Effect of Prepared Cavities on the Strength of Teeth

AUTHORS: T.D. Larson, W.H. Douglas, R.E. Geistfeld
YEAR: 1981
JOURNAL: Operative Dentistry

Summary: The force required to fracture teeth with either occlusal or MOD cavities was determined and compared with that required to fracture sound teeth. In all instances, teeth with cavity preparations were SIGNIFICANTLY weaker than sound teeth. The influential factor was the width of the occlusal portion of the cavity. This was of more influence in weakening the crown than the presence of boxes.

The purpose of this study was to compare the effect of occlusal cavities as well as MOD cavities on the strength of teeth and in both instances to compare the effect of cavities which are narrow occlusally (1/4 intercuspal distance) and those that are wide (1/3 the intercuspal distance).

Materials and Methods Summary:

60 premolar teeth (non-carious) were divided into 5 experimental groups.

Five Experiment Groups:

  1. Unprepared teeth
  2. MOD preparations with wide occlusal portions (1/3 intercuspal distance)
  3. MOD preparations with narrow occlusal portions (1/4 the intercupsal distance)
  4. Occlusal Preparations wide ( 1/3rd the intercuspal distance)
  5. Occlusal Preparations narrow (1/4th the intercuspal distance)

Preps were .5mm into dentin oclussaly. Axially, the box was .5 mm past the DEJ.

Figure 1. Preparation Designs for occlusal and MOD preparations.

Figure 1. Preparation Designs for occlusal and MOD preparations.

A force was applied axially and centrally to the occlusal surface with a steel sphere 3/16 in diameter so that both the cusps of the crown were contacted. A force of 20 lbs per second was applied until the main fracture of the crown occurred.

Figure 2.  Occlusal load applied with a steel sphere.

Figure 2. Occlusal load applied with a steel sphere.


Results:

Table 1 shows the average dimensions of the teeth within each group. The mean force needed to fracture teeth is shown in Table 2. More force was required to fracture teeth without prepared cavities than teeth with prepared cavities with the difference being statistically significant.

Table 1. Dimensions of the teeth in each of the 5 groups.

Table 1. Dimensions of the teeth in each of the 5 groups.

Table 2. Load Required to fracture teeth. Highlighted pairs are not statistically different.

Table 2. Load Required to fracture teeth. Highlighted pairs are not statistically different.


Discussion:

The greater resistance to fracture of teeth without prepared cavities indicates that even the narrow preparations weaken the tooth significantly. The surprising finding is the lack of significant strength difference between occlusal and MOD preparations. A commonly believed concept is that the mesial and distal marginal ridges form a circle of enamel that is important to the strength of the crown. Breaching the circle with mesial and/or distal boxes should weaken the crown seriously, but this did not prove to be true with the minimal preparation designs used in this study.

It appears that reduction of occlusal enamel is the first critical step towards weakening of the crown of the tooth.

This study supports the movement towards minimally invasive and conservative restorations. The conservative preparation would best preserve the strength of the crown.


Conclusions for the Biomimetic Dentist

  • Even a conservative prep with a narrow width of 1/4 ICD (intecuspal distance) weakened the tooth by 40% compared to a sound unprepared tooth. In these premolar teeth, the 1/4 ICD prep was approximately 1.35 mm wide.
  • When the width of the preparation was extended to 1/3 ICD the strength of the tooth was weakened by 60% compared to a sound unprepared tooth. The 1/3 ICD is approximately prep was approximately 1.8 mm wide.

Increasing the width of the preparation by as little as .5 mm will weaken the tooth significantly. Preparations must be as conservative as possible to preserve the strength of the tooth.

Matt Nejad About Matt Nejad

Dr. Nejad graduated from the University of Southern California School of Dentistry with recognition for a record setting amount of fixed prosthodontic procedures completed. He was the youngest dentist to become a clinical instructor at USC, at the age of 26, and he has given lectures on Morphology and Occlusion (since 2010). He is a premiere educator in Biomimetic Dentistry, presenting at numerous conferences by invitation and providing continuing education courses for dentists. He also has private practices in Simi Valley, CA and Beverly Hills, CA.

He has been teaching dentists from around the world, how to improve their adhesive dentistry and provide conservative restorations with the biomimetic approach.

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