Dental onlays are indirect restorations which can be appropriate to restore back teeth where there is damage to one or more cusps, and could be described as partial crowns. Where damage to a tooth is judged too extensive then it may be better to opt for a full crown. The dental onlay is more usually constructed in gold to allow for a thin enough covering where cusps need to be reduced and protected, but can also be made from porcelain since modern porcelains are stronger than they used to be and the appearance is generally much preferred by patients. The onlay helps to conserve as much tooth substance as possible and also to protect it from fracture due to biting forces, and so covers one or more cusps unlike an inlay which just fits into a prepared tooth cavity leaving the cusps untouched.
Porcelain inlays and onlays are sometimes used instead of crowns when there is enough sound tooth substance remaining to support them. They require much less tooth reduction than crowns and are therefore able to conserve more tooth tissue and can last as long as crowns. When provided in porcelain they provide an excellent aesthetic result. As a rule of thumb if more than a third of the biting surface is destroyed then a bonded porcelain onlay is often the best option for restoring a tooth enabling it to both appear and function virtually as it did before, and to last well.
Sometimes a dentist is confronted with cases of severe erosion and attrition causing flattening of back teeth and loss of much cuspal substance. In such a situation some dentists would prescribe a crown as the only option but because there could be insufficient natural coronal tissue remaining for retention this could necessitate carrying out elective root treatment first which is another whole complex procedure, and at the end of the day you would have a non-vital tooth. A vital tooth is much preferred and the same situation could alternatively be addressed by means of an adhesive bonded porcelain onlay, which would probably need only a minimum of preparation. Such treatment would leave the pulp of the tooth intact and vital which is a much more favourable one for the long term prognosis of the tooth.
The other advantage of a onlay over a full crown on a posterior tooth is that most of the restoration’s margins lie well away from the gum-line which removes the possible irritation to gum tissues that might otherwise occur.
The dental onlay is usually made in the same way as an inlay from impressions of the teeth following suitable removal of decay and cavity shaping by the dentist. There is period of time required for the onlay to be constructed in a dental laboratory and then it is returned to the dentist for fitting. Some dentists have invested in sophisticated computer technology called CEREC CAD/CAM which enables an onlay to be provided within one day by scanning the cavity in a tooth and then providing accurate three dimensional measurements to allow an appropriate exact shape to be milled from a block of porcelain to fit that cavity exactly.
Restoring lost vertical dimension
Another use of onlays apart from the restoration of an extensively damaged back tooth is in the restoration of lost vertical dimension. The occurrence of cases where patients have suffered a combination of attrition and erosion of their teeth appears to be on the increase and may be partly attributable to increased consumption of acidic drinks such as sodas, fruit juices, and wine. Once the surface enamel of teeth is softened by acid it is much more vulnerable to attrition from incorrect tooth brushing technique or simply from chewing. Attrition is the loss of tooth substance due to frictional wear from opposing teeth which can proceed gradually to reduce enamel thickness and eventually expose dentine. If left unchecked the process is more rapid from then on because dentine is much softer than enamel.
The habit of bruxism or grinding one’s teeth is fairly common, often put down to increased stress of various kinds in our lives today, and many people have a clenching or slight grinding habit that is entirely subconscious but can contribute to attrition of tooth substance over time. Attrition is facilitated by acid erosion that first softens surface enamel making it easier to be worn away.
Continued erosion and attrition can cause the teeth to be reduced in height and lead to loss of vertical dimension in the face, which can give a prematurely aged appearance and also lead to overclosing of the jaws and possible jaw joint problems. There are also inherent anomalies of teeth occlusion that can occur coupled with skeletal abnormalities such as the deep overbite where an overall loss of vertical dimension may be evident.
These kinds of cases often require the restoration of all or at least part of the vertical dimension as part of the treatment plan and this can be achieved on an interim basis by means of occlusal splints in order to estimate the degree of increased vertical dimension that is tolerable to the patient. An alternative method, however, is to gradually build up the teeth with composite onlays by bonding increments of composite resin material in a stepwise manner until the level of tolerance is achieved. Once this has been established a more permanent solution is necessary in order to maintain the equilibrium. This might be achieved with an overlay removable partial denture, or by means of onlays bonded to the teeth using gold or porcelain. In severe cases it may be necessary to prescribe full crowns for multiple teeth.
The so-called temperomandibular (TMJ), syndrome is another common problem which may also be linked to stress with symptoms of headache, jaw ache, or muscle spasm, and can sometime be treated initially by means of an occlusal splint with a view to relieving the joint and muscle tension. Success with this may lead to the diagnosis of a degree of overclosure as part of the problem and again onlays may form part of the treatment plan.
It is increasingly recognized that patients who show excessive tooth wear present difficulties in deciding immediately on the definitive treatment prescription since the wear has taken place over a long period of time and the precise correct occlusion and vertical dimension may have been lost along the way, and there is often no easy way to correctly establish these again. It is thus logical to proceed on a trial basis using conservative means such as removable splints and adhesive restorations so that treatment is reversible if necessary.
Therefore, the best way forward is often an initial overlay splint, provisional restoration with onlay techniques, careful observation, and finally definitive permanent restorations once a satisfactory and stable situation is achieved, which may take several months.