All medical conditions need to be evaluated individually, but in general there are some principles when tooth implants might be recommended: 

1. Missing Teeth: The most common reason for a dental implant is to replace one or more missing teeth.

2. Failed Bridge or Crown: If a previous dental restoration like a bridge or crown has failed or deteriorated, a dental implant may be a more permanent solution.

3. Unsuccessful Root Canal: If a tooth that has had a root canal fails to heal properly or becomes infected, an implant may be the next step after extraction.

4. Tooth Decay: Sometimes decay is so extensive that it is impractical or impossible to save the tooth using other treatments.

5. Fractured Tooth: A tooth that has fractured at or below the gum line may need to be extracted and replaced with an implant.

6. Bone Loss Prevention: Implants can help prevent or minimize bone loss that occurs when teeth are missing.

7. Denture Improvement: Implants can be used to stabilize or anchor a lower denture, making it more comfortable and functional.

8. Cosmetic Reasons: Some people choose dental implants for aesthetic reasons, especially if the missing tooth is visible when they smile or talk.

9. Oral Health: Missing teeth can cause a range of problems, including difficulties in eating or speaking and increased risk of gum disease and tooth decay. Implants can help restore normal function.

It's essential to consult with a qualified dental professional for an accurate diagnosis and treatment plan that suits your specific needs.