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Photoelastic studies in the edentulous human mandible* James P. Ralph,
BDS, FDS, HDD
Department of Dental Prosthetics, University of Glasgow Dental Hospital and School ABSTRACT
Replicas of an edentulous human mandible were made in a photoelastic resin material. They were suspended from a supporting frame by means of struts, representing the principal muscles of mastication, and were subjected to loading through the medium of a complete denture base. The stresses generated within the models were examined by the method of three-dimensional photoelastic stress analysis and were related to the anatomical structure of the mandible and to the design of the denture base and the way in which it was
FIRST STUDY Materials and methods An edentulous human mandible with a wellformed alveolar process and areas of recent tooth loss was selected. A mould of the mandible was prepared using silicone?, and a
INTRODUCTION PHOTOELASTIC stress analysis depends on the property, which many transparent materials possess, of demonstrating internal stresses when subjected to load and viewed in polarized light (Hendry, 1966). This technique has been used in dentistry to study internal stresses in the periodontium (Glickman et al., 1970) and in cavity preparations, dental restorations and materials (Lehman and Hampson, 1962; Craig et al., 1967; Caputo et al., 1973). In a recent study (Ralph and Caputo, 1975) photoelastic replicas of the dentate mandible were used to demonstrate the stress fields which developed under simulated occlusal loads. The aim of the present study was to examine the stresses generated in replicas of the edentulous mandible, when subjected to loading through the medium of a complete denture base.
Fig. I.-Photoelastic human mandible.
of an edentulous
replica was made in a photoelestic resin material: (Fig. I). The model was annealed to
eliminate any stresses formed during polymerization and was examined in polarized light to ensure that it was free of stress before any load was applied. *Presented at the Annual Conference of the British Society for the Study of Prosthetic Dentistry in April 1974. jSilcoset 105; ICI, Stevenston, Ayrshire. *Araldite CT200; Ciba Geigy (UK) Ltd.
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the canine and first molar regions, was constructed in cobalt-chromium alloy, lined with silicone* and fitted to the photoelastic model. Occlusal loading was simulated by suspending weights from a metal frame which was positioned over all the vertical posts on the denture base (Fig. 2). The total load was 500 g. The stresses which developed in the model under these conditions were permanently ‘frozen ’ into the structure by subjecting the material to its stress-freezing cycle while the load was maintained. The model was then sectioned in the midline, and each of the hemimandibular specimens was immersed in a fluid of comparable refractive index? and viewed in the field of a circular polariscope. A series of transverse and longitudinal sections was then prepared and examined in the same manner. Isochromatic stress patterns were recorded photographically in colour and in black and white (Rg. 3). Results Fig.
2.-Application of occlusal load to mandibular replica.
Specimen 1 The model, identified as specimen 1, was subjected to a load of 500 g distributed over
3.-Diagrammatic arrangement of polariscope.
The model was fitted into metal copings which had been cast to cover the coronoid processes and angles of the mandible and which also provided attachment for struts representing the masseter, medial pterygoid and temporalis muscles. These struts were used to suspend the mandible from a supporting frame and to simulate a condition of isometric contraction of the muscles. The condyles were covered by a layer of silicone* and fitted into articular fossae, which had been duplicated from the skull base and processed in acrylic resin. A denture base, with vertical posts in
the denture base area. The fringes which developed in response to this load formed a number of well-defined trajectories in the hemimandibular specimens (Fig. 4) : 1. Along the inferior border and up the posterior aspect of the ramus to the condyle. 2. From below the alveolar process, obliquely upwards through the ramus to the condyle. *Xantopren Blue; Bayer, West Germany. tOi1 of Cedar Wood (microscope immersion BDH Chemicals Ltd, Poole, Dorset.
Studies in the Mandible
3. From below the alveolar process, up the anterior border of the ramus to the coronoid process. 4. Between the condyle and the coronoid process. There was considerable accumulation of stress in the area underlying the denture base and also in the region of the mandibular angle, where the coping, providing attachment for the pterygomasseteric siing, was located. This
stress (Fig. 5). Longitudinal sections were cut from the left half of the model, just above the inferior border and just below the alveolar
Fig. I.-Specimen 1. Right side, medial aspect. Stress fields developed under uniform occlusal load.
Fig. 5.-Specimen 1. Transverse section, molar region, indicating cortical distribution of stress.
Fig. 6.-Specimen 1. Longitudinal section, inferior border, demonstrating Specimen supported on the buccal surface, viewed from superior aspect.
pattern was demonstrated on both halves of the model and was similar whether the specimen was viewed from the lateral or medial aspect. Transverse sections were cut from the right half of this model, in the midline, in the region of the mental foramen and in the area of the These demonstrated the molar sockets. development of fringes around the cortical area of the specimens, with minimal internal
cortical stress distribution.
crest. These also demonstrated stress concentration in the cortical areas of the specimen (Fig. 6).
DISCUSSION The stress fields which developed within the model and which were demonstrated in the hemi-sectioned specimens showed a strong similarity to the trajectories which have been
described in the mandible on the basis of examination of the external and internal structure of the bone (Scott and Symons, 1967; Sicher and Du Brul, 1970). They were also similar to the trajectories described by Seipel
areas where the bone itself demonstrates structural reinforcement. The transverse and longitudinal sections showed that, under these exnerimental conditions, the occlusal load was transmitted through the denture base and
in the edentulous mandible, redrawn after Seipel (1948).
Fig. 8.-Radiograph of the mandible demonstrating cortical reinforcement.
distributed throughout the cortical region of the model which corresponds to the thickened cortical layers of the mandible (Fig. 8).
SECOND Fig. 9.-Specimen
2. Right side, medial aspect. Stress fields developed under occlusal load on underextended denture base.
(1948), who used the impregnation technique of Benninghoff (1925), supplemented by low power microdissection and histological examination of the cortical layers of the mandible (Fig. 7).
Stresses in the photoelastic replica appeared to be concentrated in precisely those areas where the cortical structure of the mandible is reinforced: along the inferior border and at the symphysis, along the external and internal oblique ridges, at the angles and up the anterior and posterior borders of the ascending ramus. Thus, although the model is a homogeneous structure it appears to develop stress in the
The behaviour of the resin model in this experiment corresponded well with what might have been predicted from a knowledge of the anatomy of the mandible and its related structures and from the design of the denture base and the way in which it was loaded. It was therefore decided to conduct a further study in order to demonstrate : 1. The effect of modification of denture design on stress distribution. 2. The effect of locating the load predominantly in the anterior region.
Methods Two further replicas of the mandible were produced and were loaded as follows : 1. Load of 500 g distributed over all vertical posts on a reduced denture base. 2. Load of 500 g positioned over both anterior posts.
Studies in the Mandible
Results Specimen 2 This model was subjected to a load of 500 g distributed uniformly over the denture base area. The denture base in this instance was of
tip of the coronoid process. There was minimal fringe formation in the posterior aspect of the ramus. Cross-sections showed the development of internal stresses, particularly in the anterior region (Fig. 22).
3. Left side, medial aspect. Stress fields developed under occlusal load concentrated in the anterior region.
Fig. IO.--Specimen 2. Transverse section, premolar region, demonstrating fringe formation in the alveolar process.
reduced extent, covering only the residual alveolar process. The her§ioned specimen (Fig. 9) demonstrated stress fields basically similar to those described in the first model, though it appeared that there was a greater concentration of stresses in the alveolar region, especially around the molar sockets. This impression was confirmed by examination of the transverse sections (Fig. IO), which all demonstrated the development of fringes below the alveolar process and extending internally into the specimen. Specimen 3 This model was subjected to a load of 500 g applied to the two canine posts on the fully extended denture base. The fringes which developed in the hemi-sectioned specimens were grouped into well-defined trajectories (Fig. II), running from the inferior border in the anterior region, obliquely upwards through the body and ramus to the condyle and to the
Fig. 12.~-Specimen 3. Transverse section, anterior development of internal region, demonstrating stress.
is a common
fault in the construction of mandibular complete dentures and is frequently associated with discomfort and with trauma to the soft tissues overlying the alveolar process. Occlusal faults are another common finding and may cause discomfort by preventing the uniform
distribution of load to the area of the denture base. In the second study an attempt was made to simulate these clinical errors by altering the design of the denture and by changing the point of application of the occlusal load. The pattern of fringe formation in the hem§ioned specimens was similar to that demonstrated in the first study. Stresses were again concentrated in the parts of the models which corresponded to the areas of major structural reinforcement of the mandible. Both modifications, however, led to the development within the models of internal stresses, which could be seen when the transverse sections were examined. Load had been transferred from the denture bases mainly to the alveolar portion of the models. This is an area of the edentulous mandible where structural reinforcement is lacking and the application of load is not well tolerated. Comparison with the findings in the first study emphasizes the importance of adequate peripheral extension and occlusal balance in distributing the occlusal load to the cortical areas of the model, which correspond to the thickened cortical layers of the mandible, and reinforces the need for attention to these two features in the design and construction of mandibular complete dentures.
Acknowledgements The author wishes to thank Professor A. R. MacGregor, Department of Dental Prosthetics, University of Glasgow Dental Hospital and School, for his advice and encouragement during the course of this study; Dr I. MacDuff, Department of Mechanics of Materials, University of Strathclyde, for the use of laboratory facilities; Mr R. B. Paul, Instructor Technician,
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Department of Dental Prosthetics, University of Glasgow Dental Hospital and School, for his help in the production of silicone moulds and denture bases; Mr I. Stevens, Chief Technician, Department of Mechanics of Materials, University of Strathclyde, for his help in the preparation of resin models and supporting frames; Mr J. B. Davies and the staff of the De,partment of Medical Illustration, University of Glasgow Dental Hospital and School, who produced the illustrations.
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