• Inadequate bone quantity or poor bite relationship.
   
• Pathologic conditions of the bone. Eg. cysts, root fragments.
   
• Radiation therapy to the jaw region.
   
• Pathologic alterations of the oral mucosa such as Lichen planus.
   
• Dry mouth syndrome.
   
• Significantly overweight or underweight.
   
• Expect to live less than five years
   
• Systemic disorders that are not in good control. Eg. Severe diabetes, anemia, blood clotting diseases.
   
• Collegen disease like Sjogren's syndrome.
   
• Bone diseases and cardiac diseases in the severe state
   
• Patients for whom dental implants may create serious bacteria focus such as in bacterial endocarditis.
   
• Alcohol or drug abuse patients.
   
• Patients that are a problem. Those patients that tend to be blamers. Patients wanting suboptimum treatment. Those who expect perfection. Patients with psychotically problems. Patients that are demanding. Patients that do not contribute to the treatment plan or a positive outcome.
   
• If you are pregnant. Smokers . Patients on certain medications. Severe bruxism or clenching or grinding of teeth. Unfinished dentistry or gum disease that is not well managed. A very large tongue.

Just because you are missing a tooth does not mean it needs to be replaced. Second molars most often do not need replacement.