Four zones are clearly distinguishable, starting from the inner advancing front of the lesion. These are the (1) translucent zone, (2) dark zone, (3) body of the lesion and (4) surface layer.
Zones of enamel caries Translucent zone {TZ)
First recognizable zone of alteration
Advancing front of the lesion
Half the lesions demonstrate TZ, not always present
Seen in longitudinal ground sections in clearing (quinoline – RI – 1.62)
TZ appears structureless
Pore volume – I% (compared to 0.1 % of sound enamel)
Dark zone
Lies adjacent and superficial to the translucent zone Positive zone
Shows positive birefringence (in contrast to sound enamel.
Pore volume of 2-4% (polarized light)Presence of small pores; large molecules of quinoline are unable to penetrateMicropore system – gets filled with air and becomes dark
Medium like water may penetrate