Effects Of Plasma Treatment On The Bond Strength Between Silicone-based Soft Liner And Thermocycled Denture Base

With continuous and slow absorption of the alveolar bone,complete dentures may have several problems,such as lack of retention,poor stability,mucosal tenderness,and chewing disfunction,which may seriously affect the patients' physical and mental health.Complete dentures may need relining once every five years.Silicone-based soft lining materials contain no plasticizer,have a better tear strength and aging resistance,and can keep elasticity for a long time,often used as the denture liner.As the molecular structure between denture base resin(polymethyl methacrylate PMMA)and silicone-based soft denture liner(polydimethylsiloxane,PDMS)are different,it is difficult to form chemical bonding and bond failure often occurred.Many factors,such as immersion in oral saliva environment,frequent temperature change caused by food and drink,and repeat chewing stress,may accelerate the denture's aging process.Surface energy and surface hydrophilicity of aged denture base will reduce,and the bond will be more difficult to be formed with the silicone-based soft liner.In previous studies,fresh denture base resin was used.However,it cannot completely simulate the oral condition.In the present study,in vitro accelerated aging reproduced with thermocycling was used to simulate the oral condition,and oxygen plasma treatment was used to investigate the tensile bond strength between silicone-based soft liner and thermocycled denture base.This study contains three parts:(1)Two 10mm�10mm�1 mm PMMA specimens were prepared and thermocycled(5-55?,5000 cycles).They were divided into control and 4 min exposure group(n=1).After plasma treatment,an X-ray photoelectron spectrometer was used to analyze the surface elements and calculate the O/C atomic ratio.(2)30 10mm�10mm�1mm PMMA specimens were prepared and thermocycled(5-55?,5000 cycles).They were divided into 5 groups:control and 4 experimental groups(exposure to plasma for 1 min,2 min,3 min and 4 min,respectively)(n=6).After plasma treatment,a contact angle goniometer was used to measure the surface contact angle and the surface energy.(3)80 8mm�10mm�30mm PMMA specimens were prepared and thermocycled(5-55?,5000 cycles).They were divided into 5 groups:control and 4 experimental groups(exposure to plasma for 1 min,2 min,3 min and 4 min,respectively)(n=16).After plasma treatment,the silicone-based soft liner(3 mm thick)was packed against two thermocycled PMMA specimens and polymerized according to the manufacture's instructions.The tensile bond strength was tested by a universal testing machine,and the load until fracture was recorded.Experimental results:(1)After plasma treatment,the oxygen containing groups of C-O and O=C-O were increased,and the O/C atomic ratio increased from 0.324 to 0.498.(2)In the contact angle measurement,there was a significant difference between control and experimental groups(P<0.05).The highest value was obtained in the control group(59.99�10.00�),and the lowest was in the 4 min exposure group(37.32�5.31�).(3)In the tensile test,there was a significant difference between control and experimental groups(P<0.05).The lowest value was observed in the control group(0.831�0.059 N/mm2),and the highest was in the 4 min exposure group(1.998 �0.110 N/mm2).The results showed that,the value of oxygen-containing groups(C-O and O=C-O)on the thermocycled denture base resin surface was increased and the surface hydrophilicity was improved after plasma treatment,and plasma treatment was efficient in improving the tensile bond strength between thermocycled PMMA resin and silicone-based soft liner.