Lumen

Designing the inside of the crown

This step covers the design of the inside of the crown/inlay – the part that will be in contact with the preparation. This part of the construction, and the parameters involved, are crucial for proper fitting. In case you are not happy with the fitting of the produced part, it's here where you should look for possible parameter changes.

The cement gap

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You can define the thickness of the cement gap using the control [1]. It will be preset to the value defined in the DentalDB module. Changes you make here will apply only to the current construction (as opposed to changes in the DentalDB module, which will be saved as the new default values for the selected dentist). Move slider [1] to change the thickness of the cement gap – changes will be applied in real-time, so you can instantly see the effect of your actions. Slider [2] defines the start of the cement gap relative to the margin line, in millimeters. That is, if you chose a value of 1, the area [8] that is within 1mm of the margin line will not have a cement gap. The same way, you can change the end of the cement gap using slider [3], to create an area on top of the crown which has no cement gap [10]. The arrow [11] once again shows the insertion axis.

Additional spacing

By unfolding the „Additional spacing“ expander, you will get access to options [4] and [5] which allow you to add additional spacing.

  • This can be defined separately for X/Y (mesial/distal) direction – using slider [5] – and for occlusal direction – using slider [4]
  • Additional spacing is also applied in the area where there is no cement gap
  • Additional spacing is applied in addition to the cement gap in area where there is a cement gap
    Tip The higher the values here, the looser the fitting. You can also use negative values here, to create a tighter fitting. However, trying to improve fitting by changing the „Additional spacing“ values is a workaround, not a solution. A properly tuned CAM system should be able to achive good fittings with zero additional spacing.

Desiging the crown border

The second tab of the „Crown Bottoms“ dialog gives you access to parameters that define the shape of the crown border. The graphic [5] gives an illustration of meaning of the sliders [1-4].

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[1] Defines the horizontal crown border width. Typically, material properties enforce certain limits here; e.g. for Zirconia a common minimum value is 0.2mm.

[2] Defines the length of the angled part of the border. This may commonly be set to zero.

[3] Defines the angle of the angled part.

[4] Defines an additional vertical border. Commonly this is also set to zero.

Dealing with undercuts

The third tab in the „Crown bottoms“ dialog gives you access to undercut-related options and milling parameters.

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Undercuts in the preparation are blocked out by default, unless you specifically chose not to do so, by checking checkbox [1].

A certain (small) angle may be applied for blocking out undercuts, as defined by [2]. By default, a value of zero is used, meaning that undercuts are blocked out „straight down“.

Slider [3] defines an „untouchable zone“ around the preparation margin, where the crown is never blocked out, even if it should be (due to undercuts being present above).

We will further elucidate the effect of this option with an example. Consider the two images below (for this tooth, we have intentionally set an incorrect insertion axis to illustrate the problem).

Dcad 05.jpgDcad 06.jpg

The graphic on the left shows the behavior with „Don't block out zone near prepline“ set to 1mm. Observe how the upper part is blocked out, but as soon as the untouchable zone is reached, the crown bottom will go back to the preparation (note that it is not possible to produce this kind of restoration with 3-axis milling). The graphic on the right shows behavior with „Don't block out zone near prepline“ set to zero. As you can see, the crown will not be in contact with the preparation around the margin. In both cases, the result will be non-optimal and the restoration will need manual work (either by manual milling, or by adding ceramic) for a proper fit. It is up to the operator to decide which option may be better in the particular clinical case.

Tip If at this point in the construction, you find that the insertion axis is not optimal, you can still easily change it:

Right-click on the tooth for which you'd like to change the insertion axis, and chose „Set insertion direction“. You can then change the insertion direction by orienting the view to the insertion direction and clicking „Set current view as insertion axis“.

Dcad 07.jpg

In the third tab of the „Crown bottoms“ dialog, you can also change the parameter for tool diameter compensation. If you have selected a material that is to be milled, the „Anticipate milling“ check box [4] is checked by default. Under [5], you can chose the diameter of the tool to be used for milling. It is beneficial to chose a value slightly higher than the diameter of the actual tool you're using, e.g. chose 1.2mm when milling with a 1mm tool.

Tip Consider the diameter of the smallest tool used. The diameter of the roughing tool is of no interest here.

Any parameters you change in this screen will not be applied in real time, as these are computing-intensive operations. Click „Apply“ [8] to see the effect of your changes. To visualize undercuts which remain (e.g. due to usage of „Don't block out zone near prepline“), click the „Show undercuts“ button [7].

When done, click „Next“ to proceed to the next step.