Читать книгу Endodontic Materials in Clinical Practice - Группа авторов - Страница 14

Another weakness is found in the use of irrigants. It is perhaps well known that chlorhexidine reacts with EDTA‐containing products, precipitating material that will clog tubules and canals. But reaction also occurs between chlorhexidine and various other irrigants, producing with NaOCl various chlorinated substances and precipitates, which may be coloured [8]. Hydrolysis to produce 4‐chloroaniline, a toxic substance, has also been suggested [9]. Essentially, the possible chemical interaction between all substances used in any sequential treatment should be considered for adverse effects as a matter of routine. Neither independence, nor complementarity, nor synergy may be assumed. It makes sense to ensure that some rinsing occurs between each irrigant used to minimize risks. Even so, since diffusion into tubules and accessory canals must occur, the efficiency of that rinsing cannot be very great. Reactions in the deeper tissue must be expected. In fact, that is how staining occurs in the first place. Indeed, even a mixture that is advocated (Chapter 5), HEDP‐NaOCl, clearly has an oxidation reaction proceeding fairly rapidly, although the speculated details appear not yet to be verified [10].

A related issue arises in respect of the formulation of products. It is incumbent on researchers to know what they are working with, the composition of materials, and all setting, mechanical, and physical properties and subsequent degradations. Failure to do so can be considered a lapse. However, it is often singularly difficult to get such information: it does not appear in full in product literature, it does not appear in Material Safety Data Sheets because only known or expected hazardous materials need be declared, and it is often denied to enquirers by the manufacturer on grounds of trade secrets. We are owed full declaration of ingredients in manufactured foodstuffs, even if the wording is obfuscated by industry jargon, so that we can avoid adverse reactions or belief violations. We expect to know what is in cosmetics, perfumes, and anything else we put on our bodies, for similar reasons. Likewise with pharmaceuticals. So why, then, is it permissible to sell products that will be implanted in patients without a full list of ingredients and components? The possibility of direct adverse effects is certainly of great concern (especially because we differ widely in our sensitivities). Given that many materials are used in sequence or are contiguous on completion, that concern is raised to an imperative. It is surely inappropriate, if not arrogant, for a manufacturer tacitly to imply that we do not need to know because they have decided it is safe, and that no regulation is thereby contravened. The regulations must be addressed.

The word ‘activation’ is also frequently misapplied in chemical contexts. Thus, so‐called ‘electrochemically activated water’ is in fact a solution of various substances produced by electrolysis. The water as such is not ‘activated’ in any sense whatsoever. The word is used outside dentistry in a variety of similar contexts, similarly vacuously. The most relevant meaning is the switching of a system into a new state or condition, as for example the electronic transition in a photosensitizer, making it capable of the next step in a reaction. Simply raising temperature, for example, often has clear chemical rate effects – but that is not ‘activation’ (such an approach of course ignores the detrimental effect on vital tissue of temperatures above 42 °C and cannot be recommended for that reason anyway). Likewise, (ultra)sonication cannot ‘activate’ anything in this switching sense. Although it does have some remarkable effects in what is termed ‘sonochemistry’, outcomes in the present context can be attributed simply to mechanical actions, including stirring through induced flow: no specific chemistry is known to have been demonstrated. Care must be taken, of course, when discussing the standard chemical term ‘activation energy’: what is required to overcome an energy barrier to a process. Sonication may well provide that for some chemical processes by means of cavitation effects, but as far as can be ascertained, this does not apply here (cavitation is also mechanically destructive). Laser‐based techniques also appear to be purely mechanical in effect. ‘Activation’ is likewise unhelpfully applied to mere agitation of a reactive solution by stirring or pumping, such as moving an irrigating solution with a gutta‐percha point or the like. Whilst this may allow faster bulk reaction, overcoming the limitation of reliance on diffusive processes to some extent, the actual chemical kinetics of the reaction are totally unaffected. Such magic is not scientific.

Whilst on the subject of irrigation, it is often said that a solution is applied specifically to remove the smear layer resulting from instrumentation. That smear layer must, of course, be composed of the same proportions of matrix and mineral as the underlying tissue. It follows that no single solution can achieve such removal: mineral can be dissolved, and matrix oxidized, but not by the same agent. Likewise, there can be no selectivity on the part of the agents: chemically, smeared material is essentially indistinguishable from its source. Diffusion ensures that an acid or chelator, say, will reach underlying material in due course (and, of course, reach more remote areas than the canal being treated via accessory canals and so on). Very often, one sees references to so‐called ‘appropriate concentrations’, which supposedly avoid overextended reaction, without recognizing that both time and concentration – to say nothing of temperature – affect rate, whilst the extent of dissolution depends on the relative amounts of smeared material and reactant (volume × concentration), assuming that factors such as flow and streaming are not involved. Even then, one cannot assume uniformity of thickness or of any of the relevant factors over the entire space, most especially because it is tapered. Always, there is a compromise – in particular because the extent of the smeared material is unknown and progress cannot be monitored. Protocols based on the mean behaviour of a laboratory series cannot inform on the status of the individual case – an example of the fallacy of averages: sample means convey no information on distribution and thus on behaviour in the tails [11].

Following irrigation, it is necessary to dry the canal. This makes sense in that free liquid as such could interfere (mechanically) with subsequent processes, or even chemically via dilution or dissolution. However, it is wrong to imagine that water (as a substance) can be removed and then excluded from tooth tissue: desiccation is neither achievable not desirable. Dentine matrix, being proteinaceous, is hydrated. Removal of that water would be detrimental to its structure and properties. However, all tooth tissues are permeable, both grossly through patent canals and tubules and diffusively through soft and hard tissues, including enamel and cementum, and the vast majority of materials (solid ceramics and metals excluded). Water is therefore always available, everywhere, always. What matters, chemically, is its activity, not its concentration. That is, the equilibrium condition to be expected is that the 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.