activity of the water in all diffusively contiguous regions – that means everywhere in the mouth and surrounding structures – is the same. This is a thermodynamic condition that cannot be gainsaid. ‘Humidity’ is therefore 100%, always (although this really is not the proper term, except in a void, where it refers to the relative saturation of the vapour – ‘wet’ is preferable). How long it takes is a separate matter: diffusivity depends on the medium (we assume close enough to constant temperature). Even so, approach to equilibrium can be expected within a couple of weeks at most in the majority of materials and relevant circumstances [12, 13]. Any reactions that are possible (including absorption, and thus swelling) are therefore necessarily going to occur, but the extent in a given timeframe – the rate – depends on the availability of the water: gradients, diffusivity, and reaction kinetics. Avoidance of ‘leakage’, meaning actual liquid flow or diffusion through liquid pathways, may properly be the goal, but exclusion of water as a reactive substance is not possible.
In the context of leakage, there is clearly much interest in how well a material may be attached to tooth tissue. Commonly, this is referred to in terms of ‘bond strength’, yet it is acknowledged that for many materials this is ordinarily attributable only to a mechanical key – the result of the interlocking of the cast asperities of the material on those of the substrate [14]. It would seem preferable in such cases simply to refer to ‘retention’, as then it is accepted that there is nothing else going on. This thought raises an interesting point: on what is actual bond strength measured? Most systems of interest in dentistry involve a carefully prepared rough surface, whether through instrumentation, grit‐blasting, or etching, seemingly acknowledging that this is the main source of interaction. Would it not be sensible to test the adhesive qualities of materials using a smoothly polished, unetched substrate? That way, the true bond strength could be ascertained; that is, the benefit of any chemical interactions could be measured directly, instead of being confounded by the mechanical key. Proper efforts could then be directed to improving the chemistry, even if the key was to be used to augment the retention in normal service.
In passing, we may note that there is no such thing as a meaningful shear test in dentistry, as has been shown several times. Its continued use – in numerous highly idiosyncratic and ill‐controlled forms – is both pointless and bemusing: the results are uninterpretable, and certainly of no clinical relevance. Whilst that leaves axial tension as the only viable method, no material in any dental context is known to fail in that mode either: the service interpretability of all such results is problematic, therefore. A related problem occurs with ‘push‐out’ tests. The assumed interfacial shear is confounded by parasitic stresses and distortions that vitiate intent and thus interpretation. The absence of appreciation of the mechanics of such systems is disappointing.
A distinction also needs to be drawn between adhesion and seal. The latter can arise from a coating that has no specific bonding beyond the van der Waals (i.e. simple wetting) or from a material that expands (for whatever reason) and is sufficiently plastic to conform to the surface. It might help to ponder the way in which an O‐ring seal works: a purely elastic system that has no bond requirement of any kind. Quality of ‘seal’ is plainly not related to ‘bond strength’ in any fundamental fashion, although in a dental context its continued existence might be. There are evident dangers in expanding materials in what are unavoidably weakened roots, but thought must be given to what scale of gap might be considered appropriate: does it matter at the molecular scale, say of water (the answer has to be no, since this is probably unavoidable), or is it just that of bacteria that is required? Perhaps somewhere in between is acceptable. This needs thinking through.
Lack of thinking is also evident in the use of methods taken from dental International Standards (ISO) documents, showing both a misapprehension of their purpose and unfamiliarity with the subtleties – indeed, outright difficulties – of testing, especially for mechanical properties, which is an exacting field [15]. Such ‘standardized’ methods are to be understood as economically sensible means of ascertaining safety and efficacy; as quality‐control (QC) methods. To call them quick and dirty is perhaps going too far, but they cannot necessarily represent the last word for scientific studies, because the manufacturer, for example, would not be prepared to pay for such accreditation testing, and they make their views known in the drafting committees and national bodies. It is essential to give a full appraisal of a proposed method, refining and elaborating it as necessary, to avoid pitfalls and increase the value of the results in terms of clinical relevance and interpretability. The fact that there are no universally recognized methods of unimpeachable protocol speaks of the difficulties of doing a good job, but also imposes severe requirements on those doing any testing. That severity is rarely even acknowledged, let alone honoured. Crude methods are taken from the literature simply because they have been used before (sometimes for many years), and that precedent is the only defence – there is no science. But on top of that, modifications are made without justification, seemingly for convenience. Comparability between papers evaporates.
1.5 Terminology
The history of the names of chemical substances is worth some study as it reveals the development of chemical thought from the earliest attempts to study the way the world works. Some old forms persist in literary contexts (e.g. brimstone), others are retained in common speech (e.g. acetic acid). The field of chemistry itself has endeavoured to standardize a systematic approach to a variety of areas on a number of occasions since the nineteenth century, culminating in the system of preferred names developed by the International Union of Pure and Applied Chemistry (IUPAC). The point of all this effort, of course, is to be able to communicate exactly, unambiguously, the substance involved. One can understand that the literature will show the progression over time as understanding and rigour develop, and it remains necessary to be able to decode old names. Yet, when perusing a list such as that for EDTA [16], several points emerge. Firstly, the use of trade names as if they were chemically meaningful (v.s. ‘MTA’), when the cessation of the sale of the product would mean that decoding the reference might take some considerable effort in the future (and we have to assume and accept that trade products will at some point cease to be sold). Some products are the same but sold with different labels, such as the Endosequence, Totalfill, and iRoot ranges. Researchers can waste a lot of effort trying to compare these when it is not necessary. Secondly, the import of foreign‐language versions without translation or checking can only confuse. Thirdly, the arcane terms used by manufacturers in their product information might seem intelligible, but you only have to read the ingredients of certain prepared foodstuffs or cosmetics to see how they would leave even a chemist stymied and bemused (part of the reason for the introduction of the E‐number system by the European Food Safety Authority (EFSA)). Fourthly, there are several ways of being systematic. But then, looking at the dental literature, we can discern other problems. Manufacturers wish to obscure their formulations for commercial reasons, but the substance names used commonly convey very little to help understand their chemical, mechanical, or biological properties, such as interactions and allergies – points that have already been made. For these to be parroted uncritically as technically correct labels betrays many things. The fact that there are documented instances of advertising copy‐writers (presumably not chemists) garbling text in the manner we are used to from the press, only for this to be propagated by ‘research’ papers, is at best disappointing. We have a duty to communicate accurately. It is incumbent on us to check. We are obliged to review material critically, and report accordingly. In many cases, a preferable approach would be to identify a substance and state its IUPAC name, then be consistent in using a proper chemical term: the appearance of several names for the same substance in the same text underlines the complete absence of understanding. Reviewers should insist on clarity.
As we should appreciate, all materials used in dentistry represent compromise. It is simply not possible to obtain all desirable attributes (chemical, physical, mechanical, biological, economic, practical) simultaneously. We routinely trade off one thing against another, and accept some deficiency for some other benefit. There are commonly strong grounds for believing that ideality is unapproachable: physics is a hard taskmaster, and thermodynamics ineluctable. Nevertheless, it is proper to enquire as to the amelioration
