Evaluation of Relationship between the Geometry of Crown and Radial Force in a Polydioxanone Coronary Stent

YANG Jie-ying(楊潔瑩)，QUE Zhi-wen(闕志文)，CHEN Nan-liang(陳南梁)，ZHANG Pei-hua(張佩華)*

1 Key Laboratory of Textile Science and Technology，Ministry of Education，Donghua University，Shanghai 201620，China

2 College of Textiles，Donghua University，Shanghai 201620，China

Introduction

The bioabsorbable coronary stent，first introduced by Stack et al.in 1980s［1］，ushered the era of interventional cardiology.Clinically，an ideal bioabsorbable stent would provide adequate endoluminal support for a short-term period in the artery and avoid the potential hazards while degrading［2］.As one of the promising bioabsorbable materials，polydioxanone (PDS)is reported with a relatively high ductility，moderate tensile strength and modulus as well as good biocompatibility and lowtoxicity monomers after degrading［3-4］.For the case of treating refractory benign esophageal strictures，good efficacy was obtained as a PDS stent was implanted into the human body［5］.However，the research of PDS coronary stent is still on the way.When designing a structure or geometry of a PDS coronary stent，multiple parameters should be comprised to optimize the stent with the best performance.Among all the mechanical properties of a coronary stent，the radial force is a key point reflecting the ability of stent to maintain a certain size and shape in the stenosis artery［6-7］.In this paper，we explored the influence of geometry of the coronary stent，namely the length and the height of the crown，upon the variation of the radial force.And finite element method，a classical method to measure the mechanical properties of the stent，was applied for the evaluation.

1 Experimental

1.1 Stent design

The stents we designed were on the basis of the prototype of S7 (Medtronic AVE Inc.)stent and PDS-based stent.With 150 μm of the strut thickness and 3 mm of the internal diameter uniformly，the stents were established as long as 8 mm.Four groups of stents were established by Pro/ENGINEER Wildfire 5.0 (Parametric Technology Corp.，USA).Each group contained three stents，keeping all the other factors the same except the variation of Lcrownor Hcrown.Figure 1 illustrated the parameters involved for evaluation.

Fig.1 Geometry parameters of the coronary stent

1.2 Commputational simulations

All compression simulation tests were performed by ABAQUS/Standard 6.10.1 (Dassault Systemes Simulia Corp.，Providence，RI，USA).Here we adopted platform compression method for the measurement of mechanical strength of the stents，the same method as applied in our laboratory by using YG061 radial compression instrument (Laizhou Electronic Instrument Co.，Ltd.).Before simulation，several parameters of the materials in the simulation were shown in Table 1.

Table 1 Parameters setting of PDS and stainless steel

Two parallel plates made of stainless steel were above and below the models with 12 mm in length，5 mm in width，and 1 mm in thickness.Twelve models were compressed at the speed of 2 mm/min in the radial direction and the maximum compression displacement was 1.5 mm.During the process of compression，the stress was monitored，shown in Fig.2.

Fig.2 The Von Mises stress and simulation of the stent

The radial force is given by

where σ was the compression stress and S was the compressed area.Radial force will increase with the growth of stress.Thus，the relationship of two parameters with radial force could be reckoned through the stress obtained from simulation.

2 Results and Discussion

2.1 Constructed models

Twelve stents were constructed in the experiment.As shown in Table 2，stents differed greatly with the variation of Lcrownor Hcrown.When the value of Lcrowngrew gradually，the number of crown decreased in the circumference direction of the stent in groups 1 and 2.And with the increase of Hcrown，the number of the crown also dropped in the longitude direction of the stent in groups 3 and 4.

Table 2 Variations of Lcrown and Hcrown and their corresponding models

2.2 The relationship between Lcrown and the radial force

As depicted in Fig.3，stress increased continuously when Lcrownwas between 0.8 mm and 1.6 mm in group 1.However，stress slightly declined in group 2 from 1.6 mm to 1.9 mm of Lcrown，but recovered rapidly since 1.9 mm of Lcrown.On the whole，increased Lcrownwould not definitely lead to the growth of the stress.Therefore，the radial force would not always increase as the rising value of Lcrown.

Fig.3 The relationship between Lcrown and the stress

2.3 The relationship between Hcrown and the radial force

Coincidentally enough，the influence of Hcrownon the stress of a coronary stent in groups 3 and 4 were also in disparity in Fig.4.The trend of stress generally declined when Hcrownwas between 0.5 mm and 0.6 mm.But the variation tendency of the stress was hard to tell when Hcrownwas greater than 0.6 mm.In group 3，the stress recovered from the point of 0.666 7 mm，and kept rising after that;while in group 4，stress fell persistently when Hcrowngrew from 0.500 0 mm to 1.333 3 mm.Therefore，there is no definite linear relationship between the radial force of the stent and Hcrown.

Fig.4 The relationship between Hcrown and the stress

3 Conclusions

The radial force would not always increase as the rising value of Lcrownfrom 0.8 mm to 2.4 mm.Meanwhile，there is also no linear relationship between Hcrownand the radial force of the coronary stent.

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