Viscoelastic properties of the periodontal ligament and mucous membrane

Post-extraction remodeling of the adult mandible

Following tooth loss, the mandible shows an extensive loss of bone in some individuals. This may pose a significant problem in the prosthodontic restoration of function and esthetics. The many factors which have been proposed as being responsible for the inter-individual variation in post-extraction remodeling mean that a perfunctory analysis of the literature, in which well-controlled, relevant studies are scarce, may not provide the whole story. This article reviews the local and systemic factors which may play a role in the post-extraction remodeling of the mandible. Since severe residual ridge resorption may occur even when the bone status in the rest of the skeleton is good and vice versa, it is concluded that local functional factors are of paramount significance. It is now essential to determine how they can be modified and applied to help maintain ridge height and quality in our aging, edentulous population.

Displacement of a mobile tooth during impression procedure: effect of impression tray design

When fabricating prostheses in a partially edentulous arch with some mobile teeth, it is important to minimize tooth displacement during the impression procedure. The present study examined the effect of custom tray designs on the displacement of a mobile central incisor in a mandibular simulation model with six anterior teeth. Twelve custom trays were designed according to the different spaces and perforated holes in the region of the remaining teeth. As a result, the degree of displacement of the mobile tooth could be reduced according to the size of spaces and the location of perforated holes in the custom tray.

Considerations for modeling

There has been a great growth in the biophysical knowledge base of the human masticatory system, which increases the chances of success in numerical and experimental modeling of the oral environment. The chief advances in understanding are in the areas of the dentino-enamel complex, the force-movement cycle of the mandible, mastication, comminution and salivary lubrication. These areas are reviewed, and suggested average clinical conditions are offered as a starting point for experiments in laboratory clinical simulation as it relates to the studies in biomaterials and biomechanics in dental research.

Experimental occlusal interferences. Part III. Mandibular rotations induced by a rigid interference

A rigid intercuspal interference (minimum mean height of 0.24 mm) was placed on either the right or left mandibular second premolar and first molar of 12 subjects. During brisk and forceful biting on the interference, rotational electrognathography measured maximum torque of the right and left mandibular condyles in the frontal and horizontal planes of orientation. All subjects showed frontal plan upward rotation (mean of 0.7 degrees) of the mandibular condyle contralateral to the interference. In 33% of the subjects there was no horizontal plane backward rotation. In 58% of the subjects there was horizontal plane backward rotation (mean of 0.5 degrees) of the mandibular condyle ipsilateral to the interference, and in one subject (8%) there was backward horizontal plane rotation (0.1 degree) of the mandibular condyle contralateral to the interference. It was inferred that the masseter muscle, ipsilateral to the interference, generated negative work in order to decelerate frontal plane 'unseating' of the mandibular condyle ipsilateral to the interference. It was inferred that the masseter muscle, contralateral to the interference, produced positive work in order to accelerate frontal plane 'seating' of the mandibular condyle contralateral to the interference. Finally, it was speculated that the impact forces of frontal plane 'seating' of the mandibular condyle, contralateral to the interference, might lead to 'vacuum sticking' of the temporomandibular joint disc because of the formation of negative hydrostatic pressures.

Ultrastructural morphology of vascular endothelial junctions in periodontal ligament

The TEM was used to categorize vessels and their junctions in normal and tensioned rat maxillary molar periodontal ligament. In tensioned periodontal ligament mean luminal diameters of capillaries were significantly smaller (p < 0.001). Goniometer tilting of sections with apparent tight regions revealed that only 16 per cent were actual tight junctions. The other regions proved to be close junctions (85 per cent) and open junctions (4 per cent). No gap junctions were found. These findings establish that morphologically the periodontal ligament contains a microvascular bed of 'leaky' endothelium with a potentially high permeability factor.

Histomorphometric study of the periodontal vasculature of the rat incisor

This study assessed quantitatively the vascular system in the cementum-related periodontal ligament (PDL) along the rat incisor. The lower left incisors of six rats (+/- 200 g) were subjected to routine histological procedures and cross-sectioned serially (2 microns), and the distance between each section and the apex was computed. The PDL of five sections at different levels along the tooth was divided into mesial, lingual, and lateral parts. The number and area of small and terminal arterioles, capillaries (C), sinusoids (S), post-capillary venules (PCV), and connecting venules, as well as the area of the PDL, were established. Blood vessels (BV) occupied 47 +/- 2% of the PDL area in the apical half and 4 +/- 2% at the incisal end. Of the total BV area, 41%, 32%, and 27% were located on the lingual, mesial, and lateral tooth sides, respectively. The majority of BV belonged to the venous system (98.5 +/- 0.6% and 82.5 +/- 3.0% in the apical and incisal parts, respectively). The apical venous system comprised 95.4 +/- 1.6% S and 3.2 +/- 1.0% PCV, reversing to 27.2 +/- 14.2% S and 55.2 +/- 11.3% PCV in the incisal half. The number of arterial profiles increased gradually from 6.8 +/- 1.5 at the apex to 25.3 +/- 2.4 in the incisal part and that of C from 9.0 +/- 1.18 to 25.0 +/- 4.3. The extensive vascularization in the apical half of the PDL is consistent with the high metabolic demands and with the need for protective cushioning of the constantly growing dental and periodontal tissues.2+_

Occlusal force distribution in lower complete overdentures

A space between the denture base and a root cap can effectively control the distribution ratio of occlusal force to abutment teeth and an alveolar ridge. The purpose of this study on abutments was to analyse the effect of variation in space (0 mm (no space), 0.3 mm and 0.6 mm) on the above ratio. Six subjects, 38-65 years of age, each with an edentulous maxilla and several teeth remaining in the mandible, were selected for this experiment. The lower experimental denture had embedded a transducer which could detect a change in the vertical force applied to the experimental tooth. This transducer was capable of changing the vertical space between a denture base and an experimental tooth. Each subject was asked to increase the occlusal force applied to the denture from zero to the maximum loading rate of 5 kgf s-1. The occlusal force and the force exerted on the experimental tooth were recorded in each space 1 month after insertion of new dentures. The following results were obtained: (i) when the occlusal force was applied to the artificial tooth just above the abutment tooth, the mean ratio in the 0 mm space was 60% of the force applied to the denture, the ratio in the 0.3 mm space was 50%, and the ratio in the 0.6 mm space was 30%; (ii) if the occlusal force was applied to the point 10 mm distant from the point just above the experimental tooth, the magnitude of the ratio was decreased by 60-80%.

Measurement of the uniaxial elasticity of oral mucosa in vivo after CO2-laser evaporation and surgical excision

The stress-strain relation of oral mucosa of dogs was measured before and 6 weeks after surgical removal of mucosa. Both CO2-laser evaporation and excision were employed. Measurements were done with a miniature tensile tester, especially developed for this purpose. The load-strain ratio of the healed mucosa was proportional to the histologically determined thickness of the healed epithelium and scar tissue together. Laser evaporation caused a 75% increase, excision almost a threefold increase of the load-strain ratio compared with untreated tissue.

The intrusive mobility of the incisor tooth of the guinea pig

The pattern of mobility of the mandibular incisor with intrusive loads was consistent with the view that the periodontal tissues are viscoelastic. A significant time-dependency of the response was observed. The pattern of mobility was similar to that reported for axially directed extrusive loads and for horizontal loads.

Location of the centers of resistance for anterior teeth during retraction using the laser reflection technique

The location of the centers of resistance for various symmetric units of the anterior maxillary dentition for a lingually directed force was studied in two dry human skulls. The units investigated were composed of two incisors, four incisors, and six anterior teeth. In addition, the effect of change in force magnitude on the location of the centers of resistance of these units was investigated. The laser reflection technique was used to study both the direction and magnitude of the initial displacement of the consolidated teeth under loading. The results indicated that the center of resistance shifted apically with the incorporation of a greater number of teeth into an anterior segment. With a unit of six anterior teeth, the apical shift of the center of resistance was the greatest. Increasing force levels had little effect on the location of the center of resistance of a given unit. This phenomenon was observed in both the skulls tested, suggesting that general trends may exist in the displacement characteristics of the dentition when subject to controlled force systems.

A transmission electron-microscope stereological study of the blood vessels, oxytalan fibres and nerves of mouse-molar periodontal ligament

Mandibular, mesiobuccal ligament was sectioned from the alveolar crest to the apex at predetermined levels. Data collected using standard point counting procedures was analysed for main effects due to animal, side of mouth, depth and zone across the ligament. Statistically-significant variations only occurred between different lateral thirds with the middle third containing the most oxytalan fibres and the bone third the greatest vascular and neural volumes. Stereology showed that the ligament was 3-7 times more vascular than other connective tissues. Eighty-eight per cent of the periodontal blood volume was enclosed in vessels with a mean lumenal diameter of 20.9 microns and characterized by a thin endothelial lining and few perivascular cells. These vessels had a surface density of 125.1 cm2/cm3. Oxytalan fibres had a length density of 1258 X 10(3) cm/cm3 and a mean caliper diameter of 0.7 micron. Furthermore, 78 per cent of fibres were adjacent to cells, 14 per cent within principal collagen fibres and 8 per cent in the walls of blood vessels. The length density of nerves within the ligament was 255.9 X 10(3) cm/cm3. Unmyelinated axons constituting 95 per cent of periodontal nerve fibres had a mean caliper diameter of 1.4 micron.

Palatal mucosa under dentures: a qualitative histologic and histochemical analysis

These histologic observations of the epithelium and connective tissue are in close agreement with a host of earlier workers. The striking feature is an increase in thickness of epithelium due to an increase in constituent cell layers, but a decrease in keratinization, with keratin mainly of the parakeratinized variety. There is a definite reduction of collagen fiber network in regions of stress where the fibers present an irregular arrangement in contrast to the normal wavy pattern. Marked chronic inflammatory exudate is also seen. Histochemical investigation of the connective tissue under dentures revealed the presence of a highly elastic and resistant oxytalan fiber network, especially in regions that showed a reduction in collagen fibers. Thus the connective tissue response under dentures is in sharp contrast to what is expected. This abnormal behavior of the connective tissue is probably a reaction to the changes in epithelium. As a result of reduction in keratinization, a highly resistant and protective layer, the connective tissue exhibits altered metabolism and function. We hypothesize that connective tissue takes up a more dominant role of protection and absorption of stress, a role hitherto fulfilled by the epithelium. Thus stresses exerted by dentures on connective tissues are prevented from reaching the bone where they could cause resorption. We therefore feel that denture wear not only results in change within the epithelium, but in an equally significant reaction of the connective tissue. This reaction results in the appearance of a highly resistant and elastic oxytalan fiber system, which assists the connective tissue to react successfully to the stresses exerted on it.

Complete denture secondary impression technique to minimize distortion of ridge and border tissues

A technique has been described with which a physiologic and anatomic registration of the attached and unattached tissue of the denture-bearing areas can be attained. Clear acrylic resin trays aid in eliminating excessive displacement at the secondary impression phase. Inadequacies of the mucostatic concept include: Failure to register the tissues, which are important for retention and stability Certain metal bases that are part of this procedure can be implemented by only a small number of technicians Increased cost There are two shortcomings of the tissue-loading technique for complete denture impressions. Resulting retention and stability lasts only for a short period of time. Unwanted ridge resorption and tissue changes occur.

Management of loading forces on mandibular distal-extension prostheses. Part I: Evaluation of concepts for design

The viscoelastic reaction of ridge mucosa and abutment teeth, by virtue of their periodontal ligaments, is a necessary dimension in the evaluation of design concepts for removable partial dentures for patients with distal-extension ridges. The possible modalities are (1) removable partial dentures with flexible denture bases (stressbreakers), (2) use of a floating denture base impression technique, (3) use of a mucofunctional impression technique to relate the denture base to the framework, and (4) use of an endosseous implant. An evaluation of each modality has been made.

Recent advances in oral mucosal research

Major progress in investigation of the normal structure and function of the oral mucosa has been made within the last ten years and has come principally from the application of various techniques developed in basic science disciplines to specific mucosal problems. However, it is apparent that many gaps still exist in our knowledge of the oral mucosa and, although it is to be expected that different workers will have distinct views on which of these are the most significant, some basic areas for further investigation can clearly be identified. For example, little is known about epithelial control systems and their disturbance by epithelial disease processes, about the nature of the interactions occurring during development and maintenance of the oral mucosal epithelia, or about the epithelial cell surface and its role in normal function. The specific properties and behavior of the cell populations of the subepithelial connective tissues appears to be poorly understood and the existence and significance of functional changes in mucosa with age and malnutrition are uncertain. It is increasingly apparent that successful progress in such investigations involves approaches using diverse methodologies. For example, epithelial-mesenchymal interactions are likely to involve multiple mesenchymal factors acting in concert to establish and maintain epithelial form and, because of this complexity, the nature of the inductive influences is not likely to be elucidated in model systems unless individual variables can be rigidly controlled. Defining the cellular and acellular elements in mesenchyme and reconstructing a functional mesenchyme from purified components may not be a simple task, but with current methods for cultivating mucosal keratinocytes and fibroblasts, as well as for purifying various components of the ground substance, it should be possible to initiate such a program of study. Some of the most dramatic advances made over the past 5-6 years in epidermal research have come about through the utilization of newly developed biochemical investigative techniques, examples of which include the use of gene cloning to study the organization of the keratin gene family, and the use of immuno-fluorescence with monoclonal antibodies to discern when various keratin proteins appear during differentiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Movement of abutment teeth in spring cantilever bridges: a laboratory study

The extent and directions of movements of spring bridge abutments were studied on acrylic casts with resilient steep and shallow palates and containing human molar and premolar abutments with compressible 'periodontal ligaments'. Bridges with rigid bars and 'flexible' standard, long and short lengths were cast in Type IV gold, and soldered to Type III gold retainers which were pinned to the abutments. The pontic on each bar was depressed a standard 0.2 mm, and the movement of the abutments registered on a dial gauge resting against the mesio-buccal corner of each retainer. The combination of bar length and palatal contour causing least movement of an abutment was the shortest bar to the molar abutment in the shallow palate model; indeed all bars on this model deflected the molar abutment less than similar bars on the steep palate model. Differences in palatal contours did not affect the extent of movements of the premolar abutments which were comparable to molar movements in the steep palate model. Abutment teeth moved mesio-buccally when the pontic was depressed, and the elbow of the bar usually moved off the palate indicating the fulcrum was positioned along the anterior section of the bar.

A quantitative comparison of the ultrastructure of the periodontal ligaments of impeded and unimpeded rat incisors

The periodontal ligaments of impeded and unimpeded rat mandibular incisors were examined to find structural correlates for the known functional differences between the tissues. The structures quantified were fibroblasts (area and membrane length, endoplasmic reticulum, mitochondria, microtubules, lysosomes, intracellular collagen profiles, intercellular contacts), oxytalan fibres, collagen fibrils and ground substance. The only changes seen on rendering a tooth unimpeded were an increase in the number of microtubules within the fibroblasts, an increase in the number of simplified desmosomes between the fibroblasts and a decreased amount of ground substance within the extracellular matrix. The results show that it is possible for a connective tissue to undergo marked changes in function, turnover and biomechanical properties without major structural changes.

A new periodontal membrane biology based upon thixotropic concepts

The biology of the periodontal membrane is not completely understood, and tissue changes associated with orthodontic tooth movement are assumed to be the result of a recoverable pathologic process. The response of this tissue to dynamic loading may, however, be explained by changes in viscosity of the collagenous matrix.