Peter Behrens

A comparison of the mechanical performance characteristics of seven drug-eluting stent systems†

Objectives: Mechanical properties of drug eluting stents (DES) will be measured to provide comparable numerical data to assess deliverability, and thus clinical performance. Background: DES are routinely used in coronary interventions to reduce the rates of restenosis and target vessel revascularizations. Current research is primarily concerned with issues related to late stent thrombosis. However, mechanical properties of DES are a critical determinant of deliverability, and consequently the ultimate arbiter of their clinical performance. Methods: Mechanical properties (pushability, trackability, crossability) were measured under standardized in‐vitro conditions. The vessel models were derived from typical vessel anatomy but adapted to the individual tests. Additionally, profile and bending forces of the stent segment of the delivery system were measured. Seven different commercially available balloon‐expandable coronary DES systems were included. All stents were 3.0 mm diameter with a stent length from 14 to 18 mm. Results: The pushability expressed as the ratio of distal force at a specific proximal push force (4N) ranged between 38.66 and 18.53%. The trackability as the mean track‐forces ranged from 0.551 N to 1.137 N. One stent system could not pass this test. The mean crossing forces at a 1.4 mm stenosis model ranged from 0.038 N up to 0.103 N. The mean crimped stent profiles ranged from 1.055 mm to 1.198 mm and the bending stiffness of the crimped stent was 17.22 to 47.20 Nmm2. Conclusion: Better understanding of mechanical properties of DES shall improve tactile skills of the interventionists during PCI and to improve criteria for DES selection in specific clinical settings.

Preliminary Clinical Results and Mechanical Behavior of a New Double-Layer Carotid Stent:

Purpose: To evaluate the in vitro mechanical and clinical implant behavior of a next-generation double-layer stent designed for the carotid artery. Methods: The new double-layer CASPER-RX stent was implanted in 12 patients (median age 69 years; 8 men) with high-grade symptomatic internal carotid artery stenoses (mean 82%). In the in vitro experiments, the CASPER-RX stent (8-×40-mm model) was investigated with respect to its radial force on expansion and the bending stiffness of the stent system and of the stent in its expanded state, as well as the collapse pressure in a thin, flexible sleeve. The wall adaptation of the expanded stents was assessed by fluoroscopy after release in a step and curve model. Results: Technical success was achieved in all patients without complications; there was no peri- or postinterventional stroke and no stroke or restenosis after 6 months. In the experimental studies, the bending stiffness of the stent on the delivery system (154.9 Nu2009mm2) was significantly lower than when expanded in a 7-mm flexible tube (467.4 Nu2009mm2). The radial force on expansion of the stent to 7 mm was low (0.011 N/mm). The collapse pressure was relatively high (0.56 bar) as a result of the stent’s particular stent structure. The stent exhibited significant foreshortening of 27.6%. The conformability to the wall in the step model was relatively smooth; in the curve model, straightening occurred with consecutive slight stenosis. Conclusion: The first clinical results showed a safe implantation behavior without the occurrence of any ischemia. The structure of the new CASPER-RX stent creates an acceptable flexibility, low radial force, and high collapse pressure. The large foreshortening during implantation should be considered as well as the higher bending stiffness, especially when used in elongated carotid arteries.

Performance Characteristics of Modern Self-Expanding Nitinol Stents Indicated for SFA

PURPOSEnTo evaluate performance characteristics of currently available superficial femoral artery (SFA) stents and stent delivery systems (SDS).nnnMATERIALS AND METHODSnSix 7xa0mm/xa080xa0mm stent systems were included: BIOTRONIK Astron Pulsar (4F), EDWARDS LifeStent Flexstar, ev3 PROTÉGÉ Everflex, CORDIS Smart Control, BARD E-Luminexx, GUIDANT Absolute (all 6F). The SDS were evaluated for profile, flexibility in the stent region, trackability and pushability through a tortuous vessel model and release force during deployment. The stents were evaluated for flexibility, radial force during expansion and compression, and shortening.nnnRESULTSnThe 4F system had a profile of 1.45xa0mm, and the 6F stent systems had a profile of 1.96xa0-xa02.10xa0mm. The Astron Pulsar was most flexible (195 Nmm (2)) compared to 334xa0-xa0972 Nmm (2) for the 6F systems. The track force of the stiffest system (Flexstar, 0.314N) was higher than that of the Astron Pulsar (0.273N) but lower than that of the other systems (0.387xa0-xa00.579N). The release force was 1.69N (Absolute), 2.05N (Astron Pulsar) up to 13.00N (Flexstar). The radial force for a 6xa0mm stent diameter during expansion ranged from 3.95N (Absolute) and 3.99N (Astron Pulsar) up to 7.22N (FlexStar) but was higher when compressed.nnnCONCLUSIONnThe 4F system had the best flexibility and trackability. The release force was high in most systems with release handles. The radial force of all tested stents covers a broad range. These results could be helpful to find the best stent for different lesions.

In vitro performance investigation of bioresorbable scaffolds – Standard tests for vascular stents and beyond

BACKGROUND/PURPOSEnBiodegradable polymers are the main materials for coronary scaffolds. Magnesium has been investigated as a potential alternative and was successfully tested in human clinical trials. However, it is still challenging to achieve mechanical parameters comparative to permanent bare metal (BMS) and drug-eluting stents (DES). As such, in vitro tests are required to assess mechanical parameters correlated to the safety and efficacy of the device.nnnMETHODS/MATERIALSnIn vitro bench tests evaluate scaffold profiles, length, deliverability, expansion behavior including acute elastic and time-dependent recoil, bending stiffness and radial strength. The Absorb GT1 (Abbott Vascular, Temecula, CA), DESolve (Elixir Medical Corporation, Sunnyvale, CA) and the Magmaris (BIOTRONIK AG, Bülach, Switzerland) that was previously tested in the BIOSOLVE II study, were tested.nnnRESULTSnCrimped profiles were 1.38±0.01mm (Absorb GT1), 1.39±0.01mm (DESolve) and 1.44±0.00mm (Magmaris) enabling 6F compatibility. Trackability was measured depending on stiffness and force transmission (pushability). Acute elastic recoil was measured at free expansion and within a mock vessel, respectively, yielding results of 5.86±0.76 and 5.22±0.38% (Absorb), 7.85±3.45 and 9.42±0.21% (DESolve) and 5.57±0.72 and 4.94±0.31% (Magmaris). Time-dependent recoil (after 1h) was observed for the Absorb and DESolve scaffolds but not for the Magmaris. The self-correcting wall apposition behavior of the DESolve did not prevent time-dependent recoil under vessel loading.nnnCONCLUSIONSnThe results of the suggested test methods allow assessment of technical feasibility based on objective mechanical data and highlight the main differences between polymeric and metallic bioresorbable scaffolds.

Clinical Results and Mechanical Properties of the Carotid CGUARD Double-Layered Embolic Prevention Stent.

Purpose: To report early clinical outcomes with a novel double-layer stent for the internal carotid artery (ICA) and the in vitro investigation of the stent’s mechanical properties. Methods: A prospective single-center study enrolled 30 consecutive patients (mean age 73.1±6.3 years; 21 men) with symptomatic (n=25) or high-grade (n=5) ICA stenosis treated with the new double-layer carotid CGUARD Embolic Prevention System (EPS) stent, which has an inner open-cell nitinol design with an outer closed-cell polyethylene terephthalate layer. The average stenosis of the treated arteries was 84.1%±7.9% with a mean lesion length of 16.6±2.1 mm. In the laboratory, 8×40-mm stents where tested in vitro with respect to their radial force during expansion, the bending stiffness of the stent system and the expanded stent, as well as the collapse pressure in a thin and flexible sheath. The wall adaptation was assessed using fluoroscopy after stent release in step and curved vessel models. Results: The stent was successfully implanted in all patients. No peri- or postprocedural complications occurred; no minor or major stroke was observed in the 6-month follow-up. The bending stiffness of the expanded stent was 63.1 N·mm2 and (not unexpectedly) was clearly lower than that of the stent system (601.5 N·mm2). The normalized radial force during expansion of the stent to 7.0 mm, consistent with in vivo sizing, was relatively high (0.056 N/mm), which correlates well with the collapse pressure of 0.17 bars. Vessel wall adaptation was harmonic and caused no straightening of the vessel after clinical application. Conclusion: Because of its structure, the novel CGUARD EPS stent is characterized by a high flexibility combined with a high radial force and very good plaque coverage. These first clinical results demonstrate a very safe implantation behavior without any stroke up to 6 months after the procedure.

Biomechanical Aspects of Potential Stent Malapposition at Coronary Stent Implantation

Despite of the clearly improved restenosis rates with drug-eluting stents (DES), optimization of stent deployment is still important for favourable immediate and longterm results. This requirement is fed by the recently reported potential risk of late stent thrombosis which is among other factors assumed to be caused by stent malapposition. Stent malapposition appears frequently in clinical practice for DES as well as bare-metal stents (BMS).

[Performance characteristics of self-expanding peripheral nitinol stents].

PURPOSEnTo evaluate geometric-mechanical characteristics of self-expanding peripheral nitinol stents, such as alignment to the vessel wall, flexibility and radial force, in order to obtain information as to which stent is most appropriate depending on the characteristics of the arterial lesion.nnnMATERIALS AND METHODSnThe in vitro tests were done on seven stent systems: Acculink, Zilverstent, Philon, Precise, Luminexx, SelfX and Sinus Super-Flex. All stents had a diameter of 8 mm and a length from 37 - 44 mm. The stents were evaluated for traceability and profile, radial force, flexibility and radiopacity. Stents were even evaluated for alignment to the vessel wall using a zigzag-shaped model and a model with a diameter step from 5 to 7 mm. Assessment considered the distance to the vessel wall and diameter reduction and in step-mode the smooth and harmonic transition.nnnRESULTSnAt a stent diameter of 7 mm, radial forces between 1.09 (Sinus Super-Flex) and 2.59 N (Philon) were measured. The flexibility of the expanded stents ranged from 11.7 (Acculink) to 88.1 Nmm (2) (Luminexx). The Precise and Philon stents yielded the best alignment to the vessel wall, while the SelfX and Sinus Superflex stents had the highest reduction in diameter and the largest distance from the vessel wall in a tortuous vessel model. Differences in vessel diameter were best bridged by stents with short segments (Acculink, Precise). All stents were readily visible after expansion.nnnCONCLUSIONnThe alignment to the vessel wall mostly depends on the stent structure instead of radial force and flexibility. The presented results may be able to help physicians find the most suitable stent depending on the lesion characteristics.

Experimental Investigation of Modern and Established Carotid Stents

PURPOSEnThe design and material determine the mechanical properties of stents. In vitro parameters such as radial force, flexibility and wall adaptation of different stents were investigated in order to obtain evidence for clinical use.nnnMATERIALS AND METHODSnA total of 8 stents, including 2 hybrid stents with a combination of closed/open-cell design (Sinus Carotid RX, Cristallo Ideale), 3 closed-cell stents (Adapt, Carotid Wallstent, Xact Carotid) and 3 open-cell stents (Vivexx Carotid, Protégé Rx and Precise) with a diameter of 8u200amm and a length of 40u200amm, were investigated. The radial force, the bending stiffness of the stent system and of the stent, and the collapse pressure were measured. The wall adjustment of the stents was documented by fluoroscopy and assessed in a step and curve model.nnnRESULTSnThe bending stiffness of the stent systems declined significantly in the expanded state, whereby the Xact Carotid stent showed the highest value (291.1 N/mm2) in contrast to 31.6-39.4 N/mm2 for the Sinus Carotid and Cristallo Ideale. The radial force on expansion of the stents to 7u200amm was lowest for the Adapt (0.009 N/mm) and highest for the Precise (0.068 N/mm). The collapse pressure was highest for the Carotid Wallstent (0.48u200abar), compared with the other stents (0.1u200a-u200a0.2u200abar). The best wall adjustment in the curve model was shown by the Precise, the Sinus Carotid Rx and the Vivexx Carotid Stent. The diameter change from 5 to 7u200amm was smoothly adapted by the Cristallo Ideale and the Carotid Wallstent. The Adapt showed poor vessel wall adaptation in both step and curved vessels.nnnCONCLUSIONnAs a result of their design, the Sinus Carotid Rx and the Cristallo Ideale show the best wall adjustment, with comparable radial force and high flexibility, whereby the Cristallo Ideale has the advantage that it has a closed-cell design in the middle third of the stent. The other stents should be taken into consideration with their specifications in individual cases.nnnKEY POINTSnThe investigation of carotid stents provides an objective comparison of mechanical properties. Conclusions about the respective optimum uses of stents can be drawn from this. The hybrid stents show a good balance of properties for wide applicability. Citation Format: •u2007Wissgott C, Schmidt W, Behrens P etu200aal. Experimental Investigation of Modern and Established Carotid Stents. Fortschr Röntgenstr 2014; 186: 157u200a-u200a165.

Direct comparison of coronary bare metal vs. drug-eluting stents: same platform, different mechanics?

BackgroundDrug-eluting stents (DES) compared to bare metal stents (BMS) have shown superior clinical performance, but are considered less suitable in complex cases. Most studies do not distinguish between DES and BMS with respect to their mechanical performance. The objective was to obtain mechanical parameters for direct comparison of BMS and DES.MethodsIn vitro bench tests evaluated crimped stent profiles, crossability in stenosis models, elastic recoil, bending stiffness (crimped and expanded), and scaffolding properties. The study included five pairs of BMS and DES each with the same stent platforms (all nxa0=xa05; PRO-Kinetic Energy, Orsiro: BIOTRONIK AG, Bülach, Switzerland; MULTI-LINK 8, XIENCE Xpedition: Abbott Vascular, Temecula, CA; REBEL Monorail, Promus PREMIER, Boston Scientific, Marlborough, MA; Integrity, Resolute Integrity, Medtronic, Minneapolis, MN; Kaname, Ultimaster: Terumo Corporation, Tokyo, Japan). Statistical analysis used pooled variance t tests for pairwise comparison of BMS with DES.ResultsCrimped profiles in BMS groups ranged from 0.97xa0±xa00.01xa0mm (PRO-Kinetic Energy) to 1.13xa0±xa00.01xa0mm (Kaname) and in DES groups from 1.02xa0±xa00.01xa0mm (Orsiro) to 1.13xa0±xa00.01xa0mm (Ultimaster). Crossability was best for low profile stent systems. Elastic recoil ranged from 4.07xa0±xa00.22% (Orsiro) to 5.87xa0±xa00.54% (REBEL Monorail) including both BMS and DES. The bending stiffness of crimped and expanded stents showed no systematic differences between BMS and DES neither did the scaffolding.ConclusionsBased on in vitro measurements BMS appear superior to DES in some aspects of mechanical performance, yet the differences are small and not class uniform. The data provide assistance in selecting the optimal system for treatment and assessment of new generations of bioresorbable scaffolds.Trial registration: not applicable

Biomechanics and clinical experience of a 3D biomimicking vascular stent

Abstract The presented investigation was designed to assess the biomechanical behavior of a 3D biomimicking self-expanding stent with respect to general technical parameters added by first clinical tests. The test sample was a Veryan Medical Biomimics 3D with 6.0 mm expanded diameter and 80 mm length. The profile of the delivery catheter with mounted stent, the strut thickness, the bending stiffness, the axial stiffness for tension and compression and the length change during expansion were measured. The bending stiffness was 2.73 Nmm², axial stiffness at tension 7.77 N/mm and at compression 97.61 N/mm. The stent shortened during expansion by 11.54 %. Radial force at 5 mm diameter was 2.54 N. Clinical experience provided no stent fractures after 24 months. Duplexsonography showed stent patency without any binary restenosis or intimal hyperplasia.