A Primer on Soft Denture Liners

Soft denture liners are as ubiquitous in removable prosthetic dentistry as dentures are. We have been using these materials for more than a century and accept that none of the liners have a life expectancy comparable to a resin denture base.

Their purpose is to provide temporary relief to sore tissues, help to condition and provide retention, as well as distribute load more evenly on denture-bearing areas. For those patients with thin atrophied ridges, bony undercuts and non-resilient mucosa, a soft-tissue liner may be the only way a denture can be worn with comfort. We routinely place a soft liner on an immediate prosthesis, over a recent implant surgical site or to help provide tissue relief in cases of xerostomia. Clinical experience indicates almost universal tissue tolerance and patient acceptance.

The categories of soft denture liners

These resilient lining materials fall into one of two categories: silicone elastomers or plasticized acrylic resins.

• The plasticized acrylic resins contain an acrylic monomer and plasticizers (ethyl alcohol and/or ethyl acetate), which act to preserve the softness of the material. The plasticizers are not bound within the resin and therefore in time, will leach out.
• The silicone elastomers are essentially composed of polydimethylsiloxane polymers, similar to the silicone impression materials you may use. This helps to retain the elastic properties of the liner for longer periods of time. They do lack the leachable plasticizers (aren't as soft) that the acrylic resins have.

All of these soft denture liner materials have disadvantages: They are porous and not color stable, have a low resistance to abrasion and a short-term resiliency, and lack bond strength despite the necessary chemicals applied to address this. The most common reason for the failure of a soft lined denture is adhesion to the acrylic base.

Given enough time wearing a denture, there will be changes in the denture-bearing tissues. Utilizing a soft liner will improve the comfort and concomitantly the masticatory efficiency for most patients. For those patients who present with insufficient ridge height or dry thin tissues, the routine placement (and replacement) of liners may be the only way that a denture can be worn. These liners will often act as masticatory shock absorbers.

The right liner for the right patient

I utilize both of these types of soft liners depending on what issue the patient presents with.

The plasticized acrylic resins (think Coe Soft, Coe Comfort) have a polyethyl methacrylate power and esters and alcohol, but no methacylate monomers. This material is soft but short term (three to six months). I utilize it on immediate denture patients to help with the retention and comfort post surgically. The plasticzers will keep the liner soft for a number of weeks, but over time the material hardens and will have to be replaced. I will add material every seven to 10 days until the immediate denture patient is comfortable and feels the denture is retentive enough. This can usually be achieved within the first two to three weeks.

Utilizing the soft liner material can also serve to create a functional reline impression. I will keep a patient in this soft liner until healing is complete and I am ready to have a hard acrylic lab reline performed. These materials will “bond” well enough to the denture base; I will describe later how I trim the material and get it to adhere to the borders.

The silicone elastomers are usually used for longer periods of time (one year). If a patient presents with thin, atrophied ridges, undercuts, non-resilient mucosa or chronic soreness, I will typically try this type of liner. It will provide a softer base than the existing intaglio, but its properties won't change as quickly as the acrylic resin liners. This material is denser and doesn't have the pockets of water, which form when the plasticizers leach out of the acrylic based ones. If the patient is considering having a new denture made but is unhappy with the fit of their existing one, I will use a silicone elastomer material. It will hopefully render comfort and buy the patient some time before investing in a new denture. Additionally, when placed over cells in recent surgical sites, it may be more suitable biologically.¹

The problem faced with this material is establishing a strong bond to the denture base. Mechanical retention can be cut in (I will place V shaped undercuts where I can) and there are solvent-based primers added to the intaglio of the denture to help rectify this problem. That being said, patients will always show you where it's peeling away.

Given enough time, both types of liners will get hard and probably uncomfortable. The surface becomes rough and irritating and will start to peel away from the denture borders. It can also become colonized with Candida and other microorganisms leading to chronic tissue inflammation.

Denture liner adhesion

The long-term performance of the liners is heightened when there is good adhesion of the liner to the denture base. One study showed that the adhesive values obtained with a roughened surface are approximately double those of the smooth surface.

It is worth the time spent to do this.

Altering the surface of the acrylic resin denture base by running an acrylic bur over the intaglio and borders, micro etching, applying 36 percent phosphoric acid, and utilizing the manufacturer's recommended bonding agents should provide an adequate roughened surface. Once the surface for bonding of the liner is addressed, attention must be paid to the thinness of the material, as the thinnest areas will peel away first. In my experience, this happens often in the retromolar pad area and posterior maxillary border. Make sure to relieve these areas 1-2 mm before applying the liner so that there is sufficient bulk and thickness.

Dentists will frequently control the powder/liquid ratio of the mix to improve the handling properties and working time. Many will increase the liquid to change the flow of the material. Note that changing the P/L ratio can create a greater plastizing effect, so make every attempt to follow the manufacturer's recommended values for powder liquid ratios.

Sealers placed on the cured liner material will act as a mechanical barrier to control the deposits of food and chemicals that are attracted to it. Using a sealer will reduce the surface break down, which will occur with routine mechanical cleaning. It will also protect the incorporation of pigments (coffee, tea, wine, tobacco, etc.) into the material. There is a study that shows that placing a sealer coating has no significant effect on the surface roughness, whether it's an acrylic-based or silicone-based liner.² Surface roughness is inevitable with either.

Cleaning soft tissue liners

Patients will ask how best to clean the dentures once the liner is placed. Peroxide-based denture cleansers are the most commonly used. They are dispensed in powders or tablets and become alkaline solutions of hydrogen peroxide when dissolved in water. There has been a prevailing thought that denture cleanser use can damage the resiliency of the liners. One study looked at Coe Soft and Polident tablets and found that although there was a change in the weight of the resilient liner, the surface roughness and bond strengths were unaffected.³ The use of conventional denture cleansing tablets or a dilute mix of water with 0.5 percent sodium hypochlorite will be equally effective for either type of liner. Silicone-based liners show better compatibility and resiliency over the long term; however, the results are clinically insignificant.

If using an acrylic-based liner, I instruct the patients to initially rinse it with cold water and use a Q-tip to remove any food debris until the consistency of the liner has become denser. Then, a soft brush can be used and denture cleansing tablets. I do explain that the liner will have to be changed more often because of the increased hardness, which will develop. If using a silicone elastomer-based liner, a soft brush and tablets can be used from the onset.

Many dentists struggle with how to finish trimming the excess material along the denture borders.

One simple and easy technique is to heat up a #7 wax spatula with a torch or Bunsen burner and then hold the instrument perpendicular to the border.

Once in contact with the liner and denture acrylic, move the instrument along the outside periphery of the border. It will trim away the excess without pulling it off and also seals the material to the acrylic.

No further polishing is needed as it provides a smooth and comfortable surface that will adhere to the acrylic border.

How long any one liner will last depends on many factors, some of which are not clearly understood. Selection of a liner cannot be based on any single property but the clinical situation presented. There are many materials currently available, all of which will provide the intended function. Both types of soft liners have a place in your removable prosthetic practice.

Mary Anne Salcetti, D.D.S., P.C. is a member of Spear Visiting Faculty.

References

1. Journal of Tissue Engineering and Regenerative Medicine. 2014; 5:2041731414540911
2. Journal of Prosthetic Dentistry. 2016 Mar;115 (3):371-6
3. Journal of Prosthetic Dentistry. 2003 May;89 (5):489-94.