Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Arterial prosthesis – Stent structure
Reexamination Certificate
2001-06-29
2003-07-01
Milano, Michael J. (Department: 3731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Arterial prosthesis
Stent structure
Reexamination Certificate
active
06585755
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to endoprosthesis devices, generally called stents, and more particularly, to achieving desired visibility of such devices under magnetic resonance imaging (MRI) and fluoroscopy.
Stents are generally thin walled tubular-shaped devices composed of complex patterns of interconnecting struts which function to hold open a segment of a blood vessel or other body lumen such as a coronary artery. They also are suitable for supporting a dissected arterial lining or intimal flap that can occlude a vessel lumen. At present, there are numerous commercial stents being marketed throughout the world. These devices are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through the lumen. Once in position, the stent is deployed. In the case of balloon expandable stents, deployment is achieved by inflation of a dilation balloon about which the stent is carried on a stent-delivery cathether.
The stent must be able to simultaneously satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or more accurately, hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
In addition to meeting the mechanical requirements described above, there is a requirement that a stent be radiopaque or fluoroscopically visible under x-rays. Accurate stent placement requires real time visualization to allow the cardiologist or interventional radiologist to track the delivery catheter through the patient's vasculature and precisely place the stent at the site of a lesion. This is typically accomplished by fluoroscopy or similar x-ray visualization procedures. For a stent to be fluoroscopically visible it must be more absorptive of x-rays than the surrounding tissue. This is typically accomplished by the use of radiopaque materials in the construction of a stent, which allows for its direct visualization. The most common materials used to fabricate stents are stainless steel and nickel-titanium alloys, both of which are radiopaque. This factor, in combination with the relatively thin wall thickness (about 0.002 to 0.006 inch) of most stent patterns renders stents produced from these materials sufficiently radiopaque to be optimally visualized with x-ray based fluoroscopy procedures. Although both materials are generally regarded as being bio-compatible, some recent concerns have arisen regarding the long term biocompatibility of stainless steel. Over time, nickel, a constituent element of most stainless steels, tends to leach from a stainless steel stent and in some sensitive patients will produce an allergic reaction. In addition, the chromium oxide layer present on the surface of stainless steel stents to prevent corrosion may have a tendency to degrade during long term use within the body.
Alternative, non-toxic, high density metals, such as tantalum, iridium, platinum, gold, and the like, have been used in prior art stents. However, these alloys can sometimes either be excessively radiopaque or may lack sufficient strength for recoil, radial strength requirements, and long-term use in a dynamic vascular setting. Stents constructed of highly radiopaque materials appear overly bright when viewed under a fluoroscope. This tends to overwhelm the image of the tissue surrounding the stent and obscures visualization of the stent lumen. Due to the lack of an appropriately radiopaque material, simply constructing a stent wholly out of a single material has heretofore not resulted in a stent with the optimal combination of mechanical properties and radiopacity. Thus, the art has moved in the direction of combining different materials to produce a mechanically sound, biocompatible and fluoroscopically visible stent. A number of such approaches have been developed. Typically such methods have focused on increasing the radiopacity or fluoroscopic visibility of stainless steel and nickel-titanium alloy stents.
One means frequently described for increasing fluoroscopic visibility is the physical attachment of radiopaque markers to the stent. Conventional radiopaque markers, however, have a number of limitations. Upon attachment to a stent, such markers may project from the surface of the stent, thereby comprising a departure from the ideal profile of the stent. Depending on their specific location, the marker may either project inwardly to disrupt blood flow or outwardly to traumatize the walls of the blood vessel. Additionally, galvanic corrosion may result from the contact of two disparate metals, i.e., the metal used in the construction of the stent and the radiopaque metal of the marker. Such corrosion could eventually cause the marker to separate from the stent which may be problematic should the marker be swept downstream within a vessel. Discrete stent markers cannot show the entire outline of the stent which is a preferred method to determine the optimal expansion of a stent over its entire length.
The radiopacity of stents has also been increased by plating or coating selected portions thereof with radiopaque material. However, a number of disadvantages are associated with this approach as well. When the stent is expanded certain portions undergo substantial deformation, creating a risk that cracks may form in the plating or coating causing portions of the plating to separate from the underlying substrate. This has the potential for creating jagged edges that may inflict physical trauma on the lumen wall tissue or cause turbulence in the blood flowing past the stent, thereby inducing thrombogenesis. Moreover, once the underlying structural material becomes exposed to an electrolytic solution such as blood, interfaces between the two disparate metals become subject to galvanic corrosion. Over time, galvanic corrosion may also lead to separation of the plated material from the underlying substrate.
X-ray based fluoroscopy is the current preferred modality for imaging stents during an intervention and for diagnostic assessment. However, in addition to the potential disadvantages stated above, other drawbacks may exist. Exposure to ionizing radiation and nephrotoxic iodinated contrast agents are intrinsic to the technique, as well as the need to wear leaded personal protective equipment. Alternatively, magnetic resonance imaging (MRI), produced by complex interactions of magnetic and radio frequency fields, does not suffer from these drawbacks and is actively being pursued to image stents in a diagnostic mode and, in the future, to guide stent based interventions. MRI has gained an increasing role in the diagnosis and assessment of human pathology. In patients undergoing MRI, there are numerous devices which are poorly seen, if they are visible at all, on the MR image artifact. The location and course of these implanted devices is usually of great clinical importance to assure their proper function and avoid complications that malposition can cause.
Due to their small size, current metal stents are sometimes difficult to see in fluoroscopy as they attenuate the x-ray beam very little. This is particularly true in
Jackson Gregg A.
Pacetti Stephen D.
Advanced Cardiovascular
Fulwider Patton Lee & Utecht LLP
Ho Tan-Uyen T.
Milano Michael J.
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