Preston Beck

The wear of polished and glazed zirconia against enamel

STATEMENT OF PROBLEM The wear of tooth structure opposing anatomically contoured zirconia crowns requires further investigation. PURPOSE The purpose of this in vitro study was to measure the roughness and wear of polished, glazed, and polished then reglazed zirconia against human enamel antagonists and compare the measurements to those of veneering porcelain and natural enamel. MATERIAL AND METHODS Zirconia specimens were divided into polished, glazed, and polished then reglazed groups (n=8). A veneering porcelain (Ceramco3) and enamel were used as controls. The surface roughness of all pretest specimens was measured. Wear testing was performed in the newly designed Alabama wear testing device. The mesiobuccal cusps of extracted molars were standardized and used as antagonists. Three-dimensional (3D) scans of the specimens and antagonists were obtained at baseline and after 200 000 and 400 000 cycles with a profilometer. The baseline scans were superimposed on the posttesting scans to determine volumetric wear. Data were analyzed with a 1-way ANOVA and Tukey Honestly Significant Difference (HSD) post hoc tests (α=.05) RESULTS Surface roughness ranked in order of least rough to roughest was: polished zirconia, glazed zirconia, polished then reglazed zirconia, veneering porcelain, and enamel. For ceramic, there was no measureable loss on polished zirconia, moderate loss on the surface of enamel, and significant loss on glazed and polished then reglazed zirconia. The highest ceramic wear was exhibited by the veneering ceramic. For enamel antagonists, polished zirconia caused the least wear, and enamel caused moderate wear. Glazed and polished then reglazed zirconia showed significant opposing enamel wear, and veneering porcelain demonstrated the most. CONCLUSIONS Within the limitations of the study, polished zirconia is wear-friendly to the opposing tooth. Glazed zirconia causes more material and antagonist wear than polished zirconia. The surface roughness of the zirconia aided in predicting the wear of the opposing dentition.

Early Failure of a Cross-Linked Polyethylene Acetabular Liner: A Case Report

The use of highly cross-linked ultra-high molecular weight polyethylene in total hip arthroplasty has become a popular alternative to the use of conventional polyethylene. Evaluations with use of hip simulator tests1-4 and clinical evaluations of total hip replacements5-10 have independently shown that polyethylene materials processed with gamma irradiation followed by melting or annealing have extremely low wear rates. However, it also has been demonstrated that such cross-linking processes may reduce the fracture toughness and resistance to fatigue crack propagation of polyethylene11-16. Bradford et al.14 found that retrieved cross-linked polyethylene acetabular liners exhibited surface cracking that had not been predicted by in vitro hip simulator studies. Tower et al.17 observed fatigue failure at the superior aspect of the rim of four retrieved highly cross-linked acetabular bearings. They concluded that the factors contributing to these failures included thin polyethylene at the cup rim, a relatively vertical cup alignment, and compromised material properties of the cross-linked polyethylene in comparison with conventional polyethylene. In the current case report, we describe the failure of a highly cross-linked polyethylene acetabular liner less than three years after implantation. We believe that the mechanisms of failure were fracture of the cross-linked polyethylene where it was thin along the locking groove and abnormal loading of the cup because of improper seating of the liner in the shell at the time of the original procedure. This study was conducted with institutional review board approval and in compliance with HIPAA (Health Insurance Portability and Accountability Act) requirements at our institution. A seventy-four-year-old woman underwent a primary right total hip replacement through a posterior minimally invasive approach. The femoral component was a VerSys Advocate cemented stem (Zimmer, Warsaw, Indiana) with a satin surface finish, a 34-mm neck length, and a …

Polyethylene damage on the nonarticular surface of modular total knee prostheses.

Modular tibial implants submitted for retrieval analysis were examined for evidence of cold flow, wear, or polyethylene failure. All retrieved components showed areas of cold flow and wear. Significant damage, defined as pitting, gouging, or delamination was observed in 77% of the retrieved implants. Cold flow deformation commonly was observed at the junction of the polyethylene and locking mechanisms or junction of the polyethylene and the edge of the tibial tray. Control implants did not show such areas of damage. Clinical factors such as age, activity status, length of implantation, limb alignment before revision, and thickness of the tibial insert did not correlate with backside wear. No association was found between polyethylene thickness, time of implantation, limb or prosthesis alignment, or activity status or weight of the patient.

Observations of biofilm growth on human dentin and potential destruction after exposure to antibiotics

OBJECTIVES This study was performed to observe biofilm formation on dentin and to then observe effects of clinically achievable antimicrobial drug concentrations on these biofilms. STUDY DESIGN Wild-strain endodontic bacteria were anaerobically cultured from necrotic pulps of extracted human teeth and used to grow biofilms on sterilized dentin slices in an anaerobic chamber for 12 days. Then these biofilms were exposed to ampicillin, doxycycline, clindamycin, azithromycin, or metronidazole. Each day for 8 days, specimens were fixed using 2% glutaraldehyde and examined with a scanning electron microscope (SEM). RESULTS The SEM images revealed the presence of a mature biofilm after 8 days of growth and that none of the antibiotics tested was effective in eliminating the biofilm even after 8 days of exposure. CONCLUSION Biofilms are formed in a few days and are resistant to antimicrobial drugs.

Influence of particle abrasion or hydrofluoric acid etching on lithium disilicate flexural strength

STATEMENT OF PROBLEM Lithium disilicate is a translucent, glass-containing material used for ceramic restorations. Clinicians frequently use alumina abrading or hydrofluoric acid etching to create micromechanical retention in the intaglio surface before bonding a lithium disilicate restoration to the tooth. Few studies have investigated how the etching or abrasion processes affect the flexural strength of lithium disilicate ceramics. PURPOSE The purpose of this study was to measure and compare the flexural strength of e.max CAD after alumina abrasion at differing pressures and acid etching at differing concentrations and times. MATERIAL AND METHODS Bars of e.max CAD (9 groups of 10; 22×2.5×2.5 mm) were prepared, polished sequentially with 180, 320, and 600 abrasive paper, and sintered according to the manufacturers instructions. Four groups were particle abraded (30-μm alumina particles from 10 mm at 55, 100, 200, or 300 kPa for 10 seconds). Four groups were etched with either 5% hydrofluoric acid (20 seconds or 120 seconds) or 9.5% hydrofluoric acid (20 seconds or 120 seconds). The control was polished and fired only (no treatment). Specimens were placed onto an Instron (1 mm/min crosshead speed) and loaded to failure in a 3-point flexural test. One-way ANOVA and the Dunnett t test determined intergroup differences (α=.05). RESULTS Compared with the control, the 100, 200, and 300 kPa alumina abraded groups produced significantly lower flexural strengths (P<.001); however, the flexural strength of the 55 kPa abraded group was not statistically different from the control (P=.080). The flexural strength of the 5% and the 9.5% hydrofluoric acid-etched groups also were not significantly different from the control (P>.050); however, the 9.5% hydrofluoric acid at 20 seconds group was nearly statistically significant (P=.051). CONCLUSION Alumina particle abrasion at pressures of 100 kPa and higher significantly reduced flexural strength by creating stress risers in e.max CAD and should not be used. Hydrofluoric acid etching should be used to increase micromechanical retention and clean the intaglio surface of the restoration before bonding.

Surface Roughness of CoCr and ZrO2 Femoral Heads with Metal Transfer: A Retrieval and Wear Simulator Study

Metal transfer to femoral heads may result from impingement against the metallic acetabular shell following subluxation/dislocation, or when metallic debris enters the articulation zone. Such transfers roughen the head surface, increasing polyethylene wear in total hip replacements. Presently, we examined the surface roughness of retrieved femoral heads with metallic transfer. Profilometry revealed roughness averages in regions of metal transfer averaging 0.380 μm for CoCr and 0.294 μm for ZrO2 which were one order of magnitude higher than those from non-implanted controls. Scanning electron microscopy (SEM) revealed adherent transfers on these retrievals, with titanium presence confirmed by electron dispersive spectroscopy. Due to the concern for increased wear, metal transfer was induced on non-implanted heads, which were then articulated against flat polyethylene discs in multidirectional sliding wear tests. Increased polyethylene wear was associated with these specimens as compared to unaltered controls. SEM imaging provided visual evidence that the transfers remained adherent following the wear tests. Pre- and post-test roughness averages exceeded 1 μm for both the CoCr and ZrO2 heads. Overall, these results suggest that metal transfer increases the surface roughness of CoCr and ZrO2 femoral heads and that the transfers may remain adherent following articulation against polyethylene, leading to increased polyethylene wear.

Different enamel and dentin mineralization observed in VDR deficient mouse model

UNLABELLED Vitamin D plays an important role in the bone mineralization process. Enamel and dentin are two mineralized tissues of different origins that combine to form teeth, but the mechanism by which vitamin D regulates these tissues remains unclear. We hypothesized that vitamin D affects enamel and dentin mineralization through different mechanisms. OBJECTIVE To examine enamel and dentin mineralization in a vitamin D receptor (VDR) deficient mouse model by micro-computerized tomography (micro-CT) and scanning electronic microscopy (SEM). METHODS VDR wild type mice (VDR+/+) and VDR deficient (VDR-/-) littermates were sacrificed at 70.5 days old, and their mandibles were dissected. Micro-CT was used to compare mineral density (MD) of enamel and dentin of the two groups at different levels along the axis of mandibular incisors. SEM was employed to examine the ultrastructure of incisors at the levels corresponding to the levels used for the micro-CT studies. Furthermore, an accelerated eruption procedure was performed to exclude the effect of delayed eruption on enamel and dentin mineralization. RESULTS Different distribution patterns of enamel and dentin MD were observed between VDR+/+ and VDR-/- groups. Early enamel maturation, mineralization, and hypomineralization in dentin were observed in the VDR deficient mice. CONCLUSION Vitamin D may affect the mineralization of dentin systemically, and enamel mineralization may be regulated locally.

Characterization of third-body media particles and their effect on in vitro composite wear

OBJECTIVES The purpose of this study was to compare four medium particles currently used for in vitro composite wear testing (glass and PMMA beads and millet and poppy seeds). METHODS Particles were prepared as described in previous wear studies. Hardness of medium particles was measured with a nano-indentor, particle size was measured with a particle size analyzer, and the particle form was determined with light microscopy and image analysis software. Composite wear was measured using each type of medium and water in the Alabama wear testing device. Four dental composites were compared: a hybrid (Z100), flowable microhybrid (Estelite Flow Quick), micromatrix (Esthet-X), and nano-filled (Filtek Supreme Plus). The test ran for 100,000 cycles at 1.2 Hz with 70 N force by a steel antagonist. Volumetric wear was measured by non-contact profilometry. A two-way analysis of variance (ANOVA) and Tukeys test was used to compare both materials and media. RESULTS Hardness values (GPa) of the particles are (glass, millet, PMMA, and poppy, respectively): 1.310(0.150), 0.279(0.170), 0.279(0.095), and 0.226(0.146). Average particle sizes (μm) are (glass, millet, PMMA, and poppy, respectively): 88.35(8.24), 8.07(4.05), 28.95(8.74), and 14.08(7.20). Glass and PMMA beads were considerably more round than the seeds. During composite wear testing, glass was the only medium that produced more wear than the use of water alone. The rank ordering of the materials varied with each medium, however, the glass and PMMA bead medium allowed better discrimination between materials. SIGNIFICANCE PMMA beads are a practical and relevant choice for composite wear testing because they demonstrate similar physical properties as seeds but reduce the variability of wear measurements.

Comparison of root canal preparation using reciprocating Safesiders stainless steel and Vortex nickel-titanium instruments

OBJECTIVE The aim of the present study was to assess several parameters related to the clinical usage of 2 root canal preparation instruments: Vortex .06 rotary nickel-titanium instruments, and Safesiders reciprocating stainless steel instruments. STUDY DESIGN Fifty extracted mandibular molars with mesial root canal curvatures between 20° and 50° were divided into 2 groups and embedded in acrylic resin inside a modified Bramante muffle system. All root canals were prepared to ISO size 40 using either Vortex .06 rotary nickel-titanium-instruments in a low-torque motor or Safesiders stainless steel instruments in a proprietary reciprocating handpiece. The following parameters were evaluated: straightening of curved root canals, working safety issues (perforations, instrument breakages, canal blockages, loss of working length), postpreparation root canal cross-section, and working time. RESULTS The Vortex .06 instruments maintained canal curvatures well, with the mean degree of straightening recorded as 0.72°. Safesiders instruments demonstrated significantly more canal straightening, with the mean degree of straightening recorded as 15.5°. More than 90% of the root canals prepared with the Vortex .06 instruments resulted in a round or oval cross-section, whereas the Safesiders instruments produced round or oval cross-sections 60% of the time. Neither of the 2 instruments could effectively prepare 100% of the root canal circumference. The area of dentin removed and the remaining dentin thicknesses from each region were similar for the 2 groups. Six procedural incidents were recorded for the Vortex .06 group, compared with 19 for the Safesiders group. There were no instrument fractures recorded in either group. Mean working time was significantly shorter for Vortex .06 (279 s) than for Safesiders (324 s). CONCLUSIONS Vortex .06 maintained the original root canal curvatures well, whereas Safesiders instruments demonstrated significant straightening and irregular preparation shapes when used in sizes larger than ISO 20. Preparation of the complete circumference of the root canal was not possible with either system. Fewer procedural errors occurred with the Vortex instruments.

Normalisation of calcium status reverses the phenotype in dentin, but not in enamel of VDR-deficient mice

OBJECTIVE To determine the effects of vitamin D receptor (VDR) deficiency on mouse dentin and enamel mineralisation, and how normalisation of serum calcium level affects dentin and enamel phenotypes in VDR knockout mice. MATERIALS AND METHODS Groups of VDR wild-type (VDR+/+), VDR deficient (VDR-/-) and VDR-/- rescued mice were sacrificed at 70.5 days of life. The rescued group was established by a high-calcium diet feeding the VDR-/- mice from postnatal 19 days. Micro-CT was used to compare enamel and dentin mineralisation density (MD) at different levels of mandibular incisors among the groups. The scanning electron microscope (SEM) was used to examine the ultrastructure of the enamel and dentin in the corresponding levels and of surface enamel after acidic treatment. RESULTS Micro-CT showed that in VDR-/- rescued group, dentin phenotype was reversed and dentin MD was reversed to normal; however, enamel mineralisation was not reversible, and remained as hypermineralisation in molar region and apical region of the incisors. SEM also revealed enamel hypermineralisation in the VDR-/- rescued group. This early enamel hypermineralisation was more susceptible to acidic erosion. CONCLUSION Vitamin D affects dentin mineralisation systemically, and it regulates enamel mineralisation locally.