Making proper use of plaster expansion!

With conventional pin models (on a plaster base), several forms of expansion clash. On the one hand, when fabricating the horseshoe-shaped dental arch, the distance of the plaster’s linear expansion in the outer area is approximately three times greater than that in the inner area. This results in serious dimensional changes. On the other, plaster expansion during subsequent fabrication of the plaster base (full model) progresses uniformly and three-dimensionally from the centre of the model outwards, and thus has an additional effect on the pin model, in this case a negative effect.

dental arch
plaster base (full model)
dental arch on plaster base

    (Wilhelm H. Kiefer: "Mit Grips an den Gips" published by: Quintessenz Verlag)

Our model-tray model (full model) takes advantage of the expansion that takes place from the centre of the model outwards. According to scientific findings, in terms of its reference values it is only about one fifth of the peripherally-acting linear plaster expansion seen with horseshoe-shaped dental arch models.

This plaster expansion of approx. 0.08% is actually desirable in order to compensate for the contraction of the elastic impression compounds that occurs after the impression is taken (setting contraction and removal of the impression tray from the warm oral cavity).

As a result of the thermal expansion of the base former caused by the heat being released while the plaster is setting, the brand new model-tray base former also adjusts exactly to the shape of the expanded model.

No differing reference values

Different reference values of plaster expansion in full models and horseshoe-shaped dental arch models.

model-tray models, as full models, do not have differing transverse plaster expansion values between the anterior tooth areas and the posterior lateral tooth areas, as occur in horseshoe-shaped dental arch models.

reference points during impression taking

A-A / B-B

reference points with the model-tray model

A1-A1 /
B1-B1

reference points with the dental arch model

A2-A2 /
B2-B2

 

 

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