Mastering the Class I Cavity Preparation

Posted by Dr. Patrice Smith on Mon, Jul 17, 2017 @ 10:00 AM

Restoring teeth is a basic skill that all dental students must master, not only to matriculate through operative dentistry labs or pass the boards but also in order to become good clinicians. It is the bedrock of our careers. Learning how to properly remove decay and prepare teeth for restorations, all while maintaining tooth anatomy and structure to maximize strength and retention, withstand the potential of fracture, and minimize pulpal exposure is an expertise required by all dentists.

To this end, Greene Vardiman (G.V.) Black was one of many pioneers who paved the way for modern dentistry. He established the proper way to prepare teeth for fillings and, as such, developed a classification system for carious lesions. Originally consisting of five classifications, Black's system ultimately included a sixth classification that was added later. In this article we will discuss the Class I lesion and how to properly prepare it for a restoration.

The Class I Lesion

Based on the Black classification system, a Class I lesion involves the occlusal surface of premolars and molars, the occlusal two thirds of buccal and lingual surfaces of molars, and the palatal pits of anterior teeth. There are two stages in cavity preparation:

Stage 1: Initial Tooth Preparation

  • Obtain outline form
  • Obtain primary resistance form
  • Obtain convenience form

Stage 2: Final Tooth Preparation

  • Removal of remaining carious dentin
  • Pulpal protection (if indicated)
  • Obtain secondary resistance and retention form
  • Finishing of enamel walls and margins
  • Sealing

Premolar Reference

Stage 1: The Initial Tooth Preparation Phase requires the clinician to establish a proper outline form (Figure 1). This is achieved by extending the external walls to sound tooth structure while maintaining a specified, limited depth and by providing retention and resistance forms. The outline form for the Class I occlusal tooth preparation should include only defective occlusal pits and fissures. The primary resistance form is the shape and placement of walls that enable the tooth and restoration to withstand masticatory forces or stresses that are delivered along the long axis of the tooth. The convenience form is the shape that facilitates proper and adequate access and visibility during preparation and restoration of the tooth.


Figure 1. Outline form includes pits and fissures.

Based on current standards,[1] the ideal dimensions of a Class I cavity preparation should be 1.5mm to 2mm deep, and 1.5mm wide faciolingually. This prep should not extend beyond 1.6mm proximally. That is, a width of 1.6mm from the marginal ridge should be maintained for structural support.

The ideal bur(s) to use for this type of prep are a #245 or a #330 carbide. The #245 bur is 3mm in length and 0.8mm diameter, while a #330 bur is a smaller version at 1.5 mm in length.

 Preparation-03.jpg  Preparation-04.jpg  Preparation-05.jpg

Figure 2. Punch cut and extension of the preparation mesiodistally with a #245 bur.

The Class I occlusal preparation is begun by entering the deepest or most carious pit with a punch cut, parallel to the long axis of the tooth using a #245 or #330 carbide bur in a high-speed handpiece (Figure 2). The target depth is 1.5mm to the pulpal floor (i.e., one half the length of a #245 bur or the entire length of a #330 bur). The preparation should be extended from mesial to distal, taking care to leave at least 1.6mm of the marginal ridge on either side (or the diameter of two #245 burs) (Figure 3). The occlusal isthmus should be just a bit larger than the diameter of the bur. Note that a preparation with a narrow occlusal isthmus prevents fracture.


Figure 3. A 1.6mm-marginal ridge on either side of the preparation, or the diameter of two #245 burs. 

Stage 2: The Final Tooth Preparation Phase includes the removal of any remaining restorative material or defective enamel or dentin on the pulpal floor (Figure 4). This is best accomplished using a discoid-type spoon excavator or a slow-revolving, round carbide bur until the remaining tooth structure feels hard or firm. Pulpal protection should also be done if indicated (i.e., if the preparation extends close to the pulp).

Preparation-09.jpg  Preparation-10.jpg 
Preparation-11.jpg Preparation-12.jpg
Figure 4. Removal of remaining defective enamel/dentin with discoid cleiod. 

In complex preparations where retention needs improvement, secondary resistance and retention form is employed. This is usually done by adding retention grooves and cuts in the walls of the preparation. After this is achieved, then finishing of the enamel walls and margins should be completed to ensure all internal lines and angles have been smoothed. This is followed by the final steps of the procedure--cleaning and sealing the preparation with the clinician's restorative material of choice.



[1] Heymann H, Swift E, Ritter A. Sturdevant's Art and Science of Operative Dentistry, 6th Edition. Mosby, 2013. VitalBook file.


Further Reading

A Comparison of Various Adhesive Composite Restorations in the Posterior Regions

New Perspectives on Dentin Adhesion

Dentin Adhesion Bonding Methods

Direct Resin Veneer Restorations (clinical images)

Tags: cavity,, class I cavity, tooth preparation, dental filling

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