A Small Arterial Substitute: Expanded Microporous Polytetrafluoroethylene: Patency Versus Porosity CHARLES D. CAMPBELL, M.D., DAVID GOLDFARB, M.D., RODNEY ROE, M.D.

From the Biomedical Engineering Research and Education Program, The Arizona Heart Institute and Department of Pathology, St. Joseph's Hospital and Medical Center, Phoenix, Arizona

Eighty-nine grafts of expanded microporous polytetrafluoroethylene (PTFE) with a diameter of 4 mm, were placed in the carotid and femoral arteries of dogs. The animals were sacrificed at varying intervals beginning three days after operation. Four animals remain alive with patent grafts 10 months postoperatively. Twenty-four of 89 grafts were occluded, an overall patency of 73%. Fibril length (pore size) of the graft material was varied from 4 to 110 microns. Average pore size ranged from 9 to 65 microns. Wall thickness varied from 0.3 to 0.75 mm. Density ranged from 0.24 to 0.35 g/ml. Tissue ingrowth, neointimization and patency rate as compared to pore size, wall-thickness and density of expanded PTFE were observed. Pore size is the primary determinant of tissue ingrowth, neointimization and patency. Of 51 grafts with an average pore size of 22 microns or less, there were 6 occlusions, an 88% patency rate. There were 38 grafts with an average pore size of 34 microns or greater. In these 38 grafts, 18 occlusions were observed, a 53% patency rate. Patent grafts demonstrated tissue ingrowth, capillary formation and a thin neointima. Using small pore grafts, patency rates of 90% can be anticipated in the dog. Expanded microporous PTFE with its ease of handling, strength and pliability may be the vascular prosthesis of choice in man.

prosthesis composed of both absorbable and nonabsorbable material.33 Microknit, a knitted graft of fine yarn with a high ratio of interstice size to yarn diameter, was recently developed as a vascular prosthesis.34 Both concepts attempted to achieve high healing porosity allowing improvement in the development of a neointima and increased patency rate for small grafts. Both designs were attempted in Dacron and yielded minimal success as small vessel substitutes. Sharp20 and others35 have worked to promote growth of a viable neointima, reporting that the greater the porosity of the prosthesis, the better the chance for development of a viable neointima. These investigators have also noted that the neointima must remain thin if patency of small vessel replacements is to be maintained. Although formation of a viable neoinT HE HISTORY of arterial replacement dates back to 1894 tima has been achieved in animals, growth of the neoinwhen Gluck placed a vein graft in the carotid artery of tima in man has not been complete.! In 1973 Matsumoto reported excellent patency rates a patient.6 In the early 1950's Voorhees renewed interest in synthetic materials when he discovered that nylon using expanded microporous polytetrafluoroethylene could be used as an arterial substitute.32 Presently, re (PTFE)* as an arterial substitute in dogs.'3 This material placement of large vessels with synthetic materials is was first used as chemical tubing and was adapted for use common, despite their limitations, but no satisfactory as an arterial substitute. These results have not been duplicated until recently when Volder30 and Campbell2 small artery prosthesis exists. In 1963 Wesolowski described a compound vascular reported excellent patency rates using the same material in sheep and dogs. This report summarizes one year's experience in which good to excellent patency rates have Submitted for publication March 24, 1975. been achieved in a variety of experimental efforts. Tissue Supported in part by The Robert and Irene Flinn Foundation. ingrowth, capillary formation, attainment of a viable, thin Reprint requests: Charles D. Campbell, M.D., Department of Surgery, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261.

*Gore-Tex, W. L. Gore and Associates, Flagstaff, Arizona.

138

Vol. 182 NO. 2

SMALL ARTERIAL SUBSTITUTE

neointima and patency rate in dogs as compared to fibril length (pore size), wall-thickness and density of the material are the subject of this report.

139

PTFE node

Fibril

I

Materials and Methods All grafts were made of expa.nded microporous PTFE and were uniformly porous throughout their length. Electronmicrographs (Fig. 1) show the material to be composed entirely of Teflon with nodes interconnected by a multiplicity of thin fibrils. The fibrils are thin and flexible and are easily pushed aside by growing fibroblasts, allowing tissue ingrowth into the graft. For the purposes of this discussion, fibril length is synonymous with pore size. Fibril length determinations were originally made using water entry pressure or the ethanol bubble technique. Neither measurement technique was reliable, and values were much lower than the actual fibril length, possibly because of the hydrophobicity and negative charge of the material. Therefore, photomicrographs were taken of each graft. The longest and the shortest fibril lengths were determined, and the fibril length is reported as an average of these two measurements. Pore sizes ranged from 4 to 10,u. Average pore size ranged from 9 to 65,u. Wall thickness varied from 0.3 to 0.75 mm. All grafts were 4 mm in internal diameter. Density ranged from 0.24 to 0.35 g/ml. Adult greyhounds and mongrel dogs weighing between 25 and 35 kg were used. Animals were anesthetized with intravenous sodium pentobarbital, 27 mg/kg of body weight. The animals were intubated but rarely was mechanical-assisted ventilation required. Five hundred milliliters of 5% dextrose Ringer's lactate solution was administered intravenously during the operative period. One gram of cephalothin was given intravenously at the beginning and at the termination of the operation. No postoperative antibiotics were given and no heparin was

administered. The animals were shaved over the bilateral femoral and anterior cervical regions and prepped with povidoneiodine. The femoral arteries were exposed and occluded between vascular clamps. A segment of artery 3 to 4 cm in length was excised. The grafts, randomly selected, were soaked in 4% cephalothin and normal saline solution and anastomosed to the femoral arteries. The posterior portion of the anastomosis was performed using the Blalock continuous everting mattress technique. The anterior portion was completed with a simple running suture. All anastomoses were performed using No. 6-0 polypropylene suture material. A midline neck incision was then performed and both carotid arteries were exposed. Grafts were placed in the right and left carotid arteries using the identical technique. All incisions were closed in anatomic layers, and the skin was closed with interrupted mattress sutures of polyglycolic acid. No

Inter Nodal Space FIG. 1. Scanning electronmicrograph of expanded PTFE demonstrating nodes interconnected by thin fibrils (X 2275).

wound dehiscences occurred. No animals have been excluded from this series for technical or other reasons. Postoperative followups ranged from 3 days to greater than 10 months with some animals still being followed. The animals were sacrificed at frequent intervals. Grafts were examined and histologic evaluation focused on tissue ingrowth, capillary formation and neointimization as compared to pore size, wall thickness and density. Results A total of 89 grafts of expanded microporous PTFE were inserted in greyhounds and mongrel animals in the carotid and femoral arterial positions (Table 1). The animals were sequentially sacrificed at varying intervals beginning three days after operation. The grafts in 4 surviving animals remain patent 10 months after operation. Twenty-four of 89 grafts were occluded, an overall patency of 73%. In comparing patency to fibril length, the series could be divided into two groups (Table 2). Grafts in Group I animals had an average pore size of 22, or less. Fifty-one grafts were placed in Group I animals, and 6 have occluded yielding an 88% patency rate. In Group II there were 38 grafts with an average pore size of 34, or greater. There were 18 occlusions in this group, a 53% patency rate. The differences are statistically significant

Ann. Surg. August 1975

CAMPBELL, GOLDFARB AND ROE

140

TABLE 1. Results: 89 Expanded Microporous PTFE Small Artery Substitutes, 4 mm Internal Diameter

Wall Graft Series # 649-1 649-2 650-1 656-1 650-2 657-8 730-1 682-2 658-5 664-3 658-3

11 19 4 8 4 12 4 8 8 10 1

Fibril Length (Pore Size) Range (micron)

Average

Thickness (mm)

10-80 4-64 20-100 5-15 20-110 8-30 12-32 6-16 8-10 10-32 5-20

45 34 60 10 65 19 22 11 9 21 12

.50 .50 .75 .30 .75 .50 .50 .75 .75 .50 .50

Density

Per cent

(gm/mi)

Patency

.29 .24 .32 .26 .31 .34 .28 .35 .35 .29 .33

64 37 75 100 75 66 100 100 75 100 100 73

Total 89

at the 99% confidence level. The relationship is shown graphically in Fig. 2. Of the 89 grafts placed, 24 had a wall thickness of 0.75 mm. Four grafts occluded giving an 83% patency rate. Fifty-seven grafts had a wall thickness of 0.5 mm. Twenty of these occluded yielding a 65% patency rate. Eight grafts had a 0.3 mm wall thickness. No occlusions were noted. Relating patency to density of this material, there were 52 grafts ranging from 0.24 g/ml to 0.29 g/ml density. Sixteen of these occluded yielding a 69% patency rate. There were 37 grafts with densities between 0.29 and 0.35 g/ml. Eight were occluded yielding a 78% patency rate. In most instances, the smaller the pore size, the higher the density. Grossly, the grafts demonstrated a thin neointima increasing in thickness with increasing porosity (Fig. 3). Minimal tissue reaction to the graft material was noted at the time of harvest. Histologically, the rate and extent of tissue ingrowth and neointima development appeared to be related primarily to pore size of the graft material. The larger the pore size, the greater the tissue ingrowth and

the thicker the neointimal development. The smaller pore grafts appeared to diminish tissue ingrowth and produce a more compatible blood tissue interface leading to higher patency rates and thinner neointima development (Fig. 4). In every case, the neointima appeared to encompass the suture line and the entire length of the graft material. Capillary formation occurred in the wall of the graft at 4 to 5 weeks (Fig. 5). The larger pore grafts developed a thick neointima which in some instances progressively thickened and thrombosed at 1 to 2 months. Wall thickness appeared to have little influence on the amount and extent of tissue ingrowth. More importantly, tissue ingrowth is greatly influenced by the pore size of the graft material. Although several large pore grafts were used in this experiment, no extravasation of blood was noted through the graft material in the unheparinized animal.

Discussion Since 1894 surgeons have attempted to replace arterial segments with substitute materials. In 1912 Carrell reported the first use of a prosthesis in an effort to bridge

TABLE 2. 89 Expanded Microporous PTFE Small Artery Substitutes: Patency vs. Fibril Length

Graft Series

Group I

658-5 656-1 682-2 658-3 657-8 664-3 730-1

Fibril Length (Pore Size) Ave. Range (micron) 8-10 5-15 6-16 5-20 8-30 10-32 12-32

9 10 11 12 19 21 22

Total

Group II

649-2 649-1 650-1 650-2

Total

4-64 10-80 20-100 20-110

34 45 60 65

Patency No.

%

6/8 8/8 8/8 1/1 8/12 10/10 4/4

75 100 100 100 67 100 100

45/51

88

7/19 7/11 3/4 3/4

37 64 75 75

20/38

53

141

SMALL ARTERIAL SUBSTITUTE

Vol. 182 - No. 2

89 Expanded PTFE 4mm carotid and femoral arterial

substitutes: Patency vs.Fibril length(pore size)

neointima

PTFE node

-1...

::.

'r'

100

*o

%Patency: 91.4

0.676

(Fibril length)

80(t4

z w

S 20~~~~

B.4

I

10 20 30 40 50 60 70 FIBRIL LENGTH (pore size) AVG. FIG. 2. Graphical representation of patency vs. average fibril length (pore size) demonstrating increasing patency with decreasing fibril length.

arterial defects.3 This met with failure. Hufnagel in 1947 was successful, however, in permanently intubating the arterial tree with a rigid solid-wall tube.10 Finally in 1951 after Voorhees discovered that silk suture became endothelialized when hanging freely in the heart, various materials were used as arterial prostheses.32 These included Nylon, Dacron, Orlon, Teflon and Ivalon sponge.9 Currently, Dacron, either knitted or woven, is the most satisfactory fabric for large vessel arterial replacement, but its limitations indicate that there is still need for improvement in fabric prostheses. Historically there have been three basic approaches to the creation of small arterial substitutes. To date, the most satisfactory materials have been autogenous, homologous or heterologous tissues.4'15'28 The autogen-

FIG. 3.

Gross

expanded

specimen of

PTFE

removed

four months after

implantation demonstrating a thin, attached neointima.

Fibrous ingrowth FIG. 4. Photomicrograph (X 170) of expanded PFTE removed nine months after arterial substitution demonstrating complete fibroblastic ingrowth throughout the graft wall and a thin, adherent neointima.

ous vein when placed in the carotid and femoral arterial position has yielded a 62.5% to greater than 90%o patency rate in the dog.12'6'26 The bovine heterograft has yielded 0% patency when placed in the femoral artery of the dog.4 In the interest of developing an off-the-shelf substitute, protheses with an antithrombogenic,surface creating an artificial intima have been developed for small vessel replacement.8'14 19 These materials have been unsuccessful as small vessel substitutes. Also, a porous prosthetic graft allowing ingrowth and support of a viable neointima has been created for use in the heparinized patient. Wesolowski has popularized the concept of porous grafts and has established specifications for the ideal synthetic vascular graft.33 He also proposed the gossamer

142

CAMPBELL, GOLDFARB AND ROE

capillary endothelial cell

Ann. Surg.

August 1975

remained patent up to 6 months and were noted to have PTFE nod4 blood cells very smooth neointimas. Higher patency rates were reported with larger pore grafts. Recently Matsumoto reported the use of this material as an arterial substitute in dogs.13 The grafts were 3 to 4 cm in length with an inside diameter of 3 mm. A 100% patency rate was noted with a 4.5-to-11-month followup. The grafts on histologic examination demonstrated neointimization and excellent ingrowth. Efforts to duplicate these astonishing results in this country have led to many failures and a few successes.2'30 In evaluating expanded PTFE as an arterial substitute, the dog was selected because it has proved to be the most rigid testing species for vascular prostheses. Grafts were placed in the carotid25 and femoral arteries. This seemed more physiologic and reliable than the atrial sword or caval ring test.7'1 It has been demonstrated that multipotential cells from the blood stream are capable of forming an endothelium29 and, if thin enough (500 ,u), the fibrous ingrowth endothelium may remain viable.20 However, a neointima FIG. 5. Photomicrograph (X 350) of expanded PTFE removed nine vascularized by capillary growth through the graft wall months after implantation demonstrating the graft wall with fibrous would be more desirable. In early experimentation we ingrowth. Capillary formation is observed frequently beginning 4 to 5 weeks after implantation. Nucleated capillary endothelial cell is noted. used large pore grafts of 100 to 300 ,u. Frequently these grafts occluded within 24 hours. It soon became apparent that the smaller pore grafts remained patent and also concept for arterial prosthetic fabrication.36 This method allowed suitable tissue ingrowth and neointimization. used a very fine yarn in the fabrication of the Dacron In addition to pore size, another variable was wall graft material, termed Microknit, increasing the ratio of thickness of the graft material. In our series wall thickinterstice size to yarn diameter yielding more porosity for was varied from 0.75 to 0.3 mm. All were conformaness the same area of graft material. Results using Microknit in and sutured nicely, and the thinner the wall, the 28 canine carotid and femoral arteries yielded an 83% ble easier the graft was to handle. Wall thickness appeared to patency rate in 48 hours which fell to 36% in 72 hours. In a minimal play role in determining ultimate patency of the another study Jacobsen reported Dacron grafts with an graft material as ingrowth occurred quickly in both the internal diameter of 4 mm have a zero to 5% patency rate 0.75 mm the and 0.3 mm wall thickness grafts. Density in the dog.12 More recently Sauvage incorporated an exfrom 0.24 to 0.35 g/ml. There were better ternal velour surface to Dacron grafts attempting to was also varied achieve more fibroblastic reaction and better tissue in- patency rates with the more dense material, but in most growth into the graft material.18 Good patency rates were cases, the denser the material, the smaller the pore size. In observing and relating pore size to patency, it can noted in the thoracic aorta of the dog. Berger has debe seen that as the porosity of the material becomes scribed the incompleteness associated with healing of greater than 22 ,u (average), the patency rate falls preDacron prostheses in man in periods lasting up to several That is, the higher porosity developed early cipitously. years.1 There was no complete tissue ingrowth, incorpothrombosis. This has been noted in some of our earlier ration or healing on the inside of the graft except within studies (Fig. 6). The reasons for these results are un10 mm of the suture line. The initial and long-term lumiknown. It can be postulated that with the large pore graft, nal lining was primarily a fibrinous tube with or without blood in the the arterial system extravasates into the blood cell components. In the 1960's Sharp described of the interstices graft becoming static and initiating the rubber latex tubing, bioelectric polyurethane and Elecmechanism. This results in blood flowing coagulation trolour as vascular prostheses.2124 These met with lima poor interface at the luminal over coagulum, producing ited success. and occlusion. to thrombosis level leading early In 1972 Soyer reported 33% patency in the inferior Expanded PTFE appears to have the characteristics of venae cavae and 100% patency in the suprarenal and thoracic inferior venae cavae using expanded PTFE as a an ideal prosthesis. It is inert, maintains its strength over long periods of time and has a high implantation and venous prosthesis in the pig.27 Grafts ranged from 6 to 9 healing porosity, thus encouraging tissue ingrowth. The mm in internal diameter and were 3 to 6 cm in length. Pore size was reported at 0.5 to 2.5 ,u. In 1973 Volder material is extremely easy to handle, pliable, conformaexperimented with expanded PTFE as an arteriovenous ble and sutures very nicely. Many suture materials and techniques have been used shunt in sheep.31 The grafts, internal diameter 5.6 mm, c.e.c.

red

/

I/

Vol. 182 No. 2

SMALL ARTERIAL SUBSTITUTE

118 Expanded PTFE Substitutes Patency vs. Fibril length(pore size) in the femoral and carotid arterial position in dogs

143

10. Hufnagel, C. A.: Permanent Intubation of the Thoracic Aorta. Arch. Surg., 54:382, 1947. 11. Jacobs, L. A., Klopp, E. and Gott, V. L.: Studies on the Fibrinolytic Removal of Thrombus from Prosthetic Surfaces. Trans. Amer. Soc. Artif. Intern O., 14:63, 1968. is 22 11 12. Jacobson, J. H., Suarez, E. and Katsumura, T.: Influence of Pros1oo0 thesis Diameter in Small Arterial Replacement. Circulation, 13 (Abstract) 28:742, 1963. 15 13. Matsumoto, H., Hasegawa, T., Fuse, K., et al.: A New Vascular 80-I Prosthesis for a Small Caliber Artery. Surgery, 74:519, 1973. C-) 14. Najjar, F. B. and Gott, V. L.: The Use of Small Diameter Dacron z w Grafts with Wall-Bonded Heparin for Venous and Arterial Replacement: Canine Studies and Preliminary Clinical Experience. 40Surgery, 68:1053, 1970. 15. Parsonnet, V., Alpert, J. and Brief, T. K.: Autogenous O01 Polypropylene-Supported Collagen Tubes for Long-Term Arte20rial Replacement. Surgery, 70:935, 1971. 16. Perloff, L. J., Reekard, C. R., Rowlands, D. T. and Barker, C. F.: *12 The Venous Homograft: An Immunological Question. Surgery, I I ' ' ' I- b 0 72:961, 1972. 17. Phelan, J. T., Young, W. P. and Gale, J. W.: The Effect of Suture Fl BR IL LENGTH (pore size) AVG. Material on Small Artery Anastomosis. Surg. Gynecol. Obstet., 107:79, 1958. FIG. 6. Earlier experiments with 118 grafts in the carotid and femoral arterial positions. The comparison between patency and average fibril 18. Sauvage, L. R., Berger, K., Wood, S. J., et al.: An External Velour Surface for Porous Arterial Prostheses. Surgery, 70:940,1971. length (pore size) is noted. The small pore grafts demonstrated good 19. Sawyer, P. N., Wu, K. T., Wesolowski, S. A., et al.: Long-Term patency rates, but as pore size increased, patency decreased markedly. Patency of Solid-Wall Vascular Prosthesis. Arch. Surg., 91:735, 1965. 20. Sharp, W. V., Wright, J., Flaksman, R. J. and McVay, W. P.: for small vessel surgery.5"7 We elected to use No. 60 Promotion of a Viable Neointima. Vasc. Surg., 3:243, 1968. W. V., Finelli, A. F., Falor, W. H. and Ferraro, J. W.: polypropylene because of its inert qualities. Matsumoto 21. Sharp, Latex Vascular Prostheses; Patency Rate and Neointimization noted no difference when Tevdek was compared with silk Related to Prosthesis Lining and Electrical Conductivity. Supp. in his series of arterial replacements.'3 Circulation, 29:165, 1964. W. V. and Falor, W. H.: Rubber Latex Tubing as a Vascular Using PTFE, good results have been achieved with 4 22. Sharp. Prosthesis. Am. J. Surg., 105:802, 1963. mm grafts in the dog. There has been tissue ingrowth, 23. Sharp, W. V., Gardner, D. L. and Andresen, G. J.: A Bioelectric capillary formation and a viable neointima. In relating Polyurethane Elastomere for Intravascular Replacement. Trans. Amer. Soc. Artif. Intern. O., 12:179, 1966. this to man, one can only surmise that patency would be 24. Sharp, W. V., Gardner, D. L., Andresen, G. J. and Wright, J.: more easily achieved, possibly permitting use of a more Electrolour: A New Vascular Interface. Trans. Amer. Soc. Artif. porous graft while still maintaining excellent patency Intern. O., 14:73, 1968. 25. Sharp, W. V.: The Carotid Artery-A Test Site for Small Vessel rates. Prosthetics. J. Surg. Res., 10:41, 1970. Acknowledgments 26. Sharp, W. V., Gardner, D. L. and Andresen, G. J.: Adaptation of Elastic Materials for Small Vessel Replacement. Trans. Amer. The authors wish to thank Messrs. James Moore, William Salyer, Soc. Artif. Intern. O., 11:336, 1965. Dan Detton, and Peter Cooper for their technical assistance. 27. Soyer, T., Lempinen, M., Cooper, P., et al.: A New Venous Prosthesis. Surgery, 72:864, 1972. 28. Sparks, C. H.: Die-Grown Reinforced Arterial Grafts. ObservaReferences tions on Long-Term Animal Grafts and Clinical Experience. Ann. Surg., 172:787, 1970. 1. Berger, K., Sauvage, L. R., Rao, A. M. and Wood, S. J.: Healing of Arterial Prostheses in Man: Its Incompleteness. Ann. Surg., 29. Stump, M. M., O'Neal, R. M., Halpert, B., et al.: Growth Potential of Circulating Cells in the Peripheral Blood. Surg. Forum, 175:118, 1972. 14:301, 1963. 2. Campbell, C. D., Goldfarb, D., Detton, D. D., et al.: Expanded Polytetrafluoroethylene as a Small Artery Substitute. Trans. 30. Volder, J. G. R. and Kolff, W. J.: Induced Growth of Connective Tissue in Cardiovascular Prostheses. Trans. Am. Soc. Artif. Am. Soc. Artif. Intern O., In Press. Intern. O., In Press. 3. Carrel, A.: Results of Permanent Intubation of the Thoracic Aorta. 31. Volder, J. G. R., Kirkham, R. L. and Kolff, W. J.: A-V Shunts Surg. Gynecol. Obstet., 15:245, 1912. Created in New Ways. Trans. Amer. Soc. Actif. Intern. O., 4. Dale, W. A. and Lewis, M. R.: Modified Bovine Heterografts for 19:38, 1973. Arterial Replacement. Ann. Surg., 169:927, 1969. 5. Dormandy, J. A. and Goetz, R. H.: Electrically Charged Wire as 32. Voorhees, A. B., Jr., Jaretski, A. III, and Blakemore, A. H.: Use of Tubes Constructed from Vinyon-N Cloth in Bridging Arterial Suture Material for the Anastomosis of Artery. Surg. Forum, Defects. Ann. Surg., 135'332, 1952. 17:144, 1966. 6. Gluck, T.: Die Moderne Chirugie des Circulation Apparates. Berl. 33. Wesolowski, S. A., Fries, C. C., Domingo, R. T., et al.: The Compound Prosthetic Vascular Graft: A Pathologic Survey. Klin., 70:1, 1898. Surg., 53:19, 1963. 7. Gott, V. L. and Furuse, A.: Standardized Methods for the In Vivo Evaluation of Artificial Surfaces. Bull. New York Acad. Med., 34. Wesolowski, S. A. and McMahon, J. D.: Artificial Arteries. A.O.R.N. Journal, 7:35, 1968. 48:482, 1972. 8. Grode, G. A., Anderson, S. J., Grotta, H. M. and Falb, R. D.: 35. Wesolowski, S. A., Fries, C. C., Karlson, E. E., et al.: Porosity: Primary Determinant of Ultimate Fate of Synthetic Vascular Nonthrombogenic Materials in a Simple Coating Process. Trans. Grafts. Surgery, 50:91, 1961. Amer. Soc. Artif. Intern O., 15:1, 1969. 9. Harrison, H. J.: Synthetic Materials as Vascular Prostheses. A 36. Wesolowski, S. A., Fries, C. C., McMahon, J. D. and Martinez, A.: Evaluation of a New Vascular Prosthesis with Optimal Comparative Study in Small Vessels of Nylon, Dacron, Orlon, Ivalon Sponge and Teflon. Am. J. Surg., 95:3, 1958. Specifications. Surgery, 59:40, 1966. 0

22