Published ahead of print on December 16, 2010
doi: 10.3174/ajnr.A2302

American Journal of Neuroradiology 32:E33, February 2011
© 2011 American Society of Neuroradiology

C. Sherif^a
aDepartment of Neurosurgery
Cantonal Hospital of Aarau
Aarau, Switzerland

H. Plenk, Jr^b
bDepartment of Bone and Biomaterials Research
Institute for Histology and Embryology
Medical University of Vienna
Vienna, Austria

The article of Bouzeghrane et al¹ is a timely and surely important contribution to reaching standardized quality criteria for experimental aneurysm models and evaluation techniques. Nevertheless, the following comments should be added.

The authors of the above-mentioned article state that the lack of correlation between angiographic and histopathologic results is a major limitation to the validity of experimental aneurysm studies. This statement holds true if subjective estimation of aneurysm occlusion is used as in most of the cited literature. However, our studies have shown good correlation of angiographicand histopathologic occlusion rates by applying a new method of computerized quantification, first on 8 venous pouch bifurcation aneurysms in rabbits.² Because this correlation reached statistical significance in a subsequent large postmortem series of 14 human aneurysms,³ the superior precision of computer-assisted quantification for evaluating angiograms and corresponding surface-stained ground sections from polymethylmethacrylate-embedded aneurysms has been validated.

Concerning the best suitable specimen preparation for histopathologic evaluation, the authors stated that cutting-grinding techniques with the devices in situ preclude advanced histotechniques such as immunohistochemistry. This limitation applies only to the past because currently several resin-embedding techniques and materials (eg, Technovit 9100 New; Heraeus Kulzer, Wehrheim, Germany) are available, in which denaturation of proteins is avoided by low polymerization temperatures (at –20°C). Therefore, the major advantage of histotechniques preserving the implants in situ to evaluate cell reactions at implant-tissue interfaces can now be combined with immunocytohistochemistry.

As a critique of the rabbit venous pouch bifurcation model, Bouzeghrane et al¹ showed, in Table 4, that neck endothelialization was not described and thrombus organization in the aneurysm sac was not properly followed because only uncovered coils were found in the lumen and no fibrosis was visible between the loops of coils. However, all these features of aneurysm healing have been described in detail by our group² and also by several other authors in rabbit venous pouch aneurysms. Bouzeghrane et al also concluded that the rabbit venous pouch bifurcation aneurysm model resulted so far in lower rates of aneurysm patency and higher rates of morbidity, making it less appropriate for embolicdevice evaluations as in, for example, the rabbit elastase model.

However, the rabbit venous pouch model could be substantially improved.^4,5 It now offers equivalent patency and morbidityrates compared with the rabbit elastase and canine bifurcation models. Additionally, the unaltered vein transplant aneurysmwalls of venous pouch bifurcation models show many similarities to the walls of human intracranial aneurysms. With proper technique, the foreign body reactions to the sutures are only located outside the aneurysms and, thus will not interfere with reactions to embolic devices. In contrast, the wall of elastase aneurysms consists of a former elastic artery, in which the multiple elastic membranes were destroyed by elastase, causing a long-lasting inflammatory repair process in the entire aneurysm wall.

Finally, because hemodynamics may be the most important factor leading to recanalization, the hemodynamic similarity of true bifurcation aneurysm models to human aneurysms with high rupture risks should be better considered in future experimental studies.

References

1. Bouzeghrane F, Naggara O, Kallmes D, et al. In vivo experimental intracranial aneurysm models: a systematic review. AJNR Am J Neuroradiol 2010;31:418–23[Abstract/Free Full Text]
2. Sherif C, Plenk H, Jr, Grossschmidt K, et al. Computer-assisted quantification of occlusion and coil densities on angiographic and histological images of experimental aneurysms. Neurosurgery 2006;58:559–66, discussion 59–66[Medline]
3. Sherif C, Bavinzski G, Dorfer C, et al. Computerized assessment of angiographic occlusion rate and coil density in embolized human cerebral aneurysms. AJNR Am J Neuroradiol 2009;30:1046–53[Abstract/Free Full Text]
4. Sherif C, Marbacher S, Erhardt S, et al. Improved microsurgical creation of venous pouch arterial bifurcation aneurysms in rabbits. AJNR Am J Neuroradiol. 2011;32:165–69[Abstract/Free Full Text]
5. Marbacher S, Erhardt S, Schläppi A, et al. Complex bi-lobular, bi-saccular, and broad-necked microsurgical aneurysms formation in the rabbit bifurcation model for the study of upcoming endovascular techniques. AJNR Am J Neuroradiol (In press)

Reply

Published ahead of print on December 16, 2010
doi: 10.3174/ajnr.A2335

American Journal of Neuroradiology 32:E34, February 2011
© 2011 American Society of Neuroradiology

J. Raymond^a, T. Darsaut^a and I. Salazkin^a
aCentre Hospitalier de l’Université de Montréal
Hôpital Notre-Dame
Montréal, Canada

We thank Sherif and Plenk for their careful reading of our work and thoughtful comments. They gave us an occasion to read more recent material from their laboratory, which unfortunately could not be included in our review, which was limited to 1961–2008. We are also grateful to have the opportunity to elaborate on our thoughts and opinions, unhampered by the obligation of reserve and objectivity essential to a systematic review.

We do not believe (and never wrote) that “the lack of correlation between angiographic and pathohistologic results is a major limitation to the validity of experimental aneurysm studies” as the writers of this letter suggest. These discrepancies were reported by other authors (some in collaboration with the writers of the letter) working with the venous pouch rabbit aneurysm model, as an insightful discovery of the model; this has nothing to do with and does not question the validity of models in general. As we understand it, those authors were simply stating that angiographic evaluation of results was less reliable than microscopic examination of specimens. Furthermore, the items included in Table 4 were never meant to be “a critique of the rabbit venous-pouchbifurcation model,” as the commentators believe, but a selection of key characteristics extracted, as faithfully and objectively as possible, from source articles on the various published models. Similarly, the drawbacks of the rabbit model, such as a lower aneurysm patency rate and higher morbidity, were reported as they appeared in the literature; at the time of our writing, we could not include the progress report of still unpublished work on the rabbit model from the present commentators.

There are many limitations to systematic reviews, 1 being the imperfect selection of articles, presumably relevant to theresearch question. Our failure to include their 2006 article¹ in the meta-analysis was perhaps a result of the chosen title,which suggested that the topic was a computer-quantification method, not an evaluation of animal models per se. Another limitation of systematic reviews, of course, is the impossibility of accurately summarizing hundreds of heterogeneous articles, each focusing on various topics, in a short article.

It seems possible to gain precision by using computerized quantification, as suggested by Sherif et al,¹ but only to the extent that expectations do not surpass physical constraints, such as the attenuation of platinum not allowing x-rays to penetrate a coil mass. We welcome any improvement in research methods and thank the authors for proposing new, perhaps more precise quantification methods of angiographic results. Time and experience (beyond 8 animals from 1 team) will tell how helpful such methods may prove to be. However, improved precision, no matter how welcome, is not our main concern with animal models. Too often animal models are used as tools specifically designed to meet marketing tactics or to please arbitrary bureaucratic requirements. In the long run, this cannot but undermine the credibility of experimental animal work.

We certainly believe in the importance of animal models to explore hypotheses and to prevent the premature introduction of new intravascular devices for the care of human beings. However, our main concern with animal experiments is not their validity but their interpretation. Attempts to make them say and conclude more than what they are entitled to are all too frequent. The description of certain pathologic features, such as “endothelialization” of devices or “increased fibrosis,” are too hastily interpreted as signs of “improved healing,” while no one has proved that such “meanings” are reliable indicators of better outcomes for our patients.

We insist that models should reproduce the problem that the research is directed toward solving or preventing, such as aneurysm recurrences, and that the minimal criterion for a useful positive study is the demonstration that the choice of 1 action leads to improved results, compared with an appropriate control. Since Claude Bernard, this classic, yet often overlooked, criterion of experimental evidence is much more important than surrogate pathologic end points, the potential meaning of which remain, in our young field, speculative. Models designed to solve the problem of aneurysm recurrences should compare the number of recurrences found in the experimental and control groups, not surrogate end points such as degree of endothelialization, number of inflammatory cells, and so forth.

In a similar vein, discussions regarding how similar, or different, animal models are compared with humans and human aneurysms, aside from the need to always acknowledge study limitations and the uncertain nature of our results, are usually biased in favor of the author’s particular model and, in the end, are fruitless.

Finally, “hemodynamics may be the most important factor leading to recanalization” is a dogmatic statement. We believe this statement may be, at least in one sense, false and, in another sense, empty. Either way, as stated, it remains a purely conceptual statement, but this is not the place to expand on this difficulty. When we admit that mathematic models are used to simulate hemodynamics, when we recognize the diversity and complexity of aneurysmal flows, when we consider how fragmentary and speculative our current understanding is regarding the relationship between aneurysm flows and ruptures (as recent catastrophes with flow diverters have shown) and how uncertain our notions of rupture risks are, we must also admit that statements such as “hemodynamic similarities of true bifurcation aneurysm models to human aneurysms with high rupture risks” are, to say the least, shaky—if meaningful at all. We do believe in the importance of experimental models, but we must exercise restraint in our interpretations because models remain models.²

References

1. Sherif C, Plenk H, Jr, Grossschmidt K, et al. Computer-assisted quantification of occlusion and coil densities on angiographic and histological images of experimental aneurysms.Neurosurgery 2006;58:559–66[Medline]
2. Raymond J, Salazkin I, Gevry G, et al. Interventional neuroradiology: the role of experimental models in scientific progress. AJNR Am J Neuroradiol 2007;28:401–05[Abstract/Free Full Text]