it can negatively influence the patient 's quality of life and may cause important complications such as malnutrition , weight loss , and aspiration.1 benign esophageal strictures are caused by various disorders and procedures such as gastroesophageal reflux disease , surgery ( anastomotic stricture ) , radiotherapy , ablative therapy , caustic ingestion , and pill - induced injury.2,3 esophageal strictures can be divided into two types : simple and complex . simple strictures are defined as short , focal , straight , and easy to pass through with a standard diagnostic endoscope , and are mostly caused by schatzki rings , esophageal webs , or peptic injury . complex strictures are longer ( > 2 cm ) , tortuous , angulated , or have a severely narrowed diameter ( < 12 mm ) and often do not allow passage of an endoscope . complex strictures can be mainly caused by surgery , radiotherapy , or corrosive injury.1,2 of the two types of strictures , the complex type is difficult to treat and associated with a higher recurrence rate than the simple type . refractory strictures have the following characteristics : an anatomic fibrotic esophageal restriction , absence of inflammation or motility disorder , and inability to achieve a diameter of 14 mm in five sessions of dilatations at 2-week intervals or inability to maintain a diameter of 14 mm for 4 weeks once this diameter has been achieved.4 until now , serial endoscopic dilatation with bougies or balloons has been the standard treatment for esophageal strictures . although the immediate success rate for relieving dysphagia is 80% to 90% , the recurrence rate reaches 30% to 40% during long - term follow - up , especially in complex strictures , requiring repeated dilatations or even surgery.5,6 what is worse , up to 10% of patients will not experience any meaningful improvement and be unresponsive to repeated endoscopic dilatations . in these cases , repeated sessions of dilatation without long - term clinical success are closely linked to increased physical and emotional burden to patients ; thus , an alternative treatment strategy should be introduced.7,8,9 as an alternative , surgical solutions such as gastric pull - up and enteral replacement are most effective ; however , many patients are not good surgical candidates because of their comorbid conditions or unwillingness to undergo surgery because of a substantial risk of morbidity and mortality.7,10,11 endoscopic temporary placement of a self - expanding esophageal stent has been proposed as a well - accepted and effective treatment for refractory esophageal strictures owing to its longer - lasting dilatation effects , ability to maintain luminal patency , and simultaneous stretching of the strictures in comparison with dilatation procedures ; moreover , its favorable outcomes , as well as lower morbidity and mortality than surgery have been reported.12,13,14 currently , three types of esophageal stents are available : self - expanding metal stent ( sems ) , self - expanding plastic stent ( seps ) , and biodegradable ( bd ) stent ( fig . 1).2,15 this review focuses on the current experience with esophageal stents , mainly seps and bd stents , for complex and refractory benign esophageal strictures . seps can be easily removed and seems not to be inferior to metallic stent.16 initially , seps was found to provide significant improvement of dysphagia and quality of life , and to decrease the number of dilatation sessions in patients with benign strictures.2,17 on the basis of these results , the use of seps in patients with benign esophageal strictures has been proposed as an alternative to sems.18 the currently available seps is the polyflex stent ( boston scientific , natick , ma , usa ) , which is composed of a plastic wire and has a fully covered design with silicone and a proximal flare . this stent material has been suggested to reduce reactive tissue hyperplasia . as a result , the stent is easily removable and approved by food and drug administration for the treatment of benign esophageal strictures . the use of polyflex stents in malignant strictures has shown good relief of dysphagia , a major complication rate of 9% , and a recurrent dysphagia rate of 37% , mostly due to stent migration ( 13% to 29% ) , tissue overgrowth ( 10% to 30% ) , and food impaction ( 5%).2,16,19 for benign strictures , varying results have been reported . the initial study series in benign esophageal strictures showed promising results , with relief of dysphagia in 80% of patients.8,20,21,22 seps seems to result in a meaningful reduction of reactive tissue hyperplasia and safe stent removal owing to the fully covered design and the silicone material . however , results of a later series have shown a high migration rate ( up to 62% ) and low long - term relief of dysphagia ( only 17% to 30% of patients).3,4,5,10 a recent systematic review of 130 patients with a benign esophageal stricture , obtained from 10 studies , showed that after a median follow - up period of 13 months , only 52% of the patients were dysphagia - free , the early migration rate was 24% , the endoscopic reintervention rate was 21% , and major clinical complications occurred in 9% of patients.3 our systematic review of 172 patients with a benign esophageal stricture , including recent updated reports at the time of writing of this review article , shows the following results : technical success rate of 98% , clinical success rate of 45% , and early stent migration rate of 31% ( table 1).10,12,17,20,23,24,25,26,27,28,29,30 considering that recent studies on seps have shown less favorable outcomes , including a disappointing long - term efficacy , high migration rate , and frequent needs for reintervention , the bd stent has been developed as an alternative to seps . the currently available bd stent designs are the ella - bd stent ( ella - cs , hradec kralove , czech republic ) , which is composed of polydioxanone , a surgical suture material,31 and the poly - l - lactic acid ( plla)-bd stent ( marui textile machinery , osaka , japan ) , which consists of knitted plla monofilaments ( table 2).32 these stents can be degraded by hydrolysis , which is accelerated at low ambient ph . generally , stents begin to degrade after 4 to 5 weeks and dissolve during a period of 2 to 3 months . the major strength of bd stent over sems or seps is that it does not require removal , even when migrated , and it can be left until it is dissolved by gastric acid , which accelerates hydrolysis , thus avoiding further procedures and potential morbidity.31 subsequent studies have shown the availability of the plla - bd stent , with a low stent - related complication rate , compared with sems and seps.32,33,34 in these studies , there were no symptoms or need for reintervention within the follow - up period in all cases ; however , most of the stents ( 10 of 13 cases [ 70% ] ) had migrated within 10 to 21 days after insertion . because of this tendency of early stent migration found in those three studies , the natural history of degradation of the plla - bd stent within the esophagus and the tolerability of the degradation process over time were not adequately assessed . in contrast to the plla - bd stent , the ella - bd stent has shown some promising results . an initial case series with the ella - bd stent for esophageal strictures , including caustic , peptic , malignant , anastomotic , radiation - induced strictures as well as achalasia , showed a low migration rate ( 0% to 22% ) and an acceptable clinical success rate ( 33% to 60%).21,31,35,36,37 however , the ella - bd stent , not unlike commercially available sems and seps , can induce significant hyperplastic tissue reactions.31,35,36,37,38 the uncovered design of bd stent allows stent embedding to the esophageal wall and significantly reduces the migration rate ; however , it induces considerable reactive tissue hyperplasia . our systematic review of 111 patients with a benign esophageal stricture obtained from nine studies showed the following results : technical success rate of 96% , clinical success rate of 47% , early stent migration rate of 21% , and tissue hyperplasia rate of 13% ( table 3).12,21,30,32,33,34,35,39,40 recently modified ella - bd stents with a non - bd covering made of polyurethane were used in five patients with esophageal leaks or perforations.41 the initial clinical success rate was 80% ; however , the stent migration rate was 60% . therefore , the effect of the covered design on minimizing the hyperplastic tissue reaction remains an unsolved problem . recently , drug - eluting bd stents have been introduced as a solution to reactive hyperplasia . theoretically , localized delivery of drugs such as paclitaxel or rapamycin from drug - eluting stents is a promising treatment method for preventing restenosis or inflammatory cell proliferation . until now , many experimental studies have been done with animal models.42,43 more studies on the clinical application of drug - eluting bd stents for human patients are needed . the main limitations of stent placement in benign strictures are hyperplastic tissue reaction , embedding into the esophageal wall , and stent migration . to avoid these limitations , an optimal stent must have a flexible , nontraumatic design and a diameter large enough for normal food passage . in addition , the procedure for insertion , repositioning , and removal should be simple and minimize stent migration and tissue hyperplasia.2 however , for the treatment of complex and refractory benign esophageal strictures , no ideal stent is yet available . each type of stent has its own advantages and disadvantages according to the stent characteristics ( table 4 ) . to date , only uncontrolled studies have evaluated the safety and efficacy of various stent designs in patients with benign esophageal strictures . on the basis of the studies published thus far , the long - term outcome according to the stent type is not different and is unsatisfactory ( approximately 45% ) ( table 5 ) . as previously mentioned , although a partially covered sems has a high anchoring capacity owing to the uncovered design at both ends , it shows a high rate of hyperplastic ingrowth or overgrowth through the uncovered mesh , resulting in severe stent embedding and difficulty of safe removal endoscopically . on the other hand , the migration rate is higher when fully covered stents , either sems or seps , are used . meanwhile , the bd stent shows reduced migration rate owing to its uncovered design ; however , hyperplastic tissue reactions have emerged as a main problem . up to now , the question of which type of stent should be recommended for the effective treatment of complex and refractory benign strictures has no clear answer . therefore , the selection of stent type for endoscopic treatment should be individualized , taking into consideration the endoscopist 's experience as well as patient and stricture characteristics , especially including the location and cause of the stricture . in the case of sites with an increased risk of migration ( i.e. , distal esophagus or anastomosis site ) or restenting after stent migration , the bd stent may be a reasonable option over sems or seps . to minimize tissue hyperplasia after bd stent placement , steroid injection or a drug - eluting bd stent larger , randomized , prospective , well - designed studies are needed to demonstrate the long - term efficacy and safety of stents at each applicable location within the gastrointestinal tract . seps and bd stents were developed to decrease the limitations of sems for benign complex and refractory esophageal strictures . early studies on both stents reported excellent results ; however , our systematic review does not show a superior clinical outcome of these stents compared with sems . although the ideal stent has not yet been developed , seps and bd stents have their own special merits such as decreasing tissue hyperplasia and eliminating the need for stent removal . therefore , the selection of stent type should be individualized , taking into consideration the endoscopist 's experience as well as patient and stricture characteristics . in the future , a new stent that has only the merits of sems , seps , and bd stents will need to be developed .
endoscopic stent placement is a well - accepted and effective alternative treatment modality for complex and refractory esophageal strictures . among the currently available types of stents , the partially covered self - expanding metal stent ( sems ) 
 has a firm anchoring effect , preventing stent migration and ensuring effective covering of a narrowed segment . 
 however , hyperplastic tissue reaction driven by the uncovered mesh may prevent easy and safe stent removal . as an alternative , 
 a fully covered sems decreases the recurrence of dysphagia caused by hyperplastic tissue ingrowth ; however , it has a high migration rate . 
 likewise , although a self - expanding plastic stent ( seps ) reduces reactive hyperplasia , the long - term outcome is disappointing because of the high rate of stent migration . 
 a biodegradable stent has the main benefit of not requiring stent removal in comparison with sems and seps . 
 however , it still has a somewhat high rate of hyperplastic reaction , and the long - term outcome does not satisfy expectations . up to now , the question of which type of stent should be recommended for the effective treatment of complex and refractory benign strictures has no clear answer . 
 therefore , the selection of stent type for endoscopic treatment should be individualized , taking into consideration the endoscopist 's experience as well as patient and stricture characteristics .