The section is aimed to provide you with the essentials of the material concepts in the hope that when one day you pick up a dental materials book it helps understand the more complex areas.
The mouth is a continually changing environment which poses considerable challenges for our material scientists. The range of pH can be as large as 2-12, with transient temperatures of up to 70oC being tolerated. Saliva does its best to recover the mouth back to neutral with all its buffering and additional constituents, but if we don't allow sufficient time for this to occur, both the materials of the teeth and restorations will breakdown. To further add complexity to the situation are the large forces 10-15kg/cm2 being applied and occur from multiple angles during mastication and everyday function.
We don't ask for much for a material to withstand...
In Dentistry the mechanical properties are essentially the rigidity of the material and can be thought of as the material's ability to resist fracture and maintain its shape.
The point at which the material will deform and eventually reach a point of no recovery or fracture.
A lot of the materials we will place in the mouth will fail under less load than the mechanical properties shown in the lab, which is the fatigue limit. This is due to time with the additional environmental changes. Therefore, our materials need to withstand a multitude of factors.
Cyclic fatigue: small force applied continually over time. This can be thought of the materials wear resistance. Small voids incorporated in the material can propagate and eventually cause the material to fail - fatigue life.
Factors affecting fatigue:
How well does the material recover or does it remain mildly distorted?
Applied to clinic:
These factors apply to both solids and liquids. We use a lot of materials that go through chemical transformation, be it from an elastomer impression or the mixing of a base powder and catalyst (cements). These principles are key for the ease of use of a material. There is an induction period of the reaction which we call the working time, which soon becomes the setting time, as we are unable to manipulate the material. Then there is the final set, and the material is now adequate for purpose. As you read the further topics it is surprising how long some of these times can be, leaving the restoration in a vulnerable state.
An important factor to consider is temperature. Temperature can decrease the viscosity of a material and increase rate of reaction, hence at times a cold glass slab is needed for mixing.
This applies to the environment and the materials themselves. The main areas to consider clinically are, the pulp and the marginal leakage.
Nature has encased the pulp with highly effective thermal insulators in the form of enamel and dentine. However, when the tooth is in need of restoration the material we place needs to act in substitution of our own natural material. If it doesn't changes in shape due to temperature can result in the loss of the marginal integrity causing leakage or the tooth could fracture.
Conductors are materials that allow heat to to flow readily through them. However, when it comes to the mouth there are more important factors to consider; diffusivity, exothermic reactions and the coefficient of thermal expansion (CTE).
Diffusivity: This is how fast the material will reach the surrounding temperature. To have a low diffusivity when drinking tea is important for a tooth restoration in protecting the pulp, but having a high diffusivity is important when wearing an acrylic denture to allow the underlying mucosa to sense what's going on. The diffusivity to the transient temperatures that occur is also important for the materials ability to return to the normal surroundings and doesn't "store" the heat.
The majority of the materials we use are exothermic reactions. Factors that will increase this are: increase the material bulk and the speed at which the reaction occurs e.g. using light cured resins. Temperatures as high as 20oC have been measured in resin setting reactions, and this could be in close proximity to the pulp. A more obvious exothermic reaction is using acrylic when creating a jig during a jaw registration appointment, do not leave this material for long on the teeth and have cool water close by.
Coefficient of Thermal Expansion:
This is defined as the unit increase in length per degree of temperature. Thermal in my mind, always thinks hot. This isn't wrong because a high CTE could lead to excessive expansion. BUT, when it comes t mircoleakage and the potential result of secondary caries as the tooth restorative interface integrity breaks down, it is COLD we should be aware of. If the material contracts more than the surrounding tooth a gap will appear. This cold is usually transient and therefore, with the ingress of saliva and bacteria it will be impossible to cleanse when "spring back occurs."
The concepts: We want a large surface area with a rough surface structure and a material that has the ability to flow (wettability) into all the irregularities and potentially interact with the interface at the molecular level.
Where I think confusion can occur.
Adhesion, bonds and adhesives are terms that seem to get used interchangeably and sometimes incorrectly. So I will do my best to try and define them here and stick with these definitions when applying them clinically in the restorative sections.
The Tooth-Restorative Interface:
Bonding: Stick two things together
Adhesive: The material used (The bond material)
Adherents: The materials that are being held together
Sub-types of bonding:
Adhesion:Unlike substances - attraction in intimate contact = Chemical Affinity
E.g. Van de Waals, H bonds, Ionic or covalent. (GIC)
Mechanical: Macro:engaging undercuts (amalgam) and Micro: Surface texture (Composite).
Surface tension: Imagine a droplet of water on glass. There is more tension in the forces of the water droplet compared to those externally acting on; therefore, it stays as a droplet and doesn't spread. Surface tension prevents wettability.
To maximise our materials we need the forces holding them together to be weaker than the forces that pull them to the surface of the tooth, allowing them to flow into all the irregularities. This increases the contact of the adhesive system with the adherents and maximises the bond, be it mechanical and/or chemical adhesion.
Therefore, to quantify wettability, angles are used. If the angle is closer to 0 this means the adhesive is essentially completely flat (no droplet) and has very good wettability properties.
Clinical example: One of the most important factors to consider when it comes to bonding composite resin is that, it is hydrophobic. Therefore, the wet surface forces will prevent a close relationship to be formed. To overcome this problem primers were invented. Primers are essentially the same resin filling material we want to bond to the tooth with additional solvents. This is key in two ways: the solvents have ability to displace the surface water, allowing the resin infiltration of the irregularities, ready to bond our adhesive too.
Our materials need the ability to change to allow us to work with them and then set to adequate form for function.
Numerous examples can be provided: when taking impressions or shaping composite before curing. Some we may not think of are the lab process, for example, when they cast our crowns or sinter our ceramics.
Once set, we need to know how stable these dimensions are and how they might change over time, for example water sorption and degradation.
This will be covered slightly more in the ceramics sections but worth mentioning here.
Colour has three main constituents:
Appearance is also affected by the surface texture, and how well the material interacts with light. Does the light reflect, get absorbed, or refract? The composition of the particles and pigmentations can be added to mimic the affects of the natural tooth.
The chemical properties of the materials we use links to the dimensional stability required. Durability of materials is their ability to withstand the continuous attack of the oral cavity by remaining chemical stable.
The combination of abrasive activity in the mouth alongside dissolution of the material.
The loss of constituents via diffusion of water. Sometimes leaching can be wanted e.g. when we use calcium hydroxide creating an alkaline environment, or when we use GIC and fluoride is released. Leaching can also be unwanted e.g. as the flexibly of the soft liner is lost as it stiffens over time.
Leaching therefore can occur in more than one form. Ions can be exchanged and the material keeps its integrity or washout can occur leading to gradual degradation.
Corrosion is associated with the reactivity of the metal and its ability to conduct an electrode potential. Metals can be anodes (negative) or cathodes (positive). Negative anodes appear to be more reactive than cathodes. Therefore we need two metals of different properties with an appropriate medium of electrolytes (saliva). With these processes occurring the material is lost from the most negative metal. Noble metals (e.g.) are the least reactive.
An important note: We don't use a single metal in the mouth. The metals we use are alloys. The more homogenous the alloy the less reactive the metals.
When reading material books, when metals are heat treated, the aim is to reduce the amount of electrolytic reactiveness.
One exception to the rule is Chronium. This metal is highly negative but doesn't appear to corrode in the mouth. This is because it forms a thick oxide layer that protects it from decomposition.
The single metal restoration: Stress occurs on the material surface which will form pits and rough areas. As these form they can propagate and is known as stress corrosion cracking. What happens is, within the pit itself an electrical cell has formed between the different alloys and as the saliva enters ions are transferred. The environments differ within the pit as well, with a more anaerobic area building up deeper within; with less oxygen, corrosion occurs more freely.
Signs that corrosion could be occurring, that the patient may present with:
All the materials we use must be biocompatible. This means they aren't toxic or an irritant, but this isn't always the case. Some materials that are known to produce adverse effects are: nickel, beryllium and mercury (in particular vapour form). I hope the above lesson was useful and helped to clarify some of the concepts and barriers we are faced with when choosing what material to use.
McCabe JF, Walls AW, editors. Applied dental materials. John Wiley & Sons; 2013 May 7.
Bonsor SJ, Pearson G. A clinical guide to applied dental materials. Elsevier Health Sciences; 2012 Dec 5.
Von Fraunhofer JA. Dental materials at a glance. John Wiley & Sons; 2013 May 31.
