Until recently, dentists filled and sealed cavities exclusively using a silver and mercury amalgam. Unfortunately, these fillings (or restorations) often weaken teeth due to the large amount of the original tooth that has to be removed. Modern dentistry has increasingly turned to composite fillings as a strong, safe and more natural looking alternative. Composite fillings utilize a soft white plastic substance that includes a hardening agent. Replacing large fillings on back teeth often require a stronger and properly contoured restoration. These are called inlays or onlays depending on their size. These restorations need to be custom-made at an off-site dental lab and two visits may be required.