Replacing large fillings in back teeth often requires stronger, more precisely contoured restorations. These are called inlays or onlays depending on whether they are being placed inside the tooth or on its outer surface. These restorations need to be custom-made at an off-site dental lab; therefore, two visits may be required. Once the dental office receives the custom inlay or onlay, the patient returns to the dentist's office, where the restorations are bonded into place.