Crown refers to the restoration of teeth using materials that are fabricated by indirect methods which are cemented into place. A crown is used to cap or completely cover a tooth.

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:

    * fabrication of the restoration without the need for having the patient in the chair

    * the utilization of materials that require special fabrication methods, such as casting

    * the use of materials that require intense heat to be processed into a restoration, such as gold and porcelain.

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.

As new technology and material chemistry has evolved, computers are increasingly becoming a part of crown and bridge fabrication, such as in CAD/CAM technology.

Why restore with a crown?

The clinical and radiographic appearance of a maxillary molar currently restored with a complex amalgam. Should this tooth require a new restoration because of recurrent decay or a fracture, for example, and the prosthetic and periodontal considerations allowed this tooth to be restored, the remaining tooth structure after removal of the amalgam and decayed or undermined tooth structure would not leave enough for an intracoronal restoration. A crown would be indicated, as probably would a post and core.

The clinical and radiographic appearance of a maxillary molar currently restored with a complex amalgam. Should this tooth require a new restoration because of recurrent decay or a fracture, for example, and the prosthetic and periodontal considerations allowed this tooth to be restored, the remaining tooth structure after removal of the amalgam and decayed or undermined tooth structure would not leave enough for an intracoronal restoration. A crown would be indicated, as probably would a post and core.

A patient presented to a dental clinic with pain in the upper left quadrant. Tooth #5, the left maxillary first premolar, is shown in photo "A" with a large buccal abscess. With little investigation, in photo "B", it was soon discovered that the tooth had cracked and the buccal wall of the crown had fractured well below the gum line. Upon removing the excessively large composite restoration, photo "C", it was evident that the reason for the fracture was because the tooth should never have been restored with composite; the scant remaining tooth structure following the initial removal of decay should have been restored with a post and core and a crown, if it were to have been restored at all. From the radiograph, photo "D", taken prior to treating the patient, it was revealed that the biologic width had been violated when the composite restoration was initially placed.

A patient presented to a dental clinic with pain in the upper left quadrant. Tooth #5, the left maxillary first premolar, is shown in photo "A" with a large buccal abscess. With little investigation, in photo "B", it was soon discovered that the tooth had cracked and the buccal wall of the crown had fractured well below the gum line. Upon removing the excessively large composite restoration, photo "C", it was evident that the reason for the fracture was because the tooth should never have been restored with composite; the scant remaining tooth structure following the initial removal of decay should have been restored with a post and core and a crown, if it were to have been restored at all. From the radiograph, photo "D", taken prior to treating the patient, it was revealed that the biologic width had been violated when the composite restoration was initially placed.

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.

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.

Other reasons to restore with a crown

There are additional situations in which a crown would be the restoration of choice.

Implants

Dental implants are placed into either the maxilla or mandible as an alternative to partial or complete edentulism. Once placed and properly integrated into the bone, implants may then be fitted with a number of different prostheses:

     * crowns or bridges

    * attachments for either removable partial dentures, complete dentures or a hybrid sort of prosthetic appliance.

Endodontically treated teeth

When teeth undergo endodontic treatment, or root canal therapy, they are devitalized when the nerve and blood supply are cut off and the space which they previously filled, known as the "pulp chamber" and "root canal", are thoroughly cleansed and filled with various materials to prevent future invasion by bacteria. Although there may very well be enough tooth structure remaining after root canal therapy is provided for a particular tooth to restore the tooth with an intracoronal restoration, this is not suggested in most teeth. The vitality of a tooth is remarkable in its ability to provide the tooth with the strength and durability it needs to function in mastication. The living tooth structure is surprisingly resilient and can sustain considerable abuse without fracturing. Consequently, after root canal therapy is performed, a tooth becomes extremely brittle and is significantly weaker than its vital neighbors.

Fractures of endodontically treated teeth increase considerably in the posterior dentition when cuspal protection is not provided by a crown.

Fractures of endodontically treated teeth increase considerably in the posterior dentition when cuspal protection is not provided by a crown.

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, the chances of it succumbing to breakage from normal functional forces is exceedingly high and the odds of an untreatable "vertical root fracture" are all but certain. 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.

Surveyed crown

Another situation in which a crown is the restoration of choice is when a tooth is intended as an abutment tooth for a removable partial denture, but is initially unfavorable for such a task. If the abutment teeth onto which the RPD is supposed to clasp do not possess the proper dimensions or features required, these aspects can be built into what is known as a surveyed crown.

Aesthetics

A fourth possible situation in which a crown would be the restoration of choice is when a patient desires to have his or her smile aesthetically improved but when partial coverage (i.e. a veneer/laminate) is not an option for one or more of a number of reasons. If the patient's occlusion does not permit for a mildly-retentive restoration, or if there is too much decay or a fracture within the tooth structure, a porcelain or composite veneer may not be placed with any adequate guarantee for its durability. Similarly, a bruxer (i.e. someone who grinds his or her teeth) may produce enough force to repeatedly dislodge or irreversibly abrade any veneer a dentist can plan for. In such a case, full coverage crowns can alter the size, shape or shade of a patient's teeth while protecting against failure of the restoration.

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.

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