Dental implants are made of medical grade titanium. Titanium is the most bio-compatible element known to man. In fact, bone will grow right up against titanium without any kind of foreign body response from the immune system. For this reason, titanium has been successfully used for years to replace knees and hips.
Statistics from three decades of clinical experience confirm a success rate of nearly 95% over a five year period for dental implants. Some implants have been functioning well for 20 years or more but this is not an average becasue so many factors come into play (i.e. changing health conditions, genetic factors, bone density, detrimental habits, improper oral hygiene, etc.). On a positive note, if the implant does become unstable for whatever reason, the implant is simply unscrewed and the bone allowed to heal, whereas bridge failures sometimes require root canals or loss of additional teeth.