The theme of the 2013 Supplement and Product Guide is orthodontic materials. As clinicians, we rely on products produced by dental and orthodontic materials manufacturers from around the world. The safety and efficiency of these products is established by testing. Experimental testing can be performed clinically in patients (in vivo) or in the laboratory (in vitro). Obviously, tests that determine the safety of materials to be placed in the oral cavity should probably be performed in the laboratory so that any potentially harmful effects can be detected before clinical usage. However, when it comes to testing the efficiency of orthodontic materials, the results of both in-vivo and in-vitro testing are commonly seen in articles published in the AJO-DO and other orthodontic journals. Are there differences in the reliability of the results that are documented in vivo vs in vitro?
It might seem logical that the results of in-vitro testing would be more reliable, since all aspects of the experiment could be controlled in a laboratory setting. After all, in-vivo experiments performed in human subjects have many variables that could complicate the interpretation of the data. But there are 2 problems to consider. One is that the variability encountered during human experimentation must eventually be ascertained, because these variables can affect the efficiency of the product being tested. Second, it might not be possible to simulate all intraoral variables adequately in laboratory experiments.
For example, past studies have shown that nickel-titanium archwires are nearly unbreakable when tested in the laboratory by using a wide variety of methods to cause breakage. However, when nickel-titanium archwires are exposed to the oral cavity for varying periods of time and are then tested for breakage and compared with the same wires obtained directly from the manufacturer, the “used” archwires break more frequently than expected. Why? Researchers have shown that “intraoral aging” of nickel-titanium archwires exerts significant effects on their fracture resistance. So, although archwire manufacturers might provide highly favorable reports about the performance of their products, the information in those reports could be meaningless when applied to the clinical performance of the same products.
Other orthodontic products perform differently in the laboratory compared with the oral cavity. For example, we rely on the bond strength of the composite material that we use to attach brackets to teeth. Most published studies that tested the shear bond strength of various types of bonding composites were performed in laboratory experiments. However, it has been shown that mouth rinses containing alcohol that are used by orthodontic patients can decrease the bond strength of the composite because they accelerate the aging and degradation of the polymeric adhesive.
Another example of materials testing that shows differences in the laboratory compared with the oral cavity is the efficiency of elastomeric chains when used to close spaces during orthodontic treatment. These types of materials are routinely tested in the laboratory and are even exposed to artificial saliva at temperatures that mimic those in the oral cavity. But there is something missing in these laboratory experiments that cannot be duplicated in the laboratory. Humans form a biofilm on teeth, brackets, archwires, and elastomeric chains. The biofilm of one patient is different from that of another patient. Researchers have shown that in-vivo exposure of elastomeric ligatures results in the buildup of biofilm, which becomes calcified and contributes to the degradation, and therefore the performance capabilities, of elastomeric chains.
I could go on with more examples, but here is my point: in-vivo testing of certain orthodontic materials produces much more clinically reliable information about the performance of that material in patients than in-vitro experiments in the laboratory. As a result, the AJO-DO ‘s Editorial Board has a preference for the type of materials research that will be accepted and published. Although we do publish a few in-vitro investigations of orthodontic materials, we definitely give a higher priority to clinical studies that are performed in vivo.