Vertical crown preparation first try

 I have previously heard about vertical crown preparations and have watched some lectures on them but today I had a go for the first time to prepare a tooth with a vertical margin. Essentially a vertical preparation lacks a horizontal margin and therefore there is no defined stop for the prosthesis at the margin. The fit of the crown works on a slip principle where there is close adaptation of the crown and the tooth preparation. When there is shrinkage of the porcelain at the margins, a horizontal margin will have marginal discrepancies whereas a vertical preparation crown will just shrink higher up on the vertical margin and the seal will not be compromised.

I have heard of a few methods to prepare the margin but was recommended to use the 9904 014 40 fluted needle flame tungsten carbide finishing bur for the subgingival vertical preparation. The smoothness of the bur provides a smooth surface and helps to avoid the bur digging into the tooth. A very light touch and wide sweeping motions are necessary to avoid ditching the bur in. The shape of the bur will provide a long bevel vertical margin. The fineness of the bur also causes less tissue damage and avoids stripping the epithelium.

Figure 1: 9904 014 Flame TC finishing bur

 

The actual reason for using this preparation type here was there was a fracture of the patient's  post and crown leaving them with a decoronated tooth. We explored the options of orthodontic extrusion and implant replacement and settled on attempting to restore the tooth. A vertical preparation was chosen to allow gaining of ferrule effect extending subgingivally without the difficulty and inconvenience of a subgingival horizontal margin which could lead to an open margin or overhang. The tooth was previously endodontically treated so I placed a fibre post and built up a core in composite resin. I had to place the core buildup all the way to the margin of the tooth as there would be no horizontal margin. Normally I wouldn't bother taking the core all the way out, just enough to prep back slightly to recreate the margin but here I had to take the composite all the way out to avoid having a step near the gingival margin. 

Labial view of the preparation. The green part is the subgingival part of the preparation prepared with the 9904 bur

Incisal view of the preparation

After that, most of the gross reduction was completed with thick chamfer burs removing any undercuts, opening the contacts and reducing the incisal surface. One concern I have is if there is enough space at the gingival margin area to stack porcelain and hide the metal margin if we end up doing a PFM. I am considering restoring in zirconia but I do have my reservations about the fracture resistance with a very fine margin.