Biodegradable Magnesium Stent Treatment of Saccular Aneurysms in a Rat Model – Introduction of the Surgical Technique

The overall goal of this standardized method for occluding saccular sidewall aneurysms in rats using biodegradable magnesium stents is to study novel concepts in stent devices and the molecular aspects of aneurysm healing. This method can help answer key questions in the neurosurgical and neuroradiological field related to the function of new stent or flow diverter devices. The main advantage of this technique is that it represents a standardized and safe procedure with low morbidity and mortality rates.

It is also highly reproducible and thus improves the reliability of associated studies. Under general anesthesia, open the abdomen of the donor rat below the xiphoid process. First incise along the midventral line and separate the skin from the musculature.

Then dissect the abdominal musculature and identify the diaphragm above the liver at the insertion point of the thoracic cavity. Next access the thoracic cavity and euthanize the rat with an intracardiac ketamine injection. Then mobilize the left lung and fix it to the right side using a cellulose swab.

Next expose the aortic arc and above the aorta, clamp the azygos and left cranial caval veins using two mosquito clamps. Then cut between the clamps and leave the clamps attached. Now increase the access to the aortic arc and descending aorta using micro scissors and micro forceps.

Along the aorta, identify the left subclavian artery and first intercostal artery. Now place a nonabsorbable 6-0 suture just cranial of the first intercostal artery and tie it up. At the proximal end, cut the aorta right after the origin of the left subclavian artery.

Finally remove the newly created aneurysm with a cut just below the ligature. After measuring the aneurysm, immediately transplant the graft into the recipient rat. Start the surgery on the recipient rat with a mid ventral incision distal of the xiphoid process and open the abdominal muscles along the linea alba.

Then move aside the small intestines, the cecum, and the colon and attach a self-holding retractor to keep the abdominal cavity open. Place a swab beneath the liver to improve the retroperitoneal exposure. Fat masses can be moved aside as well.

Now open the parietal peritoneum to expose the abdominal aorta. First bluntly dissect the surrounding retroperitoneal fat to expose a longer length of the vessel. Be careful when handling the ureters, testicular vessels, and the superior mesenteric artery.

They are often covered by a thin layer of adipose tissue. When proceeding to a sharp dissection, only grasp the adventitia to avoid damaging the vessel wall. Once exposed, dissect the abdominal aorta.

Start at the distal segment which is the most technically challenging segment to dissect. Use micro scissors and micro forceps starting at the vena cava alongside the arterial adventitial layer between the vessels. Small lumbar arteries may arise as segmental vessels from the dorsal surface of the aorta.

Coagulate and cut these as needed. Also try to identify the right iliolumbar artery. Its origin is most frequently caudal to the left counterpart but this varies.

Once the dissection is finished, without stretching the vessel, place a small, colored rubber pad underneath it to indicate where the arterial puncture will be performed. Now proceed with the dissection of the abdominal aorta from the vena cava in the middle segment. Identify the superior mesenteric artery at its origin from the abdominal aorta and keep it intact.

Also do not damage the lumbosacral plexus. Use lots of flushing and be mindful of the short coagulation time. Ultimately establish the location of the stent in the middle segment and be sure to leave enough room for the abdominal vessels around the stent.

Now place a small piece of gauze and a colored rubber pad under the aorta for better exposure. Then dissect the proximal segment of the abdominal aorta located proximal to the renal vein. After opening the parietal peritoneum, separate the vessels with two blunt forceps.

Then after a dorsal circumnavigation of the aorta, place a small piece of a colored rubber beneath it to facilitate the future application of a proximal clip. Clamp the aorta in the middle segment. First apply the distal clamp then the proximal clamp.

Next remove the adventitia at the planned anastomosis site using micro forceps and micro scissors. Do not injure the vessel wall. Next lift a small piece of vessel wall using micro forceps and perform a linear incision with micro scissors to make a hole with the same size as the aneurysm base.

Then flush the artery with saline in both directions using a blunted needle. Now place the first two stitches of the end-to-side anastomosis at the proximal and distal end of the arteriotomy. Grasp the vessel as little as possible while placing the sutures.

Then close the incision with interrupted sutures to complete the anastomosis. To proceed, remove the distal clamp first. Place an extra suture in case of major bleeding from backflow.

Then remove both clamps and rinse the anastomosis site. Check the patency of the distal abdominal aorta using a direct milking test. Then remove the gauze swab under the colored rubber pad and wait 10 minutes before proceeding with the stent implantation.

To start the stent implantation, first clip the artery puncture site in the distal segment at the distal clamp and then the proximal clamp. Then remove the loose connective tissue and some of the adventitia at the puncture site. Next place a large temporary clamp in the proximal segment to avoid major hemorrhaging during the arterial puncture.

Now lift the vessel wall and puncture the distal arterial segment using an 18-gauge puncture needle and thoroughly flush the artery with saline. Next insert a hydrophobic guidewire into the puncture site and remove the proximal clamp from the distal segment. Advance the guidewire to the clamp in the proximal segment.

Then insert the Teflon introducer using the Seldinger technique. Advance the dilater to be proximal to the anastomosis site. Then check that the three-way stopcock is turned off to avoid unnecessary loss of blood.

Next remove the guidewire and dilater. After vacuuming the balloon system, set the system to one bar of pressure, and then insert the observable magnesium stent using the size four Fogarty introducer. When the stent is finally in place, carefully inflate the balloon to eight or nine bar while continuously checking the anastomosis sutures for ripping.

Then deflate and remove the balloon. While removing the introducer, be ready with a temporary clip as retrograde oozing from the proximal patent arteries may be significant. Now place a clip in the proximal distal segment and remove the temporary clamp in the proximal segment.

Next thoroughly flush the puncture site with saline and close it with interrupted, nonabsorbable 10-0 sutures. If possible avoid grasping the vessel wall. Check for distal artery patency with the direct milking test.

Remove the colored rubber pad. Finish by closing the surgical site as described in the tech’s protocol. The average duration of surgery was about three hours of which about 1/2 hour was needed for aneurysm creation and another half hour was needed for stent application and reconstruction of the arteriotomy.

There were no deaths during surgery. In 87%of surgeries, the desired endpoint of a patent artery was achieved. The other animals included five that experienced complications and four that died within three days due to early in stent thrombosis.

In two animals, late in stent thrombosis occurred 21 days postoperatively with no accompanying neurological deficits. One animal suffered wound dehiscence on the second postoperative day with a consecutive loss of volume. We first had the idea for this method when we recognized that biodegradable stent devices gained increasing popularity in the treatment of coronary heart disease and realized that today’s techniques of stent insertion are associated with high rates of morbidity and mortality.

Once mastered, this technique can be performed in an average time of two hours and 15 minutes if it is performed properly. Following this procedure, other methods like intra aneurysm coil embolization can be performed in order to answer additional questions about aneurysm healing.