Use of experimental aneurysms to evaluate wrapping materials

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Use of Experimental Aneurysms To Evaluate Wrapping Materials Balaji Sadasivan, F.R.C.S., Swanhoo Ma, M.B .B.S ., Manuel Dujovny, M.D ., Khang Loon Ho, M.D., and James I . Ausman, M .D., Ph.D . Henry Ford Neurosurgical Institute, Henry Ford Hospital Division, Detroit, Michigan

Sadasivan B, Ma S, Dujovny M, Ho KL, Ausman JI . Use of experimental aneurysms to evaluate wrapping materials . Surg Neurol 1990;34 :3-7 .

study the reaction of aneurysms to five different wrapping materials .

Experimental venous pouch aneurysms in rats were wrapped with muscle, bovine collagen, muslin, cotton, or polyvinyl alcohol . The rats were killed 6 or 12 weeks later, and the aneurysms were compared with control aneurysms . Bovine collagen and muscle were reabsorbed and the aneurysms were similar to the control group . Cotton, muslin, and polyvinyl alcohol caused fibrosis around the aneurysm . However, giant aneurysms were found in the muslin and polyvinyl alcohol group . Cotton appears to be the most suitable material for wrapping aneurysms . The experimental venous pouch aneurysm model in rats can be used to evaluate wrapping materials.

Materials and Methods Sixty-two Sprague-Dawley rats weighing between 250 and 350g were used . Intraperitoneal injection of pentobarbital (30 mg/kg) was used for anesthesia . The rats were placed on their back and, through a neck incision, an internal jugular vein graft was harvested . An abdominal incision was made and the aorta below the renal arteries was identified . A 1 .0-mm arteriotomy was made in the aorta and the vein graft was anastomosed end to side to the aorta . The free end of the vein graft was closed with a purse string suture to create a 3-mm aneurysm . All

Cerebral aneurysms ; Wrapping ; Muslin gauze ; Cotton ; Polyvinyl alcohol KEY WORDS :

Figure 1 . Aneurysm wrapped with cotton dissected en-bloc with aorta and surrounding scar tissue . Sections were made through the aneurysm and studied histologically . Introduction The first direct surgical treatment for intracranial saccular aneurysms was the wrapping of aneurysms with muscle [4] . Since that time, a variety of materials have been used to wrap aneurysms [5,8,11,13,15-19,211 . With the use of the microscope and better aneurysm clips, wrapping is now restricted to those few aneurysms that the surgeon feels are unclippable . Previous laboratory studies have looked at the reaction of the peripheral neurovascular bundle, neural tissue, or arteries to wrapping material [2,3,6,12,16,20,22] . These experimental models are not similar to the clinical situation where a thinwalled structure (the aneurysm sac) is under arterial pressure and wrapping is supposed to induce a tissue reaction that strengthens the sac wall . The experimental venous pouch aneurysm [1,7,14,23] more closely simulates the clinical situation and we have used this model in rats to

Address reprint requests to: James 1 . Ausman, M .D ., Ph .D., Henry Ford Neurosurgical Institute, Henry Ford Hospital, 2799 West Grand Boulevard, Detroit, Michigan 48202 . Received December 27, 1989 ; accepted February 9, 1990 . C 1990

by Elsevier Science Publishing Co ., Inc .

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Table 1 . Summary, of Results Scar tissue around aneurysm Control Muscle Bovine collagen Polyvinyl alcohol Muslin Cotton

Thickening of aneurysm wall

I) + +

Inflammation in aneurysm wall

Number of giant aneurysms

I) 0 D

1) D 0

++++

++

++

2

++

++ + +

++ +

2 0

aneurysms were created by one author (BS) using the same technique . The rats were divided into a control group and five other groups in which the aneurysms were wrapped with the following materials : (1) muscle taken from the sternocleidomastoid ; (2) cotton from

cotton balls (Med Surg Industries, Herndon, VA) ; (3) bovine collagen strips (Hemopad", Datascope Corp ., Oakland, NJ) ; (4) shredded muslin (Parke Davis, Detroit, MI) ; (5) polyvinyl alcohol granules (Laboratoire lngenor, Paris, France) . Half the rats were killed at 6 weeks and the rest at 12 weeks. The aorta, aneurysm, and surrounding tissue were dissected out (Figure 1) . Sections of the aneurysm were prepared and studied by the neuropathologist (KLH) who was "blind" to the group of rats from which the specimen was taken . The sections were stained with hematoxylin and eosin, Masson's trichrome, and Verhoeff stains . The scar tissue around the aneurysm, the thickness of the aneurysm wall, and the degree of cellular inflammation in the sac wall were graded using a semiquantitative method by the neuropathologist . Results The results are summarized in Table 1 . The aneurysm was patent in all cases after the rat was killed . There was

Figure 2 . Wall of aneurysm in bovine collagen group . I' his rat was killed at 6 weeks and a small remnant of the bovine collagen is seen on the external surface of the aneurysm . The bovine collagen is infiltrated with macrophages (hematoxylin and eosin stain x 60) .

Figure 3 . Fibrous andforeign body reaction induced b cotton (hematoxylin and eosin stain x 120) .



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Figure 4 . Greatly enlarged partially thrombased aneurysm that was wrapped with muslin (trithrame xl0) .

no difference in the results within each group between rats killed at 6 and 12 weeks . Almost all the muscle and bovine collagen had disappeared by 6 weeks and no evidence of their presence was seen at 12 weeks . The aneurysms in these two groups were similar to the control group (Figure 2) . Polyvinyl alcohol, cotton, and muslin caused thickening of the aneurysm wall . There was also a considerable amount of scar tissue around the aneurysm . Scarring was thickest with polyvinyl alcohol . Cotton caused less cellular infiltration of the sac wall than muslin or polyvinyl alcohol (Figure 3) . There were two aneurysms each in the polyvinyl alcohol and muslin groups that were greater than 1 cm in size (giant aneurysm) . These giant aneurysms were partially thrombosed (Figures 4 and 5) .

Discussion In laboratory animals, experimental aneurysms can be created in two ways . The first consists of weakening the arterial wall mechanically or by administration of a toxic agent, such as beta-aminopropionitrite (9,10] . The formation of the aneurysm is promoted by keeping the animal hypertensive . The formation of these aneurysms is unpredictable and takes a considerable amount of time. This makes these animal models unsuitable for evaluation of wrapping materials . The second method is to construct a venous pouch aneurysm (1,7,14,23) . The technique of producing these aneurysms in the cervical carotid artery has been previously described [231 . We have used the same technique to create venous pouch

aneurysms in the rat abdominal aorta. We used the abdominal aorta because we felt the peritoneal cavity is less restricted than the neck and will allow an aneurysm to grow if its wall is weakened . Some wrapping materials have been previously studied in laboratory animals . Sachs [16] found that muscle wrapping of dog internal carotid artery did not result in satisfactory fibrous tissue formation . However, he noticed that when fragments of cotton from cotton "patties" were accidently left around the artery, they induced a good fibrous reaction . Diaz et al [3] found that ethyl2-cyanoacrylate placed on the conical surface and around the femoral neurovascular bundle in cats produced inflammatory and necrotic changes both in the cortex and in the peripheral nerve . Ebina et al [6] wrapped the intracranial arteries of 43 mongrel dogs with muscle, fascia, dura mater, cotton fiber sponge, synthetic lyophilized dura mater, or cyanoacrylate. They found that muscle, fascia, and dura mater became necrotic and were absorbed . The synthetic lyophilized dura mater and the cyanoacrylate showed poor adhesiveness to the vascular wall . The cotton sponge showed collagen infiltration and was firmly adherent to the arterial wall . In our study, we found that bovine collagen and muscle were absorbed and did not produce any significant fibrous reaction around the aneurysm . The aneurysms in these two groups were similar to those in the control group . This finding supports previous studies that suggested no value in wrapping aneurysms with muscle . Polyvinyl alcohol, cotton, and muslin produce scar tissue around the aneurysm . The formation of giant aneurysms



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Figure 5 . Giant aneurysm that was wrapped with polyvinyl alcohol . The intraaneurysmal thrombus is laminated (hematoxylin and eosin stain xl0) .

in the polyvinyl alcohol and muslin groups of rats suggests that these materials may weaken the aneurysm sac wall and cause the aneurysm to increase in size . The increased cellularity in the aneurysm wall wrapped with polyvinyl alcohol or muslin as compared with cotton may be responsible for the formation of the giant aneurysms . This study shows the feasibility of using the rat venous pouch aneurysm model to evaluate various wrapping materials . Based on the results of this study, the use of muslin and muscle for wrapping should be abandoned, and cotton from cotton balls is the material of choice for wrapping . Further studies with this model may yield a better material for wrapping aneurysms . The ideal material is one that promotes a thick fibrous scar around the aneurysm without causing rupture or enlargement of the aneurysm.

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