The aim of this investigation was to determine the effect of storage in disinfectants and artificial saliva on a series of commercial soft lining materials for dentures. Changes in mechanical properties and the nature of chemicals released into these solutions were studied.
Four soft lining materials were studied (Vertex Soft ® and Villacryl Soft ® , both of which are plasticized acrylics; Molloplast B ® and Mollosil ® , both of which are silicone elastomers). All were cured according to manufacturers’ instructions, and then cylindrical specimens (6 mm height × 10 mm diameter) prepared. These were stored under various conditions then loaded in uniaxial compression and the stress measured at a strain corresponding to a 10% deformation to determine Young’s modulus. Storage involved exposure either to the following disinfectants: 2% aqueous chlorhexidine gluconate, 2% aqueous sodium hypochlorite, Corega Tabs ® cleansing tablets or 3% aqueous hydrogen peroxide; or to artificial saliva. For the latter, storage involved either immersion in artificial saliva at 37 °C for the whole study, or immersion for 16 h a day and dry at room temperature for the next 8 h each day. GC/MS was used to determine species leached into these solutions.
The acrylic materials Villacryl Soft and Vertex Soft became less elastic on storage for up to 28 days whereas the silicone materials Molloplast B and Mollosil, showed no change in elastic properties. Various compounds were found to be released from these materials, including EGDMA, methyl methacrylate and dibutyl phthalate. Generally, the silicones were more stable than acrylics, releasing smaller amounts of the various eluants. In all cases, amounts eluted were well below permitted exposure limits.
Practical denture cleansing agents affect the properties of soft lining materials, reducing their elastomeric character, acrylics being more adversely affected than the silicones. These changes are associated with the loss of various chemicals, including plasticizers and monomers, from the soft lining materials.
Soft lining materials have been used since the late 1950s in prostheses for patients with unfavorable conditions of the denture bearing area . Certain parts of the supporting regions of the alveolar ridge are sensitive to the pressure of hard prosthetic materials, and this may be more pronounced in the immediate period following operative procedures. Soft lining materials are able to form an absorbing (elastic) layer on the part of the denture in contact with the oral mucosa and this allows less traumatic transmission of occlusal forces. The result is that wearing the prosthesis becomes more comfortable for the patient .
Contemporary elastic materials are used for short- and long-term application in the oral cavity and are divided into acrylic and silicone types. These materials may be further sub-divided into room temperature or at high temperature polymerizing categories .
Elastic materials used in dental prosthetics to line removable dentures should be characterized by: biocompatibility toward the oral tissues, shape and color stability, resistance to abrasion, and durability of the junction between the lining and the denture. Moreover, these materials should be resistant to the variable conditions in the oral cavity environment and their properties should not be degraded by hygiene procedures. Knowledge of the mechanical properties and durability of elastic materials helps to make the right choice, depending on specific clinical applications .
The structure of soft lining materials and the roughness of their surface may encourage colonization by oral micro-organisms. Patients using removable prosthetic restorations lined with an elastic material should, then, carry out regular cleansing procedures to prevent such infection. However, such procedures may lead to changes in the elastic properties of the soft lining materials . Certain chemical substances are known to be leached out of soft lining materials by disinfecting solutions, resulting in deterioration of their mechanical properties. As a result, they become harder and their elasticity decreases . The loss of the elastic properties of lining materials may intensify the traumatic influence of the prosthesis on the tissues and reduce the comfort for the wearer.
The current work was undertaken to provide information on the durability of selected modern soft lining materials. Silicone and plasticized acrylic materials were studied, and experiments considered both changes to mechanical (elastomeric) properties and the nature of substances leached by various disinfecting agents, and also artificial saliva.