the static mechanical stress distribution of the stent material. To elucidate the mechanism behind esophageal stent migration we developed a simplified radially symmetric computational model of esophageal peristalsis and the stent. A thorough review of the literature on esophageal peristalsis was performed and pertinent data were implemented into the model. Similarly, mechanical properties of an existing esophageal stent were used for the stent model. A sensitivity analysis of the parameters of the model enabled identification of the key elements of stent design that influence the degree of stent migration including flares design, stent length as well as longitudinal and radial stiffness. A comparison of the model to the migration rate reported in clinical studies for various types of fully covered stents further verified our model, which can significantly contribute to the development of a more stable esophageal stent with lower rates of migration.

In vivo evaluation of a new bioabsorbable self-expanding biliary stent.

BACKGROUND: Bioabsorbable stents may offer advantages for the treatment of benign and malignant biliary strictures, including large stent diameter, decreased biofilm accumulation and proliferative changes, elimination of the need for stent removal and imaging artifacts, and prospects for drug impregnation. However, suboptimal expansion has hampered prior iterations. A new bioabsorbable biliary stent (BioStent) was evaluated in a porcine model. METHODS: BioStents were placed in 8 animals for long-term follow-up. The following were evaluated: accuracy and ease of delivery and deployment, radial expansion, and radiologic visualization. Stent function and biotolerance were assessed by cholangiography, serum bilirubin, and necropsy for histopathology performed in pairs at 2, 4, 6, and 12 months. RESULTS: Stents were delivered without sphincterotomy and were deployed easily, accurately, and with good immediate stent expansion and radiographic visualization. On follow-up, all stents were fully expanded and serum bilirubin levels remained within the normal range. Although there was no clinical evidence of biliary obstruction, filling defects were common at cholangiography. On histopathologic evaluation, there was neither bile duct integration or proliferative change. CONCLUSIONS: The BioStent bioabsorbable biliary stent, modified with axial runners, can be effectively deployed endoscopically, is self-expanding, is visualized radiographically, and remains patent up to 6 months. There was no bile duct integration or proliferative change, which are potential advantages. Stent occlusion and migration remain concerns.

Mechanical properties and in vitro degradation of self-reinforced radiopaque bioresorbable polylactide fibres.

The aim of this study was to evaluate the effect of the radiopaque filler, barium sulfate (BaSO4), on the mechanical properties of self-reinforced bioresorbable fibres. The bioresorbable polymer was a copolymer of L- and D-lactide with an L/D monomer ratio of 96:4 (96L/4D PLA). The fibres were manufactured using an extrusion and a drawing process. Three different methods of processing the composites were studied. The materials were blended prior to extrusion. In the first method, the BaSO4 powder was mixed with the polymer granulates by hand (manual blending). The blend was then processed using a twin-screw extruder. The second and third methods utilized a single-screw extruder. In the second method, the BaSO4 powder was manually mixed with the polymer prior to extrusion. In the third method, the BaSO4 powder was mechanically attached on the polymer granulates (mechanical blending) prior to extrusion. The mechanical and chemical properties of the radiopaque bioresorbable fibres were measured after processing and during in vitro degradation. The fibres were gamma, plasma or EtO sterilized. There was no statistical difference in the mechanical properties of the fibres when manufactured using the twin-screw extrusion with manual blending or the single-screw extrusion with mechanical blending. The gamma sterilization markedly decreased the initial intrinsic viscosity of all fibres, whereas the plasma and EtO sterilization methods had no effect on the initial intrinsic viscosity. During in vitro testing, the loss in the intrinsic viscosity occurred at the same rate whether the fibres were loaded with the barium sulfate or not.

Theoretical and experimental evaluation of the radial force of self-expanding braided bioabsorbable stents.

This study was carried out to evaluate if the analytical model developed by Jedwab and Clerc for calculating the mechanical properties of metallic braided stents is also valid for bioabsorbable braided stents. An analytical model could be used to shorten the development cycle of stents by reducing the amount of in vitro testing. Jedwab and Clerc derived formulae for longitudinal stiffness and radial pressure stiffness. The longitudinal stiffness was defined by measuring the stent elongation under load. The radial pressure stiffness was defined from the slope of the load-displacement curve measured with the testing method described by Agrawal and Clark where a collar is placed around the stent to compress it. The radial pressure stiffness was measured with and without lubrication to evaluate the effects of friction between the stent and collar and in the stent structure itself. Two bioabsorbable braided stents and one metallic braided stent were used in the measurements. The metal stent test results were consistent with what was reported by Jedwab and Clerc. However, the analytical model was not applicable to bioabsorbable stents. This was mainly due to the larger fibre diameter of the bioabsorbable stents, which prevents the fibres from freely collapsing when the stent diameter decreases. The analytical model is based on an assumption that the fibres behave independently. However, the testing method described by Agrawal and Clark provided a useful tool to compare the radial force of self-expanding stents.

Mechanical properties and in vitro degradation of bioabsorbable self-expanding braided stents.

The aim of this study was to characterize the mechanical and self-expansion properties of braided bioabsorbable stents. In total four different stents were manufactured from PLLA fibres using a braiding technique. The changes in radial pressure stiffness and diameter recovery of the stents were determined initially, and after insertion and release from a delivery device. The braided stents were compared to three commercially available metallic braided stents. The changes in physical and mechanical properties of the PLLA fibres and stents during in vitro degradation were investigated. After release from the delivery device, the PLLA stents did not fully recover to their original diameter. The radial pressure stiffness of the bioabsorbable stents was similar to that of the metallic stents. The in vitro degradation study showed that the stents would keep at least half of their initial radial pressure stiffness for more than 22 weeks.

Mechanical properties and in vitro degradation of bioresorbable knitted stents.

The aim of this study was to characterize the mechanical properties and in vitro degradation of bioresorbable knitted stents. Each stent was knitted using a single self-reinforced fibre made out of either PLLA or 96L/4D PLA or 80L/20G PLGA. The mechanical and physical properties of the fibres and stents were measured before and after gamma sterilization, as well as during in vitro degradation. The mechanical properties of the knitted stents made out of bioresorbable fibres were similar to those of commercially available metallic stents. The knitting geometry (loop height) had a marked effect on the mechanical properties of the stents. The rate of in vitro degradation in mechanical and physical properties for the PLLA and 96L/4D PLA stents was similar and significantly lower than that of the 80L/20G PLGA stents. The 80L/20G PLGA stents lost about 35% of their initial weight at 11 weeks. At this time, they had lost all their compression resistance strength. These data can be used as a guideline in planning further studies in vivo.

Effect of gamma, ethylene oxide, electron beam, and plasma sterilization on the behaviour of SR-PLLA fibres in vitro.

The aim of this study was to evaluate the effect of various sterilization processes on the physical and mechanical properties of self-reinforced bioabsorbable fibres made out of polylactide (PLLA). The samples were sterilized using plasma, ethylene oxide (one and two cycles), gamma (25 kGy at room temperature, 25 kGy in dry ice, and 2 x 25 kGy at room temperature), and electron beam (15, 25, and 55 kGy) sterilization. The intrinsic viscosity, crystallinity, and mechanical properties (modulus of elasticity, yield strength, and ultimate tensile strength) were tested before and immediately after each sterilization treatment, as well as up to 30 weeks in vitro. Compared with unsterilized fibres, the intrinsic viscosity was markedly decreased after radiation sterilization (gamma and electron beam) and the loss in mechanical properties was accelerated during in vitro degradation. Plasma and ethylene oxide (one and two cycles) did not markedly alter the properties of the samples after sterilization or during in vitro degradation. These data are important for determining the effect of various sterilization processes on the physical and mechanical properties of polylactide-based materials and can be used to predict how fast degradation of the mechanical properties of the self-reinforced PLLA will occur. They can also be used to tailor the degradation kinetics to optimize implant design.