Gradients of biochemical volumes in normal enamel in relation to enamel tufts

 Increasing attention has been paid to the graded mechanical properties of dental enamel, with decreasing hardness and elastic modulus and increasing toughness from outer to inner enamel (Bajaj et al., Biomaterials 2009; 30: 4037-4046; Barani et al., J Mech Behav Biomed Mater, 2012; 15:121-130)). Spatially-resolved quantitative data on the biochemical volumes from outer to inner enamel were lacking. A recent study, exploring microradiography and interpretation of enamel birefringence, reported this later data, showing decreasing mineral volume and increasing water and organic volumes from outer to inner normal enamel of human permanent teeth (Macena et al., Archives of Oral Biology, 2014, http://dx.doi.org/10.1016/j.archoralbio.2014.03.001). Mineral volume ranged from 70.6 to 98.5%, while organic volume ranged from 0.02 to 20.87%, and water volume ranged from 3.8 to 9.8%.

Figure 1. Microradiography of ground section of normal enamel showing tufts as radiolucent structures. From enamel surface to enamel-dentine junction, mineral volume decreases and water and organic volumes increase (Macena et al., Arch Oral Biol, 2014, http://dx.doi.org/10.1016/j.archoralbio.2014.03.001), creating a osmotic gradient for water transport. Enamel layer is divided into tuft area and non-tuft area, which differed markedly with regard to biochemical volumes.

     Organic and water volume are the highest ever reported for fully formed normal enamel, and indicate an osmotic gradient from outer to inner enamel. The location of the increased organica volume coincides with the organic-rich inner enamel layer shown by BSE-SEM by Dusevich et al. (Arch Oral Biol, 2012; 57:1585-1594). This is consistent with the early report that water diffuses to inner enamel following an osmotic pressure that vanishes after removal of organic matter (Atkinson, British Dental Journal, 1947, 83: 205-214). This confirms recent report that, during dehydration at room temperature, outer enamel reaches equilibrium moisture content earlier than inner enamel (Medeiros et al., J Microsc, 2013, 250: 218-227). Macena et al. also report that after heating dried enamel to 50º C and 50% of relative humidity, inner enamel rehydrated after being exposed to 25ºC and 50% RH, evidencing the osmoti pressure exerted by the graded biochemical volumes.

    The implications for transport of materials in dental enamel is that small molecules applied to the enamel surface  are expected to follow the water flow towards inner enamel. 

     Biochemical volumes were measured in relation enamel tufts. Imaginary lines were drawn from outer to inner enamel (see Fig. 1) and histological points along those lines were selected for measuring biochemicla volumes. In each ground sections, two lines were traced: one crossing a tuft (tuft line) and another (adjacent) crossing an area without tuft (control line).  Each line extended from the area where tufts could be found (tuft area; close to the EDJ, Fig 1) to the region where there were no tufts (non-tuft area). No differences were found between tuft line and control line, but the tuft areas had biochemical volumes markedly different (lower mineral volume and higher water and organic volumes) from those of the non-tuft areas. Tuft presented the same behavior of lamellae, which are crack lines lacking birefringence in ground sections. Thus, this evidence indicates that tufts are also crack lines.