The influence of heat treatments on several types of base-metal removable partial denture alloys

Trace elements can influence the physical properties of tooth enamel

In previous studies, we showed that the size of apatite nanocrystals in tooth enamel can influence its physical properties. This important discovery raised a new question; which factors are regulating the size of these nanocrystals? Trace elements can affect crystallographic properties of synthetic apatite, therefore this study was designed to investigate how trace elements influence enamel's crystallographic properties and ultimately its physical properties. The concentration of trace elements in tooth enamel was determined for 38 extracted human teeth using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The following trace elements were detected: Al, K, Mg, S, Na, Zn, Si, B, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se and Ti. Simple and stepwise multiple regression was used to identify the correlations between trace elements concentration in enamel and its crystallographic structure, hardness, resistance to crack propagation, shade lightness and carbonate content. The presence of some trace elements in enamel was correlated with the size (Pb, Ti, Mn) and lattice parameters (Se, Cr, Ni) of apatite nanocrystals. Some trace elements such as Ti was significantly correlated with tooth crystallographic structure and consequently with hardness and shade lightness. We conclude that the presence of trace elements in enamel could influence its physical properties.

Base metal alloys used for dental restorations and implants

One of the primary reasons for the development of base metal alloys for dental applications has been the escalating cost of gold throughout the 20th century. In addition to providing lower cost alternatives, these nonprecious alloys were also found to provide better mechanical properties and aesthetics for some oral applications. Additionally, certain base metal alloy systems are preferred because of their superior mechanical properties, lower density, and in some cases, their capability to osseo-integrate. The base metal alloy systems most commonly used in dentistry today include stainless steels, nickel-chromium, cobalt-chromium, titanium, and nickel-titanium alloys. Combined, these alloy systems provide a wide range of available properties to choose the correct material for both temporary and long-term restoration and implant applications.

Prediction of permanent deformation in cast clasps for denture prostheses using a validated nonlinear finite element model

OBJECTIVES

Permanent deformation is one of the most common mechanical complications that affect denture clasps. This can lead to loss of retention and stability of the prosthesis. The purpose of this study was to apply and validate a nonlinear finite element model for permanent deformation prediction in cast denture clasps. Such a model can enhance the process of design optimization and contribute to minimizing the possibility of this problem.

METHODS

Cast clasps made from Ti-6Al-7Nb, Co-Cr and Type IV gold alloys were loaded in three different directions (outside, inside and outside inclined 30 degrees ), and the resulting permanent deformation values were recorded. Nonlinear finite element analysis simulations based on the maximum distortion energy criterion for yielding, were conducted for clasp models that were reproduced according to the dimensions of each experimental specimen. Linear regression analysis for the results of the experiment and simulation was performed to verify the validity of the mathematical models.

RESULTS

Deflections required to produce specific amounts of permanent deformation were in close agreement with those recorded experimentally. The R2 value for all bending tests was 0.985 and the linear regression equation expressed in micrometers was [DeflectionFEA=0.976 (DeflectionReal)+34].

SIGNIFICANCE

Permanent deformation behavior in the cast clasps with a relatively wide range of deflections (0-2 mm) can be predicted using the proposed model, which shall enhance the design optimization process of cast clasps for denture prostheses.

Microhardness of Ni-Cr alloys under different casting conditions

This study evaluated the microhardness of Ni-Cr alloys used in fixed prosthodontics after casting under different conditions. The casting conditions were: (1-flame/air torch) flame made of a gas/oxygen mixture and centrifugal casting machine in a non-controlled casting environment; (2-induction/argon) electromagnetic induction in an environment controlled with argon; (3-induction/vacuum) electromagnetic induction in a vacuum environment; (4-induction/air) electromagnetic induction in a non-controlled casting environment. The 3 alloys used were Ni-Cr-Mo-Ti, Ni-Cr-Mo-Be, and Ni-Cr-Mo-Nb. Four castings with 5 cylindrical, 15 mm-long specimens (diameter: 1.6 mm) in each casting ring were prepared. After casting, the specimens were embedded in resin and polished for Vickers microhardness (VH) measurements in a Shimadzu HMV-2 (1,000 g for 10 s). A total of 5 indentations were done for each ring, one in each specimen. The data was subjected to two-way ANOVA and Tukey's multiple comparison tests (alpha = 0.05). The VH values of Ni-Cr-Mo-Ti (422 ± 7.8) were statistically higher (p < 0.05) than those of Ni-Cr-Mo-Nb (415 ± 7.6). The lowest VH values were found for Ni-Cr-Mo-Be (359 ± 10.7). The VH values obtained in the conditions induction/argon and induction/vacuum were similar (p > 0.05) and lower than the values obtained in the conditions induction/air and flame/air torch (p < 0.05). The VH values in the conditions induction/air and flame/air were similar (p > 0.05). The microhardness of the alloys is influenced by their composition and casting method. The hardness of the Ni-Cr alloys was higher when they were cast with the induction/air and flame/air torch methods.

Bending properties of strengthened Ti-6Al-7Nb alloy major connectors compared to Co-Cr alloy major connectors

STATEMENT OF PROBLEM

Although Ti-6Al-7Nb alloy has several excellent mechanical properties, its poor rigidity has limited its clinical use as a material for the fabrication of a removable partial denture (RPD) major connector.

PURPOSE

The purpose of this study was to develop and evaluate strengthening designs of Ti-6Al-7Nb RPD major connectors in an effort to increase rigidity.

MATERIAL AND METHODS

Four strengthening designs of Ti-6Al-7Nb alloy major connectors were developed using finite element analysis modeling: wide, thick, thick at the middle, and thick at the anterior and posterior borders. The designs had similar rigidity values to a conventional Co-Cr alloy major connector, as measured by the maximum deformation when a simulated load was applied. Next, 30 Kennedy Class II maxillary RPD specimens, using 6 different major connector designs (n = 5), were fabricated. These 6 designs included Ti-6Al-7Nb alloy RPDs with 4 different strengthening designs, a Ti-6Al-7Nb alloy RPD without strengthening designs, and a conventional Co-Cr alloy RPD as a control. The rigidity of the RPDs was evaluated by measuring strains on the major connector and force on the intaglio surface of a denture base under a 30 N loading condition. The data obtained from the 6 different RPD designs were compared using a repeated measures analysis of variance and the Bonferroni correction (alpha=.05).

RESULTS

The strains on the major connectors and the force on the denture bases measured from the Ti-6Al-7Nb RPD without strengthening designs were significantly larger than those of the standard Co-Cr alloy RPD. The strains measured from the 4 strengthening designs were not significantly larger than those of the Co-Cr alloy RPD.

CONCLUSION

The results suggest that the strengthening designs tested may improve the rigidity of the Ti-6Al-7Nb alloy major connectors and, hence, may promote clinical application for RPDs.

Deflection fatigue of Ti-6Al-7Nb, Co-Cr, and gold alloy cast clasps

STATEMENT OF PROBLEM

There is little information about the deflection fatigue of clasps in relation to stress distribution.

PURPOSE

The aim of this study was to investigate the fatigue resistance and permanent deformation of cast clasps made of titanium and other dental alloys and to relate the fatigue resistance with the calculated stress values.

MATERIAL AND METHODS

Twenty-five Ti-6Al-7Nb, 25 Co-Cr, and 15 Type IV gold alloy clasps were subjected to cyclic deflection of preset values of 0.25 mm, 0.50 mm, or 0.75 mm, for 10 6 cycles (n = 14). Finite element models were created to calculate principal stresses within the specimens. Fatigue life, retentive force, and permanent deformation were recorded, and the fracture locations were determined microscopically. The results were characterized in relation to the stress within the clasps. One-way analysis of variance and Tamhane's post-hoc tests were used to compare the results of the 9 material-deflection groups (alpha=.05).

RESULTS

Ti-6Al-7Nb clasps exhibited significantly less permanent deformation than the other clasps under relatively greater deflections, indicating better adaptation to the tooth surface. However, the fatigue life of the Ti-6Al-7Nb clasps under 0.75-mm deflection, with the stress above the alloy's 0.2% yield strength, was significantly shorter than those under smaller deflections. The gold clasps showed significantly longer fatigue life than the other clasps under the 0.50-mm deflection. High-stress areas within the fatigue clasp specimens coincided with the fracture locations. The probabilities of fatigue fracture and permanent deformation were closely related to the material strengths and the preset deflections.

CONCLUSION

To minimize fatigue failures, the cast clasp should be designed with consideration of the stresses distributions within the clasps.

Soldering and remelting influence on fatigue strength of cobalt-chromium alloys

STATEMENT OF PROBLEM

For laboratory production, it would be economically advisable to reuse excess cobalt-chromium alloys in manufacturing partial denture frameworks. Furthermore, discrepancies in fit, modifications, repairs, and also economic factors could lead to soldering procedure on the metal framework.

PURPOSE

This study evaluated the influence of soldering and remelting procedures on the fatigue strength of Steldent, Dentorium, and Biosil cobalt-chromium alloys, as new alloys and in a mixture of new and previously casted alloy and when submitted or not submitted to conventional soldering procedure.

MATERIAL AND METHODS

The soldering procedure was conducted in rod specimens 1.7 mm in diameter made with new, or with a mixture of new and reused alloy (n = 10 per group). The results were compared with those of intact rods. The fatigue strength was carried out with a cyclic testing machine simulating the insertion and the removal of partial dentures. After failure, the fracture surface was examined with a scanning electron microscope.

RESULTS

The soldered specimens revealed 1119 loading cycles before fracture. These means differed statistically from those of intact alloys, which disclosed 2733 cycles before failure (p < 0.05). Statistical analysis indicated that the soldering procedure reduced fatigue strength of all new alloys or those in a mixture of new and previously casted alloys. It was also found that the soldering procedure may cause premature failure of a removable partial denture. Photomicrographs from fracture surfaces revealed the presence of inclusions and voids. Conversely, there were no significant differences between new and reused specimens, soldered or not (p > 0.05), which suggested that it may be possible to reuse excess cobalt-chromium alloys without compromising the fatigue strength of the framework.

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, untapered 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.

Duration of induction melting of cobalt-chromium alloy and its effect on resistance to deflection fatigue of cast denture clasps

This study determined the effect of various durations of induction melting of a cobalt-chromium alloy on resistance to deflection fatigue. Commercial cobalt-chromium alloy was melted by high-frequency induction for various lengths of time before it was cast into the shape of a denture clasp. The test method used was a constant-deflection fatigue test. The fatigue fracture surface of the clasp was examined with a scanning electron microscope, and the surface hardness (Vickers hardness) of the clasps was measured. In clasps with greater fatigue resistance scanning electron microscope photomicrographs revealed a coarse grain structure. The surface hardness of the alloy was least in the group with the lowest fatigue resistance and was higher in groups with greater fatigue resistance. This study suggests that, although some porosities are formed in the middle of the clasps after the alloy is overheated, the fatigue resistance of cobalt-chromium alloy denture clasps can be increased by lengthening the induction melting period of the alloy.

Cobalt-chromium and nickel-chromium alloys for removable prosthodontics, Part 1: Mechanical properties

PURPOSE

This investigation compared the mechanical properties of three base metal casting alloys (Vitallium2, Neoloy N [Regular], and Regalloy T) for removable partial denture (RPD) frameworks with those for two base metal RPD alloys (Vitallium, Jelenko LG) whose mechanical properties had been previously reported.

MATERIALS AND METHODS

Plastic patterns with dimensions meeting ADA specification no. 14 requirements were used with appropriate casting investments to cast tensile test bars of the five alloys. Nine specimens of each alloy were loaded to failure, and the mechanical properties of modulus of elasticity, 0.1% yield strength, 0.2% yield strength, and tensile strength were obtained from five well-behaved load-elongation plots. Values of percentage elongation were also determined, along with Vickers hardness of the work-hardened alloys from polished cross-sections of fractured tensile specimens. The fracture surfaces for representative specimens of each alloy were examined with a scanning electron microscope.

RESULTS

Although the measured values of mechanical properties generally agreed with those reported by the manufacturers, differences found for Vitallium and Jelenko LG with previous investigations suggest that there may have been some changes in alloy compositions and processing by the manufacturers. The investment burnout procedure adopted for the plastic patterns and the present casting conditions may also have affected the mechanical properties of some alloys. Two alloys, Jelenko LG and Regalloy T, had mean values of percentage elongation exceeding 10%, but caution is recommended for clinicians and dental laboratories when performing clasp adjustments with all five alloys because of the substantial work hardening that can occur.

CONCLUSIONS

In general, all of the alloys met the requirements in ADA specification no. 14 for base metal RPD alloys. Further research will be necessary to determine the relationships between the mechanical properties, particularly yield strength, rate of work hardening and percentage elongation, and the clinical requirements for these alloys.

Radiographic and metallographic evaluation of porosity defects and grain structure of cast chromium cobalt removable partial dentures

A study was undertaken to determine the defects in the internal microstructure of clasps of cast chromium cobalt removable partial denture frameworks. Ninety cast frameworks were radiographically evaluated for porosity with an industrial radiographic machine. The grain structure of the clasps was analyzed with an optical photomicroscope before and after various heat treatments. The radiographic study showed a large number of internal defects in various parts of removable partial denture frameworks. The grain structure study revealed microporosities randomly distributed within the matrix. Various heat treatments were performed and resulted in no change in the grain structure of the original castings.

Metal release from plates used in jaw fracture treatment. A pilot study

This study investigated the occurrence of corrosion associated with the use of metallic implants to stabilize jaw fractures. Three different types of plates, Co-Cr and Ni-Cr alloys and Titanium, were connected in vivo to the mandibular bone surface of monkeys (Cercopithecus aethiops). The animals were killed after 3 and 6 months. The mucous membrane and bone tissue were analysed for concentrations of Co, Cr, Ni, Mo, Al, and Ti by atomic absorption spectrophometry and a radiochemical neutron activation technique. With the exception of Ti, higher concentrations of all the above elements were found in the tissue near the implants when compared with contralateral controls. However, no signs of corrosion, macroscopic or microscopic, were observed on the surface of the implants.

Flexural strength of a high-temperature soldered cobalt-chromium alloy

The flexural strength of a high-temperature soldered cobalt-chromium alloy was compared with that of the intact alloy. In one test group the specimens were soldered by Vitallium welding rods and in the other group Bego cobalt-chromium solder was used. The cylindrical portion of all specimens was mounted equally in an Instron testing machine. No statistically significant differences were found in flexural strength between the groups.

Metal release from cobalt-chromium partial dentures in the mouth

The amounts of cobalt and chromium released to the saliva in individuals wearing cobalt-chromium partial dentures have been measured. Saliva was collected during a 5-min period, both with and without dentures in the mouth, and the two samples compared. More than 80% of the 30 cases showed increased amounts of metals with the denture in the mouth. Newer and larger dentures liberated more than did older and smaller ones. The median increase in chromium was approximately twice that of cobalt.

Corrosion of Co-Cr alloys in aqueous environments

The literature relating to the corrosion in aqueous media of the Co-Cr alloys is reviewed. Data are surveyed for their corrosion in sulphuric acid, alkalis and in hydrochloric acid and chloride media. In the latter category, the information is sub-divided into single corrosion, crevice and stress corrosion, fretting corrosion and data obtained from 'in vivo' experiments. It is clear that the very low current densities which result from 'in vitro' experiments present abnormal experimental problems and may not, in every case, actually represent the dissolution process.