Bioabsorbable stents

# Overview
Bioabsorbable stents, also known as the disappearing stents, are a promising new discovery in the field of interventional cardiology.  They have been an interesting field of research over the past decade and half. As the name suggests they get absorbed completely over a period of time after their work is done. Percutaneous coronary intervention (PCI) using bioabsorbable stents has created special interest because the mechanical support for the healing artery is required only for a brief period of time, and also the presence of a bare metallic prosthesis has potential disadvantages beyond the first few months. 

# Classification
Bioabsorbable stents can be broadly classified into two types: polymeric and metallic types. The key features to be considered while selecting a polymer or an alloy for a bioabsorbable stent are:
- Strength in order to avoid potential immediate recoil
- Rate of degradation and corrosion
- Biocompatibility with the vessel wall
- Lack of toxicity
Polymers have been widely used in cardiovascular devices and are now primarily used as delivery vehicles for drug coatings. Polymers used for bioabsorbable stents are Poly-L-Lactic acid(PLLA), polyglycolic acid(PGA),Poly(D,L-lactide/glycolide) copolymer(PDLA) and polycaprolactone. The use of bioabsorbable polymer coating reduces the need for extended dual anti-platelet therapy and in turn late thrombotic events. Among the polymers, Poly-L-Lactic acid is widely used in medicine. It breaks down to lactic acid a natural metabolite in human body, which enters krebs Cycle and is metabolized to carbon dioxide and water. 
So far two bioabsorbable metals alloys have been proposed for this application: magnesium and iron. The factors that determine the biocompatibility of these alloys are their solubility and degradation products. Magnesium stents are made of 93% magnesium and 7% rare-earth-metals. Reasons for selecting magnesium include:
- It is an essential element in the body.
- The alloy induces rapid endothelialization.
- It has low thrombogenicity.
- It has a lower degradation time of 2-3 months.
- It has calcium antagonist and antiarrhythmic properties.
- Is not associated with any adverse reactions.
The following is the list of some of the bioabsorbable stents that are currently under trials.

# Supportive Trial Data
- The Igaki-Tamai stent was one of the first bioabsorbable stents to be tested in clinical trials. In the preliminary in-man prospective non-randomized clinical trials that involved 50 patients, a 4 year follow up of all patients revealed a low complication rate with 1 in-hospital stent thrombosis causing Q-wave myocardial infarction, 1 non-cardiac death, 18% repeat PCI and no surgical revascularization. This stent is no longer in development now.
- Bioabsorbable therapeutics and REVA medical are testing stents coated with sirolimus and paclitaxel respectively.
- PROGRESS-AMS study  is a prospective multicenter non-randomized study that used the Magnesium-alloy stent and was conducted on 63 patients. The results of this study are 100% device and procedural success rate and the study met it's primary endpoint of MACE(major adverse cardiac events) of <30%. The immediate angiographic results were similar to those after deployment of other metallic stents. Although the stent was absorbed completely within 2 months radial support was lost much earlier so that there was an insufficient radial strength to counter negative remodeling forces after PCI. Consequently there was high restenosis rate at 4 months of almost 50% and target vessel revascularization at 1 year was 45%. There were no deaths, MIs or stent thromboses, and the stent was no longer detectable by IVUS. The high restenosis rate may raise concern about safety.
- The ABSORB trial involving Abbott's drug eluting BVS stent was a prospective non-randomized two phase study on 131 patients. Key endpoints included assessments of safety - major adverse cardiovascular events(MACE) and treated-site thrombosis rates - at 30 days and 6,9,12,24 months with additional annual clinical follow up for up to 5 years. The first phase (cohort A) of the trial enrolled 30 patients and its 2 year results demonstrated the following key results:
A 0% rate of stent thrombosis for all patients
No new MACE between 6 months and 2 years. At 2 years the device demonstrated a MACE rate of 3.6% (1 patient)
Bioabsorption of the stent at 2 years after implantation as confirmed by an assessment of the stent struts using optical coherence tomography (OCT)
Restoration of vasomotion (ability of the blood vessel to contract and expand)
Reduction in plaque area in treated arteries, as confirmed by IVUS and virtual histology
- A 0% rate of stent thrombosis for all patients
- No new MACE between 6 months and 2 years. At 2 years the device demonstrated a MACE rate of 3.6% (1 patient)
- Bioabsorption of the stent at 2 years after implantation as confirmed by an assessment of the stent struts using optical coherence tomography (OCT)
- Restoration of vasomotion (ability of the blood vessel to contract and expand)
- Reduction in plaque area in treated arteries, as confirmed by IVUS and virtual histology
The second phase (cohort B) of the trial enrolled 101 patients and incorporated device enhancements designed to improve deliverability and vessel support. 30 day results demonstrated no cases of blood clots, no need for repeat procedures and a very low rate of MACE. At 1 year MACE was 6.9% with no reports of thrombosis.  
- In the EVOLVE trial experts compared 2 bioabsorbable polymer SYNERGY stents - one deliverig full dose of everolimus and one delivering half dose of the drug with the durable PROMUS ELEMENT metal stent delivering full dose of the drug in 291 patients. The angiographic outcomes of the study suggested that it may be possible to achieve at least comparable efficacy with a lower dose of everolimus than is used in commercially available everolimus-eluting stents. At 1 year the target lesion failure (TLF) in both the study arms were not statistically different (4.4%,4.2% and 5.1% for the full dose SYNERGY, half dose SYNERGY and PROMUS stents, respectively. TLF was defined as target-vessel-related cardiac death, target-vessel-related MI, or ischemia-driven target lesion revascularization. Follow-up at 1 year demonstrated no cardiac related deaths or Q wave MI or stent thrombosis in any of the stent groups. 

# Advantages
The potential advantages of the bioabsorbable stents over the traditional bare metal stents include:
- Reduced late stent thrombosis: The stent being absorbed completely leaves behind no stimulus to ignite a chronic inflammatory process and thus in turn reducing the rate of late stent thrombosis which is a major concern of the traditional stents
- Short duration of post-stenting use of dual anti-platelet drugs
- Absence of metal implants in vessels, leaves only healed natural vessel
- Improved lesion imaging with CT and MRI unlike their bare metal counterparts
- Easier reintervention (PCI and CABG)
- Prevention of postdilatation constrictive vascular remodeling due to the scaffolding effect of the stent
- Late expansive luminal and vessel remodeling after the test is completely absorbed
- Increased drug loading capabilities that will enable chronic drug release strategies
- Elimination of the mechanical stent deformity and strut fractures

# Limitations
The following are the limitations of these disappearing stents:
- Lower mechanical strength compared to their metallic counterparts, which can result in early recoil post implantation
- Significant local inflammation
- Impaired accurate positioning due to their radiolucent nature
- Limited mechanical performance and a recoil rate of approximately 20%, which requires thick struts impeding their delivery capabilities, especially in small vessels
- Limited use in calcified lesions due to their lack of mechanical strength

# Future Applications
The probable future applications of the bioabsorbable stents are:
- Possible use in peripheral arteries such as the femoral or tibial arteries
- Possible use of polymer stents as vehicles that transfer genes that code key regulatory pathways inside the cells of the arterial wall
- Possible use in pediatric congenital heart diseases such aspulmonary artery stenosis