The environment inside the human mouth subjects dental restorations, such as fillings and crowns, to rapid and repeated temperature swings. A common meal can expose these materials to a significant thermal contrast, such as consuming a hot beverage followed immediately by a cold dessert. This sudden change creates a mechanical challenge for any foreign material bonded to the natural tooth structure. Restorations must withstand these daily thermal stresses without compromising their integrity or the health of the surrounding tooth tissue. The constant shift in temperature is a major factor determining the long-term success of dental work.
The Physics of Expansion and Contraction
The fundamental principle governing how materials react to temperature change is thermal expansion. When any substance is heated, its atoms vibrate more vigorously and move farther apart, causing the material to increase in volume. Conversely, cooling causes the atoms to slow down and move closer, resulting in contraction. The specific measure of this dimensional change is the Coefficient of Thermal Expansion (CTE), which quantifies how much a material expands or contracts per degree of temperature change.
The main challenge for dental restorations is that the tooth structure and the restorative material possess different CTE values. Natural enamel exhibits a relatively low CTE, typically ranging from about 11.4 to 11.8 per degree Celsius. The dentin beneath the enamel has an even lower value, closer to 8.0 per degree Celsius.
Restorative materials generally have CTEs that are significantly different from the tooth, meaning they expand and contract at different rates for the same temperature fluctuation. This disparity generates internal stresses at the interface where the filling meets the tooth. The greater the difference between the material’s CTE and the tooth’s CTE, the more mechanical stress is created on the bond line with every cycle of heating and cooling.
Immediate Consequence: Marginal Sealing and Percolation
The mismatch in thermal expansion rates directly impacts the seal between the restoration and the tooth. When a person drinks hot coffee, the filling material and the tooth expand, but because they expand at different rates, a microscopic space, or marginal gap, opens up at the boundary. As the temperature drops, the materials contract, and this gap may temporarily close.
This cyclical opening and closing of the marginal gap creates a physical phenomenon termed percolation. Percolation is a pumping action where oral fluids, bacteria, and microscopic debris are drawn into the space between the restoration and the cavity wall during the contraction phase. As the material expands again, some fluid may be expelled, but the cycle repeats with every temperature change.
This ingress of substances compromises the restoration’s function, often leading to patient sensitivity when consuming hot or cold foods. The presence of bacteria and food debris within this gap accelerates secondary decay, which is the formation of new cavities beneath an existing restoration. With an estimated 10,000 thermal cycles occurring annually, this constant fluid movement can undermine the integrity of the dental work.
Cumulative Damage: Thermal Fatigue and Microfractures
Beyond the immediate problem of leakage, repeated thermal cycling causes long-term structural degradation known as thermal fatigue. Each time the restoration expands or contracts at a different rate than the surrounding tooth, internal forces are generated within the material and along the bond interface. Heating induces compressive stresses, while cooling causes tensile stresses.
Over time, this continuous alternation between tension and compression weakens the material’s internal structure and the adhesive bond. This repeated stressing eventually leads to the formation of microscopic cracks, or microfractures, within the restorative material itself. These flaws propagate with each subsequent thermal and mechanical stress cycle.
The growth of these microfractures reduces the material’s strength, making the restoration susceptible to failure. Eventually, the cracks may coalesce, leading to chipping, marginal breakdown, or a total fracture of the filling or crown. The cumulative effect of thermal fatigue, often compounded by the mechanical forces of chewing, is a primary reason for the eventual replacement of dental restorations.
How Different Materials Respond to Thermal Stress
Different restorative materials are selected based partly on how their CTE aligns with that of the natural tooth. Traditional silver amalgam, for example, has a comparatively high CTE, often in the range of 22.1 to 28.0 per degree Celsius. This high CTE mismatch means amalgam restorations experience greater dimensional changes, which historically led to significant marginal gaps and percolation.
Modern composite resins, used for tooth-colored fillings, also have a high CTE, typically between 25 and 60 per degree Celsius. While advancements in adhesive technology help mitigate the effects of the CTE mismatch, these materials are prone to thermal fatigue and microfracture formation due to internal stresses.
Dental ceramics and porcelains, often used for crowns and inlays, are generally formulated to have a CTE much closer to that of enamel, sometimes ranging from 4 to 13.3 per degree Celsius. This closer match minimizes the marginal gap and reduces stress on the bond line. However, ceramics are inherently brittle and are susceptible to the progressive growth of microcracks under the combined effects of thermal stress and chewing forces.