Research On The Mechanical Properties Of The Degradable Biliary Stent

Degradable magnesium-alloy stent is a new medical device for the treatment of biliary stenosis.The stent can be fully expanded via the balloon within the bile duct to open and scaffold the stenosed bile duct and relieve symptoms quickly after being implanted into biliary stenosis sites.The interventional treatment of the stent is easy to operate and causes little wound.The degradable stent can be degraded after a period of time,reducing the pain caused to the patient by removing the stent via a second surgery.Therefore,it is important to study the mechanical properties of the biodegradable magnesium-alloy biliary stent for the clinical treatment of the biliary stenosis disease.The expansion performance and the degradation performance are the two most important mechanical properties of degradable stents.In current,researches of these properties are not mature.Accordingly,the article focused on the expansion performance of the magnesium-alloy biliary stent,exploratory studied the degradation performance of the magnesium-alloy biliary stent,and optimized the structure of the stent.The main works and the results of the research are as follows:(1)The effects of ellipticity of the bile duct on expansion performance of the stent have been studied via finite element theory.Three kinds of bile duct models with different ellipticities were established.As a result,the maximum von Mises stress in the stent occurred in the connection part between the wave ring and the transition rod,which is well lower than the limit strength for the stent material.Therefore,the stent would not break during the expansion process.After deflation,an increase in the bile-duct ellipticity made the stent to be expanded more difficultly,thereby inducing an increment of the maximum von Mises stress occurred in the stent,as well as the radial recoil,the ellipticity and the dog bone degree of the stent,whereas the radial displacement and the foreshortening of the stent were diminished.For bile ducts,the reductions of the stenosis degrees of bile ducts were consistent with the clinical requirements after deflation,and the expansion degree of the bile duct with circular cross-section was the highest.Meanwhile,the reduction of the ellipticity of the bile duct with elliptical cross section was the largest,which may make patients uncomfortable.In addition,the larger the ellipticity of the bile duct,the larger maximum von Mises stress in the bile duct.The stress concentrations generally occurred in the short axis plane.However,the maximum von Mises stress in each bile duct was relatively smaller,and the diseased tissue of the bile duct didn't penetrate from the grid of the stent,which indicated that these bile ducts would not be injured by the stent.(2)The degradation model of the magnesium-alloy biliary stent has been established based on the continuous damage models of stents.Making use of the secondary development function of ANSYS,the finite element analysis program controlling the degradation process,including the birth and death process of elements,the update process of stent surface elements and contact elements,was written.Results confirmed that the stent began to degrade after 234 hours and degraded completely after 49 days.The maximum von Mises stress first occurred in the edge of the middle part of the stent.Therefore,the degradation propagated gradually from the middle part to the end part of the stent,as well as from the edge to the medial of the stent.In the process of degradation,the outer diameter of the middle part of the stent was well decreased,while that of the end part of the stent was slightly changed.The dog bone degree and the length of the stent increased generally,whereas the foreshortening of the stent decreased generally.Moreover,the inner diameter of the bile duct kept decreasing.From the beginning of the stent degradation to 690 h,the stent fitted well with the bile-duct wall,and the diseased tissue of the bile duct didn't penetrate through the grids of the stent.The concentrated von Mises stress in the bile duct was relatively smaller,which decreased during the degradation process of the stent,leading to a lower possibility of damaging the bile-duct wall caused by the stent.(3)The parametric modeling of the magnesium-alloy stent has been performed via parametric modeling language(APDL),and an optimized model of the stent has been established based on the optimized design theory.The optimized structure of the stent was obtained via subporder method,of which the connecting rod is thinner and shorter,the transition rod is thinner and longer,and the wave ring is smaller than the original stent.Moreover,the thickness of the optimized stent was smaller than that of the original stent.The expansion performance and the degradation performance of the optimized stent were analyzed.Results revealed a better expansion performance of the optimized stent as a consequence of smaller von Mises stress in the optimized stent and greater expansion degree of the optimized stent.Compared with the original stent,the optimized stent degraded more slowly,not only increased the expansion degree of the diseased lumen,but also decreased the radial displacement and concentration von Mises stress occurred in the healthy tissue located on the end of the bile duct.Furthermore,the variation of the length of the optimized stent was smaller than that of the original stent,thereby reducing the friction between the stent and the bile-duct wall.Accordingly,the optimized stent had a better degradation performance.Results confirmed that the optimized design was reasonable and effective.