1. Field of the Invention
The present invention relates to a stent for indwelling within a living body, which is used for improving a stricture section or a obstruction section occurring in a tubular lumen such as a blood vessel, a bile duct, the trachea, the gullet, the urethra, and other internal organs, particularly, to a stent of a balloon expansion type.
2. Description of the Related Art
A stent is generally a tubular medical instrument used for treating various diseases and indwelled in a stricture section or an obstruction section of a tubular lumen within a living body such as a blood vessel for expanding the stricture section or the obstruction section so as to ensure an inner cavity therein.
The stents include stents of a balloon expansion type and stents of a self-expansion type. The balloon expansion type stent does not perform the self-expansion function unlike the self-expansion type stent. In the balloon expansion type stent, the stent is inserted into a desired position and, then, the balloon is expanded within the stent so as to permit the expanding force of the balloon to expand (plastically deform) the stent and bring the stent into contact with the inner surface of the desired position of the tubular lumen, thereby fixing the stent to the desired position.
In general, the stent is required to perform the delivery function and the re-stricture or restenosis preventing function as basic functions. The delivery function is the function where the stent is carried to a desired position within a tubular lumen. Naturally, the delivery function is a basic function, because it is impossible to retain or indwell the stent in the desired position unless the stent is carried to the desired position. The factors relating to the delivery function of the balloon expansion type stent include the stent diameter under the state that the stent is mounted on a balloon catheter and the close contacting of the balloon with the stent under the state that the stent is mounted on the balloon catheter. Particularly important is the flexibility of the stent under the state that the stent is mounted on the balloon catheter.
The stent is required to be flexible under the mounted state on the balloon catheter in order to allow the stent to be delivered along the guide wire retained in, particularly, the bent and meandering blood vessel. It is possible for a stent poor in the flexibility in the axial direction to be incapable of following the guide wire, resulting in failure to be delivered to the lesion portion. Particularly, the difficulty is rendered prominent when it comes to a long stent. Also, when the stent is passed through a bent and calcified lesion, it is possible for the stent to be caught by the calcified hard inner membrane so as not to be moved further. Particularly, when the stent is bent, a part of the strut protrudes toward the outside so as to abut against a hard lesion portion and, thus, to fail to move further. An additional problem takes place in clinical medicine in some cases. Specifically, when the stent is brought back into the guiding catheter because the stent is incapable of passing through the diseased portion, it is possible for a part of the stent to be caught by the tip of the guiding catheter, resulting in failure to recover the stent. It is also possible for the stent to be dropped from the balloon catheter.
On the other hand, the re-stricture preventing function represents the function of preventing a re-stricture in the portion where the stent is retained. The construction of the stent that permits suppressing the occurrence of a re-stricture has not yet been clarified sufficiently because the re-stricture generating mechanism has not yet been clarified sufficiently and because it is difficult to carry out the re-stricture comparison test by the stent since the types of the disease are complicated and diversified in clinical research. However, it is said that the re-stricture tends to occur easily in the edge of the stent when it comes to a stent poor in flexibility in the axial direction. It is considered reasonable to understand that the re-stricture tends to take place easily because stress is applied to the edge of the stent if the stent is poor in flexibility so as to stimulate the blood vessel. Naturally, it is considered reasonable to understand that the stent should desirably be flexible even after expanded and indwelled. However, a stent that does not have a free portion in general is hard in the axial direction, leading to a high re-stricture occurrence in the edge portion of the stent.