There are two basic roles of balloons: first, the pre-expansion of the balloon facilitates the smooth placement of the subsequent stent; second, the post-expansion of the balloon ensures the adequate expansion or good fitting of the stent.
There are various classifications of balloons, according to the characteristics of balloon use, they are classified as OvertheWire (OTAw). Rapidexchanging. Rapidexchangesystem(①() and fixed guidewire balloons(balloononwire,such balloons are basically no longer used in clinical practice) are three types of balloons: also include specially designed balloons such as perfusion balloons, cutting balloons, double guidewire poly balloons, drug-carrying balloons According to the size of balloon diameter, it can be roughly divided into small balloons(2-5mm), common balloons( 5N12mm) and large balloons. 5N12mm) and large balloons (≥12mm). Small balloons are generally used for coronary arteries, tibiofibular arteries below the rouge artery and renal and vertebral arteries with thin diameters; common balloons are generally used for carotid arteries, renal arteries, iliofemoral N arteries, etc., while large balloons are generally used for infrarenal abdominal aorta, iliac arteries and vena cava.
I. Compliance balloon and non-compliance balloon
The compliance of balloon refers to the corresponding change in balloon shape or volume for each increase in atmospheric pressure (atm) during balloon filling, and is an indicator of balloon stretching capacity. After the balloon is completely filled, the higher the balloon compliance, the more obvious the tendency of further increase in balloon volume or shape as the filling pressure continues to increase. However, for the majority of dilated balloon catheters, the length of the balloon does not change when the filling pressure is increased, and the change in volume is mainly reflected in the change in balloon diameter.
Important parameters: Nominalpressure refers to the intra-balloon filling pressure required for the balloon to obtain a predetermined diameter, generally between 6 and 8 atm. The ratedburstpressure refers to the maximum filling pressure at which 99.9% of the balloons will not rupture when the balloons are repeatedly filled 40 times during in vitro testing. Depending on balloon compliance
The balloons currently in clinical use are broadly classified as compliant balloons, semi-compliant balloons and non-compliant balloons. The compliance of balloons mainly depends on the material of balloon production. The early balloons were made of polyvinyl chloride (PVC) and were compliant balloons. Nowadays, semi-compliant balloons and non-compliant balloons are mainly made of polyethylene (PE), polyurethane, nylon (Nylon, DuralynTM) and polyethylene terephthalate. The latter two are the main materials used in the production of semi-compliant balloons and non-compliant balloons today.
1.Compliant balloon
After the pressure of the compliant balloon increases to the named pressure or expands to a predetermined diameter, its diameter and volume can increase continuously as the filling pressure continues to increase,
When resistance is encountered in the vessel, the balloon morphology may change and expand to the point of less resistance, which can lead to two outcomes: first, the compression force exerted on the surrounding disease is significantly reduced; second, the continued increase in filling pressure and excessive expansion of the shoulders at both ends of the balloon can easily lead to damage to the normal vessel wall and cause entrapment. The susceptibility of dilated stenotic lesions to entrapment makes compliant balloons generally not used for angioplasty. However, the use of compliant balloons to adapt to the morphological shaping of the blood vessel still has a role that cannot be ignored: for example, the Mo. Ma. balloon cerebral protection device used for carotid stenting takes advantage of the shaping properties of compliant balloons, and low-pressure expansion ensures that the balloon is completely attached to the wall and does not damage the intima of the vessel wall, so as to completely block the blood flow of the internal carotid artery.
2. Semi-compliant balloon and non-compliant balloon
Semi-compliant balloon means that the balloon filling pressure continues to increase between the named pressure and burst pressure, and the balloon diameter still has a range of increase of 0.25-0.75mm above the predetermined diameter. The non-compliant balloon diameter remains the same after the balloon diameter is fully filled to the specified value regardless of the further increase in pressure. Both are more resistant to high pressure and have a greater ability to squeeze lesions than compliant balloons, while being less adaptable to vessel shape than compliant balloons. The excellent high pressure resistance of the noncompliant balloon allows it to compress lesions more than the nonhomeostatic balloon and facilitates the dilatation of severe or fibrotic hard lesions.
With semi-compliant balloons, the balloon diameter can be precisely regulated by controlling the pressure after filling beyond the named pressure. However, semi-compliant balloons are prone to dogbone phenomenon at pressures higher than 14 atm, where the squeezing pressure on the lesion is not sufficient to dilate a hard lesion, and the protruding balloon portion at the ends of the lesion may damage the normal vessel wall tissue at the ends of the lesion, causing a marginal effect that may lead to restenosis at the ends of the stent or entrapment of the lesion. At pressures as high as 20 atm, not only may the lesion not be dilated, but there is also a risk of severe vessel rupture and perforation or entrapment events. In addition, incomplete stent expansion or wall apposition is often an important influence on in-stent thrombosis and distant restenosis. Therefore, in cases of severe calcification with high resistance, stents or semi-compliant balloons are usually insufficient to fully dilate the lesion or to adequately appose the stent to the wall. In contrast, the ultra-high-pressure resistance of noncompliant balloons ensures that high pressure is continuously applied to the lesion and can fully dilate the lesion or allow adequate stent apposition. For such lesions, if the lesion indentation does not disappear with semi-compliant balloon dilatation pressure increasing to 16 atm or exceeding the balloon burst pressure, especially if there is significant calcification of the artery in the vessel wall on fluoroscopy, consideration should be given to switching to a noncompliant balloon, cutting the balloon, or using plaque spinning. Balloon pressure should not be continued to increase to avoid serious complications such as vessel rupture and perforation.
Non-compliant balloons are usually indicated for pre-dilatation before stenting for severe calcified Chi lesions or post-dilatation at high pressure after stenting to allow adequate expansion or complete apposition of the stent to the wall. Noncompliant balloons are also used for lesions that are not easily expandable such as bifurcation lesions, open lesions, stent overlaps and in-stent restenosis due to their high expansion power. Non-compliant long balloons used in long occlusive lesions can significantly reduce the incidence of entrapment due to endothelial injury after angioplasty. Post-expansion of self-expanding stents is also usually performed with noncompliant balloons to allow complete wall apposition.
II. drug-eluting balloon
The late thrombosis of drug-eluting stents is related to the inhibition of endothelial repair and healing process by the polymeric carrier of the stent. The use of drug-eluting balloons not only inhibits endothelial proliferation to prevent restenosis, but also avoids the late thrombosis caused by the long-term retention of the metal skeleton and polymeric carrier of drug-eluting stents in the vessel wall. In contrast to the slow and continuous release of drug from drug-washed stents, the drug-carrying balloon (paclitaxel-eluting balloon) is filled with paclitaxel in microscopic holes on the surface of the balloon, which is rapidly released into the local arterial wall through the contact between the balloon expansion and the lesion. The use of the collapse technique before filling the balloon prevents the drug from being washed away prematurely as the balloon moves forward in the bloodstream. Balloon expansion simultaneously allows 75% of the drug dose to penetrate into the local arterial wall, preventing intimal proliferation, while the remaining 25% of the drug dose is washed away by rapid blood flow during balloon expansion.
Cryoballoon
Cryoplasty uses a special balloon catheter that works by the dual mechanism of the mechanical dilating force of a normal balloon for angioplasty and the rapid freezing of the vessel wall. The cryoplasty effect, by generating most of the tiny fissures in the vessel wall and lesion surface, can achieve a neater dilatation effect than the normal balloon and effectively reduce the incidence of local intimal tears or entrapment; also, by changing the physical properties of collagen and elastic fibers, it theoretically reduces the elastic retraction of the vessel wall and decreases the possibility of negative vascular remodeling in the long term; in addition, it can induce apoptosis of vascular smooth muscle cells and reduce the incidence of intimal proliferation or restenosis. or the incidence of restenosis. The use of frozen balloons theoretically reduces the incidence of immediate intraoperative entrapment, reduces the need for stenting due to acute complications, alters the vascular remodeling process and induces apoptosis of vascular smooth muscle to reduce the chance of restenosis, but there is a lack of strong evidence that the long-term efficacy and endpoint event rates of frozen balloons are superior to those of conventional balloons or stents.
Section II Stents
There are many criteria for classifying peripheral vascular stents, which can be divided into balloo-expanding (balloo-expandablestent) and self-expandingstent based on the mode of stent release: bare metal stents (Baremetastent, BMS), drug-coated stents ( drugcoatedstent), Covered
stent): according to the structural design of stents, they can be divided into tubular stents (slotted-tubestent), annular stents and wound stents (coil
stent); according to the different mesh of the stent can be divided into closed-loop stent and open-loop stent. The metal skeleton material of the stent is generally stainless steel wire, tantalum wire and temperature-controlled nickel-titanium alloy and cobalt-chromium alloy, etc.
A ball-expansion type bracket.
The balloon-expanded stent itself is inelastic, and its design is that the stent is pre-installed on the balloon, and the stent is delivered to the vascular lesion through the balloon catheter. After the balloon expands to the proposed diameter, it relies on the retraction force of the vessel wall to adhere to the vessel wall and does not produce continuous expansion tension on the vessel wall. The greatest advantage of the balloon-expanded stent is its precise positioning during release, which is especially suitable for open lesions, such as open lesions of vertebral artery and renal artery, and it also has the characteristics of insignificant shortening phenomenon after release and stronger radial support than peripheral self-expanding stents. However, the ball-expandable stent itself lacks elasticity, is prone to collapse and occlusion after compression, and is less flexible, so it is less suitable for extracranial carotid artery, femoral carotid artery and other vulnerable or mobile joint sites; in the peripheral vasculature, it is only suitable for limited short-segment stenotic occlusive lesions (<3 cm) with straight alignment and non-mobile joint areas. Fewer ball-expandable stents are available for peripheral arterial disease, and the classic peripheral ball-expandable stents are represented by the Palmaz (Cordis) and Strecker (Boston) stents.
The Palmaz ball expansion stent and its derivative Genesis series are produced by Cordis and are slotted stainless steel wire mesh tubular stents with a very thin wall thickness of only about 0.15mm and a closed-loop design. The length is 15N50mm: 5-7mm in diameter for the renal and vertebral artery openings and 8-14mm in diameter for the iliofemoral artery. The advantages are strong radial support, close to the vessel wall after expansion, almost no elastic retraction phenomenon, rapid endothelialization, and less likely to block branch openings. The disadvantage is that the longitudinal flexibility is poor, it is not easy to pass through the twisted vessels, the overall tendency is straight after release, and there is some resistance to tortuous vessels.
Strecker ball expansion stent is made of O.lmm thin single tantalum wire woven into a tubular metal mesh, the surface has a negatively charged metal oxide layer, which can prevent platelet adhesion, and the X-ray fluoroscopy is clear for accurate positioning. The advantages are that it has good longitudinal and radial flexibility compared with Palmaz stent, easy to pass through the twisted vessels, can adapt to the natural curvature of the vessel wall, less shortening phenomenon after expansion, non-ferromagnetic and feasible to follow up by nuclear magnetic examination. The disadvantage is that the radial support is smaller than that of Palmaz stent, and there is a certain elastic retraction phenomenon. Therefore, the Palmaz stent with strong support should be used for heavily calcified, occluded and open lesions, and the Strecker stent with good flexibility should be used for obviously distorted lesions.
The Jostent ball-expandable stent (AbbottVascular) has the advantages of both Palmaz and Strecker stents. It is easy to be positioned, has a strong radial support, and can adhere closely to the vessel wall after expansion and is not easily retracted or displaced. It is highly maneuverable. Another feature is the wide range of stent diameter variation, which can be expanded from 6 mm to 12 mm.
II Self-expanding stents.
The release mechanism of the self-expanding 1:l stent differs from that of the ball-expanding stent in that the stent is compressed in the delivery sheath and delivered to the vascular lesion, and the sheath is withdrawn to release the stent, relying on a balanced relationship between the expansion tension of the stent itself and the elastic restriction of the vessel wall to adhere to the vessel wall. The advantages of the self-expanding stent are that it is more flexible, facilitates the passage of twisted vessels and calcified lesions, conforms to the natural curvature of the vessel wall, is less susceptible to compression and deformation, and can even be released across mobile joints. The disadvantage is that there is a forward jump and shortening phenomenon when releasing, so it is difficult to locate the release precisely.
Except for the renal artery and vertebral artery, self-expanding stents are mainly used in peripheral vessels, and there are more options than ball-expanding stents.
The classical self-expanding stents are represented by Gianturco-Z-shaped stent (Cook Company), Wallstent (Boston Scientific Company), Memotherm (Bard Company), Smart stent (Cordis Company), and the new self-expanding stents are mainly made of nitinol, including Symphony stent (BostonScientific Company), Luminexx stent (Bard Company), Zilver stent (Cook Company), and Pulmonary stent (Pulmonary stent). (Cook Corporation), Precise (Cordis Corporation), Protégé stent (EV3 Corporation), Maris stent (Invatec Corporation), Sinus stent (Optimed Corporation), etc.
Gianturco-Z-shaped bracket uses stainless steel wire with a diameter of 0.25 to 0.5mm wound into a cylindrical structure enclosed by Z-bending of various lengths and diameters, which is easy to convey. It is characterized by larger stent mesh, which is less likely to cause obstruction at the opening of vascular branches, strong radial support, and no shortening phenomenon. It is mainly used for venous system lesions, especially for inferior vena cava lesions at the opening of the hepatic vein in Buga syndrome, and is less likely to cause obstruction at the opening of the hepatic vein and the paratentorial hepatic vein. The disadvantage is that it has the phenomenon of forward jump during release, so in order to increase the stability and prevent the stent displacement caused by forward jump, three-section Z-shaped stent should be used routinely. Because of the high support, it can be used for tough, fibrotic, calcified or highly elastic retracted lesions.
Wallstent stents are made of a mesh tubular structure woven with 0.075 mm diameter stainless steel wire, which has the advantage of good longitudinal flexibility, easy placement in tortuous vessels, and can be used for transarticular placement. The stent can be retrieved and repositioned before releasing more than 80% of its full length. The disadvantages are that the stainless steel wire is slim and the fluoroscopic visibility is poor; the radial expansion force is smaller than that of the balloon-expanded stent, and it is not easy to expand certain hard fibrotic or severely calcified lesions, and post-balloon expansion is needed to ensure that the stent is close to the vessel wall; it is obviously shortened after expansion, and sometimes it is difficult to position; the mesh is smaller and denser than that of similar stents, and it may block the vessel branches.
Memotherm and Smart stents are formed by laser engraving and cutting of nickel-titanium alloy tubes, which have strong radial support, and the stents are shorter and smaller after expansion and have better visibility under fluoroscopy than stainless steel Wallstent. Except for severely calcified stiff lesions and limited (1N
2cm) short lesions, generally the iliofemoral carotid artery and carotid artery should choose the self-expanding stent with better flexibility. Nickel-titanium alloy self-expanding stents have good overall flexibility and are more likely to recover their shape after extrusion than stainless steel self-expanding stents: moreover, stainless steel wire stents used in carotid arteries may have a higher incidence of distant stent fracture due to poor fatigue resistance.
Trimetallic bare stent (baremetalstent).
The metal stent with polished surface without any coating or laminating material is called baremetalstent, which can be very effective in dealing with vascular entrapment and acute vascular occlusion and improve the success rate and safety of angioplasty. Bare stents have proven to be valuable in two ways: as an effective remedy for failure of balloon dilatation angioplasty alone and to reduce postoperative restenosis in the long term. Commonly used metal bare stents include balloon-expandable and self-expandable stents, with the characteristics and types described above. Only the therapeutic principles of stents and their inherent defects are described here.
The limitations of balloon angioplasty are its low immediate success rate in the management of eccentric, calcified, or long-segment stenoses, its high rate of acute intraoperative occlusion, and its high rate of distant restenosis. Early and distant stenosis after PTA is usually due to elastic retraction, endothelial proliferation after endothelial injury, and distant vascular remodeling. Metal bare stents provide effective mechanical support to the vessel wall through their good radial support, thus eliminating and preventing acute vascular occlusion due to elastic retraction and limited entrapment, providing a larger initial lumen area and a smoother intimal surface to allow intra-stent blood flow to hydroxylate positive laminar flow and limit restenosis due to negative distal vascular remodeling. Therefore, the introduction of metal bare stents into endoluminal angioplasty after balloons not only effectively reduces the incidence of angioplasty failure and acute vessel occlusion due to elastic retraction and flow-limiting entrapment, ensures surgical safety, and expands the indications for angioplasty, but also helps to maintain long-term patency and reduce long-term restenosis. However, metal bare stents have only mechanical support and lack intrinsic biological activity to inhibit endothelial proliferation, the latter being the main mechanism leading to distant restenosis. In contrast to balloon angioplasty, stents remain in the lumen of the body as a foreign body for a long time, which can cause excessive intimal proliferation and lead to in-stent thrombosis and long-term restenosis. The lack of biological activity of bare metal stents and the restenosis rate have led to the introduction of new products and design concepts such as covered stents and drug-eluting stents.
IV Covered stent (coveredstent, stent-graft).
Covered stent is composed of a special membrane material covered with polymer on the platform of ordinary bare metal stent, which is an effective combination of the supporting physicochemical properties of bare metal stent and the unique properties of covered material. The stent-type artificial vessel is named specifically for the clad stent used in the aorta. The polymeric membranous materials covered are mainly biodegradable polymers, mainly expandablepolytetrafluoroetnyiene (ePTFE), polyester (pojyethyleneterephthalate, PET, commonly known as dacron), polyester ( polyestPE), polyurethane (PU), silk, etc. The diameter of the target vessel has specific requirements for the selection of laminating materials with different properties: for small diameters, resistance to thrombosis is particularly important; for large diameters (≥lOmm), mechanical durability is a relatively prominent issue. Compared to dacron, ePTFE is less thrombogenic and is therefore used for vascular cladding or graft material for diameters ≤lOmm. Dacron has a more pronounced inflammatory and fibroproliferative response than ePTFE and is therefore better tolerated in large-diameter main and iliac arteries. Clad stents or stented prostheses have been widely used for endoluminal repair of dilated arterial disease, such as aortic aneurysms, aortic coarctation and peripheral aneurysms, pseudoaneurysms and arteriovenous fistulas due to vascular injury, and acute rupture perforations due to angioplasty. Since the endothelium can proliferate through the mesh of the bare stent and lead to in-stent restenosis defects, laminated stents are also used in peripheral arterial occlusive disease to restrain the proliferation of endothelium in the stent lumen by the physical barrier effect of the laminated material.
To prevent displacement, barbs are added at both ends of the stent; the length of the stent or stent-type vessel should generally exceed the length of the lesion by more than 2 cm, and the ends of the stent should exceed the ends of the lesion by more than 1 cm, in principle, it is better to be long than short: the diameter of the stent should be 15%-20% larger than the diameter of the vessel at both ends of the lesion, so that the stent fits closely with the vessel wall. Compared with the bare metal stent, the outer diameter of the overlapping branch delivery sheath is significantly thicker, usually 8F-12F for peripheral arteries and 16F-24F for aorta.
Stent-type artificial vessels for the aorta are generally self-expanding and releasing, including tubular, bifurcated, and main single iliac types, and are generally supported by means of the cladding material throughout the stent. The stent-type artificial blood vessels mainly include imported Talent (Medtronic) and Zenith (Cook), and N-made Ancura (Shenzhen Xianjian) and Aegis (Shanghai Minimally Invasive). Talent consists of nickel-titanium alloy self-expanding stent covered with polyester inside and outside, while Zenith is composed of GianturcoZ-type stent lined with ePTFE membrane. Both have bare stents proximal to the stent, which can be released across the opening of the subclavian and renal arteries without interfering with branch blood flow.
Peripheral vascular stents are released in the same manner as ordinary bare metal stents and are divided into two types: ball-expandable and self-expandable, with self-expandable stents being the most common.
Scientific), based on Wallstent stent platform covered with polyester, is one of the most commonly used overlapping stents, which retains the good radial support of Wallstent and has good wall adhesion performance. The delivery and release technique is basically the same as Wallstent, with a thicker delivery sheath of about 9-12F and less flexibility. Available diameters are 6-14 mm and lengths are 20-90 mm. Hemobahn/Viabahn
(W.L. Gore Company) also uses self-expanding nitinol stent platform with ePTFE lining on the inner surface of the stent and delivery sheath IOF. Jostent overlay stent (Jomed) is a ball-expandable Jostent stent platform with ePTFE membrane, which can be delivered via a 7-8F sheath with a dilated diameter of 4-12mm, and is often used in deep coronary arteries, renal arteries, vertebral arteries, etc. small diameter vessels.
There are problems with overlapping stents: the outer diameter of the overall delivery sheath is thick and stiff and lacks flexibility, which often makes delivery or release difficult in twisted vessels; for large arteries, arteriotomy is often required and percutaneous puncture is not possible; local vascular complications increase in general stenting procedures; crumpling, collapse and breakage of the overlapping material cause the weakness or broken area of the overlapping membrane to form stent endothelial proliferation restenosis or internal fistula; when used in peripheral small and medium caliber vessels Early thrombosis is increased, and the cladding material hinders the endothelialization process in the stent lumen leading to late thrombosis: in peripheral arterial occlusive disease stenosis at both ends of the clad stent is still unavoidable.
V. Drug-(J()-eluting stent
Endothelial injury-mediated intimal hyperproliferation is the most important aspect of restenosis. Restenosis includes 3 main mechanisms: local injury to the vessel wall triggering excessive cell proliferation and extracellular matrix synthesis (endothelial proliferation): acute elastic retraction immediately after balloon withdrawal: late vascular remodeling or remodeling leading to an overall reduction of the vessel’s internal diameter. The advent of metal endoprostheses has effectively addressed restenosis due to the latter two mechanisms. The advent of drug-eluting stents has been effective in preventing restenosis due to both early vascular elastic retraction and distant negative remodeling after balloon angioplasty and significantly reducing restenosis due to intimal proliferation. After the drug-eluting stent is placed into the lesion, the anti-smooth muscle cell proliferating drug carried by the polymer carrier encapsulated on the surface of the metal stent is released from the polymer coating in a controlled manner into the lesioned tissue of the local vessel wall to exert biological effects.
The drug-eluting stent consists of three components: the metal stent platform, the polymeric carrier, and the antiproliferative drug. The two main classes of anti-proliferative drugs include rapamycin and paclitaxel. Rapamycin, also known as sirolimus, is a natural macrolide antibiotic that binds to the FK506 protein after diffusion into cells, causing the release and transcriptional arrest of E2F and reduced synthesis of DNA and ribosomal transcriptional proteins in vascular smooth muscle cells, thereby inhibiting smooth muscle cell proliferation: paclitaxel is an anticancer drug, for promoting the binding of microtubule dimers,l preventing microtubule mitosis from proceeding. Other drugs are everolimus and zotamox (Zotorolimus).