Biomimetic Crown Transfer

This is an extended version of the same article on CEREC Digest.

 

A while back, a member of the study club came across a case report by Pascal Magne, titled:

An approach to biomimetics: the natural CAD/CAM restoration: a clinical report.

The report outlined a method that transferred a molar from one patient to another as an onlay restoration. It involved tricking the milling system into grinding a specially prepared block with the donor molar bonded using resin cement. Of all the cool aspects about this procedure, I was personally most surprised at how the resin cement held up against the forces from the milling burs; and who better to showcase the strength of resin bonding than Pascal Magne, the man who advocated and helped popularize IDS.

The procedure did not seem overly difficult, so after reading that paper it got me thinking:

Does our clinic use the same system (CEREC)? Check.

Do we have spare milling blocks lying around? Check.

Do we have too much free time? Double Check.

Is there practical clinical application? Who cares.

All aboard the train of discovery!

The Procedure

The donor molar was treated with H2O2, rinsed, dried, then cut to the appropriate size.

Our mock-patient with a prepared #46.

Instead of an onlay preparation, we opted for crown, which turned out to be more problematic. The main issue was that the recipient site is wider near the CEJ than the donor tooth, so the final restoration would end up with a thinner margin than designed.

Fitting the donor tooth onto a secondary stone model.
This configuration serves as the BioCopy for design reference.
Scanned BioCopy.

A second stone model of the patient was trimmed to fit the donor tooth, the combination of which now serves as our Biogeneric Copy design. Unfortunately, the antagonist tooth in this patient is missing, so we get a free pass at determining the occlusal table. Otherwise, we would use the bite record to determine the proper positioning of the donor tooth.

BioCopy (brown) with restoration (white) overlay.

We made sure that the occlusal height of our restoration is slightly elevated to minimize machine grinding on that surface. In hindsight, this was perhaps unnecessary because the enamel responded well to the milling burs.

Milled block with putty impression.

Once we have designed the restoration by referencing the BioCopy, the result is milled and manually halted before completion. A putty impression of the milled block was made to record the dimensional relationship between the donor tooth and the metal block holder.

Making sure that the special block can fit the donor tooth, which is set between the ceramic plates.

In the default orientation, if the restoration is small enough, the milled block will have two plates on the buccal and lingual aspects of the restoration like the one shown above. This was the basis for our special block, which was fashioned out of a previously-used Enamic block. While we noticed that in Pascal’s method the donor tooth was positioned with the occlusal surface facing away from one of the plates (a 90-degree rotation), it was decided that the our default configuration would give a better, circumferential adhesion.

Yeah… this was not going to work at all.

…and soon we discovered why our method wouldn’t work. With the way our block was oriented, the putty impression would have to be cut into an awkward shape, otherwise the special block wouldn’t fit. After some careful sectioning, it was apparently not worth the extra effort, and we repeated the previous step with the restoration rotated 90 degrees, just as Pascal did.

Our second milling attempt; notice the position of the notch on the block handle indicates that the restoration has been rotated 90 degrees.
Second attempt after a 90-degree rotation.
Ample gap between the special block and the donor tooth seated in the putty impression.

With enough clearance between the tooth and the special block, we were ready for some bonding. Note that the decision to use an Enamic block was for better resin bonding. Sounded pretty good in theory, anyway. The proximal contact area on the putty had to be trimmed because the donor molar was slightly larger (mesio-distally) than our milled restoration.

A fourth generation bonding system was used for attaching the donor tooth to the special block. According the Magne, this is the system that will generate the largest adhesive strength. Still, the amount of resin applied is directly proportional to my personal skepticism about the resin holding its own against the milling forces.

The moment of truth.
Our glorious block emerged victoriously.

After a few nerve-wrecking minutes, the donor tooth emerged milled and, most importantly, still attached to our special block. Granted, there were a few chipped enamels here and there but they were expected given the marginal thickness.

Milled biomimetic restoration from a different angle.
While the restoration has adequate marginal thickness, the donor tooth was physically smaller than our design, so the margins were much thinner than shown here.

Thoughts

The first question that people will wonder is: how’s the margin fitness? The short answer is: almost clinically acceptable but not great. From the above photos it’s fairly obvious that there were substantial marginal discrepancy both vertically and horizontally. We can explain the horizontal part more easily, since the donor tooth was physically smaller than the recipient at the margins. For vertical gaps, however, I think it would be a combination of factors including: brittleness of enamel from H2O2 infiltration, chipping due to thin margins, and incorrect position in the putty index.

A fourth-generation bonding system can generate an adhesive strength of up to 50 MPa. It may be possible that I’ve underestimated the bonding capability, or overestimated the forces exerted by the milling burs. The chaotic nature of diamond grinding, however, should be fairly damaging to the resin structure, so I guess resin cements are just awesome.

The thing about hands-on experiments is that it’s always the minute, easily-overlooked details that come back to bite you. We literally sat and discussed the first block configuration (without the 90-degree rotation) before agreeing that there was no perceivable shortcoming, and then immediately regretted our decision once our special block did not fit the putty impression.

So what clinical applications can this procedure possibly have? For one, we may have a new reason to keep our own extracted wisdom teeth in the future, just for the opportunity to use it as a restoration for another tooth. I guess if you can get over the hygiene concerns, then inter-personal transfers may be a viable treatment option. However, there are still major hurdles to overcome, the foremost being the unknown biomechanical response of milled enamel bonded to dentin with resin cement. In addition, the donor tooth has to be larger than the recipient site, and this may often not be the case if the recipient is a first molar.

Given how relatively successful our attempt was, this may honestly be a new treatment plan in the future.