Why a dental crown may placed on a tooth which has had root canal treatment?
The crown is necessary for improving the appearance of a tooth, restoring a broken tooth to its original shape, and/or strengthening a tooth. It protects the tooth from darkening and fracturing. It also creates a seal over a tooth which prevents the bacteria and contaminates from seeping back into those inner aspects of a tooth where the root canal treatment has been performed. Due to these reasons it is always beneficial to have a crown after root canal treatment.
http://www.killerstartups.com/news/tackle-the-fear-of-root-canal-with-useful-tips-2/
Crowns are dental restorations which cup over the portion of the tooth which lies above the gum line. People sometimes refer to dental crowns as “caps”. Dental crowns can either be gold or else have a porcelain surface so they look white like the tooth’s neighboring teeth. A dentist will use a dental crown as a means of improving the appearance of a tooth, restoring a broken down tooth to its original shape, and/or strengthening a tooth. After a tooth has had root canal treatment, any or all of these qualities may be needed.
