Research that matters – root canal filling and leakage studies

The necessity to reform the editorial review processes of the International Endodontic Journal (IEJ) has been highlighted recently (Zehnder 2012). As is the case with submissions relating to irrigants and disinfectants, the area of root canal filling and leakage studies also suffers from an overload of submissions, many of which are of limited value. Clearly, it is now time to reappraise simple comparative leakage studies and develop guidelines so that authors can appreciate more clearly the quality of research work that is expected. 1. The main confounding factor in the context of root filling and leakage studies is the anatomy of the root canal systems under investigation. As a consequence, most of the time leakage studies demonstrate the effect of canal anatomy rather than the variable of interest, that is, the root filling material or technique. To overcome this problem, experimental groups should be comparable anatomically, to provide a consistent baseline. A good way to create balanced experimental groups in terms of anatomical features is to use paired teeth. Although this process may limit the size of the experimental groups, it is a good way to minimize the critical effect of root canal anatomy. The easiest way to match teeth is to use contralateral specimens from the same individual, for instance premolars extracted for orthodontic reasons (Zehnder et al. 2006). In several studies, teeth were matched anatomically based on visual examination of bucco-lingual and mesio-distal radiographs. However, it is clear that high resolution X-ray tomography scans can improve the matching of teeth to enhance even more the internal validity of the experiment. Whatever the attempt to stratify test and control groups for better comparability, this procedure should be described fully in the Material & Methods section and also explored in the Discussion section. 2. Bacterial leakage studies with the popular two-chamber model must be complemented by histological controls. Despite the popularity of these studies, few authors have made an effort to confirm that systematic errors have not occurred and thus have failed to validate this approach (Rechenberg et al. 2011a,b). In other words, there is no certainty as to the real route of the contamination in the experimental groups without good histological controls being included. The routes of microbial leakage need to be traced histologically to validate the two-chamber model. 3. Endodontics still requires an experimental model that is able to rank filling techniques and materials in a reliable and reproducible way. Thus, we encourage investigators to consider methodological issues that can contribute to our knowledge base. Studies elucidating the mechanisms leading to apical leakage are more than welcome. Furthermore, studies on the potential correlation between the outcomes of laboratory-based studies and clinical outcomes are encouraged. 4. Investigations, which attempt to correlate smear layer removal procedures with sealability are pointless, as the experimental models are usually not trustworthy. Smear layer removal in single rooted, extracted teeth based on subjective/descriptive analysis of SEM scans of the root canal walls lack experimental reliability (De-Deus et al. 2011). On the other hand, quantitative and longitudinal studies on hard tissue debris accumulation and its relationship with the overall quality of root fillings are welcome. 5. Studies that use sealer penetration into dentinal tubules as a reliable outcome to rank the quality of root fillings materials and techniques can be valid. However, contrary to common belief, a positive correlation between sealer penetration into dentinal tubules and sealability has never been established (De-Deus et al. 2012). Moreover, only a poor correlation exists between sealer penetration into dentinal tubules and resistance of root fillings to dislocation. Thus, the ability of a given root canal sealer to penetrate into dentinal tubules cannot be regarded as an indication of superiority of a given root filling technique or one root filling material over another. However, methodologically sound studies assessing the relationship between sealer penetration into dentinal tubules and other aspects of the root canal treatment are welcome. 6. Simple comparative bench-top studies on the effect of antibacterial effects of root canal sealers should include micro-organisms, which are relevant to the endodontic microbiota. The microorganisms should be in mature biofilm form. Moreover, the antibacterial ability of a given sealer or canal filling should be measured in an environment that is as close as possible to the real conditions found clinically in a filled root canal space. Microbiological criteria should be established and applied only where there is a definite need and their application is practical; consideration should be given to the evidence of actual or potential hazards to health. Furthermore, study designs should state clearly the sampling plan, method and microbial quality controls. 7. Animal studies assessing biocompatibility of root filling materials need to be novel or use innovative or improved experimental models. Moreover, a power analysis is required to optimize sample size and prevent the unnecessary sacrifice of animals. 8. High resolution X-ray tomography evaluations are encouraged to develop a reproducible 3D quantitative model to reliably rank root filling materials and techniques. 9. As a more general issue, there is an overload of manuscripts, in which no significant difference was found between any of the experimental groups. In those particular cases, a type-β statistical error should be carefully avoided. Thus, a description of the sample size calculation is required to reliably trust the power of the study to reveal, whether a difference exists in the techniques/materials or not. I hope this editorial will help potential contributors to direct their studies into areas of interest. The above summary is in no way exclusive. As has been stated before, innovative and well-controlled research is always welcome. I would like to thank Prof. Erick Souza, Prof. Marco Versiani, Dr. Morgana Vianna, Prof. Patricia HP Ferrari, Prof. Luciana Sassone and in particular Prof. Matthias Zehnder for their contribution to this editorial.