253Journal of Conservative Dentistry | Jul-Sep 2012 | Vol 15 | Issue 3

address for correspondence: Dr. Ankit Arora, Department of Conservative Dentistry and Endodontics, Manipal College Of Dental Sciences Manipal, Manipal University, Karnataka – 576 104, India. E-mail: aroraankit24@gmail.com

Date of submission : 01.10.2011 Review completed : 27.12.2011 Date of acceptance : 06.02.2012

Original Article

A self-designed instrument to evaluate cavosurface angle for class I amalgam cavity preparation: A learning aidAnkit Arora, Shashi Rashmi Acharya, Vasudev Ballal, Padmaja Sharma1

Departments of Conservative Dentistry and Endodontics, 1Orthodontics and Dentofacial Orthopaedics, Manipal College of Dental Sciences Manipal, Manipal University, Karnataka, India

A b s t r a c t

Fighting the controversies, amalgam, as a posterior restorative material has survived till date. The fate of amalgam restoration is determined by geometry of the cavity preparation and cavosurface angle is one integral part of the cavity, which decides the marginal characteristics of the restoration and health of the tooth. Low edge strength of amalgam highlights the importance of cavosurface angle. However, at the same time conservation of the tooth also should be considered. This article presents a self-designed instrument to evaluate and obtain a cavosurface angle close to 110°. The design of the instrument also helps in evaluating parameters, such as depth of the cavity, undercut angle, and allows finishing of the buccal or lingual walls.

Keywords: Amalgam; cavity preparation; instrument

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DOI: 10.4103/0972-0707.97951

INTrOducTION

Millions of patients have benefited from amalgam over the last 150 years of its existence. Although its use has declined in the past decade, it is continued to be used in many developing countries. Amalgam is considered an appropriate material for many restorations because of its durability and cost-effectiveness.[1]

Increasing the longevity of amalgam restoration demands perfection in geometry of the prepared cavity in all the dimensions. One integral component of the cavity form is cavosurface angle (CSA). Preparation and finishing of cavosurface angle determines the integrity of amalgam tooth interface at the margin,[2] as improperly prepared CSA may result in fracture of the tooth or of the restoration. [3] Since long it has been thought that CSA for amalgam should be 90° to allow bulk of amalgam at the margin since it is brittle in nature[4,5]; however, Elderton suggested that for wide cavities, cavosurface angle should be in the range of 105°to 115° permitting an amalgam margin angle (AMA) approaching 70°.[6] In most of the cases, CSA obtained is higher than 90°,[7,8] hence proving that striving for a butt joint is not a realistic approach. Thus an attempt was made to reproduce the concept of achieving CSA close to 110° and a complementary AMA close to 70° using a self-designed instrument as an adjunct

to conventional rotary burs. The instrument can help evaluate the CSA, and using the information obtained, cavity can be modified accordingly with appropriate burs.

INSTruMENT dESIgN

parts of the instrumentHandle [Figure 1a]Eight sided with serrations

Shank [Figure 1b]Triple angled

Blade [Figure 1c]Length: 6 mm.Width: 2 mm.Thickness: 1 mmPart of the blade that enters the cavity is 1 mm wide and 2 mm is the maximum length to which it can be inserted inside cavity.Part of the blade that remains outside the cavity is 4 mm in length.

cutting edge• Monobeveled [Figure 1d].• 2 mm in length.• Along the long axis of the blade.

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Arora, et al.: A self designed instrument to evaluate cavosurface angle for class I amalgam cavity preparation

Journal of Conservative Dentistry | Jul-Sep 2012 | Vol 15 | Issue 3254

Angulations• To achieve an undercut of 5° [Figure 1e].• To achieve a CSA of 110° [Figure 2a].• Blade angle 19.4° [Figure 2b].

Instrument formula20 6 19.4NOMENCLATUREOrder (purpose of the instrument): Evaluation of the occlusal cavity for dental amalgam with respect to cavosurface angle and finishing of the walls.

Suborder (position/manner of use)The use of instrument for evaluation of cavosurface angle and cutting action is explained here considering the most common situation wherein the cavosurface angle is in excess of 90°.

Evaluation• Holding the instrument using modified pen grasp, the

blade is inserted into the prepared cavity with its long axis parallel to the long axis of the tooth.

• When the blade gains maximum adaptation to the tooth as dictated by the prepared cavity, CSA is evaluated.

• In Figure 3 we can see that in such a situation, instruments’ cutting edge would not gain complete adaptation along its entire length to the prepared cavity wall.

• This would demand increasing the occlusal convergence of the cavity (done using rotary burs). The instrument can be used to re-evaluate the angle once modification is done.

Cutting actionOnce modification of the CSA is done as guided by the instrument, the cutting edge gains complete contact with the internal wall [Figure 4], finishing of the walls can then be carried out by moving the instrument in pull motion [Figure 5].

Figure 3: Insertion of the instrument and evaluation of the cavity

Figure 2: (a) For achieving a cavosurface angle of 110°. (b) Angle of the blade from the long axis of the handle

Figure 4: On completion of modification using rotary burs

Figure 1: (a) Handle. (b) Triple-angled shank. (c) Dimensions of the blade. (d) Monobeveled cutting edge. (e) For achieving an undercut of 5°

a

b c

d e

a

b

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Arora, et al.: A self designed instrument to evaluate cavosurface angle for class I amalgam cavity preparation

255Journal of Conservative Dentistry | Jul-Sep 2012 | Vol 15 | Issue 3

class (form of working end): It is a double-ended instrument (right and left) with a monobeveled cutting edge parallel to the long axis of the blade .angle: triple angled.

cATEgOrY

Primarily it is an exploring instrument, however, as aids in finishing of the walls, it stands as a cutting instrument also.

Adaptation as in typodont modelAfter the manufacturing of the instrument (Acharya Dent, Ontibettu), its adaptation was analyzed in cavities prepared on typodont teeth [Figure 6a and b]. The information provided by the instrument regarding various parameters was comprehendible.

dIScuSSION

Cavosurface angle is the angle of the tooth structure formed by the junction of a prepared wall and the external surface.[9] Various theories have been proposed for the orientation of the walls and the corresponding AMA. Black recommended that walls of the cavity in occlusal portion be perpendicular to pulpal floor.[10] Kornfeld suggested CSA of 100° on the basis that occlusal surface has inclined planes and 90° cavosurface angles were incompatible with Black’s design.[11] Kantorowicz added that it may be permissible to carve surface of the amalgam flat, which may lead to higher AMAs.[12] Standard text books suggest CSA and AMA of 90°, which in turn provide for the resistance form of the cavity and the restoration.[4,5] However, studies show that the above-recommended CSA and AMA are not generally achieved and attempt to achieve 90° cavosurface angle generally leads to excess undermining of the cusps.[13] Stratis and Bryant concluded that in 47% cases, cavosurface angle is between 130° and 149°.[8] The use of cylindrical burs to prepare occlusal cavities and the bur often being held at approximately right angle to the overall surface of

the tooth results in high cavosurface angles.[14] The reason also relates to the steep cuspal slopes that automatically result in higher CSA of about 135°.[14] Cavosurface angle of about 105° to–115° is optimally practical and preparing undercuts to a greater extent might endanger the integrity of the cavity margin.[6,7] AMA should be approaching 70° or more to minimize the risk of fracture. Less than 90° AMA does not necessarily lead to marginal degradation.[15]

The instrument guides for the attainment of 110° cavosurface angle thus allowing the clinician to achieve a minimum of 70° AMA.

Other applications of the instrument include• Finishing of the walls.• Checking the depth of the cavity, whether it is equal

to, less than, or more than 2 mm

(The part of the blade that can be inserted inside the cavity is 2 mm in length).• Gives an idea about the undercut angle (if the

instrument adaptation to the internal and external wall is adequate with its long axis parallel to the long axis of the tooth, the undercut angle will be close to 5°).

Limitations of the instrument• The use of instrument involves a learning curve, as

precise adaptation and interpretation is indispensable.• Due to variation in cuspal anatomy, the adaptation

of the instrument may not be apt in all the clinical situations.

Skills of dental students have been gauged since long by evaluating the different parameters of the cavity. Although resin-based composites have replaced amalgam in majority of restorations, when it comes to teaching posterior restorations preclinically, amalgam is favored over composite restoration.[16] The cavity preparation for amalgam, which is much more demanding as against composite restorations, will be used for long for the purpose of evaluating dental students. Newly graduated dentists start their practice on the basis of teaching and clinical experience they receive during the dental

Figure 5: Finishing of the wall using pull motion

Figure 6: (a) Third quadrant, lingual wall of prepared amalgam cavity. (b) Fourth quadrant, buccal wall of prepared amalgam cavity

a b

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Arora, et al.: A self designed instrument to evaluate cavosurface angle for class I amalgam cavity preparation

Journal of Conservative Dentistry | Jul-Sep 2012 | Vol 15 | Issue 3256

curriculum. The presented instrument can help in refining the skills of cavity preparation.

The above instrument is made of medical grade stainless steel. The instrument has been named as “PACE” (Perfected for amalgam Cavity Evaluation).

rEfErENcES

1. Cook A. Amalgam: Dead or alive? Dent Update 2006;33:94-8.2. Roland W, Bryant. Marginal fracture of amalgam restorations. A review.

Aust Dent J 1981;26:162-6.3. Mahler DB, Terla LG. Relationship of cavity design to restorative

materials. Dent Clin North Am 1965;23:149-57.4. Roberson TM, Heymann HO, Ritter AV. Introduction to Amalgam

Restorations. In: Roberson TM, editor. Sturdevants’s Art and Science of Operative Dentistry, 5th ed. St. Louis, Missouri: Mosby Inc; 2006. p. 699.

5. Marzouk MA, Simonton AL, Gross RD. Class I cavity preparation for amalgam. In: Cargas HJ, editor. Operative Dentistry Modern Theory and Practice, 1st ed. St. Louis: Ishiyaku EuroAmerica Inc; 1985. p. 123.

6. Elderton RJ. Cavosurface angle, amalgam margin angles and occlusal cavity preparation. Br Dent J 1984;156:319-24.

7. Bracket WW, Browning WD, Moore DS, Gilpatrick RO, Johnson WW. A comparison of occlusal amalgam margin angle and clinical appearance of amalgam restorations. J Tenn Dent Assoc 1999;79:18-20.

8. Stratis S, Bryant RW. The influence of modified cavity design and finishing techniques on the clinical performance of amalgam restorations: A

2-year clinical study. J Oral Rehabil 1998;25:269-78.9. Roberson TM. Fundamentals in tooth preparation. In: Roberson TM,

editor. Sturdevants’s Art and Science of Operative Dentistry, 5th ed. St. Louis, Missouri: Mosby Inc; 2006. p. 294.

10. Blackwell RE. Principles of Cavity Preparation. In: Blackwell RE, editor. G V Black’s Operative Dentistry, Vol. 2, 9th ed. South Milwaukee: Medico-Dental Publishing Company; 1955. p. 112.

11. Kornfeld B. Amalgam failures. In: Wilson GW, editor. Yearbook of dentistry 1939. Chicago: Year Book Publishing Co; 1939. p. 194-9.

12. Kantorowicz, GF. Finishing amalgam restorations. Dent Pract Dent Rec 1957;3:64-9.

13. De Vree JH, Peters MC, AJM. Plasschaert Clinical science the influence of modification of cavity design on distribution of stresses in a restored molar. J Dent Res 1984;63:1217-20.

14. Morris CF, Heuer GA. Measurement of cavosurface angle in conventional cavity preparation for amalgam. J Dent Res 1979;58 A-Abstract no. 427:200.

15. Jokstad A, Mjör IA. Cavity design and marginal degradation of the occlusal part of Class-II amalgam restorations. Acta Odontol Scand 1990;48:389-97.

16. Ottenga ME, Mjör IA. Amalgam and Composite Posterior Restorations: Curriculum Versus Practice in Operative Dentistry at a US Dental School. Oper Dent 2007;30:524-28.

How to cite this article:AroraA,AcharyaSR,BallalV,SharmaP.Aself-designedinstrumenttoevaluatecavosurfaceangleforclass I amalgamcavitypreparation:A learningaid. JConservDent2012;15:253-6.Source of Support:Nil,Conflict of Interest:Nonedeclared.

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