Peter Bolhuis, DDS, PhD

Introduction

For this study a standardized test setup was developed. A milling device, a Celay machine (www.mikrona.com), was used to standardize the samples.

In conformity with a brass master freshly extracted sectioned premolars were prepared, using the “key-duplication” method. The first master, Master I, has the shape of a decapitated premolar and Master II the shape of a full crown preparation.

Procedure

The sectioned premolar was aligned with the brass master in the Celay device and Master I was copied:

Lining up.

Master I,

and the copy.

Next the premolar was built up with core material and prepared again according to the Master II. 

Building up.

Preparing from

Master II

Below the dimensions of the preparations (Masters I & II) are illustrated. The crown preparation ended in the axial dentin, 2 mm beyond the build-up material.

Master I

Master II

Finally, cementing a standard crown on this core build-up completed the sample. A “push out wax and cast” system (right) was developed to produce these standard crowns.

Testing

The resistance to fracture of the 5 groups (n=16) was evaluated. In three groups the sectioned teeth were built up with a direct adhesive material without post, the resin-based composites Photo-core and Ti-core and the glass-ionomer Ketac-Molar. In the fourth group a conventional gold cast post and core was fabricated by means of the direct method. A Duralay core build-up, supported by a burnout post, was prepared in conformity with the second master, removed, cast in metal, and cemented into the root canal. In the last group the sectioned premolar was not rebuilt with core material but only prepared according to the second master. This affected only the 2 mm axial dentine, in which a shoulder was prepared. These three core build-up designs are summarized below.

Of each group 50 % of the samples were exposed to a fatigue load in the Acta Fatigue Tester at one cycle per second. The maximum load was 57 N at an 85 degrees angle. For the duration of this fatigue loading the teeth were kept in a moist environment. During 1 million cycles of fatigue loading no spontaneous failure was found. Finally all samples were  loaded until fracture at a 45 degrees angle (left) in an Instron testing machine.

The results are illustrated in Fig 11, which still has to be produced...