JOVD 2015 #3: The Influence of Crown Height to Diameter Ratio on the Force to Fracture of Canine Teeth in Dogs

Question 1

Authors?

Answer 1

Jason W. Soukup, DVM
Caitlyn Collins, MS;
Heidi-Lynn Ploeg, PhD

Question 2

Aim of this study?

Answer 2

To investigate the influence of H/D on force to fracture and probability of fracture of canine teeth in dogs.

Question 3

Prevalence of dental fracture in dogs?

Answer 3

The prevalence is reported to be as high as 27% .

Question 4

According to literature, which of the following is false?

A) The prevalence of dental fracture in domestic dogs is 27%.

B) The most commonly fractured tooth in dog is the canine tooth with a reported frequency between 25 to 37%.

C) Canine tooth are more vulnerable to fracture due to their prominent rostral position in the oral cavity, their used as apprehension and defense and their morphology (in particular crown height (H) to base diameter (D) ratio (H/D)).

D) Finite element model revealed an inverse relationship between tooth height and both tooth strenght and fracture resistance in wild canivores.

Answer 4

B) is partialy false since the most commonly fractured tooth in the dog is the canine tooth with a reported frequency between 35.5% and 57.1%.

Question 5

Orignal Study
Study design?

Answer 5

• 30 canine teeth from Beagle
• 3 groups:
  
  • Group A: unaltered H/D
  • Group B: 10% reduction in H/D
  • Group C: 20% reduction in H/D
• Teeth potted in clear autopolymerizing orthodontic acrylic; placed in a universal materials testing machine.
• Force applied at speed of I-mm/min to the distoocclusal line angle at an angle of 45° to the long axis of the crown.
• Force to fracture (F_F) = maximum measured force at time of fracture

Question 6

To what correspond the following terms:

1. Expected Force (F_E)
2. Margin of safety (MoS)
3. Paired MoS

Answer 6

1. Biting-pulling force expected on canine teeth (F_E) (for 1 canine tooth = ½ F_E) (obtained from previous study)
2. Margin of safety (MoS) = F_F - F_E (for 1 canine tooth used ½ F_E) = probability of fracture of a canine tooth when subjected to typical biting- pulling forces. (more MoS in hight, less chance of fracture)
3. Paired MoS = impact of a 20% H/D reduction of a single maxillary canine on the fracture probability of the unaltered contralateral canine

Question 7

How would you define “moment” ?

Answer 7

• Moment:
  
  • amount of stress on a tooth when a load is placed near the cusp
  • M = F x a

M=moment; F=force; a=moment arm (= crown height when force applied perpendicular to long axis of tooth)

Question 8

Why was tissue volume calculated?

Answer 8

• Hard tissue volume
  
  • Used to minimize the variability in the size of the teeth
  • = total crown volume – pulp chamber volume
  • total crown volume calculated using formula for right elliptical cone V= πab(h/3); V = volume, a = minor base diameter, b = major base diameter, and h = crown height.
  • Pulp chamber volume calculated using formula for a cone V=1/3(πr2h); V = volume, r = pulp chamber radius, and h = pulp chamber height

Question 9

How H/D ratio of canine tooth influence its probability to fracture?

Answer 9

• MoS (probability of canine tooth to fracture)
  
  • Group A (unaltered): 36.7%;
  • Group B (10% decrease in H/D): 27.8%;
  • Group C (20% decrease in H/D): 14.5%
• 10% decrease in H/D decreased probability of fracture by 24.1%;
• 20% decreased in H/D decreased probability of fracture by 60.4% compared to unaltered teeth.

Question 10

Which of the following statement is false

A) “Factor Safety” refer to the ratio between the force required to cause a structure to fail and the force that the structure is expected to withstand in routine use.

B) Zoological studies of wild canid and felid dentition present the hypothesis that the factor of safety is larger in biological structures when their failure is associated with greater consequences.

C) Given that fractured teeth do not have the ability to repair themselves afterfracture, teeth should possess a high factor of safety that would allow teeth to withstand high forces.

D) Similar to a factor of safety calculation, the present study utilized aMoS analysis to compare the normal distributions of a structure’s strengtht of the expected loads and determine the probability of failure of the structure.

E) According to the MOS analysis of this study, the factor of safety of canine tooth of domestic dog is high.

Answer 10

E) is false

The probability of fracture of a canine tooth when subjected to typical biting- pulling forces = Margin of safety (MoS) = FF - FE (for 1 canine tooth used ½ FE) )

The MoS in the unaltered group A is 36.7%; so, probability of fracture of a canine tooth when subjected to typical biting-pulling forces of canine tooth in domestic dogs is considered low; so, typical force applied to the canine teeth during routine activity is probably less than that required to fracture the tooth in most dogs.

.

Question 11

What is the influence of H/D ratio of a canine tooth when expected biting-pulling force is distributed across 2 canine teeth according to their relative heights?

Answer 11

• Paired MoS analysis
  
  • = probability of fracture of a canine tooth and its contralateral canine tooth when expected biting-pulling force was distributed across 2 canine teeth according to their relative heights
  • 20% H/D reduction of one canine tooth reduced its probability of fracture by 86.5%, however, it increases by 54.4% the probability of fracture of the unaltered contralateral canine tooth.
  • efforts to improve the biomechanics of a single tooth by altering H/D may have a negative influence on the biomechanics on the entire oral cavity.

Question 12

Is there a canine height at which the canine is not at risk to fracture during a bitin-pulling exercice?

Answer 12

It is plausible that canine teeth of any height may fracture under the forces that dogs can generate during biting-pulling exercises.

Question 13

In which of the following situation the canine teeth are less prone to fracture?

A) When biting force in directed occlusally

B) When trumatic force is directed mesio-distaly

C) When biting-pulling forces is directed disto-mesialy

Answer 13

A) ; Canine teeth do not fracture under occlusally directed masticatory loads; canine teeth more often fracture due to a frontally-mediated traumatic force or biting-pulling forces

Question 14

What are the mean forces required to fracture teeth of each group?

Answer 14

• mean F_F:
  
  • group A : 494 N;
  • group B: 573 N;
  • group C: 630 N
• 10% reduction in H/D increased µF by 78.8 N compared to the unaltered teeth;
• 20% reduction in H/D increased µF by 136 N compared

Question 15

Does the H/D of canine tooth is solely responsible for the variance in force to fracture found in this study?

Answer 15

No

Only 13.8% of the variance in force to fracture found in this study could be explained by H/D alone; therefore, other factors must contribute to the fracture resistance of the tooth such as mechanical properties (e.g., modulus of elasticity, hardness, toughness), differences in hard tissue volume, and other unknown factors.

Question 16

Does an ideal H/D ratio of the tooth exist to maximize fracture resistance?

Answer 16

An ideal H/D may exist that maintains proper function of the oral cavity and maximizes fracture resistance of the tooth in question. However, that information cannot be determined from this study alone and further investigation into this possibility should be pursued

Question 17

Was the hypothesis concerning the benefit of H/D recduction of the canine tooth on its resistance to fracture confirmed by this study?

Answer 17

The present study support the hypothesis that reduction in the H/D of the canine tooth in dogs increases fracture resistance of a single tooth and may suggest future clinical implications but more studies are needed for specific clinical situation and impact of H/D reduction of one tooth on fracture resistance of the other teeth.

Question 18

Was the hypothesis concerning an ideal H/D exists that balances tooth fracture resistance with function confirmed by this study?

Answer 18

The clinical questions of the existence of an ideal H/D that balances proper function of the oral cavity with maximized fracture resistance and the applicability of routine crown height reduction to reduce fracture susceptibility require further investigation.

Question 19

What are the study limitations (#7)?

Answer 19

Study limitations:

1. All teeth provided from single breed of dog
2. limited and small sample of population (5 military dogs) on which F_E (bitting-puling force) was based and from which was calculated the MoS and Paired MoS.
3. Prevalence of clinical tooth fracture were not considered (breed & size of dogs, preexisting dental wear or other alteration, parafonctional habit of dog, work demand of dog, chewing/bites
4. Variability in material and mechanical properties between teeth sample even though effort to limit variability.
5. Calculation of hard tissue volume are estimates (tooth shape not uniforme).
6. Limitation to replicate the natural environment of oral cavity (10% formaline conservation (? impact on properties of teeth), experiment done on dry teeth at room temp.
7. Model do not account compliance of periodontal ligament; didn’t consider effect of crown preparation prior to full coverage on resistance to fracture according to different H/D.

Question 20

The study found all of the following except:

A) a 10% decrease in H/D resulted in 24% decrease in the probability of tooth fracture

B) a 20% decrease in H/D resulted in 60% decrease in the probability of tooth fracture

C) the probability of tooth fracture of an unaltered tooth with 100% load was 36.7%

D) A paired margin of safety analysis was performed where in the applied forces were distributed across 2 maxillary teeth according to their relative heights, which resulted in a decrease in probability of fracture of the 20% reduced H/D tooth of 86% but the contralateral unaltered tooth had an increase probability of fracture of 54.4%

E: All endodontically treated canine teeth should be reduced in H/D in order to decrease the probability of additional tooth fracture.

Answer 20

E) has not been proven by this study

Soukup suggests that there may be an ideal H/D ratio, but simply reducing the height places compromise on the contralateral canine tooth.

studies evaluating the potential advantages and disadvantages of crown height reduction as opposed to other crown strengthening methods (e.g. full coverage crown) would be warranted.

Question 21

To remember!!!

Answer 21

• The prevalence of dental fracture in domestic dogs is 27%.
• Prevalence of canine tooth fracture in dogs: 35 - 57%
• MoS (probability of canine tooth to fracture; if pb of fx is high, MoS is low)
  
  • ​Group A (unaltered): 36.7%;
  • Group B (10% decrease in H/D): 27.8%;
  • Group C (20% decrease in H/D): 14.5%
  • 10% decrease in H/D decreased probability of fracture by 24.1%;
  • 20% decreased in H/D decreased probability of fracture by 60.4% compared to unaltered teeth
• 20% H/D reduction of one canine tooth reduced its probability of fracture by 86.5%, however, it increases by 54.4% the probability of fracture of the unaltered contralateral canine tooth; so, efforts to improve the biomechanics of a single tooth by altering H/D may have a negative influence on the biomechanics on the entire oral cavity.
• Only 13.8% of the variance in force to fracture found in this study could be explained by H/D alone; therefore, other factors must contribute to the fracture resistance of the tooth such as mechanical properties (e.g., modulus of elasticity, hardness, toughness), differences in hard tissue volume, and other unknown factors.
• An ideal H/D may exist that maintains proper function of the oral cavity and maximizes fracture resistance of the tooth in question. However, that information cannot be determined from this study alone and further investigation into this possibility should be pursued; so this study did not proved that height of endo tx canine tooth should be reduced.