Different Ion Release Mechanism in Alternating pH Environment

Objectives: To test the hypothesis that immersion in alternating pH environments causes increased ion release compared with immersion in constant pH.

Methods: Nine disks of glass-ceramic veneer (IPS Eris; Ivoclar Vivadent) were fabricated and grouped into either test sequence: I (pH 10, 2, 7); II (pH 2, 7, 10); and III (pH 7, 10,2) Each specimen was placed in a polypropylene jar within a shaker bath containing deionized distilled water at 80 °C at a vibrating speed of 50 oscillations per min. Specimens were exposed to the first buffer solution in the sequence for 3days, rinsed, and immersed in the next solution for another 3days. Immersion for 27 days totaling 9 cycles. Concentrations of Al³⁺, Ca²⁺, Zn²⁺, Li²⁺ in solutions was analyzed for each 3day cycle using inductively coupled plasma atomic emission spectroscopy. Ion concentrations were compared with ion release concentrations from a previous study where discs were exposed to constant pH 2, 7, 10 for 3, 15 and 30 days. R statistical software package (V.3.0.2) was used to create mixed-effects linear models.

Results: Linear model results indicated that all ions were released faster in cycling conditions than in constant pH (P<.0001). Zn²⁺ was released 3.8 higher in 10-2-7 than in 2-7-10 (P=.0008). Li²⁺ was released 0.5 higher in 10-2-7 than 2-7-10 (P= .0009). Ca²⁺ was released 0.46 higher in 10-2-7 than 2-7-10. Al³⁺ was released 0.07 higher in 10-2-7 than 2-7-10 (P=.004). Zn²⁺, Li²⁺ and Ca²⁺ were released in pH10 more than pH 2. Al³⁺ was released only in pH 2 solution.

Conclusions: Alternating pH is more detrimental than constant pH immersion to the surface integrity of glass ceramics because of the release of network formers and network modifiers in solution.