July 2013, Volume 9, Issue 7
Published by AEGIS Communications
Why Porcelain Chips and Breaks
Address these 10 factors to decrease post-cementation failure
The margins were great, the color exact, only minor adjustments were needed at delivery, and the patient was thrilled with the result; everything seemed perfect. Then comes the phone call that results in not only frustration but also wasted time for the practice, the lab, and the patient. Those restorations that seemed perfect have chipped or broken. What went wrong? The typical suspects are the lab and the materials. But the lab says they just followed the script and the material used has been reported to be strong. What other factors could have caused the failure?
The fracturing of porcelain is often related to one of 10 factors that, when followed, result in predictable stability and beautiful esthetics. These principles produce restorations that are contoured properly to function in harmony with the joints and muscles, dramatically decreasing the chance of post-cementation failure.1
Properly Designed Centric Stops
Equal-intensity centric holding contacts distribute forces equally throughout the mouth. The centric stops also create vertical and horizontal stability of the teeth. For posterior teeth, the goal is a cusp-fossa occlusal relationship. The stamp cusps (buccal of lower, lingual of upper) should land on a marginal ridge or in a central fossa.2
With anterior teeth, the goal is to have a centric stop or an acceptable substitute, such as the tongue, that prevents eruption. Although some occlusal philosophies advocate very light central stops to prevent overloading the anterior teeth during jaw movement, the author’s philosophy calls for equal-intensity centric stops all around the arch. This allows the bite forces to be evenly distributed in the mouth and prevents eruption or drifting of the anterior teeth (Figure 1).
A porcelain restoration with a poorly designed stop will lead to an overload on the porcelain. Often the first sign of occlusal instability of an all-ceramic restoration is fracture.1
Lateral Anterior Guidance
The goal is immediate disclusion of the posterior teeth on the working and balancing sides when the mandible moves laterally (Figure 2). Williamson and Lunquist illustrated the neuromuscular advantage obtained when posterior teeth are prohibited from contact in excursive movement in their classic study, which was published in 1983.3
If the posterior morphology is flatter than the anterior guidance when the condyles are seated in centric relation (CR), the posterior teeth will disclude. With the anterior teeth, the lingual contour of the maxillary canines must be steeper than the posterior morphology, but not too steep. Work out the contours in the provisional restorations to produce a customized anterior guidance.
When posterior teeth are allowed to contact in excursive movements, increased muscle activity, combined with the increased mechanical stress, contributes to increased risk of fracture. Additionally, if the cuspids are too steep for the patient’s functional pattern, fracture, mobility, or migration is likely.1
Protrusive Anterior Guidance
The goal of any protrusive movement of the mandible is immediate posterior disclusion (Figure 3). Achieving this goal requires that the lingual contour be steeper than the posterior morphology without restricting movement of the mandible, which could lead to fractures.3
As the mandible moves forward, the goal is to have equal contact of the leading edge of the lower incisors from the centric stop to the maxillary incisal edge. Balancing the protrusive load over the lingual contours of the central and lateral incisors is key to the comfort and stability of anterior esthetic restorations.
It is not by chance that the most common anterior esthetic restorations to fracture are the maxillary lateral incisors. Proper occlusal design dictates a smooth transition to the incisal edge of the maxillary centrals as the patient moves beyond the cuspid in lateral excursive movements.4 Overlooking this positioning can place excessive loads on the distal of the lateral incisors, leading to fracture (Figure 4).
Lingual Contours and the Envelope of Function
Protecting the posterior teeth from contact in excursive movements is one of the most important functions of the anterior teeth. Working with the condylar guidance, the lingual contour must be steep enough for immediate separation of the posterior morphology. Equally as important is a lingual contour that is concave enough to be in harmony with the functional envelope of the mandible.1
Some patients’ functional patterns are steep, whereas others are more horizontal. Signs of instability such as wear, fremitus, or migration of the anterior teeth are all indications that a constriction could be occurring. Working in CR, combined with a customizing of the lingual contour for the patient’s functional pattern, is critical for long-term success.1
A four-step process can be used to adjust maxillary provisional restorations. First, create equal-intensity holding contacts in CR. Second, with the patient in a seated position, use a hands-off approach and a different colored paper to mark the articulation as the patient opens and closes. The mandible will open and close on an outside-in stroke, following the functional pattern of the patient.
Third, adjust the long centric marks, being careful not to touch the centric stops. This adjustment allows the patient to chew in CR or posture forward without running into the anterior teeth.1 When customizing the anterior guidance, many dentists fail to recognize how working in CR can provide the necessary horizontal freedom of the mandible. Fourth, once the customized lingual concavity is established, adjust for a smooth protrusive stroke from CR contact to the incisal edge. Similar adjustments will be made on the canines in lateral movements. Verify that the posterior teeth are separating with this customized lingual contour.
Bruxism, nail biting, sleep disturbances, chewing on pencils/pens, or any aberrant movement of the mandible that brings the teeth together in an abnormal pattern and creates signs of instability in any part of the system must be indentified during treatment planning (Figure 5).1 If the parafunction occurs during the day, however, every effort should be made to help the patient break the habit before restoring the teeth, and the patient should be warned of the risk of failure due to the habit.
Properly Designed Preparations
Over-reduction of the incisal edge is one of the most common causes of porcelain fracture. Proper treatment planning with mounted diagnostic casts, photographs, radiographs, and a complete examination process allows the restorative team to create a 3-dimensional diagnostic wax up for creation of preparation reduction guides.5-7
Properly Finished Preparations
Sharp line angles and rough preparations are some of the major contributing factors to fractured porcelain. Leaving a sharp line angle on a prep usually results in a porcelain restoration with a rounded internal surface, creating tremendous stress on the substructure.8 When combined with poor occlusal management, this often leads to fracture. Using reduction guides created from the diagnostic wax-up ensures reduction goals are met.
Meticulous Adhesive Technique
Poor attention to detail during the delivery phase of all ceramic restorations is a major cause of postoperative discomfort and fracture.9-13 Follow the manufacturer’s specs for adhesive materials exactly to avoid failure of the restoration.
For the patient who must perform job-related activities that pose a risk to the dentition, provide a sports guard to minimize trauma-induced failure of the restorations.14
The 10 principles reviewed in this article should be considered when treatment planning anterior and posterior restorations. Fracture is just one sign of system instability. Tooth mobility, migration, wear, sore musculature, and temporomandibular joint issues can also be consequences of not addressing these principles. Having a thorough understanding of the masticatory system is critical to obtaining predictable results.
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14. Tapias MA, Jiménez-García R, Lamas F, Gil AA. Prevalence of traumatic crown fractures to permanent incisors in a childhood population . Dent Traumatol. 2003;19(3):119-122.
About the Author
John C. Cranham, DDS
The Dawson Academy