Published in Dental Labour, No. 5, 1991

Introduction
Material & Methods
Results
Discussion Summary

Introduction

All-ceramic crowns in the form of the porcelain jacket crown have been in user for more than sixty years.

They have very good tissue compatibility and the esthetic effect is of the reconstruction is satisfactory. The drawback of jacket crowns, however, has proved to be their low ultimate strength. Though ceramic materials have high compressive strength, their tensile strength is ten to twenty times lower. When the material are subjected to shear forces, i.e. when tensile stresses are formed, fractures occur, starting from the microscopic surface defects, which are always present. Shear stress particularly occurs in the anterior region at the incisal position, spalling of the labial facet.

The ultimate strength of crowns was greatly improved with the development of metal-ceramic crowns. However, oxidation and/or corrosion products resulting from the fired alloys led to reduced tissue compatibility. The esthetic effect was also impaired by the dark metal core.

Developments over the last ten years have therefore focused on improving the strength of all-ceramic crowns by employing new ceramic materials and techniques so that these crowns can continue to be used.

While the jacket crown produced by conventional technology is built up in layers of different materials, the Dicor (DeTrey) and IPS Empress techniques (Ivoclar) use a completely different method of fabrication. In both techniques the crown is first modelled in wax and embedded in special materials. Dicor crowns are centrifugally cast with transparent glass ceramic materials, whereas IPS Empress crowns are fabricated from tooth-coloured special ceramics by pressing. The Dicor technique is followed by heat treatment (ceramming), in which - among other things - mica crystals are formed. In the IPS Empress technique, leucite crystals are already latently present. The mica (Dicor) or leucite crystals (IPS Empress) increase the mechanical strength of the ceramics materials, as well as improving opacity.

A series of experiments was conducted to study how the improved material data of these new materials affected the ultimate strength of anterior crowns, as compared with conventional jacket crowns.

Material & Methods

Given the low strength of jacket crown porcelain, step-by-step preparation with a step depth of around one millimetre is the usual procedure for porcelain jacket crowns. The processing instructions for the Dicor and IPS Empress techniques also recommend preparation of this kind.

The tooth model dies have to have sufficient strength for load values at fracture, or ultimate load, to be measured. Tooth model dies were therefore fabricated from stainless steel in such a way that they could be inserted into the loading apparatus.

Step-by-step preparation at one millimetre intervals was chosen as the method of preparation. The shape and measurements of the test dies were approximated to the upper incisors (figure 1).

Fabrication of crowns

The wall and layer thicknesses of the crowns have a decisive effect on strength, particularly in the case of all-ceramic crowns. It was therefore essential that all the crowns be modelled in identical shape. As accurate fabrication of shape was not possible visually, an auxiliary apparatus was constructed in which the external shape of the model could be checked and remodelled by means of insertable templates (figure 23>. A total of 24 crowns were modelled in wax and 12 jacket crowns fabricated using the usual technique, reworking their external shapes and employing the test templates in the frontal and sagittal directions.

Twelve wax crowns were embedded with the Dicor and IPS Empress techniques, respectively, and finished according to the manufacturers instructions. A total of 36 crowns with approximately identical external shapes were available for strength measurements (figure 3).

Measurement of crown wall thickness

Since contraction in the sintering of ceramic material - even when templates are used - makes total identity of shape very difficult to achieve, particularly in the case of jacket crowns, the labial and palatinal wall thickness and the height of the incisal build-up on the finished crowns were measured. The thickness was measured using calipers (figure 4). The resolution of the calipers was 0.01 mm (Renfert praecimeter).

Measurement of ultimate strength

In the anterior region, both an axial load and, at the labial edges, a direction of force inclined toward the axis of the tooth can occur. Both possible types of load were considered. Before the fracture test, the crowns were cemented on the metal dies. The dies were roughened with an Ivoclar fine-jet spray using aluminum oxide with a particle size of 50 - 80 um. The jacket and Dicor crowns were cemented with zinc phosphate cement - Harvard normal hardening - in accordance with the manufacturers instructions. With IPS Empress crowns, Ivoclar Dual Cement was used after silanizing the crowns.

The ultimate load values were measured with an electronically-controlled, hydraulic loading apparatus (figure 5). In such a procedure, objects to be loaded are clamped to a lifting table, lifted via pressure cylinder and forced deflecting lever and pressed against a stationary loading die. The loading die is connected to a force transducer clamped onto the frame of the apparatus, so that the force can be recorded. The fissures and fractures were recorded acoustically, using a measuring microphone and subsequent evaluation electronics, then plotted against force on an x-y recorder (figure 6). An 0.5 mm sheet of unbreakable but ductile plastic material was placed between the tooth and the die to prevent a point-shaped load between the crown and loading die. The load was increased at a rate of 50 Nsec. The first recorded acoustic fracture pulse was taken as the ultimate load.

Loading at 30*: A metal base (S), which had a hole drilled at 30* to the vertical to hold the tooth dies was inserted into the lifting table of the loading apparatus (figure 7). After the incisal edge of the tooth and the incisal edge of the loading die had been placed in parallel position, the metal base was displaced horizontally so that the point of force induction was one mm below the incisal edge. In each case, six crowns were loaded to fracture by means of oblique load application.

Axial loading: The metal dies with cemented crown were inserted into a base with a vertical hole, with the base then inserted into the lifting table and displaced horizontally so that the tooth axis and the axis of the pressure die, which was provided with a groove, coincided (figure 7). The remaining six crowns for each type of crown were tested by means of axial loading.

Results

Crown wall thickness

The crown wall thicknesses varied only slightly in valued obtained at the particular measuring points (table 1). Approximately equal wall thicknesses of 1.48 and 1.45 mm were recorded at measuring points 1 and 3 respectively.

The lowest measured value was 1.33 mm, while the highest was 1.53 mm. In relation to the mean value, therefore, the maximum variation in measured value on 0.2 mm is only about 14%. The variation for measuring points 2 and 4 with mean wall thickness of 1.64 and 1.62 mm respectively, was between 1.55 and 1.69 mm, i.e. only approximately 9%.

In the incisal region the layer thicknesses varied between 3.43 and 3.73 mm with a mean value of 3.55 mm, i.e. by approximately 8%.

Ultimate strength

Loading direction 30*: After the ultimate strength was exceeded, the labial facet was abruptly sheared off in all the crowns, with palatinal portions of crown remaining on the dies in some cases with Dicor and IPS Empress crowns (figures 8 & 9).

In the case of jacket crowns the first fracture pulses were recorded between 151 N and 212 N. The median and mean are 179.5 N.

Dicor crowns fracture at a mean value of 253 N and 196 N and 345 N. The median was slightly lower at 239 N.

The highest ultimate load values were found for IPS Empress crowns (figure 11). The measured values varied between 301 N and 385 N. The mean value and median are 335 N.

The measured ultimate load values and the mean values are listed below in Table 2:

Table 2: Mean ultimate load values, measured with induced direction of force inclined 30* to the tooth axis

Axial loading: Under axial loading, abrupt and complete spalling of the crown in front of the metal die occurs when the fracture load is exceeded. The crown breaks into several pieces. No crown residue remains on the die (figure 12). It was possible to photograph typical fracture lines for some IPS Empress crowns by suddenly interrupting the increase in load application (figure 10). The ultimate load values for jacket crowns were between 476 N and 610 N. The mean value was 545 N (table 3). Dicor crowns show an ultimate strength of 1683 N, with a variation in measured values of 1430 N to 1790 N. The median was 1565 N.

The highest mean ultimate strength was also found with IPS Empress crowns under axial loading, i.e. 2180 N. The median was also the highest at 2095 N. The measured values varied between 1950 and 2550 N.

Table 3: Ultimate load values, measured with axial loading

Discussion:

An essential criterion for assessing the usability of denture crowns is adequate ultimate strength under functional loading. A mean maximum masticatory force of 14 N can be anticipated in the anterior region. However, individual force peaks can reach values of up to 200 N. In addition, a mean angle of 30* is to be anticipated at the incisal edges between the direction of load application and the tooth axis. In this case, additional shear forces at the level of half the load applied act on the construction.

If anterior crowns are to be used unrestrictedly, their ultimate strength for both axial and oblique load application must be above individual force peak, i.e. above 200 N.

The ultimate strength of all-ceramic crowns mainly depends on the external shape of the crown, i.e. the thickness of the material over the tooth die. Since ceramics have low tensile strength, shaping is used in an attempt to prevent, or at least reduce, the occurrence of tensile zones. Step-by-step preparation is therefore necessary in the case of axial load application. Identical external shapes had to be achieved for the same test conditions to be assured for the different types of crowns. The small variation in crown wall thickness revealed that an approximately identical shape of crown would be achieved with the help of templates. Wide variations in ultimate load value because of different external shapes were therefore not to be expected.

The types of crown studied show adequate ultimate strength for axial loading. As a result of the vertical induction of force from the crown into the horizontal step, very few tensile zones are formed. Even jacket crowns with a mean value of 545 N show a high ultimate load value for the anterior region.

That value is considerably high still for Dicor crowns at 1583 N and for IPS Empress crowns at 2180 N, i.e. an order of magnitude which could never be attained in the mouth.

The mean ultimate load value of the IPS Empress crowns is approximately 38% higher than that of the Dicor crowns, confirming that IPS Empress material has higher ultimate strength. With oblique load application, all ultimate load values measured are substantially lower. Strong tensile zones form in the crown at the level of the force induction line, resulting in the labial facet-splitting off once the tensile strength is exceeded.

Since manually layered dental ceramic material have only low tensile strength because of unavoidable porosities and lack of homogeneity, the ultimate load values for jacket crowns are low. Both the mean value at 179 N and approximately 85% of the measured values are below the required minimum strength of 200 N. The conventional jacket crown thus fails to meet the requirement of ultimate strength.

A rise in ultimate strength can be anticipated if homogeneous ceramic crystal-reinforced materials are used. The mean ultimate load value for Dicor crowns at 253 N is approximately 26% above the strength limit. However, the lowest mean value is only 196 N. Starting from microscopic surface cracks possibly generated in the working of the glass crown, fractures can evidently occur in unfavourable cases at load values which are still below the required strength of 200 B. Although the Dicor crown can be termed fracture-proof for oblique load application under the selected experimental conditions, fractures can occur in individual cases at very high force peaks.

The IPS Empress crowns also shoed the highest ultimate load values in oblique force induction. The mean value of 335 N is approximately 68% above the minimum strength of 200 N. Even the lowest measured value of 301 N still has a safety margin of 101 N or approximately 50%.

The mean ultimate load value is approximately 32% higher than that of Dicor crowns. If the flexural strength of the materials used (approximately 70 MPa for layered dental ceramics) is considered, approximately 150 MPa for Dicor glass ceramics, and approximately 200 MPa for IPS Empress ceramics (figures from Ivoclar), it can be observed that the ultimate load values obtained correlate well with the material data.

The higher flexural strength of IPS Empress material guarantees correspondingly higher ultimate strength of the crowns. Assuming that the appropriate form of preparation is used, IPS Empress all-ceramic crowns thus have adequate ultimate strength with a margin of safety. Since the Ivoclar IPS Empress material is additionally coloured according to natural tooth colours, a better overall esthetics can be achieved for the restoration than with Dicor crowns

Summary:

The ultimate strength of various all-ceramic crowns was investigated in a comparative study. Using test templates, twelve anterior crowns were fabricated with identical shape and layer thickness by the conventional jacket crown technique, the Dicor method, and the more recent IPS Empress method.

Six of each type of crown were loaded with axial load applications and the remaining crowns with a direction of force inclined at 30* to the tooth axis until fracture.