Subgingival preparation limit – Step by step
Subgingival margin placement of a preparation is necessary in different clinical situations: to hide the margins of an unaesthetic tooth, to hide the margin when a more opaque restoration is placed, to change the emergence profile of the tooth and sometimes to increase the amount of tooth structure for the restoration. In other words for aesthetic reasons and for retention reasons.
When treatment planning for subgingival margin placement, it is very important to be aware of your patient’s sulcus depth. Knowing how to prep your patient’s teeth and where to place the gingival margins is essential if you want a predictable and aesthetic final result.
In my daily practice, I prefer to use these steps which work in almost every situation:
1. I perform a juxtagingival preparation.
2. I place the first cord that retracts half of the sulcus. This cord makes a VERTICAL retraction.
3. I replace the limit of the preparation from supragingival to juxtagingival.
4. I place the second cord for HORIZONTAL retraction.
5. I remove the second cord for the impression.
I will explain all these steps in detail using a clinical case.
The initial situation of the case. The chief complaint of the patient was that tooth 11 is slightly darker and in perception of the patient the composite restorations are visible in some lighting conditions.
The composite reconstructions were made 1 year ago. I suggested one more try with composite but the patient refused this option.
Because of the high aesthetic demands of this patient, two ceramic veneers were proposed as treatment choice.
As a first step, I use a preparation bur and prepare juxtangingivaly without any retraction cord in place. Of course it is mandatory at this moment that the surrounding tissues be healthy.
I place the first cord that retracts half of the sulcus. This cord makes a VERTICAL retraction.
How much should I prepare subgingivaly? Based on J. Kois and F. Spear there are two rules:
a. If probing depth is 1.5 mm or less, extend .5 to .7 mm below tissue.
b. If the probing depth is greater than 1.5 mm, go half the depth of the sulcus below tissue
So if we have a 1mm sulcus depth the preparation should go 0.5 mm and if we have 2 mm sulcus depth we prepare 1 mm subgingivally.
So, the answer to the question “What is the cord retraction cord should I use?” is: the one that retracts the gingiva half of the sulcus. If the chosen cord does not retract half of the sulcus, then we should choose a thicker one.
I repositionthe limit of the preparation from supragingival to juxtagingival.
With the finishing bur at low speed we can reposition the preparation limits juxtagingivaly.
I place the second cord for HORIZONTAL retraction.
Another question that I receive a lot is: “How should I choose the second cord?”
The second cord is for impression, and the purpose of the second cord is to do horizontal retraction, in order to create room for the impression material.
I delicately remove the second cord for impression. No bleeding should occur at this moment. To prevent this I use impregnated cords with aluminum chloride or epinephrine.
To control the preparation limits we should work when the gingiva is healthy
Always measure the sulcus depth before proceeding to subgingival preparation
The first cord is for vertical retraction and for preparation
The second cord is for horizontal retraction and for impression
Using these steps impression can become a more relaxing step
The two rules of subgingival margin placement. Frank Spear