Triangulated pedicle screw construct technique and pull-out strength of conical and cylindrical screws.

To compare the mechanical effectiveness of a new conical screw design with a conventional cylindrical screw design, the screw insertion time, torque, and pull-out strength of single-pedicle screw and triangulated-pedicle screw constructs of each type of screw were compared in human cadaveric vertebral bodies. The time required to insert the conical screws was less than that required for cylindrical screws. Regression analysis revealed a positive correlation between insertion torque and pull-out strength of single and triangulated constructs of each type of screw. The conical screw had a greater increase than the cylindrical screw in the pull-out strength of triangulated pedicle screw constructs. Application of the new conical screw design was significantly faster, and the new screw had better mechanical fixation to the vertebral body than did the conventional cylindrical screw tested.

Student productivity in a comprehensive care program without numeric requirements.

As the dental students of the Class of 2000 entered the Comprehensive Care Clinic at the University of Colorado School of Dentistry in the spring of their second year (spring of 1998), a different, competency-based set of clinical expectations was communicated to them. These students were presented a list of "Recommended Core Experiences" and told that the "requirements" for completion of the Comprehensive Care Program were 1) successful performance on all departmental competency examinations, and 2) timely completion of the comprehensive dental treatment appropriate to each assigned patient. This study examined the number of procedures completed during the six semesters in the Comprehensive Care Clinic for the Class of 2000, as compared to the clinical activity for the Class of 1999 during the same stage of their career. The overall mean number of clinical procedures performed per student was 7 percent greater for the students in the Class of 2000 than for those in the Class of 1999. These results suggest that numerical requirements are not necessary to ensure dental student productivity and that, in fact, students can complete more clinical procedures in a comprehensive care clinical environment without numerical requirements.

Alumni self-perception of competence at time of dental school graduation.

A survey instrument was administered to recent alumni to evaluate self-perceived competency in twenty-one selected areas at the time of graduation. The questionnaire was completed by 439 (62.8 percent) alumni who graduated between the years 1985 and 1994, inclusive. Graduates generally felt most competent in their ability to treat dental caries and its sequelae and least competent in their ability to recognize myofascial pain and temporomandibular joint disorders. Very little association was demonstrated between demographic characteristics of the respondents and their competency self-ratings. The survey identified areas of strengths and weakness within the present comprehensive care curriculum as perceived by the graduating dentists.

Deflection and stress distribution in three different IMZ abutment designs.

PURPOSE: The purpose of this study was to compare the stress distribution in the resin element and the retaining screw for three different IMZ prosthetic systems: 1) original threaded Intra-Mobile Element (IME); 2) Abutment Complete (ABC); and 3) Intra-Mobile Connector (IMC). This stress distribution comparison was then related to variations in deflection of the prosthetic superstructure. MATERIALS AND METHODS: Employing the finite element method, a three-dimensional model simulating a cast gold restoration attached to an osseointegrated IMZ implant fixture was generated for each system. The representation of the implant fixture, the supporting structures, and the external contours of the crown were identical in the three models, while the configuration of the abutment varied to characterize the individual systems. Each model was discretized into axisymmetric finite elements representing the crown, the various implant system components, and supporting structures. A series of harmonic functions was written to define non-axisymmetric loads of 100 N and 500 N evenly distributed over the entire occlusal surface of the crown. Each load was applied individually to the models, first in a vertical direction, and then at a 45 degree angle to the median plane. Predicted deflection and stress distributions were computed and plotted for each loading condition of each model. RESULTS: Deflections measured at the buccal cusp tip ranged from 0.002 mm (100-N load applied vertically to the ABC model) to 0.802 mm (500-N load applied at 45 degrees to the IME model). Maximum effective stresses in the retaining screw ranged from 129 MPa (100-N load applied vertically to the ABC model) to 1,315 MPa (500-N load applied at 45 degrees C to the IMC model). A correlation was observed between the peak stresses in the screw and the deflection of the superstructure. CONCLUSIONS: Deflections and stress concentrations with the IMC were predicted to be in the same range as with the IME, but much greater than with the ABC.

Finite element stress analysis of IMZ abutment designs: development of a model.

PURPOSE: This article describes the development of a model system for use in finite element stress analysis of three different IMZ abutment designs: original threaded Intra-Mobile Element (IME), Abutment Complete (ABC), and Intra-Mobile Connector (IMC). MATERIALS AND METHODS: A three-dimensional model simulating a cast gold crown restoration attached to an osseointegrated IMZ implant fixture was generated for each abutment design. Each model was discretized into axisymmetric finite elements representing the crown, the various implant system components, and supporting structures. A convergence test was performed to optimize the mesh. Convergence test mesh refinement for the IME, the IMC, and the ABC abutment models resulted in 818 elements, 2,566 nodes; 738 elements, 2,362 nodes; and 663 elements, 2,051 nodes, respectively. Progressive tightening of the retaining screw (preload) was simulated; the degree of screw tightening necessary to prevent opening of the crown-abutment interface in extreme loading (500-N occlusal load at 45 degrees) was determined individually for each system. CONCLUSIONS: Models of three IMZ abutment designs have been refined and the appropriate relative screw preloads determined. This model system is to be used subsequently in stress analysis comparison for the three systems.

Influence of bone quality on stress distribution for endosseous implants.

The present study examined the influence of bone quality on the transmission of occlusal forces for endosseous dental implants. Employing the finite element method, the study modeled a 3.75 x 10-mm threaded implant placed in a 12 x 11 x 8-mm section of bone. By varying the elastic parameters assigned to the bone elements, four bone quality categories were established. A load of 100 N was applied at the occlusal surface of the restoration at a 30 degrees angle to the vertical axis of the implant. Maximum von Mises stress concentrations (sigma Emax) were observed to be located in the marginal bone at the coronal aspect of the implant fixture in all four cases. Values of sigma Emax were 13.7 MPa for type 1 bone, 15.8 MPa for type 2 bone, 20.1 MPa for type 3 bone, and 26.5 MPa for type 4 bone. Magnitude of the stresses in bone was strongly correlated (r = 0.997) with computed displacement of the implant system. This analysis predicts that placement of implants in bone with greater thickness of the cortical shell and greater density of the core will result in less micromovement and reduced stress concentration, thereby increasing the likelihood of fixture stabilization and tissue integration.

Influence of post dimension on stress distribution in dentin.

Stress distribution studies can be helpful in determining the appropriate diameter and length of endodontic posts for specific teeth in select occlusal relationships. This study selected the finite element method to predict distribution of stresses in dentin of an endodontically treated tooth restored with cast post and cores with various post dimensions. Peak dentinal shear stresses occurred adjacent to the post at mid-root. Peak shear stresses were elevated as the length of the post decreased. Peak dentinal tensile stresses occurred in the gingival third of the facial root surface, whereas peak dentinal compressive stresses were evident in the gingival third of the lingual root surface. The distribution of tensile and compressive stresses was not affected with variation in the dimensions of the posts.

Management of the clinical curriculum in which students direct rather than perform the laboratory stages of restorations.

At the University of Iowa, senior dental students direct rather than perform laboratory construction of clinical prosthodontic work. This report highlights the fundamentals of a successful curriculum based on this concept. One critical component of the curriculum involves teaching the process of self-evaluation of laboratory work as well as the evaluation of work received from a commercial laboratory. Instructors teach students how to evaluate a product in the sophomore preclinical curriculum. A computerized evaluation form, developed by the prosthodontic faculty, allows assessment of the students' laboratory work in addition to self-evaluation. To incorporate patient care into the appraisal process, a modified preclinical evaluation framework is used during the junior clinical clerkship. This expanded framework takes into consideration patient treatment factors such as degree of difficulty, diagnosis and treatment planning, use of infection control, and students' understanding of patients' comprehensive treatment plan. In the senior clinic, the students' ability to direct the construction of a prosthesis is enhanced by the unique diversity of faculty within the Department of Family Dentistry at the University of Iowa. This department unites private practice dentists with academicians and general practitioners with specialists.

Post and core retention with different cements.

In this study, bond strengths of four cement systems--Comspan resin composite luting cement with and without Gluma dentin bond, Fleck's zinc phosphate cement, and Ketac-Cem glass ionomer--used in cementing cast post-cores were investigated. Gluma appears to enhance the bond at the post/tooth interface, resulting in decreased variability and increased strength.

Short-term fluoride release/uptake of glass ionomer restoratives.

OBJECTIVES: The short-term fluoride release/uptake of four glass ionomer restoratives was measured and compared to the fluoride release after exposure to three commercial fluoride gels. METHODS: Materials tested were: 1) Ketac-Fil (ESPE GmbH); 2) Ketac-Silver (ESPE GmbH); 3) Photac-Fil (ESPE GmbH); and 4) Fuji II LC (GC Corp.). Twenty discs of each material were fabricated and stored in deionized water at 37 degrees C. Initial fluoride release was measured at 24 h intervals for 7 d, and 24 h intervals at the end of each week for 5 wk. After 6 wk, samples were divided into groups and each group was treated for 6 min with one of the following commercial fluoride gels: acidulated phosphate fluoride (APF), neutral sodium fluoride (NaF), and stannous fluoride (SnF2). Control specimens were placed in deionized water. After exposure, fluoride measurements were carried out at 24 h intervals for 7 d, and 24 h intervals at the end of 2 wk. Fluoride exposure was repeated, and measurements were again recorded over 3 wk. RESULTS: Initial fluoride release by all materials was highest during the first 24 h and decreased sharply over the first week. After exposure to APF, fluoride release increased significantly for all materials. Exposure to NaF also resulted in increased fluoride release for all materials although it was not as high as the increase recorded after exposure to APF. Exposure to SnF2 did not result in significant fluoride release by any material. SIGNIFICANCE: The results of this study suggest that certain fluoride gels may replenish fluoride within some glass ionomers and thus prolong their cariostatic potential.

Deflection of superstructure and stress concentrations in the IMZ implant system.

The finite element method was used to model a 4.0 x 13.0-mm IMZ implant restored with a cast gold crown to examine the relationship between deflection of the prosthetic superstructure and stress concentrations in the retaining screw using both the polyoxymethylene intramobile element and the analogous titanium element under various loading conditions. A strong correlation was observed between the peak stresses in the screw and the deflection of the superstructure. Deflections and stress concentrations generally increased with increases of either the load magnitude or the load angle. Greater deflection and stress concentrations within the coronal retaining screw were predicted with the use of the resin IME than with the titanium element.

Comparison of stress transmission in the IMZ implant system with polyoxymethylene or titanium intramobile element: a finite element stress analysis.

Using the finite element method, this study modeled a 4.0 x 13.0-mm IMZ implant, restored with a cast gold crown, to examine the influence of the polyoxymethylene (POM) intramobile element (IME) on the transmission of vertical and oblique forces. Stress concentrations in the bone and in components of the implant system were much greater under a 30-degree load than under an equal vertical load. Stress transmission to bone occurred chiefly in the crestal region, and these stresses were not reduced when the IME was modeled in POM rather than in titanium. Maximum stress concentrations occurred in the fastening screw.

Organochlorine pesticide residues in human fat in the United Kingdom 1982-1983.

Between December 1982 and October 1983 samples of human body fat were taken during routine necropsies carried out on 187 persons aged over 5 years and four infants aged under 4 months. Comparison of the results of analysis with those from the previous studies in 1976-1977, and earlier, shows a continuing decline in residues of p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT) and dieldrin (HEOD) and a reduction in the amounts of p,p'-dichlorodiphenyltichloroethane [p,p'-DDE, a metabolite of p,p'-DDT] and hexachlorobenzene. The concentrations of other compounds were similar to those observed in the previous studies. The results show that the amounts of organochlorine pesticides and polychlorobiphenyl compounds contained in human fat samples from residents in the UK compare favourably with data obtained in other countries.

Organochlorine pesticide residues in human milk in Great Britain, 1979-80.

1 The concentrations of some organochlorine pesticides and polychlorobiphenyl compounds have been determined in 102 samples of human milk collected between January 1979 and September 1980. 2 Comparison of the results with those from a limited study undertaken in 1963-4 shows a decline in residues' levels. 3 The concentrations of organochlorine pesticides and polychlorobiphenyls in human milk samples from residents in Great Britain compare favourably with data obtained in other countries.