Thermal conductivity through various restorative lining materials
Introduction
Traditional teaching has advocated the use of lining materials to afford protection to the pulp and insulate the pulp from the extremes of thermal stimuli particularly after restorative procedures. The best insulating material available has been quoted as being dentine itself.1 Rationales behind lining placement, with consideration of individual materials and their possible applications, have been reviewed2, 3 but little evidence-based explicit guidance currently exists. The current rationale behind the placement of a lining would appear to be to attempt to reduce dentinal fluid flow and bacterial ingress, as these appear to be the cause of much of the post-operative sensitivity experienced by patients.4
Zinc-oxide eugenol has been the classic dental base material for many generations of dentists who were encouraged to place thick linings to ensure good thermal insulation. It has been for many years considered to be the ideal lining material.5 It has insulating properties6 and proven antibacterial properties.7 Calcium hydroxide has been used as a lining and sub-lining material for several decades.8 Its therapeutic effects on the exposed human pulp9 and ability to promote dentine bridging have been suggested as the chief reasons for using this material as a lining.10 Resin modified glass polyalkenoates (RMGP) may bond to dentine via mechanical,11 micromechanical12 or chemical adhesion.13 This group of materials have demonstrated good dentinal sealing properties in vitro.14, 15 Dentine bonding systems, which seal the tubules from external stimuli, e.g. hot and cold, should prevent hydrodynamic fluid shifts, thereby stopping the stimulation of the nerve fibres and the resulting pain. This has been demonstrated clinically16 in an investigation that also cast doubt on the need for a thick insulating layer to ‘protect’ the dentine and prevent post-operative sensitivity.
The purpose of the current study is to assess the thermal transfer of dentine and commonly used lining materials via conductivity experiments. These values can then be considered in relation to the clinical situation that involves only transient thermal extremes17, 18, 19 to determine whether a bulk thickness of lining material is necessary.
Section snippets
Materials and methods
The following materials were tested for thermal conductivity.
- 1.
Zinc oxide eugenol cement. (ZNOE), Kalzinol, Dentsply DeTrey, Milford, DE, US
- 2.
Resin modified glass polyalkenoate. (RMGP) Vitrebond, 3MEspe, St Paul, MN, US
- 3.
Dentine bonding system. (DBS) Scotchbond 1, 3MEspe, St Paul, MN, US
- 4.
Calcium hydroxide lining, chemical setting. (CAOH) Dycal, Dentsply DeTrey, Milford, DE, US
- 5.
Dentine (extracted human first permanent molar teeth)
Both the dentine bonding system and the resin modified glass polyalkenoate
Results
A typical graphical representation of the data, in this case for ZNOE, is shown in Fig. 2. The first region corresponds to when the copper and brass discs were initially brought together, separated by the sample disc, until a steady state was reached. The increase in temperature then corresponds to when the sample was then removed from between the two discs and these were placed together to allow a second steady state to be reached. The third region corresponds to when the free copper disc was
Discussion
Thermal conductivity has been defined as ‘The rate of flow of heat per unit area per unit temperature gradient when heat flow is under steady state conditions’.21 However, it is not the steady state condition at 37 °C that is of concern in the mouth but the conduction of temperature extremes to the pulp. Under these conditions thermal diffusivity may be considered more relevant. The values for thermal diffusivity can be calculated from the equation:where:
h is the thermal diffusivity; k is
Conclusions
The two adhesive lining materials tested (RMGP and DBS) that have the ability to seal the dentinal tubules also had the lowest thermal conductivity values.
The thermal conduction of dentine, as measured in this study, suggests that only extreme temperatures applied for long times would be harmful to the pulp. As these are unlikely in vivo the insulating property of a cavity lining material is not of great significance.
In purely thermal terms DBS and RMGP are the most efficient thermal insulators.
Acknowledgements
The authors would like to thank Ian Smith for technical assistance during this study.
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