The Influence of Crown Height to Diameter Ratio on the Force to Fracture of Canine Teeth in Dogs.

Previous work suggests that the tooth height to diameter ratio (H/D) may have an influence on the fracture resistance of dog canine teeth. Thus, it can be hypothesized that canine teeth with distal abrasion or teeth already requiring pulpal manipulation may benefit from a reduction in height and that an ideal H/D exists that balances tooth fracture resistance and tooth function. Therefore, a study was performed to investigate the influence of H/D on force to fracture and probability of fracture of canine teeth in dogs. Thirty extracted canine teeth from laboratory Beagle dogs were standardized by hard tissue volume and evenly distributed among three groups; unaltered H/D (group A), 10% reduction in H/D (group B), and 20% reduction in H/D (group C). The teeth were potted in clear autopolymerizing orthodontic acrylic and then secured within a universal materials testing machine. A displacement was applied at a speed of 1-mm/min to the distoocclusal line angle at an angle of 45 degrees to the long axis of the crown. The maximum measured force at the time of fracture represented the maximum force to fracture. A linear regression model showed a significant inverse relationship between H/D and force to fracture (p = 0.043; 95% CI-55.2 to -0.09). A margin of safety (MoS) analysis was performed to determine the probability of fracture by comparing normal force distributions of the measured force at fracture to that reported in a previous study, representative of normal biting-pulling loads on canine teeth. When 100% of the load was applied to a single unaltered canine tooth the probability of fracture was 36.7%. Decreases in H/D of 10% and 20% resulted in a decreased probability of fracture by 24.1% and 60.4%, respectively. A paired MoS analysis was conducted wherein the applied loads were distributed across 2 maxillary canine teeth according to their relative heights. Within the pair, a 20% decrease in H/D decreased the probability of fracture of that tooth by 86.5%, but increased the probability of fracture of the unaltered contralateral canine tooth by 54.4%. The findings of this study support the hypothesis that teeth with a lower H/D are more resistant to fracture. However, given the potential impact of crown reduction of a single canine tooth on the load redistribution to the remaining unaltered canine teeth, further investigation is needed to determine what H/D would be ideal. In addition, future studies could elucidate in which clinical scenarios the concept of H/D reduction could be implemented. The results of this study may have implications on the successful long-term management of traumatized canine teeth in dogs.