Knoop Hardness Number of enamel

 # Knoop Hardness Number (KHN) of enamel is:
A. 15
B. 55
C. 25-35
D. 350-430

The correct answer is D. 350-430 

The Knoop hardness test employs a diamond-tipped tool. The impression is rhombic in outline, and the length of the largest diagonal is measured. The load is divided by the projected area to give the Knoop hardness number (usually abbreviated as HK or KHN). 

Knoop hardness number of dentin is 68 and that of enamel is 350-459. Unfilled acrylic has KHN of 15 whereas hybrid composites have a KHN of 55-60. 

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