Compression vs tension on materials

Teeth are brittle materials. What works so well is the fact that the materials that make up a tooth work so well synergistically to give the best of all the materials. Enamel is extremely hard but extremely brittle which allows it to cut and grind through foods with not much damage to the tooth surface. Dentine on the other hand is still quite hard but due to its lower mineral content has some flexibility to it. Enamel and dentine are bonded together at the CEJ and the flexibility of dentine supports the enamel to avoid it cracking. As we age, the mineral content of dentine starts to increase, enamel becomes thinner due to wear and along with accumulation of chewing cycles cracks will start to appear in enamel and risk of tooth fracture becomes higher.

It is inevitable that things eventually will wear and break. Our mission is to try and make this happen as slowly as possible. Everything is important in this regard from preparation design, material choice and occlusal design. One trend to keep in your mind is that "dentistry tends to break in tension, not compression".

Table 1: Table off the internet showing the material properties of various dental materials

In Table 1 there is a rough idea of the material properties of dental materials. One thing to note is that the compressive strength of each and every materials is much higher than the tensile strength. This means that when the material is in tension, the breaking point is much lower than when it is in compression. Therefore if we want our materials to not break then we should avoid situations when materials are in tension and if force is required to be placed on a material (i.e in function) we should optimise situations where it is in compression. Another thing to note is that the compressive strength of porcelain is lower than composite. I'm not sure the exact porcelain and composite they are referring to but it does highlight the idea that porcelain especially in thin sections i.e veneers are extremely weak. This can be seen if you try and force on a porcelain onlay onto a tooth and it cracks. The strength of weaker porcelain (i.e lithium disilicate) comes from when it is bonded to a rigid surface i.e tooth structure. I liken it to a tile that is quite weak straight out of the packaging. If you bend it far enough it cracks. But once it is glued onto a concrete surface it is strong enough to support a car driving over it.

Compression driven design:
Care must be taken when there is the possibility of introducing tension into your dental work. These include:
-Bridge design (Figure 1): The mechanical issue with bridges is that they extend the force application away from the long axis of the tooth. The magnitude of this force is determined by the length of the bridge. From an engineering point, cantilevered bridges are a terrible idea. Any force on the pontic puts tension on the cement interface of the retainer-abutment interface and a lateral torquing force on the abutment tooth. It will also put tensioning stresses on the occlusal aspect of the connector. Fixed-Fixed bridges minimises the non axial stresses on the abutments but introduces tensional stress on the gingival side of the connector. The amount of force on a bridge is a function of the length and a small increase in length can result in a very large increase in the tensional force on the bridge. Tension can't be avoided in any bridge design but the design of the bridge can minimise the tension as much as possible. This can be achieved by narrowing the occlusal table to reduce the force on the pontic teeth, increasing the connector thickness as much as possible to increase the tensile strength of the bridge, considering restorative alternatives to avoid long span bridges (e.g implant replacement, partial denture).

 

Figure 1 Blue arrows indicate occlusal forces resulting in tensional forces (yellow)

-Deep intracoronal restorations (Figure 2): Deep restorations e.g deep MOD restorations or at its extreme, a root filled tooth have thinning of the cusps. When occlusal force is placed on these cusps it results in flexing of the cusps which puts tension at the base of the preparation. The amount of tension is dependant on the length of the lever arm. This force increases with sharp internal line angles, deeper preparations with thinner remaining cuspal structure. As a standard, we aim to reduce sharp internal line angles to reduce stress concentrations. Preparations which preserve much of the cuspal structure i.e narrow and shallow are usually strong enough to withstand normal occlusal forces with an intracoronal restoration. However when the preparation results in excessively thin cusps or excessively long cusps i.e in deep preparations or root filled teeth, alterations are required in design to protect the remaining tooth structure. Cuspal coverage has a threefold effect. Firstly it reduces the height of the lever arm and therefore reduces the tension on the tooth structure. Secondly it provides a ferrule effect which acts as a bracing ring around the tooth to bind the remaining tooth structure together. Thirdly it shifts the force distribution from outwards to inwards minimising tension and maximising compression type forces.

Figure 2: Blue arrows denote occlusal forces, green arrows denote compression and yellow arrows indicate tension forces

-In occlusal design (Figure 3): When restoring teeth whether it be a single tooth or a whole arch, the design of the occlusion is important for the longevity of our restorations. Restoring single teeth may require us to conform to the current occlusal scheme to reduce the undesirable forces on the tooth and restoring a whole arch or mouth allows us the opportunity to change occlusal schemes to our advantage. As the number of teeth we restore increases obviously the higher the risk that our work will break. Therefore we want to design the form of teeth in a way that the force is minimised. To do this we have to consider what the purpose of each tooth is. Posterior teeth are closer to the hinge and muscle force and therefore act to crush and grind food. They act mainly through vertical forces through the long axis and lateral forces are undesirable. Canines are the longest and most robust tooth in the mouth and act to tear food but also to protect the posterior occlusion by bearing the brunt of lateral forces (in laterotrusion). Thick bone buttressing allows this to occur in a safe manner. Proprioeception of the periodontal ligament and neural feedback forces the muscle forces to decrease slightly when occluding on canines and significantly when occluding on incisors.
Keeping these concepts in mind, when restoring a posterior tooth, aim to have occlusal contacts on areas on areas which will transmit force through the long axis of the tooth. These contacts exist on cusp tips on the functional cusp, in the central fissure and on marginal ridges (ideally the mesial ridge furthest from the muscle vector). When designing anterior occlusal contacts on natural teeth ideally we aim for point contact in MIP in a centralised area (2 contacts on mesial and distal marginal ridges or 1 contact in the cingulum). On laterotrusion we aim for contact only on the ipsilateral canine which causes immediate disclusion of the posterior teeth. This reduces the muscle forces slightly and ensures that there is minimal lateral force on the posterior teeth. As laterotrusion travels up the canine towards the incisal edge (which is the highest risk for fracturing the canine) we want a more anteriorised contact to be introduced (i.e to pick up a contact on the central incisal edge) which will force the muscle force to decrease even further in the high risk area. On protrusion we want to design even bilateral contacts on the slide forward on anterior teeth. Ideally we keep stresses off lateral incisors which are fairly fragile teeth with dainty root structures. To lower the stresses on anterior teeth we can design the occlusion to minimise the overbite and maximise the overjet. removal of sharp enamel edges on leading and trailing edges will avoid stress concentrations in high risk transition zones.

Figure 3: An idealised occlusion. This is not always possible but is something to strive towards. Centralised contacts reduces force in compression and posterior disclusion on excursion reduces stress for the posterior teeth. Designing the anterior occlusion should be driven towards reducing shear forces. Different skeletal relationships and tooth positions may force you to have occlusal and excursive contacts in different positions but it is important to understand the concepts underlying the design so you can manage these scenarios accordingly

These points are not recipe steps for how to design occlusions but a general conceptual overview as to what increases and what decreases the force and you will make decisions based on the current situation and the aim of treatment.

Tension in dentistry can undo many hours of planning and can be a headache for you and your patient. This reinforces the importance of designing your treatment plans and material choices in risky cases. Extra time put into considering the planning can save you significant amounts of time and money as well as reputation down the line fixing broken work. If something you have completed fails then be sure to ask yourself why this has happened and try to ensure that you take steps to avoid the same thing from happening in the future.