Effect of a resilient layer in a removable partial denture base on stress distribution to the mandible Ergun Aydinlik, D.D.S., M.S., Ph.D.,* and Hasan U. Akay, M. SC., Ph.D. ** Hacettepe University, Fat ulty of Dentistry, and hliddle East Technical University, Ankara, Turke\ L ocal and systemic factors contribute to bone remodeling. Among the local factors, pressure is of great significance. According to Beerstecher and Bell,’ when excessive pressure interferes with blood circulation in the mucosa and alveolar bone, osteo- cytes cannot supply sufficient oxygen. Thus, the pentose cycle or phosphogluconate shunt in carbohy- drate metabolism produces oxygen to a limited extent. However, some acids are also produced as a result of these metabolic reactions; and one of these is citric acid, which induces calcium to dissolve and be mobilized. Hence, a decalcification process occurs on the bone surface; as this process continues, loss of bone which is initially reversible. becomes irreversi- ble. Resilient dent ure base materials have been recom- mended to prevent the excessive pressure on residual alveolar ridges and to provide a more uniform load distribution by absorbing the “mechanical shock” caused by masticatory forces.‘-* Some investigators have constructed removable dentures containing resilient materials as a layer between the artificial teeth and the denture base.“-” The purpose of this study was to investigate the effect of a resilient layer in a denture base on stress distribution to the mandibular bone. For this, a two-dimensional finite element model” of the man- dible was constructed. Stress analyses both with and without the presence of the resilient layer were performed. MATERIALS AND METHODS Stresses developed in various dental materials, teeth, and jaws have been studied by photoelastic and electronic strain gauge techniques. More recent- ly, however, a new method of analysis, namely the *Associate Professor, Department of Prosthodontics. “Associate Professor, Department of Civil Engmeering 0022-3913/80/070017 + 04$00.4ll/O6 1980 The C V Mosby Co finite element method,” has been successfully used in dental research.“-“’ A two-dimensional model which simulates a man- dible with missing posterior teeth was used for the investigation (Fig. 1). A corresponding two-dimen- sional finite element mesh was constructed for the mandible with a removable partial denture (Fig. 2). The mesh consisted of 238 elements of triangles and quadrilaterals with a total of 265 nodal points. The triangles are the constant stress elements, whereas the quadrilaterals are the bilinear displacement elements with incompatible bending modes. A wide- ly known, general purpose finite element computer program, the Structural Analysis Program for Static and Dynamic Response of Linear Systems, called SAPI\‘“’ was used for the analyses. Plane stress elasticity conditions were assumed, and the nodal points on the boundaries of the mandibular base and ramus were prevented from movement in .r and-y directions (Fig. 2). A vertical concentrated masticatory force wit!? a magnitude of 25 kg was applied to the denture (Fig. 2). The properties of the materials used for the mandible, denture base (Acryl), and resilient layer (silicone) are given in Table I. r-\nalyses were performed both with and wlthout the presence of the resilient layer in the denture base so that a direct comparison of the results could be obtained. RESULTS AND DISCUSSION Th e principal stress distribution developed within the mandibular bone as a result of the occlusal loading of the conventional removable partial den- ture base is shown in Fig. 3. The principal stresses are plotted vectorially, using the scale indicated on the figure. The single hes represent the compressive stresses and the double lines represent the tensile stresses. Stresses with magnitudes less than 2 kg/cm’ THE JOURNAL OF PROSTHETIC DENTISTRY 17