Artificial Arteries

ARTIFICIAL ARTERIES Sigmund A . Wesolowski, M.D., and 'James D. McMahon,

M.D.

The initial success of a given arterial reconstructive procedure is a direct function of patient selection and of the surgical technique and the adequacy with which it is performed. The type of material or the procedure used to effect arterial reconstruction also directly determines the permanency of the reconstruction. It is most timely, therefore, with respect to the maturation of the field of arterial reconstructive surgery, to consider the biological Sigmund A. Wesolowski, M.D., is a well known cardiovascular surgeon and academic researcher. He is clinical professor of surgery at State University of New York, Downstate Medical Center, and a consultant in cardiovascular and thoracic surgery at Mercy Hospital, Rockville Centre, N. Y. Dr. Wesolowski was elected President of the American Society for Artificial Internal Organs for 1966. James D. McMahon, M.D., is attending surgeon at Mercy Hospital, Rockville Centre. Dr. McMahon is a member of the American Society for Artificial Internal Organs. A native of Long Island, he ettended premed at the University of Rochester Medical School and received his medical degree from Georgetown University. Dr. McMahon is a general surgeon in practice and specializes in vascular surgery.

January 1968

and engineering details of the materials used for arterial reconstruction. This communication includes a brief review of the various tissue and synthetic materials which can be used, an elaboration of the known facts of the basic healing pattern of arterial grafts and prostheses, a comparison of the various arterial prosthetic materials, observations of the healing pattern following the implantation of prostheses into the human for the correction of clinically symptomatic arterial diseases, and the developmental aspects concerned with sttempts to improve upon the presently available prostheses. A detailed discussion of the surgical technique, clinical judgment concerned with the selection of patients, or dissertation of the arterial lesions which dictate the use of reconstructive surgery, is not included in this paper. There are two major classes of materials which may be used to effect arterial reconstruction : (a) autologous tissue materials, and (b) prostheses of foreign tissues or synthetic materials.

35

On the basis of our present knowledge, there is no known substitute for autologous tissue although great strides have been made in the direction of improving host acceptance of homologous transplants. When considering the various materials which can be used to effect arterial reconstruction, autologous tissue must be considered the standards against which all other types of procedures using prostheses must be compared in terms of performance and permanency. It has been stated that the only perfect graft is “no graft” but certainly the arterial autograft is a close second choice. Endarterectomy can be considered as an application of an autologous vascular tissue material. Other type of autologous tissues that are used for purposes of arterial reconstruction are autologous veins, central tendon of diaphragm, and pericardium. All tissues of non-autologous origin, whether vascular or nonvascular, must be considered as prostheses because upon implantation there is a rapid loss of viability with production of an implant which consists of foreign proteins. The shortcomings of arterial homografts and heterografts are well known to all. Suffice it to say that, with the possible exception of the ficin-digested bovine heterograft,13 the problem of arterial prosthesis development centers about the demonstration that the synthetic prostheses exhibit healing fates superior to the foreign protein (homoand heterograft) prostheses and, hopefully, equal to autologous artery. SYNTHETIC PROSTHESES In the light of the present day demands for arterial tubes of sufficient quantity, length and diameter, autologous materials are inadequate while non-autologous tissue prostheses leave much to be desired. It has been quite natural, therefore, to search for manufactured or synthetic materials which could be used for arterial prostheses. The earliest prostheses used in an attempt to bridge arterial defects were solid-wall tubes of glass and

36

metals;l*l8 thrombosis or junctional rupture precluded success. Hufnagel was successful, however, in perfecting a rigid solid wall tube for permanent intubation of the arterial tree and soon thereafter he developed his now classical plastic aortic valvular prosthesis.* In 1952 Voorhees et d.1° introduced the concept of the porous plastic arterial prosthesis while others noted that flexible, solid-wall plastic tubes sutured directly into the aorta were not as successful as tubes that were porous.28 Following Voorhees’ discovery a number of synthetic fibers were found to perform satisfactorily as porous arterial prostheses for short periods of observation. The earliest used porous prostheses were fabricated by hand or machine sewing into suitable tubular configurations. Following the demonstration of early success with this type of fabrication, textile manufacturers were approached to construct tubes of synthetic fibers.&9, 10, 17

A decade ago we began our own investigations in order to establish an animal preparation to effect the screening of various types of vascular reconstructive materials with respect to their ultimate fate, and to define those properties of arterial prostheses which determine the ultimate fate.2* 12, l 7 The basic experimental preparation is the growing pig because not only is this species spontaneously atherogenic but produces changes in arterial prostheses during a maturation period of six months which are equivalent to changes which are being observed in human prostheses beyond two years after implantation.20 The use of the growing pig preparation, therefore, has proven to be a rapid longterm equivalent screening p r e p a r a t i ~ n . ~ ~ We have also made extensive observations in the adult dog and the 24 The latter two species exhibit certain quantitative differences in ultimate healing but the basic qualitative healing pattern remains quite similar in all three species, the major difference being simply one of chronology. 39

43

AORN Journal

Figure 1 : Basic healing pattern of synthetic arterial prostheses. The column on the left contains a series o f four diagrams interpreting the histologic sections (right column) of different stages o f healing arterial prostheses. L = lumen of prosthesis, F = fibrin, Y = yarn bundle, C = organizing granulation tissue, H = healed fibrous capsular tissue, D = degenerative fibrous capsular tissue, C = calcified capsular tissue. The middle column lists the comparative chronology o f the specific stages of healing illustrated for the growing pig ( P ) , the adult dog ( D ) and the adult human (H). a. Prosthesis recovered from abdominal aorta of a n adult human 36 hours after implantation; Masson’s Trichrome, x 100. Note the fibrous inner and outer capsules. b. Prosthesis recovered from abdominal aorta of an adult human 8 months after implantation. Masson’s Trichrome, x 220. Note that granulation tissue hm replaced only half of the intercapsular tuft and that the inner capsule consists of compact fibrin. c. Prosthesis recovered from the thoracic aorta of a growing pig 3 weeks after implantation. H & E, x 80. T h e inner and outer capsules and interstitial tufts consist of healthy fibrous tissue. d. Prosthesis recovered f r o m the thoracic aorta o f a growing pig 7 months after implantation. H & E, X 20. A t least one cycle of inner capsular degeneration has resulted in thickening o f the inner capsule, calcification of the inner capsde and the reappearance of granulation tissue as the basal layer of inner capsule.

January 1968

37

I 0

100 200 300 CALCIFICATION INDEX

Figure 2 : Relationship o f physical water porosity of arterial prostheses to calcification in the growing pig. Ordinate is water porosity in thollsands of cc. waterlminute for each square centimeter of prosthesis and the abscissa is the calcification index upon healing in thoracic aorta of the growing pig. See text. Modified from Wesolowski.10

BASIC HEALING PATTERN OF

SYNTHETIC PROSTHESES The sequence and comparative chronology of events in the basic healing pattern following implantation of a porous synthetic prosthesis into the arterial tree of the growing pig, the dog and the human is illustrated in Figure 1.20,24 At implantation the porous arterial prosthesis is leaky to blood but with preclotting the interstices of the fabric become filled with fibrin within a matter of minutes. Twenty four hours after implantation the arterial prosthetic wall consists of the porous fabric sandwiched in between a layer of fibrin on the inner surface in contact with the blood stream and an outer layer of fibrin in contact with the peri-arterial tissues. During the first few weeks after implantation the outer fibrin layer becomes completely organized by granulation tissue to form an outer encapsulating sheath of fibrous tissue which gives rise to discrete tufts of granulation tissue which grow between the yarn bundles through the interstices of the fabric and organize the inner layer of fibrin. An unsolved mystery is the observation that the inner layer of fibrin is of apparently self-limiting thick-

3a

ness, rarely exceeding 1 mm. in thickness at this stage of healing. During organization of the inner capsule blood vessels grow through the interstices to vascularize the inner capsule. In the growing pig the inner capsule becomes nearly completely organized with cicatricial tissue by the end of one month. With further passage of time this scar tissue contracts with obliteration of the vascular blood supply. In the case of a scar in the skin or deep within the body, such normally occurring maturation-contracture occurs gradually (necrobiosis) with a m ple opportunity for surrounding tissues to absorb the products of autolysis and degeneration. In the case of the arterial prosthesis the vascular blood supply via the interstices functions simultaneously as the nutrient and the absorbing blood supply. With normal scar tissue contraction, the forces of contracture are focused upon the locus minoris resistenciae in the plane of the prosthesis where the cicatrix is mechanically weakest as it penetrates the screen-like membrane of the prosthetic material itself. The resultant degeneration of tuftal interstitial tissue often is accompanied by rhexis or fracture of the tufts; the inner capsule can undergo rather acute degenerative change and autolysis without a ready surrounding blood supply to absorb the products of degeneration (necrosis). As soon as autolysis and other degenerative changes occur within the inner capsule, there is called forth a second wave of granulation tissue from the outer capsule in juxtaposition to the plane of the fabric. If the size of interstice of the fabric is large enough, there is easy access to the second wave of organizing granulation tissue and the products of degeneration are quickly reorganized. If, however, the size and number of interstices in the usual type of prosthesis is small (and, therefore, the physical porosity is small) there is a delay in reorganization of the products of degeneration ; these products can attract calcific deposits

AORN Journal

Figure 3: H & E photomicrographs of diflerent areas of a human iliac prosthesis removed surgically 354 years after implantation demonstrating the varying chronology of various complications in the same prosthesis. The lumen is to the reader’s upper l e f t in each photomicrograph. TOP: Left, x 100. Largely organized inner capsule with secondary deposit of red cell thrombus upon the luminal surface and granulation tissue in the intercapsular bridge which contains capillary buds; Right, x 70. Three-layered inner capsule with collagen and granulation tissue in the basal layer, precollagenous tissue in the middle layer, and red cell thrombus in the luminal layer. BOTTOM: Left, x 30. Area showing inner capsule which is largely organized but which has a fibrinous deposit upon the luminal surface. There has been rhexis o f the intercapsular tufts with hemorrhage in space between the fabric and outer capsule (perigraft hematoma); Right, X 30. In this area the inner capsule on the l e f t can be seen to be largely intact, as are the intercapsular t u f t s in this area. The upper right portion of the inner capsule consists entirely of fresh fibrin and in this area the intercapsular tufts have degenerated and fractured. I n this latter area the inner capsule had sloughed and new fibrin has been deposited upon the bare surface. Modified from Wesolowski et

apparently as a result of the acid reaction in the presence of low oxygen tension. Furthermore, this cycle of healing, degeneration and reorganization can be repeated to produce an increasingly thick and calcified inner capsule. Confirmatory evidence of the importance of the permeability of porosity of the fabric in determining the extent of calcification is illustrated in Figure 2. The poros-

January 1968

ity of each of the 25 individual fabrics which were studied to establish this curve was measured prior to implantation using an apparatus with a high degree of reproducible results.*O Porosity is expressed in milliliters of highly filtered water which will pass through one square centimeter of fabric during a period one minute at a hydrostatic pressure head equivalent to 120 mm. of mercury.

39

The calcification index is the product of the average calcification (on an arbitrary 0 to 4+ scale) of all animals implanted with a given material and the percentage of specimens of the same material which exhibit calcification. These observations establish the fact that in the growing pig calcification bears an inverse relationship with the physical water porosity. In addition to calcification of the inner capsule, which represents an index of the general health of the inner capsule, other specific complications of healing can accompany the above cycles of degenerative change within the inner fibrous capsule2°*21* 24 (Fig. 3). It should be borne in mind that all portions of the inner capsule undergo degeneration which, fortunately, are not chronologically synchronous. These degenerative changes can affect varying sized areas of inner capsule. When the area of degeneration remains focal there may be no specific deleterious effects, or may in some unknown manner cause the precipitation of a secondary fibrinous deposit upon the inner surface of the inner capsule to produce a septum which can cause graft stenosis or actual occlusion especially in arterial prostheses which are less than 15 mm. in diameter. Areas of inner capsular necrosis, especially in the less porous grafts, can slough producing bare areas which can be grossly and microscopically identified. If the area of slough is large enough it can produce an important distal arterial embolism, which has been reported in patients bearing thoracic aortic synthetic prostheses of unusually low porosity. Once a secondary deposit of fibrin is formed, it can propagate locally to produce graft occlusion in the pig, in the dog, and in the human. Studies of the enzymatic changes of specimens forwarded to Dr. J. E. Kirk of St. Louis, Missouri, corroborate the histologic evidence of autolysis and proteolysis within the inner capsule.ll During the phase of degeneration of the interstitial tufts of tissue, as mentioned above, the degenerating tufts can fracture

40

over a significantly wide area. There can result seepage of plasma from the raw surface of the outer capsule in juxtaposition to the plane of the fabric to produce a perigraft seroma. If tuftal rhexis involves blood vessels larger than capillaries frank blood can escape into the plane immediately exterior to the fabric to produce a perigraft hematoma (Fig. 3). Once perigraft hematoma occurs it tends to propagate the length of the prosthesis except in the most porous materials. This complication is observed to a limited extent in the growing pig and the mature dog and not uncommonly in the human. Other evidence indicates that perigraft hematoma can dissect in the periprosthetic plane to cause delayed rupture of the anastomotic suture line. This is offered as one of the causes of aortic rupture, with or without the production of aorticoduodenal fistula, following implantation of a prosthesis into the abdominal aorta. Fortunately this very serious complication, which bears a high clinical mortality, is rarely seen and is probably much rarer in the higher porosity materials which exhibit firmer bonding of the outer to the inner capsule via the interstitial tufts than do the lower porosity materials. Thus it can be appreciated that the healing of implanted arterial prostheses and the complications which it can undergo are determined largely by the physical properties of the fabrication, the specifications for which become a straightforward biological engineering problem. COMPARISON OF VARIOUS ARTERIAL PROSTHETIC MATERIALS Table I compares some of the important parameters of healing of arterial prostheses which have been screened at the National Graft Material Screening Center at the Downstate Medical Center of Brooklyn during the past ten Comparison of the various materials and their fabrications was made with respect to the porosity, the average thick-

AORN Journal

Material

REPRESENTATIVE GROSS RESULTS OF HEALING IN SYNTHETIC ARTERIAL PROSTHESES * (Sampling of Over 100 Materials Tested) Thickness Porosity Inner Capsule Calcification Index Fabrication cclsa. cm-min (mm.)

Vinyon-N Nylon Orion Fiberglass Dacron Dacron Dacron Teflon-Dacron Teflon Teflon Dacron Dacron

Woven twill

1.4 1.5 1.8 1.5 1.5 1.o 1.1 1.2 1.1 1.0 1.5 0.9

<

Broadloom. Meadox

Narrowloom Knit, Edwards Knit Teflon coated Knit: Wesoiowski Weavenlt *All of the prostheses listed have been used cilnlcaliy with the exception of fiberglass.

100 174 133 0 160 35 300 16 230 25 128 21

Minimal Residual Diameter

44 62 65 60

68

68 64 66

Table I

ness of the inner capsule in the equivalent longterm healed state of the growing pig preparation, the degree of calcification and the residual percentile lumen which is a measure of the degree of septum formation. There is considerable variation in the three indices of healing independent of the type of material. It was determined that those materials which appear to produce the thinnest inner capsule, the least calcification and the least amount of septum formation (highest percentile residual lumen) are the materials which have the highest porosity to water. As a result of our studies we have been able to establish a scale of acceptability of all the presently known graft materials as illustrated in Figure 4. The scale of acceptability is divided into three bands: acceptable longterm fate which includes only autologous tissues; acceptable short-term fate which includes many of the homografts and all of the synthetic fabrics tested in the study, the most porous materials being equivalent to the fresh homograft and the least porous materials being barely acceptable even on a short-term basis; and unacceptable fate because of rupture or occlusion soon after implantation. In addition, our observations have allowed elaboration of the specifications for the ideal simple vascular prosthesis: ( 1) absence of toxicity, allergenic potential or other overtly adverse chemical reaction; the biological reactivity of the material per se, over the range of that from Teflon to glass is not a limiting factor in the biological healing of the synthet-

J a n w r y 1968

ic vascular prosthesis; (2) the prosthesis should be durable without significant deterioration of the synthetic yarn upon prolonged implantation. Nylon, Orlon and Ivalon are disqualified on this account; Dacron, VinyonN and Teflon qualify. Dacron is preferable because of its superior mechanical handling properties during fabrication and at implantation; (3) the biological healing porosity should be of the order of 10,OOO milliliters of water per minute per sq. cm. fabric at a pressure head of 120 mm Hg. It should be pointed out that no commercially available prosthesis today meets this specification because the limit of safe implantation from the viewpoint of hemorrhage is in the vicinity of 5000 ml. of water per min. per sq. cm., at a pressure head of 120 mm Hg; (4) ideally, the material should have a low implant porosity to enable the administration of heparin or other anticoagulant: less than 50 cc. per min. per sq. cm. at a pressure head of 120 mm Hg; ( 5 ) there should be desirable handling properties which facilitate implantation which, therefore, becomes safer : (a) conformability (“scrunchability”) for ease of performance of anastomosis; (b) linear elasticity is desirable; crimping in our experience is preferable to elastic yarn because with graft shortening the latter is more likely to affect adversely the porosity; (c) the fabrication should have good pliability and good twist characteristics for traversing flexion creases and subcutaneous and subfascial tunnels without significant mechanical kinking.

41

.*--

T/T

Auto-artery PERFECT -No ORAFT Auto-tendon diqhraqm(7)

.-.'.. .- -

.^_.

Auto-pericardium (dog) Auto-vein, peripheral

Homo-artery,irrad.frozen

I

SYNTHETIG FABRIGSHomo.srtery,freezedred

p:

L I M B O

Hetero-arter y,f idn Homo-artery, alc.,glyc.= == Ivalon,unrupported Homo-artery,formalin

AG%ETXECE' Tiii%WEifi'

Homo-artery, dried Hetero-ortery,fresh (L fad. Auto-pericardium (pig) DIED ON TABLE

-

--. .

Figure 4 :' Diagram of acceptability rating of various materials for use in arteries. Modifie'd from

remaining eight prostheses were removed at operation because of some major complication: occlusion in 7 and aorticoduodenal fistula in the eighth. As a result of this study the following observations and conclusions were made:24 In the 14 patent prostheses there was consistent demonstration of one or more minor complications (dilatation, secondary deposit of fibrin or septum formation, degeneration of inner capsule, seroma formation) beyond OBSERVATIONS IN THE HUMAN We have analyzed a large series of, and re- 8 months of implantation. Progression of ported on the first 22 prostheses which had these minor complications can result in major been recovered from humans varying from complications (occlusion or fistula). Of the 8 failed prostheses, 3 demonstrated between 36 hours to 3?42 years after implantation for arterial reconstruction for complica- early occlusion probably as a result of existtions of atherosclerosis. Fourteen of these 22 ing inadequate outflow at the time of implanprostheses were patent and functional at the tation. Three exhibited late occlusion (two, time of removal at necropsy or operation. The 21,4 and 3 years after implantation) as a direct

From the practical clinical point of view, it is recommended that patients who have been subjected to implantation of porous synthetic vascular grafts be followed carefully for possible graft obstruction especially if the internal diameter at implantation has been less than 15 mms., and for distal arterial embolism, especially if the prosthetic material is of low biological porosity.

42

AORN Journal

result of changes in the prostheses per se. One developed an aorticoduodenal fistula probably as a result of changes in the prosthesis, while the cause of failure remains unknown in one. The general pattern of healing of the human vascular prosthesis is the same as in the mature dog and in the growing pig, with the following differences in the human : (a) The rate of inner capsular healing is much slower (one year is roughly equivalent to one month in the growing pig, and four months in the adult dog) ; (b) Healing or organization of the intercapsular tufts or bridges and of the inner capsule by granulation tissue arising in the outer capsule is cyclically interrupted by degeneration and/or rhexis of the intercapsular tufts which causes cyclical autolysis and local repeated accumulations of fibrin. Rhexis of intercapsular tufts may be related to the composition of granulation tissue which, in the human, is relatively rich in capillaries and poor in collagen and, therefore, has poor mechanical strength. Unlike the pig and the dog, the human does not heal the inner capsule completely prior to the onset of intercapsular tuft degeneration, but the same qualitative phenomenon of autolysis in the inner capsule would appear to occur. Inner capsular degeneration can lead to slough or to thickening of the inner capsule secondary to cyclical intracapsular deposit' of fibrin and to luminal surface deposition of fibrin to form septa which can lead to graft obstruction. Degenerate inner capsule can become reorganized. Rhexis of intercapsular tufts can cause accumulation of tissue fluids (seroma) or of blood (hematoma) in the plane between the prosthetic material and the outer capsule. Perigraft hematoma formation can accentuate tuft rhexis and inner capsular degeneration and can probably lead to delayed anastomotic leak.

January 1968

The observed changes are consistent enough to be able to construct a schema of factors which can contribute to failure of vascular prostheses in humans, i.e., the observations are reproducible (Fig. 5 ) . The outer capsule matures during the first month to consist of three layers from without, luminad: (a) a vascular layer carrying blood supply and giving rise to branches to traverse, (b) a mature collagen layer, and (c) a layer of granulation tissue immediately adjacent to the prosthetic material. This granulation tissue remains rich in capillary buds and foreign body giant cells even after three and one-half years of implantation, probably in continued response to repeated cycles of autolysis and fibrin deposition in the intercapsular tufts and basal layer of inner capsule. The basal layer of inner capsule contains accumulations of material which take specific stains for lipids and cholesterol. The inner capsule resembles to this degree the lesion of atherosclerosis which can also be characterized by containing areas of necrosis, fibrin deposition, fibrosis and accumulation of lipids and cholesterol. The only element of atherosclerosis missing from the inner capsule of the vascular prosthesis in the human is calcification which, however, has been demonstrated experimentally in the pig and the dog, and may yet be demonstrable in the human prosthesis beyond three and one-half years of implantation. Changes predicted from extensive animal experimentation are being observed in the human prosthesis within the predicted time interval. The fact that nearly all the prostheses in the present study show fresh fibrin upon the luminal surface in many areas may help to explain the inability of others to demonstrate the presence of complete endothelial lining in the human prosthesis. At the present time there is general interest in the endothelialization of implanted cardiac and vascular prostheses. There is no question that such studies have fundamental signifi-

43

I ? I PR. PT. DIS. I DECREASED FLOW

= .

\ DISTAL

I.C. SLOUGH

-

FIBRIN LAY ERlNG

/\GRAFT EblZLIZATION

OCCLUSION

DEGEYTN

FIBRIN SEPTUM

-

t

TUFTS,PERIGRAFT SEROMA 7 or HEMATOMA RHEXIS

S

t TUFT

ORGANIZATION

I

ANASTOMOTIC RUPTURE

6

ART. CH.

I?

I Figure 5: Reconstruction schema of the possible complications of healing observed in arterial prostheses recovered from the human. T h e hollow arrows represent the course o f primary failure ( P ) or secondary failure ( S ) . Pr. Pt. Dis. = progression o f patient disease; Art. ch. = change in arterial wall. Identical basic changes can be observed in both patent and occluded arterial prostheses. Extrapolation o f the aberrations observed in the patent human and experimental prostheses have predicted the changes observed in the failed prostheses recovered f r o m humans. Modified from Wesolowski.80

cance to our understanding of the healing process but they have little practical significance to our understanding at present of the healing of the arterial prosthesis. There are two major reasons: (1) Even if a prosthesis comes to the endothelially healed state this does not prevent the occurrence of changes within the inner capsular or “stromal” layer upon which the endothelium lies; and (2) Changes within the inner fibrous capsule can not only prevent endothelialization, but can lead to loss of endothelium secondary to degenerative changes and luminal and intracapsular deposition of fibrin in the growing pig, adult dog and human. From a practical point

44

of view endothelialization studies will have maximal significance in the healing of vascular prostheses when a prosthesis is perfected which will allow for a stable and permanent inner fibrous capsule to provide a stable and permanent bed upon which the endothelium may lie and function persistently.

THE DEVELOPMENTAL ASPECTS Further development of arterial prosthetic materials can be made in three possible directions: (a) to produce a synthetic arterial prosthesis which mimics the inner surface characteristics of the normal artery. This direction we have termed the “solid wall tube;”

A O R N Journal

(b) optimization of the simple arterial pros- exhibit full-lumen thrombosis.1~On implantathesis with the eventual production of a fabri- tion of platinum one cannot even indict the cation which has yarn bundles that are so fine formation of a deleterious oxide. A number of that there will be minimal competition for the solid metal tubes have been tested in the mechanical integrity of the interstitial tuft and dog thoracic aorta and the dog thoracic inferithe yarn bundle in the healed state. This direc- or vena cava where duration of patency has tion we have termed the “gossamer concept;” been shown to be directly proportional to the and (c) the development of a prosthetic vas- location of the metal in the electromotive cular material which has a very low porosity series. Those metals which are higher in the at implantation but which will have a very series, like magnesium, maintain patency for high biological porosity. This is attained by a theoretically infinite period of time while fabrication of materials containing both ab- metallic tubes of platinum, located on the elecsorbable and non-absorbable components in tromotive series below the electropotential the wall. This direction we have termed the precipitation point of blood, thrombose with66compound prosthesis.” in relatively short periods of time. The metals which are higher in the electromotive series, SOLID WALL PROSTHESIS therefore, have ‘certain antithrombotic characWe have previously noted that plastic tubes teristics, but in addition have three deleterious which were non-porous when sutured into ar- biological characteristics: (1) they demonterial defects were prone to full-lumen throm- strate a propensity to severe inflammatory bosis and/or distal From the data responses, except aluminum; (2) Sawyer l4 dealing with the characterization of the nor- has reported that these materials tend to demal and abnormal blood-vascular interface, it stroy blood cells and protein, except alumiwould appear that these plastic tubes have un- num; and (3) these same metals corrode so desirable bioelectrical properties perhaps re- rapidly that they tend not to be useful for any lated to the property of good electrical insula- consideration for long term implantation, intion. When placed into the arterial tree these cluding aluminum. solid-walled plastic tubes may act as electrical It is probable that the porous plastic proscapacitors which allow for the steady and pro- theses remain patent where the solid wall plasgressive deposition of fibrin at the blood-pros- tic prostheses do not because of permeability thesis interface. The same plastic substances to ions. Tissue permeability is probably not when fabricated in the form of a porous tex- necessary and may even be detrimental. The tile develop a layer of fibrin on the inner production of an acceptable solid-walled arsurface which rarely exceeds 1 mm. for pro- terial prosthesis must await two other developlonged periods of time. The inescapable ments: (1) the perfection of a true artificial conclusion is that the maintenance of an intima which will mimic the normal bloodacceptable stable blood-membrane interface is vascular interface; and (2) the development attainable when the vascular wall is porous, of a good method of permanent binding at the question being to what class of particles the host-tube junction, both parameters to be must the wall be permeable: electrons, ion, or considered still in the highly developmental fibroblasts. stage. The work on solid wall metallic prostheses as reported by Sawyer et al. would suggest THE GOSSAMER CONCEPT that the permeability need be greater than The production of a gossamer type of fabrithat to electrons themselves because, for in- cation is not difEcult using the process of stance, the solid-walled noble metallic tubes weaving. However, the physical properties of

January 1968

45

PHYSICAL PROPERTIES AND TATES IN 6ROWIN6 PI6 OF VARIOUS DACRON PROSTHESES AVERAGE TATES IN PHYSICAL PROPERTIES GROWING PIG

A 2 Sidebotham + c Meadox Medicals 0 Pillin 0 U. S. 0 U. S. Catheter (Meadox Medlcals0 Dacron knit "Weavenit" Bearlng Products) 0 U. S. Catheter Dacroi k n i i crimp T.D.I. 0 U. S. Catheter Dacron' knlt' crimp: T.C. a 0 U. S. Catheter Dacron' knit' open 0 U. S. Catheter Dacron: knit: open, heat p l a t i n 0 U. S. Catheter Dacron, knit, open, gelatin 8 Two dlfferent types of Teflon coating bTwo different types of gelatin coating c A t tip of long bevel only d Excessive bleeding a t Implantation e Increase in diameter secondary to "reverse Chinese finger-cuff'' effect (readjustment of wall

Dacron, narrowloom, elastic Dacron, broadloom, crim Dacron knit crimp l95P Dacron' knlt' crimp' 1960 Dacron: knit: crimp: 1962

1.0 1.5 0.9

catheter

1.1 1.4 4 3 3 36

3 3

0.9

1.5 1.3 1.4 1.2 1.4

35 160 162 300 220

62 44 66 65 37

21 128 166 88 80

66 64

79

76 47 59

60

architecture)

Table I1

a woven gossamer tube are such to make it impractical from the viewpoint of slippage of the yarn bundles and general instability of the pattern of weave. Experimentally, we have tested woven gossamer prostheses which would appear to be promising from the viewpoint of healing.22 With respect to mechanical stability of the fabrication pattern, knitting is probably the most desirable type of fabrication. The usual commercially knitted prostheses are manufactured at a linear needle density of 22 to 30 needles per inch; at these densities the fabrication is considerably more bulky than a gossamer because of the necessity to employ texturized, stretch or other type of bulk yarn to control the porosity to a reasonable figure. As the first practical step in the development of the gossamer concept Mr. Walter Golaski of Philadelphia has successfully developed machinery which can knit at 40 needles per inch;27 this is the first generally available knitted arterial prosthetic material which does not require the use of a bulk or texturized yarn in order to control the porosity. The resulting physical characteristics derive directly from this development: 1) a very finely knitted prosthesis which is soft, pliable, thin-walled and handles, in general, very much like normal arterial tissue in terms of ease of suturing and conformability at su-

46

ture lines. This fineness of knit allows for run-proofness, cutting on a bias with no tendency to fray, construction of branchings on the bias, and is as lightweight as many finely woven prostheses. 2) It has a high porosity: 4000 cc. water/sq. cm-per minute at a pressure head of 120 mm

Hg. 3) It has a high degree of efficiency of preclotting; a single external preclotting reduces the porosity of the prosthesis to ten percent of the initial porosity. Other knitted prostheses, in general, possess a twenty-five percent residual porosity after preclotting. Although other knitted prostheses are less porous initially, they can exhibit a higher porosity after a single external preclotting than the 40 needle product. 4) This new prosthesis has avoided the use of texturized, stretch and bulk yarns. This new simple prosthesis has been extensively tested and demonstrates a pattern of healing in the adult dog and in the growing pig which is superior to all other polyester prostheses tested (Table 11).2" More than 100 of these prostheses have been employed clinically for arterial reconstruction and have been found to possess mechanical handling characteristics superior to those of any other prosthesis which we have used.26 The few implants of this material which have been recov-

AORN Journul

ered from humans have demonstrated acceptable healing of the inner capsule. The outer capsules of the recovered prostheses have been the thinnest observed.26 Up to 27 months the outer capsule is thin enough to allow for excellent residual flexibility, slight residual elasticity of the prosthesis, and the presence of a unique periprosthetic surgical dissection plane : observations which are, therefore, unique to this particular prosthesis. It should be pointed out that although we have been impressed by the observations of this material in the human, further observations are necessary to establish any claim to superiority over other available vascular prostheses upon a scientific basis, but this direction has been sufficiently promising to us that we are now undertaking programs to develop even finer knitted prostheses.

thesis is valid; i.e., there is a significant change in porosity in many compound prostheses such that biological porosities 100 times in excess of the initial porosity can be observed (Fig. 6). The concept of the coated mesh is, in general, not as satisfactory as the concept of the fabricated compound material in which the absorbable component is an integral part of the fabrication. The compound vascular prostheses tested demonstrate the same general pattern of healing as the simple prosthetic materials, but the time-sequence of late changes would appear to be delayed. Previous observations are confirmed that the complications of graft calcifications and septum formation are attributable to inner capsular degeneration. Further pathologic evidence correlates periTHE COMPOUND VASCULAR graft seroma and hematoma formation to dePROSTHESIS generation of the autologous cicatricial tissue As a result of the specification which states in close proximity to the mesh of the implantthat the initial porosity should be very low to ed prosthesis. Further evidence also indicates minimize leakage at the time of operation, and that the foreign body cellular reaction in the that the biological porosity should be very prosthetic wall site is the result of simple preshigh, a series of compound vascular pros- ence of the resorbable component of the graft thetic materials were screened to test the con- wall initially, and subsequently to autolysis cept of whether a prosthetic wall containing of host collagenous tissue in the basal layer both absorbable and permanent components of the inner capsule in the presence of the would result in a functional or biological poro- implanted foreign body. In vitro tests reveal the presence of active sity significantly in excess of the initial porosity.z3*25 Three classes of materials were ionic transport across healed prosthetic vastested: (1) open mesh fabrics coated with cular grafts suggesting that the healed inner collagen derivatives; (2) woven fabrications capsule functions in a manner similar to the in which some of the polyester yarns are intima of the normal arterial wall. replaced with monofilament yarns of 5-0 catFurther evidence is cited that the inner gut or collafil;" and (3) fabrics woven of fibrous capsule of the vascular prosthesis is compound core yarn, the yarn consisting of capable of changes similar to those of the hua monofilament collafil around which is man aortic intima undergoing the process of wrapped multifilament polyester yarn. Initial atherogenesis. experience with 24 compound arterial prosThe most desirable results in terms of sigtheses over a three-year period resulted in the nificant increase in biological porosity over following observations:2s*26 initial porosity at the time of implantation The concept of the compound vascular proswere observed in the fabrications utilizing the compound yarn containing a core of re* A catgut-like continuous filament.

January 1968

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INITIAL DIGESTED BIOLOGICAL

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Figure 6 : Plot of the graphic representation of the initial (unmodified), digested (chemically) and biological porosities for 14 different compound prostheses tested in the growing pig. The initial and digested porosities were measured directly. T h e biological porosity was calculated from the calcification index (see Fig. 2). T h e dotted line, at porosity level 5000 cc./sg. em.-min., represents the maximal implantation porosity of a n otherwise untreated prosthetic material for prevention of hemorrhage. T h e cross-hatched areas represent the gain in porosity that has been achieved by inclusion of an absorbable component in the prosthetic arterial wall. There is significant divergence of the biological and digested porosities because of delay in organization of the absorbable component in these particular fabrications allowing healing o f the inner capsule prior to complete organization of the absorbable which limits the theoretically maximal attainable biological porosity (equal to the digested). Modified from Wesolowski et al.**

AORN Journal

sorbable material wrapped with multifilament polyester yarn. Further specifications dealing specifically with the compound vascular prosthesis were elaborated. These are: (a) after resorption of the absorbable material the residual pattern should be stable to prevent shifting of the residual yams which predisposes to hemorrhage; (b) the residual wall should have an interstice size which will not allow frank hemorrhage. The critical interstice size in this connection has not yet been determined; (c) the residual wall should have a certain degree of stiffness, as yet undetermined, in order to prevent fibrosis-stricture formation in growing subjects; (d) the absorbable component should have a large surface-mass ratio to allow for efficient organization by cicatrix; and (e) the organization time of the absorbable component ideally should be of the order of autologous fibrin. As a result of these experiences we developed a series of fabrications which were woven or knitted, and which contained a mixture of multifilament polyester yarns and multifilament yarns of oxidized cellulose or viscose, in an effort to increase the surface-mass ratio of the absorbable component. Initial results of these compound prostheses composed of polyester and oxidized cellulose are promising and are presently being evaluated further.

SUMMARY This communication has included review of the various tissue and synthetic materials which can be used as arterial substitutes, elaboration of the basic healing pattern of arterial grafts and prostheses, a comparison of the various arterial prosthetic materials, observations of the healing pattern of arterial prostheses recovered from the human, and the developmental aspects dealing with improvement of arterial prostheses. It is the sincere hope of the authors to leave the reader with the following philosophy concerning the ultimate results of arterial reconstruction. We should not be complacent about the results even though many patients are dramatically benefited, but should attempt to recover all implanted prostheses and to analyze them in minute detail with a view to document all of the complications which can occur, in order to apply this information to the development of better vascular prostheses. In time a prosthesis may be developed to compare favorably with normal autologous aorta. The ultimate challenge: Can we develop a vascular prosthesis which can be implanted into a child, which will grow in pace with the surrounding host artery and maintain a viable, intact, stable, and endothelialized inner fibrous capsule which will be free of any complications for the life of the individual?

REFERENCES

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1. Carrel, A.: Results of permanent intubation of the thoracic aorta. Surg. Gynec. & Obstet. 15: 245-

248, 1912. 2. Deterling, R. A., Jr.: The current status of blood vessel replacement. Surg. Gynec. & Obstet. 104: 227, 1957. 3. Deterling, R. A., Jr., and Bhonslay, S. B.: An evaluation of synthetic materials and fabrics suitable for blood vessel replacement. Surg. 38: 71, 1955. 4. Edwards, W. S.: Plastic Arterial Grafts. Charles C. Thomas, Springfield, 111., 1957. 5. Edwards, W. S., and Tapp, J. S.: Chemically treated Nylon tubes as arterial grafts. Surg. 38: 61,

1955. 6. Fries, C. C., and Wesolowski, S. A.: A polyesteroxidized cellulose compound vascular prosthesis: A preliminary report. Trans. Amer. Sm. Art. Int. Or-

January 1968

gans X. 227-230, 1964. 7. Hufnagel, C. A.: Permanent intubation of the thoracic aorta. Arch. Surg. 54: 382-389,1947. 8. Hufnagel, C. A.: Aortic plastic valvular prosthesis.

Bull. Georgetown Univ. Med. Center 4: 128-130,1951. 9. Hufnagel, C. A.: The use of rigid and flexible plastic prostheses for arterial replacement. Surg. 37: 165, 1955. 10. Julian, 0. C., Deterling, R. A., Jr., Su, H. H., Dye, W. S., and Bilio, M. L: Dacron tube and bifurcation arterial prostheses produced to specification. Surg. 41: 50, 1957. 11. Kirk, J. E.: Personal communications. 1W-1965. 12. MacPherson, A. I., and Duthie, R. B.: The healing of vascular grafts: A histological, histochemical and autoradiographic study. 1. Roy. Coll. Surg.

Edinburgh 3 : 98, 1957.

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13. Rosenberg, N., Martinez, A., Sawyer, P. N., Wesolowski, S. A., Postlethwait, R. W., and Dillon, M. L., Jr.: Tanned collagen arterial prostheses of bovine carotid origin in man: Preliminary studies of enzymetreated heterografts. Ann. Surg. 164: 247-256, 1966. 14. Sawyer, P. N.: The effect of various metal interfaces on blood and other living cells. N. Y. Acud. Sciences (in press) 1967. 15. Sawyer, P. N., Wu, K. T., Wesolowski, S. A., Brattain, W. H., and Boddy, P. J.: Long term patency of solid wall vascular prosthesis. Arch. Surg. 91: 735, 1965. 16. Scales, J. T.: Tissue reactions to synthetic materials. Proc. Roy. SOC.Med. 46: 647-652, 1953. 17. Schumacker, H. B., Jr., Harris, E. J., and Siderys, H.: Pliable plastic tubes as aortic substitutes. Surg. 37: 80, 1955. 18. Tuffier, M.: De I’intubation dans Ies plaies des grosses arteres. Bull. Acud. Med. 74: 455-460, 1915. 19. Voorhees, A. B., Jr., Jaretski, A., 111, and Blakemore, A. H.: Use of tubes constructed from Vinyon“N” cloth in bridging arterial defects. Ann. Surg. 135: 332-336, 1952. 20. Wesolowski, S. A.: Evaluation of Tissue and Prosthetic Vascular Grafts. Charles C. Thomas, Springfield, Ill., 1%2. 21. Wesolowski, S. A.: The healing of vascular prostheses. Surg. 57: 319, 1965.

22. Wesolowski, S. A., and Fries, C. C.: Unpublished data, 1%4. 23. Wesolowski, S. A., Fries, C. C., Domingo, R. T., Liebig, W. J., and Sawyer, P. N.: The compound prosthetic vascular graft: A pathologic survey. Surg. 53: 19, 1963. 24. Wesolowski, S. A., Fries, C. C., Hennigar, G., Fox, L. M., Sawyer, P. N., and Sauvage, L. R.: Factors contributing to longterm failures in human I.Curdiovmc. Surg. 45: vascular prosthetic grafts. . 544-567, 1964. 25. Wesolowski, S. A., Fries, C. C., Liebig, W. J., Sawyer, P. N., and Deterling, R. A., Jr.: The synthetic vascular graft: New concepts, new materials. A.M.A. Arch. Surg. 84: 56-72, 1962. 26. Wesolowski, S. A., Fries, C. C., McMahon, J. D., and Martinez, A.: Evaluation of a new vascular prosthesis with optimal specifications. Surg. 59: 40.56, 1966. 27. Wesolowski, S. A., Liebig, W. J., Golaski, W., and Fries, C. C.: A knitted arterial prosthesis with optimal characteristics. Circulation XXVIII, Part 11, “Abstracts” 825, 1963. 28. Wesolowski, S. A., and Sauvage, L. R.: Comparison of fates of Orlon mesh for prosthetic replacement of thoracic aorta and aorta bifurcation. Ann. Surg. 143: 65-72, 1956.

ANA GOES FOR FLEXIBLE BILLING CENTRAL BILLING, INSTALLMENT PLANS AND PAYROLL DEDUCTIONS The flexible membership plan for ANA is in effect in 46 states to date. This means that for new members, membership will run on an anniversary year basis:

New member joins November 1967 December 1967 January 1968

Membership runs to November 1968 December 1968 January 1969

Central billing, a plan providing an automated system of dues processing, now includes about half of the states. An installment plan is now in effect where annual dues can be paid in full or in three installments. Ae far as payroll deduction plans, some nurses have membership dues withheld from their paychecks throughout the year.

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AORN Journal