Structural Deformation and Fracture Analyses of Complete Dentures

Part 4. Dynamic Movements in a Complete Lower Denture

Abstract

Structural deformation and fracture analyses of complete dentures were made by use of a strain gauge. A variety of methods-brittle coating, photoelastic, strain gauge, finite element, and laser holography interference have been used to analyze dynamic movements in prostheses. Since strain gauge is very accurate and demonstrates linearity in stress vs. load, such gauges were used in these experiments.
Preliminary experiments were designed to study principal strain, maximum shear strain, principal stress, maximum shear stress, and surface strain at breaking point in experimental bite rim models. A fracture test for breaking strength in the experimental bite rim model was first carried out breaking occurred at a 127 kgf concentrated load. Next, rosette analysis of this model was performed using rosette gauges. Rosette analysis was done to determine various strain/stress parameters at the breaking point. Larger strain-stress dynamic movements appeared in the palatal than in the labio and buccal regions, with the largest deformation occurring in the molar and posterior palatal part upon median sagittal line. Rosette gauges were also concentrically arranged on the palatal area was done. The principal strain and stress were observed in the posterior palatal part of the median sagittal line; tensile strain and stress were much the same at right angles to the median sagittal line. Maximum shear strain and stress were observed in the same area on a large at the breaking point scale. Observations were made with strain gauges on surface strain along a single axis; large tensile strain was observed in the posterior palatal part of the median sagittal line.
Following the preliminary experiments using bite rim models, experiments were done using complete dentures. Firstly fracture testing at centric occlusion was done using uniformly distributed and concentrated loads. After this step, principal strain, maximum shear strain, principal stress, maximum shear stress, and surface strain at breaking point were measured and analyzed under a breaking load. Principal strain and stress were observed in premolar and posterior palatal area along the median sagittal line; tensile strain and stress were much the same right angles to the median sagittal line. And the maximum shear strain and stress were observed in the same areas on a larger scale. When a concentrated load was used, principal strain, maximum shear strain, principal stress, and maximum shear stress were also observed mainly in the molar and posterior palatal areas along the median sagittal line. The complete denture behaves like a shell structure which is subject to tensile stress and shear stress according to the membrane theory. It was concluded that deformation and fracture result from tensile and shear stress working in the median palatal region. Following rosette analysis, experiments were designed to measure the surface strain at breaking point by concentrated stress strain gauges using both uniformly distributed and concentrated loads. Dynamic movements in the posterior palatal region appeared to result from large tensile strain. It was concluded that initial destruction begins in this area. The tensile strain observed at the upper labial frenulum notch is thought to be a result of this palatal region tensile strain.
Evidence has been presented:
1) that deformation and fracture of complete upper dentures appears in the median palatal region
2) that initial destruction was caused by tensile stress and shear stress, and
3) that deformation and fracture progresses from the posterior palatal region foward to the upper labial frenulum notch.