Fillings can be replaced in various ways , from composite to ceramic fillings ,
The governing factor will be your budget .
You must first establish why they are failing be it caries or occlusal trauma , look at occlusion on this site .
Once you know why then have your treatment , the most important factor is that whatever you have done can be done to strengthen your remaining teeth ,This strength is maximised by the materials used and the fit of the materials used as well as the accuracy of the dentist ability and the way your bite is left after the restorations are done .
You must bear in mind the design and the depth , Well polished accurate tooth coloured restorations are possible and if you have the finance they can look better than your original teeth . Amalgams are the entry level , they do release mercury on placement and removal , in some cases patients are known to be ill if too many are done at once , dietary supplements to minimise the toxic effects of amalgam (mercury)should be understood . (seaweed extracts )
Composites done in careful layers with minimal shrinkage really work well with the right kind of composite that is colour stable , they must be polished well and left smooth , especially if there is glass filler in it (reduces the shrinkage ) Any rough surfaces are abrasive to opposing teeth . Well polished porcelain is the strongest with the least shrinkage as it is cemented on . Look at well made Cerec crowns and onlays , these do cost the same as a crown or on-lay or inlay .