The use of metallic endoprostheses in interventional radiology

The use of metallic endoprostheses has advanced rapidly since 1969 when Charles Dotter first described the implantation of a spiral stent via a percutaneous route to maintain a patentlumen. I Initial problems arose from early occlusion but rapid advances and the use of various materials and configurations have increased the indications and use of metal stents throughout the body. Initial use was in the vascular system with later extension to the gastrointestinal, biliary;and respiratory systems. Metallic endoprostheses in current use are manufactured from a variety of alloys and are either self expanding or balloon expandable.


Introduction
The use of metallic endoprostheses has advanced rapidly since 1969 when Charles Dotter first described the implantation of a spiral stent via a percutaneous route to maintain a patentlumen. I Initial problems arose from early occlusion but rapid advances and the use of various materials and configurations have increased the indications and use of metal stents throughout the body. Initial use was in the vascular system with later extension to the gastrointestinal, biliary;and respiratory systems. Metallic endoprostheses in current use are manufactured from a variety of alloys and are either self expanding or balloon expandable.

Balloon expandable endoprostheses
The Palmaz stent This is a balloon expandable tubular meshwork of annealed steel. This is deployed by inflation of a coaxial balloon system which has been mounted using a spedal crimping tool and does not expand furtherwhen the balloon has been deflated. It has been widely used since 1987 in the vascular systern2 (both arterial and venous) and also for Transjugular Intrahepatic Portosysternic Shunting (TIPS? The stent ismanufactured from stainless steel and has an electropolished surface which reduces thrombogenicity. When expanded a diamond lattice shape forms (Figure 1a-d), the struts of which embed in the vessel wall and limited by little longitudinal flexibility but accurate placement is aided by only a small amount of shortening. Recent development of this stent includes a heparin coating, which has potential use in low flow vessels, and articulated stents to increase flexibility.

Strecker stent
The original Strecker stents were a balloon expandable endoprostheses made of tantalum which have been in use since 1987. 4  The use of metal Iic endoprostheses in interventional radiology frompageS Strecker stent has been developed. The design for both materials isfundamentally the same. The stent is made from a metallic monofilament knitted into a tubular mesh ofloosely connecting loops. The main advantage of this stent isits increased flexibility and compressibility, both radial and longitudinal and in the expanded and non expanded state which makes its use possible in tortuous vessels. Although originally use was in the vascular system, Strecker stents have been deployed in the genitourinary; gastrointestinal and biliary systems. Tantalum is less thrombogenic than copper or stainless steel and this is enhanced by surface electropolishing.' The stentis well seen fluoroscopically asitis relatively radiodense and due to its magnetic properties itis also MRI compatible.

Nitinol stents
Nitinol is a alloy of nickel and titanium which expands and contracts under the influence of changes in temperature." Heat treatment during manufacture enables a shape memory to be gained so that it can be used to make therrnoreactive self expanding stents which include the Strecker Elasta1loyUltraflex 1M (Figure 3). Otherself tube of nitinol with slightly flared ends to reduce the risk of migration. It is highly radio-opaque and undergoes minimal shortening. It can berepositioned during deployment as long as no more than 30% of the total length has been deployed, which aids accurate placement 7,8 The Cragg stent is constructed of a spiral of back and forth bends of a single nitinol filament held by a suture ( Figure Sb). Infusion of cold saline during introduction through the introducer sheath keeps the wire maintained in the soft constrained state and aids placement. A covered stent (the Cragg Endopro) with ultrathin dacron sutured to the nitinol is also available (Fig-ure6).

Gianturco Z metallic endoprosthesis
An uncovered Gianturco Z stentis available for bili~venous and tracheobronchial applications and a polyethylene covered stent is available for oesophageal use. An uncovered endoprosthesis is also available for use in the coronary arteries. The stent is made of stainless steel which is bent into a zigzag and joined at the ends forming a cylinder? (Figure 7). This is then compressed and loaded into a catheter. Deployment is by pushing the stent out of the lumen of the catheter into the vessel where re-expansion will occur: The final size and expansibility depend on the diameter of the The Memotherm stent has been available since 1993 and is made from a single

Wallstent self expanding endoprosthesis
This is a self expanding stent with a cylindrical braided structure with variations in design depending on the application. The stent is able to conform to the shape of the vessel,the flexiblity being attributed to its design which does not include any cross points ( Figure 8). The stent is elongated for delivery on a narrow system and shortens during deployment dependent on the final diameter.This can make accurate placement difficult but it is possible to reposition the stent while it is only partially deployed. There has been rapid development of the Wallstent since 1989 and systems are currently available for vascular, 10 biliary, Il o escph age al.P tracheobronchial" and TIPS 14 use, each with differing properties and delivery systems.
The use of these various stents in differing applications will be discussed -however the final choice is often dependent on personal preference and familiarity with the delivery systems.

Use of metallic endoprostheses in the vascular system
The lower 1imb Most experience has been gained within the vascular system of insertion of metallic stents in the iliac arteries. Balloon expandable and selfexpanding stents have longtermpatencyratesof64-95%. 2,15,16,17 A study including 239 patients showed a 54 monthpatencyrateof81.5%withasignificant difference in patency rates at 4 years between stented stenoses (82.9%) and stented occlusions (76.3%).18 Both short stenoses and long occlusions have been successfully treated and several studies have shown long term patency rates to be better than percutaneous translurninal angioplasty (PTA) alone. PTA patency rates for aortoiliac lesions at 2 years are approximately 80% and at 5 years 72%.19 A randomised trial of PTA vs iliac stenting with the Palmaz stent has shown stenting to be superior giving a better morphological response and lower trans-stenotic pressure gradient with fewer technical failures and fewer complications. 20Stenting has this advantage because of the high resistance of the stent to the collapse of the vessel wall. Inaddition the wall of the stent holds any dissection or plaque fracture away from the vessellumen. Longer occlusions for which PTA is frequently unsuccessful, because of the collapse ofthrombogenic material into the opened lumen and subsequent  The use of rrieta!l ic endoprostheses in interventional radiology  Wallstentwhich has a smaller diameter delivery system (7-9F) than the Palmaz stent (8-10F).2 The FDA trial of the Wallstent included 130 iliac and 91 proximal femoral stents and showed a 96.4% primary angiographic success and a primary success rate of88-96% with no significant difference between stenoses and occlusions. The 6 month patency rate was 77% with a complication rate of 16%, although the majority were minor complications with only 2.7% requiring surgery.
There is no one specific clinical indication for stenting in the iliac vessels. The clinical situation, technical aspects e.g. site and configuration of the lesion, and the post angioplasty results need to be considered. Inour institution we have evolved our own guidelines for stent placement within the iliac vessels (Table I), which includes aneurysmal disease ( Figure Il). Placement of stents in the distal vessels of the leg isgenerally less successful because of the smaller size of the vessels and the increased turbulence, particularly in the adductor canal. Restenosis rates are 30-40% in trials using the Strecker stent, which is thought to be a consequence of intense endothelial hyperproliferation. 21 We do not routinely use stents below the inguinal  indications are acute dissection producing occlusion that cannot be controlled by repeated PTA, reocclusion of a vascular segment which has been opened by PTA due to vascular wall deficiency; and exceptionally in cases of persistent restenosis after PTA with expected imminent restenosis.

Renal arteries
In the renal vessels stents may be used in limited circumstances as restenosis occurs angiographically in 20-39% 22,23 of patients and further surgical management is difficult. Most experience has been with the Palmaz stent although other systems have been usedincluding the Wallstentand the Memotherrn stent. Suggested current indications are failure of initial angioplasty with residual stenosis >20%, renal artery dissection post PTA, restenosis after PTA, distal based flaplike plaque, or in patients who are unsuitable for surgical revascularisation. The accurate positioning of the stent is imperative as the stent must not extend into the aorticlumen.

Coeliac and superior mesenteric arteries
The mortality of acute arterial occlusion in the abdominal visceral arteries (coeliac, superior and inferior mesenteric) is between 70 and 90% 24 and the postopera-tivemortality3-20%.25 PTAofmesenteric vessels has a primary success rate of90% with redilation required in 50% of cases. 26 Stenting of mesenteric vessels requires a highly flexible stent because of the nurnerous curves between the puncture site and the site of deployment. Because of this the brachial approach may be easier to negotiate. We have used the Palmaz stent with good radiological and clinical results (Figure 12a-b). In our opinion the only indication for sten t placement is failed PTA in a patient who is inoperable or a poor operative risk. This agrees with the limited amount of information inthehterature. 27,.28

Supra-aortic arteries
The Wallstent has been successfully used in the supra-aortic vessels both as primaryprocedure and in restenosis after PTA.
PTA is a high risk procedure in these vessels because of the risk of embolisation, particularly for the recanalisation of occluded vessels. In a series of 42 carotid stents" 2 patients had transient ischaemie attacks and there were no long term neurological deficits. The rate of recurrent stenoses in these vessels islow, this isprobably because of the large vesselsize and high flow. TheWallstent is used preferentially in this region because it is flexible and self expanding so the artery is not occluded at any time during deployment. No migration of the Wallstent is reported; it is thought that the Strecker stent may potentially migrate ifthe ends are not sufficiently overdilated for anchorage. 30

Aorta
The first report of endoluminal stent placement for aortic aneurysm repair was in 1991. 30 Sincethen stentshave beenplaced successfully in both the thoracic and abdominal descending aorta .31Early experience with the tube steut-graft combinations required the aneurysm to be in the descending aorta with well defined necks at both ends, not tortuous, with no sharp angles and with no side branch involvement. However more recently bifurcated sterit-graft combinations have become available increasing the number of cases suitable for endolurninal grafting. Both balloon expandable and covered stents have been used, these have been custom made to fit the individual characteristics of each aneurysm. Careful preprocedure imaging in each patient is a prerequisite so that the stent -graft combination conforms to the aneurysm exactly. Leakage at the upper or lower end can prevent sealing of the aneurysm and enable continued expansion. Reported complications include difficulty of delivery of the large introducer system (up to 24 french), stent migration during deployment which can be partially overcome by lowering the systolic blood pressure during release, and relatively minor complications of pleuritic chest pain and pleural effusion. 32Transient weakness of the lower limbs has also been described. 32

Venous stents
PTA for the treatment of venous stenoses has been used with limited success and high rates of restenosis. 33However venous stenting has proved valuable in malignant disease,34,35stenting in benign disease should only be performed after detailed discussion with clinical colleagues. It is important to distinguish between benign 'benign' disease and benign 'malignant' disease, where life expectancy is short although no strictly malignant disease is present. Self expanding stents (Wallstent, Gianturco) are most commonly used in tumour related stenoses and predilatation is recommended. Relieffromobstructive symptoms, for example in superior vena cava obstruction, after stenting occurs within hours. Brachiocephalic stenosis complicating haemodialysis has a high restenosis rate following PTA.3 Stenting here may give more permanent relief although restenosis in adjacent areas often occurs .37,38

Metal stents in the oesophagus
The use of metal stents in the oesophagus is largely confined to the palliation of malignant disease. Balloon dilatation is generally successful in the treatment of benign strictures in the majority of cases and self expanding metallic stents should only be used with caution and after detailed discussion with clinical colleagues. 39 Self expanding metal stents can be used as first line treatment for inoperable malignant strictures and have distinct advantages over other palliative methods. Radiotherapy may relieve symptoms but may take 8 weeks to be effective. 40 ,41 Plastic endoprostheses are cheap but require general anaesthesia for placement and usually only allow a liquid diet. 42Endoscopic laser therapy is safe and effective but needs repeating every 4-8 weeks .43The metallic stents commonly used in the oesophagus are the Ultraflex, Gianturco, andWallstent ( Figure 13). All are selfexpanding, with different expansile radial forces. Balloon dilatation up to 12 mm is advised prior to stent placement and if this is performed the Gianturco and Wallstent usually maintain tapage 13 The use of rne-tall ic endoprostheses in interventional radiology from page 12 patency of the oesophageal lumen without further intervention. However post placement dilatation is often required with the Ultraflex as its radial expansile force isless. Various diameters and lengths of stent are available to match the pre procedure contrast assessment of the stricture. The release confined to lesions where the lower end of the stenthes above the cardia. Covered stents are the stent of choice where there is perforation or fistulation (Figure 14a-b). Laser can be used to treat tumour ingrowth which cannot be reheved by passage of an endoscope. A second stent can be placed Using similar principles stents can be placed to relieve obstruction in the duodenum and proximal small bowel. Figure 13: The self expanding metal stents commonly used In the oesophagus include the nitinol Elastalloy stent (/eft), the polyethylene covered Gianturco stent (centre), and the polyurethene covered Wallstent (right), mechanisms for the different devices are similar but have a number of important differences. The Ultraflex iscompressed by a gelatine coating which dissolves on contact with secretions within the lumen of the oesophagus. TheTelestop dehvery system of the Wallstent has 3 coaxial shafts the outer of which can be readvaneed over the partially released stent to allow repositioning during the procedure. The Gianturco device has a cord which needs to be severed once the stent is in the required position. Technical success using these stents is near 100% and improvement in dysphagia 90-100% .44,45 Stent related mortahty is 0-6%. Oesophageal perforation secondary to the procedure is rare, mortahty is more commonly a result of GI haemorrhage or aspiration. Morbidity includes stentmigration, tumour ingrowth and overgrowth, and food impaction. Gastro-oesophageal reflux is common espedallywhere the end of the stent lies in the fundus of the stomach. Tumour ingrowth can beminirnised by the use of covered stents but the migration rate of these is higher and their use is best Stenting for major airway obstruction in malignant disease in the trachea and bronchi has only become possible recently with the advancement of the development of metallic stents. Chnical assessment prior to stenting is vital as recanalisation of the airway may not produce significant improvementin respiratory symptoms ifthere is distal disease. Silicone stents had been used previously but they can only be deployedin the major airways,tend to migrate and occlude due to mucus impaction." Of the metallic stents available the Gianturco has been used most widely but isnot without problems .47 Distal migration may oecur as the stent ispushed out of the sheath and tumour ingrowth may occur in uncovered stents while covered stents cannot not be used across airway bifurcations. Complication rates of up to 30% are recognised and a case of tracheal perforation has been reported.f The Wallstenthas advantages over the Gianturcoin thatitis easierto reposition during deployment, migrates lessfrequently and has less tumour ingrowth while allowing aeration where side branches are crossed, 49 The nitinol stents eg.nitinol

Tracheobronchial stenting
Memotherm are also achieving good results presumably because of their great rigidity whilst maintaining flexibility.50 The use of rnetall ic endoprostheses in interventional radiology from page 13

Metal stents in the biliary system
Percutaneous transhepatic biliary drainage is well established in benign and malignant biliary obstruction 51and is currently the procedure of choice if endoscopic methods fail. Historically plastic stents have usually been placed. These have the disadvantage of a high migration rate (3-6%) and need frequent replacement. 52,53These problems may be overcome by the larger diarneter of metal stents and the epithelialisation over the mesh preventing migration. Plasticstents occlude in most instances due to bile encrustation 53 whilst occlusion in metallic endoprostheses is secondary to tumour ingrowth or overgrowth. This was significantin earlystudies,Il however more recent series,where longer stents have been placed to minimise the problem of overgrowth, have shown much lower occlusion rates (12%).54,55The onlyrandomised trial of plastic versus metal stents'" demonstrated a lower reintervention rate using metalstents (12%vs43%). Otheradvantages of metal stents mainly centre around the smaller size of the introducer sheath (7Fr-8Frvs 12Fr for plastic stents) reducing local complication rates and enabling a one stage procedure to be performed. Inour institution we routinely use metallic stents for clinical benefit to the patient in the short term (smaller introducer system, one stage procedure) and the long term (longer patency rates therefore reduced reintervention).
In addition the randomised trial 53demonstrated a financial advantage due to reduced reintervention rate in the long term. A disadvantage of metal stents isthat they cannot be removed after epithelialisation has occurred, and their use is therefore restricted to permanent stenting predorninantlyfor the relief of malignant strictures. The role in benign strictures is limited to strictures that are resistant to repeated balloon dilatation and in whom surgery is inadvisable or unwanted. 56Most of the current literature refers to the Wallstent, however the nitinol Memotherm stent isbeing increasinglyused (Figure 15a-d).

TIPS (Transjugular Intrahepatic Portosystemic
Shunt) The development of metal stents has enabled TIPS to become established in the treatment of variceal haemorrhage and portal hypertension ( Figure 16).57, 58,59 Contraindications to this procedure include raised right heart pressure, sepsis, acute liver failure or extensive malignancy in the proposed transhepatic path. The technique involves dilatation of the intrahepatic tract between the hepatic and portal veins and the placement of an expandable metallic stent. This reduces the pressure gradient between the portal vein and hepatic vein to 10-15 mm Hgwhich is sufficient to reduce bleeding but not induce encephalopathy. complication in 25-40% and is usually due to intimal hyperplasia with reintervention rates risingto 68% at 2 years.60 Regular surveillancewith Doppler and direct catheterisation is necessary. 63 Shunt malfunction can be treated with balloon dilatation or placement of a second stent with good long term results. TIPS is particularly valuable for patients awaiting liver transplant and does not increase the operative difficulty in the short term (3-6 months.)

Conclusion
Metal stents are now being used for a wide number of clinical applications in many areas. TIlls incorporates restoration of patency in many vessels in the vascular system with good long term patency and palliation of numerous malignant conditions in which a compromised 'lumen' results in poor quality oflife in many instances and a shortened life span in others. TIlls is largely due to the advances in stent research and design over the past decades and the availability of a range of different stents for these applications. TIlls has produced an improvement inpatency rates with, in many instances, reduced mortality and morbidity. The future is focussed on further improved stent design. Recent research work has centered on reducing intimal hyperplasia, particularly in the vascular system, using coated stents", exploitingmetallic properties to reduce cellgrowth locally" and even the use oflocal irradiation." We look foward to the continued application of these devices asthey become available.