When a tooth has been grossly decimated in natural structure (enamel and dentin), it typically needs to be rebuilt prior to its preparation for an indirect restoration. Even though many types of materials such as gold, amalgam, compomer, and resin/glass ionomer have been used for this purpose, the most versatile one is composite.
Since it can be bonded to tooth structure, composite cores do not usually require mechanical retention, which, of course, saves you from having to remove additional sound (and precious) tooth structure. If you choose a material that achieves enough hardness within 5 minutes of its placement, you can prep the tooth at the same appointment. If this hardness equals or exceeds the hardness of dentin, which is approximately 45 Knoop (KHN), then you don't have to alter the pressure you apply to the handpiece when you are cutting the prep. If the core material is softer, you need to ease up on the handpiece as you go from dentin to the core or you will ditch the latter.
Composite also has shading options. You can choose a tooth-colored shade if you plan to place a translucent, metal-free restoration over it or a contrast shade, which gives you better visibility of the core/tooth junction and ensures your margins will be placed on tooth structure where they belong, not on the core. In addition, composite can be used in some type of core form, matrix, or template or can be applied freehand.
However, if the entire clinical crown is decimated, then a cast core attached to a cast post is the classical alternative. This type of buildup combines strength and optimal contours that are easily perfected in the lab. Nevertheless, the additional time and cost involved with a cast post and core has relegated this procedure to dental history for most practices.
Types of Core Materials
Core materials can be divided into light-cured, dual-cured, and self-cured versions. This category is dual-cured, arguably the most versatile of the three options, since these products presumably offer both modes of curing (light and self) in one material. The main presumed advantage over light-cured-only materials is that you don't have to worry about placing them in increments, since the deeper sections are supposed to cure chemically if the light doesn't reach them. They offer an advantage over self-cured-only since you can polymerize the surface immediately, which keeps these materials from slumping.
However, all dual-cured materials do not increase to a dentin-like hardness after they have been light-cured, indicating their chemical-cure mechanism has been shutdown or severely restricted by virtue of the light polymerization process. Some of them do reach a dentin-like hardness after 24 hours, but this doesn't help you if you want to prep the tooth immediately or within 5 minutes of placing the material.
If this is the case, then the dual-cured material would be, in effect, a light-cured product and should really be compared to other light-cured materials. If it did not cure deeper via light activation than a pure light-cured material, then there would be no advantage to using a dual-cured material over a light-cured one. On the other hand, if a dual-cured material did not cure very deeply using a light, but did continue to cure chemically in very deep areas that the curing light did not penetrate, then it would have some benefits over light-cured-only products.
To give us insight into these confusing issues, we performed two tests:
Similar to the depth of cure hardness test performed on Light-Cured-Only products, but instead of testing the hardness, the unset material in the proximal box of the simulated Class II preparation is scraped away immediately after light curing using a sharp #15 scalpel blade. The resulting thickness of cured material is then measured with a caliper. If the dual-cured material polymerized deeply when exposed to the curing light, you could quickly remove the matrix or core form (if used) and proceed with the procedure, saving valuable time.
However, even if the scrape test revealed a specific material was hard enough to resist removal with the scalpel blade, it will most likely still be softer than dentin at that point. This means you will need to alter the pressure you apply to the handpiece to keep from ditching the material. You can also prep the occlusal surface, stop and light cure the material again to increase its hardness, and continue. Granted that this is not very convenient, but it will help to ensure that the core material will be harder and, at the same time, help increase the bond strength.
This test is somewhat different than the one performed for Light-Cured-Only materials, but it complements the information from the scrape test. In this hardness test, we filled a simulated Class II preparation that has a proximal box 8mm deep. For a dual-cured material to be useful, the section deep in the prep that the light doesn't reach should at least chemically polymerize to a dentin-like hardness in five minutes, since waiting any longer is not clinically practical if you plan to prep the tooth at the same visit. After filling the simulated Class II preparation, we placed it in our temperature/humidity (T/H) chamber (to simulate the mouth environment) for five minutes.
At the end of the five minutes, the simulated matrix band was removed from restored tooth specimen and the hardness of the core material was tested for hardness on the occlusal surface and in the proximal box at the 7mm level (1mm coronal to the gingival margin).
Finally, the specimen was placed back into the T/H chamber for 24 hours to analyze the self-cure mechanism and tested again for hardness on the occlusal surface and at 7mm. We also performed the exact same test without any light curing at all to see if the self-cure property was affected in any manner by the light activation of the initial test. If the self-cure property of the material is not affected by light curing, then the hardness of the material after initiating the cure with light should be at least as high as the material that relied solely on its self-cure mechanism to polymerize.
The bottom line on all this information is that dual-cured materials may not really offer any real advantages over light-cured-only products except for handling issues — dual-cured are usually more flowable than light-cured-only — and their ability to be used to bond posts at the same time you are building up the core. Please note that dual-cured materials require a bonding agent that will work without light curing. For more information on these products, see Bonding Agents .
Can You Use These Materials As Definitive Restorations?
Since strength is paramount with core materials, most of them contain large filler particles, which create a rough surface. Even though this rough surface is of no consequence with core buildups, it disqualifies these materials from being used for final restorations, should you be tempted to do so due to their lower costs compared to universal composites.
How Soon Can You Prepare the Core?
As noted previously, you can prep the core as soon as it reaches a dentin-like hardness if you don't want to alter the pressure you apply to the handpiece as you go from tooth to core. If changing the pressure you apply to the handpiece does not bother you, then you can prepare the core as soon as it "feels" hard to your explorer or other instrument. By looking at the hardness values we have posted in each product's narrative, you can get an idea on prepping time.
How Hot Do These Materials Get When Setting?
Since these materials are being placed in bulk and presumably reasonably close to the pulp, their exothermic heat during the polymerization reaction may be significant. We measured this heat for dual-cured materials, but only that produced by their self-cure mechanisms due to the limitation of our testing equipment. However, it is very probable that light-curing these materials would produce additional heat.
Selecting the Proper Material
Deciding on what type of core material to use really depends on several factors, such as when you typically place the buildup in your preparation sequence. If you usually prepare the tooth first and then decide if you need to augment deficient areas, then a light-cured-only material will probably be more than adequate, since the thickness of the core will rarely exceed its depth of cure.
The main disadvantage to this approach is being able to isolate the preparation for the adhesive procedure, since placing a rubber dam would be more difficult and there could be bleeding if you needed to extend the preparation subgingivally.
Your other main placement option is to remove old restorations and carious tooth structure first and then restore these areas before you begin the definitive preparation. With this method, you would usually be able to isolate the tooth as you would if you were placing a definitive direct restoration and you would be able to prepare the tooth as if it was intact, allowing a more ideal form.
If this method is your choice, then the depth of cure issue becomes more important. If you don't mind layering, then a light-cured-only material is still your best bet. However, if you just want to fill the defect in bulk, then you need to be sure that the material at the bottom of the defect is as hard as the surface. While our data suggest that using a self-cured material is probably the safer bet, using a dual-cured material (and light-activating it) may still be acceptable after analyzing our test results and picking the material judiciously.