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Considerations in the Application and Benefits of Soft Lining Materials

Selection of soft lining materials to support removable prosthodontic therapy influences treatment success by helping to condition edentulous soft tissues prior to complete or partial denture fabrication. Following surgical interventions (e.g., dental implant placement or preprosthetic tori reduction), liners may be used to control prosthetic force application and improve prosthesis fit against the surgically manipulated denture foundation. Soft liners can also facilitate intraoral drug delivery, although adhesion to denture bases, microorganism colonization and liner cleansability remain concerns. This issue of Prosthodontics Newsletter presents new materials and novel applications of soft lining materials.

Evaluation of Surface Treatments for Adhesion

Denture liners can be classified as either acrylic polymers, such as poly(methyl meth-

acrylate) (PMMA) and poly(ethyl methacrylate), or elastomeric polymers, known as silicone. While silicone-based denture liners are resistant to temperature change and have low degradation rates and high tear strengths, they do not adhere well to PMMA-based denture materials.

To evaluate the effect of different PMMA surface treatments on adhe-sion of silicone-based denture lin-ers, Cavalcanti et al from the State University of Campinas, Brazil, conducted an in vitro blind analy-

sis of different surface treatment protocols for PMMA resins. They prepared PMMA specimens and divided them into 4 groups:

➤ no treatment, the control group

➤ application of methyl methacrylate (MMA) monomer for 180 seconds

➤ application of an acetone (AC) solu-tion for 30 seconds

➤ application of an ethyl acetate (EA) solution for 60 seconds

PMMA disks and surface free energy PMMA bars were evalu-ated for surface roughness. Surface roughness was found to be signifi-

(continued on next page)

➤ Effect of Denture Cleaners on Soft Lining Materials

➤ Microorganism Inhibition on Soft Lining Materials

➤ Candida Colonization of Soft Liners in Patients with Diabetes

➤ Effectiveness of Chlorhexidine in Denture Soft Lining Materials

Insi

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M. Patrick Pedlar, BSc, ddS, MSd, Frcd(c) B. larry Pedlar, ddS, MSd, FacP

Restoring the Natural Appearance of Teeth310 Plains Road East • Burlington, Ontario L7T 4J2

Tel: 905-632-1882 • Fax: 905-632-1351 • www.RestorativeDentistry.ca e-mail: patrick@drpedlar.com

Certified Specialists in Prosthodontics: Crowns • Veneers • Bridges • Implants • Dentures • Aesthetic Reconstructive Dentistry

A Professional Courtesy of:

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cantly higher for the MMA group (p < .05); surface free energy was found to be significantly lower for the AC and EA groups (p < .05).

Tensile bond strength was tested using a universal testing machine after the silicone liner had been applied between 2 treated PMMA bars. The authors found that use of MMA and EA surface treat-ment protocols resulted in a bond strength (p < .05) significantly higher than those measured in the AC and control groups, which agreed with the results of other studies. These results partially rejected the study’s null hypoth-eses, in which the authors posited that PMMA surface treatments would not modify the surface properties or affect the adhesion of PMMA to a silicone-based resilient denture liner.

Comment

The use of these MMA and EA treat ment protocols appears to improve the adhesion of silicone-based denture liners to PMMA. These results may help guide the creation of improved silicone-based resilient liners. The effects of aging and the oral environment, however, were not considered in this study; these factors need to be evaluated in order to achieve ade-quate clinical performance.

Cavalcanti YW, Bertolini MM, Del Bel Cury AA, da Silva WJ. The effect of poly(methyl methacrylate) surface treatments on the adhesion of silicone-based resilient denture liners. J Prosthet Dent 2014;112:1539-1544.

Effect of Denture Cleaners on Soft Lining Materials

Since the 1950s, practitioners have used soft lining materi-als to increase comfort for

denture wearers. These soft lining materials, which can be divided into acrylic and silicone types, form an elastic layer on the area of the denture that comes in contact with the oral mucosa. Because soft lin-ing materials have the propensity to allow colonization by microor-ganisms, patients are instructed to carry out regular cleansing proce-dures. However, disinfecting solu-tions can lead to deterioration of the soft lining materials.

To provide information on the durability of 4 commonly used soft lining materials, Broz ek et al from Poznan University of Medical Sciences, Poland, studied the effects of 4 disinfectants and a saliva solu-tion on the properties of the soft lining materials. The authors stud-ied the following 4 materials:

➤ Vertex Soft (Vertex-Dental B.V.)

➤ Villacryl Soft (Zhermack SpA)

➤ Molloplast B (Detax)

➤ Mollosil (Detax)

The samples were cured according to manufacturers’ instructions. To imitate the various modes of prac-tical use by patients, each mate-rial was immersed once daily for 15 minutes in 1 of the following disinfectants:

➤ 2% aqueous chlorhexidine gluconate

➤ 2% aqueous sodium hypochlorite

➤ Corega Tabs cleansing solution

➤ 3% aqueous hydrogen peroxide

A sample of each material was also exposed to saliva. Samples were stored at either 37°C in an artificial saliva solution for the duration of the study or 37°C for 16 hours daily and at room tem-perature for the remaining 8 hours. Physical properties were measured by Young’s longitudinal elastic modulus. To measure chemical properties, gas chromatography with mass spectroscopy character-ization was employed to determine substances released from the soft lining materials.

While silicone materials showed no deterioration in elastic properties, acrylic elastomers were sensitive to disinfecting solutions. Villacryl Soft showed the greatest change in elasticity when treated with sodium hypochlorite, and Vertex Soft, which had a lower initial elasticity, showed a similar change. Chemical analysis also revealed the silicone materials to be more stable than the acrylic materials.

Comment

The authors found that strong disinfecting substances led to a reduction in the elasticity of soft liners, while liners stored dry or in artificial saliva showed little or no change. Silicone materials were less affected by disinfectants than were acrylic materials.

Broz ek R, Koczorowski R, Rogalewicz R, et al. Effect of denture cleansers on chemical and mechanical behavior of selected soft lining materials. Dent Mater 2011;27:281-290.

Evaluation of Surface Treatments For Adhesion (continued from front page)

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Microorganism Inhibition on Soft Lining Materials

While soft lining materi-als reduce the traumatic effects of dentures in

certain patients, some studies have found them to be a suitable substrate for microbial coloniza-tion, while others report they inhibit yeast growth. Because these organisms can be responsible for oral and systemic infections, Pavan et al from São Paulo State University, Brazil, evaluated the adhesion of pathogenic microor-ganisms Staphylococcus aureus and Pseudomonas aeruginosa and the yeast Candida albicans to 2 soft lin-ing materials. They also studied the inhibitory effect of these materials on microorganisms and any corre-lation to surface roughness.

The soft lining materials studied were Molloplast B (Detax GmbH & Co.) and Ufi Gel P (VOCO GmbH). Cultures of S aureus, P aeruginosa and C albicans were used to inocu-late 20 mL of Mueller Hinton agar, and prepared specimens of the lin-ing materials were placed in each agar plate. Filter paper discs soaked with 0.05% chlorhexidine gluco-nate solution served as controls. Colony-forming units per milliliter (CFU/mL) and surface roughness were calculated; significance was defined as p < .05.

For Molloplast B, the mean num-ber of adhesion cells of S aureus and P aeruginosa were significantly greater than those of C albicans. For Ufi Gel P, S aureus showed a significantly greater adherence

than did the other microorganisms studied (Figure 1). While Ufi Gel P demonstrated a greater adherence of C albicans than did Molloplast B, no significant correlation was ob -served between the adherence of microorganisms and the surface roughness of the soft lining materi-als. A visual examination found neither of the materials exhibited a zone of inhibition when compared with the control.

Comment

The authors of this in vitro study found that while S aureus and P aeruginosa adhered more to the soft lining materials tested than did C albicans, neither material had an inhibitory effect on the growth of any of the microorganisms. These findings are important be cause pathogenic bacteria may play a role in denture stomatitis and sys-temic infections. To substantiate the data, these studies should be conducted again.

Pavan S, dos Santos PH, Filho JNA, Spolidorio DMP. Colonisation of soft lining materials by micro-organisms. Gerodontology 2010;27:211-216.

Candida Colonization of Soft Liners in Patients with Diabetes

For patients with diabetes mel-litus, soft liners are often more comfortable than conven-

tional prostheses fabricated from hard acrylic resin, but their surface texture, water absorption and dif-fusion of nutrient material make them prone to microbial adhesion. This can lead to denture stomatitis, caused primarily by Candida albi-cans and related species.

To assess the prevalence of Candida species on a heat-polym-erized soft silicone lining mate-rial, Mantri et al from Hitkarini Dental College & Hospital, India, compared Can dida growth in 15 patients with noninsulin-depen-dent diabetes and in 15 patients without diabetes. The authors also examined the antifungal action of 4% chlorhexidine gluconate.

In patients who used soapy water and a soft denture-cleaning brush to clean their dentures, Candida colonization at 21 and 30 days was significantly higher in patients with diabetes than in patients without. Immersing the dentures in chlorhexidine gluconate signifi-cantly decreased the colonies at 21 and 30 days (Table 1).

This study confirmed previous re -ports that soft liners used on pros-thetics in patients with diabetes can lead to a significant increase in the density of Candida coloni-zation. Proper oral hygiene with

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C albicans S aureusP aeruginosa

Ufi Gel P Molloplast B

Figure 1. Comparison of adherent cells for each microorganism on both materials tested

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antimicrobial agents can minimize fungal/microbial colonization of soft liners. In this study, soaking the dentures in chlorhexidine gluconate for 5 minutes was found to be more effective at eliminating Candida spe-cies than was brushing.

Comment

Ensuring that dentures are free of antimicrobial contamination not only controls odors but also main-tains oral and systemic health. Further research on the effects of long-term use of chlorhexidine on soft liners is needed.

Mantri SS, Parkhedkar RD, Mantri SP. Candida colonisation and the efficacy of chlorhexidine gluconate on soft silicone-lined dentures of diabetic and non-diabetic patients. Gerodontology 2013;30:288-295.

Effectiveness of Chlorhexidine In Denture Soft Lining Materials

With previous studies show-ing the effectiveness of chlorhexidine solution to

suppress the adhesion of Candida albicans to prostheses, research-ers have investigated creating a drug delivery system that incorpo-rates antifungal or antimicrobial

agents with denture acrylic resin or soft lining materials. Bertolini et al from State University of Rio de Janeiro, Brazil, conducted an in vitro study that evaluated the growth inhibition of C albicans and the rate of chlorhexidine released from resins-based denture soft lin-ing materials modified by chlorhex-idine diacetate (CDA) or chlorhexi-dine hydrochloride (CHC), as well as any change in Shore A hardness.

The authors tested 2 denture mate-rials and 2 chlorhexidine solutions:

➤ poly(methyl methacrylate) (PMMA)

➤ poly(ethyl methacrylate) (PEMA)

➤ CDA salt hydrate

➤ CHC salt hydrate

Antifungal activity against C albi-cans was evaluated, and drug re -lease was analyzed by the change in optical density of the storage so -lution by ultraviolet spectrometry. Shore A hardness was evaluated at baseline and after 2 and 7 days of water storage at 37°C. CDA, but not CHC, was released from the PEMA and PMMA soft lining materials in proportion to concen-trations of CDA. Shore A hardness increased over time.

Comment

Incorporating antimicrobial agents into resins-based denture soft lin-

ing materials could create a drug delivery system that would interfere with the colonization and penetra-tion by C albicans. Such a system would be convenient for patients. This in vitro study provided data that could be used for a future in vivo study of a denture soft lining material containing CDA.

Bertolini MM, Portela MB, Curvelo JAR, et al. Resins-based denture soft lining mate-rials modified by chlorhexidine salt incor-poration: an in vitro analysis of antifungal activity, drug release and hardness. Dent Mater 2014;30:793-798.

Etiology and diagnosis of temporomandibular stability and dysfunction

Do you or your staff have any questions or comments about Prosthodontics Newsletter? Please write or call our office. We would be happy to hear from you.

© 2017

In the Next Issue

Our next report features a discussion of these issues and the studies that analyze them, as well as other articles exploring topics of vital interest to you as a practitioner.

Table 1. Mean ± standard deviation of Candida growth (colony-forming units/0.2 mL)

15 days 21 days 30 days 15 days 21 days 30 days Oral flora soap soap soap chlorhexidine chlorhexidine chlorhexidine

Nondiabetic patients 0.40 ± 0.61 1.86 ± 1.09 2.26 ± 1.24 2.53 ± 1.31 1.66 ± 1.30 1.20 ± 0.91 0.80 ± 0.91Diabetic patients 0.46 ± 0.61 2.46 ± 1.31 3.40 ± 0.60 3.66 ± 0.60 2.20 ± 0.83 1.66 ± 1.01 0.93 ± 0.77