From Black to Clark: How Wide Radius Bevels Create Compression in Direct Resin Restoration and Why This Matters
Every day dentistry lies in the preparation of Class I and Class II molar restorations. Interestingly, dental schools worldwide still teach principles based on G. V. Black's concepts from the late 1890s. Dr. Black's influential work has saved billions of teeth through his cavity preparation techniques.
However, modern preparation designs combined with warm overmolding techniques are now challenging these traditional precepts. The Clark preparation principles integrate bio-engineering concepts with the exceptional properties of composite resins. The Bioclear Method and Clark Class I and Class II restorations exemplify this harmony between nature and modern materials science
Black vs. Clark
Dental students are graded on G.V. Black's Class I and Class II principles, characterized by:
- Axial walls that are slightly convergent and taper towards the occlusal table
- Axial walls that slightly undermine the cusps
- A deep, narrow channel carved along the central fossa of the tooth for mechanical retention in a classic Class II preparation
- Butt joints and sharp internal angles defining the dovetail design of the preparation
- Immediate restoration of the tooth in tension, according to how occlusal forces engage the joinery
- G.V. Black's preparations are designed for metal-based restorations that rely solely on mechanical retention. However, since the advent of composite resins in the 1960s, dentistry has not significantly adapted its preparation guidelines to match the strengths and properties of this new material.
Bio-Engineering the Clark Class I and Class II
The main differences between G.V. Black and Clark preparations are:
- Clark's reliance on the concept of the radius bevel, which sits on enamel
- Non-retentive cavity preparations (Diagram A)
- Smooth and rounded internal and cavo-peripheral angles
- An enamel-to-dentin ratio of 70:30, meaning most of the restoration is bonded to enamel
- Absence of proximal boxes and clear-cut margins
- Automatic restoration in compression, greatly increasing resistance to occlusal stress
Diagram A
What Are Wide Radius Bevels and Why Are They Important?
Some dentists may hesitate to generously bevel the cavo-periphery of their preparations. However, biometric studies at the University of Minnesota confirm that despite removing more enamel during radius beveling, there is a net decrease in coronal volume loss for Clark preparations compared to G.V. Black's. A broad bevel radius increases the bonded enamel surface area, offering the best seal in direct and indirect restorations alike.
As illustrated in Diagram B, 90° butt joints disperse tensile forces along the restoration and axial walls, causing the cusps to flex outward during occlusal loading. This tension-based design can lead to fissures and fractures over time. Better bioengineering is achieved with a cavo-peripheral angle of 125° to 130°, obtained by a convex radius bevel, which places the tooth in compression and prevents the cusps from flexing during occlusal loading. This design maximizes the composite resin’s properties by making the material resistant to stress.
Diagram B
Day-to-Day Dentistry: From G.V. Black to Clark Class II
Here is a step-by-step process for converting and restoring teeth using Clark's principles:
STEP 1: Setup (FIG. 1)
Secure a rubber dam using a clamp and forceps to ensure it is leakproof.
STEP 2: Apply a Disclosing Solution
Apply the Bioclear™ Disclosing Solution to highlight biofilm and protein pellicle, which must be removed to strengthen the bond between enamel and restorative material.
STEP 3: Pre-Wedge
Pre-wedge the proximal surfaces with Bioclear™ Diamond Wedges to loosen the periodontal ligament and ensure a predictable proximal contact.
STEP 4: Disassembly* (FIG. 2)
Remove the existing restoration and disclose the tooth's intaglia to expose underlying decay, cracks, and fissures.
STEP 5: Transition from Black to Clark (FIG. 3)
Round all external and internal angles, bevel the gingival aspect of the preparation, and create a convex, wide radius bevel along the occlusal portion to place the tooth in compression. Please refer to the video at the end of this article for a full demonstration of the Clark Class II Prep technique.
STEP 6: Biofilm Removal
Remove the Diamond Wedges and use the Bioclear™ Blaster loaded with Bioclear™ Blasting Powder to clean away biofilm and protein pellicle without risking pulpal exposure.
STEP 7: Matrix Selection and Setup (FIG. 4)
Using the Bioclear™ Evolve Matrix Height Indicator, measure the proximal preparation depth and select the appropriate Bioclear Evolve mylar matrix. Secure it in place using the spot-weld technique. See Step 10.
STEP 8: Place Acid Etch
Etch for 15 seconds, then rinse and air dry. It is important to apply the etch underneath the flap of the matrix and all the way to the infinity edge area of the prep. Also, etch the enamel surfaces where the spot welds will be placed.
STEP 9: Managing Exposed Dentin
Scrub 3M™ Scotchbond™ Universal Adhesive onto the freshly cut and exposed dentin, air-thin, but do not cure yet.
STEP 10: Pre-Curing and Spot-Welding (FIG. 5)
Apply 3M™ Scotchbond™ Universal Plus Adhesive and 3M™ Filtek™ Bulk Fill One flowable resin to create your spot welds. Appose the matrix against the adjacent tooth, and light cure the spot welds and exposed dentin. Please refer to the QR code at the end of this article to learn how to use the spot weld technique for your Clark Class II composite restorations.
STEP 11: Pre-Curing and Spot-Welding (FIG. 5)
Generously wet the Bioclear Evolve matrix, etched enamel surface, and pre-cured dentin with 3M™ Scotchbond™ Universal Plus Adhesive, then air thin. Do not cure.
STEP 12: Injection Molding (FIG. 6)
Inject warm 3M™ Filtek™ Bulk Fill flowable resin followed by 3M™ Filtek™ One Bulk Fill composite paste into the pressurized system to create a monolithic restoration.
STEP 13: 3-Point Light Curing
Cure the occlusal table, buccal, and lingual surfaces based on your curing light
manufacturers recommended times to ensure proper depth of cure.
STEP 14: Cut Back and Polishing (FIG. 7)
Remove the matrix and shape the restoration with 3M™ Sof-Lex™ discs. Polish beginning with Bioclear’s Magic Mix paste, followed by a Bioclear™ Rockstar Polisher cup for a glasslike finish.
FIG. 1
Pre-operative view of tooth 15 with an MOD amalgam to be replaced. The butt-joints of traditional G.V. Black Class II restorations place the tooth in tension. This causes the cusps to flex apart during occlusal load. This leads to cracks and fissures in the proximal cervical third, hence the presence of decay under the box.
FIG. 2
The margins and boxes present decalcified/decayed enamel and dentin. The bonding ability to aprismatic enamel is nil—overmolding the tooth allows to bury these less-than-adequate surfaces without needing to amputate. Conservation of the coronal structure is maximized without compromising the peripheral seal of the restoration.
FIG. 3
All internal and external angles are rounded with a wide radius bevel, including a more modest bevel at the gingival aspect of the preparation. Selective caries removal is performed along the DEJ on the distal aspect of the preparation. Any caries remaining on the pulpal axial wall is left behind. The adjacent teeth are groomed to aim at wide and broad interproximal contacts. The final preparation is treated with Bioclear™ Blasting Powder (aluminium tri-hydroxide) using the Bioclear™ Blaster.
FIG. 4
The Bioclear™ Evolve Matrix Height Indicator determines the depth of the preparation. The rule-of-thumb is to add 1-2 mm to the reading and select the matching Bioclear Evolve matrix. Bioclear™ Diamond Wedges and Bioclear™ Universal Twin Rings are engaged and seated into place Note: the mesial and distal proximal depths may or may not coincide. Adequate matrix selection for each depth is mandatory for a successful restoration to occur.
FIG. 5
The Bioclear™ Adjustable Push-Pull Instrument serves to appose each matrix to its corresponding neighbouring tooth. Once the matrix is in position it is tacked into place using the spot-weld technique. Seen here are 4 spot-welds: 2 on the mesial (buccal/lingual) and 2 on the distal (buccal/lingual). This is critical as this will create a set-up which can sustain and resist the pressure of the injection molding process.
FIG. 6
Pre-curing the dentin is the first step. After this is done the injection molding process begins. Both tooth and matrices are generously coated with 3M™ Scotchbond™ Universal Adhesive Plus (surfactant). This allows for the warm flowable composite to seep into gaps as narrow as 4 microns by capillarity action. Warm paste is then injected, which displaces most of the flowable
resin previously injected. This resin mix of varying viscosities is then cured and will shrink-wrap around-and-over the tooth. 3-point curing (occlusal-buccal-lingual, for each the mesial and the distal aspects) is essential to ensure the composite is fully cured.
FIG. 7
The final restoration shows no margins, and the transition between enamel and composite is imperceptible. The tooth is ensconced in composite and is automatically placed in compression.
Why Injection Molding Should Prevail
Injection molding creates monolithic composite restorations, reducing the risk of density imperfections and improving material adaptation. The process of spot-welding the matrix and injection-molding into a pressurized system decreases the C-factor and places the restoration in compression, enhancing its strength and resistance to occlusal stress. Studies confirm that warmed 3M™ Filtek™ composites maintain their physical and chemical properties and pose no health risks to patients or practitioners. It is this author’s opinion practitioners should actively seek additional training in the Bioclear Method to perfect their clinical skills as they transition into its daily application.
The Bioclear Method Glossary:
*Disassembly: the act of removing the existing restoration
†Overmolding: the process by which warm composite resin is injection-molded into and around the tooth, creating an additive layer of composite that ensconces the tooth.
Bioclear Matrix | The Bioclear Class II Prep using Komet Burs
The 2-Point Spot Weld Class II Animated Video
About the Author
Marie-France Roux, DMD
Dr. Marie-France Roux owns a private practice in Downtown Montreal. A 2002 graduate of University of Montreal she underwent a General Practice Residency at Seton Hall University prior to completing her occlusion training at The Dawson Center in Tampa Bay, FL. Dr. Roux holds Bioclear certifications in Core Posterior/Core Anterior & Advanced Anterior. Combining Invisalign and Bioclear Dr. Roux teaches The Bioclear Method to Canadian dentists.
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This article was originally published in the Clinical Life™ magazine: Fall 2024 edition
Clinical Life™ magazine is a premier periodical publication by Clinical Research Dental Supplies & Services Inc. Discover compelling clinical cases from Canadian and US dental professionals, cutting-edge techniques, product insights, and continuing education events.
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