When a tooth is cracked, a filling may not seal the crack. A crown may have to be placed over the tooth to hold it and the crack together. Endodontic Treatment (Root Canal Therapy) may also be required. It is important to restore a cracked tooth before it breaks, because in some cases a broken tooth cannot be filled or crowned and must be extracted.
Learn more about cracked teeth at the AAE (American Association of Endodontists) web site.