A self-designed instrument to evaluate cavosurface angle for class I amalgam cavity preparation: A learning aid.

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.

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[45]; 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°,[78] 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 instrument

Handle [Figure 1a]

Figure 1

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

Eight sided with serrations

Shank [Figure 1b]

Triple angled

Blade [Figure 1c]

Length: 6 mm.

Width: 2 mm.

Thickness: 1 mm

Part 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.

Angulations

To achieve an undercut of 5° [Figure 1e].

To achieve a CSA of 110° [Figure 2a].

Figure 2

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

Blade angle 19.4° [Figure 2b].

20 6 19.4

NOMENCLATURE

Order (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.

Figure 3

Insertion of the instrument and evaluation of the cavity

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 action

Once 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 4

On completion of modification using rotary burs

Figure 5

Finishing of the wall using pull motion

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 model

After 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.

Figure 6

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

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.[45] 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.[67] 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 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 A malgam Cavity Evaluation).