In vivo biostability of four types of arterial grafts with impervious walls; their haemodynamic and pathological characteristics

In viva biostability of four types of arterial grafts with impervious walls; their haemodynamic and pathological characteristics J.Charara,

G. Beaudoin, C. Fortin, R. Guidoin, P.-E. Roy, A. Marble*, R. Schmittert and R. Paynterl

Departments of Surgery and Pathology, Lava1 University and Biomaterials Unit, St Francois d’Assise Hospital, Quebec, QC, Canada; *Department of Electrical Engineering, Technical University of Nova Scotia and Department of Surgery, Dalhousie University, Halifax, NS, Canada; +HOPF, International Service Technology and Trade, Birsfelden, Switzerland; and *INRS-Energie, CP 1020, Varennes, QC, Canada Received

January

1988, accepted

May

1989

ABSTRACT We describe the haemodynamic and pathological characteristics offour types of impervious arterial prostheses, two alloplastic (Mitrathane@ and Gore- Tex@), and two chemically processed bovine heterografts (Solcograf@ and Solco lx@). They were implanted in the thoracic aortae of dogs for durations of 24 hours, 48 hours, one week, two weeks, one month, three months and six months. Haemodynamic analyses showed no relation between the shear rate index, I;, and compliance, CD. The observed shear rates are 6.5 times lower than those likely to damage the endothelial cell layer. Macroscopic and microscopic observations of explanted grafts showed the presence of obstructive thrombi at the anastomoses of Mitrathane@ grafts as early as one week. Gore-Tex@ grafts develop in the area of anastomoses parietal-thrombi which reorganize and become covered with pseudo-endothelial cells, The bovine heterografts show a similar behaviour. However, whereas Solcograft@ has an irregular thin wall, Solco P@ had improved characteristics except in the graft implantedfor three months which demonstrated some manufacturing weaknesses. Both types showed the development of anastomoticpannus covered with endothelial-like cells. All grafts, whether alloplastic or chemically processed, suffered from an absence of healing of the middle part of the prosthesis. The cause of this problem will be found in the analysis of the biochemical and enzymatic reactations between the material used and its physiological surrounding. Keywords:

Haemodynamic

model,

arterial

prosthesis,

INTRODUCTION Modern vascular surgery started with the implantation of the autologous saphenous vein by Kunlih in 1948’ and of homologous arteries by Oudot in 195 I* for revascularization after arterial occlusion, aneurysms and trauma3. The autologous vein graft rapidly became the substitute of choice for small calibre vessels and is still the accepted standard4. It is, however, not always available and is unsuited to large diameters. Arterial homografts degenerate soon after implantation and lead to erratic results; they have almost been abandoned despite some interesting features5. Porous textile grafts are used mainly for surgical replacement vessels of 8 mm diameter or larger‘j. This concept was introduced by Voorhees et al. in 19527. Edwards et al. promoted Teflon textiles’, whereas DeBakeyg and Wesolowski et a1.” preferred polyesters (Dacron@). Today Teflon textiles have been abandoned” but polyesters are still popular for medium and large diameters’*. They are stable and their postoperative development corresponds with the reorganization of the blood thrombotic matrix which fills the graft interstices during preclotting. The luminal surface always remains more Correspondence Unit.

FI-304,

Qurher

and reprint St Franqois

City, GIL

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to: Dr R. Guidoin, Hospital,

Eng.

& Co (Publishrrs) I3 $03.00 1989, Vol.

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3L5, Canada

ii:) 1989 Buttcrworth 0141-5425/89/05041fG 416

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Ltd

1 I, Srptt~mbrr

Strwt,

compliance,

biostability,

histologic

evaluation

thrombogenic than a normal artery since complete healing is never achieved13 and the blood flow must exceed the thrombotic threshold velocity’4. In diameters smaller than 8 mm, when no autologous vein is available, impervious, haemocompatible, stable grafts are preferred. Ideally, in vivo, the luminal surface should be inert with no thrombotic accumulation’5. The wide range of available types either alloplastic (microporous Teflon and polyurethanes) IsI7 or chemically processed biological (bovine heterografts, human umbilical veins) ‘sJg proves that the ideal substitute does not yet exist. The most important factors for the production of satisfactory arterial graft have not yet been identified. It is a multifaceted challenge involving mechanical, haemodynamic and pathological influences. In order to contribute to the understanding of these factors, we present a comparative study of four types of impervious grafts of 8 mm diameter, two microporous synthetics (polyurethane and PTFE)*O,*’ and two chemically processed biological grafts (bovine heterografts)22J3. Implanted as thoracic aorta substitutes in dogs, their physical, haemodynamic and pathological characteristics”4 were observed for pre-established periods between 24 hours and six months. The physical and haemodynamic characteristics were recorded immediately prior to graft removal: they included pressure at

several sites, the external diameter and the mean blood flow. This enabled us to calculate axial and radial velocity, shear stress at the vessel wall, compliance, longitudinal and circumferential distensions, internal diameter, and wall thickness. The rheological parameters used in this study are taken from a haemodynamic model. The use of a haemodynamic model to investigate the cardiovascular system is of particular interest. A suitable model is one which includes the essential characteristics of the real system by way of parameters of physical significance. The haemodynamic model is based on local resolution of the non-linear equations of‘ ,Navier--Stokes by the numerical scheme of Crank-Nicholson, chosen for its unconditional stability”. After removal, the grafts were analysed morphologically using a binocular scope and histologically using light and scanning electron microscopy. The subject of the study is considerable practical importance since in the province of Quebec, population seven million, the number of peripheral vascular interventions have reached more than 6000 per year (according to Rkgie de l’Assurance-Maladie du Qut?bec) of which over a third are reoperations. The number of these cases requiring grafts of more than 8 mm diameter (for which polyester grafts are acceptable) is 2500-3000. The reminder of the cases, ! 3000-3500) require impervious tubes of smaller calibre when no autologous vein is available which is the case in at least 30 per cent of the patients. For the graft to have a longer durability than the life expectancy ofthe patient, it would have to behave like a healthy artery. The challenge is considerable fhr diameters of 6 mm and above, but is even greater Li)r vessels ofsmaller calibre. In addition, due to more sophisticated non-invasive methods ofvascular screening, evidence of occlusive disease is observed in younger patients. Furthermore, the disease spreads to several sites and is progressive; by-pass surgery does not stop it. This kind ofsurgery is very gratifying in that, if it is successful, it enables many patients to maintain their mobility and resume their occupation, even if it cannot cure the disease itself’(j. The best performing grafts must be identified in order to prevent the evolving complications caused by the prosthesis. The deterioration of the patient’s physical condition due to progressing atherosclerosis by itself can bring about enough evolving complications. GRAFT

SELECTION

Four types ofgraft, each with a wall that is impervious under normal pressure, were selected: two synthetic models and two chemically processed biological grafts. Synthetic,

microporous

prostheses

Mitruthane~K’. This is a hydrophilic polyurethane graft of 8 mm diameter, manufactured by Mitral Medical International, Wheat Ridge, Colorado, USA, sterilized by irradiation and stored in physiological saline (9 g/l, pH 7). It is produced by a phase separation technique in order to obtain a microporous structure.

The hydrophilic properties of the graft are improved with the use of a hydrophilic solvent in the storage medium”. Reinforced Gore-Tex@. The structure of the microporous polytetrafluoroethylene consists of Teflon nodes connected by microfibrils. It is manufactured and distributed by WL Gore and Associates, Flagstaff, Arizona, USA, and is supplied in a dry and sterile form; sterilization is by ethylene oxide2s.

Chemically

processed

biological

grafts

Solcogrqj?‘“.The bovine heterograft is prepared from carotid arteries of calves by Solco Basle Ltd, Birsfelden, Switzerland, according to the slightly modified Rosenberg method. After the anatomical preparation, a ficin treatment eliminates the muscle and elastic fibres. The hydrolysis is stopped by the addition of sodium chloride, and crosslinking is obtained by continuous flow treatment with dialdehyde starch. The graft is sterilized in 1 per cent propylene oxide, rinsed and stored in a balanced salt solution’“. Solco PH.‘. The graft has the same origin and manufacturer as Solcograft’K’. It is manufactured using a completely different chemical method which consists of dehydrating the carotid artery with tetrahydrofuran and crosslinking (via peptide bonds) with adipyl dichloride. Sterilization is by ethylene oxide. The resulting graft has the same wall thickness and its longitudinal and circumferential elasticity is only slightly inferior to that of the original artery”“.

IMPLANTATION

OF GRAFTS

Adults dogs of both sexes, weighing between 15 and 26 kg, were selected and conditioned according to the Canadian Council on Animal Care. The grafts were implanted as thoracic aorta substitutes for periods of 24 and 48 hours, one and two weeks, and one, three and six months, with one animal for each period and graft type3’. The dogs were fasted for 24 hours prior to the operation. They were anesthetized by intravenous administration of 32 mg/kg sodium pentobarbital (Somnotol’w, MTC Standard, MTC Pharmaceuticals, Mississauga, Ontario). Ketamine hydrochloride Rogar STB Inc, Montreal) (10 mg/kg) (Ketalar”‘, was administered as a supplement when needed. The dogs were mechanically ventilated. After shaving, washing and disinfection, the thorax was opened and the thoracic aorta dissected. Anticoagulation was achieved with 0.5 ml/kg heparin (sodium-heparin 1000 USP U/ml, Allen & Hanburys, Toronto). In addition, at least 500 ml of solution of NaCl was infused to compensate for dehydration. A 5 cm segment of each type of graft was implanted into the thoracic aorta of each dog by the same surgeon. DeBakey clamps were used and end-to-end anastomoses were created by continuous suture using polypropylene monofilament (Prolene@, Ethicon Sutures, Peterborough, Ontario).

.J. Biomed.

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1989. \‘ol.

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In viva bioslability of arlerial grafts:J. Chnrara et al.

After the operation the dogs were returned to their cage and fed a regular diet. For the first five postoperative days, the dogs were given IM antibioinjectable, Schering Canada tics daily (Garamycin@ Inc., Pointe Claire, QC).

the graft. The probe was calibrated in vitro using a canine aorta, held in an experimental set-up, through which was passed heparinized blood flow at a known flow and at 37°C. Numeric

PHYSICAL AND HAEMODYNAMIC MEASUREMENTS Collection

of data

At the end of the pre-established period, each dog was again anesthetized with Semnotol@, immobilized on the operation table equipped with a heating blanket and maintained on a respirator. The thorax was widely opened in order to facilitate access to the graft and the placement of the probes. The implant and the adjacent segments of the thoracic aorta were dissected. The dog was then heparinized. The following measurements were taken on the aorta distal to the graft, at the distal anastomosis, at the centre of the graft and at the proximal anastomosis (Figure I). Pressure. Two pressures (P, and P2) were measured 3 cm apart on either side of the site under consideration using a Millar catheter (model PC-76 1, Millar Instruments Inc., Houston, Texas,) with double pressure reading. These measurements give the pressure gradient as well as the pressure at the graft under test. The catheter was introduced into the thoracic aorta via the femoral artery to the diaphragm level (Figure I). The measurement sites, marked on the catheter so that it could be displaced without handling the vessel, were the distal aorta, the distal anastomosis, the centre of the prosthesis and the proximal aorta. External diameter. The measurements were carried out after removal of the external capsule by means of an extensometer developed in our laboratory and adapted from the models of Murgos2 and Baird33. Blood Jaw . Flow was measured with an electromagnetic flowmeter (Gould Statham, model SP 2202, Gould Measurement Systems Division, Oxerard, California) placing the probe proximal to

data processing

Data processing was carried out on an microcomputer HP 9000 model 2 16. The physiological signals needed for the calculation were recorded by means of a data input interface (DIGI, D.E.C.) operating at 70 Hz or twice the highest frequency in the signals. Axial and radial velocity. Modelling and instrumentation make it possible to obtam axial and radial velocity profiles of the pulsatile blood flow. Numerical integration of the axial velocity yields the instantaneous flow which can then be compared withthe measured flow . Thus, the difference between the model and the measurement can be quantifiedby the normalized square error*. The haemodynamic and mechanical parameters obtained directly or by deduction from these measurements are discussed below. Shear. Parietal

shear stress is defined as:

(1) where G is the shear stress in dynes/cm’, 9 is the viscosity in poise+, w is the axial velocity in cm/s, r is the radial position and R = radius to the wall in cm. 6w/&, usually listed as Q, is the shear rate in s-‘. In order to compensate for the differences of shear rate due to flow and geometry between the studied cases, a normalized shear rate index is defined as:

(2) where WaVgis the average axial velocity during the cardiac cycle and Ravg the average radius. Since WaVg= Q,,,/nR&( Poiseuille’s law), this index can then be wrttten as: 3 - a”%

6W -

T;=Tl;

(3)

avg ( 6r > r=R

where cycle.

Qavgis

*The normalized

NSE = [j,

Figure

418

1

Sites of measurements

J. Biomed.

Eng. 1989, Vol. 1 I, September

the average

square

(Q,(k)

flow during

error (NSE) is defined

- 9,(+,

Q:.(k)]

the cardiac

by the formula:

x 100

where C&,(k) is the measured flow and Q,(k) is the calculated instantaneous flow expressed in ml/s. NSE is expressed in per cent. +The viscosity was measured at 37” with a capillary viscometer, considering the blood as a Newtonian fluid34.

Compliance.

Dynamic

compliance

is defined as:

where the compliance Cn is expressed in per cent per mmHg ‘. AR is the variation of the internal radius in proportion to the variation of pressure AP. Distension.

Longitudinal

distension

is defined as:

where I, is the length of two reference points observed in vivo and L, that same distance measured after removal of the graft. Circumferential distension is determined from:

AZ=; I)

where R is the internal radius in vivo and R. the same radius at zero pressure and without circumferential or longitudinal extension. Internal radius. The internal radius, R, is calculated by subtracting the thickness of the graft wall from the external radius in vivo R,, using the formula:

where H is the wall thickness in viuo, HO is the wall thickness at zero distension, as measured with a micrometer model CS-49-055 (Custom Scientific Instrument Inc., Whippany, NJ) after removal of the graft with the adjacent arterial segments. COLLECTION

OF GRAFTS

After completion of the haemodynamic measurements, the prostheses were excised, including 1 cm of the host arteries. They were rinsed in isotonic buffer and the wall thickness measured. The grafts were cut, opened longitudinally and fixed in isotonic buffer containing 2 per cent glutaraldehyde for further examination. ANALYSIS

OF THE GRAFTS

Macroscopy The grafts were first examined for gross morphology and photographed by a special designed system essova+, Zeiss, Oberkochen, West Germany) with (T magnifications of 0.4 to 12.8. Representative samples ofvarious segments were selected for light microscopy and scanning electron microscopy (SEM). Histology Light microscopy. The samples were immersed buffered 20 per cent formaldehyde and divided

in in

two parts. The first part was embedded in paraffin and the sections were stained with haematoxylin and eosin and the Weigert preparation for elastin. The second part was embedded in Epon 821 (Ladd Research Industries Inc., Burlington, Vermont). The method consists of rinsing in sodium cacodylate containing 11.15 per cent (0.33 M) sucrose and 0.025 per cent calcium chloride (CaC 1, * 2H,O), pH 7.4; fixation in 1 per cent osmium tetroxide in cacodylate buffer 0.5 M, pH 7.1; rinsing in cacodylate buffer; and dehydration in increasing concentrations of ethanol and acetone. After embedding, semi-fine cuts were prepared and stained with toluidine blue. Scanning electron microscopy. Representative segments were selected and fixed in a 1 per cent TCH (thiocarbohydrazide) solution and osmium tetroxide. They were rinsed five times in distilled water and dehydrated in solutions of ethanol of concentrations increasing to 100 per cent and then in absolute acetone. Final drying was by critical point drying using liquid carbon dioxide as transfer medium. The samples were gold-palladium coated under vacuum using a model type Hummer V (Technics, Alexandria, Virginia) before being examined at acceleration voltages of 15-25 kV in a Jeol 35CF (Soquelec, Montreal, QC).

RESULTS The surgical aspect presented no difficulties, all grafts were easy to handle and to suture. All anastomoses, both proximal and distal, were sutured in about the same time ( 18 min f 4 min), regardless of the type of graft. Five dogs died in the postoperative period. A sixth dog was used in the immediate postoperative period for the calibration of the measuring techniques. Therefore, we operated on 34 dogs in order to have one dog for each type of graft for each pre-established period of observation, i.e. 24 hours, 48 hours, one week, two weeks, and one, three and six months.

Physical

and haemodynamic

measurements

The proposed model allows a complete representation of the macroscopic rheological properties of blood flow in arteries34. Shear. The average parietal shear stress at the centre of the graft, for all prostheses studied was 60.9 f 23.3 dynes/cm’. As for the correlations between the shear rate (p ) or the shear rate index (Z;) and the other parameters, we only considered the cases where the NSE between the measured and calculated flow was less than 2.5 percent. In these cases, we found no correlation between the shear rate index, particularly its maximum reading, and the graft compliance (r = 0.2). Based on this data, we established a direct correlation between the maximum parietal shear 0, derived from the stress, omax, and a parameter, measured blood viscosity q, the maximum flow Qm,, and the average radius of the vessel Ravg, according

J. Biomed. Eng. 1989, Vol. II, September

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In viva biostability of arterial grafts: .I. Charara et al.

to the equation:

where 8 is expressed in dynes/cm’. Figure 2 shows that the real value of the maximum parietal shear stress can be obtained by the simple equation crmaX= 30 + 7, with a correlation coefficient of r = 0.94. This correlation is of particular interest when non-invasive methods are used to measure Q,, and Ravg.

50

25

10

20

40

30

cT(DYNE/CM~)

Figure 2

Linear correlation between shear stress and 5, parameter derived maximum flow and the average radius

the maximum pa&al from the viscosity, the

Compliance. The longitudinal variation in compliance (according to site) is shown in Figure 3. Note that the compliance was lower than that of arteries which was always above 20 x 100 x mmHg‘. At the aortic junctions, Solco Ps and Mitrathanem were more compliant than the other models. Gore-Tex@, Solcograft@ and Solco P0 showed similar behaviour at both ends, Mitrathane@ showed an increased compliance at the proximal site. A local decrease in compliance was observed in the middle of the graft for the four types. The compliance of Mitrathanea was essentially equal to that of Gore-Tex@ and Solcograft@, whereas Solco P@ was more compliant. The evolution of the compliance of the host artery occurs in two ways depending on the type of graft

AORTA

B C D

DISTAL

M:

MITRATHANER

G:

GORE-TEXR

OF THE GRAFT

S:

SOLCOGRAFTR

ANASTOMOSIS

SP:

ANASTOMOSIS

CENTER PROXIMAL

SOLCO

PR

T

1

M -

EdI

;P -

M

in compliance

according

anastomosis

420

J. Biomed. Eng. 1989, Vol. 11, September

to site: a, aorta;

S

SP

MGS i

C

B Figure 3. Variation

G

b, distal

anastomosis;

c,

mid-graft

segment;

d, proximal

SP _-

In viva biostability of arterial gmfis: J. Charara et al.

(Figure 4). All the graft types except Mitrathane showed a decrease in compliance in the days following implantation and thereafter showed an increase returning to the initial level after about two weeks. For Mitrathane, however, the aorta joining the graft showed evidence of erratic behaviour probably related to aortic lesions observed in two cases.

;

MITRATHANE

I

.

I

I

0.1

10

Time (WWIS)

Figure 4 Evolution implantation

ofhost artery compliance

A, : T x1 :

LONGITUDINAL CIRCUMFERENTIAL DISTENSION

versusduration of

T:

Aorta

M:

MltrathaneR

G:

Gore-texR

s:

solcogroftR

T

SP: solco

1.2 5-

PR

P

1.o-

T

0.7

SOLCO

PR

-

LO

Time(week)

Figure 6 Variation implantation

in

wall

thickness

with

duration

of

vessel. The capsule covered the entire graft. The increase in wall thickness of the Solcograft@ was erratic. In one instance, it was 3.5 times the original size. For Solco Pm, the wall seemed to reach its limit at 1.33 times the original size as early as the first week implantation, and did not change afterwards.

examination

of removed

grafts

The healing process is illustrated in Figures 7A and 7B. The microscopic observations are shown in Figures 8 to 11. The external encapsulation of the grafts was unpredictable and varied with time. Their stabilities depended on the formation of a neo-adventitia consisting of fibroblastic collagen. The graft wall was permeated by cellular elements which did not reorganize. The luminal surface showed a varied morphology depending on the graft type and on the post implant period. After 24 hours of implantation, few cells appear on the middle part of the luminal surface of the Solco P@ graft. The other g rafts showed an accumulation of thrombotic material at anastomosis which consisted of fibrin entrapping numerous blood cells. The rest of the luminal surface was free of accumulation and showed only isolated cellular elements. At 48 hours, most of the wall thrombi were lysed, due to the fibrinolytic activity, and a few blood elements remained on the surface. After one week, the wall deposits persisted only at the anastomoses; they contained cellular elements immersed in a fibrin network, while the Mitrathane@ graft showed the presence of obstructive thrombi at the anastomoses and began to degrade. By this time, newly formed cells appeared on the pannus of the Solco P@ graft. After two weeks, each anastomosis showed a pannus. They were covered by a thin layer of cells which desquamated a little. The wall and the internal surface of the Mitrathane@ graft showed yellow spots of bilirubin retention and signs of damage to the integrity of the material. Between one and three months, healing progressed further and extended over a few millimetres to the central part, while the Solco P@ graft demonstrated some weaknesses and the Mitrathanem graft showed

Tr T

5 2

0

T

_dO

0

SOLCOGRAFTR

/._+/f--j-----j----‘--

,‘9

c

’

0

Morphological

.

‘F\,

._ u)

.2-

MITRATHANER

0 GORE-TEX

.

.

1. 5-

-

1

_OTHER

25

.05

Wall thickness in vivo. The results are shown in Figure 6. The Mitrathane@ graft showed an unpredictable encapsulation with time; however, the capsules adhered poorly to the graft and were limited to the anastomotic areas. In the case of the Gore-Tex@ graft, the encapsulation was gradual. At three months, the wall was twice as thick as the original

0

.

Distension. Distension (Figure 5) was independent of the duration of implantation. The longitudinal as well as the circumferential distension measured on all grafts were very inferior to that measured on the aorta.

.

.I

I

l-L G SSF

II

x2

Figure 5 Comparison of distentions of the host artery and the four types ofarterial grafts. The distention was independent of the duration of implantation

J. Biamed. Eng. 1989, Vol. 1 I, September

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In viva bioskzbility of arterial grafts: J. Chamra IA al.

an obstructive thrombus. By six months, the pannus had spread over almost the entire surface by comparison with the previous grafts. There were numerous surface cells, but they detached easily. The degradation of the Mitrathane@ graft was more pronounced. DISCUSSION In vivo studies conducted over many years suggest that haemodynamic forces influence arterial wall physiology and pathology in a number of ways35. In systemic and pulmonary circulation, vascular endothelium cells comprise the interface between flowing blood and the vessel wall and are subjected to many haemodynamic factors, a major component of which is wall shear stress. It has been proposed that extremes of shear stress, either high3’ or 10w~~, can contribute to vessel wall pathology. This particular haemodynamic factor (shear stress) has been implicated also in the atherosclerotic process because a strong correlation exists between the location of developing arterial lesions, and regions where large variations in wall shear stress occur. Endothelial cell geometry and orientation appear to be determined by haemodynamic-related events. Disruption of endothelial cell alignment has been observed in areas where flow separation is expected to occur3’, and such areas appear to be more susceptible to endothelial damage, intimal cell proliferation and the development of atherosclerotic lesions4’. Sites of hig hest shear stress are located near the flow dividers. These in vivo observations suggest, therefore, that both time average magnitude of wall shear stresses, as well as temporal variations in magnitude and direction may modify endothelial function. Mismatch in mechanical properties (compliance mismatch) between host and prosthetic graft has also been suggested as a cause of graft failure4’,42,43, but no mechanism linking the two has been identified. Decreased compliance has been implicated as one of the causes of long term occlusion in small internal diameter vascular replacements44. Abbot and Bouchiers-Hayes have demonstrated a significantly increased thrombosis of air dried, banded femoral arterial segments (70 per cent) as compared to air dried, normally compliant segments (10 per cent). Lyman et al. 46 have confirmed this observation using copolyetherurethane grafts with various expansible properties. They demonstrated that grafts which closely match the elasticity of native artery had the highest patency rate at one month following implantation. The method used in this first general study allows the estimation of the perietal shear stress, with a good precision, by the simple derivation of the axial velocity profile. This method seems to be easy to set up, in spite of five failures out of 34 cases, which is unusual. The dogs recover well after this surgical intervention. Haemodynamic measurements and the pathological tests follow the classical standards for the evaluation of arterial grafts. We have not been able to establish a correlation between the shear rate index I; and compliance Cn.

422

J. Biomed. Eng. 1989, Vol. 1 I, September

The longitudinal profiles of compliance revealed that compliance increases at anastomoses. Our results are in good agreement with those of Hasson et aL4’. They used a simplified model based on isocompliant arterial grafts, pulsed ultrasound was used to generate detailed longitudinal profiles of compliance near the anastomoses. Although arterial diameter decreases monotonically to a minimum at an anastomosis, compliance first increases by approximately 50 per cent before decreasing to 60 per cent ofthe control value. A para-anastomotic hypercompliant zone was found to be centred 3.6 cm from the anastomosis. If the long term success of such arterial implants depends on the compliance, the mechanism for the improvement of the healing process is independent of the shear rate values at the boundaries ofour cases. The maximum shear stress was established at 60.9 dynes/cm2 with a standard deviation of 23.3 among the implants, thus always inferior to the 400 dynes/cm2 considered to be the critical level for inflicting damage to endothelial cells48~4g. Furthermore, it is independent of the type of implant and the duration of the implantation. Therefore, the absence of endothelial-like cells in the centre of arterial grafts cannot be due to excessive shear stress. More likely, the explanation has to be sought in the analysis of the biochemical and enzymatic relationship between the implanted material and its physiological environment. The preceeding analyses of the shear rate index 1, or the shear stress rr are all general and the conclusions independent of the graft studied or of the duration of implantation, as long as the rafts were patent. The microporous Gore-Tex 8 Teflon grafts, the bovine heterografts Solcograft@ and Solco P@ graft heal adequately and remain generally patent for an duration of implantation, whereas the Mitrathane (8 grafts evolve in a more unpredictable fashion and confirm earlier results obtained at the level of the abdominal aorta and carotid in dogs50,5’. The development of very obstructive thrombi at the proximal anastomosis in the Mitrathane@ graft at one week, one month and six months, revealed by the macroscopic observations, seems alarming. The same phenomenon is observed in the bovine heterografts at 24 hours and after one month. However, by limiting the study to one dog for each type of graft for each period, we must consider reactions of the individual dog and the specificity of the material. The occurrence ofocclusive thrombi at different times of implantation suggests alternation of the fibrinolytic and thrombogenic phases. After one week, the Gore-Tex@ grafts show a pannus, whereas the Mitrathane@ grafts display it after two weeks and only at the proximal anastomosis. The Mitrathane@ shows an inferior healing process which may be explained mainly by the progressive degradation of the material beginning as early as the first week. At six months, the bovine heterografts show the thickest and most extended pannus. The microscopic details of the grafts at the proximal anastomosis demonstrated that the Mitrathane@ graft shows the fewest cellular elements, regardless of the length of time of the implant. As

In viva biostability

a

qf art&d grafts: J. Charara et al.

MICROPORUSSYNTHETIC GRAFTS h

r

l_.] r-1

prosthesis

m

r$,:tood

artery

I]

:;!:I’,’ blo0di-jTG-5)

MITRATHANE

F+-]

platelets

m]

surface cells

fibrin

[BTl

desquamation

8

GORE-TEX

@

a-

Figure 7 biological

Healing grafts

process

of the four

types

of arterial

early as the second week, its synthetic structure seems to favour the occurrence of yellow plaques which could be interpreted as bilirubin deposits caused by hemolysed red blood cells. The Gore-Tex@ graft contains more blood cells, particularly at 24 and 48 hours and after two weeks, whereas fibrinolysis is highest at one week. This is probably also due to

grafts:

a, microporous

synthetic

grafts;

b, chemically

treated

properties of the material. The bovine heterografts have a similar behaviour regarding the retention of blood cells except for the Solco P@ graft implanted for 48 hours where the clots are not lysed. Surface cells appeared early after 24 hours in the Solco P@ graft. By two weeks, surface cells are present in all four materials. In the bovine heterografts, the

J. Biomed.

Eng. 1989, Vol. II, September

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In viva bidability

of arterial grafts: J. Charara et al.

CHEMICALLYPROCESSED BIOLOGICALGRAFTS1 3

5

m]

prosthesis lzi

“&y,itood

v]

artery

$1:

1x1

-1

“ood~l

platelets

[ml

surface Cells

fibrin

[-I

desquamatiol n -

SOLCOGRAFT

@

SOLCO P 0

>

) ,

1 1 ,

, I

Figure

7

continued

cells cover a larger area during the first and sixth month for Solcograft@ and during the third month for Solco P@ graft. In spite of the extent of the surface cell coverage at two weeks, Mitrathane@ does not reach a quantity of cells at their full development comparable to that of bovine heterografts. Newly formed cells which are limited to the anastomosis,

424

J. Biomed. Eng. 1989, Vol.

11,September

show the smallest spread in the Gore-Tex@ grafts. Solcograft@ manifested an uneven wall thickness at 24 hours. We can surmise that the proteolytic action of the ficin was interrupted and/or that the crosslinking process with dialdehyde starch was irregular. In our opinion, such an uneven wall thickness could bring about long term complications

In viva biostability

Figure 8 SEM photomicrographs showing the healing sequence of the MitrathaneB graft: A, the thrombotic deposits resulting from the accumulation of blood cells were very abundant and organized after 24 hours ( x 2000); B, the graft structure begins to degrade after one week ( x 2000) ; C, After three months the endothelial-like cells have extended from the anastomosis, but were not sufficient to completely cover the luminal surface ( x 780) ; D, After six months the endothelial-like cells were well developed on the pannus along their anastomoses ( x 200)

of arterial grafts:

J. Charara et al.

Figure 9 SEM photomicrographs showing the healing sequence of the Gore-tex@ graft: A, after 48 hours the graft structure was exposed, although few blood elements remained on the surface ( x 720); B, after two weeks isolated cells were observed below the anastomoses ( x 720); C, after one month cellular growth had progressed and the cell layer had acquired a thin fibrin network ( x 480) ; D, after six months a layer of fibroblastic collagen has covered the entire surface of the graft ( x 720) J. Biomed. Eng. 1989, Vol.

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lg the healing s’ A, af’ter 48 hours the thrombotic quence of the Solcograft@: matrix had disappeared due to the fibrinolytic mechanism except for a few red cells over a thin and condensed fibrin network ( x 720) ; B, the first endothelial-like cells appeared after two weeks but were limited to the vicinity of the anastomoses ( x 720) ; C, the pannus growth was extended after three months ( x 720) ; this pannus was more covered by the endothelial-like cells after six months ( x 200) Figure 10

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Figure 11 SEM photomicrographs showing the healing sequence of the Solco F’@. A, few cells were developed in the middle part of the surface after 24 hours ( x 2000); B, more cells were observed at the anastomoses by one week ( x 720) ; C, after three months the endothelial-like cells have extended over a few millimetres from the anastomosis ( x 720); D, but they did not cover the entire surface which was well covered by a thin layer of fibroblastic collagen ( x 200)

In viva biostabilityof’ arterial grafts: J. C’horaroet ot.

since the encapsulation observed after removal was variable as well. Also, in the middle segment of the Solco P@ implanted for three months, we have seen depressions illustrating the difficulty encountered with quality control. However, some clinical failures have led Solco Basle Ltd voluntarily to suspend clinical application for the time being *.

CONCLUSION The conclusions based on characteristics are as follows. 1.

2.

3.

4.

the

haemodynamic

For similar flow conditions (voiume and velocity), parietal shear stress is only slightly different from that in the aorta; thus, all conditions being equal, shear rate is independent of compliance. In some other studies success in vascular surgery was related to compliance. The explanation for this will not be found in the shear mechanism. At no time did we observe critical shear stress in implanted grafts. Thus, the absence of endothelial cells in the middle segment of the graft is not a consequence of too strong tangential forces, but rather due to the nature of the material and the state and characteristics of its surface. Using non-invasive measurements of radius and arterial flow, it is possible to calculate the maximum shear stress, with good precision, using the equation:

The problematic behaviour of the Mitrathane@ graft appears to be related to its hydrophilic and microporous surface, since its in vivo mechanical behaviour (distension and compliance) differs little from that of Gore-Tex@.

ACKNOWLEDGEMENTS This study was supported by the MRC of Canada (Grants MA 9429 and MT 7879). The prostheses were kindly provided by WL Gore & Associates, Mitral Medicals and Solco Basle Ltd, sutures by by Alien and Hanburys and Ethicon, heparin antibiotics by Schering. We cordially thank J. Bastien, K. Horth, D. Martel, L. Martin, N. Massicotte, G. Mongrain and B. Napert for their valuable technical assistance. We are indebted to Drs C. Gosselin, J.C. Forest, G. Roy and J. Couture for help and guidance.

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