Elsevier

Journal of Dentistry

Volume 27, Issue 2, February 1999, Pages 89-99
Journal of Dentistry

Review
Thermal cycling procedures for laboratory testing of dental restorations

https://doi.org/10.1016/S0300-5712(98)00037-2Get rights and content

Abstract

Objectives: Exposure of restorations in extracted teeth to cyclic thermal fluctuations to simulate one of the many factors in the oral environment has been common in many tracer penetration, marginal gap and bond strength laboratory tests. Temperature changes used have rarely been substantiated with temperature measurements made in vivo and vary considerably between reports. Justification and standardization of regimen are required.

Data, sources and study selection: An assessment of reports describing temperature changes of teeth in vivo is followed by an analysis of 130 studies of laboratory thermal cycling of teeth by 99 first authors selected from 25 journals. A clinically relevant thermal cycling regimen was derived from the in vivo information, and is suggested as a benchmark standard.

Conclusions: Variation of regimens used was large, making comparison of reports difficult. Reports of testing the effects of thermal cycling were often contradictory, but generally leakage increased with thermal stress, although it has never been demonstrated that cyclic testing is relevant to clinical failures. However, should this be done, the standard cyclic regimen defined is: 35°C (28 s), 15°C (2 s), 35°C (28 s), 45°C (2 s). No evidence of the number of cycles likely to be experienced in vivo was found and this requires investigation, but a provisional estimate of approximately 10 000 cycles per year is suggested. Thermal stressing of restoration interfaces is only of value when the initial bond is already known to be reliable. This is not the case for most current restorative materials.

Introduction

Restorative materials are routinely used to obturate dental cavities but later pain, marginal staining and caries often occur. These conditions may be associated with an inadequate cavity seal [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], which is thought to be exacerbated by the effects of thermal changes [18]. Intraoral temperature changes may be induced by routine eating [19], drinking [18], [20], [21], [22], [23], [24] and breathing [25]. Thermal stresses can be pathogenic in two ways. Firstly, mechanical stresses induced by differential thermal changes can directly induce crack propagation through bonded interfaces [18], [26], [27]. Secondly, the changing gap dimensions are associated with gap volume changes which pump pathogenic oral fluids in and out of the gaps. This cyclical flow has been incorrectly termed `percolation' [18].

Laboratory simulations of clinical service are often performed because clinical trials are costly and time-consuming. Thermal cycling is an in vivo process often represented in these simulations, but the regimens used vary considerably and, with few exceptions [28], [29], [30], [31], [32], [33], are always proposed without reference to in vivo observations. Standardization of conditions is necessary to allow comparison of reports. The aim now is to analyse the limited number of reported in vivo observations, summarize previous thermal cycling regimens, and recommend a single substantiable regimen to enable comparability between the results of future tests.

Section snippets

Temperatures recorded in vivo

Some experimental work has attempted to measure the routine limits of temperature change induced by eating and drinking. It is difficult to be precise about such events, as eating and drinking are very erratic habits and large variations are expected between occasions, subjects [22] and locations in the mouth [22], [24]. Air temperature, humidity and air velocity when breathing can radically alter even resting mouth temperature [25]. However, with no thermal load and no mouth breathing,

Temperature regimens previously used for in vitro tests

Thermal cycling is common in tracer penetration, shear bond strength and tensile bond strength tests of dental materials. Some 130 thermal cycling experimental reports from 25 journals with 99 first authors made up the sample reviewed here (Table 2). Of these, 110 involved a tracer penetration test, 26 a shear bond strength test, nine a tensile bond strength test and one recorded enamel crack length. The mean low-temperature point was 6.6°C (range 0–36°C, median 5.0°C). The mean

Investigation of effects of thermal cycling regimens

Various aspects of a thermal cycling regimen have been tested experimentally. The dependent variable was often some measure of tracer penetration. Comparison of cycled and uncycled specimens [56], [62], [69], [77], [83], [86], [109], [121], [128], [167], temperature range [168], number of temperatures in the cycle [57], [69], number of cycles [3], [9], [49], [53], [99], [118], [168], dwell times [57], [69] and whether the cycles were in tracer [9] have all been made. Gage and Clarke [87] used

Recommendations for thermal cycling simulation for in vitro interface testing

Past thermal cycling regimens are almost all unreferenced to in vivo observations [171], despite the general expectation that methods are substantiated in publications. In addition, despite great variations in temperature fluctuations and their tolerance in vivo, a standard thermal cycling simulation is required to allow comparison of materials and procedures between reports. The essential element is ordinarily assumed to be the generation of mechanical stresses and fluid flows. The thermal

The usefulness of thermal cycling

It is to be noted that there is no concrete evidence that failures in practice occur because of thermal stresses, notwithstanding the theoretical expectation. However, the distinction must be made between the equivalent static stress test (i.e., increase steadily until collapse occurs) and fatigue failure, where repeated loading to a stress below the static strength occurs. A less severe test would in fact improve discrimination of this point, so long as stresses were below those which would

Acknowledgements

Financial assistance in the form of a postgraduate studentship for Martin Gale from The University of Hong Kong is gratefully acknowledged. This work was done in partial fulfilment of the requirements for the degree of Ph.D. for the first author.

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