Article Peter Neirynck for Zahn Dental Magazine

 

Dental Investments certainly belong in the category of products that have very high expectations in every lab. Casting is one of the most essential, and often feared, stages in the making of dental restorations. At the same time there is very little knowledge about the products used in the casting process, especially about investments. We know that today we use phosphate bonded investments, but what does this really mean ?

Furthermore what are the consequences, the conveniences and inconveniences? How can we fit these investments in the lab and how can we influence the way they act and react? Though we are all aware of the importance of dental investments, perhaps as a result of their unglamorous appearance, we tend to treat them with very little respect. Let me try to clarify and put dental investments right where they belong : high on our list of unmistakable friends.

What are dental investments? Basically they are high-heat, fireproof materials consisting of quartz, cristobalite and binders. Mixed with water they can be poured around a wax pattern and after hardening they result in a negative copy of the wax pattern. The negative form can be filled with metal creating a positive duplication of the wax pattern.

This looks simple but on daily bases we are confronted with the frustrating complexity behind all this.

The complexity is a result of the common physical and chemical properties of every product involved. Not to take these properties into account can lead to disaster while not understanding why?

Control through understanding is the key. Physically, products that change from one form to another change dimensions : this is the case with gypsum, waxes, metals and investments. In this article we only look at what part investments take in this whole train of transformations.

What qualities should investments have when they leave the factory and can you use these qualities to obtain the desired result?

Manufacturing investments can perhaps better be called : composing dental investments. Investment being a mixture of quartz sands and cristoballites ( cristoballites being quartz melted at very high temperatures and after cooling grinded to specific dimensions.) have to be very well balanced. This means that the grain sizes and forms of the different quartzes and cristoballites for this mixtures have to be carefully selected. This is to ensure that after hardening they will form a solid construction whereby spaces between coarser crystals are filled with fine structures. It is thus obvious that the famous proverb - look how fine my investment is - has little or no saying about its quality. It's finding the right balance that counts to finally end up with a solid construction where all particles are perfectly bound.

To do so binders, products that are able to bind through a chemical reaction, are added to the mixture. In the case of phosphate-bounded investments these are mainly Mono Amonium Phosphate (MAP) and Magnesium Oxide (MgO). Both these products mixed with water create a violent reaction resulting in a linked chain of MgO and MAP holding all the cristals together, but this also results in a bigger volume then all the components separate. By adding extremely fine quartzes to the water (expansion Liquid for investments) the resulting volume (in other words the expansion) can be manipulated.

This all said it is clear that composing investments is work for specialists and that for the lab what matters is to be able to rely on the consistency of the investment of your choice. Indications for a stable, well balanced investment is that after hardening it does not flock out or gets "powdery" on the surface and that the cylinders can still be used in rapid-cast procedure even days after hardening.