Why Your Porcelain is Breaking by Dr. Leonard Hess

The top reasons for failed anterior and posterior restorations—and how to address them

by Leonard Hess

Many dentists at one time or another experience failed restorations with their patients. It may be a first instinct to assume the failure is caused by the laboratory’s mistake—which does happen sometimes, but often there can be other reasons for the failure.

The main reason that restorations fail and porcelain breaks is a lack of understanding functional and occlusal principles. The right preparation and analysis can prevent the major causes of broken porcelain, leading to fewer remakes and a better result for the patient.

It’s important to understand that failure happens. But you can prevent it by having a solid understanding of how the masticatory system functions in harmony … and what happens when it’s not in harmony.

Why porcelain breaks on anterior teeth
One of the most common areas to see porcelain breaking on anterior teeth is the incisal edge (Figs. 1–3, p. 76). Breakage and failure here usually have three main causes:

• Interferences to the envelope of function. Functional jaw movement is an important factor, and must be considered whenever anterior teeth are moved, restored or equilibrated. Failures can occur when the incisal edges are too far to the lingual for the patient’s envelope of function. A patient can have sufficient anterior guidance, but that does not mean it is in harmony with the envelope of function.

• The incisal edge of lateral incisors involved in excursive movements. When a person goes into a lateral excursive movement, the incisal edge of lateral incisors should not be handling the excursive load. In movement to a right canine guidance or right lateral excursion, there should be a pass-over of contact from the canine onto the central incisor. When the lateral incisor is involved in a crossover transfer, this is when breakages and failures can occur.

• Excessive loading of the anterior teeth. When a patient has posterior interferences in a protrusive or excursive guidance, the back teeth are still touching. This unfortunately allows for excessive loading of the anterior teeth. The interferences on the posterior teeth allow the full force of the muscles of mastication to be available. The front teeth become overloaded because of the back teeth touching in an excursive movement.

These causes can result in failures such as stress fractures in the porcelain, large porcelain failure, and incisal edge breakage and chipping.

The second area where dentists see anterior porcelain breaking is on the lingual contours, whether it’s a fracture or breakage on the lingual surfaces. There are two main clinical causes of porcelain failures on the lingual contours:

• Centric relation/maximum intercuspation (CR/MI) interferences. These interferences cause posterior/anterior hit-and-slides. This results in excessive loading of the linguals of the maxillary anterior teeth or the incisal edges of the mandibular anterior teeth.

• Envelope-of-function issues. When a patient has a constricted envelope of function, the maxillary anterior teeth have the incisal edges placed too far to the lingual. This can result in overcontouring or excessive bulk of the lingual surfaces of the maxillary anterior teeth.

There are two additional causes of anterior porcelain breaking that relate more to technical issues:

• Improperly placed centric stop: This is related to preparation design flaws and underreduction of tooth structure. Improper centric stops can cause excessive force on the teeth, causing breakages.

• Anterior restorations being fabricated on a simple, single hinge articulator. When cases aren’t mounted on a semiadjustable articulator using a face bow and in centric relation, problems occur frequently. This incorrect arc of closure will often dictate that the centric stop is placed in the wrong position. This creates a discrepancy between where the centric stop would be in the mouth versus on the articulator. This often relates to excessive adjusting of restorations and commonly creates the envelope-of-function interferences that can lead to the porcelain fracture.

In addition, dentists must also be aware of the effects of sleep-related disorders. Issues such as sleep apnea can have a direct impact on the stability and long-term success of anterior restorations. Dentists should also be aware of the implications of sleep-related disorders and be capable of screening for sleep apnea and similar issues.

Why porcelain breaks on posterior teeth
A very common cause of posterior porcelain breaking is CR/MI discrepancies, which create incline interferences on posterior teeth. These interferences cause the teeth to become loaded in an oblique or transverse fashion, rather than the long axis of the teeth and restoration. This sheering type of force, in turn, often leads to porcelain fracture and failure (Figs. 4–6).

The next-leading cause of posterior porcelain failures is lateral or excursive interferences on posterior teeth. Dentists are often very astute at adjusting crowns into a patient’s habitual or maximum intercuspation. However, when the patient goes into an excursive movement, working and nonworking side posterior interferences again cause a sheering type of force in the porcelain that leads to failures.

The final common cause of posterior porcelain breaks relates more to a technical issue. Poor or bulky morphology is created by two main issues: Either the lab technicians aren’t creating the teeth that are anatomically correct, or the dentist is creating errors by underreduction in the preparation design. This leads to overcontoured crown restorations and poor or bulky morphology. This is a primary reason for CR/MI interferences and excursive interferences.

Ensuring quality control
Considering the many possible causes of anterior and posterior porcelain breakage, it’s helpful to follow The Dawson Academy’s five requirements of occlusal stability. Meeting these requirements is a key to ensuring long-term functional and aesthetic success:

1. Stable and equal-intensity stops on all teeth in centric relation.
2. Anterior guidance in harmony with the envelope of function.
3. All posterior teeth disclude during mandibular protrusive movement.
4. All posterior teeth disclude on the nonworking side during mandibular lateral movement.
5. All posterior teeth disclude on the working side during mandibular lateral movement.

In addition to the five requirements, the pro also utilizes two-dimensional and three-dimensional checklists during treatment planning and diagnostic wax-ups. These help dentists place tooth structure in the proper functional and esthetic positions.

Following the occlusal stability requirements and 2D/3D checklists allows dentists to be more confident in treatment decisions. It also will help dentists institute quality control through:

Determination. Using the checklists and programmed sequence allows dentists to see where the tooth structure is supposed to go in space and meet the five requirements of stability.

Verification. Utilizing the provisional restorations to verify that the aesthetic, functional and phonetic success has been achieved. The proposed changes must be verified as correct by the dentist and approved by the patient.

Communication. Once the dentist is able to verify the success of the changes, the provisional restorations are used to communicate the position of the functional contours of the anterior teeth to the lab. This will allow stents to be fabricated to duplicate the position in the permanent restorations (Figs. 7–9, p. 76).

Using the tested provisionals and communicating data to the lab are important aspects to ensuring success in restorations. In this provisional phase, dentists have already proved that the changes are successful by determining tooth structure placement, then verifying the changes, and communicating the tested provisionals to the lab for permanent restorations. (Figs. 10–11).

Technical reasons for porcelain breaking
Proper communication with the lab is an essential part of successful restorations, but there are also several technical issues that can cause porcelain to break or restorations to fail.

Choosing the right material based on the patient’s functional risk and aesthetic demands is essential to the success of a restoration. Different materials have different strengths and aesthetic qualities. Dentists should evaluate and recognize patients who are at higher risk for breaks, which leads to choosing a material that has better resistance to fractures.

The proper bonding criteria is another important technical factor to understand. This also dictates material selection, depending on if the material needs to be conventionally cemented or resin-bonded. If a patient has poor-quality tooth structure, whether it’s enamel or dentin, a dentist shouldn’t risk the long-term bond strength, which could lead to a higher risk of fracture.

Consideration of materials and bonding also directly ties into the preparation design of restorations. Because every material has a recommended thickness for proper strength and durability, dentists must be sure that adequate reduction is being performed. It’s also up to the dentist to design preparations to allow the lab technician enough room to create the proper thickness, morphology and anatomic contour. One of the principles taught at The Dawson Academy is proper preparation design and how to create reduction stents to help guide the preparation in the clinical phase.

In addition, if the dentist is prescribing a restoration that is nonmonolithic—meaning it has a coping layer and a porcelain layer—the lab needs to design the coping or substrate layer correctly, as to not leave excessive amounts of unsupported porcelain (Fig. 12).

There are multitudes of other miscellaneous technical issues that could cause porcelain failures, including restorations that are heated excessively in the finishing/polishing phase (creating stress fractures), and improper handling in the lab when materials are being cycled and baked at the wrong temperatures. However, these technical issues and failures will most likely be seen in the first one to three months of service in the patient’s mouth, making them easier to identify and fix.

Figs. 1–3: Examples of chipped or broken incisal edges,and a failed lateral incisorfrom excessive lateral forces

Figs. 4–6: Examples of posterior porcelain fractures.

Figs. 7–9: Examples of stents, preparation and lab communication stents.

Figs. 10–11: Examples of provisionals being used to verify and communicate.

Fig. 12: Incorrect coping design leaving excessive unsupported porcelain.

Conclusion
While having zero failures is an unattainable goal, we can drastically decrease chances of mistakes by knowing the potential causes of failed restorations and following the provided checklist and quality control processes. The Academy’s philosophy can help fix failures while creating functional and esthetically pleasing restorations for patients.

In addition, when dentists restore teeth, it’s vital to recognize signs of instability in the patient’s occlusion. Dentists must understand risk factors for parafunctional activity, and to identify whether a patient is at high functional risk. It’s also key to understand the times when it might be prudent for a patient to have nighttime protection, such as a nocturnal bruxism guard, to protect restorations.

Knowing the potential causes of failures, signs and symptoms, and properly communicating with the lab are all key pieces to creating a successful, long-lasting, functional and aesthetically pleasing restoration for your patient.