OHRI pH Cycling Models: pH cycling models were designed to simulate the dynamic variations in mineral saturation and pH associated with the natural caries process (White, 1995). They reproduce specific events of the caries process under controlled conditions allowing the investigation of individual mechanistic variables that could not be possible to do or would be extremely difficult to do under in vivo conditions. At the same time, it is important to recognize that because of not being able to reproduce the whole complexity of caries dynamics, in vitro experiments provide only limited information on the effects of different variables on the caries process. While there are several pH cycling models commonly used, the one developed by White (1987, 1988) has been shown to be an excellent model to evaluate the remineralization potential of fluoride dentifrices. This model is able to demonstrate a dose-response (0, 250 ppm and 1100 ppm F) in both enamel and dentin (Dunipace et al., 1992; Faller, 1992; Schemehorn et al., 1994) and can be used with either bovine or human enamel (Schemehorn et al., 1992).
The Indiana Oral Health Research Institute has also the capabilities and experience to conduct other pH cycling models, such as the one developed by Featherstone et al. (1986).
The main difference between these two models is the net outcome – the model developed by White (1987, 1988) is a net remineralization model, whereas the one developed by Featherstone et al. (1986) is a net demineralization model. Thus, both aspects of in vitro caries lesion remineralization and prevention can be studied at the Indiana Oral Health Research Institute.
The Indiana Oral Health Research Institute employs the following techniques to characterize caries lesions:
- surface microhardness (SMH)
- cross-sectional microhardness (CSMH)
- transverse microradiography (TMR)
- quantitative light-induced fluorescence (QLF)
- confocal laser-scanning microscopy (CLSM)
- enamel fluoride uptake (EFU)
Dunipace AJ, Zhang W, McClure RJ, Stookey GK. An in vitro model for studying human dentin. J Dent Res 1992;71:200.
Faller RV. In vitro fluoride dose response below 1100 ppm F (NaF). J Dent Res 1992;71:186.
Featherstone JDB, O'Reilly MM, Shariati M, Brugler S. Enhancement of remineralization in vitro and in vivo. In: Leach SA. Factors relating to demineralisation and remineralisation of the teeth. Oxford: IRL Press Ltd, 1986;23-34.
Schemehorn BR, Farnham RL, Wood GD. Fluoride uptake and remineralization in human and bovine enamel. J Dent Res 1992;71:186.
Schemehorn B, Roberts JA, Wood GD. An in vitro remin/demin model showing a fluoride dose response. J Dent Res 1994;73:241.
White DJ. Reactivity of fluoride dentifrices with artificial caries. I. Effects on early carious lesions: Fluoride uptake, surface hardening and remineralization. Caries Res 1987;21:126-140.
White DJ. Reactivity of fluoride dentifrices with artificial caries. II. Effects on subsurface lesions: F uptake, F distribution, surface hardening and remineralization. Caries Res 1988;22:27-36.
White DJ. The application of in vitro models to research on demineralization and remineralization of the teeth. Adv Dent Res 1995;9(3):175-193.
Enamel Solubility Reduction Model: The procedure used in this model is the FDA Test #33 for the determination of the enamel solubility reduction of different products. Extracted human teeth are cleaned and exposed to a lactic acid buffer. The amount of phosphate dissolved from the teeth is quantified. The teeth are then exposed to the treatment and demineralized again. Again, the amount of phosphate in the lactic acid buffer is determined. Finally, teeth are exposed to the lactic acid buffer, and the amount of phosphate dissolved from the teeth is quantified once again. The percent of enamel solubility reduction is then computed as the difference between the amount of phosphorus in the pre- and post-treatment lactic acid solutions, divided by the amount of phosphorus in the pre-treatment solution and multiplied by 100.
Equipment: Atomic Absorption Spectrophotometer
Model: AAnalyst 200, Perkin-Elmer
Capabilities: This instrument is used to determine the total concentration of calcium in samples collected from dental specimens, bone and food and beverages.
Equipment: Microhardness Tester (3 units)
Model: MHT-230 and LM247 AT, LECO
Capabilities: Measurement of surface hardness of flattened and polished dental substrates. Equipped with Knoop and Vickers diamond tips and connected to a computer where the indentations are analyzed using an image analysis software.
Equipment: Visible Spectrophotometer
Model: Thermo Scientific GENESYS 20
Capabilities: Determination of the concentration of phosphorus in samples collected from dental specimens and beverages. Calculations are performed by a colorimetric assay based on the molybdenum reaction, at a wavelength of 650 nm.
Equipment: Hard Tissue Microtome
Model: Deluxe 1000 series
Capabilities: The microtome machine cuts extremely thin slices of materials into standardized sections. Important in science, microtomes are used in microscopy, allowing for the preparation of samples for observation under Transverse Microradiography (TMR). Microtomes use diamond wafer blades for slicing thin sections of hard tissue such as bone and teeth. Samples are sliced by moving the specimen across a recessed rotating saw. The cut thickness is > 50µm.
Equipment: X-ray Unit for Microradiography (TMR)
Model: PW1830/40, PW1327/00, PW2233/20, Philips
Capabilities: This X-ray unit is used to irradiate specimens previously submitted to experimental caries and erosion challenges. Radiographic plates with the images of the specimens can be generated and the mineral content of the lesions can be quantified using dedicated software (TMR, Inspektor Research Systems BV)