Natural tooth should not be substituted by any other means if it can be retained. But if the condition or prognosis of natural tooth is poor, the better alternative is an Implant.
Implants are made up of inert material, Titanium and so it can be placed in human body (Bone) without any reaction or allergy. Implant replaces root part of natural tooth/teeth and connecting part of implant holds prothesis (Crowns, Bridges or full arch).
Implant/Implants is/are placed inside the Jawbone and left there for few weeks or months to get integrated inside the bone. Abutments/ connectors are placed connected to the integrated implants in second surgery after 2-4 months. Prothesis can be prepared over abutments with 5 days.
