A three-dimensional finite-element stress analysis of an endodontically prepared maxillary central incisor.

This study is an application of a three-dimensional Finite-Element Method to investigate the changes in stress characteristics of a prepared maxillary central incisor. The purpose of this study was to analyze stress distributions in this tooth after simulated canal preparation and static loading. A maxillary central incisor was embedded in acrylic, sectioned, photographed, and digitized. A three-dimensional finite-element model was generated by a computer and appropriately modified to simulate canal preparation. Data identified the highest stress magnitudes to be located between the middle and coronal thirds of the root; an area clinically observed to be prone to fracture during treatment. In addition, the magnitude of generated stresses was directly correlated with the simulated prepared canal diameter. The development of a validated three-dimensional finite-element method could identify areas that may predispose a tooth to structural failure during condensation loads.

Regional variation in permeability of young dentin.

Variation in the permeability of dentin in people 19 years and less in age was evaluated. The regions compared were the occlusal third versus the middle third versus the cervical third of approximal surfaces. Also compared were the mesial approximal surface versus the distal. The comparison was done by obtaining 1 mm-thick dentin discs from the area of dentin close to the dentinoenamel junction. It was seen that the cervical area was significantly more permeable than the occlusal area. The middle third, though not statistically significant, had mean values almost twice those in the occlusal third and almost half of the values in the cervical third. No difference was seen in permeability between the mesial and distal surfaces. The reasons for these regional variations and their clinical implications are presented.

Effect of cavity depth on stresses in a restored tooth.

Restorative procedures commonly replace lost tooth structure, but redistribution of functional stresses after treatment is not fully understood. Many restorative methods are dictated by the integrity of the remaining tooth structure, because sparse tooth structure can lead to fracture. It is essential to prevent fractures by having a clear concept of the designs for cavity preparations, and to anticipate the stresses of mastication on the remaining tooth structure. Knowledge of various internal parameters of cavity designs would facilitate selection of the appropriate cavity preparation for a specific clinical situation. Three cavity designs and restorations were examined in this study for stresses using the finite element technique. After placement of restorative materials, the dentin experienced a dramatic change in stress gradient immediately below the pulpal wall, and this response was magnified in deeper cavity preparations. Enamel also exhibited major alterations in the stress gradient in all three designs of cavity preparations. The combination of the changes can cause cracks in the remaining tooth structure, leading to cusp fracture immediately adjacent to the deepest portion of the cavity.

Stresses at the dentinoenamel junction of human teeth--a finite element investigation.

A three-dimensional, linear, elastic finite element model of a maxillary first premolar from longitudinal ground sections was developed to investigate stress variation in the enamel and dentin adjacent to the dentinoenamel junction (DEJ). The effect of regional variation in the contour of the DEJ on the stress patterns for enamel and dentin was also analyzed. The normal (compressive or tensile) and shear stresses in the dentin and enamel surfaces of the DEJ were computed for a vertical load of 170 N acting on the entire occlusal surface of the model. The normal stresses in dentin and enamel were maximum on the occlusal surface of the model and diminished along the buccal and lingual surfaces of the DEJ. However, the magnitude of the normal stresses increased at the cervical enamel, which also showed increased values for shear stress distribution. The normal and shear stresses were markedly affected by the contour of the DEJ and the thickness of enamel in the occlusal third on the buccal and lingual surfaces. The results suggested that because the mechanical interlocking between enamel and dentin in the cervical region is weaker than in other regions of the DEJ, enamel in this region may be susceptible to belated cracking that could eventually contribute to the development of cervical caries.

Parameters of MOD cavity preparations: a 3-D FEM study, Part II.

Stresses induced by the influence of variations in the dimensions of three different parameters of MOD cavity preparations were studied using the finite element technique. The parameters studied were the depth, isthmus width, and the thickness of the remaining interaxial dentin. A total of eight different cavity designs, divided into three groups, were compared with the normal (unprepared) tooth and other cavity designs in the same group. Enamel and dentin experienced large regional variations in compressive stresses even in normal teeth. Teeth with prepared cavities showed even more variation, and stress combinations became critical from a tooth fracture point of view. It was seen that narrow but deep cavity preparations and wide and equally deep cavity preparations experienced a similar, damaging combination of stresses. It was deduced from the stress patterns observed in this study that the depth of the cavity preparations is the most critical factor in fracture of the tooth or cusps, whereas the width of the isthmus alone is the least critical.

Clinical implications of the response of enamel and dentin to masticatory loads.

The success of restorative procedures is dependent on comprehension of the responses of enamel and dentin, including responses to masticatory forces. The regional variation resulting from masticatory forces is critical because clinically it relates to the thickness of enamel and dentin occlusogingivally. Three-dimensional finite element models of an intact mandibular molar were developed to analyze stresses in enamel and dentin occlusogingivally, buccolingually, and mesiodistally. There were dramatic regional variations in the magnitude and character of different stresses caused by masticatory forces, and despite being organically "bonded," enamel and dentin responded independently. This unique behavior with regional variations of these tissues could have serious clinical implications during restorative procedures.

Adaptability of two amalgams to finished cavity walls in Class II cavity preparations.

Adaptability of two amalgams--the dispersed-phase Dispersalloy and the spherical-particle-type Sybralloy--to cavity walls in a Class II cavity preparation was investigated. Four techniques for finishing cavity walls were compared, as was the adaptability of alloys at different occluso-gingival levels. The issues were examined by direct measurement of microleakage between cavity wall and the amalgam without the use of cavity varnish. There was no statistically significant difference in the adaptability of the two amalgams directly to the cavity walls, except those finished by a sharp hand instrument (hatchet) and restored with Dispersalloy. This combination demonstrated the best overall adaptability of amalgam to cavity walls. Sybralloy demonstrated best adaptability to walls finished with sandpaper discs. Although not significant, the gingival 1/3 of the Class II cavity preparation had more microleakage than the middle 1/3 or the occlusal 1/3, suggesting greater potential for recurrent decay. Overall dispersalloy demonstrated significantly better adaptability than Sybralloy.

Mounting natural teeth for preclinical exercises.

Plastic teeth are commonly used to help teach students who are taking preclinical courses in restorative dentistry. Unfortunately, plastic teeth do not have the texture, anatomy, and other characteristics of natural teeth. To provide more realistic clinical experience for dental students, a technique is described for mounting natural teeth in a dentoform.

Shortie matrix and complex Class IV composite resin restoration.

Clinicians face a dilemma treating anterior teeth severely damaged by caries or fracture. The ideal treatment in many cases would be a full veneer crown. A restorative procedure that satisfies biologic and esthetic considerations is generally not difficult for the experienced clinician. But such a treatment path may be reevaluated because of financial considerations, tooth prognosis, and planned treatment of teeth. Perhaps the most critical issue in the restorative procedure is the role the faciolingual contour of the junctional epithelium (epithelium attachment) plays in matrix adaptation. This article offers the clinician the rationale and technique to utilize the Original Tofflemeier matrix #2 in large Class IV cavity preparation.

Marginal leakage: entry side and exit side-clinical implications.

The marginal leakage along the proximal walls of Class II amalgam restorations was studied. The influence of the direction of the bur rotation to tooth structure in cavity preparation was the primary concern of investigation. Also, the effect of different cavity wall finishing techniques created by exit and entry action of the rotating bur and that of two different types of amalgam on the marginal leakage was studied. The degree of marginal leakage was not significantly different among any of the treatment groups.

Dentinal crazing and interpin distance.

All three types of self-threading plus cause dentinal crazing. The frequency and degree of damage done to the dentin varies significantly between the three types of pins and depends upon the differences in the diameters of the drill and the pin. When two or more pins are placed in dentin, the potential for dentinal damage increases. Minikin pins (0.019-inch diameter) caused the least damage, in terms of both degree and frequency. The optimum and safe interpin distance using Minikin pins suggested by this study is 3 mm. Minim and Regular pins (0.023- and 0.031-inch diameters, respectively) caused more injury than Minikin pins. The Regular pins showed more damaging effects on dentin than Minim pins, both in terms of degree and frequency. On the basis of this study, the minimum interpin distance using Minim or Regular pins should be 5 mm. These minimum interpin distances are applicable only in situations where the dentin is not dehydrated or embrittled due to endodontic treatment. In teeth that have been endodontically treated, it would be advisable to use only the smallest self-threading pins and possibly increase the interpin distance if two or more pins are to be used.

Microleakage and enamel finish.

Cavity preparations that are finished with hand instruments allow significantly less marginal leakage than those which are completed only with rotary instruments. This does not mean that hand-instruments walls are perfectly smooth; it only means that these walls allow less marginal leakage. Cavity preparations restored with composite resin showed more marginal leakage than those restored with amalgam. Also, the degree of marginal leakage was greater in cavity preparations restored with composite resin than in those restored with amalgam. Cavity preparations are more prone to marginal leakage at sharply defined cavosurface acute angles than at the smooth or straight surfaces of the cavity. This pattern of marginal leakage was true for both amalgam restorations and composite restorations, but the degree of marginal leakage was greater when composite resin was used as a restorative material. Since less marginal leakage was found in the finished cavity preparations, it is obvious that all cavity preparations must be finished with hand instruments. This may not completely eliminate the problem of marginal leakage, but it will reduce the potential secondary caries both in degree and frequency. Also, since sharply defined acute angles did exhibit more marginal leakage, this part of the cavity preparation needs special attention. With amalgam and composite restorations at least, these angles should not be sharply defined but should be slightly round, so that better condensation of restorative materials can be obtained. This may reduce the degree of marginal leakage.