A filling is used to treat the effects of tooth decay: formation of a cavity in a tooth.
Root canal treatment is done when the nerve of a tooth is diseased. Thus, resulting in a toothache or an abscess. A filling is placed after the root canal treatment is completed to close the cavity caused by tooth decay and the root canal treatment access cavity.
Whether a filling alone will be enough or if root canal treatment is needed will depend not only on X-rays but also clinical symptoms (such as pain, presence of a swelling, tenderness on chewing etc).
