Cavity is a defect in hard dental tissues resulting from caries or trauma.
Preparation is dissection of defective (e.g. weak hypoplastic enamel), injured (e.g. thin sharp edges in case of partial tooth cracking) or diseased (e.g. caries) tooth structure to prepare a space for restorative material that will reestablish dental function and esthetic appearance.
Principles of tooth preparation could be different; they depend on restorative material to be placed. It might be divided into two types.
► Conventional approach to preparation means to prepare the cavity in specific retentive form. The approach is used for amalgam fillings, mineral cements. These materials have no adhesion or chemical connection with tooth structure; so to keep it in cavity (to prevent dropping out) we need to prep to a special design (90 degrees walls, additional extensions, boxlike cavity shape, etc.). The walls should be smooth because these materials also have no good adaptation, and in case of any irregularities on walls after placement of, for instance, amalgam there will be pores in it, as hard amalgam will not come into irregularities.
► Modified approach to preparation for bonded restoration (composite, compomers and glassionomers) has less need for retentive form because of adhesion/chemical bond to tooth structure. The walls also can have some concavities because adaptation of this material is much better than in amalgam or mineral cements, and it will fill irregularities better (especially flowable material).
It does not mean that in case of bonded restoration we can totally give up the idea of retentive cavity preparation because strength of adhesion alone does not suffice to withstand occlusal loads (especially horizontal loads), and the filling may drop out after some time. Thus in working with bonded restorations we combine two approaches. Dental adhesives enable clinicians to generate more conservative tooth preparations because (1) most of the retention is obtained through adhesion and micromechanical retention, (2) resistance form