Assessment of Total Occlusal Convergence for Crown Preparation: A Comparative Study

INTRODUCTION

Complete crown coverage is highly recommended for badly destructed and endodontically treated teeth.¹ Several factors play important roles in retention and resistance of the crown, such as height of the prepared tooth, type of dental cement, and convergence angle of tooth preparation.^2–8

The features that prevent dislodgment of a crown along its path of insertion is called retention, e.g., forces acting along the long axis of the tooth. The features that prevent crown dislodgment when oblique (nonaxial) forces act on the tooth called resistance.⁹

The total occlusal convergence (TOC) angle of a tooth preparation is the combined angle made by opposing axial walls when measured against the vertical long axis of the tooth.⁵ Greater retention occurs when we have more nearly parallel opposing walls of a preparation.^10,11

The ideal taper recommended by fixed prosthodontics textbooks and different dental schools is 2° to 7° per axial wall or 4° to 14° total TOC.² Based on a systematic review by Goodacre et al. in 2001, a clinically accepted TOC is 10° to 20°.⁹

However, in real-life situations, idealistic preparations in oral cavity with no undercuts are not always achievable. Many techniques have been described to evaluate the TOC angle of tooth preparation using devices such as photocopy machines, three-dimensional laser scanners, and overhead projectors,^12–14 but none of these devices can be available during clinical evaluation of a tooth preparation in clinical setting, in addition many specialists been evaluating their preparation by guessing by sight and sometimes computer-aided design/computer-aided manufacturing (CAD/CAM) software can give them an idea after sending their impression to fabrication.

The convergence angle method definitely influences crown retention. Therefore, it is essential that dental students become competent in achieving acceptable abutment taper. Theoretically, the more parallel the opposing walls of a tooth preparation are, the greater the retention and the more conservative is the tooth preparation. It is widely accepted that the convergence angle of a full-veneer crown preparation should be as close to parallel as possible to attain adequate retention and resistance.

The main aim of the present study was to compare the overall value of TOC of crown preparation in Princess Nourah Bint Abdulrahman University between dental students, interns, and specialist prosthodontists using guessing by sight and ruler methods, and these were compared with snapshots for accuracy.

MATERIALS AND METHODS

A cross-sectional study was conducted to assess and measure the total convergence angle of tooth preparation for complete crowns done by Princess Nourah Bint Abdulrahman University undergraduate students and specialist prosthodontists at College of Dentistry, Princess Nourah Bint Abdulrahman University, utilizing the different methods; guessing by sight, the manual convergence angle ruler (TOC Ruler) and compared with digital snapshots.

RESULTS

The results for TOC comparisons between three main groups, namely, dental students, interns, and SP using two different methods of assessment (by guessing and TOC Ruler) are shown in Table 1.

About 49% of the interns along with 31% of the students achieved the recommended TOC angle (10° to 20°) for the respective MD surface by sight method, while 28% of the interns and 33% of the students obtained it by ruler method.

For BL surface, 59% of the interns have achieved it by sight and 26% by ruler method. In comparison, for the MD surface, the reading was done by specialists; about 49% achieved the recommended TOC angle by sight method, whereas 44% obtained by ruler method. In addition, BL surface results show 63% by sight and 50% by ruler method.

The result of comparison in general between undergraduates and specialist as presented in Table 2 shows that 47% of the specialist observation achieved the recommended TOC angle, while 29.25% was obtained by undergraduates.

Table 3 presents the result of comparison of the accuracy of using manual convergence angle ruler and guessing by sight to the digital snapshots done by the three groups (dental students, interns, and specialists); 10.17% of the reading by guessing method matched the digital snapshots, while the ruler reading matched it by 7.67%.

The inference of the present study indicates that overall 47% of the specialist observation achieved the recommended TOC angle, while 29.25% obtained by undergraduates.

DISCUSSION

One of the factors that affect the acceptance of a crown preparation is the TOC. As mentioned in the textbooks, achieving the idealistic TOC (4° to 14°) in the oral cavity is nearly impossible. Goodacre et al. recommended in his systematic review that an angle from 10° to 20° can be accomplished in real-life situation and it is highly acceptable to achieve a better retention.⁹

The findings in this study showed that 30% of the prepared dies reached the idealistic angle recommended by textbooks, while 50% of the TOC of tooth preparations done in PNU dental clinics met the clinically recommended TOC (10° to 20°), according to Goodacre et al. review.⁹

Specialist group showed maximum of their expertise in guessing by sight method compared with the ruler method; this could be because they do not have much knowledge or they have not practiced convergence angle ruler. On the contrary, 44% of the students observed properly (10° to 20°) using sight, but around 71% did not observe properly (10° to 20°) under ruler method, this led to an agreement that the students tend to give an overestimation readings. And the specialists can give a positive effect on their judgment or reading during their clinical crown preparations (Table 4).

The retentive features mainly include biologic considerations. These affect the health of the oral tissues, which include avoidance of overcontouring, conservation of tooth structure, and harmonious occlusion. And mechanical considerations affect the durability and integrity of the restoration.¹⁵ It can be divided in to the following three categories: (1) providing retention form: it is the ability of the tooth preparation to resist the removal of the crown restoration along its path of insertion. (2) Providing resistance form: it is the ability of the preparation to resist the dislodgment of the restoration by forces directed obliquely or horizontally to the restoration. (3) Preventing deformation of the restoration: a restoration must have sufficient strength to prevent permanent deformation during function, so the restoration must have structural durability and marginal integrity.¹⁶

If the crown height is very less, then some of the measures can be taken to improve the retention such as placement of foundation restorations, alteration of tooth preparation design and placement of auxiliary retention and resistance form features, endodontic treatment and overlay removable partial dentures, orthodontic eruption, and surgical crown lengthening.¹⁷ The vestibular observation point shows a correctly tapered preparation to avoid undercuts.

In the present study, about 49% of the interns along with 31% of the students achieved the recommended TOC angle (10° to 20°) for the respective MD surface by sight method, while 28% of the interns and 33% of the students obtained it by ruler method. These results are in contrast with Ow et al.¹⁸ who found 38% of their students preparing tapers greater than 20°. Our finding is also in contrast with Rafeek et al.¹⁹ who have achieved 62% (MD) and 36% (BL) taper. Also El-Mubarak et al.²⁰ showed 100% of their samples out of the ideal range (they prepared their first clinical case), and this could be due to the minimal preclinical experience of the students because of either lack of proper teaching aids or supporting staff, which had negative impact on the performance of the students.

Overall there was a marginally significant difference between ruler and sight guessing in their outcome comparing to the digital snapshots.

It is recommended to do more training on how to assess TOC angle. When in doubt underestimation is recommended to achieve a better outcome because majority of the literature and lab studies recommended some features of the preparation design. For example, restricting the preparation to enamel is considered to be a critical factor for a favorable bonding strength, thus more durable outcome. In addition, more accurate ways are available to asses TOC, such as AutoCad digital software.

The limitation of this study include (1) no equal distribution of the sampled TOC; (2) no proper training on how to use the ruler (first time users); (3) participants’ availability and access to the digital software.

CONCLUSION

Within the limitation, the present study concluded that specialists’ observation was better in both ruler and sight guessing methods compared to the observations of dental students and interns. And further studies should be conducted with larger sample size to assess TOC.

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