Three-dimensional Photoelastic Investigations of Bite Stresses on the Abutment Tooth, the Alveolar Socket and Plate-supporting Alveolar Ridge Bones Part.2

Abstract

The present study is concerned with a consolidated analysis through the photoelastic method of mechanical properties of physical stresses which are responsible for pathohystological changes on the abutment tooth, alveolar socket and plate-supporting alveolar ridge bones due to bite stresses when a partial denture is inserted in a patient. By the use of Epoxy resin B hardener 901 product of Ciba Company, Switzerland, the author constructed through a casting method of his own design study models which would fit intercalary denture (56 missing), one-side free-end denture (76 missing), as well as both side free-end denture (76 67 missing). On these models there were constructed one-piece cast denture bases and clasps of platinum-gold having an elastic coefficient of 12, 300Kg/mm2, on which dentures were completed with artificial porcelain teeth and resin gingiva. On the other hand, the upper jaws were made of hard plaster on which were planted artificial teeth made of cobalt-chrome alloy having an elastic coefficient of 12, 000Kg/mm2. When the upper and lower jaws were firmly mounted on the articulator in the position of centric and lateral occlusion, bite load was administered to them while they were placed in the stress freezing apparatus. Then, sections were prepared of the jaws and a three dimensional photoelastic analysis was conducted on them by means of photographic fringe method. As a result of these experiments, the following findings came to light.1) As regards bite load working on the abutment teeth, a stress concentration on a load point on the tooth crown, and a stress distribution on the tooth surface except the load point were observed. With reference to the roots and alveolar socket bones, stresses were dispersed by the stress braking action of the periodontium. Therefore, under these conditions the uniform load was obtained which gave little fatigue on the abutment teeth.2) When bite load was exerted on the single-rooted abutment teeth in the axial direction, there was produced an equilibrium between a load point on the crown and the root apex, and thus stresses were uniformly distributed all over the surface of root and alveolar socket bone. In case more than two stresses in different directions acted on one another and consequently in balance, the working center of force took place inside the dental substance of the root portion. The posion of the working center of force varied in terms of the direction, angle and size of stresses. As the working center of force acted as a fulcrum of inclination of the abutment tooth, a compression stress was distributed locally on the surface of the abutment tooth as well as on the alveolar socket bone surface in the inclined direction of the tooth.3) When bite load was exerted on the multirooted abutment teeth in the axial direction of the one root, an equilibrium was produced between a load point on the crown and the apex of the root. Moreover, when more than two loads were experted in different directions and they were in balance, the working center of force was produced inside the dental substance of the crown center portion. On the other hand, when more than two loads working in different directions were out of balance, the working center of force tended to be located inside the dental substance of the side portion of the crown on which a stronger stress acted, a compression stress being distributed locally on the surface of multiroots in the direction of that stress and the opposing surfaces of alveolar sockets and the inter-alveolar septum.4) As compared with the single-rooted abutment teeth, the multirooted abutment teeth had a larger surface to bear a stress because of inter-alveolar septum and, for this reason, a load on the alveolar socket bone becomes less. Also the resistance against torsion was correspondingly large.