Occlusal Region Research Articles

Histogenetic aspects of the composition and structure of human ectopic enamel, studied by scanning electron microscopy

This study was made on 12 enamel projections and 12 enamel pearls on human permanent molars to compare their structure and composition, which might elucidate possible histogenetic differences between these two forms of ectopic enamel. The enamel projections contained approx. 2 (weight)% less Ca and P than control occlusal and mid-coronal enamel. Their mineralization was less homogeneous and probably defective. Their enamel structure was markedly irregular in the prismatic as well as the aprismatic layer, but the composition and structure were very similar to those of the control enamel close to cervical border. The enamel of the pearls largely corresponded in composition and structure to the control occlusal and mid-coronal enamel. The enamel of large pearls also resembled coronal enamel in its compositional and structural variability in the occlusal-cervical direction. These findings suggest that amelogenesis in enamel projections is a continuation of amelogenesis in the cervical region. In contrast, amelogenesis in enamel pearls follows the same pattern as that in a dental crown, from the occlusal region to the cervical enamel border. Therefore, the enamel pearl may well be regarded as an attempt at new tooth formation.

Chondroitin sulfates in developing mouse tooth germs

Abstract The role of glycosaminoglycans and proteoglyans during ontogenesis is not known. The developing tooth offers a potentially important model for studies of structure-function relationships. In this study, we have analysed the temporal and spatial expression of chondroitins of differing sulfation patterns in embryonic molars and incisors. For this purpose, we have used monoclonal antibodies (Mabs) specific for unsulfated, 4-sulfated, and 6-sulfated forms of chondroitin in conjunction with indirect immuno-fluorescence or immunoperoxidase labeling. Unsulfated chondroitin was not detected in embryonic teeth. Chondroitin 4- and chondroitin 6-sulfates were present in the stellate reticulum but otherwise they were confined to the dental mesenchyme. The 3B3 and MC21C-epitope, which are markers of 6-sulfated chondroitin, were uniformly distributed in the dental mesenchyme during the bud stage; they disappeared from the dental papilla of the cusps and of the anterior region of the incisor as development proceeded. These epitopes were absent from the basement membrane and from the predentin. In the odontoblastic cell lineage, the 3B3 and MC21C-epitopes were detected only between preodontoblasts at an early stage of differentiation. The monoclonal antibody 2B6 served as a probe to localize chondroitin 4-sulfate. This glycosaminoglycan was detected as early as the dental lamina stage but its expression was restricted to the basement membrane of the teeth until the late bell stage. After the onset of cusp formation, strong staining was also observed over the occlusal region of the dental papilla while the cervical region of the dental papilla remained 256-negative. Incisors at the bell stage exhibited a decreasing gradient of immunostaining by 2B6 from their anterior region to their posterior end. The extracellular matrix surrounding preodontoblasts reacted with 2B6 and the predentin, produced by the odontoblasts, was also intensely labeled with this antibody. Comparison between immunostaining with 3B3 and 2B6, on consecutive sections revealed a mutually exclusive pattern of distribution of the corresponding epitopes during odontogenesis. Furthermore, in the continuously growing incisor, a striking positive correlation was found between the immunostaining patterns produced by 3B3 and MC21C and the mitotic indices along the anterior-posterior axis of the tooth. Hence, sulfation of chondroitin seems developmentally regulated. We postulate that changes in the sulfation pattern of chondroitin might play a role in ontogenesis by locally altering the functional properties of the extracellular matrix.

A mathematical treatise on the fit of crown castings

A mathematical analysis was given to calculate the marginal cement thickness, Dm, of a cemented crown casting as a function of the minimum cement film thickness, F; the width, W, of the base of the tapered region; the width, W', at the bevel (if present) of the tooth preparation; the taper angle, theta; the marginal angle, beta; and the compensatory expansion, X, of the metal investment system: Dm = [F/sin theta - 0.5(X - 1) (W cot theta - W' cot beta)] sin beta. The elevation of the cemented casting, delta H, above the tooth preparation may also be obtained from this equation by substituting beta = 90 degrees, in which case Dm = delta H. The effect of the preparation of a chamfer margin or bevel on the marginal film thickness may be calculated from this equation. An additional analysis showed that the minimum marginal cement thickness, Dm = F, can be obtained for a shoulder preparation and X values of common metal-investment systems by applying a spacer of appropriate thickness on the entire occlusal and axial walls of the die. This renders the preparation of bevels superfluous. The thickness of the spacer should allow for the cement film thickness, roughness of the tooth and casting surfaces, dimensional inaccuracies of the die, and distortions of the wax pattern. If the spacer is thicker than required, a minimum value of Dm is still obtained for preparations without bevels. If the horizontal part of the die shoulder is covered as well with the spacer, a greater spacer thickness is required to obtain a minimum value of Dm than if the spacer extends to the end of the axial walls. In this case the spacer thickness should be greater on the occlusal die wall than on other regions of the die for values of X smaller than 1. The spacer should also be thicker in the occlusal region if restriction of the investment setting expansion by the wax pattern occurs mainly in this region.

Quantitative analysis of connective tissue resorption in the supra-alveolar region of the mouse incisor ligament.

During eruption of the mouse incisor part of the periodontal ligament moves along with the tooth in the occlusal direction and is degraded in the supra‐alveolar region.In the present study stereologic analysis at the ultrastructural level has shown that significant reduction of the amount of extracellular collagen is confined to the most occlusal region of the ligament, just apically to the functional epithelium. In connection herewith a relatively high amount of homogeneous material of moderate electron density was found in the intercellular space, suggesting a process of extracellular collagen breakdown near the epithelium‐connective tissue interface.With respect to the distribution of collagen phagocytosis by fibroblasts, it appeared that the volume density of cytoplasmic vacuoles containing collagen fibrils was considerably higher in the supra‐alveolar region of the ligament than in its subcrestal part, particularly in the zone along the tooth surface. This phenomenon is interpreted as being indicative of increased phagocytic activity of fibroblasts leading to the detachment of the periodontal ligament from the cemeatum before it enters the region of final degradation near the apical termination of the junctional epithelium.In an attempt to identify the cells involved in the degradation of (fragments of) periodontal fibroblasts in the supra‐alveolar region, periodontal ligament cells were labelled with tritiated thymidine at their site of origin. By means of both autoradiography and electron microscopy, it was shown that, following the arrival of Jabelled fibroblasts in the supra‐alveolar region, at least part of the degradation of cellular constituents is accomplished via a process of heterophagocytosis among fibroblasts.

ワックス・クラウン作製操作時の外力にともなう寸法変化についての実験的研究

This experimental study has been performed to investigate the distortion of wax patterns caused by the loading force, and to examine the influence of the angle of the axial wall of the crown, the kind of inlay wax, and the manipulation for waxes on the distortion.a) Wax patterns for crowns were made on the plaster dies, employing metal original dies. The axial dimensional change caused by the loading force on the occlusal surface and the axial wall of the wax patterns was measured by use of the optical fiber transducer for detection.b) The metal sprue pin 2 mm in diameter was inserted in the center of the occlusal surface, and the tension produced when the sprue pin was pulled out was measured. The results obtained were as follows:(1) A large amount of distortion on the axial surface of the wax pattern caused by the vertical load at the central region was observed at the occlusal, central, or marginal region. Furthermore, the distortion of both the occlusal and marginal regions was conspicuous in comparison with that in the central region, and a concave-shaped distortion was found on the axial surface.(2) Concerning the relationship between the distortion, which was brought about when the wax pattern was pressed between the thumb and index finger, and the duration which was left after the preparation of the wax pattern, the distortion observed when the wax pattern was left at 1 hour or 24 hours after taking off was smaller in amount than that observed immediately thereafter.(3) The tension loaded for pulling out the wax pattern from the plaster die was approximately 3kg, and the axial surface of the wax pattern was noticed to shrink.

Changes of lateral soft tissue profile after surgical correction of mandibular prognathism

The lateral soft tissue profile was recorded in 10 patients with slight to moderate degrees of mandibular prognathism, preopera-tively, and 6 weeks after subcondylar sliding osteotomy had been performed. The recording method was mechanical. Only small and insignificant profile changes were found in the submandibular and occlusal plane regions as well as in the ramus region. The only significant change of profile was found in the mandibular body region, probably as a result of firm connection between soft tissue and underlying distally moved bone. The lack of significant profile change in the osteotomy region was in accordance with subjective observations and was probably due to local remodelling and adaptation processes. The facial width was thus found to be unaltered 6 weeks postoperatively.

Autoradiography of 90Sr in developing rats

The distribution patterns of 90Sr in five littermate, 8-day-old Wistar rats were studied by whole body autoradiography. Rats were killed 15 min, 1, 4, 24, and 72 h after a single intraperitoneal injection of the isotope. Immediately after administration, 90Sr was distributed throughout most of the soft tissues of the body. The soft tissue deposits had practically disappeared after 4 h. In the hard tissues of the body 90Sr accumulated up to 24-72 h. Fifteen minutes after injection the uptake of 90Sr in the enamel of the teeth was highest in the occlusal and incisal regions. 90Sr gradually accumulated throughout the enamel and after 72 h its distribution in this layer was fairly uniform. Immediately after injection a narrow zone of radioactivity appeared in the dentin near the pulp. This zone broadened with time towards the dentinoenamel junction and included the entire dentin layer 72 h after injection. Initially, the uptake of 90Sr was higher in the dentin thatn in the enamel, particularly in the cervical areas of the crown. This difference became less apparent with time. There was good correlation between the uptake in teeth and bones, supporting the use of teeth as indicators of the 90Sr body burden.

Pattern commonality in the facial orientation of the occlusal region: Form-functional significance and clinical potentiality

The orientation of facial components, including dentition, to anterior cranial base reference points and planes is characterized by a wide range of variability determined by both angular and linear measurements of cross-sectional roentgenographic cephalometric material. In contrast, the intrafacial orientation of the occlusal aspect of the dentition can be defined within very narrow parametric estimates, characteristic of both clinical cephalometric and nonhuman primate craniometric data. These angular and linear orientation characteristics of small variability indicate a high order of morphologic integration between face and dentition. The resulting pattern commonality reflects the integration of numerous growth and neuromuscular control mechanisms which function to attain and maintain biologically normal occlusal relationships. The uniquely invariant nature of these occlusofacial relationships and their intimate correlation with function provide a frame of reference whereby both normal and abnormal growth, development, form, and function can be critically evaluated in a total facial context.

Photoelastic stress analysis of buccolingual sections of Class II cavity restorations

Previous investigations by Lam pshire1 and Haskins2 showed tha t if certain me­ chanical principles are incorporated in the Class I I cavity preparation, they can alter significantly the resistance of the silver amalgam restoration to fracture and affect the stress pattern produced in the restoration. Haskins’2 photoelastic study of stress distribution was concerned with mesial-distal sections of the Class I I cavity restoration. There rem ained the question of how the buccolingual cross section of the Class I I cavity should be shaped in order to make the most effi­ cient use of the restorative m aterial and produce the lowest possible stresses in the restoration. The basic purpose of this investigation, therefore, was to study the distribution of stress on the occlusal step portion of the Class I I restoration as a result of different cavity designs. I t was immediately apparent that an investigation of the stresses on buccolin­ gual planes by two-dimensional pho­ toelastic methods would introduce a problem not encountered in Haskins’ research. The problem arises from the fact that all mesiodistal sections of the Class II restoration are geometrically similar, but buccolingual sections experi­ ence an abrupt change in shape a t the axial wall. This abrupt change of shape may affect the distribution of stress in the buccolingual sections on the occlusal step as far removed from the axial wall as a distance equal to the width of the oc­ clusal isthmus (de St. V enant’s p rin­ ciple) .3 Thus, the results of this study are limited to a qualitative investigation of the stress in the buccolingual section and to a quantitative comparison of stresses on sections removed a t least the width of the occlusal isthmus from the axial wall. This limitation, however, is largely compensated for by the fact that buccolingual sections of the occlusal step portions of Class I I restorations are simi­ lar in shape to axial sections of Class I restorations placed in the occlusal region. T he results of this study, therefore, can be applied to Class I restorations in the occlusal region as well as to Class I I restorations. In this investigation, buccolingual sec­ tions of five different Class I I cavity forms, with shallow and deep pulpal