The influence of cyclic stress on surface properties of soft liners.

Soft polymers used under tissue-supported prostheses have limited service life because of surface degradation. PURPOSE: This paper evaluates the changes in surface properties and softness of soft relining materials after cyclic loading in water. MATERIALS AND METHODS: 3 polysiloxane (Silagum AM Comfort, Molloplast B, Mollosil Plus) and 2 acrylic-based (Vertex Soft, Astron LC Soft) proprietary soft relining materials and a vinyl polysiloxane (Imprint 2 Garant) as the reference impression material evaluated. A stainless steel block for detail reproduction was used on the basis of the apparatus recommended by the ISO 4823:1992 test method for preparing a standardized surface. A control group from each material was only subjected to water immersion. Non-destructive cyclic loading was carried out with a strain of 16.6 % and a frequency of 1.6 Hz 200,000 times in distilled water at 37 degrees C. The specimens were then duplicated and compared with the controls using roughness measurements, detail reproduction and Scanning Electron Microscope. Shore A hardness values before and after cycling were also measured. RESULTS: After degradation in the form of mechanical cycling in water, loss of substance and loss of surface detail was observed. Surface properties and Shore A hardness values of acrylic-based soft liners changed significantly (p < 0.05). CONCLUSIONS: The chemical composition of the soft relining materials seemed to affect their potential of preserving the surface texture and softness after mechanical cycling. Mechanical cycling influences the surface degradation process leading to changes of the surface texture. Polysiloxane-based materials preserved their softness, surface texture and surface smoothness better under cyclic loading compared to acrylic resin -based plasticized materials.