A root canal treated tooth is devoid of its blood vessels and as time passes tends to get dehydrated. They are usually teeth which are extensively decayed and has hence lost considerable amount of tooth bulk as well! Some more tooth structure is usually removed to gain access to the inside of the tooth to perform the root canal procedure and relieve pain! As years pass by due to the physical changes in the tooth structure there can be shift in the refractive index of the tooth and the tooth becomes yellowish to brown or even gray. Due to all the above mentioned reasons a root canal treated tooth tends to be weaker than a healthy tooth and hence requires additional coverage restorations like Onlays / Oberlays or Crowns ! Such restorations enhances the ability of the tooth to take the biting stresses more efficiently, masks the dark shade and thus enhances the life of the tooth and makes it look more beautiful.
