Comparative analysis of rupture resistance between glutaraldehyde-treated bovine pericardium and great saphenous vein

Abstracts

BACKGROUND: Carotid endarterectomy using bovine pericardium is an acceptable alternative to great saphenous vein patch. Bovine pericardium is easily obtained and provides a shorter operative time and lower rupture rate. OBJECTIVE: To evaluate rupture resistance of glutaraldehyde-treated bovine pericardium patch in comparison with great saphenous vein patch. METHODS: The sample was divided into two groups: bovine pericardium patch (group I, n = 20) and great saphenous vein patch (group II, n = 20). Both bovine pericardium and saphenous vein patches were prepared in the same dimensions (50 mm x 5 mm) and tested using standard procedures. The patches were tested in the longitudinal axis until the point of material failure. The following parameters were addressed: failure force, ultimate force and failure stress. Statistical analysis was conducted using the Student t test and Pearson's linear correlation. RESULTS: Failure force and ultimate force parameters were significantly higher in the bovine pericardium patch group: 1.97 vs. 1.36 kgf (p = 0.001230) and 2.27 vs. 1.51 kgf (p = 0.0001087), respectively. Mean failure stress in the bovine pericardium patch group was also significantly higher than that in the great saphenous vein group (193.99±43.05 vs. 49.19±22.96 kgf/cm², p = 7.603e-16). The correlation between thickness and failure force was considered moderate (r = 0.5032993) for the bovine pericardium group and low (r = 0.3062166) for the great saphenous vein group. CONCLUSION: The failure stress related to the bovine pericardium group was considered appropriate in this study, and was significantly higher than that observed in the great saphenous vein group. In addition, patch thickness in both groups did not show a good correlation with rupture resistance.

Pericardium; saphenous vein; carotid endarterectomy

CONTEXTO: O uso do pericárdio bovino como remendo na endarterectomia de carótida é uma alternativa à veia safena magna. As vantagens do pericárdio incluem facilidade de obtenção, menor tempo operatório e principalmente menor índice de ruptura. OBJETIVO: Avaliar a resistência tensional do pericárdio bovino tratado com glutaraldeído e compará-la com a da veia safena magna. MÉTODOS: Os remendos de pericárdio bovino (grupo I, n = 20) e de veia safena magna (grupo II, n = 20) foram recortados em dimensões iguais (50 x 5 mm) e preparados de modo habitual a sua utilização. Os grupos foram submetidos a ensaio de tração e comparados em relação a força de ruptura, força máxima e tensão de ruptura utilizando-se o teste t de Student. A correlação da espessura do remendo com a força de ruptura também foi analisada utilizando-se o coeficiente de correlação linear de Pearson. RESULTADOS: Os parâmetros força de ruptura e força máxima foram significativamente maiores no grupo dos remendos de pericárdio bovino: 1,97 versus 1,36 kgf (p = 0,001230) e 2,27 versus 1,51 kgf (p = 0,0001087), respectivamente. A tensão de ruptura média para o material pericárdio bovino também foi maior (193,99±43,05 versus 49,19±22,96 kgf/cm², p = 7,603e-16) do que na veia safena. A correlação entre a espessura e a força de ruptura foi considerada moderada (r = 0,5032993) para o pericárdio bovino e baixa (r = 0,3062166) para o grupo da veia safena. CONCLUSÃO: Os autores concluem que a resistência do pericárdio bovino à ruptura foi considerada adequada neste estudo, e é significativamente maior que a da veia safena magna, retirada da região da coxa. Além disso, a espessura do remendo em ambos os grupos não apresenta boa correlação com sua resistência a ruptura.

Pericárdio; veia safena; endarterectomia de carótida

ORIGINAL ARTICLE

Comparative analysis of rupture resistance between glutaraldehyde-treated bovine pericardium and great saphenous vein

Marcio Miyamotto^I; Carlos Eduardo Del Valle^II; Ricardo Cesar Rocha Moreira^III; Jorge R. Ribas Timi^IV

^ICirurgião vascular e endovascular, Serviço de Cirurgia Vascular Prof. Dr. Elias Abrão, Hospital Nossa Senhora das Graças (HNSG) e Hospital Universitário Cajurú (HUC), Pontifícia Universidade Católica do Paraná (PUCPR), Curitiba, PR, Brazil. Mestrado, Clínica Cirúrgica, Universidade Federal do Paraná (UFPR), Curitiba, PR, Brazil. Área de atuação em Angiorradiologia e Cirurgia Endovascular, SBACV e Colégio Brasileiro de Radiologia (CBR). Diretor de publicações, SBACV - Regional Paraná

^IICirurgião vascular, Serviço de Cirurgia Vascular Prof. Dr. Elias Abrão, HNSG e HUC, PUCPR, Curitiba, PR, Brazil. Mestrado, Clínica Cirúrgica, UFPR, Curitiba, PR, Brazil

^IIIDoutor, Clínica Cirúrgica, UFPR, Curitiba, PR, Brazil. Chefe, Serviço de Cirurgia Vascular Prof. Dr. Elias Abrão, Curitiba, PR, Brazil. Área de atuação em Angiorradiologia e Cirurgia Endovascular, SBACV e CBR. Membro titular, SBACV

^IVCirurgião vascular e endovascular, Núcleo Integrado de Cirurgia Endovascular do Paraná (NICEP), Curitiba, PR, Brazil. Chefe, Departamento de Cirurgia Vascular, Hospital das Clínicas, UFPR, Curitiba, PR, Brazil. Doutor, Clínica Cirúrgica, UFPR, Curitiba, PR, Brazil. Área de atuação em Angiorradiologia e Cirurgia Endovascular, SBACV e CBR. Membro titular, SBACV

Correspondence

ABSTRACT

Background: Carotid endarterectomy using bovine pericardium is an acceptable alternative to great saphenous vein patch. Bovine pericardium is easily obtained and provides a shorter operative time and lower rupture rate.

Objective: To evaluate rupture resistance of glutaraldehyde-treated bovine pericardium patch in comparison with great saphenous vein patch.

Methods: The sample was divided into two groups: bovine pericardium patch (group I, n = 20) and great saphenous vein patch (group II, n = 20). Both bovine pericardium and saphenous vein patches were prepared in the same dimensions (50 mm x 5 mm) and tested using standard procedures. The patches were tested in the longitudinal axis until the point of material failure. The following parameters were addressed: failure force, ultimate force and failure stress. Statistical analysis was conducted using the Student t test and Pearson's linear correlation.

Results: Failure force and ultimate force parameters were significantly higher in the bovine pericardium patch group: 1.97 vs. 1.36 kgf (p = 0.001230) and 2.27 vs. 1.51 kgf (p = 0.0001087), respectively. Mean failure stress in the bovine pericardium patch group was also significantly higher than that in the great saphenous vein group (193.99±43.05 vs. 49.19±22.96 kgf/cm², p = 7.603e^-16). The correlation between thickness and failure force was considered moderate (r = 0.5032993) for the bovine pericardium group and low (r = 0.3062166) for the great saphenous vein group.

Conclusion: The failure stress related to the bovine pericardium group was considered appropriate in this study, and was significantly higher than that observed in the great saphenous vein group. In addition, patch thickness in both groups did not show a good correlation with rupture resistance.

Keywords: Pericardium, saphenous vein, carotid endarterectomy.

Introduction

The use of vein patch closure after carotid artery endarterectomy has reduced the occurrence of perioperative complications, such as thrombosis and ischemic neurologic events, as well as late complications such as restenosis.^1,2 Patch materials most commonly used include great saphenous vein, polytetrafluoroethylene, polyethylene terephthalate (polyester) (Dacron^®), and bovine pericardium. Great saphenous vein has low thrombogenicity and low restenosis rates, but it increases operating time and requires an additional incision.^1-4 Occasionally, saphenous vein may not be suitable (history of phlebitis, deep venous thrombosis, varicose degeneration) or may not be available (previous saphenectomy). In these situations, a synthetic patch is a feasible alternative. Polytetrafluoroethylene and Dacron^® patches, despite reducing operating time, show lower resistance to infection, higher costs, longer suture line bleeding (polytetrafluoroethylene), and higher rate of restenosis (Dacron^®).^5-8

Bovine pericardium is a biological graft material and its use as a patch in carotid artery surgery is steadily growing. Bovine pericardium has shown weak immune reaction, good integration with adjacent tissues, and nonthrombogenic inner surface, in addition to lower costs when compared to synthetic patches. The material is easy to handle, with excellent suture retention, resulting in hemostatic anastomosis, and is pliable, i.e., adjustable to several shapes, including tridimensional shapes. In the last decade, several studies were conducted and published analyzing the use of this material as a patch in carotid artery endarterectomy, with results comparable to other types of patch material.^9-14 Complications related to the various types of patch material are basically the same: bleeding, infection, thrombosis, embolization, restenosis, and pseudoaneurysm formation Patch rupture is a severe complication associated with high morbidity and mortality rates, occurring almost exclusively in vein patches. Cases of patch rupture occurred mainly in saphenous vein patches harvested from its distal segment.^2,15-17 Based on these reports, some authors recommend that venous patches should be harvested from the great saphenous vein at the level of the thigh, where the vein is more resistant to tensile stress and, thus, less likely to disrupt.¹⁸ The growing use of bovine pericardium in carotid artery endarterectomy has pointed to the need for an evaluation of the mechanical properties of this biological material, mainly due to risks of patch rupture and pseudoaneurysm formation. A comparative evaluation of rupture resistance of bovine pericardium and other patch materials may yield important predictive data for prevention of such complications. Therefore, the objective of this study was to analyze the rupture resistance of bovine pericardium in comparison with great saphenous vein harvested from the proximal thigh.

Methods

This prospective study comparatively analyzed two patch materials used in carotid endarterectomy: bovine pericardium and great saphenous vein from the thigh. Samples were obtained between January 2005 and December 2006 from surgeries performed at Hospital Nossa Senhora das Graças, in the city of Curitiba, southern Brazil.

Bovine pericardium segments were obtained from remaining pericardial plaques used in patients submitted to carotid endarterectomy over a 2-year period. We collected 20 segments of 0.5% glutaraldehyde-treated bovine pericardium, kept in 4% formaldehyde solution until the traction test was performed. All samples were analyzed before the expiration date specified by the manufacturer. Before resistance testing, segments were cut out of the corresponding sample and prepared in a 5.0-mm-wide and 50-mm-long rectangular format, dimensions similar to those of patches used in carotid artery surgeries (Figure 1).

During 2 years, saphenous vein segments were harvested from seven eligible patients, after written informed consent was obtained from these patients. Eligibility criteria included: patients with an indication for varicose vein surgery with total saphenectomy, Doppler ultrasound detection of venous reflux along the whole length of the great saphenous vein except for the proximal third (segment to be tested). The sample was entirely composed of women aged between 25 and 40 years. All patients had leg venous reflux at least in the distal thigh, reason why they underwent harvesting of the entire saphenous vein. Great saphenous vein stripping was performed through three cross incisions: at the level of the inguinal crease, anterior to medial malleolus, and medial to the level of the knee joint. Segments of proximal saphenous vein were atraumatically dissected, harvested before stripping, and sent to traction testing. Specimens were harvested and placed in 0.9% saline solution until their preparation for the traction test, which was conducted no later than 60 to 120 minutes after harvesting. The seven segments of proximal great saphenous vein were dilated with saline solution, and a pressure less than 0.5 atm (or 360 mmHg) was maintained. After being longitudinally sectioned in a rectangular patch, the veins were cut out in similar dimensions, 5 mm wide and 50 mm long (Figure 1). A total of 20 test grafts were considered appropriate for the traction mechanical testing, which was conducted using an Instron^®, universal automated testing machine model 4467 (Instron, London, UK), equipped with pneumatic wedge action grips and a data acquisition system (Instron software IX, version 7.26.00) (Figure 2).

Two variables were obtained through traction testing of each test graft: force and failure force. Ultimate force is expressed as absolute values, in kilogram-force (kgf), and represents the maximum force exerted by the tensiometer throughout the traction testing. Failure force, also expressed as absolute values in kgf, represents the force that, once applied on the test graft, results in its rupture. Failure force was the variable used to calculate failure stress in both patch materials. Since tensile stress (T) is calculated as force (F) per unit area (S), failure stress is calculated using the formula T = F/S. Letter F represents failure force of each test graft, calculated by the universal automated testing machine. Although Newton (N) is the unit of force adopted by the International System of Units, kgf is the unit of force used in Brazil, since most machines used in traction tests are calibrated in this unit. Letter S represents the area of cross section of the test graft, calculated as S = L.E, where L = length of patch graft in millimeters and E = patch thickness in millimeters (Figure 3).

The width of bovine pericardium and saphenous vein patches was standardized as 5.0 mm. Thickness, in the case of bovine pericardium, was specified by the manufacturer as 0.3 mm on average (ranging from 0.25 to 0.35 mm). However, a cross segment was extracted from each test graft and sent to a pathologist in order to confirm pericardium thickness. The thickness of great saphenous vein patches was obtained as follows: after vein dilatation and patch preparation, a cross segment was extracted from an end of each test graft prior to traction testing. Such segments were sent to a pathologist for measurement of vein wall thickness. Measurements were performed using a ruler for light microscope at 40x magnification (Figure 4).

Results

Comparison of failure force between groups

The mean failure force found in the bovine pericardium patch group was 1.97±0.51 kgf (failure force ranged from 1.17 to 3.79 kgf). The mean failure force observed in the great saphenous vein patch group was 1.36±0.59 kgf (ranging from 0.69 to 2.72 kgf) (Table 1).

Table 1
– Comparison between groups I and II in relation to parameters under study

Parameters	Group I (BP)	Group II (GSV)	p
FailF (kgf)	1.97±0.51	1.36±0.59	0.001230
UltF (kgf)	2.27±0.58	1.51±0.53	0.000108
FailF/UltF	0.880±0.117	0.894±0.162	0.7565
Area (cm2)	0.0104±0.0022	0.030±0.01	2,047e-10
FailS (kgf/cm2)	193.99±43.05	49.19±22.96	7,603e-16

Comparison of ultimate force between groups

The mean ultimate force calculated in the bovine pericardium patch group was 2.27±0.58 kgf (ranging from 1.18 to 3.93 kgf). The mean ultimate force observed in the great saphenous vein patch group was 1.51±0.53 kgf (ranging from 0.81 to 2.72 kgf) (Table 1).

Comparison of failure force/ultimate force ratio between groups

Mean failure force to ultimate force ratio was 0.880±0.117 kgf for the bovine pericardium patch group, ranging from 0.56 to 1.0. In the saphenous vein group, mean ratio was 0.894±0.162, ranging from 0.48 to 1.0 (Table 1).

Comparison of failure stress between groups

The mean area used to calculate failure stress in the bovine pericardium patch group was 0.010374±0.00219 cm². In the great saphenous vein patch group, the mean area used to calculate failure stress was 0.030±0.010 cm². The width of bovine pericardium and saphenous vein patches was constant (0.5 cm). The mean thickness of bovine pericardium was 0.02075±0.0044 cm (ranging from 0.015 to 0.030 cm). The mean thickness of saphenous vein patches was calculated as 0.06±0.02 cm, ranging from 0.02 to 0.09 cm. Failure stress calculated for bovine pericardium patches in this sample was 193.99 kgf/cm². Standard deviation was ±43.05 kgf/cm², in the range of 117 to 260 kgf/cm². In the great saphenous vein patch group, failure stress was calculated as 49.19±22.96 kgf/cm². Values ranged from 18 to 108.8 kgf/cm² (Table 1).

Correlation between patch thickness and failure force

In the correlation between pericardium thickness and failure force, Pearson linear correlation coefficient was 0.5032993 (Figure 5). In the great saphenous vein patch group, specimens were harvested from seven different patients. Mean thickness and mean failure force parameters of the seven subgroups are shown in Table 2. In the correlation between great saphenous vein thickness and failure force, Pearson linear correlation coefficient was 0.3062166 (Figure 5).

Table 2
– Comparison between subgroups of group II, in relation to mean thickness and mean failure force

Patient	n	Mean thickness (cm)	Mean FailF (kgf)
IIa	3	0.090±0	1.61±0.9
IIb	3	0.050±0	1.91±0.72
IIc	3	0.046±0.006	0.98±0.31
IId	3	0.067±0.006	1.37±0.82
IIe	2	0.060±0	1.06±0
IIf	3	0.030±0.01	0.99±0.08
IIg	3	0.076±0.006	1.48±0.36

Discussion

Complications related to pericardial grafts used in the construction of prosthetic heart valves, due to calcification, have not been described with patch grafts used in carotid endarterectomy. The same occurs for aneurysmal dilatations in arterial grafts of bovine pericardium. Such dilatations, previously attributed to degeneration of bovine pericardium, in part as a result of antigenicity, have not been reported. Since bovine pericardial tissue is mostly acellular, due to improved harvesting procedures and patch preparation, immune reaction is weak and dilatation is currently a rare occurrence.¹⁰

Due to the growing use of bovine pericardium as a patch material in carotid procedures, reports on pseudoaneurysms have been published in the literature.^14,19-21 In the case reported by Miyamotto et al.¹⁴, there was no structural damage to the pericardium patch, and an incisional pseudoaneurysm developed between the artery and the patch. The cases reported by Lin et al.²⁰ and Hertz et al.²¹ were treated with endoluminal placement of covered stent-grafts, thus preventing an analysis of the patch structure. Pseudoaneurysms may originate from the anastomosis between the vessel and the patch or from the patch itself. Their occurrence was attributed, in the past, to compliance mismatch between the two materials: host artery and prosthetic graft.²² Comparative studies have shown that bovine pericardium compliance is twice as high as that of Dacron^® and polytetrafluoroethylene, which could play a role as a protective factor against pseudoaneurysm formation at the suture line. A greater pericardial compliance also acts on the structure surrounding the hole through which the suture was passed, which gets back to its original form, reducing the occurrence of bleeding.²³

The fixation method applied to the pericardium may also affect its rupture resistance. In our study, all bovine pericardium patches used in the traction test were fixed with 0.5% glutaraldehyde solution, leading to permanent changes in their molecular structure. Glutaraldehyde permits the cross-linking of aldehyde to amine groups of collagen fibers, increasing tissue stability and, consequently, increasing material resistance more effectively than other alternative substances that may be used for this purpose: diphenylphosphorylazide and ethyldimethylaminopropyl carbodiimide.^24,25 Furthermore, studies using 0.6% glutaraldehyde-treated bovine pericardium showed that these grafts had a better endothelialization of the blood-contacting surface. Glutaraldehyde-treated pericardial grafts also showed a slower fragmentation of the collagen layers than the fresh grafts.²⁶ Although the concentration of glutaraldehyde to be used in fixation still lacks standardization, its use in the preparation of pericardium is of paramount importance.

In a review of the literature, there were no cases of bovine pericardium patch rupture, as described for saphenous vein patches. The prevalence of saphenous vein patch rupture in carotid endarterectomy, mainly when the vein is harvested from the ankle, ranges from 0.5 to 2%.^{2,4,15-17-27-29} Due to the catastrophic consequences related to patch rupture when used in the carotid, some authors studied the mechanical properties of the great saphenous vein concerning its resistance, in order to find rupture-related predictive factors.

The vein is considered a non-isotropic material, therefore it exhibits different responses when stretched into different directions. This property of the saphenous vein makes it more likely to tear along the axis, which was observed by experimental studies and also in cases of great saphenous vein patch rupture.^18,29-31 In all the reported cases, rupture occurred longitudinally along the vein. This vein-rupture behavior is explained by Archie & Green³⁰ as follows: the cells within the intimal layer are aligned parallel to the direction of flow, as well as smooth muscle cells in the subendothelial layer. The adventitia is composed of collagen fibers also aligned longitudinally. The media, in addition to a thin layer of smooth muscle cells arranged longitudinally, is composed of circular layers of smooth muscle cells responsible for resistance against circumferential tensile stress-induced rupture. This structural formation increases vein resistance along the longitudinal axis at levels higher than those in the circumferential axis. Moreover, circumferential tension caused by intraluminal pressure is twice as high, favoring vein rupture along the axis. The experimental study by Donovan supports such concepts.³² In that study, all force and tension parameters obtained were significantly higher when measured in the longitudinal direction of the saphenous vein. Bovine pericardium, on the other hand, does not show such a distinct orientation of its collagen fibers. In the present study, in order to apply the same method of traction testing to both materials, we chose to apply tensile force to the longitudinal orientation of the saphenous vein. Traction testing in a circumferential model would not be an appropriate choice for bovine pericardium. Such an attempt would require the use of sutures for manufacturing the test grafts, thus adding one more variable to the tests.

Patch width may also affect its rupture resistance, after its implantation in the carotid artery. The wider the patch, the larger the total artery radius will be after implantation, thus increasing wall tensile stress, in accordance with Laplace's law. Patches should not exceed 4 mm wide, or carotid bulb plus patch total diameter should not exceed 13 mm.^18,27

In the case of bovine pericardium, a flat and non-cylindrical material unlike the vein, a uniaxial approach proved to be the most appropriate way to analyze rupture resistance. Since only recently has pericardial patch been used as a patch material in carotid endarterectomy, data concerning its rupture resistance are still scarce in the literature. As the saphenous vein has been the most extensively studied material in this aspect, it was then used as a comparison group. In the present study, failure force was the main parameter used to compare differences in tissue resistance between both patch groups, although ultimate force represents the maximum force exerted during the traction test. Ultimate force is a mechanical property analyzed in the plastic phase of tissue deformation, in which some fibers are likely to be frayed and others may already be permanently deformed. Even if we fail to achieve ultimate force, once the material exceeds the elastic limit, deformation will be irreversible. Since these are biological materials, whose integrity is essential to prevent ruptures and pseudoaneurysms, any frayed fiber bundles can lead to the occurrence of severe complications. Mean failure force and mean failure stress of bovine pericardium patches were 1.97±0.51 kgf and 193.99±43.05 kgf/cm², respectively. Failure stress expressed as N/mm² was 19.02±4.22, above the value considered adequate by Baucia et al.³³, of 17.6 N/mm². In the literature, there is no study with traction testing that defines a safe failure stress limit for bovine pericardium as an arterial patch material. In the study by Baucia et al.³³, it is not clearly described how the value of 17.6 N/mm² was obtained, considered as an adequate limit.

The mean failure force of the saphenous veins in that study was 1.36±0.59 kgf or 13.34±5.78 N. In the study by Donovan et al.³², the mean failure force of the 48 saphenous veins tested under similar conditions was 24.46±6.75 N, almost twice as high. However, failure force may vary according to the venous segment thickness and width. It is believed that wider and thicker patches require a greater fraying force to break them through. In the present study, all saphenous vein patches were the same width (5 mm) and mean thickness was 0.6±0.2 mm. The width of the vein segment tested in the article by Donovan et al.32 was not specified and mean thickness was 0.38±0.13 mm. A marked difference can be observed in relation to the mean saphenous vein thickness (0.6 vs. 0.38 mm) found in both studies. A possible explanation for such a difference in thickness lies in the fact that all saphenous veins used in the present study were harvested from the thigh and almost all veins used by Donovan were harvested from the leg. However, disregarding width (not specified), we should expect a greater failure force in our sample, since mean thickness is almost as twice as that found by Donovan. It is important to point out that, in the present study, all saphenous vein samples were harvested from women, whereas in the study by Donovan, 72.7% of the patients were men. This could explain, at least in part, the marked difference in longitudinal failure force between both studies. These sex differences in tissue resistance to rupture pressure were observed in studies by Archie & Green,³⁰ in which rupture pressure in saphenous veins of men was 5.39 atm, whereas in women it was 3.45 atm (p = 0.001). Another factor to be considered is the presence of clinically evident varicose veins in all patients from whom saphenous vein segments were harvested. Although the veins were free of reflux, clinically non-evident structural changes might have reduced rupture resistance in these venous segments. Such correlation between the presence of varicose veins and the occurrence of rupture due to vein wall weakness was pointed out by Van Damme et al.³⁴ Even when we compare mean failure stress between the veins in both studies, we observe a marked difference between the two groups (4.8 vs. 13.22 MPa). Bearing in mind that MPa stands for N/mm², that is, force per unit area. Since width is not specified, a comparison of tensile stress translates into a correction factor to negate the variable cross section area of the test graft. Although longitudinal failure stress in the study by Donovan et al. is greater than failure stress in group II of the present study, it is still lower when compared to the bovine pericardium patch group (13.22 vs. 19.02) tested in our study. When failure force is compared between groups I and II, the difference is significant (p = 0.001230). The same occurs when other parameters are compared to ultimate force (p = 0.0001087) and failure stress (p = 7.603e^-16).

In most samples submitted to traction testing, ultimate force was greater than failure force. When failure force is equal to ultimate force, failure force/ultimate force ratio is equal to 1. This ratio classifies materials into ductile or brittle. As this ratio gets closer to 1, the material is considered more brittle (such as: cast iron, glass, stone), being more easily disrupted when submitted to tensile stress that exceeds the elastic limit. When comparing both samples, there was no statistically significant difference between both groups, regarding failure force/ultimate force ratio; 0.880±0.117 vs. 0.894±0.162 (p = 0.7565) (Table 1). This reveals a similar behavior pattern in both materials, regarding this characteristic. If we analyze the results separately, we observe a great variation among samples within the same group, both in the pericardium and vein groups (ranging from 0.56 to 1.0 and 0.48 to 1.0, respectively). This might occur due to the anisotropic features of biological materials and the multifactorial nature of their rupture. Failure stress is a measurement related to failure force in relation to the material cross section area in cm². Failure stress, then, is a force exerted on a fixed and known area. The great variation observed in failure stress within the same group also corroborates the theory on the multifactorial nature of resistance of these biological patches to rupture (ranging from 78 to 252.67 kgf/cm² for pericardium and from 18 to 108.8 kgf/cm² for saphenous vein). This variation suggests an influence of other factors, in addition to patch thickness and width. Since they are biological materials, composed of different types of cells and different extracellular matrix components (in the case of veins) as well as different types of collagen fibers (pericardium), the behavior pattern of these materials is viewed as justifiable.

By analyzing the relation between patch thickness and its rupture resistance, we observe the following: In group I, there was a positive correlation between bovine pericardium patch thickness and failure force, i.e., thicker pericardia tend to show greater rupture resistance. However, according to Pearson linear correlation coefficient, correlation was only moderate (r = 0.5032993). In the saphenous vein patch group, the correlation between vein thickness and failure force, although also positive, was considered low (r = 0.3062166). Comparison of failure force between groups reflects practical situations, since failure force is expressed as absolute values, which does not need a known patch cross area to be calculated, as failure stress does. In clinical practice, when we use a patch, there is no practical way to appropriately determine its thickness. Even in the case of bovine pericardium, in which mean thickness is specified by the manufacturer, we verified that more than half the samples showed thickness below the specified value. Nevertheless, failure force, ultimate force and failure stress obtained from bovine pericardium traction testing were significantly higher than those from the group of saphenous veins harvested from the thigh. Even when compared to the literature, bovine pericardium force parameters were higher than rupture resistance of veins in general, even when tested along their axis, which is considered more resistant to rupture. Despite playing a significant role in tissue resistance in both materials, morphometric characteristics are not the only factors affecting the risk of patch rupture. Other factors such as age, sex and presence of hypertension or diabetes may influence tensile strength of veins.³² The presence of tributaries and valves is an example of vein non-homogeneous regions which might have properties different than those observed in regions free of such characteristics. Even in bovine pericardium, differences in preparation and storage techniques might affect tensile strength, leading to rupture. This study suggests that further investigations should be conducted to evaluate other factors involved in rupture resistance of great saphenous vein and bovine pericardium, since the results obtained still vary greatly among specimens within the same group (Table 2). The cause of patch rupture is probably multifactorial; however, physical forces and material properties play a significant role.

Conclusions

Based on our findings, we concluded that bovine pericardium rupture resistance was significantly higher than that of great saphenous vein harvested from the thigh. In addition, patch thickness in both groups did not show a good correlation with rupture resistance.

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Publication Dates

• Publication in this collection
  02 Oct 2009
• Date of issue
  June 2009

History

• Received
  06 Feb 2008
• Accepted
  06 Mar 2009