How far does an E2 interproximal caries penetrate the enamel?

• A. Less than a quarter of enamel
• B. More than half the enamel thickness
• C. Exactly half the enamel thickness
• D. Through the enamel into dentin

Answer: (B)

E2 is about how far the lesion has progressed within enamel. It means the caries has reached the inner half of the enamel, i.e., more than half of the enamel thickness, but has not yet crossed the dentinoenamel junction into dentin. This distinguishes it from E1, which is in the outer enamel (less than half), and from lesions that have entered dentin (which would be classified as D1, D2, etc.). So the best description is that the decay extends more than halfway through the enamel.