To explore the fracture patterns of cracked tooth syndrome (CTS) models with different crack size in vitro, and to explore the clinical risk of cracked teeth.

A total of 50 maxillary premolars were randomly divided into five groups by simple random sampling. The first four groups were used as cracked tooth models, which was adjusted to the buccal cusp inclination 59° and the palatal cusp inclination 50° by cusp inclination measurements and grinding. All the cracked tooth models were grouped according to the set pre-cracked size, i.e., the distance between the center points of the outermost edge of the mesio-distal middle margin ridge was set to the crack length a, and the distance between the plane where the two points were located and the most convex point of the cemento-enamel junction of the mesial adjacent surface was the crack depth h. The four groups were divided into (a/3-h/2) , (2a/3-h/2) , (a/3-h) , (2a/3-h) , with Group Ⅴ as blank control. All samples were subjected to compressive stress tests to record the anti-folding force, fracture mode and clinical risk classification. Single factor t-test was used to compare the fracture resistance of each group. Statistical chart was used to analyze the fracture mode. Nonparametric rank sum test was used to calculate the clinical risk level of fracture samples. The significant level was set at 0.05 (α = 0.05) .

The fracture resistance of the four groups was (1126 ± 126) N, (974 ± 159) N, (1114 ± 240) N, (608 ± 105) N, (1205 ± 216) N, respectively. There was a statistical difference between the maximum resistance of group Ⅱ, Ⅳ and the other groups (P<0.05) . No significant difference was observed among the other groups. For the fracture pattern, more palatal oblique fracture rather than buccal fracture was observed in all experimental groups. The proportion of crown and crown-root fracture for each group was (50%~50%) , (40%~60%) , (20%~80%) , (0~100%) , (100%~0) , respectively. For clinical risk assessment, there were significant statistical differences between all CTS model groups (groups Ⅰ-Ⅳ) and the control (group Ⅴ) (P<0.05) . For CTS model groups, there was a statistical difference between group Ⅰand Ⅳ (P_1-4 = 0.003) . No significant difference was observed among the other groups. The samples of group Ⅰ may be reserved. A non-reservable crown-root fracture may occur to 20% samples of group Ⅱ and III, while the risk for group Ⅳ was 40%, with a high crack proportion in the apical area.

The teeth with CTS were easier to fracture than normal teeth due to their much lower anti-crack ability. The fracture pattern of teeth with CTS seemed to be related to the crack size. The deeper and wider the crack, the easier the crown-root fracture, the higher the clinical risk. gradually increases on this account.