Continuing the series on the Nancy Case Study on malalignment, once the mock-up is approved, depth cuts and tooth preparation can begin. Whenever you're prepping malaligned teeth that you desire correcting the alignment on, it's impossible to know how much you have to reduce them to get the correct amount of space for the final restoration without doing a bonded mock-up of the desired form and alignment.
It is now possible to do depth cuts into the mock up for the incisal reduction, I prefer 2- 2.5mm, and also the facial reduction, 1 – 1.4mm for full crowns, depending upon the underlying tooth color and material chosen.
Once the depth cuts are complete you can prep the rest of the incisal edge and facial surface. The distal ended up getting most of the reduction and the mesial was less due to the natural rotation of her teeth. Now the technician will have the correct amount of space to produce ideal restorations as long as they match the contour of the bonded mock-up. Completing the incisal reduction and facial reduction removes all of the bonded mock-up.
Going through the interproximal
The next step entails going through the interproximal utilizing a 1.2mm diameter KS1 bur to avoid accidentally damaging the adjacent teeth. Whenever I'm doing anterior preparations I like going through the interproximal at the midline first, straightening the burr to match the vertical alignment of the patient's facial midline rather than the alignment of the teeth, then adjust the distals to the correct taper relative to the mesials.
I then prepared to the gingival crest with the same KS1 bur to create a chamfer right at tissue height. My next step was to use a KS5 bur to reduce the lingual for adequate clearance and to complete any build-ups necessary. The gold restoration popped out during this process allowing me to address the decay that was hidden underneath it.
I could now check my tooth reduction and occlusal clearance using the copyplast as a preparation guide. Often times when there are malaligned teeth, there can be root proximity due to the tooth rotation, which happened to be the case on the distal of Nancy's right central. Even though the margin was dropped to papilla height the contact still hadn't broken. One of the tricks that I use in this case is to use a small carbide bur like a 169L to break contact so I don't damage the adjacent tooth, and then come back with a diamond burr to create the chamfer on the interproximal.
Placing the margins subgingivally
At this point, Nancy's preps were done except for the final gingival margin placement. With the discoloration on the right central, I wanted to place the margins subgingivally to be sure the color difference between the restoration and her tooth would not be visible. However, Nancy's sulcus was too shallow to drop the margin any more than .5mm below tissue. I decided to place an Ultradent #0 cord around both centrals and push it .5mm deeper than the existing margin. This allowed me to come in with a KS1 diamond burr and drop the prep right to the top of the cord, which placed the margin right at .5mm below tissue.
I then pushed the cord slightly deeper to expose the margin and left the cord in place during the final impression. Before sending the information to the lab, I made sure to take shade photos of the preps and to temporize using the copyplast as a matrix.
In cases of malalignment like Nancy's, it's extremely important to get the tooth preparation correct. In the next installment, I'll go over what needs to be sent to the laboratory in cases like this one.