Porcelain Bonded (no metal) Crowns
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 a subsequent 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.
Longevity
Although no dental restoration lasts forever, the average lifespan of a crown is around 10 years. While this is considered comparatively favorable to direct restorations, they can actually last up to the life of the patient (50 years or more) with proper care. One reason why a 10 year mark is given is because a dentist can usually provide patients with this number and be confident that a crown that the dental lab makes will last at least this long. It should be noted that many dental insurance plans in North America will allow for a crown to be replaced after only five years.
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 well 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.
Advantages and Disadvantages
Whenever considering any irreversible process, especially in the field of surgery, one must conduct a thorough cost-benefit analysis.
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:
• Fixed partial denture (bridge)
• Removable partial denture
• No treatment
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 natural tooth's crown (A) meets the root (B) at the cementoenamel junction, and it is roughly at this point that the gingival attachment begins at the base of the gingival sulcus (G). The margin of the prosthetic crown may not violate the 2 mm of biologic width from the base of this sulcus to the height of the alveolar bone (C) if complications are to be avoided.
