How to avoid fracture/chip in all ceramic crowns

Most fractured porcelain is a sign of an occlusion interference.  When not properly corrected may lead to tooth mobility, tooth wear, TMJ problems, tooth migration, sore facial muscles.

Some of the principles that may help prevent fractured porcelain are:

 1. Benefits of Properly Designed Centric Stops are:

• Distribute forces equally throughout the mouth through equal intensity centric contacts. 
• Create vertical and horizontal stability during occlusion, e.g., for posterior teeth, a cusp-fossa occlusion.

An all-ceramic crown that is high or with a poorly designed centric stop will lead to porcelain overloaded, and eventually fracture.

 2. Establish occlusal guidance

• Correct Anterior Guidance in Lateral Excursion 
  The goal is to have disclusion of the posterior teeth on the working and balancing side.  The only pair that is in contact when the jaw moves laterally is upper and lower canines, e.g., when sliding the jaw to your right, only your left upper and lower canines contact and vice versa.
  
  Disclusion is a separation of the occlusal aspects between the upper and lower posterior teeth.
  
  Therefore, creating a correct contour on the lingual of the upper canines is one of the most important things in preventing the fracture in all porcelain crown, especially in severely worn dentition.


• Correct Protrusive Anterior Guidance
  The objective is to establish incisal guidance between the upper and lower anterior teeth while disclusion of posterior teeth during protrusive movement of the jaw that is a key to the stability of the anterior crowns.


• Correct Crossover Disclusion
  The objective is to achieve a smooth transition to the incisal edge of the upper central incisors.  To prevent excessive load on the maxillary lateral incisors when the jaw moves laterally beyond the canine.


• Dental envelope of function is in harmony with neuro-muscular envelope of function
  The objective is to create a path of lower jaw forward action that follow the contours of the lingual aspect of the maxillary anterior teeth.  This pathway is in balance with the mandibular movement created by the muscular action. This harmony prevents breaking of porcelain in crown lengthening and full mouth rehabilitation patients.

3. Identify and resolve occlusion-related problems before all-ceramic crowns are fabricated:

• Periodontal disease
• TMJ problems
• Facial muscle pain
• Problem with muscle's coordination

4. Proper design of tooth preparation:
A tooth preparation for ceramic crown should not have any sharp edge and must have the appropriate thickness for a particular porcelain according to the manufacturer guideline. When there is a sharp line angle on a tooth preparation, it can not accurately transfer to the die. The result is a porcelain crown with a rounded internal surface abutting the sharp line angle on the tooth. This phenomenon creates tremendous stress on the porcelain substructure and leads to fracture of porcelain.

5. Avoid having a large portion of unsupported porcelain.  Unsupported porcelain will break off.

6. Select the recommended adhesive cement for a particular type of porcelain and use proper technique during cementation.  Following the manufacturer's instruction helps avoid the problems.

7. Recommend a mouth guard to the patient who participates in any activities that pose a risk to the teeth.