Dehydration of normal enamel: new paper (Medeiros et al., J Microsc 2013)

          What is the parameter to define air dried enamel at room temperature? It is reasonable to say that it is when water content is stabilized over time at a certain temperature and relative humidity of the air.  This subject is addressed in a recent paper (Medeiros et al., Journal of Microscopy, 250, 2013, DOI: 10.1111/jmi.12037; http://onlinelibrary.wiley.com/doi/10.1111/jmi.12037/abstract) that explores enamel birefringence to obtain quantitative volumetric data of water loss at normal enamel histological points during air drying. The paper analyzes ground sections of normal enamel and reports an inward water diffusion, driven by osmotic pressure of a protein gradient, following prisms paths, similar to what was reported by Atkinson (British Dental Journal, 83: 205-214, 1947).  The main findings are:

- during air drying at 25 C and relative humidity of 50%, normal enamel looses water from the outer to the inner layers, following prisms paths;
- loosely bound water is lost until a plateau is reached, i.e. when an equilibrium moisture content is established;
- the equilibrium moisture content at the histological points along the enamel thickness is reached following an apparent diffusion rate (median) of 3.47 x10 -8 cm2s-1;
- outer enamel reaches equilibrium moisture content earlier than inner enamel;
- equilibrium moisture content is achieved after 40 h and 90 h for depths of 1 mm and 2 mm in enamel, respectively;  
- due to the lower water volume in dried enamel, and a corresponding lower ratio of water to organic matter, a lower diffusion rate for materials is expected in dried enamel compared to wet enamel;
- the inward water diffusion (related to the osmotic pressure exerted by an increasing protein gradient from the outer to the inner layers) has implications in the transport of materials in enamel (including acid, bleaching agents, etc), and in the optical and mechanical properties;
- the observation of such phenomenon in ground sections is explained by the fact that prisms are partially surrounded by the prisms sheaths (the main pathways for transport of materials in enamel), which do not connect each other in cross section, and that intraprismatic pores house the least mobile water of enamel due to a higher capillary force compared to that in the prisms sheaths (which are larger pores);
- the paper describes a method using the combination of a lambda filter plus a quarter wave filter (yielding a retardance of 680 nm) to detect the sequence of qualitative changes in enamel birefringence during air drying of ground sections;
- it is now open the possibility of studying the role of this inward water diffusion in the transport of materials in normal enamel.