Indications for cuspal coverage

Dental caries and restorative interventions

Dental caries and subsequent restorative interventions result in substantial loss of tooth structure, weakening the tooth and increasing the likelihood of fracture of remaining tooth tissue. The three-dimensional extent of dental caries in a tooth can affect the residual tooth strength. For example, caries may undermine cusps of teeth, resulting in a predisposition to undergo excessive flexure with an increased likelihood of subsequent cuspal fracture.³ Such undermining of cusps may also be a feature of conventional amalgam cavity preparation for the purposes of retention of the restoration. However, advances in the use of adhesive technology and a shift towards minimally invasive dentistry have encouraged an increase in the clinical use of composite as a posterior restorative material and a reduction in cavity size due to the obsolescence of retention and resistance form.⁴

Reeh et al demonstrated the effect that restorative and endodontic procedures have on the stiffness, and therefore fracture resistance, of remaining tooth tissue.⁵ Their results indicated that occlusal and mesio-occlusal-distal (MOD) cavity preparations reduced tooth stiffness by 20% and 63%, respectively. The additional reduction in tooth stiffness as a result of a subsequent endodontic preparation was approximately five percent. There is some limited in vitro evidence that premolar teeth after endodontic treatment and restoration are particularly prone to cuspal fractures without a cuspal-coverage restoration, possibly due to their overall shape.⁶

Mondelli et al prepared Class I and Class II cavities in freshly extracted premolar teeth. They demonstrated, in vitro, the protective effect of preserving the marginal ridges and recommended that Class II cavities, where the occlusal isthmus was half the intercuspal width, or more, should receive indirect restorations with cuspal coverage.⁷

The mechanism of failure results from the application of a ‘wedging’ force on the tooth, exerting buccal and lingually directed lateral forces on the remaining tooth walls.⁸ Fisher et al demonstrated areas of high stress concentration in conventional inlay restorations that increased the likelihood of fracture of remaining tooth walls when compared to the ‘protective’ force distribution exhibited with onlay restorations.⁹

Traditionally, photoelastic stress analysis has been used to analyse the distribution of forces through restorations and teeth.¹⁰ This involves the analysis of colourful chromatic fringes, seen on cross-sectional slices of teeth when viewed in polarized light, and how these fringes change when stresses are applied to teeth.¹¹ Although this has been a valuable method of stress analysis, in the current literature, finite element analysis is more commonly used due to its accuracy, ease and adaptability in stress analysis. This involves the analysis of complex structures using a computer-based mathematical model. Structures are broken down into simple blocks and equations of how these behave individually are generated and combined to assess the overall response of a structure to stress. Finite element analysis demonstrates greater sensitivity in the analysis of structural response to stress, although this is dependant on the accurate description of several parameters, such as the structure of the outer surface of the tooth.¹⁰

Care must be taken with the final restoration contour. Simultaneous occlusal contact with axial loading may not occur if the occlusal surface is restored incorrectly. A poorly contoured occlusal surface of an intracoronal restoration may result in a premature contact which, if on the incline of a cusp, will result in lateral forces to that cusp with the possibility of subsequent fracture.¹

Previous endodontic treatment

Lack of pulp tissue in an endodontically treated tooth will reduce the opportunity for preventive biofeedback under occlusal loading and could partly explain the increased fracture rate in posterior root-filled teeth when restored with an intra-coronal restoration.¹² A retrospective cohort study, involving 220 endodontically treated teeth over an 11-year period, concluded that composite restorations offered a degree of improved fracture resistance, whereas amalgam restorations provided little or no resistance to cuspal movement. However, when compared, conventional direct restorations have been shown to be less successful with regard to overall tooth survival than restorations with cuspal coverage.¹³

A further factor that must be considered with regard to endodontic treatment is the fracture pattern and subsequent tooth restorability. Fennis et al demonstrated that endodontically treated teeth are more often associated with a subgingival fracture location, potentially impacting on the subsequent restorability of the tooth.¹⁴

Reports of improved survival rates of endodontically treated teeth restored with a crown are widely cited in the dental literature.^13,15,16 Although it must be noted that a Cochrane systematic review in 2012, appraising ‘the success rate of single crowns versus conventional fillings for the restoration of endodontically treated teeth', concluded that there is insufficient high quality evidence to endorse the clinical effectiveness of crown placement for the long-term survival of endodontically treated teeth.¹⁷

Cuspal-coverage restorations facilitate the distribution of masticatory forces down the long axis of the tooth, providing an element of support for residual cusps.¹⁸ This finding was corroborated by Craig et al, who demonstrated the protective dissipation of forces evident when the restoration overlays the cusps of a tooth. The authors analysed the stress distribution, using photoelastic two-dimensional stress analysis, when teeth restored with MOD inlays and crown preparations were loaded. They concluded that cuspal-coverage restorations reduce the compressive stresses elicited by MOD restorations and redistributed them towards more protected areas of remaining tooth tissue.^19,20 It was also noted that a rounded cuspal reduction prevents high compressive stresses being focused on the fitting surface of the restoration, possibly leading to restoration failure.²⁰ Farah et al demonstrated substantially lower compressive stresses were present when cusps were overlaid with gold restorations. Furthermore, the authors evidenced, through an in vitro study, that when a cusp was completely protected by a restoration, the average compressive stress exerted on residual tooth structure was significantly lower and more uniformly distributed than with restorations that did not overlay the residual cusps of a tooth.²¹

Tooth loss

Tooth loss may result in tilting, rotating or drifting of teeth and a subsequent change in the occlusal forces applied to them. In many cases, the change in position of teeth may result in the application of non-axial forces during function.²² The application of this force may result in excessive flexure and fracture of cusps previously weakened by caries or restorative intervention. Indeed, there may be forward movement of the mandible to achieve best occlusal contact in cases where tipping or tilting has resulted in an abnormal occlusal scheme, further influencing the application of non-axial forces to teeth.²³ Craig et al also demonstrated that stress concentration was greater in tilted teeth than their non-tilted counterparts and this may increase the likelihood of fracture.¹⁹

Parafunctional activity

Increased occlusal loading on all teeth may occur due to various types of normal activity, for example, clenching the teeth together when lifting heavy loads, or due to abnormal activity such as clenching or grinding habits, otherwise known as parafunction or bruxism. Such forces on a tooth that has already been compromised by caries or extensive or poorly contoured restorations may result in crack formation or fracture. Additionally, bruxism may result in the application of non-axial forces to teeth and further increase the likelihood of tooth fracture due to suboptimal distribution of occlusal forces.²²

The amount and distribution of remaining coronal tooth tissue (tooth restorability)

Any restorative procedure requires an initial diagnosis to facilitate the planning of treatment. A key stage in this is the assessment of the restorability of compromised teeth and planning for appropriate reconstruction. In order to plan the placement of restorations appropriately, practitioners must be able to appraise the value of remaining tooth tissue prior to definitive restoration. Ideally, this should be done before undertaking any endodontic treatment for a tooth. However, this is not always possible.

The evidence base for appraising tooth restorability is not extensive. A common method of assessing tooth restorability involves the visualization of the remaining tooth tissue following preparation for a cast restoration.^13,16

In an attempt to increase objectivity, Bandlish et al formulated the Tooth Restorability Index (TRI), an assessment methodology to determine the strategic value of remaining tooth tissue.²⁵ This technique divided the remaining tooth structure into sextants and assigned a score to each sextant based on the thickness, height and distribution of coronal dentine present. The scores allocated to each sextant ranged from 0 to 3. Individual sextant scores are added together to give a score from 0 to 18, indicating the restorability of the tooth. Lower scores represent a more guarded prognosis for restoration. This study also highlights the utility of assessing teeth in this way, as there was fair-to-good or substantial agreement between assessors (Kappa = 0.63).^26,27 Although this assessment criterion is a useful objective tool in the appraisal of tooth restorability, it is necessary to consider that advances in adhesive dentistry have also facilitated the restoration of many teeth which may have low TRI scores.²⁸

The type of restorative material to be used

Cuspal-coverage restorations can be created using a variety of dental materials. Most commonly, gold, composite, amalgam, porcelain or porcelain-fused-to-metal restorations are utilized. Other cast restorations, including nickel chromium, may also be considered but have fallen out of favour with practitioners, possibly due to concerns regarding possible allergic reactions.²⁹

Gold

Gold onlay restorations are commonly utilized as a suitable alternative to full or partial coverage crowns when maintaining residual tooth tissue is a priority. These restorations have the advantage that they are usually less destructive to tooth tissue, as less preparation is required, although a significant laboratory cost may be incurred to the practitioner. Nagasiri and Chitmongkolsuk demonstrated that the strength of teeth restored with gold onlay restorations was significantly higher than teeth restored with amalgam or composite restorations¹⁵ (Figure 1).

Ceramic

Advances in milled and pressable ceramics have encouraged an increase in their use, although there is currently insufficient evidence as to their long-term success rates.³⁴ Krämer et al demonstrated a 7% and 8% failure rate of ceramic inlays and onlays after four and eight years, respectively.^35,36 Additionally, Ozyoney et al demonstrated a 92.5% success over a four-year period with glass ceramic onlays restoring endodontically treated teeth, although it is evident that current literature lacks high quality long-term studies.³⁷ The increasing use of CAD/CAM technology within modern dentistry is encouraging a growing number of practitioners to use ceramic restorations. Although the evidence base for their use is limited and lacking long-term follow-up, ceramic onlay materials have been shown to achieve restoration strengths suitable for use in cuspal overlay restorations³⁸ (Figure 2).

One concern regarding the use of all-ceramic restorations is the potential for abrasion of opposing tooth substance. Although commonly referenced, a systematic review carried out by Hmaidouch and Weigl demonstrated that there is currently insufficient evidence to conclude that all-ceramic restorations are associated with increased toothwear. This study concluded that some ceramic restorations are as wear friendly as metal ceramic restorations, although all-ceramic restorations may have an abrasive nature due to suboptimal surface treatment, such as failure to polish adequately or re-glaze following occlusal adjustment.³⁹

Restoration of teeth

Conventional cavity preparation, where the marginal ridge is lost and where the isthmus is more than half the occlusal width, would benefit from cuspal protection by either bonding the restoration within the cavity or cuspal coverage.⁷

Endodontically treated molars and premolars where cavity preparation has resulted in the loss of one or more marginal ridges would probably benefit from a final restoration with cuspal protection.^6,7

Cuspal coverage may be used to increase the resistance form of an indirect restoration thus reducing the stress on any cementation agent.

Materials

Gold is still a popular choice of material for cuspal coverage when aesthetics are not a priority. It requires minimal tooth preparation and has a long track record of success.^15,18

Indirect composite restorations may be used with some success.^30,31

Ceramic is a popular choice for cuspal-coverage onlays, but there are no long-term data available for their survival or the effect of the newer ceramic restorations on the opposing tooth structure.³⁹

With the increasing popularity of tooth-coloured restorative materials and the skill required for placement, amalgam is less favoured than gold or restorative materials with better aesthetic properties.

Method of preparation

The current literature lacks clarity on: