The orientation and direction of rods in dental enamel Claudio Pinheiro Fernandes, D.D.S., M.D.S.,* and Orlando Chevitarese, D.D.S., Ph.D.** Lund University, Maim6, Sweden; and Federal University of Rio de Janeiro, Rio de Janeiro, Brazil Enamel rods are commonly accepted as perpendicularly oriented to the tooth surface and this concept has influenced the design of cavity preparations. This investigation studied the orientation and direction of enamel rods at the outer third of the enamel plate. Fifteen human molars were randomly selected and sectioned in the buccolingual, mesiodistal, and coronal directions. Angulations of the rods at the outer surface were measured by use of a protractor superimposed on the micro- graphs taken by scanning electron microscope. The mean angulation for axial surfaces was 60 degrees. The enamel rods were consistently parallel to the long axis of the tooth with 55-degree mean angulation. The conclusion was that enamel rods are not perpendicular to the surface except at the edge of the cusps and occasionally at the cervical area. (J PROSTHET DENT 1991;65:793-800.) Operative dentistry texts, replete with statements regarding tooth preparation, emphasize that enamel rods are often perpendicular to the surface of the enamel. The question that arises is whether this is an accurate statement that can be verified histologically. Several studies have demonstrated opposing views because the rods are often oriented toward the occlusal surfaces. 15 This histologic alignment of the enamel rods influences the cavity preparation for several restorative materials. *Graduate student, Department of Prosthetic Dentistry, Lund University. **Professor of Dental Materials, Research Division for Dentistry, Federal University of Rio de Janeiro. 10/1/27554 Unsupported enamel rods are considered a hazard for composite resin restorations because of their brittleness. 6-8 Composite resins are also affected by unsupported enamel because of polymerization shrinkage during the setting period. 9-11 Conversely, histologic appraisal of enamel before tooth preparation is not widely done. Perhaps the proposals of Marzouk et al. 6 regarding cavity preparations should be considered histologically supported cavosurface configu- rations. This investigation determined the orientation and di- rection of the enamel rods at the outer third of the enamel plate of axially and coronally sectioned molars. MATERIAL AND METHODS Fifteen permanent, intact human maxillary and man- dibular molars were randomly divided into three groups. Group I. Five teeth were axially sectioned buccolingual- ly, parallel to the long axis of the tooth with a carborundum Fig. 1. Buccolingual section at light microscope (original magnification ×30), framed areas indicate location of SEM micrographs. Fig. 2. Mesiodistal section at light microscope (original magnification )<30), framed areas indicate location of SEM micrographs. THE JOURNAL OF PROSTHETIC DENTISTRY 793