THE TOP TEN CLINICAL MISTAKES IN CROWN AND BRIDGE – PART 1
I am getting some great feedback from our Summit clients and our BEST for Dentistry members (www.bestfordentistry) about one of our alliance partners, Bayshore Dental Studio. The quality is great and the pricing even better.
Just the other day, I was speaking with Jaimee Borden, their operations manager, about the wide range of good and not so good clinical work they see on a daily basis. I sympathized with her because I was a dental lab technician for 6 years before becoming a dentist and did most of the crown and bridge lab work for the doctors that were our teachers at Baylor Dental School. Even our proctors varied widely in the quality of work I received. So much so that I was forced to have them re-prep or retake impressions so that I could give them the work they wanted. With most commercial labs this is impossible. They are tasked with the almost impossible job of taking marginal impressions and trying to compensate for the lack of excellence that they would prefer to have in order to create the best product possible. The difficulty is that if they complain, the offending doctor just sends their impressions to someone else. It’s a no win situation: The lab loses business and the doctor fails to improve. The take away here is that the relationship between doctor and lab is a partnership where there is a fiduciary responsibility for both parties to work together. If you fail to deliver an excellent clinical result, followed up by a perfect impression, the result will suffer and it is your fault. Do just the opposite and you can expect a restoration from your lab that you could hook shot from the corner of the room in a couple of minutes at the seat appointment.
I would like to take a couple of weeks to go over the most common things I see in dentistry when it comes to marginal clinical excellence. It will be a simple 1-10 countdown and, like so many other things, is not an exhaustive list.
1. Inadequate occlussal or incisal reduction: Whether it’s an anterior or posterior preparation, there has to be adequate reduction. With the rather large number of options we have for crowns, this can be problematic. The old porcelain fused to metal has gone the way of the dinosaur. Over 90% of crowns today are E.Max or Zirconium. The problem has arisen because some dentists are not following the manufacturers recommendations on preparations, and dentists in general are not doing enough reduction. Occlussally, a porcelain to metal or E.Max crown needs 1.5-2mm of reduction. Keep in mind two millimeters on a tooth is not a flat top. Your reduction should yield a result resembling the surface contours of the unprepped tooth. The Zirconium has created a gross misunderstanding that you need little or no reduction. Once again, you will need over 1mm reduction and 1.5 would be best. This involves not just a structural reason, but also a cosmetic and occlussal need. Zirconium crowns have traditionally been “butt ugly” but really tough. I feel like Zirconium crowns have contributed to bad clinical habits due to its resiliency. This perpetuates less than ideal standards of care. This has led to using them in areas they were never intended and subjecting patients to a compromise on cosmetic excellence. I would routinely use the E.Max crown for most situations other than full coverage second molars. For a single anterior I would choose an Empress crown for its ability to almost disappear appearing just like a natural tooth.
2. Margins too deep (Biologic width): Those high-speed handpieces screaming along coupled with the cutting ability of modern burs, and you could vaporize a tooth in 20 seconds. As a dental student, I remember seeing preps come in to my lab with margins that were buried in bone and/or obvious lacerations of the attached gingival. Your preps should follow the contour of the gingiva. Believe it or not, most preps from the bicuspids back can be “supra-gingival, with the use of an E.Max or all ceramic crowns. There is no longer a need to bury the margin. If you are forced to go deep mesially and distally, you probably need a crown lengthening procedure. If you cannot give the lab at least a 1.5 mm collar around the tooth with a well-defined easily seen margin, you have failed to deliver a standard of care that allows any lab to produce an adequate restoration.
3. Poorly defined margins (loops, illumination, and retraction): Once again, the lab needs a clean, well-defined margin at least 1mm above the soft or hard tissue. Using loops and adequate lighting are precursors to adequate preps. Involving your assistant who has a different perspective on the tooth helps also. Once the proper reduction and retraction is done, the impression has to be spot on. I don’t find that Expasyl or similar products effect an adequate retraction for perfect impressions. They are efficient but lack effectiveness. In addition to a minimal retraction effect, it seems to leave a smear layer on the tooth if it is not vigorously scrubbed prior to taking the impression. I would recommend every filling and every crown be scrubbed with Chlorohexidine to clean and disinfect the tooth prior to impressions, seats, fillings or buildups. I advise all of our clients to place a single or double cord prior to even beginning the preparation to insure you don’t encroach on the biologic width, to control bleeding, and to achieve maximum dryness and retraction.
4. Too little mesial-distal reduction: This is especially true in an anterior tooth creating a “tepee” prep. Once again, follow the margins of the soft tissue and do not go straight through. If using an E.Max crown we need about a 1-1.5 axial reduction in the form of a deep chamfer or flat margin and a 1.5-2mm occlussal reduction. The Zirconium crown needs at least a 1-1.5mm occlussal reduction, and a .5-1mm axial reduction with a deep chamfer margin. (Buccally I would do a 1 mm margin for both structural and cosmetic profile) Both crown preps need a 6-8 degree prep for retention and rounded line angles.
5. Prep not long enough: I would suggest that anything less than 2mm in length is a “short clinical” crown prep and not sufficient for adequate retention. We have all faced this and even with the advent of bonding this creates a less than ideal long-term solution to crown retention. For most of us we need to consider a crown lengthening procedure. If in a grey area of length, I have found that adding grooves on at least two opposing parallel surfaces will work most times. Buildups are not the ideal solution. In a way, they really are a far cry from the strength of the crown by itself. Buildups are really there to insulate and protect the pulp. Secondly they were originally needed in a fired porcelain crown to make sure that the thickness was the same on every surface to minimize the stress created by firing the crown repeatedly. With milled blocks of ceramic this is not a problem.
More next week.
Michael Abernathy, DDS