At the Margins
In this section I should like to review the placement of margins for crown preparations in relation to the gingival margin.
Our periodontal colleagues advise that, wherever possible crown margins are best placed supragingivally for optimum periodontal health. In addition, at the cementation stage, the accuracy of the fit of the crown to the prepared margins can be directly visualised. It is also easier for our technicians to make these crowns because they do not have to consider trimming the model and the associated guesswork that accompanies some of the impressions they have been sent to work on. What is also clear is that once crown margins are placed subgingivally, there is a loss of control of these margins unless other clinical steps are taken (see below). There is also the risk of encroachment on the biological width which will lead to a persistent gingival hyperplasia and a possible claim against you.
Additionally, when maxillary anterior crowns are being made, the palatal margin must be finishedsupragingivally because the gingivo-palatal mucoperiosteum is tightly bound down to the underlying alveolar bone, and it is extremely difficult, and often painful for the patient, to place retraction cord at this anatomical site.
There are exceptions:
- Where a pre-existing restoration is already placed subgingivally. Here the preparation must finish on sound tooth apical to the restoration margin. A crown lengthening procedure may be necessary first.
- Where there is a relatively short clinical crown and the patient has declined a crown lengthening procedure.
- A discoloured root filled tooth.
- Where aesthetics dictate that the margins are better placed subgingivally, for example maxillary anterior crowns for a patient with a high lip line.
Here, in (4) above lies the seeds of a clinical dilemma. Should the margin be placed for periodontal health, or aesthetics and an increased risk of periodontal disease? There is no easy answer, but what is clear is that, to avoid future complaints and possible litigation, this must be discussed with your patient before starting on the crown preparations. For our American colleagues, this can be a Catch-22 legal question. Did you discuss marginal placement – no – guilty; did you place the margins subgingivally knowing that this increases the risk of future periodontal problems – yes – guilty. Some US dentists are now recording their discussions with patients for this reason. We have it here in the UK in a different form when calling many companies. ‘Calls may be recorded for quality and training purposes’. Really! They are recorded so that, in the event of a dispute the company has a record of what was said for their defence.Consultationtape recordings may not have arrived yet in the UK, but when they are recommended, remember you read about it first in Dental Practice.
However, before you can have this discussion you need to know the depth of the periodontal pockets labially on the teeth you are planning to crown, and how deeply into the anatomy you plan to place your margins. Many patients have no periodontal disease and present with tight and shallow pockets. If you have planned and agreed with your patient a subgingival marginal placement of say 0.5mm, and the pocket is only 1.0mm deep you are in trouble.
How deep should a subgingival margin be? The concensus, but by no means the only answer, from delegates to my hands on courses, is 0.5mm. The next question is can you prepare a tooth,then measure this depth with accuracy?
You can. It helps to visualise small dimensions if you use magnifying loupes. During preparation run a CPITN probe, which is manufactured with a precise 0.5mm diameter tip, along the crown margin, and look where it is in relation to the gingival margin.
In figure 1, the preparation is too shallow, figure 2 exactly right, and figure 3 the tooth has been prepared too deeply.
Once preparations have been made subgingivally, control of the margins has been lost unless some form of gingival retraction is carried out. This may be by:
- Retraction pastes – follow the manufacturer’s instructions
- Retraction cords – my preferred method
In too many cases this stage is omitted because gingival retraction cord placement is perceived as a difficult technique to master. I have found this works.
You need to purchase a cord placement instrument rather than use a plastic instrument which is too thick, a pair of sharp straight iris scissors, and a supply of different diameter retraction cords. Then, choose the correct diameter cord, thin for shallow pockets, thicker for deeper pockets. Only clinical experience will tell you which is the correct diameter to choose when!
- Get the area dry, and keep it dry.
- If the pocket is tight, a gentle flow of air from a triple syringe often helps to open it up.
Start placing the cord interproximally, and ease it into place always in a clockwise direction, because that is the way the material is woven. This direction closes the weave and therefore the cord is easier to insert around the tooth. Anticlockwise placement opens the weave and makes it nearly impossible to place.
- Finish by cutting the cord to make a butt joint with the starting point using the sharp scissors. Blunt scissors risk pulling the cord out of the gingival sulcus!
- Leave the cord in place whilst you take your impression.
The correct way to hold scissors for precise cutting is shown in Figure 4.
Finally, if your technician asks for another impression, take his/her word that this is necessary. It is, in this case, a reasonable euphemism to tell your patient that the original impression may have distorted and you want to be sure that the best possible crowns are going to be made.