English
中文Introduction
Today, people are quite aware regarding the appearance and position of their teeth. They want to have perfectly erect, even and whitish teeth. So, what they expect from their artificial tooth is that they would replicate the natural ones in appearance as well as in strength. Initially, metal substructure was used to replace the missing tooth which at times resulted in grey discoloration on top of the gum line making them appear quite unpleasant. Therefore, this technique of tooth replacement and restructure was developed in which ceramic core was pushed in order to create the crown. These all-ceramic crowns, without any metal touch provide high degree of customisation. Staining and blemish can be rectified with the help of ceramic substructures. Moreover, the porcelain over the crown unites better with ceramic instead of a metal core and leads to a well-built bonded crown.
Dental ceramics is helpful in repairing the teeth and even replacing them if required with their substitutes like metals, or other substances capable enough to provide them strength (Zohairy & Feilzer, 2005) to endure routine wear and tear and possessing similar physical properties as the natural one (Noort, 2002). Ceramics have become quite acceptable alternative for such procedures due to its consistency, natural appearance and attractiveness when used in place of a natural dental crown (Albakry, et al., 2003). But, ceramics are inclined to be frail and therefore are not a permanent solution for the patient and often they break down when significant burden is put on the teeth by the patient (Guess et. al., 2009).
Clinical procedure for Preparing a Crown
Smashed teeth in the mouth require “caps” for guarding them and preserving their capability to perform in a beneficial style. These ‘caps’ are the dental crowns which are fixed on the decomposed or spoiled teeth. They are also used for replacing the old caps that have deteriorated due to natural or other factors. They act as protective sheath for susceptible teeth. It is however, necessary to use such materials for making the crowns which would be suitable for the patients and capable of providing total security of the damaged teeth. The procedure normally employed involves all-ceramic (all-porcelain), gold and porcelain-fused-to-metal. The final selection of the material will depend upon the strength requirements, the aesthetic and clinical demands, the space available in the jaw and the durability of the material.
A dentist, after the preliminary examination of the oral cavity, will make a molded impression of the present position and structure of the teeth before going for crown procedure. The mold is carefully framed and forwarded to the laboratory where a permanent crown is constructed to secure or replace the damaged tooth or teeth on the basis of the mold of the teeth. As soon as the new crown is ready, it is cemented on the damaged tooth. New computer-aided design/manufacturing technology (CAD/CAM) demonstrates a 3-D image of the teeth which helps a dentist to build a restoration comfortably.
Crowns used with Fixed Partial Dentures and Dental Implants
Dental bridge or the fixed partial denture a technique in which the teeth besides the missing tooth are also capped or crowned and attached to a false duplicate of the absent teeth. The technique was employed prior to the initiation of dental implants which is the most popular option even today for replacing missing teeth. Under dental implant procedure, a model of the tooth is prepared using titanium which is positioned and amalgamated with the jawbone with the help of titanium paste. The part of the tooth replica that is above the gum line is then fitted with a crown. Dental implants eliminate the need for shielding the adjacent teeth and yet offer considerable assistance in biting. The substance which are normally used for make crowns are Gold crowns,Porcelain-Fused-to-Metal crowns and All-Ceramic crowns.
Gold Crowns
Those with the habit of grinding their teeth while sleeping or scrunching them under stress should prefer such teeth due to their toughness and strength. Gold crowns are also appropriate for molars as they are involved in intense chewing and they also required less preparation time. Though, the colour of the Gold crowns fails to give a natural appearance to the teeth and is therefore not preferred by all.
Porcelain-Fused-to-Metal Crowns
Porcelain fused to metal crowns provides durable, tough and enhanced appearance to the teeth. The efficiency of these crowns primarily depends upon correct preparation of the fundamental teeth model, so that there is enough space to allow the thickness of the material being used to cap the tooth to settle comfortably. The skill of the person making the crown will add to its visual beauty and the manner it will blend in with other teeth. One of the drawbacks in this porcelain-fused-to-metal crown is that over a period of time, the gum line may recede and thus expose the gold margin. A number of patients choose this type of crown, but later decide to replace the crown in order to maintain better visual appeal. Choosing a Porcelain-fused-to-metal crown with an all porcelain collar can get rid of this minus point.
All-Ceramic Crowns
All-ceramic dental crowns are basically crowns that are formulated using a dental ceramic like porcelain. The technique of porcelain jacket crowns was instituted around 100 years ago and it offered quite enhanced cosmetic appearance to the teeth but at the cost of strength. These porcelain-jacket crowns were normally positioned on the anterior teeth.
All-ceramic crowns are advantageous as they appear more natural as compared to crowns that result from porcelain-fused-to-metal (PFM). Enamel is a translucent layer on a tooth with quite light managing capability. All-ceramic dental crowns can offer a same kind of covering for a tooth and manage light in a manner that strongly replicates the dental enamel. Moreover, as there is no frame of metal substructure there is less probability of the damage to the crown’s cosmetic veneer even when the gum level of the tooth alters with the passage of time.
Different types of dental materials used for all-ceramic restoration include felspathic porcelains which are used as veneers (Willand, et al., 2001), luecite systems that are utilised as dental ceramics for changing thermal extension of the ceramic (Hobkirk, et al., 2003), castable glasses which along with tetra silicic fluromica crystals produce strong glass ceramic (Willand, et al., 2001) and alumina based systems which act as shield for pure almina core, porcelain jacket crown (JPC) and glass-infiltrated alumina system (Bartlett & Fisher, 2004).
All-ceramic crowns utilise substances like zirconia and aluminous but does not require the support of metal core. The breadth of the tooth therefore reduces significantly and hence they can be helpful when little space is available in the mouth. Also, as the metal core is not present, light can easily pass through the porcelain giving the artificial tooth a more natural appearance.
During the past few decades numerous companies have come up with different novel substance and techniques for constructing all-ceramic crowns offering similar kind of appearance like that of porcelain-jackets crowns but with enhanced strength. Some of the well-known brands for all-ceramic dental crowns are Optec, Empress, Vitablocs, Dicor, ProCad, Cerapearl, Inceram, Vitadur, Hyceram, Cerestore, Paradigm, Procera and Cerec. The materials used in all-ceramic crowns continue to progress in terms of their sturdiness and strength, nevertheless one should be careful in using these where too much chewing and biting is required. Research are still being done wherein the susceptibility of porcelain in heavy-duty areas of the mouth are being explored.
A dentist needs to be consulted before opting for all-ceramic crowns. These crowns are the best preferred under conditions where the cosmetic glimpse of the patient’s front teeth is the sole concern. But as these crowns are not as strong as the other kinds of crowns, the discretion of employing an all-ceramic dental crown on the teeth where cosmetic appearance is of less importance is still to be confirmed.
Cementation of all-ceramic crowns
The cementation procedure for all-ceramic crowns is very helpful for its success. IPS Empress restorations and feldspathic porcelain jacket crowns can be conveniently sketched with the help of hydrofluoric acid and later fixed by means of a resin cement. Investigations have confirmed the involvement of this technique in boosting the probable scientific lifetime of the restoration (Kern, 2005; Vult von Steyern, 2005).
A dentist may use quick dentin seal technique which employs a substantially packed dentin bonding agent (like Kerr, OptiBond FL, Orange, Calif.) for sealing after preparing tooth and before creation of the structure (Curtis, Wright, & Fleming, 2006; Sailer, Fehér, Filser, et al., 2007; Studart, et al., 2007). Also, he may apply self-adhesive dual-cure resin cement and omit the bonding procedure completely. It is quite an easy procedure, yet it may involve certain insignificant uneasiness, regarding probable enduring hydrolysis because of hydrophilic character of self-adhesive cement.
All-ceramic substance with excessive strength (zirconia and alumina) cannot be scratched and connected promptly. This allows the performance of the procedure with the involvement of more typical resin-modified glass ionomer cements (like St. Paul, Minn, RelyX Luting Plus Cement, 3M ESPE) as they are relatively less sensitive towards the procedure. Adhesive cementation is not required while using the traditional preparations offering perfunctory preservation and resistance form.
Some experts feel that employment of cementation of zirconia-based restorations and air abrasion along with tribochemical bonding or with cement consisting of special bonding agents (like Panavia F2.0, Kuraray America) (Luthardt, Holzhüter, Sandkuhl, et al., 2002) can be applied for adhesively cement zirconia restorations. Dentists are required to be quite cautious while implementing this technique as it may notcompulsorily prove successful as the results of the air abrasion of zirconia-cored structures procedure so far have been erratic (Aboushelib, et al., 2007; Kou, Kou, Liu, & Sjögren, 2007). Air abrasion of the intaglio region of zirconia crowns may cause a change to monoclinic phase from the tetragonal one, severely affecting the period of restoration and may even shrink its life. This is generally due to the size of its particle and its dependence upon the pressure. To keep the patient secured from such problems, dentists should give a nightguard (Comfort Zone Bite Splint, Drake Precision Dental Laboratory) to the person alleged to carry out any parafunctional activities, like nocturnal and diurnal bruxism as it can give support to the gums and teeth Aboushelib, et al., 2005).
Majority of the patients for dental implants prefer a restoration technique without involving metal basically due to aesthetic reasons. They also desire for a technique which offers least deterioration in the structure of the tooth restored. Failure in the restorations is also due to clinical problems of the patient, smoking habit, or absent of crucial bone may compel the surgeon to perform different surgeries in the patient in order to improve his tooth settings. The region of the mouth located on its posterior surface demands high-power restorative substances for replacing the absent teeth. In such cases, metal-based systems are generally suggested but they can also be substituted with fiber-resistant compounds or ceramics (Rosentritt, Behr, & Handel, 2003). Day-by-day tougher ceramic substances including oxide ceramics are being employed for improving the life of the tooth (Vult von Steyern, 2005).
The application of these crowns is generally restricted to the front teeth as they are not as stronger as the ceramo-metallic counterparts. These all-ceramic crowns involve unique technique known as shoulder preparation, signifying that the edges are prepared at the dental collar which provides strength to the crown and protects the gums as well. The advantages all-porcelain crownsinclude better aesthetic effect, non-allergic effect due to the absent of metals, absent of grey-colouration of gingival cleft and translucent ceramic quality gives it a natural look.
As per the ADA ethics, Procera® AllCeram crowns is a novel advancement towards prosthetic dentistry concerning computer based copings and is quite reliable precision fit technique. Before Procera® technique all-ceramic restorations, resin cements were used for bonding. Studies prove that marginal fit of Procera® CAD/CAM process copings (May, et al., 1996; May, et al., 1997; Persson, et al., 1996) resulted in minor cavity slit with dimensions of not more than 100 microns which was quite acceptable range. As compared to other all-ceramic crowns, flexural-strength examination on Procera® AllCeram has given more satisfactory outcomes. An investigation comprising of 10 samples of In-Ceram, Procera® AllCeram ceramics and IPS Empress4 were arranged as per the directions obtained from the manufacturers and the results of the investigations were quite in favour of AllCeram. Both In-Ceram and AllCeram exhibited similar fracture toughness considerably greater than Empress ceramic. Also, the flexural strength of these three disclosed that it was 687 MPa for AllCeram, 352 MPa for In-Ceram and 134 MPa for Empress (Wagner & Chu, 1996).
All the resin cements bonds may not show exclusively great strength. But if due care is taken while preparing it with proper resistance and retention form then it may be possible to obtain enduring bond strength of the cement. Many times it is desirable to get such crowns replaced in future, and keeping this in mind dentists as well as the patients may not desire to have any such treatment procedure which would be difficult to remove. Adding resin to glass ionomer cements enhances the positive features of the cements irrespectively of the type of cement applied besides providing excellent bond strength and propelling fluoride discharge. It also reduces compassion and solubility (Dwan et al., 1996; Snyder et al., 1999). In general, the functioning of these all-ceramic restorations has been quite successful for the last 25 years with marginal rate of failure and hence this technique is gaining popularity. The fact has been confirmed that the strong and sturdy ceramics are thicker and less lucid as compared to aesthetic ceramics (Kelly, 2004).
The Zirconia Process
Cercon zirconia is the most beneficial among the available all-ceramic materials due to the strength and consistency it offer. A straight wall of zirconia which is 0.3 mm in width, with the border region in the frontal portion of 0.2 mm for aesthetic reasons provides sufficient strength to the tooth. Cercon also provides a supreme aesthetic benefit with connector dimensions of 9 mm2 to 11 mm2. These dimensions are adequate to provide enough aesthetic restorations along with agreeable magnitude. The Cercon support substance is available in two colours which are natural tooth and neutral white shade (Luthardt, Holzhüter, Sandkuhl, et al., 2002).
Veneer Restorations
Ceramics are mostly suitable for veneer restorations. The failure rate in such a procedure is nearly 5% at five years and inclusive failures due to fractures and loss of retention (Walls, 1995; Peumans, et al., 2000). The investigations in this directions has also proved long lasting survival chances with the use of feldspathic porcelain veneers In these two investigations (n = 3,047 and n = 1,828) calculations were made using Kaplan-Meier statistics and the results of one of the study revealed the success rate of 96% at 5 to 6 years, 93% at 10 to 11 years and 91% at 12 to 13 years. The second study showed over 94% at 12 clearly indicating high rate of long-term success of the restorations (Fradeani, Redemagni, & Corrado, 2005). Major complications were connected with aesthetics and mechanical problems (31% each) while the others were basically due to periodontal support and loss of retentions (12.5% each). At last caries and tooth fracture both constituted 6% towards the failure rates of the procedure (Layton, & Walton, 2007). It should be however noted that periodontal support and caries the natural processes and are not caused due to the substances used during restoration.
Inlay and Onlay Restorations
Feldspathic ceramic or mica-packed glass ceramic are extensively utilized for the purpose of inlay and onlay restorations with the aid of CAD/CAM system (Pallesen, & van Dijken, 2000; Fasbinder, 2005; Sjogre, Molin, & van Dijken, 2004) hot and compressed leucite-reinforced ceramic from Ivoclar Vivadent (IPS Empress Esthetic) is also widely used for this process (Stoll, et al., 2007; Lohbauer, et al., 2008).
Again, Otto and De Nisco (2002) with the help of Kaplan-Meier statistics, revealed a survival rate for 200 restorations which was over 90% at 10 years. Reasons behind the unsuccessful restorations were ceramic fracture (53%), tooth fracture (20%) and endodontic complaints (7%). Further investigations in this direction proved similar kinds of results. IPS Empress (Ivoclar Vivadent) employed during inlay/onlay restorations registered survival rates between 96% at 4.5 years and 91% at 7 years (El-Mowafy & Brochu, 2002) with 92% at 8 years (Krämer & Frankenberger, 2005). A research conducted few years ago involving (Stoll, et al., 2008) 1,588 IPS Empress inlay/onlay restorations on crucial teeth disclosed 97% at 10 years. The results of all these research were evaluated using Kaplan-Meier statistics.
A systematic review of 22 clinical studies that used the CEREC system to produce inlay and onlay restorations and crowns from Vitablocs Mark I and II and Dicor ceramics reported a survival probability of approximately 97 percent at five years and 90 percent at 10 years (Fasbinder, 2006). One of these studies (Sjögren, Molin, & van Dijken, 2004) reported data about 66 CAD/CAM inlays that had an estimated survival rate of 89 percent after 10 years-77 percent for the inlays luted with a dual-cured resin-based composite and 100 percent for those luted with a chemically cured resin-based composite. This difference in performance on the basis of the cement used was statistically significant.
Single-Unit and Multiple Unit Crowns
Single-unit crowns involve glass-infiltrated ceramics (In-Ceram Alumina and In-Ceram Spinell, Vita Zahnfabrik), polycrystalline alumina (Procera Alumina, Nobel Biocare, Göteborg, Sweden) and leucite-reinforced glass-ceramic (IPS Empress). Although, all of them were distinct in their microstructure, constitution, processing technique and intraoral area, yet majority of them registered survival rates of over 90% independent of the period in service with the only omission of glass-ceramic introduced in the 1980s (Dicor) which is not longer is use now.
The research also revealed that the fracture rates were higher in the case of the molar crowns as compared to the anterior crowns. The primary reasons for the failure were catastrophic fractures (crown shattering into two parts), fragmentation of the veneer ceramic and caries (natural phenomenon).
In a separate study, McLaren and White (2002) explored that 223 crowns (In-Ceram Alumina) presented a survival rate of 96% after 3 years. A scrutiny of 546 In-Ceram Alumina restorations displayed survival rate of over 99% after 6 years of service (Segal, 2001) and 135 restorations (Procera Alumina) with excellent survival rate of 100% percent in the anterior region and approximately 99% in the posterior region (with single crown fracture) after 5 and 7 years (Zitzmann, et al., 2007). Recently, two studies involving restorations using lithium disilicate-based glass-ceramic (IPS Empress 2) have displayed high survival rates of 95% (Toksavul, & Toman, 2007) and 100% (Marquardt, & Strub, 2006) after 5 years of research.
Few manufacturers feel that all-ceramic systems for anterior three-unit pros-theses which include a glass-infiltrated alumina (In-Ceram Alumina) and a lithium disilicate-centered glass-ceramic (IPS e.max Press]) is quite effective in tooth restorations. Many scientific studies have been conducted for fixed partial dentures (FPDs) wherein the In-Ceram Alumina has been applied during the procedure concerning posterior teeth. A reserech conducted over the period of 3 years and concerning 61 three-unit FPDs (In-Ceram Alumina) revealed a success and survival rate of 100% in case of anterior teeth and approximately 83% in case of posterior ones (Sorensen and colleagues). The remaining 7% showed FPDs fracture in the connector region which was repaired using glass-ionomer cement.
All-ceramic crowns are quite beneficial as it has the capacity to give better aesthetics. They also have various weaknesses as well when compared to metal-ceramic crowns. The drawbacks of the all-ceramic crowns are diminished marginal consistency, more-hostile tooth grounding, probable deterioration of the contrasting dentition, improved procedure-sensitivity, and complexities in staging with a tooth grounding that differs considerably from the model (Cho, Donovan, & Chee, 1998).
Many all-ceramic systems and ceramic margins on metal-ceramic crowns have been effective to give satisfactory marginal consistency. Despite of various assertions forwarded by the manufacturers regarding greater marginal consistency with particular all-ceramic systems, the investigations in this direction has revealed that metal margins (Boenig, & Wolf, 2000). However, most of the machined margins utilizing different CAD/CAM technologies have failed to give excellent marginal integrity as proclaimed.
All-ceramic tooth preparations are regarded as more brutal as compared to metal-ceramic ones. All-ceramic preparations require the removal of around 1.5 mm of tooth structure and approximately 2 mm from the occlusal position of posterior teeth for attaining its greatest force and best aesthetics. In case of metal-ceramic crowns, somewhat less reduction is necessary on the surface of the mouth and radically less reduction can be obtained on the lingual region.
Another drawback of all-ceramic crowns is their incapability to give enough hold with imperfect preparations. Metal-ceramic crowns provide a complete contour wax model after a regulated cut-back procedure to support the ceramic veneer, detached from the basic preparation. This procedure also leads towards expected aesthetics. Due to the formation of consistent deposit porcelain, negligible pressure is produced after firing at the ceramic or metal border as the restoration cools down and thus, better metal-ceramic bonding is obtained.
In people with multiple lost teeth, the dentists would preferably advice a dental bridge. Various kinds of dental bridges are available in the market but an all-ceramic dental bridge at dental implants can be regarded as one of the finest alternatives for good aesthetics and best function. A dental implant offers support to the dental bridge functioning in a manner similar to the natural root. All-ceramic dental bridge on dental implants appears and works just like the absent teeth.
The patient has to make a selection from the different options available for restoration from the different products, after he is advised all-ceramic crowns procedure. Given that the primary indication for use of all-ceramic crowns is improved aesthetics, the clinician should analyze available systems in terms of their ability to deliver on that promise. Numerous all-ceramic systems have succeeded in obtaining better strength along with an interior opaque core. But, these techniques are likely to offer less effective outcomes than the metal-ceramic restorations. Therefore, it is suggested to adopt metal-ceramic technique. Also, for getting highest aesthetic probability, the color must replicate the tooth structure. In some of the techniques, colorants are tinted on the outer shell and thereby no or modest light diffusion takes place. But, the aesthetic probability of these procedures is quite restricted and hence should not be preferred. One should opt for this procedure after carefully picking up the fundamental substances which allow light diffusion and the one in which major tint of the restoration is verified within.
The next key factor which should be used in picking up an all-ceramic crown system is its scientific facts in favour of the system. Laboratory researches performed to establish the substantial characteristics or strength of these crowns is almost useless in forecasting medical act (Kelly, 1999). These crowns crash due to faults created at the time of creation or adjustment proliferation of microscopic damage known as Griffith’s flaws (Anusavice, 1993).
These errors leads to stagnant exhaustion and pressure-decay in a humid surroundings, and split proliferation can happen due to the lack of surplus occlusal pressure (White, Zhao, et al., 1995). These details put an extra obligation on the manufacturers to give substantial evidences by performing appropriate and extensive scientific assessment for a period of minimum 3 years and maximum 5 years with the success percentage of above 95% and prove the efficiency of their product (Scherrer, De Rijk, et al., 2001).
As stated above number of researches have been performed and published during the past few years. In reality, it takes a long period of time before a particular technique can really prove to be completely successful because these researches cannot provide substantial evidences regarding the success of a fresh restoration procedure within few years time. Hence, if a dentist wishes to adopt a new technique, he has no concrete record with him to guarantee the success of such a technique. All a dentist can do is to use a fresh technique with care and vigilance. The new technique can be assessed on the basis of its capacity to grant better aesthetics and long life to the tooth. These dentists can also consult the experts in the discipline and try to obtain maximum knowledge about the fresh technique besides discussing its manufacturing processes with the manufacturers. After being fully satisfied with the detailed investigation regarding the technique, the dentists use it in some of his patients and review the results for sometimes. Only after being satisfied with the initial results, he should move forward and apply it on significant number of patients. Initial investigation on a large group of patients may be an unjust to patients and also prove costly for the dentists.
In people with multiple lost teeth, the dentists would preferably advice a dental bridge. Various kinds of dental bridges are available in the market but an all-ceramic dental bridge at dental implants can be regarded as one of the finest alternatives for good aesthetics and best function. A dental implant offers support to the dental bridge functioning in a manner similar to the natural root. All-ceramic dental bridge on dental implants appears and works just like the absent teeth.
Failures of All-Ceramic Crowns
All-ceramic crowns appear quite attractive due to their improved biocompability, composure and stillness. Yet, the complete ability of these restorations has not been comprehended due to their fairly excessive failure rates under extremely stressful purposes like posterior bridges or molar crowns. A team of experts from 5 different universities of USA conducted research in order to explain the injury spheres and exhaustion mechanisms functioning in all-ceramic full crowns. The primary objective of the research was to obtain a planned pattern that can be effectively employed when latest substances are utilised in molar crowns construction with CAM/CAD techniques. The centers of attention are molar crowns regions because they present with the peak design complexity and a number of functional cyclic acquaintances.
Ceramics used for crowns is generally segregated into 3 categories viz. structural ceramics, glasses and glass-ceramics. The major constituent for coating the metals, glass-ceramics and structural ceramics is Feldspathic glasses. The feldspathic ceramics is generally used for maxillary incisors and monolithic crowns as the pressure in these regions is comparatively limited.
The introduction of Dicor, a glass-ceramic, assisted in providing greater strength as compared to the usual ceramic. It resembled a flexible modulus just like the enamel and could be applied for molar restorations as well. The use of this monolithic substance also failed and proved unsuccessful techniques (Malament, & Socransky, 1999; Sjogren, & Lantto, 1999). Later, a new high strength glass-ceramic, Empress II, was explored and used for layered crowns which comprised of needles of ceramic, a lithium disilicate glass. Till date, no long term clinical investigations on the layered molar crowns have revealed the failure rates of this technique.
Structural ceramics was considered as for all-cermics restoration due to the high rates of failure witnessed in the molar crowns. Another major reason for considering structural ceramics was to expand all-ceramic restorations to permanent prostheses. Partial sintered alumina core was the first one to be considered for this purpose and was permeated with a glass at an elevated temperature. The core was thereafter covered with appropriate ceramic to obtain proper coefficient of thermal extension. The temporary results of such a technique were quite successful and hence it was widely employed for several years (Segal, 2001). The percentage of failure for molar crowns for the past 5 years was estimated to be just 1% to 2% every year (McLaren, & White, 2000). CAD/CAM is been introduced recently for In-Ceram cores with less than 1% probability of failure annually (Bindl & Mormann, 2002). The long term investigation comprising of more than 200 molar crowns revealed the percentage of failure as 3.5% annually for 10 years. The results clearly indicated an accelerated rate of failure in this technique with increased time period.
A different structural ceramic layer crown Procera crown consisting of a gush cast, densified alumina core and covering of feldspathic porcelain was examined for 5 years and revealed the failure rate of about 1.2% annually (Oden, 1998). Excessive strength structural porcelain yittria alleviated zirconia (YTZP) have been presented to compete with Procera although the success of the zirconia core crowns in case of molar crown is yet to be confirmed.
The primary reason behind application of all-ceramic crowns are often bulk fractures, which is a catastrophic collapse form recorded for monolithic (e.g., Dicor) as well as layered crowns (Thompson, et al., 1994; Kelly, Tesk, & Sorensen, 1995). The fracture commences from the interior surface of the ceramic where tensile force is maximum. It then spreads via the substances towards the external surface, eventually causing fracture (Kelly, 1999). The strategy for preparing ideal preparation design includes reducing the tooth consistency circumferentially as this will lead to proper dispersion of stress and minimize the probability of fracture in the region. Sufficient reduction provides the dentist with enough room helps in creating an aesthetic restoration.
CONCLUSION
All-ceramic restorations have proved to improve longevity of the restorations and provide other long-term aesthetic benefits as well. Confirmations have been received from different scientific researches and its efficiency. The procedure is adopted only after confirming the aesthetic requirements of the patient for veneers and full or partial restoration. The technique has been quite successful is restoring molars. Modified and toughened zirconia is likely to become the most booming amongst different all-ceramic system. However, whittling of the veneering ceramic on zirconia restorations remains a concern.
With one absent tooth, the best alternative would be to get a dental crown placed over a dental implant which would provide strong foundation to the dental crown and it would also function like the absent tooth. A dental implant present on the top of natural root offers better support and a natural appearance. If many teeth are absent, then the dentist will suggest a dental bridge with multiple options available to the patient. Besides giving the teeth a natural look, a dental bridge helps to stabilise the teeth beside the absent teeth and makes the smile of the patient more pleasant. It also checks the shrinking of the jawbones due to the reduction in the size of the jaw.
The dentists are required to obtain an informed consent of the patient regarding before applying all-ceramic restorations. The primary reasons for the failure of the all-ceramic restorations may be interior splits, erroneous ceramic layers, structure of the oral cavity or teeth and deterioration in the material caused due to excessive heat or cold.
