A preliminary study of prosthesis damage by laser energy. James M. Seeger, M.D., George S. Abela, M.D.,* and Nina Klingman, B.S., Gainesville, Fla.

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1 Laser radiation in the graft stenosis treatment of prosthetic A preliminary study of prosthesis damage by laser energy James M. Seeger, M.D., George S. Abela, M.D.,* and Nina Klingman, B.S., Gainesville, Fla. Transluminal laser recanalization is potentially an important new treatment of anastomotic intimal hyperplasia. However, currently used grafts or sutures may be damaged by laser radiation at power and energy levels required for plaque removal. To investigate this problem, two commonly used grafts (Dacron and polytetrafluoroethylene [PTFE]) and two types of vascular suture (polypropylene and PTFE) were exposed to argon laser radiation in vitro. Dacron and PTFE grafts recovered from amputations were also studied to determine whether graft "healing" affected graft resistance to laser damage. Power and energy levels required to perforate atherosclerotic superficial femoral arteries were determined for comparison. PTFE grafts were significantly (1.5 to 7 times) more resistant to perforation by laser energy than atherosderotic arteries under all conditions. In contrast, Dacron grafts perforated at power and energy levels one half to one third of that required for vaporization of atherosclerotic plaque. PTFE sutures remained intact at power and energy levels above the levels that perforated atherosclerotic arteries, whereas polypropylene sutures were destroyed by very low levels of power and energy (0.5 joules at 0.5 watts). Because of the variable levels of power and energy that damage different types of prosthetic grafts and sutures, laser angioplasty should only be investigated clinically as a therapy for anastomotic intimal hyperplasia when PTFE grafts and sutures are present. (J VAsc SURG 1987;6:221-5.) Prosthetic graft stenosis resulting from anastomotic intimal hyperplasia remains an important cause of prosthetic graft failure3 However, surgical correction of the anastomotic stenosis seldom produces long-term graft patency since surgical trauma appears to be an important factor in the genesis of recurrent anastomotic fibrous overgrowth. Balloon angioplasty has also been disappointing in the treatment of prosthetic graft stenosis caused by anastomotic intimal hyperplasia, presumably because of the arterial wall damage that occurs during balloon dilatation. From the Departments of Surgery and Medicine, University of Florida, and Vascular Surgery Section, Veterans Administration Medical Center. Presented at the Eleventh Annual Meeting of the Southern Association for Vascular Surgery, Scottsdale, Ariz., Jan , Supported in part by W. L. Gore & Associates, funds from the Research Service, Veterans Administration Medical Center, Gainesville, Fla., and grant R01-HL36320 from the National Heart, Lung, and Blood Institute, Bethesda, Md. Reprint requests: James M. Seeger, M.D., Surgical Service (112), VA Medical Center, GainesviUe, FL *Recipient of Research Career Development Award K04-HL01817 from the National Institutes of Health, Bethesda, Md. Laser recanalization is potentially an attractive new treatment of graft stenosis caused by anastomotic intimal hyperplasia. Fibrous plaque is easily vaporized by laser radiation and currently available systems allow transluminal delivery of laser energy through fibers that can be introduced percutaneously3 '3 The tissue effects of clinically available lasers are primarily thermal, however, and temperatures between 160 and 310 C are needed for plaque vaporization. ~ Such high temperatures applied to an anastomosis could damage prosthetic grafts or sutures and lead to anastomotic disruptions. Thus, before laser angioplasty can be investigated in the treatment of anastomotic intimal hyperplasia, the respouse of commonly used prosthetic grafts and sutures to laser energy must be determined. MATERIAL AND METHODS The resistance of commonly used prosthetic grafts (Dacron and polytetrafluoroethylene [PTFE]) and suture (polypropylene and PTFE) to damage by laser radiation was investigated with a 20-watt argon laser (Model 20 Endocoagulator, HGM Medical Systems Corp., Salt Lake City, Utah). This type of laser 221

2 Journal of VASCULAR 222 Seeger, Abda, and Klingman SURGERY TaMe I. Lowest energy and power required for prosthetic graft perforation in air Fiber distance~fiber pressure 2 ram~ogre 1 ram~ogre 0 ram~2 gm 0 mm/7ogm 0 ram~200 gm Plain PTFE 24.5 at at 4 12 at 4 1 at 1 1 at 1 Explanted PTFE 48 at 4 10 at 5 5 at at at 0.5 Cadaveric artery 30 at 3 9 at 3 3 at at Explanted Dacron 4.3 at at at at 0.5 <0.5 at 0.5 Preclotted Dacron 7.5 at at at at 0.5 <0.5 at 0.5 was chosen because it is currently being used as part of our clinical trial of laser angioplasty in the treatment of arterial occlusive disease, s Pulsed energy (pulse duration 1 second with a 0.2 second delay between pulses) was transmitted to the graft sample through a 600 wm silica optical fiber. The power at the fiber tip was confirmed before laser application with a power meter (Laser Power Meter, Laser Precision Corp., Utica, N.Y.). Power was initiated at 1 watt and up to ten 1-second pulses were delivered. If no perforation of the graft sample occurred, the power was increased by 1 watt increments and the sequence repeated until perforation occurred. If perforation occurred with the first pulse at 1 watt, power was decreased to 0.5 watt and the sequence repeated until the energy level at which perforation of the graft sample occurred was determined. Samples were then restudied at power levels 0.5 and i watt lower than that which caused perforation to confirm that samples were resistant to perforation by laser radiation up to that level. Results were recorded as the amount of energy (joules [watts x number of 1-second pulses]) required for graft perforation at the lowest power setting at which perforation occurred. Perforation of the sample was determined by gross inspection. All experiments were repeated three times. Graft samples were tested in air and in saline solution (37 C) with the fiber held by a micromanipulator at 90 degrees to the graft surface. Lasing was done at a distance of 2, 1, and 0 mm from the sample surface. When in direct contact with the sample, lasing was done with pressure on the fiber of 2, 70, or 200 gm. This was achieved by placement of the sample on a scale (Mettler PC 440 balance, Mettler Instrument Corp., Hightstown, N.J.) and with the use of the micrometer arm to push the fiber on the surface of the sample to maintain a steady scale reading. The experimental setting of the fiber in contact with the sample (0 mm fiber distance) with 70 gm of fiber pressure and lasing being done under saline solution most closely simulates currently used techniques of clinical laser angioplasty. Graft samples exposed to laser radiation included (1) plain PTFE, (2) preclotted knitted Dacron, and (3) recently explanted portions of PTFE and Dacron grafts. Explanted graft samples were portions of either PTFE or Dacron grafts removed from eight patients with thrombosed arterial grafts who required amputations because of peripheral vascular disease untreatable by surgical reconstruction. Portions of superficial femoral artery removed from similar patients at the time of amputation were also lased for comparison. Sutures lased included polypropylene and PTFE. Lasing was done in air at fiber-to-suture distances of 1 and 0 mm with 5 gm of tension on the suture. Commonly used suture sizes were tested. Fiber pressure against the suture at 0 mm fiber distance was 2 gin. Data analysis. When similar samples of grafts or arteries were repeatedly lased under the same experimental conditions (constant fiber distance, fiber pressure, delivered wattage, and surrounding medium [air or saline solution]), the amount of power and energy required for sample perforation was constant. This was consistent for each of the graft types and arterial segments studied through the three repetitions of each experiment. In addition, when the power and energy delivered to a sample was reduced to any level below the "threshold level" at which perforation occurred, samples were consistently resistant to perforation by laser radiation. Thus identical values were obtained from each repetition of each experiment and all data are presented as a single value, which represents the mean of identical values with a standard deviation of zero. No statistical analysis of differences between mean values that had no variance was performed. RESULTS PTFE graft samples were significantly more resistant to damage by laser radiation than were Dacron graft samples (Tables I and II). In addition, PTFE graft samples were also more resistant to perforation

3 Volume 6 Number 3 September 1987 Laser radiation and prosthetic graj~ damage 223 Table II. Lowest energy and power required for prosthetic graft perforation in saline solution Fiber distance/fiber pressure 2 ram~ogre 1 mm/ogm 0 mm/2gra 0 mra/70gra* 0 mm/200gm Plain PTFE >70 at 7 35 at 7 5 at 5 3 at at 0.5 Explanted FTFE >70 at 7 35 at 7 10 at 1 10 at 1! at 1 Cadaveric artery 30 at 3 9 at 3 6 at 1 1 at at 0.5 Explanted Dacron 22 at at at at 0.5 <0.5 at 0.5 Preclotted Dacron 6 at at at at 0.5 <0.5 at 0.5 *Most closely simulates current techniques used in clinical laser angioplasty. Table III. Lowest energy and power required for suture disruption in air (5 gm tension) Suture size Fiber distance (tara) PTFE 0 3 at 1 6 at 1 9 at 3 >95 at 8 1 >50 at 5 >50 at 5 >50 at 5 >50 at 3 Polypropylene at at at at latl 1at 1 1at1 lat 1 during lasing than atherosclerotic superficial femoral arteries. This is in marked contrast to the power and energy levels that caused perforation of Dacron graft samples, which were consistently less than the power and energy levels that perforated atherosclerotic artery segments. In general, the level of power and energy that perforated explanted grafts compared with the corresponding plain graft was higher for both graft types. However, this pattern was more consistent with Dacron grafts than with PTFE grafts. In any event, explanted PTFE grafts were more resistant to perforation than explanted Dacron grafts, preclotted Dacron grafts, or atherosclerotic arteries. The amount of argon laser power and energy required to perforate both types of prosthetic graft material decreased as the distance between the fiber and the graft sample decreased and also decreased as the pressure on the fiber increased once the fiber was in contact with the graft sample. The amount of power and energy required for graft perforation also increased when the lasing was done under saline solution compared with results obtained in air. These findings are consistent since power declines exponentially as the distance from the fiber tip increases. In addition, since saline solution is a better thermal conductor than air, the heat generated by lasing will be more rapidly dissipated when samples are tested under saline solution. Gross inspection of lased Dacron graft samples showed a central area of vaporization surrounded by a rim of charring (Fig. 1). In contrast, inspection of lased PTFE showed an area of vaporization surrounded by a much larger area of thermal damage (Fig. 2). Incomplete thermal combustion of PTFE graft samples was seen consistently. This resulted in strands of charred material in the irradiated area. Comparison of the power and energy required to break the two types of suture material tested was even more marked. PTFE sutures remained intact at power and energy levels equal to or significantly above the energy levels that perforated arterial segments, whereas polypropylene sutures were vaporized by very low power and energy levels (Table III). As would be expected, the amount of power and energy required to break a PTFE suture increased as the size of the suture increased. In contrast, low levels of argon laser power and energy destroyed all sizes of polypropylene suture tested. Variations in the power and energy requirement for polypropylene suture disruption were only seen when the distance from the fiber to the sample of suture was increased. DISCUSSION The effect of argon laser energy on tissue and presumably prosthetic material is primarily thermal. Excess absorbed energy is converted into heat and causes thermal damage. Factors listed by Fuller s to determine the degree of damage that occurs after exposure of tissue to laser radiation include (1) absorption or scattering of the laser light by the tissue, (2) the power density of the radiation, (3) the du-

4 224 Seeger, Abela, and Klingman Journal of VASCULAR SURGERY Fig. 1. Photomicrograph of lased knitted Dacron graft (original magnification 62.5). A central area of vaporization surrounded by a small rim of charring is seen. (Lasing done with direct fiber sample contact, 0.5 joules at 0.5 watts.) Fig. 2. Photomicrograph of lased plain PTFE (original magnification x 62.5). The central area of vaporization can be seen to contain strands of incompletely vaporized graft material. (Lasing done with direct fiber sample contact, 5 joules at 4 watts.) ration of the exposure, (4) the size of the irradiated area, and (5) any cooling component that may be present. In addition, when laser radiation is applied to prosthetic graft or suture materials, the different responses of these materials to high temperatures must be considered. Since all but the first of the factors listed by Fuller s were constant in each of the experimental settings in this study, the absorption or scattering of the laser light by each prosthetic graft or suture and the thermal sensitivity of the prosthetic materials studied are the likely determinants of the variable degree of damage to these grafts and sutures reported herein. The surface of PTFE grafts is very reflective and appears to scatter light to a significantly greater degree than the surface of Dacron grafts. In addition,

5 Volume 6 Number 3 September 1987 Laser radiation and prosthetic graft damage 225 preclotting of Dacron grafts with blood places hemoglobin, which readily absorbs argon laser energy, s on the graft surface and within the graft interstices. There also may be a preferential absorption of the argon laser light by polypropylene suture material. These differences in reflectance and absorption probably decrease the percentage of laser radiation absorbed by PTFE grafts or suture and increase the percentage absorbed by Dacron grafts and polypropylene suture. The temperatures at which Dacron, PTFE, and Polypropylene melt or decompose are also quite different. Polypropylene melts at 165 C and Dacron at 250 C, whereas PTFE only begins to melt at 400 C. 6 Since Welch et al. 4 have determined that tissue ablation by laser radiation occurs at tissue temperatures of 160 to 310 C, it is not surprising that PTFE prostheses are more resistant to perforation by laser energy than superficial femoral arteries and that Dacron grafts and polypropylene sutures are damaged or destroyed before plaque vaporization. The integrity of an anastomosis between a prosthetic arterial graft and an artery is always dependent on the strength of the prosthetic graft material and the suture used in the anastomosis. 7 Fracture of the suture or breakdown of the prosthesis will lead to pseudoaneurysm formation. The preliminary in vitro results presented herein suggest that attempted laser vaporization of anastomotic intimal hyperplasia at a Dacron graft/artery anastomosis, particularly if polypropylene suture has been used, could lead to significant weakening of that anastomosis. Clearly, this in vitro finding must be confirmed in vivo. However, the very low levels of laser energy required to damage polypropylene suture and Dacron grafts must be considered at least a relative contraindication to the use of laser recanalization to treat anastomotic hyperplasia in an anastomosis containing those prosthetic materials. However, our results suggest that anastomotic intimal hyperplasia that develops within an anastomosis containing PTFE graft and suture can probably be safely treated with the laser. Again this conclusion must be confirmed by in vivo studies. Regardless, anastomoses containing these materials appear to be the ones where the use of laser energy in the treatment of prosthetic graft stenosis caused by anastomotic intimal hyperplasia should be first investigated. REFERENCES 1. Whittemore AD, Clowes AW, Couch NP, Mannick JA. Secondary femoropopliteal reconstruction. Ann Surg 1980;193: Abela GS, Normann S, Cohen D, Feldman RL, Geiser EA, Conti CR. Effects of carbon dioxide, Nd: YAG and argon laser radiation on coronary, atheromatous plaques. Am J Cardiol 1982;50: Abela GS, Seeger JM, Barbieri E, et al. Laser angioplasty with angioscopic guidance in humans. J Am Coil Cardiol 1986; 8: Welch AJ, Valvano JW, Pearce JA, Hayes LJ, Montamedi M. Effects of laser radiation on tissue during laser angioplasty. Lasers Surg Med 1985;5: Fuller TA. Fundamentals of lasers in surgery and medicine. In: Dixon IA, ed. Surgical application of lasers. Chicago: Year Book Medical Publishers, 1983: Windholz M, Budavari S, Stroumtsos LY, Fertig MN. The Merck index. Rahway, NJ: Merck & CO, Inc, 1976: Evans WE, Hayes JP, Vermilion B. Anastomotic femoral false aneurysm. In: Bernhard VM, Towne JB, eds. Complications in vascular surgery. Orlando: Grune & Stratton, Inc, 1985: