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中文Dental impression of 36th tooth and crowning procedure
Dental impressions have become fairly common procedures now-a-days. The process and different types of dental impressions, however, change with the type of the teeth, problems and pathophysiological conditions associated with them (Livaditisa, 1998). The procedure starts with numbing of the teeth and gums by local anesthesia. In most situations, dentists take impressions of teeth using a putty-like substance, known as ‘Impression material”. This impression will then be processed in the dental laboratory where a plaster cast will be generated. This cast will be used to make dental crowns which will be almost an accurate fit to the tooth in consideration since the cast fits snugly on the same tooth. The entire process takes about a week or so to complete. In some situations the dentist may choose to use a dental ceramic milling machine which results in the entire process being completed in a single visit.
During the two to three weeks waiting period, the tooth is covered with a temporary crown usually made of a thin sheet of plastic or even metal sometimes. This temporary crown is cemented in place until the new crown is ready. Subsequent to impression and crown preparation, the dentist will compare the color of the crown in the case of porcelain crown, so as to match the color with neighboring teeth. This process is called shade grade and involves comparison of pieces of porcelain of different colors with the teeth to select the appropriate color. Once a suitable match is found, the particular color of porcelain is used to generate the final crown. The final step is cementing the crown into place. This step involves numbing the teeth and gums and removal of the temporary crown if already placed on the tooth. Before the crown is cemented, the dentist follows certain steps to ensure proper placement. This involves inspection of proper fit by dental floss and usage of dental tools. The dentist usually asks the patient to bite down, and then removes and adjusts the crown until the patient is comfortable. Since the 36th tooth is not clearly visible in smiling or laughing, there is no need to determine the color match with neighboring teeth in these situations but dentists usually ask patients to self evaluate color match so as to make it as esthetically better as possible. The final step involves cementing the crown over the tooth. Once there is consensus between the dentist and the patient over crown and color and blending, cement is placed inside the crown and it will be subsequently seated on the tooth. Once the placement is complete the cement will be allowed to dry. The dentist then uses a dental tool to scrape of pieces of cement the overflow outside the edges of the crown. This completes the impression and crown placement procedure.
2. Type of impression methods used for the 36th tooth
There have been tremendous advances in fixed prosthodontic impression methods over the past four decades. The different impression methods in use are:
2a. Copper tube and resin coping methods
The copper tube and associated modeling compound method is rarely in use at present but it has been modified and reintroduced on a regular basis. Thus this method includes the copper tube with modeling compound along with variations in the method such as copper tube variations with elastomer combinations; resin copings with elastomer; resin (interim restoration type) crown or fixed partial denture (FPD) with elastomer and polycarbonate crowns with elastomer. In spite of the variations, this method had an underlying basis. They all have a hard substance as carrier in common for the impression material. The carrier finally becomes a part of the impression (Livaditisa, 1998). A lot of care is taken in preparing the carrier which is made in such a way that it rises a little above the preparation finish line. Finally finish line registration is accomplished in the carrier. In all these variations, an elastomeric modeling compound is used to achieve the actual finish line impression. This method is very efficient in delivering impression material into the sulcus.
2b. Syringe/tray procedure
This method followed the tube/coping procedures and became popular after the introduction of elastomers. Briefly this method consists of predefined steps such as retraction or pulling to push the gingival tissue followed by hemostasis to curtail bleeding from the sulcus. This is followed by injection of impression into the sulcus through a syringe dispenser and teeth and associated structure registration with a full arch impression. Retraction and hemostasis are closely linked. A chemically impregnated retraction cord is the one chosen in this method for hemostasis. The drawback associated with this method is that it is laborious and inconsistent. Retraction, hemostasis and syringing time increases proportionally with increase in the number of teeth. Also it is very hard to maintain gingival retraction indefinitely and pressure of from the syringe tip/bevel usually leads to some sulcal bleeding. The advantage of this procedure is that it affords better directional control. The other advantage is that using this method it is possible to deliver the impression material directly into the sulcus thereby preventing entry of air.
2c. Putty/wash or impression/reline method
This method differs from the syringe/tray method mainly through the process of placement of the impression material into the sulcus. Most of the process of retraction and hemostasis are similar but there is no syringe process involved. Instead this method used closely adapted trays, heavier bodied material and relieved preliminary impressions to push the wash or reline material into the sulcus. This method is gentler than the syringe method but there the disadvantage is that a little bit of air enters the sulcus. The partially seated impression acts to trap fluid and air into the sulcus. This method was improved later to efficiently deliver the impression material (Lococo, 1986). Success is less predictable and more random in this technique. Control in impression material delivery is less in this method. The advantage of this method is that it is gentler than the syringe method and leads to reduced sulcal bleeding. This method may lead to gingival tissue collapse in a case where the impression material is heavy in consistency.
2d. Matrix impression system
This is a relatively new system that utilises three steps and / or three different viscosities of impression materials. The main aim behind this method is to rectify the problems and pitfalls associated with the earlier methods on one hand and to include their advantages on the other. In this method, a matrix of occlusal registration elastomeric material is used over the tooth preps. The matrix is fabricated in single or multiple pieces depending on requirement. Subsequent to this, the matrix is trimmed, and the retraction cord is removed. Definitive impression is then made using a highly viscous elastomeric material. Once matrix impression is seated, an elastomeric material of medium viscosity in a stock tray is kept over the matrix and remaining teeth and the impression of the entire arch is then generated. This system is effective because it can be used to efficiently control four important forces such as retraction, collapsing, displacement and relapsing. Tearing is also eliminated in this method.
2e. Interim fixed prosthodontic restorations
This is not an impression method per se but in some situations it shares steps common for impression procedures. The main aim of this method is to register finish lines and prepared abutments accurately. In this method, temporary crowns are produced using manufactured shells (celluloid or polycarbonate crowns), vacuum assisted moulds of the generated teeth and associated procedures (sectional impressions, split-wax etc.) and direct restoration carving completely. This method simulates the cooper tube and modeling compound method especially with respect to extension into sulcus and becoming a part of the sulcular flange. In this method, it is seen that after pattern removal, there is finish line registration even in the absence of retraction procedures. The only disadvantage of this method is that the resin from the vacuum carrier comes out and creates an unfavorable force.
3. Problems associated with impression and crowning
There have been significant improvements in impression methods and crowning procedures over the past three decades. There are two main problems that cause concern during and after impression/crowning procedures. These issues are gingival tissue recession which usually ends by exposure of the restorative margins on one hand and chronic marginal fingival inflammation on the other. The history of these problems dates back to the late 1950’s when metal ceramic crowns were introduced for use post-impression and they were thought to offer optimal fit (Donovan and Prince, 1985). For esthetic value, the metal margins were covered and placed inside the gingival sulcus. This procedure however, turned out to be unpredictable and led to the evolution of porcelain based ceramic crowns (Toogood and Archibald, 1985). In spite of these advancements, there were still problems seen with respect to gingival recession and gingival inflammation.
3a. Gingival recession
Gingival recession is not associated with normal ageing but is indicative of pathology. If good gingival health is present prior to impression and crowning, then with the usage of adequate clinical technique, then a very stable relationship can be maintained between the gingival tissues and the restorative margin of the crown. Although, there are a number of methods available to suppress gingival recession, most of it lies in the hands of the dentist. The most important factor that ensures success of impression and crowning is the ability to predict health of the gingival tissues prior to making impression and definitive margin placement. A large number of patients who present with gingival tissue problems usually require extensive restoration and treatment of before the initiation of the impression procedure (Nemetz, 1974). It is almost always impossible to prep and perform impression and crown restoration in tooth with extensive gingival inflammation in the same appointment and even if done, is not associated with good prognosis. However, if the process is continues over a large temporal span, it is usually possible to instill proper oral hygiene in the patient. Additionally the gingival inflammation may reduce and resolve over time thus resulting in a successful impression and crown restoration. The problem however, in this situation is that the gingival tissues move along the apical route thus exposing the restorative margins. This may lead to esthetic failure during the provisional or restorative phase of the entire procedure.
Most dentists decide to wait until peak gingival health is attained in order to determine final margin location. In most impressions and restorations, it is usually advisable to leave the margins in a supra-gingival location after the tooth is prepared. Optimal crown and gingival tissue contours are usually achieved by fabrication of provisional restorations, which aid in proper oral hygiene and are good predictors of successful restorations (Preston, 1974). In situations of gingival recession, numerous periodontal procedures are performed to bring the tissues to optimum health status. The patient is usually recommended Peridex for a period of two weeks after determining that the gingival tissue health is optimum and suitable. Subsequent to this gingival margin location is determined and prepared. The patient continues to use Peridex throughout the impression and crowning procedure (Sorensen et al, 1991). However it is very vital to determine that tooth impression and prep does not damage the gingival tissues. Preparatory procedures involving a retraction cord o pre-pack the gingival sulcus usually has the ability to stop iatrogenic damage. Subsequent to cord removal, the margin can be dropped through the created space and this usually prevents trauma. As an additional measure to minimise trauma, rotary instruments are usually recommended.
Another important criterion to keep in mind during gingival displacement procedures is damage to the attachment apparatus. An easy way to achieve this is to define the gingival margin and place it at a small distance from the gingival sulcus which usually helps in the atraumatic retraction process. Suitable retraction cords immersed in hemostatic agents are usually used in these procedures to place them into the gingival sulcus for time frame of eight to ten minutes (Nemetz et al, 1984). The cord is then moistened with water prior to removal to prevent damage to the delicate epithelial inner lining of the sulcus (Anneroth and Nordenram, 1969). Fabricating quality provisional restorations must be completed as early as possible in the sequence of restoration during the healing process or after finalization of preparation. In all situations the provisionals must satisfy criteria such as esthetic quality, marginal integrity and crown contour physiology. In problems associated with gingival recession, conventional dental luting agent is indicated and the one commonly used is Zinc oxide-Eugenol temporary cement which affords a perfect seal of the prepared tooth and abolishes sensitivity in the provisional phase. However, the trade off is that zinc oxide is an irritant and excess luting agent must be cleaned before discharging the patient since residual agent may lead to inflammation and could compound the problem.
3b. Gingival inflammation
Gingival recession is the important cause of esthetic failure most impressions and crown restoration procedures, long term gingival inflammation of gingival tissues represents an equal and serious problem. Historically the inflammation was attributed to the poor oral hygiene status of the patient. However, recent studies show that in many patients the surface of the cervical marginal configurations may be rough by birth and can lead to greater incidence of accumulation and retention of plaque. This is a very important factor that could lead to the development of marginal inflammation (Keenan et al, 1980). This suggests that glazed porcelain and highly polished metal surfaces can prevent inflammation and could be placed inside the gingival sulcus. However, there could be multiple biological causes of chronic inflammation. It is very difficult for the clinician to decide whether to place the crown margin with the gingival sulcus or not in order to prevent the possibility marginal exposure during recession. It is known from clinical studies that proximity of restorative margin to the attachment is usually associated with adverse periodontal outcome and vice versa (Newcomb, 1974).
Studies have shown that if the restorative margin is placed 0.5 millimeters from the healthy free gingival margin, which suggests a minimum distance of 3 millimeters from the alveolar crest, it usually results in better prognosis. In majority of cases it is seen that the cause of inflammation is violation of the biologic width which happens when margins are placed too deep into the sulcus. This most often results due to improper anatomical reshaping of the gingival tissues with the interproximal regions placed in close proximity to attachment. While the general recommendation is supra-gingival for crown margin placement, it is seen that sub-gingival margins offer better esthetic value. A number of studies shows that with this type of an arrangement esthetic quality is preserved with normal or in situations of exaggerated smiles (Crispin and Watson, 1981). In all situations, a proper procedure results in substantially improved and simplified operative procedures, periodontal response and evaluation of marginal integrity.
4. Discussion
Impression for crown of teeth with sub-gingival margins is difficult because in some cases there could be a defect that extends deep down into the sub-gingival region or the impression is indicated for generating a full coverage crown for solving esthetic issues (Kazuhiko et al, 2005). In the classical case it is always better for impression to be carried on teeth with supra-gingival margins because there are several studies that show that the gingival health in teeth with supragingival margins is almost same as the original teeth. However, there may be situations where the margins are sub-gingival and in these situations, the margins require retraction so that the impression material can penetrate and have access to the prepared margins (Livaditisa, 1998). A subgingival margin is only suitable when the attached gingival is free of inflammation on one hand and there is enough gingival tissue. In a case where the gingival tissue is not enough, a periodontal graft may be needed (Bassiouny and Yearwood, 1996). In most cases the appropriateness of the impression procedures should be evaluated based on the pros and cons of each procedure. Additional factors that should be taken into account are the extent of the sub-gingival margin present and the level of esthetic crown desired (Livaditisa, 1998). In most cases traditional fixed prosthodontic impression procedures are used. But with the advent of latest impression procedures which have the ability of better retraction , reduced sulcal bleeding and improved penetration of the impression material, these methods are more suitable for sub-gingival margins in the 36th tooth. One such system is the matrix impression system which combines the positive aspects of traditional methods with superior healing and esthetic appearance (Livatidisb, 1998). This method is suitable and indicated in situation were there are gingival recession and inflammations, two major processes that are a major hurdle for the impression procedure. The choice of the exact impression method however, rests in the hands of the dentist who considers the benefit of each method and customises is according to the situation.
5. Conclusion
Impression of the 36th tooth in the subgingival margin is very critical and requires the choice of correct impression methods. A careful selection of impression methods is necessary to promote efficient gingival retraction, reduce sulcus bleeding, assist in efficient impression material entry into the sulcus at proper pressure and prevention of air and fluid entrapment. In addition it must be kept in mind that there might be problems associated with gingival health such as gingival recession and gingival inflammation that require special treatment at the sub gingival margin. In these cases it is important to defer the procedure and promote oral hygiene and reinitiate the procedure once gingival health is regained. Thus it is vital to tailor or customise impression procedures based on the health status of each tooth and plan judiciously in such a way so as to achieve maximum prognosis and will prevent relapse in the future.
