# Risk Factors Associated with Cusp Fractures in Posterior Permanent Tenth--A Cross-Sectional Study
## 4 Discussion  
Lingual cusps were the most fractured, and showed a significant association with severe fractures, regardless of their functional or nonfunctional role. These data were based on the results here obtained, since fracture type and fracture height were the only factors significantly associated with fractures on lingual cusps. Total and subgingival fractures were more likely to occur on lingual cusps than partial and supragingival fractures. Indications of classic protection on functional cusps (LUBL), covering of all cusps for weakened teeth and for endodontically treated teeth [5; 6; 7; 11; 13; 20; 21], were also assessed in this study, since, theoretically, functional cusp would fracture more than nonfunctional ones and, therefore, should be covered [14]. However, obtained results question this statement, since in lower molars the majority of fractures occurred on non-functional cusps (lingual), and for the upper molars and lower premolars, there were no differences between functional and non-functional cusps fractures. Maximillary premolars were the only teeth showing a higher frequency of fractures on functional cusps (lingual). Given that, when a cusp is already weakened by cavity preparation, lingual are generally the most involved in fractures, deserving special attention [4; 5; 6; 7; 15]. Possible explanations rely on Wilson curve of occlusion, in which the posterior teeth are lingually oriented and, thus, the occlusal force components favor these cusp fractures. Nevertheless, the most adopted and disseminated reason is based on the fact that functional cusps are more voluminous, except for the upper premolars. These data support the idea that fractures do not occur due to the cusp role (functional or nonfunctional), but under the assumption that smaller cusps are weaker and more likely to fracture. Authors also observed that nonfunctional molar cusps and the upper premolars were the most inclined, and this might also contribute as a correlating factor. Thickness of the enamel was also related to fractures. In addition, functional cusps lie between two other cusps during intercuspation, which may avoid fractures [4; 5; 6; 7; 15; 8; 9; 10; 11; 12; 13; 14; 15]. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{3}{c}{**Fracture of Lingual Cusp**} \\ \hline
**Variables** &
\begin{tabular}{c} **Odds Ratio** \\ **CI (95\%) \({}^{*}\)** \\ \end{tabular} & \(p\) \\ \hline Arch & & \\ (ref) Mandibular & 0.54 (0.26–1.11) & 0.09 \\ Maximillary & & \\ \hline Fracture type & & \\ (ref) Partial & 3.20 (1.42–7.21) & \textless{}0.01 \({}^{**}\) \\ Total & & \\ \hline Teeth condition & & \\ (ref) Restored & & \\ Unrestored/caries or & & \\ Higid tooth & & \\ \hline Restorative material & & \\ (ref) Resin & 0.86 (0.40–1.87) & 0.71 \\ Amalgam & & \\ \hline Restored Surfaces & & \\ (ref) 1 surface & 0.71 (0.21–2.48) & 0.60 \\
2 or more surfaces & & \\ \hline Fracture height & & \\ (ref) Supragingival & 3.62 (1.32–9.89) & 0.01 \({}^{**}\) \\ Subgingival & & \\ \hline Number of cusp & & \\ (ref) 1 cusp & 1.18 (0.46–3.05) & 0.97 \\
2 or 3 cusps & & \\ \hline \({}^{*}\)CI 95\% = Confidence interval 95\%. \({}^{**}\)\(p<\) 0.05–statistically significant differences. & & \\ \end{tabular}
\end{table} 
Table 3: Multiple-model logistic regression with the variables that presented \(p<\) 0.20 in the univariate regression (Table 2). Fractures were more common in teeth where the restoration had an isthmus wider than 1/3 of the intercuspid distance and/or involved more than and/or involved more than 3 restored surfaces. Overall, less than 1% of sound teeth fractured, reinforcing the notion that teeth undergoing cavity preparation are weakened and prone to fracture [2,13]. In this sense, [PERSON] et al. (2001) [10] reported that 97% of fractures involved teeth with restorations. This may be related to the fact that cusp deflection increases with cavity size, causing fatigue of the dental structures over time, resulting in microleakage and dental fractures [3,11,28,29].