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DE GRUYTER Current Directions in Biomedical Engineering 2020;6(3): 20203114 Paula Rosam*, Michael Stiehm, Finja Borowski, Jonas Keiler, Andreas Wree, Alper Öner, Klaus-Peter Schmitz and Wolfram Schmidt Development of an in vitro measurement method for improved assessment of the side branch expansion capacity Abstract: The expansion capacity and accessibility of the Keywords: stents, side branch accessibility, expansion side branch is essential for the stenting of complex capacity, strut deformation, bifurcation bifurcations. Since previous measurement methods only provide limited informationb ased on geometrical data ofh ttps://doi.org/10.1515/cdbme-2020-3114 stent cells, a new measurement approach was developed which considers the mechanical deformation capacity of the stent design. This approach provides essential information on 1 Introduction the stent with regard to the application of bifurcation stenting. Four different commercially available coronary The term over-expansion capacity of stent designs is used to stents (nominal diameter 3.0 mm) were dilated and a central evaluate how the stent geometry changes when over- strut cell was over-expanded by means balloon catheters of dilatation is performed [1,2]. This parameter is also increasing nominal diameter (2.0 to 5.0 mm). After balloon suggested in the context of bifurcation stenting for the inflation, the remaining cell size was investigated for selection of suitable stents , including a description of how maximum cell diameter and strut fractures. Large expansion side branch accessibility changes when over-dilatation occurs capacity without cell damage is taken as a measure of the (cell opening). accessibility of the side branch. In none of the expansion This is a common approach in various kinds of experiments the desired target size could be achieved, which bifurcation stenting, in which the side branch has to be is due to the elastic recoil of the stent cells. Deviations from stented additionally. High grade stenoses as seen in figure 1 the target diameter between 14-38% were determined. obstruct the blood flow extensively along the bifurcation and However, larger diameters also showed a constriction of the must be treated. balloon, so that in some cases the target diameter could not be achieved at all. No strut fractures occurred even at maximum balloon diameter and pressure (5.0 mm non- compliant balloons). As a result the side branch accessibility differs depending on the individual stent designs. No particular risk for the stent was found by extensive over- dilatation. _____ *Corresponding author: Paula Rosam: Institute for ImplantatTechnology and Biomaterials e.V., Friedrich-Barnewitz- Str. 4, 18119 Rostock-Warnemünde, Germany, mail: email@example.com Figure 1: MicroCT scan of an anatomical bifurcation preparation Finja Borowski, Michael Stiehm, Klaus Peter Schmitz: Institute from the Department of Anatomy (Rostock). Arrows are indicating for ImplantatTechnology and Biomaterials e.V., 18119 Rostock- flow direction. LAD: left anterior descending artery, D1: (diagonal) Warnemünde, Germany side branch. Jonas Keiler, Andreas Wree: Rostock University Medical Center, Department of Anatomy, Rostock, Germany This side-branch accessibility was tested by an approach Alper Öner: Rostock University Medical Center, Department for Cardiology, Rostock, Germany similar to above-mentioned, namely without extra Wolfram Schmidt: Institute for Biomedical Engineering, Rostock mechanical stress by a penetrating balloon / stent system University Medical Center, 18119 Rostock-Warnemünde, (catheter) for the “side-branch-struts”. Germany Open Access. © 2020 Paula Rosam et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License. Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 2 However, the result is only one criterion for success. If the passage with wire and catheter is successful, the dilatation of the side branch also causes a considerable deformation of the cell. The question arises how far a stent cell can be dilated without breaking and thus not becoming a risk to balloon and vessel. Balloon ruptures at sharp-edged strut fractures prevent the expansion, but also the safe evacuation and withdrawal of the balloon. This aspect has already been investigated for bioresorbable scaffolds (BVS Absorb, Figure 2: Xience Sierra 3.0/15 with guide wire ChoiCE PT and Abbott Vascular) and has been evaluated as critical . balloon Pantera 2.0/20 in a Y-silicone model There is a lack of systematic studies for permanent stents. After balloon expansion and retraction, the cell size was determined optically (light microscope SZX16, magnification 2 Material and method 1.25x, Olympus). The maximum inscribed circle was measured with image analysis software (Stream Start, V1.7, For the following coronary permanent drug eluting stents Olympus). The diameter of this circle was used as a measure DES (each n = 1) of the achieved cell size (see Figure 3). ▪ Abbott Xience Sierra 3.0/15 ▪ Boston Scientific Promus PREMIER Select 3.0/16 ▪ BIOTRONIK Orsiro 3.0/15 ▪ Medtronic Resolute Integrity 3.0/15 the resulting cell sizes were determined after probing one TM mesh each with a guide wire (ChoiCE PT 0.014" x 182 cm, REF H74912160011, Boston Scientific). This mesh was then dilated using ascending diameter balloon catheters (see Table 1). The balloon pressure corresponds to the nominal pressure of the balloon (NP) and the expected maximum balloon diameter corresponds to the nominal diameter of the balloon. The used non-compliant balloons for larger test diameters allow at the same time maximum pressure and represent a highly challenging case. The procedure is shown in Figure 2. Figure 3: Xience Sierra 3.0/15 - Mesh expanded with balloon Table 1: Balloon catheters used for over-expansion of the stent Pantera Leo 5.0/30. Diameter measurement of the inscribed circle cells; balloon parameters are: dimension, balloon pressure and was used as a measure for the mesh size compliance Due to the three-dimensional deformation of the stent, a strict Dimension Ballon pressure Ballon catheter comment [mm] [atm] vertical top view was not possible. Horizontal distortions may occur due to the position of the stents under the semi- Biotronik Pantera 2.0/20 7 microscope. As long as the inscribed circle is vertically compliant limited by the stent structure, the diameter of the circle is still semi- Biotronik Pantera 3.0/20 7 correct, as no distortions occur in this direction. compliant semi- Biotronik Pantera 3.5/25 7 compliant 3 Results Biotronik Pantera non- 4.0/30 14 LEO compliant During diameter increase of the dilating balloon, an Biotronik Pantera non- 5.0/30 14 LEO compliant increasing constriction of the balloon through the mesh was detected. This effect was strongest at the maximum diameter (d = 5.0 mm) Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 3 The following Figure 4 shows an example of the increasing The cell diameters reached during the over-expansion process deformation of a stent mesh when stretched from 2.0 to of all stents are shown in Figure 5. 5.0 mm. As a result of the severe deformation of the cell, in none of the investigated stents were torn. However, at maximum expansion an almost circular cell without visible reserve for further diameter increase was seen. A strong deformation of adjacent cells also occurred. Figure 5: Stent cell diameter in relation to the diameter of the expanding balloon. 4 Discussion The achieved cell diameters were mostly about 0.5 mm smaller than the targeted diameters. On the one hand this is due to the constriction of the balloons which was especially seen with large balloon diameters. This is remarkable because especially for the diameters 4.0 and 5.0 mm non- compliant balloons were used, which were dilated with high balloon pressure (14 atm). The result indicates considerable mechanical stress on the cell. On the other hand, every plastic deformation is accompanied by an elastic component, which leads to a restoring movement after mechanical loading. Thus, the cell size also shows elastic recoil. At the target diameter of 2.0 mm, the difference to the achieved mesh size was maximal (28 - 38%). The mesh structure was only slightly deformed and the proportion of elastic deformation dominates. As the target diameter increases, the difference initially becomes smaller (at 3.5 mm: 14 - 17%) since the proportion of plastic deformation increases. At the largest target diameter of 5.0 mm, there is a clear obstruction of the balloon expansion (constriction), so that the stent mesh was not expanded to this target diameter. Figure 6 provides an overview of the measured deviations from the target diameter. Over-expansion of stent cells larger than the diameter of Figure 4: Side branch expansion of the Resolute Integrity 3.0/15. the main branch is of little relevance to clinical practice. Increasing deformation of a stent mesh when stretched with However, since the stents investigated were examined at balloons from 2.0 to 5.0 mm. nominal diameter, they have a capacity for over-dilatation up to 3.5 - 4.75 mm according to the manufacturer's specifications [5–8] Paula Rosam et al., Development of an in vitro measurement method for improved assessment of the side branch expansion capacity — 4 Orsiro Resolute Integrity Xience Sierra Promus Premier Select These values show no correlation with the new in vitro measurements. They are much lower than needed for probing the side branch. However, since an additional widening and deformation of the meshes occurs, especially with regard to the stenting of bifurcations, the side branch expansion capacity should also be measured in the future. Author Statement Research funding: Financial support by the European Regional Development Fund (ERDF) and the European 2 3 3.5 4 5 Social Fund (ESF) within the collaborative research between Target diameter of the balloon [mm] economy and science of the state Mecklenburg-Vorpommern Figure 6: Difference between target diameter and achieved mesh within the project “TheraMagna” is gratefully acknowledged. size as a function of the diameter of the expanding balloon Conflict of interest: Authors state no conflict of interest. On the other hand since it is assumed that secondary References branches are smaller than the main vessel, dilatation up to 5.0 mm is a very extreme case. The absence of strut failure is  Foin N, Sen S, Allegria E et al. Maximal expansion capacity a sign of high stent quality and material strength. It facilitates with current DES platforms: a critical factor for stent selection low-risk use in bifurcation stenting from mechanical point of in the treatment of left main bifurcations? EuroIntervention 2013; 8: 1315–1325; DOI: 10.4244/EIJV8I11A200 view. The deformation of adjacent cells including the kinking  Ng J, Foin N, Ang HY et al. Over-expansion capacity and of the stent would probably have been less severe in a closed stent design model: An update with contemporary DES vascular model, thus representing a worst-case scenario. In platforms. Int J Cardiol 2016; 221: 171–179; DOI: such a model, however, the measurement could not have 10.1016/j.ijcard.2016.06.097 been performed with the same accuracy.  Foin N, Alegria E, Sen S et al. Importance of knowing stent design threshold diameters and post-dilatation capacities to The novel investigation shows a new relevance optimise stent selection and prevent stent compared to previous measurements of side branch overexpansion/incomplete apposition during PCI. Int J accessibility. In a common benchmark of different stents, as Cardiol 2013; 166: 755–758; DOI: it is also performed in our test laboratory, only the size of the 10.1016/j.ijcard.2012.09.170  Foin N, Lee R, Bourantas C et al. Bioresorbable vascular expanded mesh is measured after conventional stent scaffold radial expansion and conformation compared to a dilatation to nominal pressure. The corresponding metallic platform: insights from in vitro expansion in a measurement results are shown in Table 2. coronary artery lesion model. EuroIntervention 2016; 12: 834–844; DOI: 10.4244/EIJV12I7A138 Table 2: Tested stents and their side branch accessibility  Medtronic Inc. Resolute Integrity™ Zotarolimus-Eluting Coronary Stent System. Instructions for Use. 2015  BIOTRONIK. Orsiro: Ultrathin struts. Superior patient Tested Stents side branch accessibility [mm] outcomes.; 2019  Boston Scientific. Promus PREMIER™ - Directions for Use (if Abbott Xience Sierra 0.967 your product does not contain CLIPIT™ Hypotube Clips): 3.0/15 91054414. Aufl.; 2015 Boston Scientific Promus  Abbott Vascular. Abbott Vascular XIENCE Xpedition, 0.891 PREMIER Select 3.0/16 XIENCE Xpedition SV, and XIENCE Xpedition LL Everolimus BIOTRONIK Orsiro Eluting Coronary Stent System - U.S. 2015 0.871 3.0/15 Medtronic Resolute Integrity 0.640 3.0/15 Deviation of the opened meshes from the target diameter in %
Current Directions in Biomedical Engineering – de Gruyter
Published: Sep 1, 2020
Keywords: stents; side branch accessibility; expansion capacity; strut deformation; bifurcation
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