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How to get the most from your dental lab
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How to get the most from your dental laboratory Below is a list of general procedures for the restorative patient whether it be cosmetic or reconstructive Initial patient visit includes a thorough oral exam noting all conditions of existing teeth, tissue, acquired bite, centric bite, and any mobility of TMJ concerns. You should also note the patients initial concerns or immediate problems. Along with your patient and assistant you should be able to put together a shot term plan and a long term plan. Short term plans would address the patients first reason for coming in, other than a routine check up, but would also include your professional diagnosis of underlying conditions that may contributed to the patients immediate problems or concerns. Long term plans would be to get the patients oral health to an optimum condition where only routine check ups and hygiene visits are required. Long term plans are the most important and should be developed based on your professional diagnose, the patients concerns and financial situation. The long term plan should have some desired guidelines built into the plan, such as the order that existing oral health problems should be addressed and a time frame for which procedures should be done. Long term plans should be based on 1 year, 3 year, 5 year time frames. Most patients should be able to have optimum oral health within five years, but should not be limited to 5 years. The time frame for most long term plans will be based on the patients financial abilities but mainly on the patients understanding of their oral health. A patient that is toughly educated on their oral health and can understand the benefits of your professional services will generally be more willing to stay with a plan and pay for it. Short and long term plans should be broken down into four categories: doctors concerns, patients concerns, laboratory concerns, and financial concerns. Why laboratory concerns? Because so often a case is sent to the lab with very minimal information. ( Make PVC, Shade, accompanied by a triple tray impression that includes a prep and one or two adjacent teeth). In most cases by the time a patient needs some restorative work they will have other conditions that ether caused the breakdown or will be effected by the restoration. We have seen hundreds of cases where a restoration was placed and a year or two later we restore another tooth next to or opposing a previous restoration. Many times the first restoration could have been greatly improved upon had there been a plan. Now we have two restorations that are both a compromise. This holds true for single restorations, cosmetic restorations, and larger reconstruction cases that may involve removable appliances or implants. 1. Inform you laboratory of the overall treatment plan, short term and long term. The more information the better. Many times a crown can be made to maintain an adjacent contact or an opposing contact that can be easily re-contoured when the adjacent or opposing tooth is restored. Also informing the patient and the laboratory that and adjacent or opposing tooth may need to be adjusted so that the new crown will not be compromised in function and esthetics will cause less confusion at a later date. 2. Prior to any cosmetic or restorative procedure, take a set of study models and a full arch bite. Study models of broken down teeth or bad restorations give us a lot of information when building a case providing us with a starting point. Depending on the case, take a bite registration in a full closed acquired centric position. If the case involves a full mouth reconstruction, TMJ problems, or changing the VOD then the bite registration should be take accordingly. 3. Taking accurate bite registrations is one of the most important procedures to avoid occlusal adjustment at seat time. Whether you or your assistant take the bite registration it is important that the patient understand why you are taking a bite and how you need them to bite. Many patients when asked to bite in to a registration material automatically bite in a protrusive position by nature leaving the posterior open. It is also a good idea to take a bite before administering any anesthetic so that the patient can have a better feeling of the natural acquired centric or the centric position you may be restoring to. While Triple Tray impressions are popular for the all in one bite and impression, we find that many are inaccurate. This is partially due to the patient but some of the inaccuracy also comes from the processes of the model work. Once the triple tray impression is poured up, the stone model that is to be used for the restoration must be removed to have further work done, such as trimming, pinning, and basing to create an accurate sectioned removable die model. Once this model is created it must be placed back into the triple tray impression in order to be articulated. Models generally never seat accurately back into any impression causing slight errors in the bite registration. For the above reasons it is always better to take a bite registration before starting any restorative procedures. Materials for bite registration can be of choice. We prefer silicone type bite registrations over composite, duralay, or wax bite due to the accuracy and durability during shipping in any weather conditions.
4. Prep design is very important for esthetics and durability of any restoration. Back in the day when high gold content alloys were used for full cast crowns and dental cements where primitive compared to today's, the taper knife edge margin prep design was very practical for a lot of reasons.
This was the preferred prep design for most gold restoration and gold/acrylic restorations. One of the benefits was the wedge design. As the crown was seated on to the preferred 6 degree tapered prep the crown would have extra retention as it is wedged in place. With primitive dental cements there was always a concern for open margins that any exposed cement would wash out leaving a gap under the crown where decay could reoccur . With the high gold content alloy (18% AU +) that where relatively soft compared to today's ceramic alloys, and the tapered knife edge prep design, open margins where easily avoided. The soft alloy of the crown would form to the tooth as the crown was seated over a 6 degree tapered wedge/prep. A 6 degree tapered wedge is a simple machine having a mechanical advantage (MA) of 9.5. The simple equation for the force exerted at the margin is MA X the occlusal force (OF) = the outward margin force (MF). If we assume the occlusal force to be 500lb/sq.in. the outward force at the margin would be 9.5 X 500 = 4750lb/sq.in. Surly enough to distort any restoration forcing the margin to have an excellent seal. These are close approximations and a simple equation was used. More complex equations would factor in friction and the hydraulic resistance from the cement during seating. We are assuming that these forces are minimal. Reduction was kept to a minimal mainly to conserve on the amount of gold use keeping the cost down. Also more tooth structure the better, stronger prep, less chance for sensitivity, etc, and lastly today's demand for ethics was not that prominent. As dental materials and techniques advanced we are able to provide a life like restoration that in many cases can not be detected when placed in the mouth. This increase in technology and materials has created a high demand for every restoration to be esthetic regardless of it location in the mouth. In order to achieve beautiful esthetics, strength, longevity, and function we must have the proper prep design. The tooth when prepped ideally will have a taper from 6-10 degrees producing a wedge as describe above. All restorations involving ceramic are prone to poor esthetics and breakage when placed on a prep with knife edge margins. Taking a look at the diagram below of a standard porcelain fused to metal crown on a taper knife edge margin prep you will be able to easily understand the inherent problems with this prep design.
First every ceramic restoration must have either a rounded chamfer or a butt shoulder margin 2mm in depth for a couple of reasons. Even though the prep with a 2 mm rounded chamfer or butt shoulder margin will still taper ranging from 6 to 10 degrees, we can eliminate the potential for porcelain fracture as described above, by simply using a couple of coats of die spacer on the tapered portion of the working die. Die spacer will leave room for cement and allow all of the occlusal forces to rest on the butt portion of the margin during seating. This will also provide a very tight marginal seal with virtually no cement line providing the impression and crown are accurate. This is illustrated in the drawing below.
Click on Margins for more details. Another advantage a 2mm butt shoulder or rounded chamfer margin is the body porcelain depth at the margin is 1.5-1.7mm thick. This provides enough porcelain for a very esthetic and strong margin and with today's dental cements a very durable long life restoration.
The next important part of prep design that is required to achieve maximum strength and esthetics is the 2 plane prep. All teeth have virtually 2 planes. Depending on which tooth or teeth are being prepped determines where the 2 planes change angle. We refer to these planes as 2/3 and 1/3 plains. These planes are illustrated below.
Incorrect prep designs of straight plane preps produce thin, week, and opaque unaesthetic crowns, or over contoured crowns as illustrated below.
5. Taking an accurate impression will insure a well fitting restoration with virtually no adjustment. It is imperative to have readable margins. Please read, " Final Impression" a reprint from the Journal of Prosthetic Dentistry. At D.D.S. all of your dies are trimmed under a Bausch & Lomb Stereo Zoom Microscope to insure the highest quality restoration. Impressions need to be detailed at the margin but also for the surrounding teeth. Triple tray impressions seem to be the technique preferred by most Dentists, but full arch impressions give us so much more information when building a restoration. With an accurate impression of the surrounding and opposing teeth we can better asses the size, contour, angle, plane of occlusion, the patients lateral movement by observing wear marks, perio condition noting bone loss and mobility. The more information we have the better we can make the restoration and possibly make suggestions that will make you and your patient happier.
6. Taking an accurate shade is another very important part of getting the most from you dental laboratory. At D.D.S. Laboratory we have every shade guide available and we do not charge extra for custom shading. Using multiple shade guides and recording accurate shade details will give us the information need to produce beautiful, natural, esthetic looking restoration. This is easily done by using the convenient shade diagram on our prescriptions. Photos are also very helpful. Photos are not accurate for shades but when taken with the shade tab next to the tooth, we are able to see the slight differences between the tooth and the shade tab. The shade tab may be a close match to the general body shade of the tooth but the amount of tooth enamel and the translucency are not always depicted by the shade tab. With photos including the shade tab and the tooth we can see these differences and make adjustment in the build up of the porcelain to make a more natural looking restoration. The more details the better and with today's digital cameras photos are inexpensive, easy to manage, and can easily be sent via the internet. For more information on how to send us your photos, please click Here.
7. Writing a clear, concise, and informative prescription to accompany the case is also a necessary procedure to getting the most from your dental lab. Our prescription pad is designed to gather the most information about the case with the minimal time and effort. Any additional information that is included is always welcome. Believe it or not having the patients age and gender is useful information. This information helps us determine shade characteristics and crown contour when other information is not present. Also communication about patients appointment also helps us in scheduling our work so that we have a consistent number of units per day which helps in keeping our consistent quality and on time deliveries. Communication is the key to a successful working and business relationship with your dental lab. You may not always have the time to pick up the phone, so at D.D.S. laboratories we have a simple questionier on the back of each prescription that allows you to let us know how we are doing, or you can always click Here.
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