Table 1
Full size table

Commentary

The full crown restoration is recommended to improve the longevity of teeth significantly broken down from underlying decay or trauma.1 The standard geometric and technical criteria of the prepared tooth have not changed much in over fifty years.2,3 Recent reviews by Tiu et al. and Podhorsky et al. have summarised the literature with the aim of better understanding the achievability of the recommended standards in clinical practice and the effects they may have on the treatment prognosis.4,5

Tiu et al. limited their question to the geometric design of the prepared tooth, with the vast majority of included studies reporting on the TOC. Interestingly, they found the TOC values typically achieved clinically far exceed historically recommended angles quoted in standard textbooks.1

Acute TOC angles are shown to optimise retention and resistance of the final restoration and are believed to minimise the mechanical stress on the luting cement. But these recommendations were made at a time when crowns were often luted with zinc-phosphate cement. With the significant advancements in dental materials over the last half century, today's crowns are often cemented with materials (eg resin and glass ionomer) which have higher compressive and tensile strengths, as well as tooth bonding properties.6

Podhorsky et al. aimed to evaluate if the conventional wisdom of tooth preparation was evidence based. The review analysed how various geometric, technical and biologic factors affect clinical outcomes, eg preparation geometry, preparation depth into dentin, location of a preparation's margin to periodontium, grit of diamond bur and the subsequent heat generated during tooth preparation and dental crown design. Nineteen clinical recommendations that the authors made were supported with low to very low quality evidence. They were only able to find moderate clinical evidence recommending that crown margins be placed supra-gingivally.

Both reviews failed to describe clear methodologies and validated standards of how they evaluated (i) the quality and (ii) the risk of bias of the included studies. For example, neither review presented detailed descriptions nor standardised critical appraisal processes for the individual studies they included. Tiu et al. did not describe how a lack of evaluator blinding in each study may have contributed to expectation bias. Furthermore, Podhorsky et al. did not assess how the potential bias from likely censored data (patients lost to follow-up) in each study may have threatened the validity of their recommendations.

Conventional wisdom behind the recommended parameters of tooth preparation and crown design seeks to balance two objectives: long term survival of restorations relative to risk of harm to the endodontic or periodontal complex. Both reviews, despite the limitations I have noted, did acknowledge the lack of good quality clinical evidence connecting the former to the latter.