Inlays/Onlays are called microprosthetics.
In the case when there is loss of tooth structure(due to decay or failed previously placed chair-side restorations)when the restorative area of the tooth takes more then 1/3 of the width of the tooth it is generally recommended to restore the tooth with the lab fabricated Gold, porcelain or special composite inlay/onlay.
Those restorations do not harm the teeth by polymerization shrinkage as it is seen in the chair-side polymerised composite restorations.

To repair damage to the tooth’s biting surface, rather than using a simple filling, or a crown, a dentist will often use an inlay, or an onlay. Inlays and onlays can be made from porcelain, gold, or composite or ceramic resin, although porcelain is now becoming the material of choice because of its strength and potential to match the natural color of your tooth.

An inlay is similar to a filling and lies inside the cusp tips of the tooth. They are custom-made to fit the prepared cavity and are then cemented into place. An onlay is a more extensive reconstruction that covers one or more cusps of a tooth. Onlays are indicated in situations where a substantial reconstruction is required. However, more of the tooth structure can be conserved compared to the placement of a crown. Inlays and onlays are applied in two dental visits.  At the first visit, the old filling, or decay, is removed, and the tooth is prepared for the inlay / onlay.  The dentist will then make an impression of the tooth, and send this impression to a dental laboratory.  This impression will be used by the laboratory to construct a custom-made porcelain, or gold inlay / onlay.  At this time the dentist will place a temporary sealant on your tooth and schedule a second appointment.				Inlay Onlay
			At the second visit, the temporary sealant is removed.   Your dentist will then ensure that the inlay / onlay fits properly in the tooth and does not interfere with your bite.  The inlay / onlay is then bonded into the tooth with a strong bonding resin, and polished smooth.
Porcelain Inlay or Onlay
TREATMENT TIME		Two appointments of approximately one to two hours per tooth.
PATIENT MAINTENANCE		Avoid biting hard objects in order not to fracture the porcelain. Normal brushing and flossing. Use fluoride mouth rinse and toothpaste as prescribed by your dentist. Same dietary restriction (as above) for the longest restorative life.
RESULTS OF TREATMENT		Porcelain inlays/onlays can successfully achieve both esthetic and functional results in restoring discolored or metal posterior teeth. More conservative than full crown.
Advantages & Disadvantages of Inlays and Onlays
		ADVANTAGES DISADVANTAGES Highly esthetic   No metal shows Strong once bonded to tooth Well-sealed tooth Will not stain Will insulate the tooth Well suited for large cavities Long lasting More costly than amalgam or composite   Can fracture Takes two appointments
Before and After Dental Inlays / Dental Onlays treatment
Before Dental Onlays		After Dental Onlays
Before Dental Onlays		After Dental Onlays