THREE-UNIT ALL CERAMIC PREPARATION GUIDELINES

The introduction of stronger and more durable ceramics over the past few years has greatly increased the clinician’s choice of materials when fabricating an all-ceramic three-unit bridge. The latest generation of zirconia is suitable for three-unit bridges and can restore teeth anywhere in the mouth, and IPS e.max is suitable for anterior teeth.

Suitable Materials for Three-Unit All Ceramic Bridges

Options include high translucent (HT) zirconia for anterior three-unit bridges and layered zirconia. Zirconia Solid is an extremely strong material suitable for restoring posterior teeth, and it can be an excellent choice when the patient has bruxism or tooth grinding issues. IPS e.max is a lithium disilicate ceramic that provides exceptional esthetics. It is suitable for three-unit anterior bridges and for posterior bridges where the second premolar is used for an abutment tooth. IPS e.max lacks the structural strength required for bridges extending past the second premolar.

If you are unsure which material will provide the best treatment outcome for the patient, contact the dental lab to discuss the case with experienced technical staff. Once you have selected the appropriate material for the patient’s restoration, it is essential to prepare the abutment teeth correctly. Tooth conservation is always an important consideration; preserving the maximum amount of healthy tooth structure is one of the main aims of modern dentistry.

With all bridge preparations for all-ceramic restorations, incisal and occlusal edges must be rounded. There should be no undercuts, the walls of the preparation must be parallel. The abutment teeth must not diverge or converge, preventing a clear path of insertion. The minimum thickness for the walls of a bridge preparation is 0.5 mm. During tooth preparation, the minimum connector cross-section for an anterior restoration should be 7 mm2, while a posterior restoration should have a connector cross-section of 9 mm2.

Three Unit All Ceramic Preparation for Layered Zirconia and HT Zirconia Bridges

Layered and high translucency zirconia bridges can provide the patient with an extremely high-quality restoration. The high strength of zirconia means that it handles similarly to PFM restorations. Supragingival margins are acceptable, because of the tooth-colored framework. Zirconia margins make it simple to maintain excellent gingival esthetics, even after these restorations have been in the mouth for a long time.

Ideally, when preparing teeth for a layered or HT Zirconia bridge, ensure that the preparations have a continuous and easily visible chamfer around the entire circumference of the tooth. Avoid bevelling. The vertical and horizontal preparation must have an angle of at least 5 degrees.

When shaping anterior teeth ensure that:

• The anatomical shape is evenly reduced by between 1 and 1.5 mm.
• The incisal third of the crown should be reduced by 1.5 mm, and the incisal edge should be reduced by between 1.5 and 2 mm.

When shaping posterior teeth then ensure that:

• The anatomical shape is evenly reduced by between 1 and 1.5 mm.
• The occlusal surface should be reduced by between 1.5 and 2 mm.

Three Unit All Ceramic Preparation for Zirconia Solid

Frequently, the patient requiring Zirconia Solid restorations will have a well-documented history of nighttime grinding and may have previously broken the cusps on molar or premolar teeth. Monolithic, or solid, zirconia is an extremely strong material that provides an excellent fit. The material exhibits opacity, which causes it to be unsuitable for selecting a proper shade for anterior restorations. Tooth preparation for solid zirconia restorations will be similar to high-translucent zirconia: it is recommended to be more liberal during preparation, even in cases that the manufacturer advises to prep like a full cast.

Ideally, there should be 1 to 1.5 mm occlusal clearance, with a 3 to 4 mm axial wall height. A 0.5 mm chamfer is preferable and preparations should have a 4 to 8 degree taper. Once the teeth are prepared, check that the margins are clearly visible and that the design allows for easy insertion. When the margins are supragingival, they are easier to identify, and an impression is more efficient to capture.

Three Unit All Ceramic Preparation for IPS e.max

IPS e.max is a good material to select when minimal restoration thickness is important. It provides good biocompatibility, high mechanical strength, and optimum translucency.

When preparing teeth for IPS e.max, the following guidelines apply:

• Tooth preparations should have no sharp angles or edges
• The shoulder preparation must have rounded inner edges
  
  

When shaping anterior teeth ensure that:

• The anatomical shape is evenly reduced while adhering to the correct minimum thickness required.
• A shoulder is prepared around the entire circumference of the tooth with a chamfered edge or rounded inner edges and with an angle of approximately 10 to 30 degrees. The overall width of the shoulder or the chamfered edge must be at least 1 mm.
• The tooth is reduced incisally by at least 1.5 to 2.0 mm. The oral or vestibular area should be reduced by at least 1.5 mm.
• For successful bonding to, the preparation height should be at least 4 mm and have retentive surfaces.

The preparation guidelines for posterior teeth are similar. You must ensure that:

• The anatomical shape is evenly reduced, and the correct minimum thickness is achieved.
• The prepared tooth has a circular shoulder with rounded inner edges. Alternatively, it may have a chamfer with a 10 to 30 degree angle. The width of the shoulder or chamfer must be at least 1 mm.
• The occlusal surface of the tooth is reduced by at least 1.5 mm.
• The buccal, palatal, or lingual surfaces are reduced by at least 1.5 mm for lithium disilicate e.max bridges. If e.max zirconium oxide is chosen, then at least 1.5 mm must be removed from these surfaces.
• The preparation has adequate retentive surfaces and is at least 4 mm in height for successful self-adhesive or conventional cementation.

Ensuring that the Pontic Width is Acceptable

For three-unit bridges, the maximum width of the pontic differs for anterior and posterior bridges. For anterior bridges up to the canine, the width of the pontic shouldn’t be more than 11 mm. For posterior bridges up to the second premolar, the pontic width should be no more than 9 mm. Once an impression is taken of the abutment teeth, check that all the details are captured before sending it to the dental lab.

Please be reminded that should you wish to discuss a case in more detail, our experienced technical team is here to assist you.

Click here to schedule a consultation with our technical team › 

References:

https://multimedia.3m.com/mws/media/598803O/clinical-handling-guidelines-for-dentists-ebu.pdf

http://glidewelldental.com/wp-content/uploads/2016/02/all-ceramic-emax-prep-guide.pdf

https://www.aegisdentalnetwork.com/cced/2013/03/combining-monolithic-zirconia-crowns-digital-impressioning-and-regenerative-cement-for-a-predictable-restorative-alternative-to-pfm