Clinical Advice: How To Cement PFM Crown

Crown cementation is a critical step in clinical dentistry. When incorrect material or technique is used, it can lead to premature failure of the crown which is costly for dentist and patient alike. There are currently numerous cement brands and types on the market, each of which may be best utilized in particular situations. 

The dental clinician must understand the nuances, select an appropriate cement for the current patient and follow cementation best practices for that cement to achieve the best possible clinical outcome.

Choosing the Right Cement

When definitively cementing a PFM (porcelain fused to metal) crown, there are several dental cements available. Composite resin-based cements are contemporary dental cements which include self-adhesive composite resin cements and composite resin cements, the latter of which requires an additional bonding step. These quickly growing cement classes utilize a chemistry based on current composites and bonding agents, making them compatible with other dental materials.

Water-based cements, which utilize an acid-base reaction during set-up, are also a good option for PFM crown cementation. These include the older “legacy” cements including zinc phosphate, zinc polycarboxylate and conventional glass ionomer cements, as well as the more contemporary resin-modified or resin-reinforced glass ionomer cements. 

Composite Resin Cements

Composite resin cements include self-adhesive resin cements and multistep resin cements. These cements are based on resin composites and are resistant to wear and abrasion, have:

• high flexural strength,
• high compressive strength,
• high stiffness,
• fracture toughness,
• and a low coefficient of thermal expansion and contraction.

Self-Adhesive Resin Cements

Self-adhesive resin cement is a subcategory of composite resin cements, featuring the greatest ease of use for all-metal and PFM crown cementation. They do not require separate bonding steps and are easy to clean up in the gel-phase (dual-cure). 

They are typically available in auto-mix or triturated capsule configurations. Self-adhesive resin cements can be used with a wide variety of materials, including zirconia and alumina core ceramics, PFM and cast metal restorations, cast-metal post and cores and pre-fabricated metal or fiber posts.

Glass Ionomer Cements

Conventional glass ionomer cements are water-based, and work through an ionic bond between the glass ionomer and the calcium and phosphate ions in enamel and dentin. Glass ionomer cements also contain fluoride, which creates the bonus characteristic of caries prevention. Cementation with glass ionomers requires that the tooth structure not be overdried – the dentin should be lightly rewet, and no pooling water or saliva should be present. 

Conventional glass ionomer can be hand-mixed as a powder and liquid combination, or triturated in single use capsules. Excess cement should not be cleaned up until it is fully set. Glass ionomers are great for use with PFM or all metal restorations on teeth and metal implants, cast-metal post and cores, and prefabricated metal posts.

Resin-reinforced Glass Ionomer (RRGI)

Resin-reinforced glass ionomer, also known as resin-modified glass ionomer, have similar properties to the conventional glass ionomer with a few notable improvements. This class of cement has approximately 10% resin added to the conventional glass ionomer formula, which leads to improved physical properties, reduced early moisture contamination, and allows for clearing of cement once it reaches the gel stage (or after it is completely set up). RRGIs allow for easier mixing than conventional glass ionomers, while still maintaining the properties of fluoride release and self-adhesion of the conventional glass ionomer cements. RRGIs are useful in the same situations as conventional glass ionomer cements but have been found to provide better PFM cementation than zinc phosphate or conventional glass ionomer cement.

PFM Crown Best Practices

When cementing a PFM crown, the provider must be detail oriented and adhere to best practices. 

Step One

To start crown preparation, the tooth surface should be clean. After removing the temporary crown and temporary cement, the prepared tooth should be polished with prophy paste or pumice. 

Step Two

The tooth should then be well isolated with a rubber dam or cotton rolls to prevent fluid contamination. If the PFM restoration is made with non-precious metal, air abrasion is recommended to improve retention. When using a glass ionomer cement specifically, the prepared tooth should be rewet slightly to avoid an over-dry tooth. 

In the case of multi-step composite resin cements, the appropriate bonding agent should be placed on the tooth at this time. When using hand-mixed powder/liquid cements it is critical to measure the correct amounts of powder and liquid accurately. 

Step Three

As all cements are different, it is important to always read instructions for the specific cement you are using to ensure correct usage and proportions. Self-mixing dispensers and auto-mix capsules are often more ideally mixed than hand-mixed cements, but even in these cases a small amount of cement should initially be dispensed onto a mixing pad prior to direct placement in the crown to ensure catalyst and base are equally dispensed. 

Step Four

Cement should be evenly spread within the crown and around all crown margins using a small brush. Once a crown with cement has been placed on the prepared tooth, it is advisable to have the patient bite down on a stick to maintain the crown in the correct position until cement has set-up. 

Step Five

In the case of dual cure cements, an initial “tack cure” with the curing light will set cement into the gel phase and allow for initial clean-up. Conventional glass ionomer cement should not be cleared until it is completely cured.

Superior PFM Restorations With Stomadent Dental Lab

While there are numerous options for cementing PFM crowns, the key is to start with a quality restoration. At Stomadent Dental Lab we are here to help. We fabricate beautiful PFM restorations with the durability and quality to last many years. Discover premium all ceramic crowns, metal crowns, zirconia restorations, and more. 

Check out our website for more information.