Longitudinal deformation bench testing using a coronary artery model: a new standard?

Abstract

Objectives: To compare susceptibility of five different stent platforms with longitudinal stent deformation (LSD) using a clinically relevant bench testing model simulating both short and long malapposed lengths.

Background: Recent data suggest that design modifications to the Promus Element stent which led to the Promus Premier stent has reduced susceptibility to LSD. However, susceptibility to LSD at long malapposed lengths has not been tested. Furthermore, the mechanisms behind susceptibility to LSD are as yet unclear.

Methods: The Omega, Integrity, Multilink 8, Biomatrixand Promus Premier stent platforms were tested. The Omega, Integrity and Multilink 8 platforms were used in place of their drug-eluting equivalents. 3.5 mm stents were deployed in a stepped tube with the distal portion fixed and the proximal test section exposed. The force required to compress stents by a fixed distance at different exposed lengths was compared. Symmetrical and point loading were used.

Results: The Promus Premier was longitudinally as strong as Multilink and Integrity at a short exposed length (4 mm) but weaker, in between Omega and the other platforms, at longer exposed lengths (12 mm). As previously noted, the Omega (Promus Element) platform was significantly weaker than the other stents and Biomatrix was the strongest stent.

Conclusion: Susceptibility to LSD varies depending on length of malapposed segment when tested using a clinically relevant model as in this study. The mechanisms behind the susceptibility are likely multifactorial, including connector number, strut thickness, connector alignment and ring orientation but remain to be elucidated.

Keywords: coronary artery disease; longitudinal stent deformation; percutaneous coronary intervention.

Figure 1

Compression testing. Diagram showing symmetrical compression with a rod (A), point testing with a fork mid-connector (B) or over connector (C).

Figure 2

Panels  A–C shows a 7 mm exposed Biomatrix stent prior to point compression, the first ring-to-ring contact (0.7 mm compression) and following 4 mm compression. Panels D–F shows the Integrity stent prior to point compression, the first ring-to-ring contact (0.5 mm compression) and following 4 mm compression. Panels G–I shows a 7 mm exposed Multilink stent prior to point compression, the first ring-to-ring contact (3 mm compression) and following 4 mm compression. Panels J–L shows a 7 mm exposed Omega stent prior to point compression, the first ring-to-ring contact (2.5 mm compression) and following 4 mm compression. Panels M–O shows a 7 mm exposed Premier stent prior to point compression, the first ring-to-ring contact (2.5 mm compression) and following 4 mm compression. Arrows indicate the ring-to-ring contacts. The proximal three rings of the Premier stent (O) have remained intact with the compression occurring below, and there is less recoil than the other platforms. Visual inspection of the inlet angle (α) shows it is steeper for the Biomatrix, Integrity and Multilink stents that the Omega and Premier stents.

Figure 3

Panels A–D show a 12 mm exposed Biomatrix stent prior to symmetrical loading (A), at 4 mm (B) and following 7 mm compression in orthogonal views (C,D). Panels E–G show a 7 mm exposed Multilink ML8 (Xience V) stent exposed 7 mm prior to symmetrical loading (E) and at 2 mm (F) and following 4 mm compression (G). Panels H–J show an Omega (Element) stent exposed 4 mm prior to symmetrical loading (H), at 1.5 mm (I) and following 2.5 mm compression (J). Panels K–N show a 12 mm exposed Integrity (Resolute) stent prior to point loading (K), at 4 mm (L) and following 7 mm compression in orthogonal anteroposterior (M) and lateral (N) views. Panels O–Q show a 7 mm exposed Multilink ML8 (Xience V) stent exposed 7 mm prior to point loading (O) and at 2 mm (P) and following 4 mm compression (Q). Panels R–T show an Omega (Element) stent exposed 4 mm prior to point loading (R), at 1.5 mm (S) and following 2.5 mm compression (T). The appearances of the stents following point loading closely resemble cases of longitudinal stent deformation.