Laboratory communication: Is it possible to achieve minimal to no adjustment bonding appointments?

The laboratory must also possess a strong understanding of how to properly equilibrate correctly mounted stone models and understand solid model verification.

Basic cusp to fossa occlusion occurs when cusp inclines on posterior teeth do not touch other posterior inclines. Cusp tips must hit static stops in central fossa.

In laboratory model equilibration there is nothing more than the removal of all incline interferences and allowing cusp tips to occlude at 90-degree angles to opposing marginal ridges and central fossa. Anterior guidance should allow complete freedom from maximum intercuspation, immediately with lateral guidance on the canines. This will not activate the elevator muscles, therefore de-creasing any chances of TMD.

Equilibrating mounted casts is crucial to achieving adjustment-free delivery appointments. Less than 1 percent of technicians understand why we need to, much less, how to perform this task. Because of this, the following are common techniques used to try to achieve adjustment-free cementation appointments:

• Placing metal foil under the working die to create a space. If too much foil is used, the crown may be shy of occlusal contact. When that tooth does erupt into occlusion, it may work into an incline interference, creating an avoidance pattern for the mandible.
• Pushing die up so crown appears out of occlusion on the model. After all, dentists do not want to adjust occlusion, so just leave it out of contact altogether.

The flaw with both of these techniques is there is no way to gauge how much to leave the crown out of occlusion so it is correct in the mouth.

These techniques can never be exactly correct, and they both create problems for proximal contacts because they raise the proximal contact up, which makes the proximal contact shy at the delivery appointment.

This also creates an unstable situation because that tooth can now drift either mesially or distally, creating possible occlusal interferences.

The only way to have predictable, adjustment-free delivery appointments is to correctly equilibrate the accurately mounted working casts. This will take a trained technician approximately five to six minutes per case.

This technique should be used for all restorations whether Emax, Empress, LAVA, Cristobal+, BelleGlass, Implants, PFM, etc.

It can also be used on all partial denture cases. Anything involving natural teeth, from full-arch impressions or double bite trays — this technique should be employed, always!

An example of model equilibration for a #30

Centric equilibration

1) Opposing model and working models are both poured in liquid/powder ratio measured die stone. (Crucial!)

2) After mounting accurately, verify the mounting. Bite should not be taken with base plate wax but with a polyvinyl bite material that can be trimmed to allow only cusp tip show through. Use double-sided, Exacta-film red/black of 19-micron thickness, use black for centric, tap models together. Notice not all teeth are in contact.

3) Initial incline contacts should be removed. Do not ever re-move cusp tips. Remove only inclines, as would be done for intraoral equilibration.

3a) A black dot stable holding contact should be found in the fossa of adjacent teeth; #31 and #29 in this example.

3b) There should be no contacts found on inclines, only on cusp tips and fossa. These holding contacts are found on all teeth. You can now proceed to the anterior guidance equilibration pro-cess.

Lateral equilibration

4) There should be no change in vertical dimension of the equilibrated models in centric because this replicates a “power clinch” of all teeth. (Periodontal ligaments are fully depressed.)

4a) With the red side of Exacta-film, move models laterally and remove all red marks except those on canines, without remov-ing black holding contacts on posterior teeth. The goal is to have black dots on all posterior teeth and red marks on the anterior teeth.

4b) At Williams Dental Laboratory, we go one step further to absolutely ensure no posterior interferences. We know all healthy teeth intrude into their periodontal ligament and move laterally. In this example, imagine the canine will move laterally 56 to 75 microns in a clinching lateral force.

4c) We safely remove approximately 5 degrees off of canine disclusion to further “shallow” the guidance to ensure no posterior interferences. Posterior interferences must then be rechecked. Remember: the key is that lateral equilibration will not change the vertical dimension, only shallow the disclusion, further ensuring no posterior interferences.

Now, and not until now, are the models ready to be utilized as an accurate portrayal of the mouth.

This system, along with the use of a solid proximal contact model and soft-tissue model, should be employed on all cases in the laboratory regardless of material choice.

About the author

Bob Clark, CDT, LVIM, is the first and only lab technician in the world to receive mastership status with LVI. He is co-owner of Williams Dental Laboratory, a small family-operated, full-service lab located in Gilroy, Calif. He and his team have been working and training with LVI dentists for many years. Clark may be reached at (800) 713-5390 or bob@williamsdentallab.com.