Teeth Preparations and Impressions
"The optimal preparation design and finish line can vary according to the initial clinical situation, shape, and arrangement of intact hard lissues...The preparation design for bonded porcelain restorations (BPRs) should Simultaneously allow optimal marginal adaptation of the final restoration and reflect an utmost respect for the hard tissue morphology. "
Deep Margin Elevation
Deep Margin Elevation is a useful tool when sub gingival interdental margins are encountered in indirect adhesive restorations. Subgingival margins, of coarse, can complicate the use of adhesives and composites due to the inability to maintain the proper environment. Isolation, impression taking, and delivery are subsequently hindered, leading to a poor prognosis of the restoration.The margins on these restorations also have poor durability and their relationship with the periodontal tissues can lead to increased inflammation and possible bone loss. This article proposes a technique involving placement of a modified Tofflemire matrix followed by immediate dentin sealing and coronal elevation of the deep margin to a supragingival position using a direct bonded composite resin base. The deep margin elevation technique may be a useful noninvasive alternative to surgical crown lengthening. Dr. Magne outlines the procedure and Dr Matt Nejad performs a clinical case which can be found in those links. They are both a very interesting read and are worth your time.
How to Achieve Good Results for a Final Impression
All prosthetic treatments are required to have different steps which can determine the success of the restoration. Proper impression taking along with the pre-prosthetic phase (conditioning and health of soft tissue, tooth preparation, provisional restoration, etc) require proper execution to guarantee the success. To have a successful impression, precise reproduction of the the intrasulcular preparation margins in the esthetic zone, the gingival displacement, has to be preformed atraumatically and carefully.
Being aware of the physicochemical properties will allow the technician to obtain the best performance and limiting possible distortion and dimensional instability. Being conscious of how these impression materials also interact with surrounding materials, dental products, tissues, etc is also important. For example, there has been reported polymerization inhibition of PVS impression material with the use of latex products, gloves and rubber dams. It has been reported that vinyl gloves have no adverse effect on the polymerization process. It also has been noted that none of the hemorrhagic medications used on the retraction cords, had any inhibitory effect on the impression material and its setting process. Epinephrine, aluminum sulfate, aluminum chloride, aluminum potassium sulfate and ferric sulfate have all been tested.
Bonded porcelain restorations have become popular in recent years. The practitioner needs to be cautious of exposing the dentin during the preparation. If dentin is exposed, immediate dentin sealing is suggested. This will help avoid any possible interaction with impression materials. The inhibition layer must be eliminated by curing the adhesive through glycerin gel.
In order to obtain low trauma gingival displacement, technique and product choice are important. Retraction cord seems to be less traumatic when compared to electrosurgery, or gingival curettage. Retraction cord also lowers the risk of gingival recession. Waiting about two minutes after placement of cord is recommended so that crevicular fluid can be absorbed. If you are wanting a cord that has a hemodynamic property, it is not recommended to use epinephrine as it can induce local necrosis to the surrounding tissues. Two cords of differing sizes are suggested for proper technique. The thinner is placed deep in the sulcus around the preparation with light pressure. The point of this is to “seal” the environment and avoid contamination of the margins from blood or crevicular fluid. The second is then placed with the goal to expand and allow for gingival displacement. Waiting a few minutes is recommended prior to impression taking.
The quality of the final impression plays an important role in the success of a prosthetic rehabilitation. The preparation for the ideal environment for the impression, the quality and stability of the soft tissue, and even the selection of the proper impression material are of the utmost importance. The gingival displacement must be preformed atraumatically in order to have a successful restoration.
Some terms to remember:
-Yield strength: The ability of the impression material to withstand the stress without permanent deformation
-Strain at yield point: The amount of undercut that the impression material can overcome without permanent elastic deformation
-Tear energy: the resistance to tear of the impression material
For more information on the properties of the impression materials and the rest of the article, you can read the full article here. The article written Dr. Nikolaos Perakis et al, has a lot more important information. Please read, to further understand the basic yet important concept of takin final impressions.