Crowns to protect teeth
A crown is a protective covering for a tooth. They can be used to re-enforce a tooth or to improve the appearance of a tooth. To learn more about crowns for cosmetic improvements click here.
Crowns can be seen as a protective helmet over a tooth. There are variations on crowns such as an onlay- a crown extends all the way to the gum level where as an onlay can just be over the biting surface of the tooth. The purpose of both of these is to re-enforce a tooth and reduce the risk of tooth fracture and overall failure of the tooth.
When biting and chewing, teeth (particularly molar and premolar teeth) are subjected to huge forces (350-700N) for on average 17.5 minutes per day. If you clench or grind your teeth, the teeth can be put under even more stress.
Research shows that loss of tooth structure through historical decay and a filling can weaken the tooth and increases the liklihood of tooth fracture.
When teeth come together, the biting forces cause the teeth to flex and bend - in teeth with large fillings, this puts the tooth at a risk of breaking in an unpredictable way. A filling running through the middle of a tooth has been found to reduce the stiffness of a tooth by over 60%. (Reeh et. al. 1989).
A common cause of root filled teeth failing is a crack forming so it is recommended most root filled molars and premolar teeth are crowned to prevent a crack developing. A crack in a tooth allows bacteria to enter it and cause infections to develop.
This molar tooth with a large amalgam filling has a snapped corner (or cusp). A crown would have prevented the tooth fracturing. More catastrophic cracks in teeth can extend into the nerve space of a tooth causing acute pain or even cause the tooth to split in half which would leave a tooth un-savable and need to be removed.
A crown (or onlay) is a protective thimble which sits over a heavily filled tooth. This changes the way that biting forces go through the tooth reducing the risk of fracture or future failure of the tooth. (Shillingburg 1997)
Crowns can be made from different materials and there are pros and cons which should be discussed with your dentist. Gold or metal crowns can be thin are very strong but aesthetically can be seen in your mouth. Ceramic crowns have better aesthetics but require more tooth to be removed to allow thickness of material which gives them strength.
Risks of crowns
In teeth which have a live nerve, preparing the tooth for a crown risks causing the nerve to be damaged. The nerve can be temporarily irritated causing sensitivity or occasionally the nerve can die off which would lead to root canal treatment being needed. Research suggests 4-9% of teeth with alive nerves may require further treatment 10 years following preparation for a crown. Removing less tooth structure by using more contemporary techniques such as an onlay reduces the risk of irreversible damage to the underlying nerve tissue.
A PFM crown is a metal based crown with porcelain fused to the metal base. Because of the metal in these crowns, they do not let light pass through them the same way a natural tooth does so the aesthetics of these crowns is not as good as all ceramic crowns. All ceramic crowns allow optimal control of aestheitcs whilst still protecting the tooth.
All metal crowns such as gold crowns aesthetically are compromised but are very strong and less tooth removal is required for the crown to fit in place.
Reeh ES, Meser HH, Douglass WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endod 1989 15(11) 512 - 516
Shillingburg HT. Fundamentals of fixed prosthodontics Chicago. Quintessence 1997
Bergenholtz G, Nyman S. Endodontic complications following periodontal and prosthetic treatment of patients with advanced periodontal disease. J Periodontol 1984; 55: 63-68
Landolt A, Lang N P. Erfolg und misserfolg bei extensionsbrucken. Schweiz Monat Zahnmed 1988; 98: 239-44. [German with English abstract] 11.
Reichen-Graden S, Lang N P. Periodontal and pulpal conditions of abutment teeth. Schweiz Monat Zahnmed 1989; 99: 1381-1385. 12.
Gonzalez G , Wier D J , Helm F , Marshall S J, Walker L, Stoffer W. et al. Incidence of endodontic treatment in teeth with full coverage restorations. J Dent Res 1991; 70 (special issue): 446. 13.
Valderhaug J, Jokstad A, Ambjornsen E, Norheim P W. Assessment of the periapical and clinical status of crowned teeth over 25 years. J Dent 1997; 25: 97-105.