A statistical approach to understand the role of inclusions on the fatigue resistance of superelastic Nitinol wire and tubing.

Superelastic wires and diamond-shaped stent surrogates were manufactured from Nitinol rods and tubing, respectively, from five different mill product suppliers - Standard VAR, Standard VIM, Standard VIM+VAR, Process-Optimized VIM+VAR, and High-Purity VAR. High-cycle fatigue tests up to 10(7) cycles were conducted under tension-tension conditions for wires and bending conditions for diamonds. These materials were compared under both testing methods at 37°C with 6% prestrain and 3% mean strain (unloading plateau) with a range of alternating strains. The High-Purity VAR material outperformed all alloys tested with a measured 10(7)-fatigue alternating strain limit of 0.32% for wire and 1.75% for diamonds. Process-Optimized VIM+VAR material was only slightly inferior to the High Purity VAR with a diamond alternating bending strain limit of 1.5%. These two "second generation" Nitinol alloys demonstrated approximately a 2× increase in 10(7)-cycle fatigue strain limit compared to all of the Standard-grade Nitinol alloys (VAR, VIM, and VIM+VAR) that demonstrated virtually indistinguishable fatigue performance. This statistically-significant increase in fatigue resistance in the contemporary alloys is ascribed to smaller inclusions in the Process-Optimized VIM+VAR material, and both smaller and fewer inclusions in the High-Purity VAR Nitinol.

Bard Denali inferior vena cava filter fracture and embolization resulting in cardiac tamponade: a device failure analysis.

A 46-year-old woman underwent inferior vena cava filter placement before bariatric surgery and returned within 6 months for routine removal. She complained of a 1-week history of severe chest pain, and during retrieval, two fractured filter components were identified including one arm in the right ventricle. The filter body and one fragment were successfully retrieved, but the fragment in the right ventricle was refractory to percutaneous retrieval. During open-heart surgery, the fragment was found traversing through the ventricular wall resulting in cardiac tamponade. Electron microscopic fragment analysis revealed high-cycle metal fatigue indicating the filter design failed to withstand this patient's natural inferior vena cava biomechanical motions.

Influence of microstructural purity on the bending fatigue behavior of VAR-melted superelastic Nitinol.

The bending fatigue resistance of commercially-available Standard versus High Purity Nitinol was evaluated at 3% mean strain and a range of strain amplitudes with the simple wire Z-specimen geometry. The Standard grade Nitinol demonstrated a 10(7)-cycle fatigue strain limit of 0.50% alternating strain, comparable to results reported elsewhere in the literature. Conversely, the High Purity grade VAR Nitinol demonstrated a 5-fold improvement in fatigue resistance with an impressive 10(7)-cycle fatigue strain limit of 2.5% alternating strain. The High Purity Nitinol has an oxygen+nitrogen content of 60wppm, maximum wrought-material inclusion length of 17µm, and inclusion volume fraction of 0.28%, all substantially less than industry standards. With all processing variables held constant except for inclusion content, it is clear that this marked fatigue superiority is due exclusively to the reduction in both size and area fraction of inclusions.

Vena Tech LGM filter retrieval 16 years after implantation: piecemeal removal by intentional mechanical fracture.

A 48-year-old man presented with symptomatic inferior vena cava (IVC) occlusion from a chronically thrombosed and embedded Vena Tech LGM filter resulting in exercise intolerance from diminished cardiac preload and postthrombotic syndrome from chronic venous insufficiency. The patient was treated using a new PRIME technique--Piecemeal Removal by Intentional MEchanical fracture--to achieve successful filter retrieval 16 years after implantation. Removal of the obstructing filter permitted endovascular IVC recanalization with restoration of venous outflow and alleviation of venous obstructive symptoms. Cardiac preload was restored, allowing the patient to resume long-distance running, and he successfully completed a half-marathon 3 months after treatment.

Complex retrieval of fractured, embedded, and penetrating inferior vena cava filters: a prospective study with histologic and electron microscopic analysis.

PURPOSE: To evaluate clinical outcomes, characterize adherent tissue, and analyze inferior vena cava (IVC) filter fractures in patients undergoing complex retrieval for management of filter-related complications. To elucidate mechanisms of filter fracture by radiographic and electron microscopic (EM) evaluation. MATERIALS AND METHODS: Over 2.5 years, 50 consecutive patients with fractured and/or penetrating filter components were prospectively enrolled into a single-center study. There were 19 men and 31 women (mean age, 42 y; range, 15-73 y). All patients underwent complex filter retrieval after failure of standard methods, and retrieval indications along with resultant clinical outcomes were evaluated. Specimens with adherent tissue underwent histologic analysis, and all fractured components were studied with EM. RESULTS: Retrieval was successful in all 50 cases (mean implantation, 815 d; range, 20-2,599 d) among the following filters: G2X (n = 23),G2 (n = 9), Eclipse (n = 3), Recovery (n = 4), ALN (n = 1), Celect (n = 7), OptEase (n = 2), and Simon Nitinol (n = 1). Mean indwell time in fractured filters (n = 31) was 1,082 days, versus 408 days in nonfractured filters (n = 19; P = .00169). Neointimal hyperplasia/fibrosis was seen in 46 of 48 specimens with adherent tissue (96%). Among 61 fractured components from conical filters, 35 had extravascular penetration whereas 26 remained intravascular (11 free-floating in IVC, 15 embolized centrally), and EM revealed fracture modes of high-cycle fatigue (n = 53), overload (n = 6), and indeterminate (n = 2). Following retrieval, previously prescribed lifelong anticoagulation was discontinued in 30 of 31 patients (97%). Filter-related symptoms from IVC occlusion, component embolization, and penetration-induced abdominal pain, duodenal injury, and/or small-bowel volvulus were alleviated in all 26 cases (100%). There were no long-term complications at a mean follow-up of 371 days (range, 67-878 d). CONCLUSIONS: The risk of filter fracture increases after 408 days (ie,>1 y) of implantation and is associated with symptomatic extravascular penetration and/or intravascular embolization. Complex methods can be used to safely remove these devices, alleviate filter-related morbidity, and allow cessation of anticoagulation.

Bard Recovery filter: evaluation and management of vena cava limb perforation, fracture, and migration.

PURPOSE: To report on the evaluation and management of Bard Recovery filter limb perforation, fracture, and migration. MATERIALS AND METHODS: In 2007, all patients who received a Bard Recovery filter at a single institution were contacted for consultation and evaluation by noncontrast computed tomography. Rates of limb perforation, fracture, and migration were evaluated on early (<180 days) and final images. Retrieval success and complications were evaluated. RESULTS: Fourteen of 16 patients with Bard Recovery filters were evaluated. The early images in nine patients (mean, 30 days; range, 0-126 d +/- 40) demonstrated arm perforations in 56% (n = 5), leg perforation in 11% (n = 1), and no early fractures or migrations. Final images (mean, 899 days; range, 119-1,218 d +/- 320) demonstrated filter arm perforations in all 14 patients. Leg perforations were seen in 36% of patients (n = 5), and there were a total of four fractures with migration in 21% of patients (n = 3). All fractures occurred in arms that had perforated the vena cava on early images. Two patients had already had their filters retrieved at 119 and 302 days, respectively; the remaining 12 patients elected to have their filters retrieved after evaluation. All 12 filters were retrieved at a mean of 1,021 days (range, 119-1,242 d +/- 134). Leg hook fractures occurred during eight of 12 filter retrieval procedures (67%). CONCLUSIONS: Recovery filter limb perforation of the vena cava increases over time and is associated with a 21% incidence of filter arm fracture and migration. Follow-up imaging is recommended. Filter retrieval has a high success rate and is the authors' preferred management strategy.

Biomechanical response of stented carotid arteries to swallowing and neck motion.

PURPOSE: To examine the effects of swallowing and side-to-side head turning on stents in the internal carotid artery. METHODS: Seven patients (4 men; mean age 76.9 years) who underwent carotid artery stenting for the treatment of atherosclerotic cervical carotid artery disease were examined with cine fluoroscopy. Geometric processing techniques were used to quantify carotid stent deformations due to head turning and swallowing forces. The variables measured included radial, axial, and crush deformations, as well as radii of stent curvatures during tested maneuvers. RESULTS: Radial deformations of the stented vessels were significantly less than axial and crush deformations, ranging from -10.2% to 15.5%. Axial deformations in response to both swallowing and head turning were positive (average 4.5%, range -14.5% to 14.1%), indicating a general lengthening of the stented vessel due to biomechanical motions. Crush strains exhibited the largest range of all of the deformation modes during both swallowing and head turning. Strain values ranged from -18.7% to 25.9% in the anteroposterior direction and from -25.6% to 21.9% in the lateral direction. Head turning produced fairly symmetrical crushing of the stent. Conversely, swallowing resulted in a preferential medial crush of the stented artery due to contraction of the pharyngeal constrictor muscles. Curvature measurements revealed a tightest radius of curvature of approximately 1.5 cm during ipsilateral head turning, with average values during both swallowing and head turning of approximately 10 cm. CONCLUSION: In general, head turning toward the stented artery produced greater deformation in the vessels than swallowing. Since patients are expected to undergo far more swallowing cycles than head turns, however, the accumulated deformations from swallowing may be more significant and should be considered in the design of fatigue resistant stents for carotid arteries.

Right renal artery in vivo stent fracture.

The authors describe an incident of a type I single strut fracture in a right renal artery (RRA) stent resulting in approximately 90% restenosis. Fracture was observed just distal to the ostium approximately 1 year after implantation in an 83-year-old man with a history of systemic cardiovascular disease. In addition, a statistical analysis of the clinically reported cases of left renal artery (LRA) and RRA stent fracture is provided, which suggests a greater susceptibility to fracture in LRA stents as demonstrated by the greater occurrence (67%) in the left side.

In vitro fatigue-crack growth and fracture toughness behavior of thin-walled superelastic Nitinol tube for endovascular stents: A basis for defining the effect of crack-like defects.

Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even "single-event" overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue-crack growth and R-curve fracture toughness) behavior in thin-walled ( approximately 400microm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite finish temperature of A(f) approximately 25-30 degrees C, identical to self-expanding Nitinol stents. Fatigue-crack growth behavior, measured in Hanks' Balanced Saline Solution over a wide spectrum of growth rates (down to 10(-10)m/cycle) and at a range of positive load ratios (R=0.1-0.7), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.