CASE REPORTS
`
`Detection of isolated hook fractures 36 months
`after implantation of the Ancure endograft: A
`cautionary note
`
`Sasan Najibi, MD,a Jerilyn Steinberg, RN,a Barry T. Katzen, MD,b Gerald Zemel, MD,b Peter H. Lin,
`MD,a Victor J. Weiss, MD,a Alan B. Lumsden, MD,a and Elliot L. Chaikof, MD, PhD,a Atlanta, Ga,
`and Miama, Fla
`
`Two cases of delayed (36-month) Ancure hook fracture are reported in patients who experienced a decrease in aneurysm
`size and no evidence of endoleak. Both devices used redesigned hooks and are otherwise identical to those devices cur-
`rently used in clinical practice. Notably, hook fractures were not visualized on all abdominal radiographic views, nor were
`they noted on the final “institutional” report by the reviewing radiologist. Careful clinical follow-up with multiple-view
`abdominal radiographs remains essential for all patients treated with an endovascular graft, with particular attention
`directed to the integrity of the metal components. The broader clinical significance of this observation with respect to
`the Ancure endograft remains to be defined. (J Vasc Surg 2001;34:353-6.)
`
`In 1993, EndoVascular Technologies, Inc, initiated a
`clinical trial sponsored by the Food and Drug
`Administration (FDA) of the EndoVascular Grafting
`System (EGS) for the treatment of abdominal aortic
`aneurysm.1 Fixation of the endograft to the host aorta was
`dependent on a series of hooks located at proximal and
`distal extents of the graft. In the initial phase of this trial
`endovascular repair was successfully completed in 39
`patients. However, explantation of a device, due to a per-
`sistent endoleak with aneurysm enlargement 12 months
`after implantation, revealed single-hook fractures of both
`proximal and distal attachment systems.2 This observation
`prompted a review of all the plain abdominal radiographs,
`which identified eight additional patients with fractured
`attachment systems. As a consequence, additional patient
`enrollment in the trial was discontinued in January 1995.
`In order to reduce the maximum force on any given
`hook, the EGS superior attachment system had been ini-
`tially designed as a self-expanding cylindrical frame con-
`sisting of four sets of V-hooks for a total of eight hooks.
`The superior and inferior attachment systems are identical
`
`From the Department of Surgery, Division of Vascular Surgery, Emory
`University School of Medicine and the Emory University Hospital,a and
`the Miami Cardiac and Vascular Institute.b
`Competition of interest: ELC and ABL have received grant/research sup-
`port as well as an honorarium from Guidant Corporation.
`Reprint requests: Elliot L. Chaikof, MD, PhD, 1639 Pierce Drive, Rm
`5105, Department of Surgery, Emory University, Atlanta, GA 30322 (e-
`mail: echaiko@emory.edu).
`Copyright © 2001 by The Society for Vascular Surgery and The American
`Association for Vascular Surgery.
`0741-5214/2001/$35.00 + 0 24/4/117865
`doi:10.1067/mva.2001.1178865
`
`in the tube graft configuration, whereas in the bifurcated
`endograft, each iliac attachment system was composed of
`a self-expanding cylindrical frame with three metal endo-
`hooks. All of these metal components are composed of
`Elgiloy, an alloy of cobalt, chromium, and nickel that pro-
`vides a combination of high strength, high performance,
`as well as fatigue and corrosion resistance.3 Nevertheless,
`close analysis of the initial hook design revealed an approx-
`imately 90-degree angle that likely subjected a discrete
`segment of the hook to maximal stress and introduced a
`potential site for microcrack formation. Therefore, the
`hooks were redesigned with a larger radius of curvature
`that increased stress distribution along a larger hook seg-
`ment, reducing the magnitude of the maximal stress and
`sites at risk for microcrack formation (Fig 1). In vitro test-
`ing performed by the manufacturer suggests that the
`attachment system can withstand at least 15 years of in
`vivo cyclic loading.4 After the device was reengineered, the
`clinical trial was resumed in November 1995.
`Completion of the trial and analysis of 1-year implant
`data led to FDA approval of the redesigned Ancure endo-
`graft (Guidant, Inc) in September 1999.2 Overall,
`midterm clinical results continue to be promising with
`approximately 75% of patients demonstrating a reduction
`in aneurysm diameter of at least 5 mm 3 years after graft
`implantation.5 Nevertheless, the number of carefully ana-
`lyzed patients who have been followed for a substantial
`time period after implantation remains relatively small.
`Fundamentally, the long-term durability of this device
`and, for that matter, all other available endografts remains
`unknown. Indeed, metal component fractures in AneuRx
`and Vanguard endografts have been observed.6-8
`353
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`354 Najibi et al
`
`JOURNAL OF VASCULAR SURGERY
`August 2001
`
`A
`
`B
`
`Fig 1. Schematic representation of forces exerted on endo-
`hooks: current (A) and initial (B) hook designs. Arrows represent
`qualitative estimate of force magnitude and direction.
`
`Therefore, close scrutiny of device durability by all
`implanting clinicians is mandatory to determine the inci-
`dence and clinical significance of component failure. We
`report herein the observation in two patients of isolated
`hook fractures 36 months after implantation of Ancure
`bifurcated endografts. Both devices used redesigned hooks
`and are otherwise identical to those devices currently used
`in clinical practice.
`
`CASE REPORT
`Patient 1. A 67-year-old man with an infrarenal abdominal
`aortic aneurysm underwent successful exclusion with the Ancure
`device. The maximum aneurysm diameter was 59 mm with supe-
`rior neck diameter and length of 21.9 mm and 36 mm, respectively.
`The aneurysm was treated with a 24-mm × 16-cm bifurcated
`Ancure graft. Follow-up computed tomography (CT) scans and
`abdominal x-ray films at 1-, 3-, 6-, 12-, and 24-month post-implant
`time points demonstrated relatively little change in aneurysm size,
`no evidence of endoleak, and an intact endograft. Abdominal CT
`scan and four-view abdominal x-ray films obtained 36 months after
`graft implantation showed a reduction of aneurysm diameter (54
`mm), increased angulation, and the presence of a single endo-hook
`fracture of the superior attachment system (Fig 2). Neither
`endoleak nor device migration has been observed.
`Patient 2. A 77-year-old man with an infrarenal abdominal
`aortic aneurysm underwent successful exclusion with the Ancure
`device. The maximum aneurysm diameter was 55 mm with supe-
`rior neck diameter and length of 23.5 mm and 21 mm, respec-
`tively. The aneurysm was treated with a 24-mm × 16-cm
`bifurcated Ancure graft. Approximately 7 months after graft
`implantation, the patient received bilateral renal artery stents.
`Follow-up CT scans and abdominal x-ray films at 3-, 6-, 12-, and
`24-month post-implant time points demonstrated little change in
`aneurysm size, no evidence of endoleak, and an intact endograft.
`Abdominal CT scan and four-view abdominal x-ray films obtained
`36 months after graft implantation showed a reduction of
`aneurysm size of more than 5 mm and the presence of a single
`endo-hook fracture of the superior attachment system. Neither
`endoleak nor device migration has been observed.
`
`Fig 2. Abdominal x-ray films demonstrating aneurysm sac
`changes and a hook fracture detected 36 months after endograft
`implantation. Arrow illustrates hook fracture.
`
`DISCUSSION
`
`Metal components are critical constituents of all aortic
`endografts, serving a number of device-specific functions,
`including fixation of the endograft to the host aorta and
`provision of an adequate seal at the proximal and distal
`extents of the device. Among certain classes of devices,
`appropriate coupling of modular components and mainte-
`nance of lumen patency are also dependent on metal com-
`ponents. In this regard, bench models for accelerated
`fatigue testing are important preclinical tools for assessing
`acute component integrity and for providing an initial pre-
`diction of long-term endograft durability. In vitro models,
`however, are based on the assumption that both in vivo
`forces and the biological microenvironment are accurately
`approximated in the test system. In large measure, how-
`ever, these factors remain incompletely defined for aortic
`endografts. Furthermore, limited clinical data exist regard-
`ing the long-term durability of endografts that is required
`for the validation of any predictive experimental system.
`Indeed, the development of accurate in vitro models for
`the assessment of heart valve substitutes evolved over a
`period of several decades through a combination of exper-
`imental, computational, and clinical investigation.9
`The ultimate performance of any attachment system
`hinges on material composition, system design, and other
`
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`
`Najibi et al 355
`
`factors that influence the magnitude and distribution of
`the applied stress field, including the position of the device
`relative to the axis of blood flow and the host aortic wall.
`Elgiloy is a metal alloy that is a common component of a
`variety of endovascular devices, including the Ancure
`endograft. On the basis of conventional mechanical test-
`ing, Elgiloy has a high-yield strength and elastic modulus
`and is corrosion and fatigue resistant.3 However, it is note-
`worthy that published material properties usually have
`been derived from fatigue studies of relatively large sam-
`ples. For example, constant amplitude axial fatigue testing
`often involves the use of metal components with diameters
`ranging from 5 to 25 mm, and the measurement of fatigue
`crack growth rates has often relied on the testing of spec-
`imen bars with widths of 10 to 100 mm.10 It is significant
`that the extrapolation of data obtained from large speci-
`mens to predict the behavior and mechanical properties of
`smaller components may yield invalid projections, a phe-
`nomenon that has been termed a size effect.11 Thus, life
`cycle data for a given material will be accurate only when
`assessed in a clinically relevant configuration. This is par-
`ticularly important for fine wires, which, as a consequence
`of material processing, may have fine grain microstruc-
`tures oriented along the axis of wire that can lead to sig-
`nificant anisotropy in fatigue properties.
`In an investigation of the durability of Elgiloy wire,
`Schmidt et al10 defined a fatigue life curve for 0.78-mm
`diameter wire in air. Given an applied load of 22.2 N and
`a maximum stress at the wire apex of approximately 1500
`MPa, failure was noted after 1 × 107 cycles. Studies at
`lower levels of applied force were not conducted. In this
`report, upper hooks with a wire diameter of approximately
`0.4 mm fractured after 36 months (approximately 108
`cycles). Thus, an appropriate estimation of the in vivo
`stress level that may have been associated with hook fail-
`ure is not possible. Nevertheless, the in vivo forces that
`lead to hook failure were undoubtedly considerably lower
`than 20 N per hook. Current estimates of the physiologic
`loading forces on the juxtarenal segment of aortic endo-
`grafts are limited with most experimental studies directed
`at defining the longitudinal forces required to cause endo-
`graft migration.12,13 However, Morris et al14 have recently
`developed a series of theoretical estimates of the resultant
`forces on the proximal portion of bifurcated endografts, as
`related to device diameter and iliac leg angle. For a 24-mm
`× 16-cm bifurcated endograft, a total force of 12 to 15 N
`was estimated or, in the case of an eight-hook attachment
`system, approximately 1.5 to 1.9 N per hook. This esti-
`mate, however, may be limited because the model did not
`account for neck or endograft angulation. Regardless, as
`an initial calculation, this work and the prior study by
`Schmidt et al10 suggest that in vivo forces may be of suffi-
`cient magnitude to induce hook failure over the long-term
`life of the implant. Moreover, additional mechanical and
`chemical processes in vivo may play a significant role in
`microcrack initiation and propagation.9 Further experi-
`mental and computational analysis will be essential to
`accurately define the physiologic stress profile on these
`
`devices and the associated fatigue life curve at relevant
`force levels for this and other material compositions.
`It is noteworthy that the hook fractures noted in this
`report were not visualized on all abdominal radiographic
`views, nor were these fractures noted on the final “institu-
`tional” report by the reviewing radiologist. Thus, it is essen-
`tial that abdominal radiographs include anteroposterior,
`lateral, and oblique images and that these studies be directly
`reviewed by the clinician responsible for patient follow-up.
`Approximately 8000 Ancure devices have been
`implanted in the United States since FDA approval was
`given in September 1999. However, as of February 2001,
`only 167 Ancure endografts had been followed under an
`FDA clinical trial protocol with implant periods approach-
`ing or exceeding 36 months (Don Schwarten, Guidant
`Corporation, oral communication, February 2001). To
`date, isolated hook fractures have been detected in two
`(1.2%) of these 167 devices. In both cases, clinical seque-
`lae have not been noted. However, a risk for additional
`hook fractures, endoleak, or device migration is present,
`and careful follow-up is mandatory.
`
`CONCLUSIONS
`Two cases of delayed (36-month) Ancure hook frac-
`ture are reported in patients who experienced a decrease in
`aneurysm size and no evidence of endoleak. Nevertheless,
`these events raise a risk for later endoleak or graft migra-
`tion. Although initial in vitro bench models have provided
`reassuring estimates for device durability, these experimen-
`tal systems have not been validated. Thus, patient moni-
`toring and frequent examinations with CT or ultrasound
`scanning, along with multiple-view abdominal radiographs,
`remain essential. Particular attention should be directed to
`the integrity of the proximal and distal attachment sites.
`Further follow-up will be required to determine if the
`redesigned Ancure attachment system has substantially
`eliminated the risk of hook failure noted in the initial endo-
`graft design or simply shifted the fatigue to failure curve to
`a later, though clinically relevant, time point.
`
`REFERENCES
`
`1. Moore WS, Vescera CL. Repair of abdominal aortic aneurysm by trans-
`femoral endovascular graft placement. Ann Surg 1994;220:331-9.
`2. Moore WS, Rutherford RB. Transfemoral endograft repair of abdom-
`inal aortic aneurysm: results of the North American EVT phase 1 trial.
`J Vasc Surg 1996;23:543-53.
`3. Clerc CO, Jedwab MR, Mayer DW, Thompson PJ, Stinson JS.
`Assessment of wrought ASTM F1058 cobalt alloy properties for per-
`manent surgical implants. J Biomed Mater Res (Appl Biomater)
`1997;38:229-34.
`4. Ancure Endograft System Physician Training Manual. p. II:1-5.
`5. Makaroun MS. The Ancure endograft system: an update. J Vasc Surg
`2001;33:129-34.
`6. Bohm T, Soldner J, Rott A, Kaiser WA. Perigraft leak of an aortic stent
`graft due to material fatigue. AJR Am J Roentgenol 1999;172:1355-7.
`7. Device alert. AneuRx stent graft system: nitinol frame fracture after
`implantation. Available from: www.medical-devices.gov.uk/da2000(06).
`htm 2000. Accessed Feb 2001.
`8. Device alert. Vangaurd endoprosthesis: upper stent row separation
`with or without nitinol wire fracture. Available from: www.medical-
`devices.gov.uk/da2001(01).htm 2001. Accessed Feb 2001.
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`JOURNAL OF VASCULAR SURGERY
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`9. Teoh SH. Fatigue of biomaterials: a review. International Journal of
`Fatigue 2000;22:825-37.
`10. Schmidt PA, Blair RR, Earthman JC. Development of a novel speci-
`men geometry for fatigue: testing of fine wire. Journal of Testing and
`Evaluation 1995;23:73-9.
`11. Dieter GE. Mechanical metallurgy. New York: McGraw-Hill; 1986.
`p. 406-12.
`12. Malina M, Lindbald B, Ivancev K, Lindh M, Malina J, Brunkwall J.
`Endovascular AAA exclusion: will stents with hooks and barbs prevent
`stent-graft migration? J Endovasc Surg 1998;5:310-7.
`13. Lambert AW, Williams DJ, Budd JS, Hurrocks M. Experimental
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`assessment of proximal stent-graft (inter-vascular) fixation in human
`cadaveric infrarenal aortas. Eur J Vasc Endovasc Surg 1999;17:60-5.
`14. Morris W, Delassus P, McGloughlin T. Forces on aortic stent grafts.
`Proceedings of the 5th Annual Hilton Head Workshop on the
`Engineering of Human Tissues; 2001 Feb 22-25; Hilton Head, SC.
`Atlanta: Georgia Institute of Technology EKC for Tissue Engineering;
`2001. p. 87.
`
`Submitted Apr 10, 2001; accepted May 28, 2001.
`
`21ST ANNUAL WILLIAM J. VON LIEBIG FOUNDATION AWARD FOR EXCELLENCE IN
`VASCULAR SURGICAL RESEARCH FOR RESIDENTS, FELLOWS, AND MENTORS—
`FIRST PLACE $5,000 (AUTHOR) AND $10,000 (SUPPORTING MENTOR)
`Additionally, an unlimited number of $2,500 awards will be given for each manuscript achieving a score within the 1.0 to 2.0
`range, with $5,000 being awarded to each of their research mentors.
`PURPOSE
`• Motivate physicians, early in their training, to pursue their interest in research
`• Recognize and support research professionals who supervise this critical function
`ELIGIBILITY
`• Author must be a Resident or Fellow on staff at an accredited vascular surgery program in the United States, Canada, or Mexico
`with senior collaborators acting in a consultative capacity.
`• Manuscripts must be postmarked no later than September 4, 2001. Selection results will be conveyed to all applicants by
`October 30, 2001.
`
`RESEARCH REQUIREMENTS
`• The research may be experimental or clinical in nature dealing with some fundamental or clinical aspect of vascular surgery. Both
`basic and clinical research papers are especially encouraged.
`• The manuscript must be an original, unpublished work {not submitted elsewhere for publication, except to the ACS Surgical Forum.
`• The submission must be in English, 10 copies of the typed manuscript and 10 original copies of illustrations (photographic
`prints or original computer-generated images). The manuscript may also be submitted electronically in PDF format on a PC
`computer disc or e-mailed to liebigfoundation@draxgroup.com. All submissions must comply with “Instructions to Authors” of
`the Journal of Vascular Surgery and include an abstract of 250 words or less.
`• Accompanying each submission should be: —A cover letter from the Resident or Fellow indicating the manuscript is to be con-
`sidered for “The 21st Annual William J. von Liebig Foundation Award for Excellence in Vascular Surgical Research for
`Residents, Fellows and Mentors” —The author’s full curriculum vitae—A signed letter from the author’s mentor attesting that
`the author performed all the essential parts of the experimental work reported
`SELECTION PROCESS
`A select committee of vascular surgeons appointed by the Foundation will review the manuscripts submitted. The 2001-2002
`Committee Members include Chairman, Bauer E. Sumpio, MD, PhD; Colleen Brophy, MD; Elliot L. Chaikof, MD, PhD; William
`H. Pearce, MD; Michael Sobel, MD; Jean A. Goggins, PhD, Secretary; Thomas C. Naslund, MD; SAVS Ex-Officio and D. Emerick
`Szilagyi, MD, Consultant.
`The first-prize winner will be a guest of The von Liebig Foundation, and the award will be presented at the annual meeting of the
`Southern Association for Vascular Surgery on January 16-19, 2002 in Miami Beach, Florida. Expenses incurred by the winning author
`at the meeting will be reimbursed according to the travel policy of the Foundation. The winning manuscript will be submitted to the
`Journal of Vascular Surgery for consideration of publication. The William J. von Liebig Foundation reserves the right to withhold the
`granting of the award at the sole discretion of the Award Committee whose judgment with respect thereto shall be final and conclusive.
`HISTORY
`Since the award’s inception in 1982, Eighty-two percent (82%) of previous first-place award recipients have pursued careers in
`Vascular or Cardiothoracic Surgical Research. Five (5) recipients have become Fellows of the American College of Surgeons, and
`three {3) are Associate members of the College. Nine (9) have become successful peer-review funded researchers in vascular surgery.
`Past Award Winners include such well-known researchers as Howard Greisler, MD, Michael Marin, MD, and Kenneth Ouriel, MD.
`CONCLUSION
`It is the desire of the Foundation to encourage the movement of technical innovation and relevant clinical findings from the
`laboratory to the vascular surgical community. It was Mr. von Liebig’s hope that those who pursue this award and those who win it
`will contribute to the advancement of medical care.
`Further inquiries may be directed to the Foundation as follows:
`
`JEAN A. GOGGJNS, PHD Executive Director
`The William J. von Liebig Foundation
`8889 Pelican Bay Boulevard, Suite 403 Naples, FL 34108
`Telephone (941) 513-2229
`Facsimile (941) 513-2239
`www.vonliebigfoundation.com liebigfoundation@draxgroup.com
`
`Page 4
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`IPR2014-00100 Pat. Owner Ex. 2027
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