THREE DIMENSIONAL CHARACTERIZATION OF INDENTATION IMPRESSIONS ON HUMAN ENAMEL     Guangming Zhang, Stanley J. Ng, Dung T. Le, and Shelly R. Tucker Department of Mechanical Engineering & the Institute for Systems Research University of Maryland College Park, Maryland 20742   Dianne E. Rekow Department of Orthodontics New Jersey Dental School Newark, New Jersey 07103      
  
Abstract
   This paper presents results from investigating indentation impressions on human enamel under micro-hardness tests. The experiments of hardness testing were performed on a microhardness indentation machine under different loading conditions. Images of indentation impressions were obtained using an environmental scanning electron microscope. Geometrical shapes of hardness indentations were visualized in three-dimensional space using computer graphics. Quantitative information was obtained through atomic force measurements to characterize "pile-up", "sink-in", and elastic recovery of enamel. Special efforts have been made to study the microstrucutual effect of the calcified rods orientations on the fracture patterns formed during the hardness tests. Significant findings include that the occlusal surface demonstrates much stronger resistance to the indentation force than does the buccal surface and shows 40% elastic recovery after indentation. A new formula to determine hardness value has been proposed. By incorporating the reversible deformation into the evluation, a normalized hardness measurement can be made to form a basis for comparison and other investigations where hardness has its unique role to play.