Relationships between birefringence and mineral content in artificial caries lesions of enamel

The microradiographic mineral content and birefringence in water and Thoulet's solution were measured at selected points in sections of caries-like lesions. Birefringence was not related to mineral content in sound superficial enamel immersed in Thoulet's solution or in the lesion body immersed in water. For the surface layer of the lesion, birefringence in water could be used to obtain qualitative information about mineral content. For the lesion body, birefringence in Thoulet's solution was linearly related to mineral content and can be used to estimate the latter with a standard error of about 7 vol%. The intrinsic birefringence for the lesion body was estimated as -34.5 (+/- 2.3) x 10(-4); This value may also be valid for other parts of the lesion. From consideration of ionic sizes, it is argued that Thoulet's solution cannot necessarily penetrate all enamel pores accessible to water. It appeared that there is a progressive fall in the content of pores inaccessible to Thoulet's solution in the surface layer of the lesion as demineralisation increases.