All About That Base: What to Know About Bases and Liners

Restorative dentistry poses a few threats to optimal outcomes. One is postoperative sensitivity. Placing a base or a liner underneath your restoration might help reduce or eliminate it in some cases. For these cases where you want to mitigate this sensitivity, clinicians should be all about that base.

Bases and liners are terms sometimes used interchangeably but there are distinctions. For example, thin liner layers seal overexposed dentin and protect the pulp, serving as thermal insulation and stimulating reparative secondary dentin formation. By contrast, bases are placed in a thick layer on the preparation floor. Like liners, bases protect pulp, but they also absorb more of the bite force. In addition, bases sometimes line undercut areas for indirect restorations.¹

Nathaniel Lawson, DMD, PhD, associate professor and director of the Division of Biomaterials at the University of Alabama at Birmingham School of Dentistry, says that sometimes when people say base/liner they mean a thin layer of flowable material prior to their packable composite. 

“I do this routinely. The first layer of flowable composite adapts to the irregularities of my prep and fills in deep areas and areas that are hard to adapt paste composite to,” Dr Lawson says regarding this definition.

“Liner/bases can also be calcium hydroxide, calcium silicate, and glass ionomer materials,” Dr Lawson continues. “There are a couple reasons why someone may do that. First, if the preparation is close to the pulp, liners are placed to stimulate reparative dentin if there is a microscopic pulp exposure. Second, these liner materials can be placed on affected dentin to help remineralize this dentin from ion release within the liner material. If the preparation is not deep or close to the pulp, liners do not seem necessary as they have a weaker bond to the tooth than adhesively bonded composite.”

The materials used for liners and bases include the following:

• Calcium hydroxide (CaOH; pH 12; liner): It is easy to work with, sets quickly when placed in thin layers, and has positive effects on decayed dentin or exposed pulp. However, it is highly soluble, not very strong, and does not work with resin-based restoratives.
• Zinc oxide-eugenol (ZOE; pH 7; base): This is the least inflaming of the dental materials but is toxic to pulp.
• Zinc phosphate (ZOP; pH 2; typically, a base): Although this is easy to mix, strong, and proven, it is also brittle, cannot adhere to the tooth structure, lacks antibacterial properties, and can irritate pulp.
• Zinc polycarboxylate (ZPC; pH 1.7; base): ZPC is biocompatible, can adhere to tooth structure, and is strong and easy to use. However, it requires accurate measuring when placing, and has a high viscosity and short working time. Plus, one should clean the prep to facilitate bonding.
• Glass ionomer (GI; liners, bases): GI bonds to tooth structure, has fluoride release that protects against demineralization, is antibacterial, and expands when in contact with moisture, which may offset problems with shrinkage and could reduce microleakage.
• Resin materials (liners, bases): These materials vary by the types of filler in them or how they are cured. These materials bond to teeth but require the area to be free of contaminates to facilitate the bond. Some resins reduce microleakage due to their adaptive properties. Others can release fluoride. When placed as a base, the resin can be shaped and contoured, and will be a temperature buffer.²

Not Everyone Is All About That Base

The authors of a study in Clinical, Cosmetic, and Investigational Dentistry are not proponents of bases in shallow to moderate preparations. Their literature review from the 1970s to 2020 suggests there is “very little evidence” to support routine use of bases under composite restorations. They concluded that using bases often compromised rather than enhanced composite restorations. One study determined composite restorations without bases even had improved adaptation. Regarding GI, several studies the researchers reviewed suggest the material’s results were questionable or had no effect on the survival rate.³

However, bonded resin-modified glass ionomers (RMGI) were an acceptable base in deep cavities. RMGI could serve as an appropriate base in deep cavities provided the pulp was capped with CaOH liners.³ The calcium hydroxide introduces bioactivity and antimicrobial effects in the deep areas, and the RMGI provides protection from the drawbacks of CaOH.⁴

Speaker and educator Jeffrey Lineberry, DDS, AAACD, owner of the Carolina Center for Comprehensive Dentistry in Mooresville, North Carolina, agrees that liners and bases are not as critical as they once were. Originally, liners and bases served as an insulator between the pulp and amalgam materials. As materials changed to composite resins, bases and liners became more appropriate in an indirect or direct pulp cap with some type of calcium-based or bioactive liner, and not just because a cavity is deep.

“As long as the nerve is not exposed and it’s not shining through, so to speak, then I don’t really see the point of doing a liner,” Dr Lineberry says. “The composite doesn’t adhere to the base unless it’s a resin-modified glass ionomer or something where the composite can stick to it. Otherwise, if it’s just a nonresin base or liner, then it doesn’t stick to it.”

Sometimes Deep Preps Benefit From Liners and Bases

It is important to note that the literature review that casts doubt on whether liners and bases are effective concluded that they were not great in shallow to moderate preparations. Deep preps are another story. Many clinicians still use them for specific cases.

For example, John Flucke, DDS, the chief dental editor and technology editor at Dental Products Report®, recommends TheraBase® (BISCO). TheraBase is a dual-cured, calcium- and fluoride-releasing base and liner. In addition to its bioactive properties, the dual cure material also contains MDP (10-Methacryloyloxydecyl dihydrogen phosphate), which facilitates bonds to enamel and dentin. It also generates an alkaline pH in minutes, enhancing the pulp health.

Dr Flucke appreciates how the product goes where he wants and stays there. Also, because the material is dual cure, meaning it light cures and auto cures, you can set it with the light but then it will continue to cure down in the dark of the prep. In addition, after you set the base with the light and then condense the restorative material on top, it stays put.

With TheraBase, you also do not have to etch and bond first, Dr Flucke explains. You clean the decay, place the base, light cure it, and it is set enough to move onto the restorative material. Moreover, TheraBase releases calcium and fluoride for the life of the restoration.

Dr Flucke says TheraBase also has excellent radiopacity, so it shows up well on bitewing x-rays. He says clinicians will not have to wonder what is underneath the restoration when their patient returns for additional care.

When Dr Lineberry does use a base, he uses Calcimol LC (VOCO), a light-cured, resin-modified, calcium ion–releasing base liner and pulp capping material.

“It’s a light-activated calcium hydroxide, which has a long-standing history in this area. It’s not only bactericidal, but then it helps stimulate the pulp and creates a dentin bridge with calcification over it,” Dr Lineberry says. “A lot of time I will put a flowable over the top of it because I know it’ll adapt readily over the top of that.”

Not everyone agrees that GI is ineffective, either. Gregor Connell, VOCO’s North American director of clinical education and international business development manager for Australia, New Zealand, the United Kingdom, Ireland, and Malta, says GI is often a great choice of material for the initial increment placed, especially in cases where the clinician has a deep prep with proximity to the vital pulp chamber.

Connell says that preparations close to the pulp mean a larger amount of dentin has been removed and an opportunity and desire now exists for the clinician to strengthen what tooth structure remains. Connell also says that GI materials offer a fluoride exchange process like that of a rechargeable battery.

“The GI can release F^- ions into the tooth structure; then, once ionically depleted, the GI material ‘recharges’ to continuously redeposit fluoride into the tooth. This provides the remaining tooth structure with much needed remineralization and offers a cariostatic barrier, which can eliminate the possibility of further damage due to carious lesions,” Connell says.

GI selection requires some careful considerations prior to use, Connell explains. Unlike most composite restoratives, GI materials possess much lower physical properties, specifically in the areas of compressive, flexural, and transverse strength. Depending on the patient’s oral dynamics regarding mastication force, bruxism, and/or potential malocclusion, a clinician needs to determine whether their GI of choice will provide the required support for long-term restorative success.

“Care needs to be taken to ensure that their selected GI can provide the required strength. I stress this only because most GI materials in the industry present with rather low physical values,” Connell says. “VOCO’s GI materials offer some of the highest physical property values currently available in the industry.

“As for use of the materials, they will require trituration or mixing prior to their being dispensed onto the preps,” Connell continues. “Not all amalgamators are the same and very few are ‘new units.’ I always suggest ‘test mixing’ a few of the caps to find the optimal mixing time, then adjusting the mixer accordingly. This tends to save any potential embarrassment in front of a patient, should the material not mix properly.”

References

1. 7: liner and bases. Pocket Dentistry. January 1, 2015. Accessed September 8, 2023. https://pocketdentistry.com/7-liner-and-bases/
2. Weiner R. Liners and bases in general dentistry. Aust Dent J. 2011;56(suppl 1):11-22. doi:10.1111/j.1834-7819.2010.01292.x
3. Arandi NZ, Rabi T. Cavity bases revisited. Clin Cosmet Investig Dent. 2020;12:305-312. doi:10.2147/CCIDE.S263414
4. Arandi NZ. Calcium hydroxide liners: a literature review. Clin Cosmet Investig Dent. 2017;9:67-72. doi:10.2147/CCIDE.S141381