Microsurgical sidewall aneurysms in rats are created by end-to-side anastomosis of an aortic graft to the abdominal aorta. We present step-by-step instructions and discuss anatomical and surgical details for successful experimental saccular aneurysm creation.
Experimental saccular aneurysm models are necessary for testing novel surgical and endovascular treatment options and devices before they are introduced into clinical practice. Furthermore, experimental models are needed to elucidate the complex aneurysm biology leading to rupture of saccular aneurysms.
Several different kinds of experimental models for saccular aneurysms have been established in different species. Many of them, however, require special skills, expensive equipment, or special environments, which limits their widespread use. A simple, robust, and inexpensive experimental model is needed as a standardized tool that can be used in a standardized manner in various institutions.
The microsurgical rat abdominal aortic sidewall aneurysm model combines the possibility to study both novel endovascular treatment strategies and the molecular basis of aneurysm biology in a standardized and inexpensive manner. Standardized grafts by means of shape, size, and geometry are harvested from a donor rat's descending thoracic aorta and then transplanted to a syngenic recipient rat. The aneurysms are sutured end-to-side with continuous or interrupted 9-0 nylon sutures to the infrarenal abdominal aorta.
We present step-by-step procedural instructions, information on necessary equipment, and discuss important anatomical and surgical details for successful microsurgical creation of an abdominal aortic sidewall aneurysm in the rat.
Rupture of a saccular cerebral artery aneurysm causes life threatening hemorrhage leading to stroke, permanent neurological damage, or death. Rupture can be prevented by either microsurgical clipping or endovascular aneurysm occlusion. A medical treatment to prevent aneurysm growth and rupture has not yet been established.
Experimental models for saccular aneurysms are needed to study the biology of arterial aneurysms and for the testing of novel therapeutic devices and strategies. For these purposes, several different models in different species have been developed and published1. Larger aneurysm models in pigs, dogs, and rabbits are preferably used to test endovascular innovations in complex aneurysm architecture1,2. Murine aneurysm models, on the other hand, allow testing research questions in genetically modified species3,4 and facilitate clarification of aneurysm biology at cellular and molecular level far better than larger species1. Although endovascular trans-carotid and trans-iliac device deployment is limited to bigger rats (>400-500 g) and stents smaller than 2.0 mm and 1.5 mm in diameter5,6, stents can also be placed through direct insertion into the abdominal aortic segment harbouring the experimental aneurysms. Previous work using the rat microsurgical abdominal aortic sidewall aneurysm model demonstrated its feasibility in testing novel embolic devices and its use in studying the molecular basis of aneurysm biology3,7.
Many of the currently published experimental saccular aneurysm models require expensive equipment, special environments (e.g. sterile operation rooms with fluoroscopy capabilities), interventional radiology competence, or use of expensive species. These requirements limit the widespread use of these models, and lead to the use of different models in different laboratories, which makes data comparison and meta-analysis difficult, if not impossible. A simple, robust, and inexpensive experimental model is needed as a standardized tool that can be used in a standardized manner in various labs in order to get comparable results from different institutions. For this purpose, we created the rat aorta sidewall saccular arterial aneurysm model.
The aim of this report is to present step-by-step procedural instructions, information on necessary equipment, and to discuss important anatomical and surgical characteristics for successful microsurgical creation of abdominal aortic sidewall aneurysms in the rat.
NOTE: Male Wistar rats (mean body weight: 356 ± 44 g; 10-14 weeks old) were housed in the animal room at 22-24 °C and twelve hour light/dark cycle with free access to pellet diet, regular tap water and also received humane care in conformity with institutional guidelines. The experiments were reviewed and approved by the Committee for Animal Welfare at the University of Helsinki, Finland.
NOTE: In the following demonstration our surgical method is as follows: Anesthetize the rat by weight-adapted subcutaneous injection of medetomidine hydrochloride (0.5 mg/kg) and intraperitoneal injection of ketamine hydrochloride (50 mg/kg). Test for the lack of a toe-pinch reflex to confirm that the rat is fully anesthetized. Apply eye ointment, clip the surgical site, and clean the skin with a suitable disinfectant, for example Chlorhexidine, either in alcohol or water. Wash hands, put on protective clothing, a head cover and facemask, and sterile surgical gloves. Have a surgical assistant aid in maintaining aseptic surgery conditions and to document the surgical characteristics (as listed in Table 1). Monitor the depth of the anesthesia every 15 min during surgery by following respiratory rate, heart rate, and reaction to noxious stimulation (toe pinch test). Subcutaneous injection of buprenorphine (0.03 mg/kg) was given for postoperative analgesia and repeated if necessary every 12 hr.
1. Hardware, Consumables, and Positioning
2. Graft Harvesting
3. Graft Decellularization
4. Aneurysm Creation
A pilot series comprised 14 rats. Subsequently a total of 84 animals were operated according to the presented protocol for several research projects between March and September 2012. Additional 29 animals served as donors for arterial saccular grafts. The remaining experiments were performed using pre-treated grafts harvested and stored from previous experiments using rats of the same gender, strain, weight, and age.
Body weight, overall operation time, aortic clamping time, time for anastomosis creation, time to hemostasis after anastomosis creation, graft ischemia time, and aneurysm dimensions at the time of creation (aneurysm width and length) were recorded and extracted from written case report forms. All characteristics are summarized and visualized in Table 1 and Figure 1.
With the exception of one animal that underwent a second operation due to thrombosis of the abdominal aorta distal to the anastomosis site there were no periprocedural mortality or morbidity. Mean operation time was less than 52 min (52 ± 12 min). In animals with transplantation of syngeneic aneurysms (n = 21) mean graft ischemia time was 29 ± 7 min. Overall mean aortic clamping time was 25 ± 7 min. Aneurysm dimensions revealed to be constant with low deviation of size (mean width 2.5 ± 0.2 mm and mean length 3.8 ± 0.2 mm).
The collected data underwent descriptive analysis and visualization using statistical software. Values are expressed as mean ± standard deviation (SD) and 95% confidence interval (CI).
Figure 1: Non-Decellularized or Decellularized Grafts. Untreated native donor grafts from the thoracic aorta are immediately re-implanted into recipient rats (1). Grafts to be decellularized are treated with sodium dodecyl sulfate (SDS) and stored at -4 degree Celsius until re-implantation (2). The histological panel depicts longitudinal section through an untreated (left) and decellularized (right) graft wall. Hematoxylin-eosin staining.
Figure 2: Surgical Characteristics. The graphs visualize the distribution of single data values (small black dots), data mean (bold long bar), and standard deviation (error bars). Please click here to view a larger version of this figure.
Characteristic | Mean | ± SD | 95% CI upper – lower |
Mean body weight (grams) | 363 | 47 | 350 – 373 |
Mean operation time (minutes) | 50 | 11 | 48 – 53 |
Mean aortic clamping time (minutes) | 25 | 7 | 23 – 27 |
Mean anastomosis time (minutes) | 18 | 6 | 16 – 19 |
Mean time of hemostasis (minutes) | 2 | 2 | 2 – 3 |
Mean graft ischemia time (minutes) | 29 | 7 | 26 – 32 |
Mean aneurysm width (millimetre) | 2.5 | 0.2 | 2.4 – 2.5 |
Mean aneurysm length (millimetre) | 3.7 | 0.5 | 3.5 – 3.8 |
Table 1: Surgical Characteristics. SD = standard deviation; CI = confidence interval
Progress in our understanding of the complex biology of saccular cerebral artery aneurysm depends on analysis of epidemiological and clinical data, complemented by laboratory studies on patient samples and experimental work in animal models3,12,13.
Small animals such as the rat are inherently associated with lower costs of experiments and housing, and reduced need of specialized equipment. An average total operation time of less than 60 minutes for microsurgical creation of a sidewall aneurysm in rats is much shorter than the time used for creation of more complex microsurgical venous pouch arterial bifurcation aneurysm in rabbits and dogs2,14,15. The advantages of low costs and faster methods of aneurysm creation may facilitate conduction of studies with larger number of experiments and subsequent increased statistical power. In addition, current murine models have been successfully implemented to answer research questions needing more sophisticated laboratory methodology, including transgenic animals3,4. When using mice for creation of sidewall aneurysms, one need to keep in mind that the needed interrupted 11-0 sutures require additional microsurgical skills. Performance of the presented aneurysm model in mice is also associated with higher mortality rates (30%; mainly due to complications in fluid balance and anaesthesia and smaller diameter (0.5-1 mm) of the mouse aorta)3.
Basic principles of the rat aneurysm model can be mastered in a short period of time. An introductory course in rodent microsurgery is recommended for those researchers unexperienced in performing dissections and suture techniques under an operating microscope. Highlighted key steps in the presented protocol will further simplify the procedure. Particular caution should be exercised during dissection of the abdominal aorta from adjacent large veins.
The small peripheral vasculature diameter of a medium sized rat makes trans-carotid and trans-iliac endovascular device deployment difficult5,6. However, devices can also be placed through direct abdominal aortic insertion or direct placement into the experimental aneurysm before end-to-site anastomosis7,16. Volumetric changes in neck remnants and aneurysm geometry can be follow-up with serial and non-invasive high-frequency ultrasound, micro-CT, or high resolution magnetic resonance angiography16. Previous experiments revealed high overall patency rates of 92.5% at a median follow-up of six weeks after creation without peri- or intra-procedural anticoagulation and antiaggregation3,7,16. With the exception of a single case significant growth or dilatation of the experimental aneurysms was not observed and none of them ruptured3.
However, if the harvested grafts are decellularized the aneurysms demonstrate a heterogeneous pattern of thrombosis, recanalization, growth, and eventual rupture11. Growing aneurysms in the latter study demonstrated marked adventitial fibrosis and inflammation, complete wall disruption, and increased neutrophil accumulation in unorganized intraluminal thrombus. In this way the model allows to study aneurysm growth and rupture and could be potentially used to assess biological responses induced by embolization devices in growing and rupture-prone aneurysms. None of the available aneurysm models that can be embolized ideally represent a human saccular cerebral artery aneurysm or reproduce the exact pathobiology behind aneurysm formation or rupture.
It remains a matter of debate to what extent the choice of graft (venous or arterial pouch) and choice of angioarchitecture (sidewall or bifurcation construct) influence the translation of results into clinical practice. Certainly different models are optimal for different purposes, and can be optimized to a very high level in certain institutions. The presented model will not make other models obsolete. It will remain necessary for an investigator to choose from a wide range of different technical models and animals the one that suits best the experimental goals, practical considerations, and laboratory environment.
However, some experiments should ideally be performed in a same standardized model in various institutions and labs, in order to allow better comparison of data and of devices or treatments. To date there are no guidelines for standardized testing of endovascular devices prior to clinical application, and animal models remain underused1. Standardized models will gain importance once multicenter randomized preclinical trials also emerge in this field of research.
Microsurgical aneurysm creation allows standardization of graft origin, volume-to-orifice ratio, and parent vessel to aneurysm long axis angle. The presented technique is aimed to generate standardized aneurysms with minimal variation in aneurysm dimension, location, and relation to the parent artery. This high degree of standardization and the relatively low costs make the model a good tool to test embolization materials and devices that are then tested in other more complicated and expensive models.
In conclusion the presented microsurgical sidewall rat aneurysm model is a fast, affordable, and consistent method to create experimental aneurysms that are standardized by means of size, shape, and geometric configuration of the aneurysm in relation to the parent artery.
The authors have nothing to disclose.
The authors are solely responsible for the design and conduct of the presented study. Dr. Marbacher was supported by a grant from the Swiss National Science Foundation (PBSKP3-123454). The authors declare no conflict of interests.
Author contributions to the study and manuscript preparation include the following. Conception and design: SM, JM, JF. Acquisition of data: SM, EA, JF. Analysis and interpretation of data: SM, JF, JM. Drafting the article: SM, JF, JM. Critically revising the article: JH, MN. Statistical analysis: SM, JF. Study supervision: JF, JH, MN.
Name of Material/ Equipment | Company | Catalog Number | Comments/Description |
Medetomidine | Any genericon | ||
Ketamin | Any genericon | ||
Buprenorphine | Any genericon | ||
Phosphate buffered saline | |||
Sodium dodecyl sulfate (0.1%) | |||
3-0 resorbable suture | Ethicon Inc., USA | VCP824G | |
5-0 non absorbable suture | Ethicon Inc., USA | 8618G | |
6-0 non absorbable silk suture | B. Braun, Germany | C0761060 | |
9-0 nylon micro suture | B. Braun, Germany | G1118471 | |
Spongostan | Ethicon Inc., USA | MS0002 | |
Operation microscope | Leica , Germany | M651 | |
Digital microscope camera | Sony, Japan | SSC-DC58AP | |
Standard surgical instruments | B. Braun, Germany | Multiple | See protocol 1.4 |
Microsurgical instruments | B. Braun, Germany | Multiple | See protocol 1.5 |
Vascular clip applicator | B. Braun, Germany | FT495T | |
Temporary vascular clamps | B. Braun, Germany | FT250T | |
Graph Pad Prism statistical software | GraphPad Software, San Diego, California, USA | V 6.02 for Windows |