The Distribution of Occlusal Load in the Human Mandible: A Photoelastic Study

Ralph, James Patrick (1975) The Distribution of Occlusal Load in the Human Mandible: A Photoelastic Study. Master of Dental Surgery thesis, University of Glasgow.

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This study is an investigation of the distribution of occlusal load in replicas of the dentate and edentulous human mandible by the method of three dimensional photoelastic stress analysis. The study was stimulated by an interest in the occlusion of the teeth and in the relationship between disorders of the occlusion and the development of pain or dysfunction in the stomatognathic system. A further interest was in the association of faults in the design of complete dentures with pain in the oral tissues underlying the denture base. The distribution of load in any bone is related to the form and structure of the bone and to the type of loading to which the bone is normally subjected. The anatomical and biomechanical techniques which may be used to investigate these factors were developed for the study of the long bones of the limbs and in particular for the study of the human femur. The methods have been outlined, their limitations highlighted and their results compared. The general agreement of the results is quite striking. The application of these methods to the structural analysis of the human mandible has demonstrated the way in which the bone is reinforced to meet the loads imposed by masticatory function. Relatively few studies have been reported in which the hypotheses developed from anatomical investigations have been tested by other methods. Photoelastic stress analysis was selected as a method which would provide a visual demonstration of the distribution of load in the mandible and which offered the additional benefit that stresses could be studied in sections cut from the photoelastic models. As one of the interests in this study was the distribution of load in replicas of the edentulous mandible when load was applied in a number of different ways to a mandibular complete denture base, the theories of complete denture design have been reviewed, with emphasis on the relationship between the design of the dentures and the transmission of forces to the underlying tissues. The anatomy of the mandible and of the muscles of mastication and the temporo-mandibular joint has also been described. In order to relate the experimental procedures in this study to the clinical conditions under which load is applied to the mandible, it was necessary to construct a frame in which the photoelastic models could be supported in a position simulating centric relation of the mandible to the cranium. The lines of action of these muscles were determined by radiographic and cephalometric methods and a supporting frame was constructed to represent the base of skull, with metal struts aligned to correspond to the angulation of the muscles. While the shortcomings of this method were recognised, the model system provided adequate support for the models and no movement of the models or of the supporting elements was observed during the loading cycle. The principles of polarisation, birefringence and photoelasticity have been explained in detail. The methods of photoelastic stress analysis have been outlined, the preparation of models described and the techniques of stress freezing and three dimensional stress analysis presented. A detailed description has then been given of the preparation of photoelastic replicas of a dentate and of an edentulous human mandible. Weights were suspended from the dentate replicas to simulate bilateral loading in the molar region, loading in the incisor region and unilateral loading in the molar region. The same loads were applied to the edentulous mandible through the medium of a complete denture base. The distribution of load beneath an underextended denture base and of a subperiosteal implant were also studied. Isochromatic fringe patterns were recorded on the right and left halves of each replica and on sections cut from selected areas of the mandibular body and ramus. Isoclinics were studied in sections from the neck of the mandible. Fringe patterns in the hemisectioned specimens of the dentate and edentulous replicas were basically similar and indicated that stress had been generated in those areas of the models corresponding to the sites of major reinforcement of the structure of the mandible. Changes in the manner of loading were reflected in alterations in the pattern of stress distribution which suggested that greater stresses were transmitted to the condylar region when load was applied in the incisor region or when unilateral loading was used. Sections through the edentulous models suggested that when the experimental conditions were in accord with accepted principles of complete denture construction, load was distributed to the outer portion of the model, corresponding to the thickened cortical areas of the mandible. The simulation of occlusal faults and the use of an underextended denture base produced unfavourable distribution of load and provided experimental confirmation of accepted techniques of complete denture construction. A strain gauge study was then undertaken to confirm the results of the photoelastic method and a study of the optical properties of bone was also made, which suggested that a pattern of birefringence exists in bone and that it may be modified by the application of load in a manner similar to that in which fringe patterns are generated in photoelastic models.

Item Type: Thesis (Master of Dental Surgery)
Qualification Level: Doctoral
Keywords: Dentistry
Date of Award: 1975
Depositing User: Enlighten Team
Unique ID: glathesis:1975-78736
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:57
Last Modified: 30 Jan 2020 14:57

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