Abstract
Objectives
To compare on the bench the physical and mechanical properties of Magmaris, a magnesium bioresorbable scaffold (BRS), with Absorb and DESolve polymeric BRS and a permanent metallic stent.
Background
Understanding the mechanical and physical properties of BRS is crucial for appropriate implantation and postdilatation.
Methods
Testing was performed in fluid at 37°C and in silicone bifurcation phantoms with a 30° angle between main branch (MB) and side branch.
Results
The 3.0‐mm Magmaris BRS did not fracture after MB postdilatation up to 4.4 mm in contrast to the Absorb where the safe postdilatation diameter was 3.7 mm. For dilatation through stent cells, there were no Magmaris fractures with 3.0‐mm noncompliant (NC) balloons inflated to nominal pressure. Mini‐kissing balloon postdilatation with two 3.0‐mm NC balloons up to 17 atm was without fracture except for an outlier. Longitudinal and radial strengths were similar for Magmaris and Absorb BRS. The crossing profile for the Magmaris was larger than other devices. Recoil 120 min after deployment was the greatest for Magmaris but 120 min after 3.5 mm postdilatation all devices had similar diameters.
Conclusions
The Magmaris BRS was more resistant to strut fracture than Absorb. It had a larger crossing profile than other devices and similar radial and longitudinal strengths to Absorb. While recoil after deployment was greater with Magmaris, 120 min after 3.5 mm postdilatation all devices had similar diameters.
Keywords: bifurcations, bioresorbable scaffolds, mechanical properties, percutaneous coronary intervention, stent, stenting technique, strut fracture
1. INTRODUCTION
Permanent metallic drug‐eluting stents (DES) have a late incidence of adverse events of a few percent annually indefinitely. It is hoped that full resorption of a bioresorbable scaffold (BRS) will reduce this. 1 , 2 , 3 Current BRSs have important limitations including inferior deliverability, lower radial strength, lower expansion capacity, and higher rates of scaffold thrombosis when compared with permanent DES. 4 , 5 , 6 , 7 As different BRS platforms have different properties, the interventional cardiologist needs to be aware of these differences to ensure safe and effective deployment. The magnesium‐based Magmaris BRS (Biotronik AG, Bülach, Switzerland) has shown promising results in recent trials 8 , 9 Mechanical properties of the Magmaris magnesium alloy are closer to permanent metallic DES than those of poly‐L‐lactic acid (PLLA) scaffolds, and this may be reflected in improved properties of the Magmaris over PLLA scaffolds.
In this study, we compared the mechanical properties of the Magmaris BRS with the Absorb (Abbott Vascular, Santa Clara, CA) and DESolve BRS (Elixir Medical Corporation, Sunnyvale, CA) and with the permanent metallic DES MultiLink8 (ML8)/Xience Xpedition (Abbott Vascular). Our focus was on safe postdilatation thresholds and side branch (SB) dilatation strategies. We compared our Magmaris results with those for Absorb, DESolve, and Xience using the same methodology. 10 , 11 , 12
2. MATERIALS AND METHODS
2.1. Scaffolds and stents
The Magmaris BRS is a sirolimus‐eluting magnesium alloy scaffold and is the first metallic BRS in clinical use. The alloy degrades to magnesium hydroxide, magnesium phosphate, and amorphous calcium phosphate. The degradation process is relatively fast compared with other BRS with 95% of the magnesium resorbed after 12 months. The design of the scaffold is in‐phase sinusoidal hoops with two connecters per hoop linking valleys and peaks (Figure 1). There are two radiopaque tantalum markers at each end. The strut thickness and width are 150 μm for the 3.0‐mm scaffold. 13
FIGURE 1.
Comparison of strut dimensions and vessel coverage for bioresorbable and permanent stents. Measurements provided by manufacturers
The Absorb BRS was everolimus‐eluting and constructed from PLLA. It had in‐phase sinusoidal hoops with six peaks and valleys per hoop and three connecters. The strut thickness including polymer coating was 157 μm, the hoop width was 191 μm, and the connector width 140 μm for the 3.0‐mm scaffold (Figure 1). 10 Absorption time was around 3–4 years.
The DESolve 150 BRS is a novolimus‐eluting PLLA‐based scaffold with a resorption time of 1–2 years. The design is sinusoidal hoops with nine peaks and valleys and three connecters per hoop. Strut thickness including polymer coating is 150 μm. Hoop width is 165 μm and connector width 100 μm (Figure 1). 10
The DESolve Cx is also a novolimus‐eluting PLLA‐based scaffold with a similar design but thinner hoops at 120 μm (Figure 1).
The ML8 is the bare metal platform of the everolimus‐eluting Xience Xpedition DES. It is constructed from cobalt chromium with in‐phase sinusoidal hoops. Each hoop has six peaks and valleys and three connecters. Strut thickness including the polymer coating according to the manufacturer is 89 μm (Figure 1).
2.2. Bench test setup
Bench testing of the Magmaris BRS in this study is compared with the previously published results for Absorb, DESolve 150, and ML8. 10 , 11 , 12
Tests were performed in a 37°C simulated body fluid (SBF) bath except for SB dilatation, which were performed in a 37°C water bath. Crossing profile was measured in air.
Deployments and postdilatations were recorded fluoroscopically. Photographs were recorded with an EOS‐5D Canon digital camera (Canon, Inc., Tokyo, Japan) and a Leica Z6 APO microscopic lens (Leica Microsystems, Wetzlar, Germany). All measurements were performed on photographs using the Image Pro Plus software (Version 7.0.0.591 Media Cybernetics, Inc., Bethesda, MD). Calibration was performed before each measurement. Selected scaffolds were imaged by micro‐CT (SkyScan1172, Sky‐Scan, Belgium). All balloon dilatations were performed using noncompliant (NC) balloons.
The number of devices for each design (3.0 mm nominal diameter) in each test is shown in Table 1.
TABLE 1.
Number of devices of each design for each test
| Magmaris | Absorb | DESolve | DESolve cx | Xience/ML8 | |
|---|---|---|---|---|---|
| Crossing profile | 10 | 6 | 5 | 5 | 3 |
| Recoil | 10 | 10 | 5 | 5 | 5 |
| Main branch postdilatation | 11 | 13 | 5 | 5 | 2 |
| Side branch dilatation | 10 | 24 | 14 | 9 | 5 |
| Mini‐kissing postdilatation | 5 | 10 | 4 | 5 | 5 |
| Radial strength | 5 | 5 | — | — | 5 |
| Longitudinal strength | 5 | 5 | — | — | 5 |
Abbreviation: ML8, MultiLink8.
2.3. Scaffold crossing profile
The devices were photographed in air from two orthogonal projections. Crossing profile was measured as the diameter of the undeployed scaffold mounted on the delivery system.
2.4. Recoil
Recoil was the percentage change in device diameter measured on the inflated balloon at nominal pressure and after balloon deflation (Figure 3). The scaffold was in SBF for 1 min before unconstrained deployment at nominal pressure. One minute after deployment, the scaffold was removed from the bath, without being deflated and was photographed for diameter measurements. The scaffold was then returned to the bath before deflation. Measurements were performed 1 and 10 min after deflation. After 10 min, the devices were separated into two groups. Group 1 underwent postdilatation with a 3.5‐mm NC balloon at nominal pressure and was measured immediately and at 1 and 10 min and at 1 and 2 hr. Group 2 was measured postdeployment at 1 and 10 min and at 1 and 2 hr before postdilatation. After postdilatation measurements were performed at 1 and 10 min. The results for postdilatation measurements at 1 and 10 min were pooled from the two groups as device diameters were stable 10 min after deployment (Figure 3).
FIGURE 3.
Recoil after deployment and after postdilatation. (a) Recoil assessed by measuring the external diameter of 3.0‐mm scaffolds/stents at deployment and over 2 hr following balloon deflation is plotted. The diameter seems stable 10 min after inflation except for DESolve, which is known for self‐expansion. (b) Recoil of devices over 2 hr following postdilatation with a 3.5‐mm noncompliant balloon at 12 atm. Half of the devices were postdilated 10 min after inflation and the other half after 2 hr. The results for 1 and 10 min after postdilatation were pooled as measurements were stable after 10 min. While Magmaris had the greatest recoil over the 2 hr following deployment, at the end of 2 hr following postdilatation, all devices had similar external diameters
2.5. Phantom design
These were three‐dimensional (3D) printed in with a 30° angle B between the main branch (MB) and the SB. 14 The diameter of the proximal MB was 3.5 mm, distal MB was 3.0 mm, and SB was 3.0 mm.
2.6. MB postdilatation and strut fracture
Devices unconstrained in an SBF bath at 37°C were postdilated to 18 atm with increasing diameter balloons (3.5, 4.0, 4.5, and 5.0 mm) or until strut fracture. Photographs were taken at every 2 atm pressure increase to document fracture. When fracture occurred, the scaffold and balloon were removed from the bath and rephotographed for diameter measurements. The percentage of scaffolds with fracture was plotted against balloon diameter (Figure 4).
FIGURE 4.
Scaffold strut fracture and balloon diameter. Percentage of 3.0‐mm scaffolds with fracture with increasing main branch balloon diameter
2.7. SB dilatation
Evaluation of balloon inflation into the SB through scaffold/stent cells after MB implantation was performed under fluoroscopic control in a 37°C water bath. Scaffolds and stents with a nominal diameter of 3.0 mm were deployed in the MB of the silicone phantoms. Proximal optimization technique (POT) was performed using 3.5‐mm NC balloons with slow inflation at approximately 2 atm/s to nominal pressure. The SB was dilated with increasing balloon diameters (2.0, 2.5, and to 3.0 mm) at 14 atm. If no fractures were observed, the 3.0‐mm balloon was inflated to 18 and 22 atm. The scaffolds were observed for strut fracture at each incremental step in pressure. Images were recorded by both fluoroscopy and photography (Figure 5a).
FIGURE 5.
Scaffold fracture with SB dilatation and with mini‐KBPD. On the horizontal axis is balloon pressure and on the vertical axis different devices. The red stars indicate strut fractures, and green dots indicate the pressures where inspections were made and where there were no fractures. Black arrows indicate outliers. (a) Strut fracture in individual scaffolds or stents dilated through a cell toward the SB with a 3.0‐mm balloon. (b) Strut fracture after mini‐KBPD with two 3.0‐mm balloons at increasing pressures. Mini‐KBPD means simultaneous inflation of a balloon in the MB and a balloon in the SB with minimal overlap of these balloons in the MB. MB, main branch; mini‐KBPD, mini‐kissing balloon postdilatation; SB, side branch
2.8. Mini‐kissing balloon postdilatation and strut fracture
Mini‐kissing balloon postdilatation (Mini‐KBPD) refers to simultaneous inflation of a balloon in the MB and an SB balloon that protrudes only a few millimeters into the proximal MB. 11 Mini‐KBPD was performed in phantoms of the same design as above using two 3.0‐mm NC balloons that were inflated slowly and simultaneously until fracture was observed or 20 atm pressure achieved (Figure 5b).
2.9. Radial strength
The radial strength of the stents and scaffolds was measured by the cross‐sectional area reduction of an expanded device exposed to increasing external pressure at 0.1 mmHg increments in a 37°C SBF bath. An Intravascular ultrasound (IVUS) catheter within the scaffold or stent measured cross‐sectional area loss with the Clearview IVUS quantitative analysis, which was plotted against increasing pressure. Pressures required to reduce cross‐sectional area by 10 and 25% and to cause collapse were plotted (Figure 7).
FIGURE 7.
Scaffold radial strength. This is depicted as the pressure to reduce device cross‐sectional area by 10 and 25% and to cause device collapse
2.10. Longitudinal strength
Longitudinal strength was assessed with an Instron 5866 Universal Testing Machine employing a previously published setup. 12 Devices were deployed in the testing rig with proximal malapposition. A hypotube was lowered to touch the valley of the proximal hoop (Figure 8). The scaffold was compressed to 4 mm in 37°C SBF with a stop each 1 mm for photography while the Instron recorded force applied in Newtons.
FIGURE 8.
Point longitudinal compression testing. (a) Force is applied to a point on the most proximal hoop of the Magmaris bioresorbable scaffold by a rod (R) attached to an Instron. Hoops were pushed together (X) on that side of the lumen, and malapposition was prominent on the opposite side (double‐headed arrow). (b) Point compression up to 4.0 mm for four stents/scaffolds. With compression up to 3.0 mm, the Magmaris and Absorb showed less longitudinal strength than the MultiLink 8
2.11. Statistics
Descriptive statistics are presented as mean ± SD or as counts (%). Differences in categorical variables were analyzed using the Fisher's exact test and differences in continuous variables by the Mann–Whitney test. A p value of .05 was considered significant. All p values resulted from two‐sided tests. Statistical analyses were performed using the Microsoft Excel or STATA (version 12; StataCorp LLC, Texas).
3. RESULTS
3.1. Crossing profile
The crossing profile of the Magmaris BRS (1.48 ± 0.01 mm) was larger than Absorb BRS (1.43 ± 0.02 mm) and ML8 permanent stent (1.14 ± 0.01 mm), p = .01 and p = .01, respectively (Figure 2).
FIGURE 2.
Crossing profiles. The crossing profiles (mm) of bioresorbable scaffolds and a permanent metallic drug‐eluting stent are shown
3.2. Recoil
Stent and scaffold diameters seemed stable 10 min (min) after inflation (Figure 3a), except for the DESolve device which is known to self‐expand. Recoil measured 120 min after deployment was the largest in the Magmaris and Absorb BRS but postdilatation with a 3.5‐mm balloon at nominal pressure, while there was some recoil with all devices, external diameters were greater when compared with the strategy of inflating to nominal pressure without postdilatation. Resulting external device diameters 120 min after postdilatation of Magmaris and Absorb BRS were similar at 3.62 ± 0.03 and 3.61 ± 0.02 mm, respectively, p = .35 (Figure 3).
3.3. MB postdilatation and strut fracture
The 3.0‐mm Magmaris BRS did not fracture during postdilatation with balloon diameters up to 4.4 mm, and we observed no fractures with postdilatation with a 4.0‐mm NC balloon at any pressure. Compared with other BRS, the Magmaris was less likely to fracture than the Absorb but more likely than the DESolve (Figure 4). All Magmaris fractures after MB postdilatation were in scaffold hoops.
3.4. SB dilatation
We observed no fractures with inflation of 3.0‐mm SB balloons up to 22 atm, except for on outlier, which occurred at a pressure less than 14 atm (Figure 5a). This outlier fracture was in a connector at its junction with the hoop and was likely caused by a damaged balloon tip catching on the strut during difficult passage of the balloon through the side of the scaffold.
3.5. Mini‐KBPD, strut fracture, and correction of distortion
With mini‐KBPD of the Magmaris with two 3.0‐mm balloons, all fractures occurred at 18 atm or more except for one unexplained outlier at 5 atm (Figure 5b). All these fractures were at the junction of connector and hoop. Distortion of the scaffold due to SB dilatation (malapposition opposite to the SB and metallic narrowing in the scaffold distal to the SB) was abolished with mini‐KBPD pressure at 10 atm or more (Figure 6).
FIGURE 6.
Magmaris scaffold distortion after SB dilatation and correction with mini‐KBPD. (a) A 3.0‐mm Magmaris bioresorbable scaffold after SB dilatation with a 3.0‐mm noncompliant balloon at 14 atm is distorted. Notice the malapposition (double‐headed red arrows) opposite to the SB and metallic narrowing (double‐headed yellow arrow) just distal to the SB. (b) Mini‐KBPD at 5 atm partially corrects the malapposition and narrowing. (c) Mini‐KBPD at 10 atm was the lowest pressure to correct distortion. (d) Mini‐KBPD at 15 atm. mini‐KBPD, mini‐kissing balloon postdilatation; SB, side branch
3.6. Radial strength
The pressure required to reduce the cross‐sectional area of the Magmaris scaffold by 10% was similar to that of Absorb (1.26 ± 0.23 vs. 1.27 ± 0.14 bar, p = .67). The pressure needed to reduce Magmaris by 25% was greater than that for Absorb (1.8 ± 0.08 vs. 1.37 ± 0.18 bar, p = .01) (Figure 7).
3.7. Longitudinal strength
The force applied by the rod causing 4‐mm compression resulted in scaffold hoops being pushed together, obstructing the lumen and dragging opposite struts in toward the lumen causing malapposition. We found no difference in the force required to compress the scaffold 4 mm when comparing Magmaris and Absorb BRS (0.58 ± 0.12 and 0.56 ± 0.11 N, p = .75). The ML8 required numerically higher pressures (0.71 ± 0.15 N); however, this did not reach statistical significance (p = .08).
4. DISCUSSION
In this bench study of the 3.0‐mm Magmaris BRS compared with previously tested scaffolds and stents, the main findings were as follows:
MB postdilatation with balloon diameters less than 4.4 mm did not cause fracture.
SB dilatation with a 3.0‐mm balloon up to 22 atm did not cause fracture except for one outlier at 14 atm.
Mini‐KBPD with two 3.0‐mm balloons was safe with pressures up to 18 atm, except for a single outlier at 5 atm. Mini‐KBPD at 10 atm or more corrected distortion after SB dilatation.
Radial strength (pressure to reduce cross‐sectional area by 10%) was similar for Magmaris and Absorb. For 25% reduction in cross‐sectional area, Magmaris required a greater pressure than Absorb and hence had greater radial strength.
Magmaris had more acute recoil after deployment than Absorb, but 120 min after postdilatation with a 3.5 mm NC balloon, the diameters of devices were similar.
Concerns about late outcomes with durable metallic DES prompted the development of BRS that leaves no foreign material in the coronary artery after a scaffold‐specific resorption time. Absorb BRS was the first fully bioresorbable device evaluated in randomized trials powered for clinical outcome. 10 , 11 , 15 , 16 , 17 , 18 The crossing profile of the Absorb reduced its deliverability with less device success than durable DES in randomized trials. 15 , 16 , 19 The Absorb had a higher rate of thrombosis than contemporary durable metallic DES. 4 , 20 Possible causes include implantation in small diameter vessels, under expansion, intraluminal strut degradation, and loss of radial strength or fracture leading to scaffold detachment or collapse. In addition, the scaffold strut thickness and width along with the relatively large volume of foreign material likely caused increased platelet activation and predisposed to stent thrombosis. 21
The Magmaris BRS was evaluated in the BIOSOLVE studies and showed good clinical results and no definite scaffold thrombosis. 9 , 22 The magnesium alloy has mechanical properties that are somewhat closer to cobalt chromium compared with PLLA used as the Absorb backbone. Thrombogenicity of the Magmaris is lower than that for Absorb BRS. 23 , 24
Our bench evaluation of Magmaris BRS showed higher expansion capacity without scaffold fracture compared with Absorb both in the main lumen and through the side of the scaffold. The outliers where Magmaris fractured at low balloon pressures (Figure 5) may be related to the position of the rewired cell across the SB ostium. If the rewired cell is only partly across the SB ostium, it is possible that struts are trapped and unable to move with the balloon expansion resulting in fracture. 25 The DESolve BRS and the ML8/Xience Xpedition did not fracture during SB dilatation or mini‐KBPD.
The stent distortion observed after SB dilatation occurs with all devices. 26 Mini‐KBPD corrected distortion without losing the positive aspects of SB dilatation, specifically the SB ostial size and protrusion of struts into the SB (Figure 6). The fractures after mini‐KBPD were single connectors and were minimally displaced without luminal protrusion and unlikely to have clinical significance. Single connector fractures are less likely to be detected by 2D IVUS or 2D OCT and even by 3D OCT due to minimal displacement of fractured ends. 27 An in vivo bifurcation study performed with implantations in rabbits found no scaffold fractures by the provisional approach with predilatation, POT, SB opening, and KBPD strategies. 27
A major limitation of the Absorb BRS was increased rates of scaffold thrombosis. 4 , 7 Magmaris has a smaller footprint within the artery than Absorb. The percentage vessel coverage for a 3.0‐mm device was 20% for Magmaris and was 27% for Absorb. In addition, the Magmaris struts are thinner and narrower (Figure 1). These features along with the lower thrombogenicity of the magnesium alloy may lead to a lower incidence of scaffold thrombosis for Magmaris. However, large clinical studies with Magmaris would be required to determine this.
Achieving sufficient acute radial strength while keeping strut thickness and width as small as possible is a design challenge for BRS. Magmaris has higher radial strength than Absorb but less compared with permanent metallic stents. The evaluated scaffolds showed differences in recoil over the first 2 hr with a rather large loss by Magmaris when delivered at nominal pressure. Routine postdilatation of the 3.0‐mm Magmaris with a 3.5‐mm NC balloon countered this difference so that there was little difference between all evaluated devices after 2 hr (Figure 3).
5. LIMITATIONS
Bench tests of devices may not predict clinical performance. The phantoms in which devices were deployed were disease free, and elasticity may not be similar to human arteries. The number of tested scaffolds and stents of each design was limited thus affecting power of statistical comparison. As data were collected at different timepoints, this could potentially cause variations in test conditions. Still, tests were performed using the same protocol and testing equipment, in the same laboratory by the same personnel. The results apply to the tested scaffolds and do not apply to other BRS.
6. CONCLUSIONS
The mechanical properties of the Magmaris BRS were superior to the Absorb BRS but inferior to durable metallic stents. The acute recoil with a 3.0‐mm Magmaris was countered with routine postdilatation with a 3.5‐mm balloon. For Magmaris, postdilatation of a scaffold with a balloon up to 4.4 mm diameter was safe. SB dilatation up to 14 atm was generally safe as was mini‐KBPD up to 15 atm. Stent and scaffold distortion due to SB dilatation can be corrected by mini‐KBPD at 10 atm.
CONFLICT OF INTERESTS
T.Ø.B. and B.W. declare no conflict of interests. N.R.H.: Institutional research grants and speaker fees from Abbott, Biotronik, Boston Scientific, and Reva Medical. J.A.O.: Advisory Board and minor honoraria from Boston Scientific.
ACKNOWLEDGMENTS
The data on Absorb, DESolve, and Xience were previously published. 10 , 11 , 12 Figures 2, 3, 4, 5, and 7 are amended with new data on Magmaris and DESolve Cx and reprinted from EuroIntervention 2015/Vol 11, John A. Ormiston, Bruce Webber, Ben Ubod, Olivier Darremont, Mark W.I. Webster, An independent bench comparison of two bioresorbable drug‐eluting coronary scaffolds (Absorb and DESolve) with a durable metallic drug‐eluting stent (ML8/Xpedition), Page 60‐7, 2015, with permission from Europa Digital & Publishing. The study was supported by Biotronik with investigators having full control of the data.
Barkholt TØ, Webber B, Holm NR, Ormiston JA. Mechanical properties of the drug‐eluting bioresorbable magnesium scaffold compared with polymeric scaffolds and a permanent metallic drug‐eluting stent. Catheter Cardiovasc Interv. 2020;96:E674–E682. 10.1002/ccd.28545
Funding information Biotronik, Grant/Award Number: Scaffolds for testing were provided
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