Evaluation of Stresses and Forces in Selected I-Bars Using the Finite Element Method

Purpose Three-dimensional models of half-round, tapered and full-round, untapered I-bar clasps of varying configurations and material properties were constructed. The purpose of this study was to examine the stresses and reaction forces produced within each model upon deflection to 0.01 in (0.254 mm), 0.02 in (0.508 mm), and 0.03 in (0.762 mm) at 1 mm from the tip using the finite element method. Materials and Methods Three-dimensional computer models of half-round and full-round clasps were constructed using solid eight-node brick elements. The half-round, tapered I-bar clasp model was 2.4 and 1.4 mm in diameter at the base and tip, respectively. The full-round, tapered I-bar clasp model was 1 mm in diameter. Three design groups were created for each clasp form. Group A had 25% of the total length in the straight anchor end of the I-bar clasp, B had 35%, and C had 50%. All models were 31 mm in length and had a radius of curvature of 5 mm. Different material properties were incorporated into the models. Each model was deflected at a point 1 mm from the tip to 0.01 in (0.254 mm), 0.02 in (0.508 mm), and 0.03 in (0.762 mm). Results The stresses and forces produced as a result of the deflection applied to each clasp were viewed and displayed graphically. The maximum von Mises stresses in megapascals and the reaction force in newtons (N) were recorded. Stresses varied in each clasp in the range of 0 to 154.3 MPa for the half-round, tapered I-bar clasp models, and 0 to 100.9 MPa for the full-round I-bar clasp models at 0.01-in deflection. Reaction force measured near the tip of the clasp models was between 1.60 N and 6.31 N for the half-round, and between 0.22 N to 2.13 N for the full-round I-bar clasp models. For all clasps studied, as the deflection increased, the location of stress within each group remained the same regardless of the material properties; however, the stress and force values increased linearly. Conclusions The location of maximum stress varied with the length of the anchor portion of the clasps studied. Maximum stresses were located on the flat side of the half-round, tapered I-bar clasp model.     

Placement and Restoration of Implants by Predoctoral Students: The Creighton Experience

Teaching of implant dentistry in the predoctoral dental curriculum has evolved dramatically over the past 20 years. In 1974, only one third of American dental schools addressed the topic of implants. Today, 48 of the 54 American dental schools have predoctoral curricula. The Creighton University experience offers some unique and instructive insights into a 10-year process of developing and implementing a predoctoral implant dentistry curriculum. All interested students may perform both the surgical placement and restoration of implant prostheses. Clinical instruction involves all restorative and surgical faculty members. Favorable 3-year (91%) and 5-year (87%) surgical success rates have been maintained. This article presents one university's program for examination and discussion.     

A review of malocclusion among individuals with mental and physical disabilities

Oral health disparities between individuals with disabilities and the general population are widely reported in the literature, and malocclusion is no exception. As the number of people living with disabilities grows, so does the need to explore their oral health status. This review examines the reported prevalence of malocclusion in individuals with Down syndrome (DS), cerebral palsy (CP), cleft disorders, mental disabilities, and physical disabilities from 1976 to 2004. Malocclusion was assessed according to Angle's classifications, the Dental Aesthetic Index (DAI), and selected occlusion characteristics. The prevalence of malocclusion was higher in individuals with disabilities than in controls without disabilities. Malocclusion was more frequent when the handicap was mental rather than physical in origin. Class II and Class III malocclusions were common in individuals with CP and DS, respectively. Crowding, anterior diastema, and > 1/2 cusp antero‐posterior molar relations were frequent among people with disabilities. Findings varied according to disability, but were attributed to musculoskeletal abnormalities, altered cranial‐base relationships, premature tooth eruption, corrective surgery, and lip incompetence. Only a deep bite was more frequent in controls compared to the individuals with disabilities.     

In Vitro Changes in Hardness of Sealed Resilient Lining Materials on Immersion in Various Fluids

Purpose: During clinical use, resilient lining materials undergo changes in hardness that make them ineffective. The aims of this investigation were (1) to determine the effect of a resilient lining sealer on the hardness of four resilient denture liners; and (2) to determine the effect of the sealer on hardness after immersion in various solutions.
Materials and Methods: Two sets of specimens of four resilient liners, Coe-comfort (CC), PermaSoft (PS), Tokuyama soft reline (TK), and Total-Soft (TS), 6-mm diameter by 4-mm thickness, were fabricated. Two coats of Permaseal, a soft reline sealant, were applied to one set of specimens of each material according to manufacturers' instructions. Sealed and unsealed samples were divided into four groups (n = 10). Each group was immersed in one of the following solutions: artificial saliva at 37°C, Efferdent, Efferdent with once daily scrubbing with a soft toothbrush, and 50% ethanol. Shore A hardness numbers were obtained at 0, 1, 3, 7, 30, and 90 days. A two-way ANOVA test was performed using materials (treated and untreated) and immersion solution as independent variables. The percentage change in hardness after the 90-day immersion period was the dependent variable.
Results: The results show that the application of sealant significantly improved the durability of CC, PS, and TS in immersion solutions by maintaining hardness close to preimmersion values or delaying the softening effect of the solutions. The hardness of sealed and unsealed TK showed the significantly ( p < 0.05) lowest change after immersion in the test solutions. Ethanol caused the most severe decrease in hardness of all solutions, followed by saliva. Immersion in Efferdent and daily brushing after immersion in Efferdent showed only a mild effect on the hardness of the soft reline agents.
Conclusion: The use of a sealer can play an important role in the preservation of the hardness of some resilient lining materials.     

Caries Prevalence in Head Start Children, 1986–87

Since the early 1970s, caries prevalence among school-aged children in the United States has declined. It appears that a small percentage of the children experience most of the caries increment. In addition, a large proportion of children in the US who need dental care receive it. An important factor in the amount of treatment received by children is the socioeconomic status of the family. Data on caries prevalence among preschool populations are limited. The Head Start program serves low-income families in the US and offers a unique opportunity to look at individuals who may be at greater risk of health problems and may experience less access to health services. A survey of 1,796 three- to five-year old Head Start children from low-income families was conducted in 1986-87. Caries prevalence, baby bottle tooth decay prevalence and relative need for dental care are reported for fluoridated and non fluoridated communities in California, Hawaii, and Micronesia. The data reveal scores that are higher in the sample population than in five-year-olds in national surveys and among Head Start children in previous surveys.     

The Bond Strength of an Adhesive Resin Luting Cement to a Variety of Surface Treatments of a High-Palladium Copper Alloy

Purpose This study evaluated the tensile bond strength of a bisphenol glycidyl methacrylate (Bis-GMA) resin luting cement with four different surface treatments of a high Pd-Cu alloy. Materials and Methods For each surface treatment type (tin-plated, porcelain furnace oxide, air-abraded, and finished-only), 15 opposing half-dumbbell-shaped samples were cast and prepared in new Pd?Cu alloy. Samples were luted with a Bis-GMA resin luting cement at a film thickness of 80 μm using a custom alignment apparatus. Samples were stored in distilled water at 37°C for 24 hours, thermocycled for 1,000 cycles, and then stored for 30 days in distilled water at 37°C. Samples were then subjected to fracture in tension at a loading rate of 0.5 cm/min with the bond strengths calculated in megapascals (MPa). The fractured surfaces were examined using stereomicroscopy and scanning electron microscopy at various magnifications ranging from 5.5x to 500x to determine the type of bond failure (adhesive, cohesive, or mixed). Results Tensile bond strengths (mean ± SD MPa) were: tin-plated, 30 ± 15.7; porcelain furnace oxide, 23 ± 8.6; air-abraded, 8 ± 8.1; and finished-only, 4 ± 4.5. Statistical analysis of the tensile bond values using an ANCOVA and Tukey's multiple comparison test at a significance level of 0.05 indicated that there was no difference between the tin-plated and the furnace oxide groups, as well as between the air-abraded and the furnace oxide groups. However, there was significant difference between the tin-plated, the air-abraded, and the finished-only groups. The observed bond failures were predominantly mixed and cohesive in nature with only one adhesive failure. Conclusions There was no significant difference in the tensile bond strengths between the tin-plated group or the porcelain furnace oxide surface group. This suggests that the less-technique-sensitive porcelain furnace oxide surface treatment offers an alternative for achieving high metal-resin bonds to a high Pd?Cu alloy.     

Optical mechanisms regulating emmetropisation and refractive errors: evidence from animal models

Our current understanding of emmetropisation and myopia development has evolved from decades of work in various animal models, including chicks, non-human primates, tree shrews, guinea pigs, and mice. Extensive research on optical, biochemical, and environmental mechanisms contributing to refractive error development in animal models has provided insights into eye growth in humans. Importantly, animal models have taught us that eye growth is locally controlled within the eye, and can be influenced by the visual environment. This review will focus on information gained from animal studies regarding the role of optical mechanisms in guiding eye growth, and how these investigations have inspired studies in humans. We will first discuss how researchers came to understand that emmetropisation is guided by visual feedback, and how this can be manipulated by form-deprivation and lens-induced defocus to induce refractive errors in animal models. We will then discuss various aspects of accommodation that have been implicated in refractive error development, including accommodative microfluctuations and accommodative lag. Next, the impact of higher order aberrations and peripheral defocus will be discussed. Lastly, recent evidence suggesting that the spectral and temporal properties of light influence eye growth, and how this might be leveraged to treat myopia in children, will be presented. Taken together, these findings from animal models have significantly advanced our knowledge about the optical mechanisms contributing to eye growth in humans, and will continue to contribute to the development of novel and effective treatment options for slowing myopia progression in children.