CDEN0310

22 I I opinion _ impression materials _The most popular classifications of im- pressionmaterialsforprecisionrestorationssuch as inlays, onlays, crowns and bridges are poly- ethers (PE) and vinyl polysiloxanes (VPS). But would you be amazed to know that PE were first introducedbyESPE(beforethecompanywaspur- chased by 3M) in 1965? Yes, Impregum has been around that long! How about DENTSPLY Caulk leading the way with VPS materials by bringing Reprosil to the market in 1982? A quick math check shows that there have been no other major category advancements on the material side of impression-taking in 28 years! So what has changed and which of these changes really affect your chances of taking the perfect impression the first time? _Hydrophilicity One of the main advantages of the PE over VPS products is the inherent hydrophilicity of the for- mer. Actually, hydrocolloid, which still shares a very small segment of the market, is the epitome of this type of material. It is generally considered that the more hydrophilic a material is, the less likelihood that fluid in the sulcus or really any- where else on the preparation will distort the impression. The hydrophilic material will merely absorb the fluid and continue with its mission of registering an accurate and detailed impression. This property also goes hand-in-hand with the ability of the impression material to ‘wet out’ on the preparation and capture better detail. This latter property has enhanced my own personal experience over the years with PE, especially Permadyne(3MESPE),whichhaslongbeenoneof my favourite materials. ButDENTSPLYCaulktrumpedthemarketagain with the first ‘hydrophilic’ VPS (Aquasil) in 1997. Since that time, there has been a race amongst manufacturers to create their VPS materials with as much hydrophilicity as found in PE. Note that hydrophilic properties in VPS products need to be additives, since these materials are not inherently hydrophilic as are PE. This race has escalated recently by several manufacturers showing what happens when you place a drop of water on a set or even unset mix of impression material. Presumably, if it beads up like water on a freshly waxed car, the material is not hydrophilic. But if it flattens out, it will do the same on a prepara- tion in the mouth, showing it has enhanced hydrophilicity and wetting out ability. The Reality Research Lab (RRL) has developed a more clinically relevant test, albeit more labour intensive. An acrylic model with prepared and intact extracted teeth is impressed with different materials after the teeth have been dried, coated with a glistening layer of water, or coated with a rather thick film of freshly captured saliva. Not only are the impressions and models from them examined closely, but full cast crowns are fabri- cated and marginal gaps measured under a stere- omicroscopeat50x.Arecentproductcomparison demonstrated virtually no differences between two popular materials. On the other hand, bucking the hydrophilicity trendisoneVPSmarketedas‘hydrokinetic’,which breaks down to simply mean ‘moving water’. Well, you can’t move water if you also love it, which is theessenceofthemeaningof‘hydrophilic’.There- cosmeticdentistry 3_2010 Dr Michael B. Miller Impression materials— Are there any REALLYnew ones? Author_ Dr Michael B. Miller, USA