dental restorative design

Dentists are conservative. This is generally an admirable trait when holding a rotary instrument and removing tooth structure. As with any virtue, an excess of it is rarely beneficial and can be down right counter productive. Bob Winter and I teach a course here at Spear about preparations, the Restorative Design Workshop.  At the start of that course we ask participants to design two preps and then complete them with the armamentarium they use at home to prepare teeth for full coverage restorations. Following the exercise we ask if the participants would like to donate their models and plans to a study that we are undertaking to determine whether dentists actually do take it off.  Not all of the data has been recorded, but I have gathered enough to share a couple of findings with you. These are the basis of a JPD article that I am submitting for publication, but I want to share it with my Spear Education and Spear Online family first.

If restorative materials are to be appropriately designed almost every case will require removal of some tooth structure. How much is required will depend on the case, but the desired outcome always drives the design.  There are two reasons why the removal may be inadequate. 

The first reason is one of vision. The dentist plans to remove too little.  If the manufacturer of a dental restorative material has recommended 1.0 mm of space and the dentist creates 0.3 mm there is an obvious, and potentially troubling, lack of space for the restoration  The result is either an over-contoured restoration or an under-fabricated (this) restoration. Over-contouring has the potential to create disappointment, under fabrication has the potential to create failure.

The second reason is one of execution. The dentist does not remove what was planned; the dentist does not take it off. The final result of either of these occurrences is an under-prepared tooth that is less predictable than possible if the appropriate guidelines are followed and executed. Let’s look at what we discovered.

The Experiment

Plan a full coverage crown preparation for the maxillary right first molar to be fabricated from lithium disilicate (E-max) and cemented (not bonded). According to the manufacturer, this would require a 1.0 mm buccal shoulder with a rounded internal line angle and a minimum thickness of 1.5 mm on the occlusal at the thinnest point. The teeth are in ideal position and no color or other changes are planned. In other words, the outcome desired is exactly what is there now.

338 molars were planned for preparation, prepped, and then measured. Here is a graph showing the planned reduction by our workshop participants for the buccal gingival reduction.

The Plan

The range - 0.3 mm to 3.0 mm

The tallest bar is at 1.0 mm (143), the second tallest at 1.5 mm (98)

The average planned reduction was 1.19 mm


gingival buccal planned reduction


The Execution

The range - 0.1 mm to 1.8 mm

The tallest bar is at 0.7 mm (57), the second tallest at 0.6 mm (55)

The average actual reduction was 0.73 mm

gingival buccal actual reduction


gingival buccal actual reduction figure 2


Here is the bar graph comparison … BLUE is the plan, GREEN is the do.


gingival buccal adequate reduction

There is a lot green to the left of the red line.  The plan was to remove enough … the execution shows that most dentists just do not take it off.

Here are the results for the occlusal:

The Plan

The range – 1.0 mm to 4.0 mm

The tallest bar is at 2.0 mm (234), the second tallest at 1.5 mm (68)

The average planned reduction was 1.92 mm


occlusal reducation planned


occlusal reduction adequate

The Execution

The range - 0.1 mm to 2.5 mm

The tallest bar is at 0.7-0.8 mm (59), the second tallest at 1.3-1.4 mm (53)

The average actual reduction was 0.96 mm

occlusal reduction actual 1


occlusal reduction actual 2


occlusal reduction actual 3

Again, a lot green to the left of the red line. The plan to remove adequate tooth structure is there … the execution is not.  Most dentists just do not take it off.

I realize that this digest will absolutely alter our research from this point forward because you will work harder at taking it off appropriately every time. That would be a small price to pay if you can enjoy the predictability that comes when plan and execution reach the same conclusion.

Gary DeWood, D.D.S., M.S., Spear Faculty and Contributing Author