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Learn about the significant impact of stents in the development of endovascular surgery and their various applications and potential future advancements.
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Θεματική ενότητα: Stenting Stenting: Τεχνική Μ. Ματσάγκας, MD, PhD, FEBVS Σάββατο 9 Φεβρουαρίου 2013
Impact of Stents Vascular stents have made it possible to reline a diseased artery Stents have had a major impact on the development of endovascular surgery that is manifested in four ways
Impact of Stents The complicationsof balloon angioplasty, such as dissection and residual stenosis, may be immediately treated Lesions that otherwise would have required open surgery, such as occlusions or long lesions or recurrent stenoses, may be treated with endovascular surgery The overall spectrum of arterial lesions that can be approached with endovascular techniques has broadened dramatically Combining stents with graft material to create covered stents has permitted the endovascular treatment of more advanced disease
Impact of Stents Each stent application has its own cost and complication risks • the sheath must usually be upsized • a foreign body is implanted • the procedure time is often extended somewhat • stents have their own unique complications
Impact of Stents • The cost of a single stent substantially increases the cost of an endovascular intervention • The placing of stents may be motivated by the wish • to extend the short- or long-term success of balloon angioplasty • to avoid surgery • to avoid repeat balloon angioplasty • But should be considered in each case !!
Impact of Stents Future applications of stents may include further miniaturization the ability to release antithrombotic agents emit irradiation prevent intimal hyperplasia through bioengineering design changes
Type of Stents • Stents are either balloon-expandable or self-expanding • The Wallstent is a special flexible self-expanding, wire mesh tube
Type of Stents The balloon-expandable stent design This is a straight, metal, rigid, and balloon-expandable cylinder The stent is premounted onto a standard angioplasty balloon It is deployed when the balloon is inflated
Type of Stents The balloon-expandable stent design The rigid balloon-expandable stent has excellent hoop strength but can be crimped by external forces
Type of Stents The balloon-expandable stent design • Perform best when placed in locations that have no mobility • Perform best when they are relatively short in length since they are rigid • Shorten slightly as they expand in diameter • Most renal artery stents are between 1 and 2 cm, and most iliac stents are 3 to 4 cm • These are the places where balloon-expandable stents are most useful
Type of Stents The self-expanding stent design More commonly constructed of Nitinol, a nickel-titanium alloy Have thermal memory and a high degree of contourability Are packaged on their own delivery catheters Are deployed by retracting the covering sheath
Type of Stents The self-expanding stent design Maintain continual outward radial force after deployment Are not as susceptible to damage from external forces since they are more flexible Have much less hoop strength
Type of Stents The self-expanding stent design are intentionally oversized at the time of deployment in the artery, usually by 2 to 3 mm cover more distance are more difficult to place with great accuracy
Type of Stents Balloon-expandable Self-expanding Wallstent
Type of Stents Self-expanding and balloon-expandable stents tend to play complementary roles Deciding which type of stent to use may be somewhat subjective from one practice to another Endovascular specialists must become facile with the use of each of these two general stent types In addition, there are numerous stents, both balloon- and self-expanding, that are available
Indications for Stents • Primary stent placement • the operator knows ahead of time that a stent will be placed • Selective stent placement • the selective approach where the operator must decide during the procedure
Indications for Stents • The concept of primary stent placement presupposes that the patient is better off with a stent, regardless of the results of treatment of the lesion with balloon angioplasty alone • Appears to work best for some lesions that were treated with balloon angioplasty alone in the past with marginal to mediocre results, including recurrent lesions, occlusions, orifice lesions, and others • renal artery origin lesions • carotid bifurcation • aortoiliac occlusive disease
Indications for Stents The idea behind primary stent placement is that the short- and/or long-term results are generally improved with stent placement to the point where it justifies the up-front increase in risk and cost Taken to its fullest extent, however, every lesion in every patient would receive a stent, and this would be expensive and unnecessary
Indications for Stents • Reasonable long-term results with balloon angioplasty for many aortoiliac, infrainguinal, nonorifice renal lesions, and some upper extremity lesions • The concept of selective stenting. Balloon angioplasty is performed. The results are assessed. If the results are not acceptable, stent placement is performed • residual stenosis • persistent pressure gradient • significant dissection
Indications for Stents The temptation with stents is to continue to lay them in place until the entire arterial tree appears to be perfect The “stack of stents” phenomenon should be avoided
Which Lesions Should Be Stented? Although each specialist must decide what the appropriate level of stent placement aggressiveness is, there are specific situations where stents are useful, or even obligatory
Which Lesions Should Be Stented? Post-angioplasty dissection Stent placement should be considered for any significant dissection after angioplasty, even if there is no gradient Stents should be placed for any false channel or for any intimal flap that impede flow, increase in size during the procedure, or extend into a previously uninvolved segment of artery
Which Lesions Should Be Stented? Residual stenosis after angioplasty The concept of preventing recurrence by eliminating residual stenosis makes empiric sense A 30% postangioplastystenosisis used as a general threshold for continued intervention
Which Lesions Should Be Stented? Pressure gradient A pressure gradient (>10mm Hg systolic) after angioplasty usually indicates a residual stenosis or dissection that requires treatment The threshold for treatment is somewhat arbitrary
Which Lesions Should Be Stented? Recurrent stenosis after angioplasty Treating recurrence with stent placement after previous angioplasty is an empiric approach with reasonable results
Which Lesions Should Be Stented? Occlusion Balloon angioplasty alone for occlusions has only fair results and these may be improved with stent placement Stent placement may make the procedure safer by stabilizing residual thrombus that could embolize from the lesion site, especially if covered stents were used Stent placement in the treatment of iliac and superficial femoral artery occlusions is widely accepted
Which Lesions Should Be Stented? Embolizing lesion Stent placement at the site of an embolizing lesion is thought to trap the embologenic plaque and prevent further embolizationduring intervention. The use of covered stents could be considered in such cases
Placement Techniques Balloon-Expandable Stents
Placement Techniques: Balloon-Expandable Stents The selection of the diameter is an important decision in the placement of balloon-expandable stents The stent size is selected based on the anticipated diameter of the reconstructed artery If the selected stent is too small in diameter, it may not adhere to the vessel wall after deployment and could migrate If the stent is too large, it will overstretch the artery and may cause rupture
Placement Techniques: Balloon-Expandable Stents If selective stent placement is performed, the inflated balloon profile from the initial angioplasty may be used to size the artery When primary stent placement is performed, sometimes it is necessary to dilate the lesion with the balloon alone to size the lesion and to create enough space for the stent delivery catheter to be placed across the lesion
Placement Techniques: Balloon-Expandable Stents Balloon-expandable stents can be dilated a few millimeters larger than the intended specifications But as the diameterincreases, the lengthdecreases The shortest stent that covers the lesion (usually 1–4 cm) is placed Longer balloon-expandable stents are available (up to almost 8 cm) but there are disadvantages to the rigidity of these stents over longer distances. They do not conform to any tortuosityor any change in vessel diameter along the length of the stent.
Placement Techniques: Balloon-Expandable Stents Most premountedballon-expandable stents can be placed using a 6-Fr sheath Larger vessel stents, such as that used for large iliacs up to 12mm are placed through larger sheaths
Placement Techniques: Balloon-Expandable Stents The general approach to balloon-expandable stent placement has been to pass the appropriate sheath and dilator combinationthrough the lesion and proceed to stent placement If the lesion has a residual lumen of less than the diameter of the sheath (for a 6 Fr sheath it is approximately 2.0mm and for a 7 Fr sheath it is about 2.3 mm), the lesion should be predilated or the sheath and dilator will dotter the lesion The sheath must be of adequate length to pass from the skin entry site to near the lesion
Placement Techniques: Balloon-Expandable Stents The sheath is withdrawn, exposing the balloon and stent Before deployment, it is important to make sure that the stent is still in the correct place on the balloon and that it is well positioned to cover the lesion The balloon is then inflated to expand the stent The stent should be slightly overdilatedto embed its metal struts into the plaque
Placement Techniques: Balloon-Expandable Stents • If there is a sense that the stent is loose in the lesion, either because it is under-dilated or because it migrated during deployment to an area that is slightly less narrow • Advance the tip of the sheath up to the expanded end of the stent and catch the edge of the stent and support it so that it cannot move • Re-advanced the balloon and over-dilateit
Placement Techniques: Balloon-Expandable Stents • Many times after placement of a balloon-expandable stent, the balloon wings will stick a bit, perhaps getting caught under the tines of the stent • If this is the case, first try a more than a gentle pull • Do asuper-aspiration, implosion level negative pressure on the balloon. Support the stent by advancing the sheath as described • Tryrotatingand/oradvancingthe balloon catheter before withdrawing it • Sometimes, re-inflating the balloon will loosen the balloon material
Placement Techniques: Balloon-Expandable Stents • Precise stent deployment is challenging • The stent may be difficult to visualize in larger individuals, especially if it is in a location with a lot of ventilatory motion, such as the visceral and renal arteries • Bony landmarks may be useful, especially the vertebral bodies • If there are no suitable landmarks, use an external marker (adherent, radiopaque measuring tape). Be cautioned that external markers are susceptible to parallax error if the field of view is modified. They are also in error if there is any change in angle of view, or any significant ventilatory motion • Road mapping may also be used, but the roadmap image degrades with time and motion
Placement Techniques: Balloon-Expandable Stents Guidewire control must be maintained across the stent until the reconstruction is complete If additional stents are required, the dilator is placed back through the sheath and advanced into the appropriate position If numerous overlapping stents are required, the distal stent is placed first and built proximally to create a “telescope” effect A balloon-expandable stent does not taper well but can be dilated to a slightly larger size on one end if necessary to match vessel size and taper (newer balloon-expandable stents that are constructed of lighter metal)
Placement Techniques: Balloon-Expandable Stents A completion arteriogram is performed by placing the tip of the sheath at the distal end of the stent and injecting contrast so that it refluxes through the area of stent placement Another option is to place a 5-Fr straight catheter through the sheath, over the guidewire, and position the tip of the catheter at the location proximal to the stent
Placement Techniques Self-Expanding Stents
Placement Techniques: Self-Expanding Stents Self-expanding stents must be oversized by 1 to 3mm so that they exert continuous outward radial force at the site of deployment These stents cannot be dilated beyond their maximum list diameter (if in doubt, go a little bigger) The prepackaged stent delivery catheter is placed through a 6-9 Fr sheath, as recommended by the manufacturer Self-expanding stents are manufactured in multiple lengths, from 20 to 120mm and beyond They are generally simple to place, and they adapt to tortuosity, calcification, and ectopic atherosclerosis
Placement Techniques: Self-Expanding Stents • TheWallstent is a closed cell structure having a relatively smooth outer surface without any “v” shaped stent joints extruding beyond the profile of the open or partially open stent • It’slength changes significantly at deployment depending upon the final resting diameter • The constrained length of the Wallstent (in the package) is longer than the deployed length (partially constrained by the artery), which in turn is longer than the stent would be if it were to be completely expanded (unconstrained) • The Wallstent in practice is never completely expanded, since it is oversized for the artery into which it is placed
Placement Techniques: Self-Expanding Stents The self-expanding stent delivery catheter is advanced over the guidewire without the need of a larger sheath The stent is marked by radiopaque markers on its proximal and distal ends, which are observed using fluoroscopy To deploy the stent, use the release mechanism, which removes the covering membrane so the proximal end of the stent begins to expand Fluoroscopy is used because it is easy to move the stent with minimal force A road map can be used to assist in placement
Placement Techniques: Self-Expanding Stents After stent deployment, the delivery catheter is removed and balloon angioplasty is performed of the length and ends of the stent, especially in sections where there is residual crimping of the stent by the lesion It is sometimes difficult to assess whether the stent is fully expanded The guidewireis maintained across the stented segment until after satisfactory completion studies are performed Completion arteriographyis performed in the same manner as with balloon-expanded stents
