E.max Cycle Times

Sameer Puri D.D.S.

13 years ago

In our courses and on the message boards of www.cerecdoctors.com, there seems to be a lot of discussion on the shortened cycle to crystallize e.max. Many of you might be thinking that I'm referring to the 19-minute cycle that is officially approved by Ivoclar. I'm not. I'm referring to the roughly 12 1/2-minute cycle as described by noted researchers, which showed that there is really no loss of strength at this shortened firing time.

I get asked all the time whether this shortened cycle is a safe cycle to use for e.max restoration. My answer to the question is really the same as my answer to anything in dentistry - there is no right or wrong and there is no one size fits all. What the user has to do is look at the clinical situation and determine what route they want to take to get the job done.

Those of you who are contemplating the cycle that's been discussed on the message boards need to realize that this cycle is not approved by Ivoclar. Their internal testing has been done on the 19-minute cycle and Ivoclar for various reasons doesn't feel that going shorter on the cycle is worth it.

Having said that, I think with anything one must look at not only manufacturer studies but make an independent and educated assessment as to the best way to use the products and materials in their practices. Fact of the matter is that six to seven minutes of savings may not be a big deal for many practices. If it is not, then stick with the regular cycle. However, if your practice is like mine and you are doing four to five CEREC restorations a day, seven minutes per restoration can be a significant amount of savings.

One common argument is that the lithium metasilicate in e.max does not fully convert to lithium disilicate in the shortened cycle. The effect of this seems to be a slight difference in color which is only evident in the LT blocks and not HT blocks. Other than that, there does not appear to be any adverse affect of the incomplete conversion. Even in the 19-minute cycle, there is incomplete conversion of the lithium metasilicate. I have yet to have someone who talks about this incomplete conversion explain to me what it really means.

Another argument is that the restorations are weaker when not fully crystallized. While independent studies by Child and Giardano refute this, let's assume for a bit that there is a 20% loss of strength and the restorations are 300 MPa in strength instead of 360 MPa. At this number, the restorations are still twice as strong as any other porcelain block made for the CEREC. In fact, if this is such a huge problem, Ivoclar should remove Empress from their product offering because it's roughly 150 MPa in strength.

Bottom line is that the debate about the shortened cycle is not about to end anytime soon. I've used the shortened cycle many times over the years with no adverse affects as have hundreds of other clinicians. While more formal testing needs to be completed (and is underway), clinicians must use their own judgment as to the materials that are offered and how best to use them. Manufacturers are a good guide to help us get started but often it's the ingenuity of the individual clinicians that leads to great strides in clinical science and technique.