The materials currently used to make degradable coronary stents are mainly polymers (polylactic acid) and metals (magnesium, iron). The former has a relatively mature manufacturing process, while the latter is still difficult to be widely used in clinic due to problems such as degradation speed and inflammatory response. Biodegradable stents that have been clinically studied worldwide include Igaki-Tamai stent (Igaki Medical, Japan), Abbott BVS (Abbott Vascular, US), ART18Z stent (ART, France), REVA stent and ReZolve stent (Reva Medical, US) ), DESolve scaffold (Elixir, US), Ideal scaffold (XenogenicsCorp, US), XINSORB scaffold (Shanghai Weite Biotechnology Co., Ltd., China), and DREAMS scaffold (Biotronik, Germany). This article reviews the clinical progress, current problems and future development directions of degradable stents.

In September 1977, Dr. Gruentzig performed the first percutaneous coronary angioplasty (PTCA), ushering in a new era in interventional cardiology. Since then, the discipline has flourished and has experienced the era of PTCA, bare metal stent (BMS) and drug-eluting stent (DES). Although the widely used DES has successfully solved the problems of acute vascular occlusion after PTCA and restenosis after BMS, DES is still not the most perfect stent. Antiproliferative drugs coated on the surface of DES inhibited the proliferation of vascular smooth muscle on the one hand, and delayed vascular endothelial repair on the other hand. In addition, metal implants that are permanently present in blood vessels can cause inflammation of the vessel wall and limit normal vasomotor activity. The BASKET-LATE study, reported at the 2006 European Congress of Cardiology, brought awareness to late stent thrombosis (ST), a dreaded complication associated with DES. In order to reduce the incidence of late ST, researchers have made various improvements to coronary stents. Experiments show that vascular remodeling is completed 6 to 9 months after stenting, so theoretically, a permanent vascular support is not necessary. The fully degradable stent is designed based on this concept. The stent can support the diseased blood vessel immediately after implantation, and completely degrade after the negative remodeling of the blood vessel is completed. The advantage of degradable stents is that after the stent degrades, it can complete the tasks that metal stents cannot, such as restoring the normal physiological function of blood vessels, relieving inflammation of the blood vessel wall, not imprisoning side branch vessels, repeating interventional therapy in the same lesion, and combining with magnetic resonance imaging. Check compatibility, etc. In addition, long-term follow-up can still find the phenomenon of late lumen enlargement. The materials currently used to make degradable coronary stents are mainly polymers (polylactic acid) and metals (magnesium, iron). The former has a relatively mature manufacturing process, while the latter is still difficult to be widely used in clinic due to problems such as degradation speed and inflammatory response. Biodegradable stents that have been clinically studied worldwide include Igaki-Tamai stent (Igaki Medical, Japan), Abbott BVS (Abbott Vascular, US), ART18Z stent (ART, France), REVA stent and ReZolve stent (Reva Medical, US) ), DESolve scaffold (Elixir, US), Ideal scaffold (XenogenicsCorp, US), XINSORB scaffold (Shanghai Weite Biotechnology Co., Ltd., China), and DREAMS scaffold (Biotronik, Germany). This article reviews the clinical progress, current problems and future development directions of degradable stents. 365 Medical Network Ming

1. Clinical progress of degradable stents

(一) Igaki-Tamai bracket

The Igaki-Tamai stent is the earliest degradable PLA stent implanted in human coronary arteries. The stent is self-expanding, composed of PLA single fibers, without drug coating, and the stent is completely degraded within 3 years. Since September 1998, Dr. Tamai in Japan used this stent to treat 19 coronary lesions in 15 patients. The coronary angiography and IVUS were reviewed immediately and 24 hours after the operation, and no immediate elastic recoil of the stent was found. There were no major adverse cardiac events (MACE) and stent thrombosis within 30 days after surgery. The 6-month follow-up results showed a restenosis rate and target lesion revascularization rate of 6.7%, no stent thrombosis and no major adverse cardiac events. During the 4-year clinical follow-up, there were 1 case of death, 1 case of Q-wave myocardial infarction, and 1 case of stent thrombosis, and a total of 9 patients required re-interventional therapy. At this time, intravascular ultrasound (IVUS) has been unable to detect the stent. So far, the 10-year follow-up of the Igaki-Tamai stent has been completed. Coronary CTA, coronary angiography and IVUS have all shown that the stent has been completely degraded, and the blood vessel where the stent was originally implanted remains open. The 10-year cumulative survival rates for all-cause death, cardiac death, and MACE were 87%, 98%, and 90%, respectively, and the 1-year, 5-year, and 10-year target-lesion revascularization rates were 16%, 18%, and 10%, respectively. 28%. A total of 2 confirmed thrombotic events occurred during follow-up (1 subacute and 1 very late). Since the implantation of the Igaki-Tamai stent requires the use of an 8F guide catheter, and the release process is complicated (the blood vessel needs to be continuously flushed with a contrast medium heated to 80°C, and then pressurized to 6-14 atm for 30 seconds to release the stent), it may be possible to release the stent. Causes arterial wall necrosis, intimal hyperplasia or platelet adhesion activation, etc., so the stent is difficult to be widely used in clinic. The improved new-generation Igaki-Tamai stent overcomes the above shortcomings, can be delivered through a 6F guide catheter, and is modified to be released by balloon expansion. The stent is currently undergoing preclinical evaluation in Germany.

（二）Abbott BVS

At present, the only biodegradable PLA stent that has carried out large-scale clinical studies and has obtained CE certification is Abbott BVS of Abbott in the United States. Abbott BVS uses PLLA to form the stent platform, PD, L-LA as the drug-loading coating, and the rapamycin derivative Everolimus as the anti-proliferative drug. The first-generation Abbott BVS has an outer diameter of 1.4 mm and a stent thickness of 150 μm. There are two radiopaque metal markers at each end of the stent to indicate the position of the stent in the blood vessel. The stent, which was used in the First-in-Human study (30 patients) in 2006, showed an immediate elastic recoil of 6.9%, slightly higher than that of the cobalt-chromium alloy XIENCE V stent (4.3%). The incidence of MACE at 1-year follow-up was 3.3% (1 non-Q wave myocardial infarction), and no target lesion revascularization and stent thrombosis occurred. The 6-month angiographic results showed that the late in-stent diameter loss was 0.44±0.35 mm. IVUS showed that the neointimal area was 0.30±0.44 mm2, and the area stenosis rate was 5.5%. During the 2-year follow-up, there was no cardiac death, ischemia-driven target lesion revascularization, or stent thrombosis. There was only 1 case of non-Q-wave myocardial infarction (same as before), and the late stent diameter loss was 0.48±0.28 mm. The rate of diameter stenosis was (27±11)%, which was not significantly different from the results of 6-month angiography. At this time, the blood vessels in the stent implanted segment had returned to normal diastolic and systolic activities, and optical coherence tomography (OCT) showed that 34.5% of the stent beams were no longer visible. The 4-year clinical follow-up showed that there was only 1 case of ischemia-driven MACE (non-Q-wave myocardial infarction), and no cardiac death, target lesion revascularization, or stent thrombosis occurred. The first-generation Abbott BVS had a large elastic recoil due to the structural problem of the stent, and the 6-month IVUS follow-up showed that the stent area was reduced by 11.8% compared with the immediate postoperative period, resulting in a large late tube diameter loss. Abbott then modified the first-generation Abbott BVS stent configuration, using the same MultiLink configuration as the XIENCE series stents, and improved the manufacturing process to slow the degradation of the stent. Compared with BVS1.0, the improved Abbott BVS1.1 has stronger radial support, less elastic retraction and longer degradation process. A total of 101 patients were enrolled in the ABSORB cohort B study, implanted with Abbott BVS1.1 (3.0 × 18 mm). The study was divided into B1 group (n=45) and B2 group (n=56), group B1 underwent vascular imaging examinations (angiography, IVUS) at six months and 2 years after operation, and group B2 at 1 year and 3 years after operation, respectively. , OCT). The results showed that compared with Abbott BVS1.0, there was no significant stent elastic recoil in Abbott BVS1.1. During the 6-month follow-up in group B1, the late lumen loss in the stent segment was 0.19±0.18 mm. IVUS showed that the stent area was only reduced by 2%, and OCT showed that 96.8% of the stent beam had been covered by intima. At 2-year follow-up, the late caliber loss was 0.27±0.20 mm, the neointimal area detected by OCT and IVUS were 0.68±0.43 mm2 and 0.17±0.26 mm2, respectively, 99% of the stent beam was covered by the intima, and MACE at 2 years The incidence was 6.8%, and no stent thrombosis occurred. During the 1-year follow-up in group B2, the late intra-segmental diameter loss was 0.27±0.32 mm, and neither IVUS nor OCT found that the stent area was significantly reduced compared with the immediate postoperative period. Two patients in this group had perioperative MI and another 2 patients had MI due to procedural reasons, with a MACE rate of 7.1% at 1 year. At 3-year follow-up, the late in-stent diameter loss was 0.29±0.43 mm, the stent area remained unchanged, and the plaque area was significantly reduced. The incidence of MACE in Cohort B group was 10% within 3 years, and no stent thrombosis occurred.

At present, in addition to ABSORB Cohort Group A and Group B continuing clinical follow-up, there are still ABSORB Extend, ABSORB II, ABSORB PHYSIOLOGY, ABSORB FIRST, ABSORB China, ABSORB Japan and other studies underway. Among them, the ABSORB Extend study is a non-randomized, single-group trial conducted around the world. A total of nearly 100 centers participated in the study. It is planned to select nearly 1,000 patients to receive Abbott BVS treatment at 30 days, 6 months, and 1 year after surgery. Clinical follow-up was performed at 2 years, 2 years and 3 years, and angiographic and OCT follow-up was performed at 2 years. The first patient was enrolled in January 2010, and by October 2012, the study had enrolled 585 patients, 93% of the lesions were ACC/AHA type B lesions, 92.6% of the patients had only 1 lesion, 81.8 % of lesions were treated with an Abbott BVS 3.0 × 18 mm stent. The instrumental success rate was 98.5% and the surgical success rate was 96.8%. At 6-month follow-up, 1 patient had cardiac death (0.2%), 14 myocardial infarction (2.8%), and 3 (0.6%) patients underwent target lesion revascularization due to ischemic events (1 CABG, 2 PCI), the MACE rate was 3.0%, and stent thrombosis occurred in 3 patients (0.6%). The purpose of the ABSORB PHISIOLOGY study was to explore the effects of Abbott BVS and XIENCE V stent implantation on vascular function, including vascular compliance, dilatation performance, endothelial cell response and shear force distribution. The main purpose of the ABSORB Ⅱ study is to compare the efficacy and safety of Abbott BVS and XIENCE PRIME stents in the treatment of primary coronary lesions. It is planned to enroll 500 patients, and all patients have been enrolled in this study. The ABSORB China study enrolled Chinese patients with coronary heart disease who were treated with Abbott BVS. The first patient was enrolled in the study in July 2013, and enrollment of all patients has been completed. Angiography review is planned to begin in July 2014.

（三） Please indicate if ART18Z stent is reproduced on 365 Medical Network

The ART18Z stent is composed of PDLLA, the stent shape is similar to the first-generation Abbott BVS, it does not carry anti-proliferative drugs, and the stent coverage rate of the vessel wall is less than 25%. Animal experiments showed that the stent had a similar degree of elastic recoil as the Multi-Link VISION stent (2.94% vs 3.32%), and the late caliber loss within 9 months of follow-up was also similar to the Multi-Link stent, and no MACE and thrombotic events occurred. The stent begins to degrade 3 months after being implanted into the blood vessel, and completely degrades after 18 to 24 months. The First-in-Human study (ARTDIVA study) of this stent was launched in 5 foreign heart centers in the second half of 2012. The primary endpoint was the incidence of MACE in 6 months, and the secondary endpoints were immediate device and surgical success rate. 1 , 3, 6, 12, and 18 months of stent thrombosis and clinically driven target lesions, target vessel revascularization, and late caliber loss at 12 months of angiographic follow-up and OCT follow-up of stent surface intimal coverage and neointimal area. Preliminary results show that the stent has a high surgical success rate, with good angiographic results immediately after surgery, and no MACE has occurred. However, due to the rapid degradation of the stent and the absence of anti-proliferative drugs, the stent needs to be further improved before large-scale clinical research can be carried out.

（四）REVA mounts and ReZolve mounts

REVA stent is a polyiodotyrosine alkyl carbonate stent developed by REVA Medical in the United States. It can be degraded into water, carbon dioxide, ethanol and iodinated tyrosine alkyl in vivo. The degradation products are almost non-toxic to the human body. Side effects can be absorbed or excreted by the body. The REVA stent adopts a unique "sliding and locking" design, which can provide sufficient radial support after stent placement, and there are radiopaque metal markers at both ends of the stent, which is helpful for later follow-up. It takes 18 to 24 months for the stent to degrade completely, and at the same time, the degradation rate can be changed as needed. Animal experiments showed that 55 months after the REVA stent was implanted into the porcine coronary artery, the degree of stenosis of the stent segment was significantly smaller than that immediately after the operation, and the lumen was significantly enlarged. First-generation REVA stents have completed the First-in-Human study (RESORB study). Angiography showed that the minimum lumen diameter increased from 0.88±0.39 mm to 2.76±0.36 mm in 25 patients with single-vessel coronary disease after implantation of this stent, which was similar to the immediate diameter obtained with the current metal stents. However, the follow-up results of 4 to 6 months showed that the target lesion revascularization rate increased, mainly related to the mechanical properties of the stent itself. To solve this problem, the company designed the second-generation REVA stent, the ReZolve stent. The surface of the stent carries the anti-proliferative drug rapamycin, and it takes about 30 days for the drug to be completely released and one year for the stent to degrade completely. The ReZolve stent further enhances radial support through increased polymer strength and an improved "helical slide and lock" design to reduce stent elastic recoil. The ReZolve stent is underway in the RESTORE study, which plans to select 50 patients with primary endpoints of ischemia-driven target lesion revascularization at 6 months postoperatively and QCA and IVUS measurements at 12 months. The second-generation ReZolve stent was designed and used in the clinic with an increased dose of rapamycin and a slower drug release system. The stent manufacturing process has also been improved, and the outer diameter of the stent has been reduced on the premise of ensuring mechanical properties. The RESTORE II study with 125 patients planned to be enrolled was launched in April 2013. The trial mainly evaluates the safety and efficacy of the second-generation ReZolve stent. The initial clinical trial results are expected to be available this year. Currently the stent is applying for CE certification.

（五） DESolve bracket

The DESolve stent is composed of PLLA and contains two novel anti-proliferative drugs (Novolimus and Myolimus), and its radial support force is similar to that of Elixir BMS, and it takes about 2 to 3 years for the stent to degrade completely. The stent First-in-Human study has enrolled 16 patients so far. The reference vessel diameter of the lesion is 3.0 mm, and the length of the lesion is less than 10 mm. The results showed that 1 patient underwent emergency CABG due to spiral dissection 30 days after surgery, and 1 patient underwent target lesion revascularization within the follow-up period from 30 days to 180 days after surgery. No cardiac death, myocardial infarction and stent thrombosis occurred within 180 days. QCA showed a late lumen loss of 0.19±0.19 mm at 6 months, and the 6-month IVUS results showed a mild decrease in lumen area (5.35±0.78 mm2 vs. 5.10±0.78 mm2), but a mild increase in stent area (5.25 mm2 vs. 5.61±0.81 mm2), OCT results showed that 98.68% of the stent beams had been completely covered by the intima, and the neointimal area was 0.71±0.36 mm2. Based on this, the research party started the DeSovle NX study, which planned to enroll 120 patients with a single primary coronary artery lesion, and the end point was 6-month late in-stent loss to evaluate the efficacy and safety of the DESolve stent. The DeSolve NX II study intends to evaluate stent efficacy in a larger sample of patients and apply for CE certification.

(六) Ideal bracket

The Ideal stent is a fully degradable balloon-expandable stent consisting of a core backbone and a surface coating. The former consists of a mixture of polylactic acid and a trimer containing 2 molecules of salicylic acid and 1 molecule of sebacic acid. The surface of the scaffold is coated with salicylic acid and contains rapamycin. The WHISPER study evaluated the efficacy of this stent. Eleven patients were enrolled in the study, and neither IVUS nor OCT found elastic recoil of the stent. However, follow-up found that the stent had obvious intimal hyperplasia, which may be related to the low drug content and rapid elution. So the stent was redesigned to increase the drug load and prolong the drug elution process. A new generation of Ideal stents is currently undergoing animal testing and will soon enter clinical studies.

（七）XINSORB bracket

The XINSORB stent is a fully degradable rapamycin-eluting stent composed of PLLA ReZolve. The surface of the stent is coated with a PDLLA coating containing anti-proliferative drugs. This scaffold is the first fully degradable PLA scaffold independently developed in my country. The stent is released by balloon expansion, and there is an X-ray-opaque marker on each end of the stent to indicate the position of the stent in the blood vessel. The stent thickness was 160 μm and the drug loading was 8 μg/mm, and in vitro tests showed that 80% of the drug was eluted within 28 days. Early animal experiments showed that the degree of elastic recoil of the stent immediately after implantation into porcine coronary artery was similar to that of 316L stainless steel stent (0.7±4.3% vs. 1.4±3.8%, P=0.45). IVUS showed the degree of elastic recoil of the stent at 1 month after operation 4.3±3.7%, similar to EXCEL stent (1.4±3.4%, P=0.27). At present, the stent has completed long-term animal follow-up. The angiographic follow-up showed that the late lumen loss in the XINSORB stent at 1.5 years was significantly reduced compared with 3 months and 6 months after operation, similar to that at 1 month after operation, while the minimum lumen diameter was significantly increased. , Stent diameter stenosis was significantly reduced. OCT also found that the lumen increased significantly at 1.5 years, which was associated with a reduction in the neointimal area within the stent. Since September 2013, the XINSORB FIM study has been carried out in two centers, Zhongshan Hospital Affiliated to Fudan University and the Chinese People's Liberation Army General Hospital (Beijing 301 Hospital). cavity is lost. By January 2014, the study had completed enrollment of all 30 patients, and the immediate device and surgical success rates were 100%. All patients have completed 3-month clinical follow-up, and no MACE and thrombotic events occurred. So far, the first 6 patients who received XINSORB stent implantation have completed 6-month angiographic follow-up. The results showed that the minimum lumen diameter and stenosis degree in the stent at 6 months were similar to those immediately after surgery, and the late lumen loss was 0.11±0.13 mm. . OCT showed that the lumen area was 5.64±0.62 mm2, the stent area was 7.68±0.87 mm2, and the neointimal area was 1.94±0.36 mm2 at 6 months. The 6 patients had no MACE and stent thrombosis. At present, the XINSORB FIM study is still in the follow-up stage, but the preliminary results have shown the excellent efficacy and safety of the stent, and the expanded clinical study will be carried out in the near future.

(八) AMS

AMS is the earliest magnesium alloy stent used in clinical practice. The stent is laser-engraved from tubular magnesium alloy and has mechanical support similar to metal stents. The first generation of AMS (AMS1.0) has no anti-proliferative drugs. Due to the rapid degradation rate, it is completely degraded after 2 months of implantation, resulting in late lumen retraction. At present, the stent has been redesigned, and the degradation process of the new generation AMS (DREAMS stent) has been extended to 6 months. The PROGRESS-AMS study evaluated the efficacy and safety of AMS1.0. The study was a non-randomized multicenter study, and a total of 63 patients were enrolled. All lesions were primary coronary lesions, with a length of 10-15 mm and a reference vessel diameter of 3.0-3.5 mm. At 4-month follow-up, late lumen loss was as high as 1.08±0.49 mm, and 23.8% of patients underwent target lesion revascularization due to ischemic symptoms. During the follow-up period of 4 to 12 months, only 1 new TLR patient was added. The MACE rate at 12 months was 26.7%. Such a high late-stage caliber loss is mainly due to stent retraction caused by significant intimal hyperplasia and a rapid decrease in mechanical support. The redesigned AMS (DREAMS1.0) carried the antiproliferative drug paclitaxel, and changed the scaffold shape and content of various components of magnesium alloy, thereby enhancing the scaffold support and prolonging the degradation process. The BIOSOLVE-1 study evaluated the efficacy and safety of the DREAMS 1.0 stent. A total of 46 patients with a single primary coronary lesion were enrolled in the study, and 47 stents were implanted. The 1-year follow-up rate of MACE was 7% (2 cases of TLR and 1 case of MI), and the late lumen loss was 0.52±0.396 mm, which was significantly better than AMS1.0. The second-generation DREAMS stent (DREAMS2.0) uses rapamycin as an antiproliferative drug, replacing paclitaxel previously used in the DREAMS1.0 stent. Animal experiments showed that compared with the previous generation of AMS, the stent significantly reduced vascular inflammation and had a higher endothelial coverage. Clinical studies are currently being planned for the DREAMS2.0 stent.

In addition, there are other degradable PLA stents, such as Amaranth stent (Amaranth Medical, US), Acute stent (OrbusNeich, US), MeRes stent (Meril, India) and FADES stent (Zorion Medical, US) and so on. Most of these stents are currently in the preclinical stage, so only a brief introduction is given. Amaranth stent is composed of tubular PLLA. After implantation, the complete configuration of the stent can be maintained for 3 to 6 months, and the degradation process is 1 to 2 years. Animal experiments show that the stent has high radial support force and stable mechanical properties, with only slight elastic recoil. Late lumen loss at 28 and 90 days was similar to that of the Liberte stent (BMS, BOSTON Scientific), but the late lumen loss was significantly reduced after 90 days of follow-up, while the Liberte stent continued to increase. The stent plans to carry out FIM research and apply for CE certification. The Acute stent is composed of 3 kinds of PLA polymers with asymmetric coating. The wall of the stent is coated with anti-proliferative drugs to inhibit smooth muscle proliferation. The lumen surface has CD34 antibody, which can capture circulating endothelial progenitor cells to accelerate endothelial repair. Animal experiments show that the scaffold performs well. The MeRes stent is constructed of PLA and carries the new anti-proliferative drug Merilimus. The stent has strong radial support force. Animal experiments showed that the anti-proliferative drug washout period was as long as 30 days, and there was no obvious inflammatory reaction of the vessel wall during the degradation of the stent. The FADES stent is made of a mixture of PLA and magnesium alloy, and animal tests show that the stent is completely degraded within 90 days without obvious inflammatory response. Other degradable stents include Avatar stent (S3V Vascular Technologies, India), Sahajanand stent (Sahajanand Medical, India) and Stanza stent (480 Biomedical, US). These stents are currently in the early stages of development, and data from physical or clinical trials are still needed to demonstrate stent performance, efficacy, and safety.

二.The current problems of degradable stents

With its excellent efficacy and good safety, degradable stents (mainly Abbott BVS) have initially had the potential to replace the currently widely used metal DES, and are even known as the fourth revolution in the history of interventional cardiology. It has become one of the main executors of the more popular vascular repair therapy. However, like its predecessors, degradable stents are still imperfect and subject to factors such as mechanical properties, stent thickness, degradation rate, inflammatory response, and drug elution rate, so there is still room for further improvement. Although polylactic acid has good biocompatibility, the degraded lactic acid will stimulate local blood vessels and cause an inflammatory response, and the larger the molecular weight of the degraded polylactic acid fragment, the stronger the inflammatory response. Restenosis is associated with stent thrombosis. Therefore, it is still necessary to find an ideal degradation substance in the future, which will not cause intimal inflammation or hyperplasia, and the degradation products are also harmless to the human body, and fundamentally solve the rejection reaction between the stent and the vascular endothelium after implantation.

Compared with cobalt-chromium alloy or stainless steel, the mechanical properties of polymer are inherently insufficient, and its elastic modulus is 100 times lower than that of metal, and magnesium alloy is 5 times lower than that of cobalt-chromium alloy, and the magnitude of elastic modulus is closely related to radial support force. Therefore, if a polymer stent or a magnesium alloy stent is to provide the same supporting force as a cobalt-chromium alloy stent, its thickness needs to be 240% and 50% larger than the latter, respectively. However, even increasing the thickness of the stent does not mean improving the radial support force. Abbott BVS1.0 and AMS1.0 both showed severe elastic recoil in clinical studies, resulting in significantly larger late lumen loss. Since the degradable stent is significantly thicker than the cobalt-chromium alloy stent, the outer diameter of the degradable stent is larger, which reduces the passing performance, and the severe distortion and calcified lesions may limit the application of the degradable stent. In addition, the elastic modulus of degradable scaffolds is low, and scaffold fracture is also one of its potential drawbacks. The fracture strain of PLLA and magnesium alloys is 1% to 5% and 2%, respectively, while the cobalt-chromium alloy with the same elastic modulus is as high as 40%. The lower the fracture strain, the easier it is to fracture when subjected to the same stress. Appropriate degradation rate is also critical to the performance of degradable scaffolds. The intrinsic properties of the material and the state of the environment at the implant site are the two main factors that determine the rate of degradation. Current research shows that the negative remodeling of blood vessels is basically completed 6 months after stent implantation, so it is required that degradable stents should be able to maintain support for at least 6 months. At present, the degradation cycle of degradable stents is usually more than 2 years. If the stent degrades too quickly, it will face the problem of severe elastic recoil and late lumen loss. However, if the stent exists in the body for a long time, it is easy to cause vascular inflammation and restenosis. There is no clear conclusion on how long the degradation cycle of degradable stents is needed. At present, the slow degradation rate of polymer stents and the fast degradation rate of magnesium alloy stents are both problems faced by the field of stent materials and the focus of research. In addition, how the blood vessels are repaired after the stent is completely degraded, and what mechanism is involved in the repair of blood vessels is still unknown, and a large number of animal experiments and clinical trials are needed to explore. Drug elution rate is another important factor affecting the efficacy of degradable stents. The early Ideal stents resulted in large late lumen loss due to too low drug loading and too fast elution rate. However, researchers currently have considerable experience in the drug loading and elution rate of metal DES. In the future, this problem can be solved by continuously improving the drug release system and increasing the drug loading, and can further explore drug elution and stents. The relationship between complete degradation has established a perfect degradable scaffold system. In addition, different drugs have different effects. In the future, different anti-proliferative drugs can be loaded on the stent for comparison, and the drug with good anti-proliferation effect and complete elution time can be matched with the complete degradation time of the stent. Selecting the most suitable drug for the response to the drug needs to be proved and perfected by a large number of experiments.

The properties of polymers or magnesium alloys themselves also limit the use of degradable stents in interventions. As mentioned above, degradable stents are thicker and have lower elastic modulus and fracture strain, which affect the passing performance of stents. Severe distortion and calcified lesions limit the use of degradable stents. It is prone to fracture, so post-expansion after implantation of degradable stents is usually strictly limited in clinical practice. The currently used PLA stents and magnesium alloy stents have poor visibility or are invisible under fluoroscopy. Usually, X-ray-opaque markers need to be added to both ends of the stent to indicate the position of the stent in the blood vessel, which increases the difficulty of positioning the stent during follow-up. difficulty. Although degradable PLA stents have been used to treat various complex lesions including left main trunk, chronic total occlusion, calcification, and small blood vessels, bifurcation lesions are still unbearable for degradable stents. Degradable stents cannot perform CRUSH or CULOTTE procedures like metal DES. Once the blood flow of the side branches is affected after the degradable stents are implanted, expanding the side holes of the stents will inevitably lead to deformation of the stents and cannot be repaired by balloon kissing like metal stents. , limiting the use of degradable stents in bifurcation lesions.

三. Future prospect and development direction

Degradable stents have good efficacy and safety, and are suitable for vascular repair treatment. Although there are still flaws, they still represent the development direction of coronary interventional therapy. In the future, it is still necessary to find suitable materials to overcome the shortcomings of the current degradable scaffolds. The ideal material used as a degradable scaffold skeleton should meet the following conditions: 1. Good biocompatibility, the degradation products are harmless to the human body, and do not cause inflammatory reactions; 2. Have sufficient mechanical properties and good radial support force and delivery performance; 3. Appropriate degradation process to facilitate vascular repair. Despite the harsh conditions, a variety of compounds have now entered the attention of researchers, such as tyrosine-derived polycarbonates, salicylic acid-based polyanhydrides, and polyurethane compounds. These substances can improve the mechanical properties of the stent, and the degradation products are harmless or even therapeutic.

The thickness of degradable stents is also the focus of current research. Thicker stents increase the outer diameter of passage and reduce the passage performance of the stent, also associated with restenosis. At present, biodegradable stent manufacturers including Abbott and Shanghai Weite Biotechnology Co., Ltd. are committed to reducing the thickness of the stent and improving the passing performance of the stent without reducing or even improving the mechanical properties of the stent. It is believed that a new generation of degradable stents will appear in the near future.

In addition, current guidelines recommend that dual antiplatelet therapy must be used for at least 12 months after DES, which increases the risk of bleeding and increases the cost of treatment. If degradable stents can take advantage of their degradation advantages to shorten the course of antiplatelet drugs, it is another advantage of DES, and relevant clinical studies should be designed to explore in the future. At present, degradable stents are still mostly used to treat simple coronary lesions, and metal DES still seems to be the unshakable overlord. To completely replace the status of metal DES, degradable stents must further improve their performance and prove their efficacy and effectiveness in the real world. safety. Only by accumulating more clinical research data can we prove the value of degradable stents in the interventional treatment of coronary heart disease and truly create an era of degradable stents.