n part 1 of seven deadly sins, we looked at the first three sins which

were tooth movement, dirty teeth and acrylic, denture base fractures and how to avoid them.

Now we will examine the other four deadly sins of heat cure processing; plaster inclusions, poor tooth/acrylic bond, increased OVD (occlusal vertical dimension) or raised bite, granular, gaseous and shrinkage porosity. Plaster Inclusions: Bad news, repairs will be necessary, prevention is better than cure. At the boil out stage trim off any rough, sharp or suspect edges which may break off into the acrylic during packing (fig.2), work in a clean area and only add acrylic from the mixing vessel, not the discarded flash off the packing bench.

Skinning Cats

Seven Deadly Sins of Heat Cure ProcessingPart 2 – The last four

By Jordan Nikoloski

Tooth/acrylic bond: When processing dentures with acrylic teeth we don’t need to waste time with diatoric holes, retention grooves, bonding agents or cleaning the teeth with monomer for that matter. Acrylic teeth and denture acrylic will bond chemically, if the outer hard smooth layer of the teeth (fig. 3) is removed and the softer inner layer is exposed! Procedure: after the separator has dried (tooth grindings won’t stick to dry separator), take a typhoon bur (fig. 5) and run it over the teeth, so as to remove any separating solution and expose a clean surface then gently blow off grindings. Pack acrylic at the doughy stage to maximize adhesion. Simple, quick and easy! Increased OVD (occlusal, vertical dimension): A raised bite commonly occurs when the denture flask has not been pressed correctly, that is, space or lack of contact between the two flask halves,

resulting in a greater height between the incisal/occlusal surface and the border extensions of the denture. Another way OVD can be increased (unintentionally) is during the transfer of the flask, from the press to the clamp. The flask will not be clamped at the same pressure as that under the hydraulic or bench press. Solution: use denture injection system or trial pack 2 or 3 times and use stainless steel flask clamps (fig. 7), as these clamps can be placed under the press at the final pack and tightened whilst under pressure. Transfer of flasks from the hydraulic press to the hand tightened clamp is redundant, due to this technical improvement. While on the subject of flasks and processing, let’s have a quick look at loss of centric occlusion (shift of midline). Battered and ill fitting flasks that allow sideways twisting rotation, backward & forward movement can cause a change in

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Place flask in hot water 6 - 8 mins, wax to soften but not melt. You won’t block the sink with wax.

A good separation. Fill flask with separator to allow good absorption into stone.

Trim off any sharp fragile looking bits of stone around peripheries so they don’t end up in the acrylic.

Let sep dry & roughen teeth. Lightly blow off tooth grindings. This is a 30 second operation.

Boil out. I’ve used D/D wash out solution & plenty of hot water. Note glazed surface of teeth.

Mix acrylic, per manufacturer’s instructions and pack at doughy stage, trial pack 2-3 times.

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relation between the teeth & the model due to the play in the flask halves altering the intended centric relation of the denture. Look after your flasks, discard & replace any that don’t fit together precisely, you’ll save yourself money in the long run. Porosity – Granular: Occurs when the polymer in the acrylic mixture has not been adequately saturated with monomer and appears as dry, irregular patches usually a lighter colour in the denture base when processed. As well as being unsightly, granular porosity is also brittle, which can greatly affect the strength of the denture base. Solution: follow manufacturers mixing ratios, gradually adding powder to liquid and mix thoroughly. Porosity – Shrinkage: Occurs when the acrylic (packing) density in the flask isn’t high enough to counteract polymerization shrinkage. Contraction or shrinkage

porosity appears as voids or concavities in the denture base, more prevalent on thicker sections, such as lingual areas on full lower dentures. Solution: Trial pack 2-3 times to achieve a greater density of acrylic. Porosity – Gasseous: Occurs when (1) the volume of monomer is greater than, that which is necessary to wet the polymer (volume). (2) When the temperature of the processing bath is raised too quickly causing the monomer to boil. Monomer has a boiling point just over 100°c and any remaining unreacted monomer (excess) would boil, generating gasseous porosity, remember that the temp is greater within the flask than the water bath because the acrylic is under pressure. Gasseous porosity appears as bubbles within the denture base, usually in the thicker sections. The worst case scenario is a totally bleached appearance of the denture base which is not only unsightly but would also reduce the

strength of the denture. Solution: follow manufacturers mixing ratios and increase the water bath temperature gradually (35 – 45 minutes) before bringing to boil. A good practice is to allow the clamped flask to sit on the bench 15-20 min prior to immersing into the water bath. As Technicians we do need to work smarter, not harder, we need to examine techniques, change and improve systems. Spend more time in preparation and less time on damage control. A process should be simple and straight forward, created by genius and implemented by dummies. However anomalies can and do happen!, eg: when you know that you have followed a technique precisely and it didn’t work, like it worked every other time. What has happened defies logic, you could lose sleep or just accept that these things happen and put it down to experience. We can only aspire to perfection - although 99% success rate should suffice.

Final press, tighten whilst under pressure. These clamps are excellent. No tools required.

No loss of pressure (no transfer from press to clamp). Ready for processing.

Processed, tap flask and remove plaster/stone. The double top section ready to remove – no sweat.

Using saw, cut at 30-50mm intervals along plaster covering buccals & labials of denture and remove.

Remove stone from around the model & check occlusion.

Section the model and remove from the denture. Note the indexes in model base.

Model removed from denture. Greatly reduced risk of denture fracture even in undercut areas.

A smooth fitting surface on the denture improves cohesion, which aids retention.

This technique applies to lowers as well. Note the exposed occlusals after double top removal.

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