All ceramic posterior onlays are a fantastic restorative solution. For patients in need of cuspal coverage, these restorations provide a number of benefits, including:

  1. Conservative tooth reduction
  2. Supra-gingival restoration margins
  3. Ease of patient maintenance
  4. Improved esthetics
  5. Adequate strength for longevity

Success with posterior onlays is dependent upon adequate tooth reduction to provide for long-term strength and durability. Teaching in Spear’s “Restorative Design” workshop, Dr. Bob Winter and I commonly hear frustrations from attendees that they desire to integrate posterior ceramic onlays into their practices on a routine basis but are concerned about ceramic fracture.

Many participants have attempted ceramic onlays for their patients in the past but, unfortunately, have experienced onlay fracture within months of insertion. These experiences prevent the dentist from pursuing further posterior ceramic onlay restorations.

It’s an issue many of us have faced while exploring new technologies, materials or procedures. Understanding how and why these failures occur enables us to move forward with newer concepts with greater confidence and success.

There are two main components to predictability with ceramic onlay success. The primary component is adequate thickness of ceramic material to withstand the forces of occlusion and function. This is the primary topic of this article.

Second is ceramic adhesion. Long-term success with ceramic restorations is also dependent upon proper bonding and adhesive protocols. This topic is covered extensively in both the “Restorative Design ” and “Anterior Restorative Dentistry” workshops at the Spear Campus. I will limit this article to the key concept of tooth preparation and ceramic thickness.

While teaching a recent “Restorative Design” workshop, I was having yet another conversation about ceramic onlay predictability with Dr. Matt Nadler of New York. He wanted to integrate ceramic onlays into his restorative practice but was concerned about restoration fracture and longevity.

Additionally, Dr. Nadler and I happened to be discussing food and food preparation. It occurred to me that proper preparation was the key that would lead to great results for both food and ceramic onlays. The concept of the “ideal reduction sauce” for ceramic onlays stems from this conversation with Dr. Nadler (thanks, Matt!).

In the culinary world, the definition for reduction sauce is “the process of intensifying and ‘thickening’ a liquid mixture, such as soup, wine or juice to improve the flavor and enhance the product.” This definition is totally appropriate for application to ceramic onlays.

To improve or enhance the product, “thickening” is the key ingredient! In other words, adequate ceramic thickness provides predictability and success for the final restoration.

The “Restorative Design” workshop provides three full days of understanding and application of preparation design and process for all types of ceramic restorations. Veneers, crowns and partial coverage restorations are covered in extensive detail. In this article, I am going to emphasize the importance of adequate occlusal reduction for posterior partial coverage ceramics. Again, it’s all about the “thickness!”

In most circumstances, partial coverage ceramics are utilized for the replacement of old amalgams, leaking composite restorations or worn and eroded teeth. When there is significant tooth structure remaining, a more conservative partial coverage restoration is the ideal choice for repair and rejuvenation over full-coverage crowns.

The first step in the preparation process is to design the proposed restoration contours of the finished product. We term this “outcome-based design.” The ideal tooth form, cusp and fossa heights and contours are established in the diagnostic wax-up. This preview is studied and utilized as a model to prepare and adequately reduce the tooth structure to provide proper thickness of the ceramic material.

side by side comparison
Tom R. – Erosion (GERD) #12 and #13 – occlusal and buccal cusp.

The photos above show the pre-treatment tooth and the required cuspal coverage due to enamel erosion. The diagnostic wax-up provides the preview for the desired outcome. Note the cuspal height changes that are to be incorporated into the final restoration. This is a key observation! Failures of ceramic restorations occur due to inadequate reduction related to final-outcome contours.

Adequate reduction is the key to long-term ceramic, partial-coverage restorations. Whether the ceramic is Zirconia or Lithium Disilicate (EMax), the minimal thickness required for material strength is 1 mm (bonded to enamel). Many variations as to material, thickness, tooth position and function are contributing factors to altering the overall material thickness, but 1 mm is considered the bare minimum.

Ideally, 1.5 mm provides greater predictability and longevity, especially if the restoration is bonded to dentin. Thicker is not always better, but conservative dentistry is a key factor in application and implementation of partial coverage ceramic restorations.

Over-reduction has its limitations in some circumstances, as thicker ceramic may cause a loss in perceived value. Again, the many nuances of ceramic preparation and design are covered in detail in the “Restorative Design” workshop.

a side view  and a top view of a molded molar section
Tom R. – Diagnostic wax-up, #12 and #13.

I refer you to the following diagrams:

diagram showing buccal and Lingual with a 1.5mm gap area at the top
Diagram No. 1: Lower molar – “Thumbprint” amalgam.

Diagram No. 1 represents a typical tooth in need of occlusal restoration due to wear, erosion, or old, under-contoured restoration. The occlusal aspect of the tooth is “bowl-shaped.” Sometimes, old amalgam restorations are referred to as “thumbprint amalgams” due to their lack of occlusal anatomy.

diagram showing buccal and Lingual with a large v-shaped area at the top over the 1.5mm gap
Diagram No. 2: Diagnostic wax-up of proposed outcome for “thumbprint” amalgam.

Diagram No. 2 is of the diagnostic wax-up, or the “proposed outcome” contour of the final restoration. Significant changes in occlusal anatomy and cups height have been integrated into the restoration proposal. Consideration of these changes must be incorporated into the tooth preparation in order to provide adequate thickness for the partial coverage ceramic onlay.

diagram showing buccal and lingual with a large v-shaped area with a red circle in the middle showing .75mm a difference
Diagram No. 3: Inadequate occlusal reduction – potential for fracture.

The bowl-shaped restoration was prepared to a 1.5-mm depth. Certainly, this is adequate thickness for ceramic bonded to dentin. However, no depth consideration was given to a change in anatomy of the final restoration. Placing a final restoration, with improved anatomy, will create a potential for fracture and failure due to the inadequate thickness in the central groove of the occlusal surface.

diagram showing buccal and lingual with a large v-shaped area showing a 2mm a difference in the middle and 1.5mm on both sides.
Diagram No. 4: Adequate occlusal reduction for ceramic longevity.

Diagram No. 4 shows that a greater reduction (2.0 mm) was utilized in the occlusal groove to compensate for the more ideal anatomy integrated into the final ceramic onlay. This preparation design factor improves the longevity of the partial coverage restoration.

top view of molded teeth, side view of a single tooth with a ridge through the middle
side view of molded mouth
Tom R. – Prep stone dies and final restorations, #12 and #13.

By incorporating proper planning and design into the posterior partial coverage ceramic restorations, long-term predictability may be achieved. This patient had significant erosion due to gastroesophageal reflux disease.

He had lost cuspal height and contour. Using the diagnostic wax-up as a guide for outcome-based tooth preparation, the full-contour EMax ceramic onlays were fabricated with adequate thickness and ideal anatomy. These restorations have now been functional for more than five years.

The following photos show the final ceramic onlays for the patient's #12 and #13 teeth:

top view of molded top with the two overlays in place of the molded teeth
side view of patients mouth with the overlays in place

The “ideal reduction sauce” for these cases is to provide adequate space for anatomic contour and material strength. Proper planning and execution create results that are predictable and long-lasting.

Whether we are talking about ceramic onlays or great food, success stems from proper planning. The ideal “reduction” is all about the preparation!

Jeffrey Bonk, D.D.S., is a member of Spear Resident Faculty.