Laboratory simulation of longitudinally cracked teeth using the step-stress cyclic loading method

Abstract

Aim: To simulate in a laboratory setting longitudinal cracking in root filled premolar teeth, using cyclic mechanical fatigue.

Methodology: Mesial-occlusal-distal (MOD) cavities were prepared in twenty root filled, single-rooted, mandibular premolars restored with fibre posts and resin composites. The samples were randomly divided into two groups based on the loading approaches: static loading with a crosshead speed of 0.5 mm/min and step-stress cyclic loading (1 Hz) with increasing amplitude. The loads and numbers of cycles to failure were recorded. Micro-CT was also used to identify the fracture modes. Statistical analysis was performed using Student's t-test. The level of significance was set at 0.05.

Results: The mean fracture loads for the static loading and cyclic loading groups were 769 ± 171 N and 720 ± 92 N, respectively. There was no significant difference between the two groups (P > 0.05). The proportions of longitudinal, cuspal and mixed-mode fractures under cyclic loading were 50%, 20% and 30%, respectively. Longitudinal fractures occurred with larger numbers of cycles and higher average loads per cycle compared with the other fractures. Static loading produced only cuspal fractures.

Conclusions: Longitudinally cracked premolar teeth with root fillings were successfully produced using the step-stress cyclic loading method. This provides a more clinically representative methodology for studying cracked teeth in a laboratory setting.

Keywords: cracked tooth; cyclic loading; longitudinal fracture; static loading.

Figure 1

Micro-CT images showing different fractures modes in the cyclic-loading group: (a, b) longitudinal fracture, (c, d) cuspal fracture, (e, f) mixed-mode fracture. The yellow triangles indicate the positions of the main failure. The red dashed arrows indicate the possible positions of crack initiation. The white dashed line in each section indicates the position of the other corresponding section of the same fracture mode.

Figure 2

Micro-CT images showing representative cuspal fractures in the static-loading group. The yellow triangles indicate the positions of the main failure. The red dashed arrows indicate the possible positions of crack initiation. The white dashed line in each section indicates the position of the other corresponding section of the same fracture mode.

Figure 3

(a) Average load per cycle against number of cycles to failure for the cyclic-loading group. (b) Failure probability against fracture load for both groups. (c) Failure probability against average load per cycle for the cyclic-loading group.