These products typically include the cements in automix syringes and separate syringes that are hand-mixed. You merely clean the prep, apply the primers, fill the restoration with the cement, and seat it. The main reason for buying one of these products is that they give you the simplicity of a self-adhesive product, but by adding one additional step (applying the primers), their performance is presumably enhanced.
The main indication for these products is to lute metal-based restorations or thick, metal-free types. This includes all types of inlays, onlays, crowns, bridges, and endodontic posts. Since they have light curing properties, this type of cement can be tacked-cured for 1-2 seconds to keep the restoration from drifting on the prep and to allow the cement to be gelled at the margins for fast removal. And, in the areas of the restoration that light presumably cannot penetrate, the self curing properties will hopefully assure complete polymerization.
Most of these products require you to store them in the refrigerator to preserve their shelf life. This means you need to remember to remove them at least one hour before use. Of course, if your practice is very busy and you use all the material in a kit within 1-2 months, you don't need to refrigerate it.
Most important property of any cement is to retain the restoration on or in the tooth. While crown pull tests are about as clinically relevant as possible for full coverage restorations, they are fairly labor-intensive and may not simulate the stresses applied to partial coverage restorations. Therefore, our tests are conducted using simulated disc-like restorations that are bonded to flattened teeth, very similar to the method we use to test bonding agents.
Some of the results may be viewed as unfavorably low, but it is still likely that even these products would be successful when used to cement full crowns. After all, cements with low adhesive values such as zinc phosphate have been retaining full coverage restorations for many years. However, partial coverage restorations, many of which sit on preparations without any mechanical retention whatsoever, may require cements that demonstrate higher levels of bond strength.
These products were tested with and without pre-etching with 35% phosphoric acid (PA) on both enamel and dentin using a 2mm thick simulated metal-free restoration and light curing for 40 seconds from the "occlusal" position.
We also tested their bond strengths to dentin using a simulated nonprecious metal "restoration" to ascertain their self-cure adhesive capability.
Consistency and Handling
These properties refer to whether the cement is easy to load into a crown, for example, and easy to clean off a restoration. Included in this property is the degree of stickiness.
If a material is too thick, it may prevent positive seating of the restoration as well as possible fracture from the force needed to seat it. A material that is too thin may not have enough body to fill the gap between the restoration and the tooth. We performed flow tests on all the products in this category. Flow is graded on a grid going from 0-5, with 0 being very runny and 5 having virtually no flow.
Extraoral Working Time
If you have an assistant, are using an automix material, and only seating 1-2 restorations, you will rarely if ever have to worry about the cement prematurely setting up in the restoration before you can seat it. However, if you are working alone, seating multiple restorations at one time, and/or using a hand-mixed version, you may find yourself not being able to fully seat the restoration.
If this occurs and you don't discover it before the cement goes through its final setting curve, you may find yourself sectioning the restorations to remove them and then you will be faced with the embarrassing, demoralizing, and expensive process of remaking them. Therefore, having adequate working time is very important in saving you from the aforementioned scenario.
Extraoral working time could be especially critical when you are working alone. In this scenario, you (the dentist) would most likely mix and load the restorations with cement and then place them on the bracket table or cart. You would then redirect your attention to the patient, hoping he or she has not closed his or her mouth in the interim, which would require you to repeat most if not all of the intraoral adhesive steps on the preparations once again.
You then retrieve and seat the restorations in the patient's mouth, hoping the cement has not prematurely set. Therefore, the time between mixing, loading, and seating the restorations could be critical.
To test this working time, we used a test restoration and standardized preparation. For the control, we loaded cement into the test restoration (crown) and immediately seated it on the preparation and visually assessed the marginal integrity. We then repeated the test, but instead of seating the restoration immediately after mixing and loading the cement, the crown was allowed to sit on the countertop for a duration consistent with the manufacturer's stated extraoral working time (if the manufacturer gave a range, we tested the shortest and longest times).
Then the crown was seated and the marginal integrity was again assessed. If there was no difference between the control and test specimens, we concluded that the stated working time was accurate. However, if the working time was not correct and a marginal gap was created by the partially set cement, we made note of that fact. In this instance, you would be well advised to seat your restorations before the working time expired. The results can be found in each product's commentary.
Simulated Intraoral Set Time
Length of time each cement requires to cure sufficiently hard so that it could not be penetrated with a sharp explorer using hand pressure. Note that we did not light cure at all since the idea behind a dual-cured cement is that it will still polymerize if no light reaches it. These times may not match the setting times in the mouth, although the cements were allowed to cure in our temperature/humidity chamber set at 37 degrees C/95% humidity. But they do offer an "apples to apples" comparison.
2-3 shades of a resin cement is usually more than sufficient. This is due to several factors. In the first case, there are minimal differences in cement shades when they are viewed in clinically relevant thicknesses. This can be demonstrated when we perform the translucency/opacity test.
Second, the effect of the shade of the cement is highly dependent on the thickness of your restoration. The thicker the restoration, the less effect the shade of the cement will have. In restorations thicker than 1.0mm, there are instances where the cement has no effect whatsoever.
Third, the opacity of the restoration may block out any shading effect of the cement. Therefore, this is one category where buying a few shades a la carte makes much more sense than investing in an entire kit.
The most utilized shade will definitely be clear, which some manufacturers may call translucent or something similar.
Discs of all products in 100µ thicknesses (to simulate the thickness of the cement under a restoration) were measured for relative degrees of translucency/opacity in a spectrophotometer. The scale was 0-100, with 0 being totally clear and 100 being totally opaque. Our findings are in each product's commentary.