This week the Prosthodontips team take a closer look at the tricky area of crown preparations and give their top tips for preparing the tooth.
For a long time crown preparations were my least favourite part of dentistry. I found them difficult, time consuming, and struggled to get consistent results.
Over the past 10 years I have worked hard to become comfortable with preparing teeth for crowns. In this month’s column I will share with you some of my prosthodontips to get the perfect crown preparation every time
In order to create an excellent crown preparation, we first need to know what an imaginary prefect crown preparation would look like.
Once we have this in our minds eye, we can try and work towards this.
Features of a perfect crown preparation
The amount of tooth reduction required is down to material science.
Certain materials require more reduction than others. This is due to physical or optical properties.
The amount of preparation required is best gleaned from the manufacturer of the material you intend to prescribe for the crown fabrication. It is also invaluable to discuss reduction requirements with your technician. They can tell you how much space they need (and therefore what reduction you need to carry out) to optimise the crown you receive from them.
Typically, dental materials require a reduction of between -0.7mm and 2mm for a crown.
The correct taper will result in a crown preparation with adequate retention and resistance.
Some clinicians suggest preparations require a taper of between 2° and 6°. However, these numbers are not achievable in clinical practice.
A taper of between 10° and 20° is usually adequate and achievable. In reality this means a preparation that is as parallel as you can make it, without any undercuts.
Attempting to prepare a tooth with a taper (even a very small one) will leave you with an over-tapered preparation.
Impression materials and intraoral scanners don’t like sharp edges. We might abrade them away on a model, and it is difficult for an intraoral scanner to pick them up accurately.
Therefore, it’s important to polish your preparations. This is done with fine diamond burs, abrasive disks, or white stones.
Margins should be smooth and continuous with no steps. This is difficult to achieve. I find that a fine grit bur (yellow or white) is very helpful in refining the margins.
Additionally an end-cutting only bur is brilliant for smoothing steps along the margin.
Prosthodontips for an excellent crown preparation
Prep for the crown you want, not the tooth you have
If a tooth is broken, worn, or in the incorrect shape, then it is very easy to lose your bearing and get taken away from the perfect preparation you imagine.
Use a putty matrix of the ideal tooth shape and position (ideally from a wax-up) as a guide to keep you aware of what you are aiming for.
Prepare from crown down
The most difficult aspect of a crown preparation is the interproximal margin. Therefore it makes sense to reduce the amount of preparation required in the area.
Prepare the occlusal surface first, then the buccal and palatal/lingual. Prepare as close as you can to the contact point. Then you will have reduced the height and width of interproximal preparation required.
My preparation sequence is shown in Figures 1-12.
Figure 11 shows what the tooth will look like from the occlusal after Figures 1-10 are complete.
Now you only need to prepare the area shown in green on Figure 12 to finish the preparation.
It doesn’t take long for a piece of tooth, or old restorative material to get caught between the diamond grits in a bur.
Prepare teeth with a new, or nearly new bur. This will make the cutting more efficient, reduce appointment times, and reduce the heat you transmit to the pulp, reducing the risk of pulpal complications.
Good retraction and good vision
In order to prepare a smooth margin you need to see the margin. This is difficult if there is blood, saliva or gingiva encroaching on your margin.
Do your best to control these by using retraction cord, haemostatic agents, or possibly even surgery (electrosurgery, or crown lengthening surgery) to give you a clear, clean field.
Good retraction and moisture control will also aid in your impression taking.
Magnification and good lighting are essential. If I’m not wearing my loupes and my light, I feel as if I might as well prepare the tooth with my eyes closed.
I hope you enjoyed this month’s column, and, as always, please get in contact if you have any questions.
Please keep the questions coming for the Prosthodontips team. You can contact us on Instagram (@sharplingdental and @prostho_zo) and also email (email@example.com).
If there are specific topics you would like us to cover in a column, please let us know.
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