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Year : 2013  |  Volume : 24  |  Issue : 2  |  Page : 216-219
Evaluation of resistance form of different preparation features on mandibular molars

Department of Fixed Prosthodontics, Faculty of Dentistry, Esfahan University of Medical Sciences, Iran

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Date of Submission16-Jan-2011
Date of Decision29-Feb-2012
Date of Acceptance06-Feb-2013
Date of Web Publication20-Aug-2013


Aims: Resistance form of full metal-ceramic crown is an important feature that determines longevity of these restorations. This study evaluated the resistance form of full metal-ceramic prepared with four different design features.
Materials and Methods: An acrylic tooth was prepared with 20° total occlusal convergence (TOC) angle, 2.5 mm of occlusocervical dimension and a shoulder finishing line. This design lacked resistance form. The crown preparation was subsequently modified by preparing Mesial Occlusal Distal isthmus, placing occlusal inclined plane, and reducing TOC. Four metal dies from these designs were constructed by lathe machine and then 10 metal copings were fabricated for each preparation. Metal coping were cemented on metal dies with temp-bond cement. Force was applied at 45° from lingual to buccal direction with universal testing machine. Statistical analysis used: The data were evaluated by Kruskal-Wallis and non-parametric Mann-Whitney test.
Results: All features increased resistance form when compare to control group. However, reduce TOC group showed greatest value of resistance.
Conclusion: Within the limitation of this study, reducing the tapering of occlusocervival dimension is the most effective way in increasing resistance form, although, other features were also effective.

Keywords: Mesial occlusal distal, occlusal convergence, resistance form, tooth preparation

How to cite this article:
Farshad B, Ehsan G, Mahmoud S, Reza K, Mozhdeh B. Evaluation of resistance form of different preparation features on mandibular molars. Indian J Dent Res 2013;24:216-9

How to cite this URL:
Farshad B, Ehsan G, Mahmoud S, Reza K, Mozhdeh B. Evaluation of resistance form of different preparation features on mandibular molars. Indian J Dent Res [serial online] 2013 [cited 2023 Mar 21];24:216-9. Available from:
Longevity of full veneer crowns are of paramount importance in fixed prosthodontics. The most mechanical failure of fixed prosthodontic restorations occurs after several years of services. [1],[2],[3] Walton et al. found that mechanical failure accounted for 69.5% of failures compared for 28.5% of failures due to oral diseases. [2]

Resistance form of prepared teeth has been stressed as a key factor in determining the mechanical failure of crown and fixed partial dentures. [4],[5] Dislodgement of crown or retainers of fixed partial dentures can compromise the function or esthetics of patient's dentition.

Resistance form is defined as the features of a tooth preparation that enhance stability of a restoration and resist dislodgment along an axis other than the path of placement. [6]

A number of authors have attempted to determine resistance by subjecting restorations to oblique or lateral forces. [4],[7],[8],[9]

Reisbick, et al. were the first to investigate the features of a crown preparation that increased resistance form. They reported that placement of interproximal grooves and boxes increased the resistance form and boxes were more effective than grooves. [10] Other preparation design features my affect the resistance form. The total occlusal convergence (TOC) [7],[11],[12] the occlusal cervical dimension of full veneer crowns, [12] and diameter of the crowns [13] have reported as important factors that affect the resistance form.

Zuckerman has shown that the placement of inclined planes on the occlusal surface rather than a flat surface can increase the resistance form. The same can be achieved by placement of an occlusal isthmus. Another way to improve resistance form was re-preparing the apical portion of the axial walls, at a reduced TOC. [14],[15]

The purpose of this study was to evaluate the effect of auxiliary preparation features and occlusal surface modifications and reduced TOC in apical portion of axial wall on the resistance form of a metal ceramic crown.

   Materials and Methods Top

An acrylic tooth was prepared using a milling machine (AF 30, Type 1369; Nurnberg, Germany) with 10° tapered bur (cone cutter bur H 35 65-obo, Brasseler USA, savannah, Ga) resulting in a 20° TOC and occlusocervical dimension of 2.5 mm and 0.8 mm finishing line depth. A 0.5 mm wide bevel was placed between the axial wall and occlusal surface of the preparation at 45°. The occlusal surface was prepared with a flat form by using a 0°, non-tapered bur (Parallel cutter carbide milling bur H364E-023, Brasseler USA) by placing the tooth at a 90° to the long axis of the bur. This form of tooth preparation had no resistance form according to Good acre criteria. [16]

This prepared acrylic tooth was now converted to metal die made up of brass by lathe machine (Mini metal lathe machine 5278, spring hill, Kansas, USA) and was dimensionally verified by digital caliper (mitutoyo, Japan) to the original Acrylic tooth preparation.

The ivorine tooth which was used for the initial group was modified by preparing two interproximal boxes connected with occlusal isthmus. the boxes were prepared using a carbide bur (171; Darby dental supply, co) and occlusal isthmus with 4° tapered finishing bur (cone cutter H 3565-031; Brasseler, USA) this resulted in proximal boxes with 3 mm facio-lingual dimension and 1 mm mesiodistal depth and isthmus with 1 mm occlusocervical depth and 1.5 mm in faciolingual dimension. [17]

This acrylic tooth preparation was again converted to metal die by lathe machine similar to previous procedure. Wax was placed to seal the boxes and isthmus of the acrylic tooth. Occlusal preparation following original cusp inclination that resulted in 30° angulations to occlusal plane was done, and with previous method metal die was constructed.

Apical portion of the preparation was altered by 4° tapered finishing bur on milling machine which resulted in reduce TOC of 8° In cervical 1.5 mm of axial surface [14] and this acrylic preparation was again duplicated to a metal die by milling machine.

This additional reduction resulted in finish line with 1 mm shoulder depth.

All features of preparation were shown in figure [Figure 1].
Figure 1: Control, Mesial occlusal distal isthmus, inclined occlusal surface and total occlusal convergence groups

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Die lubricant ("keen lube"; Belle de st Claire, chats worth, calif) was applied to the surface of the metal die.

A wax pattern (kerr, orange, California, USA) was made on this die. [18] A site for the tip of the universal testing machine was carved at the lingual incline of buccal cusp. A vinyl polysiloxane material (Aquasil HV; Densply International Inc.,) in a special tray was used to make an impression of the wax pattern on each metal die. This impression was used as a mold to standardizing the fabrication of 10 copings using the same investment, special liquid to distilled water ratio and Base metal alloy (will-ceram, lite cast-B, Composition: Ni 77.4%, Cr 12.8%, Mo 4%, Al 3.3%, Be 1.8%, Fe 1%; pro-Art Williams). [19]

The intaglio surface of the castings was checked with fit-checker and pressure spots were adjusted. The copings and metal dies were subjected to airborne particle. Abrasion was done by 50-μm Al 2 O 3 before cementation. Crowns in each group were cemented on metal die with temp-bond cement (kerr Co. orange, California, USA). A 5-kg load was applied to the copings while the cement was setting (for minimum of 5 min). [20] The cement was allowed to set for 24 h. The cemented copings were gradually loaded with increasing force (kg) applied at 45° angulations from lingual to buccal direction by a universal testing machine (model 1125; instron corp., canton mass). [4] The force was applied at the lingual inclined plane of the buccal cusp [Figure 2].
Figure 2: Force was applied with Dartec machine at 45° angle to metal crown

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The force required for crown dislodgement in the universal testing machine was recorded.

   Results Top

The data was obtained from universal testing machine.

The means and standard deviations for resistance are listed in [Table 1].
Table 1: Resistance value of the four groups (MPa)

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The data were evaluated by Kruskal-Wallis and non- parametric Mann-Whitney test.

All crown preparations in all groups showed increase in resistance values compare to control group. However, the resistant value in group with reduced TOC was the highest compare to other three groups. Occlusal inclined plane group was the next; the difference between the occlusal inclined planes and Mesial occlusal distal (MOD) inlay groups was statistically not significant.

   Discussion Top

In this laboratory study, the crown preparation that had the greatest resistance form was the one with 8° of occlusal convergence at the apical 1.5 mm of axial wall. There are many articles with similar results.

Zuckerman observed that reduce TOC reduces Buccolingnal diameter at the cervical part of teeth and this area acts as a resistance zone. [21] Proussaefs demonstrated only crown preparation that significantly enhanced the resistance form was reducing the TOC at the cervical area of the axial walls. [22]

Rosentiel explained that reducing TOC will cause paralleling of axial wall and therefore, increases the resistance form. [23]

Goodacre concluded that occlusal convergence angle (the angle of convergence between two opposing axial surfaces was one the first aspect of tooth preparations for complete crowns to receive adequate resistance. He concluded TOC ideally should range between 10° and 20°. [16]

Next design that had the highest resistance form after TOC was the internal MOD inlay. This design was not investigated in previous studies but box and isthmus design was studied separately.

According to proussaefs study grooves and boxes were not effective at increasing resistance form for a short tooth preparation with 20° TOC. Perhaps, they would have been effective if they had been placed with less TOC. [22]

Reisbick, et al. concluded that resistance values increased significantly with the addition of grooves or boxes. [10] Next design was the occlusal incline plane design used in teeth with lacking resistance form, which increased the resistance form compared to the control group. How ever there was no significant difference between MOD inlay group and occlusal incline plane group. In proussaefs study, preparing occlusal incline plane in teeth lacking resistance form did not increase resistance significantly. [22] This difference in result may be due to using two different cements in these two studies. In proussaefs study, resin modified glassionomer cement was used, in our study temporary cement (temp-Bond) was used, and this may be the cause of this slight difference in this part of results.

   Acknowledgement Top

The authors wish to thank Isfahan Torabinejad research center for their supports.

   References Top

1.Schwartz NL, Whitsett LD, Berry TG, Stewart JL. Unserviceable crowns and fixed partial dentures: Life-span and causes for loss of serviceability. J Am Dent Assoc 1970;81:1395-401.  Back to cited text no. 1
2.Walton JN, Gardner FM, Agar JR. A survey of crown and fixed partial denture failures: Length of service and reasons for replacement. J Prosthet Dent 1986;56:416-21.  Back to cited text no. 2
3.Foster LV. Failed conventional bridge work from general dental practice: Clinical aspects and treatment needs of 142 cases. Br Dent J 1990;168:199-201.  Back to cited text no. 3
4.Potts RG, Shillingburg HT Jr, Duncanson MG Jr. Retention and resistance of preparations for cast restorations. J Prosthet Dent 1980;43:303-8.  Back to cited text no. 4
5.Woolsey GD, Matich JA. The effect of axial grooves on the resistance form of cast restorations. J Am Dent Assoc 1978;97:978-80.  Back to cited text no. 5
6.Academy of Prosthodontics. The glossary of prosthodontic terms. J Prosthet Dent 1999;81:39-110.  Back to cited text no. 6
7.Weed RM, Baez RJ. A method for determining adequate resistance form of complete cast crown preparations. J Prosthet Dent 1984;52:330-4.  Back to cited text no. 7
8.Maxwell AW, Blank LW, Pelleu GB Jr. Effect of crown preparation height on the retention and resistance of gold castings. Gen Dent 1990;38:200-2.  Back to cited text no. 8
9.Rosenstiel SF, Land MF, Fujimoto J. Principles of tooth preparation.Contemporary fixed prosthodontics. 4 th ed. St. Louis: Elsevier; 2006. p. 209-58.  Back to cited text no. 9
10.Reisbick MH, Shillingburg HT Jr. Effect of preparation geometry on retention and resistance of cast gold restorations. J Calif Dent Assoc 1975;3:51-9.  Back to cited text no. 10
11.Wiskott HW, Nicholls JI, Belser UC. The relationship between abutment taper and resistance of cemented crowns to dynamic loading. Int J Prosthodont 1996;9:117-39.  Back to cited text no. 11
12.Cameron SM, Morris WJ, Keesee SM, Barsky TB, Parker MH. The effect of preparation taper on the retention of cemented cast crowns under lateral fatigue loading. J Prosthet Dent 2006;95:456-61.  Back to cited text no. 12
13.Wiskott HW, Nicholls JI, Belser UC. The effect of tooth preparation height and diameter on the resistance of complete crowns to fatigue loading. Int J Prosthodont 1997;10:207-15.  Back to cited text no. 13
14.Zuckerman GR. Resistance form for the complete veneer crown: Principles of design and analysis. Int J Prosthodont 1988;1:302-7.  Back to cited text no. 14
15.Zidan O, Ferguson GC. The retention of complete crowns prepared with three different tapers and luted with four different cements. J Prosthet Dent 2003;89:565-71.  Back to cited text no. 15
16.Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: An art form based on scientific principles. J Prosthet Dent 2001;85:363-76.  Back to cited text no. 16
17.Kono A, Fusayama T. Casting shrinkage of one-piece-cast fixed partial dentures. J Prosthet Dent 1969;22:73-83.  Back to cited text no. 17
18.Fusayama T, Ide K, Kurosu A, Hosoda H. Cement thickness between cast restorations and preparation walls. J Prosthet Dent 1963;13:354-64.  Back to cited text no. 18
19.Eden GT, Franklin OM, Powell JM, Ohta Y, Dickson G. Fit of porcelain fused-to-metal crown and bridge castings. J Dent Res 1979;58:2360-8.  Back to cited text no. 19
20.Teteruck WR, Mumford G. The fit of certain dental casting alloys using different investing materials and techniques. J Prosthet Dent 1966;16:910-27.  Back to cited text no. 20
21.Zuckerman GR. Factors that influence the mechanical retention of the complete crown. Int J Prosthodont 1988;1:196-200.  Back to cited text no. 21
22.Proussaefs P, Campagni W, Bernal G, Goodacre C, Kim J. The effectiveness of auxiliary features on a tooth preparation with inadequate resistance form. J Prosthet Dent 2004;91:33-41.  Back to cited text no. 22
23.Ayad MF, Maghrabi AA, Rosenstiel SF. Assessment of convergence angles of tooth preparations for complete crowns among dental students. J Dent 2005;33:633-8.  Back to cited text no. 23

Correspondence Address:
Ghasemi Ehsan
Department of Fixed Prosthodontics, Faculty of Dentistry, Esfahan University of Medical Sciences
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0970-9290.116686

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  [Figure 1], [Figure 2]

  [Table 1]

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