The basic premise of tooth restoration is to conserve as much tooth tissue as possible whilst taking into account the need to protect remaining tooth substance from damage from either further decay or occlusal stress. Inlays and onlays are the same type of restoration, but they incorporate different proportions of the tooth.
Where a cavity has most of its margins in a self-cleansing area of the tooth, i.e., a part of tooth surface that can be reached with a toothbrush, then it is fine to finish the cavity margin in that area. It is only in the case of a very caries prone patient that the dentist would opt for full crown coverage when an inlay would do the job.
The greyer area comes when one or more cusps of a back tooth are seen to be compromised by being undermined and weakened. In such cases an inlay may not be ideal because removing any undercuts to allow for the required path of insertion might undermine the cusp further, and in any event such a cusp is vulnerable to fracture in time due to occlusal stress. In cases such as these it becomes necessary to cover the cusp or cusps in order to provide protection against occlusal stress whilst chewing, and an onlay is more appropriate. This is similar to an inlay but also takes in one or more cusps, or even covers the entire biting surface of the tooth.
An onlay in such cases can be the answer if it is not too extensive, and a rule of thumb might be that it doesn’t involve more than two cusps. Any more than this and most dentists would agree that it is better to go the whole hog and provide protection for all four cusps by constructing a full crown. Thus an onlay could also be described as a partial crown, but because of the length of the margin compared to that of a full crown is also a more demanding restoration in terms of achieving accuracy of fit than a full crown.
The disadvantage of the full crown is the potential threat to gingival health and here again it is necessary to achieve an accurate fit to avoid encouraging plaque accumulation that would cause gum inflammation. It is technically desirable to place crown margins supra-gingivally to avoid gum irritation but this is not always possible because of the overriding need from the patient’s point of view not to show the margins, and so a compromise has to be made. Inlays and onlays are more conservative of tooth tissue than crowns, but not always as retentive in cases of shallow preparations or short teeth. The onlay allows for conservation of tooth structure when the only alternative is to totally eliminate cusps and perimeter walls for restoration with a crown. A caries prone patient would be better off with a crown because of the relatively high risk of recurrent caries round an inlay or onlay. It requires a very high level of workmanship with regard to both dentist and laboratory technician to achieve good results with onlays because of often complex shape, and thus where there is any doubt with a damaged or broken down tooth most will opt for a full crown.
The aesthetic requirements and relative costs often decide the kind of material to be used for inlays or onlays, with the choice generally narrowing down to gold alloy or porcelain. Gold requires the least amount of tooth reduction and is generally the longer lived because there is always a risk of porcelain cracking eventually.
Porcelain inlays and onlays have the advantage of not only looking much more aesthetic to most people than gold, but are also able to be bonded to the tooth with a resin luting cement which provides a strong chemical bond and a degree of sealing against micro-leakage and recurrent caries. This makes porcelain the preferred choice in many if not most cases in the typical dental practice today.