The influence of various core designs on stress distribution in the veneered zirconia crown: a finite element analysis study

Abstract

Purpose: The purpose of this study was to evaluate various core designs on stress distribution within zirconia crowns.

Materials and methods: Three-dimensional finite element models, representing mandibular molars, comprising a prepared tooth, cement layer, zirconia core, and veneer porcelain were designed by computer software. The shoulder (1 mm in width) variations in core were incremental increases of 1 mm, 2 mm and 3 mm in proximal and lingual height, and buccal height respectively. To simulate masticatory force, loads of 280 N were applied from three directions (vertical, at a 45° angle, and horizontal). To simulate maximum bite force, a load of 700 N was applied vertically to the crowns. Maximum principal stress (MPS) was determined for each model, loading condition, and position.

Results: In the maximum bite force simulation test, the MPSs on all crowns observed around the shoulder region and loading points. The compressive stresses were located in the shoulder region of the veneer-zirconia interface and at the occlusal region. In the test simulating masticatory force, the MPS was concentrated around the loading points, and the compressive stresses were located at the 3 mm height lingual shoulder region, when the load was applied horizontally. MPS increased in the shoulder region as the shoulder height increased.

Conclusion: This study suggested that reinforced shoulder play an essential role in the success of the zirconia restoration, and veneer fracture due to occlusal loading can be prevented by proper core design, such as shoulder.

Keywords: Dental crowns; Dental prosthesis designs; Dental stress analyses; Finite element analyses; Zirconia.

Fig. 1

Schematic representation of the shoulder variations in the zirconia core created in CAD software. The shoulder (1 mm in width) variations in core were incremental increases of 1 mm, 2 mm, and 3 mm in proximal and lingual (PL) height, and buccal (B) height respectively. A: no shoulder, B: PL 1mm, C: PL 1 mm and B 1 mm, D: PL 2 mm, E: PL 2 mm and B 1 mm, F: PL 2 mm and B 2 mm, G: PL 3 mm, H: PL 3 mm and B 1 mm, I: PL 3 mm and B 2 mm and J: PL 3 mm and B 3 mm.

Fig. 2

CAD designed tooth/veneered zirconia crown system components. A: veneer porcelain, B: core, C: cement layers and D: tooth preparation.

Fig. 3

Loading points and directions simulating maximum bite force (A and B) and masticatory force (C and D). A: Three points on the outer inclines of the buccal cusps, three points on the inner inclines of the buccal cusps, and two points on the inner inclines of the lingual cusps were loaded. B: A total load of 700 N was applied from the axial (vertical) direction. C: Three points on the outer inclines of the buccal cusps and two points on the inner inclines of the lingual cusps were loaded. D: A total load of 280 N was applied from three directions.

Fig. 4

Maximum principal stress distributions of 10 models subjected to maximum bite force. Maximum principal stress concentrated in the areas around loading points on the crown surface. A: Model 1, B: Model 2, C: Model 3, D: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.

Fig. 5

Lingual side view of maximum principal stress distributions of 10 models subjected to maximum bite force. A: Model 1, B: Model 2, C: Model 3, D: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.

Fig. 6

Maximum principal stress distributions of 10 models subjected to masticatory force (under the application of loads from three directions). (1) load of 280 N at 0° to the to oth axis (vertical direction), (2) load of 280 N at 45° to the tooth axis, towards the lingual margin, and (3) load of 280 N at 90° to the tooth axis, towards the lingual surface (horizontal direction). A: Model 1, B: Model 2, C: Model 3, D: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.

Fig. 7

Lingual side view of maximum principal stress distributions of 10 models subjected to masticatory force. (1) load of 280 N at 0° to the tooth axis (vertical direction), (2) load of 280 N at 45° to the tooth axis, towards the lingual margin, and (3) load of 280 N at 90° to the tooth axis, towards the lingual surface (horizontal direction). A: Model 1, B: Model 2, C: Model 3, B: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.

Fig. 8

Lingual side view of minimum principal stress distributions in the 10 models subjected to maximum bite force. A: Model 1, B: Model 2, C: Model 3, D: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.

Fig. 9

Lingual side view of minimum principal stress distributions of 10 models subjected to masticatory force (under the application of loads from three directions). (1) load of 28 0 N at 0° to the tooth axis (vertical direction), (2) load of 280 N at 45° to the tooth axis, towards the lingual margin, and (3) load of 280 N at 90° to the tooth axis, towards the lingual surface (horizontal direction). A: Model 1, B: Model 2, C: Model 3, D: Model 4, E: Model 5, F: Model 6, G: Model 7, H: Model 8, I: Model 9 and J: Model 10.