Stent Strut Protrusion – An Uncommon Complication of Stent Placement in a Fistula Adrian Sequeira,* Shukhrat Artikov,† and Kenneth Abreo* *Interventional Nephrology Section, Division of Nephrology and Hypertension, Department of Medicine, Louisiana State University Health Sciences Center, Shreveport, Louisiana, and †Atlanta Access Care, Vascular and Interventional Specialists, Atlanta, Georgia
ABSTRACT Stent strut protrusion through the skin is a rare and a potentially dangerous complication from the cannulation of stents placed within arterio-venous fistulas and grafts. Such cases are usually managed surgically. We present a case wherein strut penetration of an arterio-venous fistula
was noted at the distal (uncovered) end of a Fluency Plusâ tracheo bronchial stent graft. After analyzing the various reasons why this may have happened, a nonsurgical approach was taken to preserve the access and manage strut protrusion.
Self-expanding stents are predominantly made of nitinol. However, different brands of nitinol stents differ from one another in terms of their structure, whether they are covered or bare and in their delivery system. The design of a stent is important as it confers various properties like flexibility, rigidity, conformability, fracture resistance, etc. It is important to know these properties as not all nitinol stents behave similarly. We present a case where the deployment of a Fluency Plusâ tracheo bronchial stent graft at the “swing site” (junction of the body of the fistula and outflow) of a transposed brachio– basilic fistula led to stent strut protrusion through the cannulating segment of the fistula.
7 mm. The stent chosen was an 8 mm 9 80 mm Fluency Plusâ tracheo bronchial stent graft (Bard Peripheral Vascular, Tempe, AZ). It was chosen based on availability. An 8 mm 9 40 mm Conquest balloon (Bard Peripheral Vascular) was used to dilate the stent. Most of the stent was positioned within the outflow vein while a small segment was within the fistula (Fig. 1B). Following stent placement, there was good fistula blood flow and the patient was successfully dialyzed the next day. Ini-
Case Report A 66-year-old African American male on chronic hemodialysis was referred for a clotted left upper extremity transposed brachio-basilic fistula. During the thrombectomy procedure, an 80% stenosis was noted at the “swing site” (Fig. 1A). Despite multiple attempts at angioplasty, the stenosis persisted with poor flow through the fistula. Hence, a decision to place a stent was made. The diameter of the fistula on either side of the stenosis was approximately Address correspondence to: Adrian Sequeira, Division of Nephrology and Hypertension, Department of Medicine, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130, Tel.: 318-675-7402, Fax: 318-675-5913, or e-mail: [email protected]. Seminars in Dialysis—Vol 27, No 5 (September–October) 2014 pp. 529–532 DOI: 10.1111/sdi.12193 © 2014 Wiley Periodicals, Inc.
Fig. 1. (A) Swing site stenosis in the transposed brachio- basilic arterio-venous fistula shown by balloon inflation. (B) Fluency Plusâ tracheo bronchial stent graft located at the swing site of the transposed brachio-basilic arterio-venous fistula. 529
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tially, cannulation of the stent was avoided, but later, the dialysis unit nurses began to cannulate the fistula through the stent. Six months later, he was referred for protrusion of a stent strut through the cannulating segment of his fistula (Fig. 2A and B). On examination of the fistula, a single stent strut protruding through the skin was visible. No local evidence of infection was noted. While a repeat angiogram was normal, it was observed that the strut penetration through the skin was only at the distal flared end of the stent (Fig. 2B and C). To avoid surgical ligation and resection of the access, another approach was taken to preserve it. After vancomycin was administered, a 10 mm 9 40 mm ProtegeTM GPSTM (Covidien Peripheral Vascular, Plymouth, MN) was deployed within the distal flared segment of the Fluency Plusâ tracheo bronchial stent graft. The diameter of the body of the fistula in this region was approximately 9–10 mm. A 10 mm 9 40 mm Conquest balloon was used to gently inflate the ProtegeTM GPSTM stent. While inflated, the area over the protruding stent strut was covered with gauze and massaged to bury the stent strut within the subcutaneous tissue. This was done until the strut was no longer visible. Care was taken to avoid perforating the glove while this was performed. The final angiogram postprocedure is shown in Fig. 3. The patient was prescribed mupirocin ointment to be applied over the affected area until the skin healed. In addition, vancomycin was given on dialysis for 2 weeks and strict instructions were given to avoid cannulating the stented area. Two years later, his fistula is functioning well with no visible struts.
Fig. 3. ProtegeTM GPSTM bare metal stent within the distal end of the Fluency Plusâ stent graft.
Discussion The Fluency Plusâ tracheo bronchial stent graft is a covered self-expanding nitinol stent. The nitinol skeleton is sandwiched between two ultra thin ePTFE fabric layers. The distal 2 mm at either end of the stent are uncovered and flared. This flared segment is also wider by 2 mm as compared with the central diameter of the stent and has been so designed to prevent stent migration. The flared segment in addition has tantalum markers for radio visibility. The stent has a closed cell structure, which makes it relatively rigid as compared with a stent with an open cell design, which is more flexible. When choosing a stent for deployment, it is recommended that the stent diameter be no more than 1–2 mm greater than the diameter of the vessel. This is known as over sizing and is defined as: Over sizing (OS) ¼
Fig. 2. (A and B) Image showing penetrating strut near the skin surface (arrows). (C) Corresponding fluoroscopic image. Note straightening of the fistula. Arrowheads show migration of the flared ends of the stent.
Nominal stent diameter (SD) Vessel diameter (VD)
Generally, OS ratio should be 1.1–1.4 for adequate wall apposition (1). Appropriate OS prevents both foreshortening of the stent and structural damage to the vessel, and minimizes exuberant neointimal hyperplasia. For the Fluency Plusâ stent, the recommendation is 0.5–1 mm over sizing for vessel diameter less than 5 mm and 1–2 mm over sizing for vessels 6–12 mm in diameter. When a self-expandable stent is deployed, it should be dilated gently using a balloon that is of the same diameter as the stent to prevent structural damage to the stent. Nitinol stents exert a low chronic outward force (COF) that is proportional to the OS ratio (1). This is the force a stent exerts on a vessel as it expands to its nominal diameter. COF keeps the stent open and prevents migration. Excessive outward force, however, can damage the vessel. Whenever an inappropriately large diameter stent is chosen or a larger balloon is used to expand the Fluency Plusâ stent, the flared ends of the stent are driven into the wall of the vessel. The stent will continue to exert its COF over time (months) as it dilates to its nominal diameter. It has been shown in animal studies that the struts of self-expanding oversized bare metal stents migrate through the
531
STENT STRUT PROTRUSION
Fig. 4. The three wave peak design of the ProtegeTM GPSTM stent. Every fourth peak is connected.
wall of the vessel and lay within or beyond the adventitia! (1–3). The migration of the struts is accompanied concomitantly by neo-intimal hyperplasia. The ProtegeTM GPSTM stent is a nitinol bare metal stent. It has an open cell, 2 wave peak design (every third peak is connected to its opposite) for stent diameters 6–10 mm and a 3 wave peak design for stent diameters 12–14 mm (Fig. 4). This provides flexibility while maintaining a low delivery profile. In our case, protrusion of the stent strut was likely secondary to the intrinsic property of the Fluency Plusâ tracheo bronchial stent graft, although repeated cannulation may also have been responsible. Even though an appropriate-sized stent was chosen, the flared ends of the stent were mildly oversized (OS ratio of 1.43). Consequently, as the flared ends of the stent expanded and exerted their COF, they penetrated the wall of the fistula. This can be seen in Fig. 2 C when compared to Fig. 1 B. In addition, as the stent was relatively rigid, it tended to straighten out. This was noticeable as the initial curved path taken by the stent (Fig. 1 B) was replaced by a relatively straight path (Fig. 2 C). The straightening caused the distal end of the stent to move upward (toward the skin) and the proximal end downward (Fig. 2 B). Consequently, it was in this area that the struts penetrated the wall of the fistula. Due to its deeper location, the flared proximal end did not penetrate the skin. There appeared to be no migration of the covered area of the stent through the fistula wall. The management of protruding struts through a fistula is essentially surgical removal of the stent along with antibiotics (4). In this case, as there was
no evidence of infection, an attempt was made to divert the straightening at the distal end of the stent by placing another stent within it. The proximal end of the ProtegeTM GPSTM stent formed an excellent funnel within the distal end of the Fluency Plusâ tracheo bronchial stent graft. Besides sealing off the distal end of the stent graft, it prevented the distal flared end from further migration toward the skin. The unique structure of the ProtegeTM GPSTM also ensured good conformability to the contour of the vessel. Conclusion This case teaches us that the interventionalist must be aware of the intrinsic properties of different stents that are used. Not all self-expanding stents are the same. Each has certain unique properties. Understanding this enables one to choose an appropriate stent for a given situation. One must also be aware of the effect of excess oversizing and the ability of relatively rigid stents to straighten a vessel. A flexible covered stent such as the Viabahnâ endoprosthesis (W. L. Gore and Associates, Newark, DE) or a flexible bare metal like the ProtegeTM GPSTM as the initial stent choice may have prevented this problem. Finally, cannulating through a stent should be avoided. References 1. Zhao HQ, Nikanorov A, Virmani R, Jones R, Pacheco E, Schwartz LB: Late stent expansion and neointimal proliferation of oversized niti-
532
Sequeira et al.
nol stents in peripheral arteries. Cardiovasc Intervent Radiol 32(4):720– 726, 2009 2. Andrews RT, Venbrux AC, Magee CA, Bova DA: Placement of a flexible endovascular stent across the femoral joint: an in vivo study in the swine model. J Vasc Interv Radiol 10(9):1219–1228, 1999
3. Duerig TW, Tolomeo DE, Wholey M: An overview of superelastic stent design. Minim Invasive Ther Allied Technol 9(3–4):235–246, 2000 4. Yevzlin A, Arif A: Stent placement in hemodialysis access: historical lessons, the state of the art and future directions. Clin J Am Soc Nephrol 4:996–1008, 2009
ABSTRACT Stent strut protrusion through the skin is a rare and a potentially dangerous complication from the cannulation of stents placed within arterio-venous fistulas and grafts. Such cases are usually managed surgically. We present a case wherein strut penetration of an arterio-venous fistula
was noted at the distal (uncovered) end of a Fluency Plusâ tracheo bronchial stent graft. After analyzing the various reasons why this may have happened, a nonsurgical approach was taken to preserve the access and manage strut protrusion.
Self-expanding stents are predominantly made of nitinol. However, different brands of nitinol stents differ from one another in terms of their structure, whether they are covered or bare and in their delivery system. The design of a stent is important as it confers various properties like flexibility, rigidity, conformability, fracture resistance, etc. It is important to know these properties as not all nitinol stents behave similarly. We present a case where the deployment of a Fluency Plusâ tracheo bronchial stent graft at the “swing site” (junction of the body of the fistula and outflow) of a transposed brachio– basilic fistula led to stent strut protrusion through the cannulating segment of the fistula.
7 mm. The stent chosen was an 8 mm 9 80 mm Fluency Plusâ tracheo bronchial stent graft (Bard Peripheral Vascular, Tempe, AZ). It was chosen based on availability. An 8 mm 9 40 mm Conquest balloon (Bard Peripheral Vascular) was used to dilate the stent. Most of the stent was positioned within the outflow vein while a small segment was within the fistula (Fig. 1B). Following stent placement, there was good fistula blood flow and the patient was successfully dialyzed the next day. Ini-
Case Report A 66-year-old African American male on chronic hemodialysis was referred for a clotted left upper extremity transposed brachio-basilic fistula. During the thrombectomy procedure, an 80% stenosis was noted at the “swing site” (Fig. 1A). Despite multiple attempts at angioplasty, the stenosis persisted with poor flow through the fistula. Hence, a decision to place a stent was made. The diameter of the fistula on either side of the stenosis was approximately Address correspondence to: Adrian Sequeira, Division of Nephrology and Hypertension, Department of Medicine, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130, Tel.: 318-675-7402, Fax: 318-675-5913, or e-mail: [email protected]. Seminars in Dialysis—Vol 27, No 5 (September–October) 2014 pp. 529–532 DOI: 10.1111/sdi.12193 © 2014 Wiley Periodicals, Inc.
Fig. 1. (A) Swing site stenosis in the transposed brachio- basilic arterio-venous fistula shown by balloon inflation. (B) Fluency Plusâ tracheo bronchial stent graft located at the swing site of the transposed brachio-basilic arterio-venous fistula. 529
530
Sequeira et al.
tially, cannulation of the stent was avoided, but later, the dialysis unit nurses began to cannulate the fistula through the stent. Six months later, he was referred for protrusion of a stent strut through the cannulating segment of his fistula (Fig. 2A and B). On examination of the fistula, a single stent strut protruding through the skin was visible. No local evidence of infection was noted. While a repeat angiogram was normal, it was observed that the strut penetration through the skin was only at the distal flared end of the stent (Fig. 2B and C). To avoid surgical ligation and resection of the access, another approach was taken to preserve it. After vancomycin was administered, a 10 mm 9 40 mm ProtegeTM GPSTM (Covidien Peripheral Vascular, Plymouth, MN) was deployed within the distal flared segment of the Fluency Plusâ tracheo bronchial stent graft. The diameter of the body of the fistula in this region was approximately 9–10 mm. A 10 mm 9 40 mm Conquest balloon was used to gently inflate the ProtegeTM GPSTM stent. While inflated, the area over the protruding stent strut was covered with gauze and massaged to bury the stent strut within the subcutaneous tissue. This was done until the strut was no longer visible. Care was taken to avoid perforating the glove while this was performed. The final angiogram postprocedure is shown in Fig. 3. The patient was prescribed mupirocin ointment to be applied over the affected area until the skin healed. In addition, vancomycin was given on dialysis for 2 weeks and strict instructions were given to avoid cannulating the stented area. Two years later, his fistula is functioning well with no visible struts.
Fig. 3. ProtegeTM GPSTM bare metal stent within the distal end of the Fluency Plusâ stent graft.
Discussion The Fluency Plusâ tracheo bronchial stent graft is a covered self-expanding nitinol stent. The nitinol skeleton is sandwiched between two ultra thin ePTFE fabric layers. The distal 2 mm at either end of the stent are uncovered and flared. This flared segment is also wider by 2 mm as compared with the central diameter of the stent and has been so designed to prevent stent migration. The flared segment in addition has tantalum markers for radio visibility. The stent has a closed cell structure, which makes it relatively rigid as compared with a stent with an open cell design, which is more flexible. When choosing a stent for deployment, it is recommended that the stent diameter be no more than 1–2 mm greater than the diameter of the vessel. This is known as over sizing and is defined as: Over sizing (OS) ¼
Fig. 2. (A and B) Image showing penetrating strut near the skin surface (arrows). (C) Corresponding fluoroscopic image. Note straightening of the fistula. Arrowheads show migration of the flared ends of the stent.
Nominal stent diameter (SD) Vessel diameter (VD)
Generally, OS ratio should be 1.1–1.4 for adequate wall apposition (1). Appropriate OS prevents both foreshortening of the stent and structural damage to the vessel, and minimizes exuberant neointimal hyperplasia. For the Fluency Plusâ stent, the recommendation is 0.5–1 mm over sizing for vessel diameter less than 5 mm and 1–2 mm over sizing for vessels 6–12 mm in diameter. When a self-expandable stent is deployed, it should be dilated gently using a balloon that is of the same diameter as the stent to prevent structural damage to the stent. Nitinol stents exert a low chronic outward force (COF) that is proportional to the OS ratio (1). This is the force a stent exerts on a vessel as it expands to its nominal diameter. COF keeps the stent open and prevents migration. Excessive outward force, however, can damage the vessel. Whenever an inappropriately large diameter stent is chosen or a larger balloon is used to expand the Fluency Plusâ stent, the flared ends of the stent are driven into the wall of the vessel. The stent will continue to exert its COF over time (months) as it dilates to its nominal diameter. It has been shown in animal studies that the struts of self-expanding oversized bare metal stents migrate through the
531
STENT STRUT PROTRUSION
Fig. 4. The three wave peak design of the ProtegeTM GPSTM stent. Every fourth peak is connected.
wall of the vessel and lay within or beyond the adventitia! (1–3). The migration of the struts is accompanied concomitantly by neo-intimal hyperplasia. The ProtegeTM GPSTM stent is a nitinol bare metal stent. It has an open cell, 2 wave peak design (every third peak is connected to its opposite) for stent diameters 6–10 mm and a 3 wave peak design for stent diameters 12–14 mm (Fig. 4). This provides flexibility while maintaining a low delivery profile. In our case, protrusion of the stent strut was likely secondary to the intrinsic property of the Fluency Plusâ tracheo bronchial stent graft, although repeated cannulation may also have been responsible. Even though an appropriate-sized stent was chosen, the flared ends of the stent were mildly oversized (OS ratio of 1.43). Consequently, as the flared ends of the stent expanded and exerted their COF, they penetrated the wall of the fistula. This can be seen in Fig. 2 C when compared to Fig. 1 B. In addition, as the stent was relatively rigid, it tended to straighten out. This was noticeable as the initial curved path taken by the stent (Fig. 1 B) was replaced by a relatively straight path (Fig. 2 C). The straightening caused the distal end of the stent to move upward (toward the skin) and the proximal end downward (Fig. 2 B). Consequently, it was in this area that the struts penetrated the wall of the fistula. Due to its deeper location, the flared proximal end did not penetrate the skin. There appeared to be no migration of the covered area of the stent through the fistula wall. The management of protruding struts through a fistula is essentially surgical removal of the stent along with antibiotics (4). In this case, as there was
no evidence of infection, an attempt was made to divert the straightening at the distal end of the stent by placing another stent within it. The proximal end of the ProtegeTM GPSTM stent formed an excellent funnel within the distal end of the Fluency Plusâ tracheo bronchial stent graft. Besides sealing off the distal end of the stent graft, it prevented the distal flared end from further migration toward the skin. The unique structure of the ProtegeTM GPSTM also ensured good conformability to the contour of the vessel. Conclusion This case teaches us that the interventionalist must be aware of the intrinsic properties of different stents that are used. Not all self-expanding stents are the same. Each has certain unique properties. Understanding this enables one to choose an appropriate stent for a given situation. One must also be aware of the effect of excess oversizing and the ability of relatively rigid stents to straighten a vessel. A flexible covered stent such as the Viabahnâ endoprosthesis (W. L. Gore and Associates, Newark, DE) or a flexible bare metal like the ProtegeTM GPSTM as the initial stent choice may have prevented this problem. Finally, cannulating through a stent should be avoided. References 1. Zhao HQ, Nikanorov A, Virmani R, Jones R, Pacheco E, Schwartz LB: Late stent expansion and neointimal proliferation of oversized niti-
532
Sequeira et al.
nol stents in peripheral arteries. Cardiovasc Intervent Radiol 32(4):720– 726, 2009 2. Andrews RT, Venbrux AC, Magee CA, Bova DA: Placement of a flexible endovascular stent across the femoral joint: an in vivo study in the swine model. J Vasc Interv Radiol 10(9):1219–1228, 1999
3. Duerig TW, Tolomeo DE, Wholey M: An overview of superelastic stent design. Minim Invasive Ther Allied Technol 9(3–4):235–246, 2000 4. Yevzlin A, Arif A: Stent placement in hemodialysis access: historical lessons, the state of the art and future directions. Clin J Am Soc Nephrol 4:996–1008, 2009
