| Abstract|| |
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
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. ,, Walton et al. found that mechanical failure accounted for 69.5% of failures compared for 28.5% of failures due to oral diseases. 
|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 2019 Nov 14];24:216-9. Available from: http://www.ijdr.in/text.asp?2013/24/2/216/116686
Resistance form of prepared teeth has been stressed as a key factor in determining the mechanical failure of crown and fixed partial dentures. , 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. 
A number of authors have attempted to determine resistance by subjecting restorations to oblique or lateral forces. ,,,
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.  Other preparation design features my affect the resistance form. The total occlusal convergence (TOC) ,, the occlusal cervical dimension of full veneer crowns,  and diameter of the crowns  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. ,
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|| |
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. 
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. 
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  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.  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). 
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).  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).  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|
Click here to view
The force required for crown dislodgement in the universal testing machine was recorded.
| Results|| |
The data was obtained from universal testing machine.
The means and standard deviations for resistance are listed in [Table 1].
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|| |
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.  Proussaefs demonstrated only crown preparation that significantly enhanced the resistance form was reducing the TOC at the cervical area of the axial walls. 
Rosentiel explained that reducing TOC will cause paralleling of axial wall and therefore, increases the resistance form. 
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°. 
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. 
Reisbick, et al. concluded that resistance values increased significantly with the addition of grooves or boxes.  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.  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|| |
The authors wish to thank Isfahan Torabinejad research center for their supports.
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Department of Fixed Prosthodontics, Faculty of Dentistry, Esfahan University of Medical Sciences
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]