Dental cements 101

There are many ways to affix a restoration to a tooth, with adhesion approaches mainly divided into adhesive bonding techniques or conventional cementation. While the end goal is the same, cementation and adhesive bonding differ in several ways.

Adhesive bonding works on a chemical level to produce a chemical bond and micromechanical retention between the restoration and the tooth. This is accomplished through substituting inorganic tooth materials with resin monomers; essentially, minerals in the enamel and/or dentin are replaced by resin monomers. Through polymerization, these become micromechanically interlocked into the resulting porosities.¹ These adhesive bonding procedures can be completed through 2 different approaches: etch-and-rinse and self-etch.

However, cementation uses good prep design and resistance to connect underlying tooth structure with a restoration. Essentially, the cement creates a hard cement layer to adhere to the 2 surfaces. With cementation, no matter how good the cement, it’s critical that there is adequate preparation, including good retention form and resistance.

If conventional cementation is the chosen method, clinicians still have to decide which cement is the right one for the job—and with so many options available on the market, it can be a tricky choice. The best way to narrow down the pool is to consider the numerous factors that affect cement selection. These include required bond strength, prep design, chosen restorative material, ability to isolate, and the importance of esthetics.

Dental cements can be broken down into five primary categories: zinc phosphate, zinc polycarboxylate, glass ionomer, resin-modified glass ionomers (RMGIs), and resin cements.

Zinc phosphate

An oldie but a goodie, zinc phosphate was the first type of permanent dental cement to hit the market. This 2-bottle system, composed of a powder mixture of magnesium oxide combined with zinc oxide and a liquid phosphoric acid, became available in the early 1900s. While the use of this godfather of modern cements dramatically increased with the introduction of more advanced options, it’s standard to compare new cements to zinc phosphate.

Ideal for cementation of inlays, orthodontic appliances, crowns, and post systems, zinc phosphate cement is known for its high compressive strength and, when applied properly, thin film thickness. While it’s also known for moderate tensile strength, zinc phosphate has drawbacks: a low initial pH can increase pulpal irritation, and the cement cannot chemically bond to the tooth itself.

Zinc polycarboxylate

First introduced in 1968, zinc polycarboxylate was the first cement to form a chemical bond to tooth structure. By swapping out the phosphoric acid with polyacrylic acid, the pulpal irritation associated with zinc phosphate cements was vastly reduced, thanks to the larger size of polyacrylic acid molecules.²

Polycarboxylate cements are indicated for many of the same uses as zine phosphate and bond to most alloys (though not to gold). However, a short working time and high film thickness have seen these cements fall out of favor with the increased use of ceramic restorations.³

Glass ionomer

Glass ionomer cements followed closely behind their polycarboxylate peers, emerging on the market in 1977. First introduced by Wilson and Kent in 1972, they designed these cements to be a hybrid that combined silicate cements’ fluoride release and translucency with polycarboxylates’ chemical bond to tooth structure and good seal. As a result, glass ionomers have excellent translucency and very low film thickness. They are used primarily for metallic and porcelain-fused-to-metal (PFM) restorations, as they can chemically bond to stainless steel, tin-plated noble metals, and base metals.²

Glass ionomer cements are some of the most resistant to salivary contamination, as they are extremely moisture tolerant, have low solubility, and are water-based. In fact, their bond strength is considerably reduced when the tooth’s surface is dry. This, along with a low initial pH level, can increase the rate of postop sensitivity. Although glass ionomers can have a remineralization effect (due to their fluoride release), their use has seen a decline, as their moderate retention rates are similar to that of zinc phosphate.²

RGMIs

RGMI cements built upon their glass ionomer predecessors but substituted part of glass ionomer cements’ polyacrylic acid with hydrophilic methacrylate monomers.⁴ Thanks to their insolubility, RGMI cements are ideal for cases where isolation is difficult, including areas where salivary flow, crevicular fluid, or tongue control can be problematic. Additionally, when applied to moist dentin, they incur little postop sensitivity and have low microleakage.

Like glass ionomers, RGMIs are indicated for metallic and PFM restorations, as well as zirconia and alumina-based ceramics and lithium-disilicate CAD/CAM inlays and onlays. However, they are contraindicated for all-ceramic restorations, as there is a risk of clinical fracture.

Resin cements

The most popular option on the market today, resin cements were available in the early 1990s. Their popularity stems from an array of advantages, including their mechanical properties, high translucency, shade selection, insolubility, high retention, and low film thickness.

Since they are methacrylate-based, resin cements require that the tooth surface be pretreated with 37% phosphoric acid. This acid-etch technique to enamel and dentin produces high adhesion levels as a result of the polymerization process. Resin cements need to be paired with a bonding agent and it is critical that the cement and bonding agent are compatible. Bonding agents come in total-etch or self-etch varieties.

With total-etch bonding systems, phosphoric acid is applied to the enamel, while the inside surface of the restoration is treated with hydrofluoric acid. While this technique provides high levels of adhesion, it can also induce postop sensitivity. Self-etch systems are simpler since they don’t require any pretreatment of the tooth, making them appealing to many clinicians. However, self-etching cements do not provide bond strengths as high as total-etch systems.

The adhesion process with resin cements is facilitated through polymerization through light, chemicals, or a dual-cure process. Resin cements come in light-cure, dual-cure, and self-cure varieties. Light-cured cements are ideal when a restoration is located in an easily accessible location that provides adequate isolation. Ideal for ceramic restorations with thin thickness, most manufacturers offer multiple shades of these cements, enabling them for use with esthetic restorations.

Dual-cure cements are technique-sensitive but are a good choice for restorations that are not easily accessible (making them difficult to light cure) or are too thick for effective light penetration. Alternatively, self-cured cements do not cure through light but rather achieve polymerization through a chemical reaction. While these cements typically have lower bond strength than light- or dual-cure options, self-cure cements are easier to use. Since self-cure cements are not available in a wide range of shades or translucency, they are best indicated for use with metal or opaque ceramics restorations.

Postop sensitivity is a concern with resin cements, with one study finding that 37% of patients reported sensitivity in the first year after a resin-cemented crown.⁵ The sensitivity is largely attributed to a failure to seal the dentinal tubules exposed by the acid etching process. Additionally, although resin cements provide good bond strength and retention, multiple steps and challenging cleanup can make the process cumbersome.

Conclusion

Regardless of cement selection, it is critical to remember that without proper preparation, a cement is likely to fail. Even with advances in cements and increased retention and bond strength, inadequate prep can still spell disaster for restorations. While resin cements have exploded in popularity due to the current prevalence of all-ceramic restorations, clinicians should always be deliberate in selecting cements and the indications of each particular case.

References

1. Yoshida Y, Inoue S. Chemical analyses in dental adhesive technology. Jpn Dent Sci Rev. 2012;48(2):141–152. doi:10.1016/j.jdsr.2012.03.001

2. Burgess JO, Ghuman T. A practical guide to the use of luting cements. Accessed October 19, 2020. http://www.ineedce.com/courses/1526/PDF/APracticalGuide.pdf​

3. Shillingburg H. Fundamentals of Fixed Prosthodontics. 3rd ed. Carol Stream, IL: Quintessence Publishing Co; 1997.

4. Rosensteil SF, Land MF, Crispin BJ. Dental luting agents: a review of the current literature. J Prosthet Dent. 1998;80(3):280-301. doi:10.1016/s0022-3913(98)70128-3

5. Christensen GJ. Resin cements and postoperative sensitivity. J Am Dent Assoc. 2000;131(8):1197–1199. doi:10.14219/jada.archive.2000.0357