Dental implants can last forever but this is not always the case. It is best to make your decision on the assumption that you will need some form of maintenance of your implant and restoration in your lifetime – the younger you are, the more likely this will be.
Complications related to dental implants can be categorised as short and long term, and biological or mechanical.
In the short run, it takes a few months for your body to grow bone onto the implant surface – a process known as osseointegration. The success rate is excellent, in excess of 95% if certain protocols are followed.
In the long run, biological problems can cause the development of gum disease around your implant. This is more likely to happen if you smoke, have gum disease (periodontitis), do not clean your implants thoroughly and regularly (with toothbrushes and interdental brushes), do not visit your dentist and hygienist regularly or have certain medical conditions (for example diabetes).
Mechanical complications can lead to chipping, staining and wear of your restoration. Over time, as with anything manmade, more extensive maintenance may be needed, such as replacing the crown/bridge/denture. Dentures are often cheaper than ceramic restorations such as crowns and bridge, but the softer materials will need replacing more frequently.
If the implant under the gum is healthy, then replacing the restoration on top is normally straightforward but can be costly and should be factored into your decision.