A Comparative Evaluation of the Effectiveness of Added Auxiliary Features, Occlusal Surface Modifications, and Reduction of Total Occlusal Convergence on the Resistance of Full Veneer Metal Crowns on a Molar Tooth with Inadequate Resistance Form: An in vitro Study.

INTRODUCTION: Single crowns or fixed partial dentures retainers usually get dislodged due to inadequate resistance form. Hence, it is prudent to evaluate features of a tooth preparation, which can prevent these failures. AIM: To evaluate the effect of auxiliary features, occlusal surface modifications, and total occlusal convergence (TOC) on the resistance of a full veneer crown.
MATERIALS AND METHODS: An ivorine mandibular molar tooth was prepared with features of inadequate resistance form, i.e., 2.5 mm axial wall height and TOC of 20°. Seven auxiliary preparation features were subsequently added one by one to it. They were mesiodistal grooves, buccolingual and mesiodistal grooves, buccolingual grooves, mesiodistal boxes, occlusal inclined planes, 8° reduced TOC in the cervical aspect, and mesiodistal grooves added to 8° reduced TOC in the cervical aspect. Ten dies with their respective crowns were prepared for each group. Resistance testing of all the samples was performed on the INSTRON testing machine.
RESULTS: Modification of the overtapered die preparation by reducing the TOC to 8° in the cervical 1.5 mm of the axial wall and then subsequently adding mesiodistal grooves to the reduced TOC cervically offered the greatest resistance to dislodgment statistically.
CONCLUSION: For an overtapered preparation, reducing the TOC to 8° in the cervical aspect and subsequently adding proximal grooves can provide maximum resistance form. Copyright:

INTRODUCTION

General dental practice includes a significant amount of fixed partial dentures and crown work. Long-lasting retention and resistance of the fixed prostheses contribute toward their success. The function or esthetics of a patient's dentition are undesirably affected by premature loosening of a restoration.

Resistance form is a concept, which has been analyzed theoretically, mathematically, as well as clinically. Trier et al.[1] found that the rate of failure of the castings due to decementation was over 95%. Lack of resistance form was an important feature of these castings and molars contributed about 63% to these failures. The importance of resistance form as an essential element in preparation design can be gauged from these facts. These clinical results support the basic prosthodontic principle that resistance form is an essential element in preparation design. Shillingburg recommends a taper of 6°. However, the studies by Woolsey and Matich[2] and Weed and Ohm and Silness[3] confirm that total convergence angle of 20°–25° is the rule rather than exception.

If taper of the tooth appears to be too extreme, additional auxiliary features, e.g., grooves and boxes in the prepared teeth, may be effective in achieving adequate resistance form. Therefore, this study aims to study the effectiveness of different modifications on the resistance of a complete metal crown on the molar tooth.

MATERIALS AND METHODS

A mandibular first ivorine molar was mounted in a block of autopolymerizing acrylic resin. The tooth preparation needed to have following features for inadequate resistance form:

2.5 mm occlusocervical dimension

8.0 mm internal faciolingual dimension

10.0 mm external faciolingual dimension

A 1 mm wide shoulder finish line

A total occlusal convergence of 20°.

The tooth preparation was done using an attachment of the air-rotor handpiece to a surveyor [Figure 1]. Occlusal reduction involved preparation of flat occlusal surface, with a wheel-shaped diamond point. For axial reduction, the head of a long, flat-ended, straight diamond point was marked at 2.5 mm to achieve a constant height of 2.5 mm for all the axial walls. The pointer was adjusted to 10° on the protractor, thus giving the same angulation to the handpiece and the bur. This ultimately achieved a 10° taper for all axial surfaces, resulting in a total occlusal convergence (TOC) of 20°.

Figure 1

Customized clamp to orient air-rotor handpiece on surveyor. Protractor affixed for determining angulation of the straight diamond point

The ratio calculation of the external tooth dimensions resulted in an occlusocervical/faciolingual (OC/FL) ratio of 0.26. Similarly, when the faciolingual dimensions of the preparation were used to calculate the ratio, a value of 0.31 was obtained. Both of these ratios could not satisfy the minimally acceptable value proposed for molars.[4] A digital caliper was used to measure the cervical faciolingual distance at the mesiodistal center of the tooth to further evaluate the potential resistance form of the master die. According to Zuckerman,[5] this measurement gave the radius of the boundary circle within which the preparation design could not offer any resistance form.

An autopolymerizing acrylic resin special tray was fabricated over the tooth preparation with the required space (5–6 mm) for the polyvinylsiloxane impression material. An impression was made using one-step putty/light body technique. It was used to duplicate 10 wax models of the tooth preparation. Casting, finishing, and polishing of the dies were carried out using standard protocol. Each metal die was mounted in an autopolymerizing resin block using a specially fabricated mold to standardize the position of the die in relation to the testing pin of the jig.

The 10 acrylic blocks with the metal dies thus obtained were numbered from 0 to 9. The die was used to prepare a wax pattern of the metal crown. A recipient site (groove of 2 mm dimension) was carved on the cuspal incline of the pattern for the tip of the testing pin of the universal testing machine.

Impression of the crown wax pattern was made using a polyvinylsiloxane impression material in a custom tray. Casting, finishing, and polishing of the crowns were carried out using standard protocol. Glass ionomer cement was used to cement the crowns onto the metal dies. To maintain a constant seating force during cementation, the crown was subjected to a 5 kg load for 2 min using a spring-loaded metal press machine. Hence, control Group A specimens were ready. The same protocol was followed for all the groups after the addition of specific auxiliary features to the die as mentioned below.

Group B involved addition of mesiodistal grooves to Group A. The mesiodistal and faciolingual depth of the grooves was 1.0 mm. Group C involved addition of mesiodistal grooves and buccolingual grooves to Group A. Group D involved addition of buccolingual grooves to Group A. For this, the mesiodistal grooves were blocked out with sculpturing wax leaving only two buccolingual grooves. Group E involved addition of mesiodistal boxes to Group A. Sculpturing wax was used to block out the buccolingual grooves. The same flat-ended tapered carbide bur and angulation was used to transform the mesiodistal grooves into the boxes. The boxes were 3.0 mm wide faciolingually having a depth of 1.0 mm. Group F involved addition of occlusal inclined planes at a 30° angulation to Group A. Sculpturing wax was again used to seal and block out the mesiodistal boxes. The occlusal surface was again restored to original dimensions of Group A with sculpturing wax. Group G involved modification of Group A by reducing TOC to 8° in the cervical 1.5 mm of the tooth preparation. The air-rotor handpiece with flat-ended, straight diamond point was attached to the surveyor device and then given an angulation of 4° and preparation was done. Group H involved addition of mesiodistal grooves to Group G.

The INSTRON automated universal testing machine was used for the resistance testing of all the 80 samples. The cemented crowns were subjected to a gradually increasing force applied on the cuspal incline at a 45° angulation by the machine. The dislodgment of the casting indicated failure for that casting and corresponded to the maximum force value for that specimen. The data obtained from testing were organized and prepared for statistical analysis.

RESULTS

Figure 2 shows the mean resistance force values for the eight groups. The mean resistance force exhibited by Group A specimens to dislodgment was 115.63 kg/s2, which is lowest. The highest value was recorded with Group H, i.e. 418.42 kg/s2.

Figure 2

Bar diagram showing the mean resistance force values for all the groups

The mean differences between all the eight groups were evaluated using one-way analysis of variance test. The calculated “F” value, 58.23, is much higher than the critical “F' value, 2.9, for 1% level of significance. This implies that some tooth preparation features exhibited greater resistance to dislodgment forces than other features. Student's unpaired t-test was used for individual comparison of control Group A with other groups in the study (Group B to Group H). No significant difference between means of Group A was found when statistically compared with Group B, C, D, E, and F.

The calculated values of “t” for Group G and Group H were 9.643 and 15.00, respectively, which were larger than critical value of “t” (2.552) for 1% level of significance when compared with Group A. This means that there is a significant difference between mean resistance force values of Group A when statistically compared with Group G and Group H. The “t” test comparison between Group G and Group H showed that there is a statistically significant difference between mean resistance force values of the two groups caused by the addition of mesiodistal grooves to Group G.

DISCUSSION

Over the years, many authors have recommended minimum taper angles ranging from 2° to 5°/side. However, it has been determined that routinely such minimal angles are not achieved. Rather, the mean TOC angles in the range of 12.2–27° have been reported by many studies.[3678] It has been proposed by many studies that posterior teeth are inevitably prepared with greater TOC than anterior teeth.[91011] Many factors such as restricted access due to unfavorable position of the tooth in the arch, limited visibility and mouth opening, and decreased height to base ratio can contribute to the difficulties experienced during preparation of posterior teeth.

In the present study, resistance was evaluated for a preparation which was inadequate to provide optimum resistance. However, this situation is routinely encountered in clinical practice. To increase the resistance form, various auxiliary features such as occlusal plane modification and reduction of the TOC of the axial wall were prepared by modifying the control Group A.

In this study, statistically significant resistance force values were obtained for Group G and Group H. Thus, the present study demonstrated that modifying the TOC to 8° in the cervical 1.5 mm of the tooth preparation (Group G) and subsequently adding mesiodistal grooves to the cervical aspect of the tooth preparation (Group H) offered the greatest resistance.

The increased resistance of the Group G could be attributed to the fact that the occlusocervical/faciolingual ratio was 0.31 for 20° of TOC, which was unable to provide adequate resistance form. After the preparation of 8° TOC at the cervical 1.5 mm of the tooth preparation, the ratio became 0.33. This ratio was capable of achieving adequate resistance form for the preparation. Furthermore, this resulted in locating the outline of the crown preparation beyond the margins of the boundary circle as described by Zuckerman,[5] thus providing adequate resistance.

In general, it is believed that the resistance to displacement for a short-walled preparation on a large tooth (molar) can be improved by placing grooves in the axial walls. However, contrary to the general assumption, neither mesiodistal grooves nor buccolingual grooves individually provided significant increase in resistance form. The mean resistance force values were relatively increased to 171.75 kg/s2 when mesiodistal and buccolingual grooves were used in combination. However, this value was also statistically insignificant when compared to the control group. The addition of mesiodistal boxes also could not provide adequate resistance form to the tooth preparation.

The studies by Teteruck and Mumford[12] have evaluated the degree of adaptation of crowns to tooth preparations with different modifications. Increased gap or loss of adaptation between the die and the crown was consistently found along grooves and interproximal boxes. This may have accounted for no increase in resistance testing of tooth preparation with these features.

The mean resistance force recorded for occlusal inclined planes (Group F) is 118.6 kg/s2, which is statistically insignificant. Several studies[101112] have shown that such a uniform contact does not exist between the crown and the die as space is required for the cement and also due to casting limitations. This may have resulted in inadequate resistance.

Thus, it was observed that reducing the TOC to 8° in the cervical aspect and subsequently adding proximal grooves in the reduced TOC provided maximum resistance to dislodgment as compared to the remaining added auxiliary features.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.