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16 I I clinical report _ crown preparation The uniformity of the axial reduction and the posi- tionofthegumsinrelationtothecord,andthecord in relation to the finish line are all evaluated. The axial reduction should have uniform thick- nessthroughoutthedifferentpositions,asdifferent areas need more reduction, while others need less, based on material and aesthetic demands. There should be no areas where the gingiva is over the cord. If this does occur, that area is refined with the radiosurgical unit to ensure a full view of the cord 360° around the tooth of tooth-tissue-cord. One of the main reasons we use polyvinyl-silox- ane impression materials is because they are re- pourable.Ifadequatestrengthandthicknessofthis materialarenotobtainedthroughtheproperradio- surgical troughing technique, then the impression may tear upon separation of the model. Having an impression tear after the first pour limits the ability to fabricate a well-fitting restoration. When a clear tooth-tissue-cord and a visible, sharp finish line are present, the rubber dam is removed and the preparation is evaluated in all dimensionswiththenakedeye.AttimestheOMcan createa‘cannot-see-the-forest-for-the-trees’type of situation, so it is always valuable to take another look from a different perspective without the OM. This can allow one to identify sharp angles or irreg- ularities in the preparation. _Full-arch impressions A full-arch impression is taken with a single tray for the arch that contains the prepared tooth. For the opposing arch, a full-arch alginate impression is taken. With full-arch impressions, a bite registra- tion is usually not required. Most often, one chair- sideassistantisutilisedfortheentireprocedure,but for difficult and challenging impressions, a second assistant may be utilised for saliva or tongue control. Oncealltheimpressionshavebeentaken,aprovi- sional is fabricated, refined, polished and cemented. Shades are taken and the patient is released with post-operative instructions._ _Reference 1.CsempeszF,VágJ,FazekasA.Invitrokineticstudyofabsorbency of retraction cords.J Prosthet Dent. 2003 Jan;89(1):45–9. roots3_2010 Dr Craig M. Barrington is a 1996 graduate of the University of Texas Health Science Center San Anto- nio. He practices general dentistry in Waxahachie, Texas, with his wife, and has a particular interest in endodontics and micro- scope dentistry. Dr Barrington was also part-time clinical Associate Professor in the Advanced Education in General Dentistry Residency Pro- gramme at Texas A&M University’s Baylor College of Dentistry in Dallas. He has lectured to a variety of dental societies and study clubs. He has also authored and co-authored a number of articles for various dental journals. _about the author roots Fig. 10d Fig. 10e