Gold Onlays - Posting #8

Color me old fashioned, but there are times where the choice between a PFM and an onlay preparation in a patient's mouth is difficult. Into this mix we have to add the Cerec/full ceramic crown also. Speaking as a dentist to a dentist, when you walk into your dentist's office with a broken cusp, are you thinking gold onlay, ceramic onlay or full ceramic crown? Naturally, the most conservative possible, with the best margins and that people don't see a lot of gold in your mouth?

I, myself, have lots of crowns - several full and partial gold veneers, several PFMs and a Cerec molar onlay. I frequently ask my students after they have studied with me for a week or two how many crowns I have and how many lower crowns do I have with occlusal gold. They never have any idea, because, conversationally, it really doesn't show for me. I guess I don't yawn in public or scream a lot either. But for many people it really doesn't make any difference whether they have gold on the occlusal or not, especially in the upper arch. But often, as dentists, we make patients think it will make a difference. Many dentists do this because they aren't confident in being able to do conservative gold preparations, like the onlay, or because they have a Cerec unit and the monthly "nut" must be covered.

But, for whatever the reason, the gold onlay can be a very useful preparation and most dentists these days don't really understand it. My dental practice was in an area where there were many older patients, so most of the crowns I did were gold, because, even if the facial surface were prepared, it didn't make much difference to them. I could always restore the tooth with an absolute minimum of removed tooth structure, generally using just what was already missing for retention of the occlusal restoration, replacing one or more cusps. The principles that we learn from 3/4 crowns, inlays and onlays will allow design of preparations that are very conservative, even though they may not have names! I used to have a die from a patient case that I passed around in my classes and defied students to figure out what kind of preparation it was. It is lost now, but as I recall it replaced only the ML cusp on a maxillary molar, had both a box and groove for retention and axial reduction only on part of the lingual surface with a chamfer margin. Now, realistically, one can do the same restoration with adhesive technology - using porcelain - but the reduction would have to be much more. Don't forget that adhesive technology can be used with GOLD as well, but it is of little advantage since we can get good retention generally for this material with less reduction.

The other misconception about gold onlays is that the preparations are more difficult than for the PFM. I have done very few gold onlays where I had to make the boxes for retention in areas where there were NOT previous restorations. In other words, almost always, there is a restoration with boxes already on the tooth and these areas can be readily used, in most circumstances, for retention of the gold restoration. You need but to make the boxes DRAW, put on some bevels and a couple other refinements, and you are ready to take the impression - AND the impression is much easier than for the PFM!

So, at the very least, we owe it to our patients to know this restoration - and occasionally it will make our lives much easier too. But, of course, to be fair, for a badly broken down tooth, we may not have enough tooth structure left for an onlay or even a traditional crown of any sort, without a major buildup first. These days, the buildup/crown combination is rarely used, since we can often prepare the remaining tooth structure for an adhesive porcelain restoration without removing much more structure. So, realistically, as usual, there are occasions where either type of restoration may be the most appropriate - don't be one sided!

By the way - another important issue that we should cover in more detail is porcelain abrasivity. I have studied this in the lab extensively, and don't miss a chance to study it in patient's mouth also. When a traditional PFM with occlusal porcelain is in the mouth for some time, a couple of years or even less, you can feel that the surface is quite rough. There are reasons for this which I won't go into here unless I get a specific question about it. I have a Cerec crown on #30 that was placed about 9 months ago, and I just checked it with an explorer and it is much smoother in the grooves than at the cusp tips, where it wears against the opposing teeth. It should be theoretically possible for the Cerec crown procelain to be less abrasive, but I'm pessimistic in practice. The abrasivity of porcelain crowns in certain patients can be a major factor in ruling them out.

For the MOD gold onlay, where there are boxes on both the mesial and distal sides, there are pretty well circumscribed preparations designs that work quite well. The most important aspect is the depth of the boxes gingivally, because this is where the retention comes from, and also, depending on the gingival recession, we might not be able to get a good impression of the gingival margin if there is not enough gingival clearance to allow passage of the set impression material. As indicated before, often the boxes are already there, but they need to be made to diverge occlusally in the faciolingual direction and converge axially, in order to have draw, AND we may need to deepen them gingivally.

When working with a tooth with a prior amalgam restoration, or even MOD composite, it is often found that this preparation was done with a great deal of faciolingual convergence. Maybe this was done to remove decalcification at the F and L proximal margins near the gingival floor, but more often it is done out of careless on the part of the operator, or because they were taught that way. It only takes a degree or two of convergence to retain amalgam, not the 30 degrees that I often see, when it is absolutely unnecessary. Also, with this much convergence, in order to convert the direct restoration prep into an indirect prep we have to make it DIVERGE, and this will increase the faciolingual size of the box, often too much to be practical. In this case we might have no choice but for full coverage or extracoronal retention.

Again - I encourage you to read through these postings and be stimulated by the topics I have presented and bring up questions that occur to you. In this blog I am only dealing with those issues that relate to preparations that may show up on advanced standing admissions practical exams - but there are many dental topics that are widely misunderstood and would be fun to discuss as well. If you are interested in the general subject of cast gold restorations, I will refer you to the best place for this - and it is not dental schools any more. Try the Richard V. Tucker Cast Gold Study clubs. They have a website now, and many study clubs throughout the world.