The class II amalgam preparation design is based on the need to remove caries and the need to take into account the physical characteristics of the amalgam material, which should not have an angular bulk at any margin less than 90 degrees. In some areas of certain teeth it may be difficult to avoid an acute angle of amalgam, for example the facial occlusal proximal margin when the contact area is shifted unusually much to the buccal. This also makes it difficult to CARVE the amalgam in that area because the buccal margin is so much higher than the lingual margin in the area of the marginal ridge.
But, generally, we can always get close to a 90 degree angle of amalgam. Remember the material is very hard, but brittle. I had a student in a clinical course back in 1993 who had taken the Cal Board and done an amalgam prep on her daughter - a lower second premolar. She had neglected to extend into the lingual groove for the occlusal outline and was failed because of that. When she brought her daughter into the course it was interesting to see that the amalgam that had been done during the exam had fractured completely through in the isthmus area where the wall was short due to the lack of extension into the lingual groove. We need to adhere pretty well to the principles design for this material in order to do a conservative preparation and restoration that will not compromise the strength of the filling itself.
Remember that the lifetime of an amalgam restoration with todays materials should be a minimum of 50 years, probably 60 or more when the preparaton is done with reasonable precision. There was a study done in Chicago 20 years ago where they recalled many patients who had been given amalgam restorations 50 years earlier and most of these restorations were still there and doing fine! And those were old type materials!
I'd like to hear some discussions under this posting about amalgam preparation design and carving, or anything else you may be confused about for class II restorations. For example, when and where do you think the "reverse curve" should be done?
DMD
Thursday, September 10, 2009
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hi,
ReplyDeletei have a question regarding class 2 amalgam,
what should be the ideal width of the isthmus?
particularly for practical exams.
The isthmus width is generally not critical, except that it should be larger than the smallest condenser, which is usually 1 mm - but not too much larger. So 1.1 or 1.2 would be fine - it could be slightly larger for a molar, especially a mandibular molar. Generally students make the isthmus MUCH too broad faciolingually because they are looking in the wrong direction and don't know where it is being placed relative to the central groove (keep it right on the groove), and don't know that they are angling the bur to the facial or lingual, and when corrected things get much too big. Look down the central groove (check to be sure the bur is vertical mesiodistally) and cut right through the groove without leaning it faciolingually, except slightly to the lingual only on the mandibular preps. Keep it slightly larger than a 55 bur and you will be just fine.
ReplyDeleteSir, are you still available to answer questions about class 2 here?
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ReplyDelete