Traditionally, the teeth to be crowned are prepared by a dentist and records are given to a dental technician to fabricate the crown or bridge, which can then be inserted at another dental appointment. The main advantages of the indirect method of tooth restoration include:
The restorative materials used in indirect restorations possess superior mechanical properties than do the materials used for direct methods of tooth restoration, and thus produce a restoration of much higher quality.
When decay is first detected in a tooth, the usual action taken by the dentist is to provide the tooth with an intracoronal restoration: a restoration consisting of a dental material that will exist totally within the confines of the remaining tooth structure. The restoration commonly referred to as a "cavity filling", or more colloquially as a "filling", is an intracoronal restoration, and can consist of a number of materials, including silver-colored amalgam, tooth-colored resin or gold. Inlays are also intracoronal restorations.
In a situation where there is not enough remaining solid tooth structure after decay and fragile tooth structure is removed, or the tooth has fractured and is now missing important architectural reinforcements, the tooth might very well require an extracoronal restoration: a restoration consisting of a dental material that will exist around the remaining tooth structure to a varying degree. Restorations that fall into this category include the various types of crowns and onlays, and these can consist of a number of materials as well, including gold, ceramic, or a combination of the two. Ceramic crowns are increasingly being substituted in place of gold crowns for aesthetic and structural reasons. In a recent study, only 1.7% of the ceramic crowns needed to be replaced after 2 years, with 3.7% showing occlusal chipping without need of replacement.
The circumstance of the damaged tooth defines the restoration. In other words, based upon factors such as remaining solid tooth structure, aesthetics, the location of the tooth within the dental arch and the consequent forces of function that said tooth will have to deal with once restored, the dentist will then decide on the proper way to treat the tooth.
Things are not always straightforward when it comes to restoring a tooth. An advantage of crowning a tooth over restoring the tooth with an excessively large pin-supported amalgam or composite restoration is that crowns provide much more protection against future fracture or recurrent decay. The indirect techniques of crown fabrication translate into a more adapted tooth-restoration margin, and thus a better seal against the decay-causing bacteria present in saliva.
The average person can exert 150-200 lbs. of muscular force on their posterior teeth, which is approximately nine times the amount of force that can be exerted in the anterior. If the effective posterior contact area on a restoration is .1 mm², over 1 million PSI of stress is placed on the restoration. Therefore, posterior teeth (i.e. molars and premolars) should in almost all situations be crowned after undergoing root canal therapy to provide for proper protection against fracture (mandibular premolars, being very similar in crown morphology to canines, may in some cases be protected with intracoronal restorations). Should an endodontically treated tooth not be properly protected, there is a chance of it succumbing to breakage from normal functional forces. This fracture may well be difficult to treat, such as a "vertical root fracture" . Anterior teeth (i.e. incisors and canines), which are exposed to significantly lower functional forces, may effectively be treated with intracoronal restorations following root canal therapy if there is enough tooth structure remaining after the procedure.
Makeover shows such as Extreme Makeover make extensive use of crowns, as the time-frame of the makeover period is too short to allow up to 18 months for orthodontic treatment to treat problems that might otherwise be corrected more conservatively.
Preparation of a tooth for a crown involves the irreversible removal of a significant amount of tooth structure. All restorations possess compromised structural and functional integrity when compared to healthy, natural tooth structure. Thus, if not indicated as desirable by an oral health-care professional, the crowning of a tooth would most likely be contraindicated. It should be evident, though, that dentists trained at different institutions in different eras and in different countries might very well possess different methods of treatment planning and case selection, resulting is somewhat diverse recommendations for treatment.
Traditionally more than one visit is required to complete crown and bridge work, and the additional time required for the procedure can be a disadvantage; the increased benefits of such a restoration, however, will generally offset these considerations.
If there is not enough tooth structure to properly retain the prosthetic crown, the tooth requires a build-up material. This can be accomplished with a pin-retained direct restoration, such as amalgam or a resin like fluorocore, or in more severe cases, may require a post and core. Should the tooth require a post and core, endodontic therapy would then be indicated, as the post descends into the devitalized root canal for added retention. If the tooth, because of its relative lack of exposed tooth structure, also requires crown lengthening, the total combined time, effort and cost of the various procedures, together with the decreased prognosis because of the combined inherent failure rates of each procedure, might make it more reasonable to have the tooth extracted and opt to have an implant placed.
The most coronal position of untouched tooth structure (that is, the continual line of original, undrilled tooth structure at or near the gum line) is referred to as the margin. This margin will be the future continual line of tooth-to-restoration contact, and should be a smooth, well-defined delineation so that the restoration, no matter how it is fabricated, can be properly adapted and not allow for any openings visible to the naked eye, however slight. An acceptable distance from tooth margin to restoration margin is anywhere from 40-100 μm. However, the R.V. Tucker method of gold inlay and onlay restoration produces tooth-to-restoration adaptation of potentially only 2 μm, confirmed by scanning electron microscopy; this is less than the diameter of a single bacterium.
Naturally, the tooth-to-restoration margin is an unsightly thing to have exposed on the visible surface of a tooth when the tooth exists in the aesthetic zone of the smile. In these areas, the dentist would like to place the margin as far apical (towards the root tip of the tooth) as possible, even below the gum line. While there is no issue, per se, with placing the margin at the gumline, problems may arise when placing the margin too subgingivally (below the gumline). First, there might be issues in terms of capturing the margin in an impression to make the stone model of the prepared tooth (see stone model replication of tooth in photographs, above). Secondly, there is the seriously important issue of biologic width. Biologic width is the mandatory distance to be left between the height of the alveolar bone and the margin of the restoration, and if this distance is violated because the margin is placed too subgingivally, serious repercussions may follow. In situations where the margin cannot be placed apically enough to provide for proper retention of the prosthetic crown on the prepared tooth structure, the tooth or teeth involved should undergo a crown lengthening procedure.
There are a number of different types of margins that can be placed for restoration with a crown. There is the chamfer, which is popular with full gold restorations, which effectively removed the smallest amount of tooth structure. There is also a shoulder, which, while removing slightly more tooth structure, serves to allow for a thickness of the restoration material, necessary when applying porcelain to a PFM coping or when restoring with an all-ceramic crown (see below for elaboration on various types of crowns and their materials). When using a shoulder preparation, the dentist is urged to add a bevel; the shoulder-bevel margin serves to effectively decrease the tooth-to-restoration distance upon final cementation of the restoration.
All who are familiar with dentistry will agree that the most important factor affecting the lifespan of any restorative is the continuing oral hygiene performed by the patient. Similar to almost anything, a poorly-made object can last way past its predicted lifetime if it is properly cared for, and even a well-made item can last only a day if handled improperly. Other factors depend on the skill of the dentist and his lab technician, the material used and appropriate treatment planning and case selection.
Full gold crowns last the longest, as they are fabricated as a single piece of gold. PFMs, or porcelain-fused-to-metal crowns possess an additional dimension in which they are prone to failure, as they incorporate brittle porcelain into their structure. Although incredibly strong in compression, porcelain is terribly fragile in tension, and fracture of the porcelain increased the risk of failure, which rises as the amount of surfaces covered with porcelain in increased. A traditional PFM with occlusal porcelain (i.e. porcelain applied to the biting surface of a posterior tooth) has a 7% higher chance of failure per year than a corresponding full gold crown.
When crowns are used to restore endodontically treated teeth, they increase the life of the tooth not only by preventing fracture of the brittle devitalized tooth but also by providing a better seal against invading bacteria. Although the inert filling material within the root canal blocks against microbial invasion of the internal tooth structure, it is actually a superior coronal seal, or marginal adaptation of the restoration in or on the crown of the tooth, which prevents reinvasion of the root canal.
The main disadvantages of restoration with a crown are extensive irreversible tooth preparation and higher costs than for direct restorations such as amalgam or Dental composite. The benefits, as described above, include long-term durability and evidence-based success as compared to other restorations or no treatment.
It is important to bear in mind that it is usually the damage to a tooth that dictates the need for a crown, and alternative treatments are usually less effective. However, it is also important to realise that even if risks and benefits are objectively analysed, their significance depends on the priorities of the patient. An example of this occurs when a patient would like to restore an edentulous area between healthy adjacent teeth. Before implants, there were three options:
Those who could afford it were usually told by their dentists that a bridge was their best choice, because it is much sturdier than removable dentures and requires less looking after. When implants became available, however, they were recommended as the best possible treatment, because the virgin teeth adjacent to the edentulous area no longer needed to be cut in order to fit the bridge. The affluent are thus told that a fixed partial denture is no longer desirable, now that implants are available.
The cost of a crown in the United States and Canada can range from $600 to $3,100, depending on many factors, including exactly which materials are to be used, where the practicing dentist is located and if a prosthodontic specialist is performing the procedure. However, dental care provided by students at U.S. and Canadian dental schools can often be obtained for as low as $300. Porcelain may cost about 20% more than full gold crowns, and the time-consuming processes of the operation is one reason for this high cost. Cost seems to be the limiting factor that precludes greater use of the procedure.
Full gold crowns are cast metal restorations that are made using the lost-wax technique. After the dentist prepares a tooth for a crown, he or she will take an impression of the prepared tooth, the adjacent teeth in the same arch and the opposing teeth in the opposing arch. With all of the necessary boundaries of the future cast crown defined in three dimensions within the impression material (i.e. the necessary height, width and depth of the crown is now recorded in impression material), the impression(s) are sent to a dental laboratory where they will be poured up in various types of dental stone or plaster. After the stone models are formed, they are ditched, died and articulated so that the laboratory technician can see how the two arches meet and properly access the tooth replicates to perform his tasks. (See photographs at the beginning of the article to see the stone model dies and the completed crown on the die.) The lab technician will then apply wax to the die (analog of the prepared tooth) and manipulate and craft the wax until he or she has built it up into what appears like and conforms to the specific dimensions of the tooth being restored. Prior to applying the wax, though, a die spacer is applied to the die. This is a thin coat of material that is painted onto the die to provide a space between the gold crown and the actual tooth structure to be filled with cement upon final cementation. A lubricant is also applied so that the wax pattern, as the wax-up of the crown is referred to, can be easily removed when completed.
The wax pattern is removed from the die and invested in a sort of plaster while connected to a short plastic stick, called a "sprue former", which will stick out of the investing plaster. The investment, as it is called now, is placed in a furnace, which will completely burn off the wax and plastic that formed the wax pattern/sprue complex. What is left is a hollow within the investment material, known as an "investment pattern". Because the sprue former stuck out a little bit from the investment material, there is a communication between the outside and the investment pattern. The investment pattern is then placed in a sort of simple centrifuge where pennyweights of gold are melted down and rapidly shot through the communication in the investment pattern, through the sprue that was formed by the sprue former, and into the hollow that used to be inhabited by the wax pattern of the crown waxed-up by the technician, thus called the lost-wax technique. After properly cooling, the single piece crown-and-sprue of gold is sectioned, and the sprue can be recycled in another casting. The crown is touched-up in the location of the sprue attachment, finished and polished to a high shine, and delivered to the dentist so that he or she can try it in the mouth, make certain it has all of the proper contacts with the adjacent and opposing teeth, and cement it to the prepared tooth.
Typically, over 95% of the restorations made using CEREC and Vita Mark I and Mark II blocks are still clinically successful after 5 years. Further, at least 90% of restorations still function successfully after 10 years. Advantages of the Mark II blocks over ceramic blocks include: they wear down as fast as natural teeth, , their failure loads are very similar to those of natural teeth, and the wear pattern of Mark II against enamel is similar to that of enamel against enamel.
Herbert T. Shillingburg. FUNDAMENTALS OF FIXED PROSTHODONTICS, 1979.
Fermin A. Carranza. CARRANZA'S CLINICAL PERIODONTOLOGY, 9th edition, 2002.