Owen Addison

Resin Elasticity and the Strengthening of All-ceramic Restorations

Resin luting of all-ceramic restorations results in increased performance; however, the strengthening mechanism and the role of the mechanical properties of the resin are not fully understood. The hypothesis tested is that ceramic strength enhancement is dependent on the elastic modulus of the resin. Three-point flexural moduli of a flowable, luting, and hybrid composite resin were characterized. Two hundred forty porcelain discs were air-abraded. One group acted as a control, and 3 additional groups were coated with 120 ± 20 μm of each resin prior to bi-axial flexure testing. All resins significantly increased in mean strength, and the associated strength increase was related to the elastic modulus of the resin (R2 = 0.9885), so the hypothesis was accepted. The combination of Poisson constraint and the creation of a resin-inter-penetrating layer sensitive to the elastic modulus of the resin may provide an explanation of the strengthening mechanism.

Do ‘passive’ medical titanium surfaces deteriorate in service in the absence of wear?

Globally, more than 1000 tonnes of titanium (Ti) is implanted into patients in the form of biomedical devices on an annual basis. Ti is perceived to be ‘biocompatible’ owing to the presence of a robust passive oxide film (approx. 4 nm thick) at the metal surface. However, surface deterioration can lead to the release of Ti ions, and particles can arise as the result of wear and/or corrosion processes. This surface deterioration can result in peri-implant inflammation, leading to the premature loss of the implanted device or the requirement for surgical revision. Soft tissues surrounding commercially pure cranial anchorage devices (bone-anchored hearing aid) were investigated using synchrotron X-ray micro-fluorescence spectroscopy and X-ray absorption near edge structure. Here, we present the first experimental evidence that minimal load-bearing Ti implants, which are not subjected to macroscopic wear processes, can release Ti debris into the surrounding soft tissue. As such debris has been shown to be pro-inflammatory, we propose that such distributions of Ti are likely to effect to the service life of the device.

Lymphoid aggregates that resemble tertiary lymphoid organs define a specific pathological subset in metal-on-metal hip replacements.

Aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) has been used to describe the histological lesion associated with metal-on-metal (M-M) bearings. We tested the hypothesis that the lymphoid aggregates, associated with ALVAL lesions resemble tertiary lymphoid organs (TLOs). Histopathological changes were examined in the periprosthetic tissue of 62 M-M hip replacements requiring revision surgery, with particular emphasis on the characteristics and pattern of the lymphocytic infiltrate. Immunofluorescence and immunohistochemistry were used to study the classical features of TLOs in cases where large organized lymphoid follicles were present. Synchrotron X-ray fluorescence (XRF) measurements were undertaken to detect localisation of implant derived ions/particles within the samples. Based on type of lymphocytic infiltrates, three different categories were recognised; diffuse aggregates (51%), T cell aggregates (20%), and organised lymphoid aggregates (29%). Further investigation of tissues with organised lymphoid aggregates showed that these tissues recapitulate many of the features of TLOs with T cells and B cells organised into discrete areas, the presence of follicular dendritic cells, acquisition of high endothelial venule like phenotype by blood vessels, expression of lymphoid chemokines and the presence of plasma cells. Co-localisation of implant-derived metals with lymphoid aggregates was observed. These findings suggest that in addition to the well described general foreign body reaction mediated by macrophages and a T cell mediated type IV hypersensitivity response, an under-recognized immunological reaction to metal wear debris involving B cells and the formation of tertiary lymphoid organs occurs in a distinct subset of patients with M-M implants.

Quantifying the Strength of a Resin-coated Dental Ceramic

Resin luting all-ceramic restorations increases clinical performance; however, the strengthening mechanisms are not fully understood. The authors have previously proposed the existence of a resin-ceramic hybrid layer, and the hypothesis tested was that ceramic strength enhancement was conferred by the characteristics of the resin-ceramic hybrid layer. Dentin porcelain discs were polished with a P4000-grade abrasive paper, and half were centrally indented at 9.8 N. Further discs were alumina-air-abraded. Groups of 30 specimens were coated with resin cement thicknesses varying from 0 to 250 ± 20 μm before bi-axial flexure testing. Following investigation of residual stresses by annealing, regression analysis enabled us to calculate the magnitude of ’actual’ strengthening for a theoretical ’zero’ thickness of resin cement on each surface texture. Accounting for resin bulk strengthening, resin cement coating significantly increased the mean strength that was attributed to a resin-ceramic hybrid layer sensitive to surface texture.

Initial fracture resistance and curing temperature rise of ten contemporary resin-based composites with increasing radiant exposure.

OBJECTIVES The principal objective of this study was to determine whether the bulk fracture resistance of ten light activated composites varied over a clinically realistic range of radiant exposures between 5 and 40 J/cm(2). METHODS Ten operators were tested for clinically simulated radiant exposure delivery from a Bluephase(®) (Ivoclar Vivadent, Schaan, Liechtenstein) LED light to an occlusal cavity floor in tooth 27 in a mannequin head using a MARC(®)-Patient Simulator (Bluelight Analytics Inc., Halifax, NS) device. Notch disc test samples were prepared to determine the torque resistance to fracture (T) of the composites. Samples were irradiated with the same monowave Bluephase(®) light for 10s, 20s or 40s at distances of 0mm or 7 mm. After 24h, storage samples were fractured in a universal testing machine and torque to failure was derived. RESULTS Radiant exposure delivered in the clinical simulation ranged from 14.3% to 69.4% of maximum mean radiant exposure deliverable at 0mm in a MARC(®)-Resin Calibrator (Bluelight Analytics Inc., Halifax, NS) test device. Mean torque to failure increased significantly (P<0.05) with radiant exposure for 8 out of 10 products. The micro-fine hybrid composite Gradia Direct anterior (GC) had the lowest mean (S.D.) T between 10.3 (1.8)N/mm and 13.7 (2.2)N/mm over the tested radiant exposure range. Three heavily filled materials Majesty Posterior, Clearfil APX and Clearfil Photo-Posterior (Kuraray) had mean T values in excess of 25 N/mm following 40 J/cm(2) radiant exposure. Mean T for Z100 (3MESPE) and Esthet-X (Dentsply) increased by 10% and 91% respectively over the tested range of radiant exposures. CONCLUSIONS Individual products require different levels of radiant exposure to optimize their fracture resistance. Light activated composites vary in the rate at which they attain optimal fracture resistance. CLINICAL SIGNIFICANCE Unless the clinician accurately controls all the variables associated with energy delivery, there is no way of predicting that acceptable fracture resistance will be achieved intra-orally.

Titanium phosphate glass microspheres for bone tissue engineering

We have demonstrated the successful production of titanium phosphate glass microspheres in the size range of ∼10-200 μm using an inexpensive, efficient, easily scalable process and assessed their use in bone tissue engineering applications. Glasses of the following compositions were prepared by melt-quench techniques: 0.5P₂O₅-0.4CaO-(0.1-x)Na₂O-xTiO₂, where x=0.03, 0.05 and 0.07 mol fraction (denoted as Ti3, Ti5 and Ti7 respectively). Several characterization studies such as differential thermal analysis, degradation (performed using a novel time lapse imaging technique) and pH and ion release measurements revealed significant densification of the glass structure with increased incorporation of TiO₂ in the glass from 3 to 5 mol.%, although further TiO₂ incorporation up to 7 mol.% did not affect the glass structure to the same extent. Cell culture studies performed using MG63 cells over a 7-day period clearly showed the ability of the microspheres to provide a stable surface for cell attachment, growth and proliferation. Taken together, the results confirm that 5 mol.% TiO₂ glass microspheres, on account of their relative ease of preparation and favourable biocompatibility, are worthy candidates for use as substrate materials in bone tissue engineering applications.

Lipopolysaccharide inhibits or accelerates biomedical titanium corrosion depending on environmental acidity

Titanium and its alloys are routinely used as biomedical implants and are usually considered to be corrosion resistant under physiological conditions. However, during inflammation, chemical modifications of the peri-implant environment including acidification occur. In addition certain biomolecules including lipopolysaccharide (LPS), a component of Gram-negative bacterial cell walls and driver of inflammation have been shown to interact strongly with Ti and modify its corrosion resistance. Gram-negative microbes are abundant in biofilms which form on dental implants. The objective was to investigate the influence of LPS on the corrosion properties of relevant biomedical Ti substrates as a function of environmental acidity. Inductively coupled plasma mass spectrometry was used to quantify Ti dissolution following immersion testing in physiological saline for three common biomedical grades of Ti (ASTM Grade 2, Grade 4 and Grade 5). Complementary electrochemical tests including anodic and cathodic polarisation experiments and potentiostatic measurements were also conducted. All three Ti alloys were observed to behave similarly and ion release was sensitive to pH of the immersion solution. However, LPS significantly inhibited Ti release under the most acidic conditions (pH 2), which may develop in localized corrosion sites, but promoted dissolution at pH 4–7, which would be more commonly encountered physiologically. The observed pattern of sensitivity to environmental acidity of the effect of LPS on Ti corrosion has not previously been reported. LPS is found extensively on the surfaces of skin and mucosal penetrating Ti implants and the findings are therefore relevant when considering the chemical stability of Ti implant surfaces in vivo.International Journal of Oral Science advance online publication, 30 January 2015; doi:10.1038/ijos.2014.76

Adherence of oral streptococci to nanostructured titanium surfaces.

OBJECTIVES Peri-implantitis and peri-mucositis pose a severe threat to the success of dental implants. Current research focuses on the development of surfaces that inhibit biofilm formation while not inferring with tissue integration. This study compared the adherence of two oral bacterial species, Streptococcus sanguinis and Streptococcus mutans to nanostructured titanium surfaces. METHODS The samples included TiO2 nanotubes formed by anodization of titanium foil of 100, 50 and 15nm diameter (NT15, NT50, NT100), a nanoporous (15nm pore diameter) surface and compact TiO2 control. Adherent surviving bacteria were enumerated after 1h in an artificial saliva medium containing bovine mucin. RESULTS Lowest numbers of adherent bacteria of both species were recovered from the original titanium foil and nanoporous surface and highest numbers from the Ti100 nanotubes. Numbers of attached S. sanguinis increased in the order (NT15<NT50<NT100), correlated with increasing percentage of surface fluoride. The lowest adhesion of S. sanguinis and S. mutans on TiO2 nanostructured surfaces was observed for small diameter nanoporous surfaces which coincides with the highest osteoblast adhesion on small diameter nanotubular/nanoporous surfaces shown in previous work. SIGNIFICANCE This study indicates that the adherence of oral streptococci can be modified by titanium anodization and nanotube diameter.

Adding functionality with additive manufacturing: Fabrication of titanium-based antibiotic eluting implants.

Additive manufacturing technologies have been utilised in healthcare to create patient-specific implants. This study demonstrates the potential to add new implant functionality by further exploiting the design flexibility of these technologies. Selective laser melting was used to manufacture titanium-based (Ti-6Al-4V) implants containing a reservoir. Pore channels, connecting the implant surface to the reservoir, were incorporated to facilitate antibiotic delivery. An injectable brushite, calcium phosphate cement, was formulated as a carrier vehicle for gentamicin. Incorporation of the antibiotic significantly (p=0.01) improved the compressive strength (5.8±0.7MPa) of the cement compared to non-antibiotic samples. The controlled release of gentamicin sulphate from the calcium phosphate cement injected into the implant reservoir was demonstrated in short term elution studies using ultraviolet-visible spectroscopy. Orientation of the implant pore channels were shown, using micro-computed tomography, to impact design reproducibility and the back-pressure generated during cement injection which ultimately altered porosity. The amount of antibiotic released from all implant designs over a 6hour period (<28% of the total amount) were found to exceed the minimum inhibitory concentrations of Staphylococcus aureus (16μg/mL) and Staphylococcus epidermidis (1μg/mL); two bacterial species commonly associated with periprosthetic infections. Antibacterial efficacy was confirmed against both bacterial cultures using an agar diffusion assay. Interestingly, pore channel orientation was shown to influence the directionality of inhibition zones. Promisingly, this work demonstrates the potential to additively manufacture a titanium-based antibiotic eluting implant, which is an attractive alternative to current treatment strategies of periprosthetic infections.

A synergistic effect of albumin and H2O2 accelerates corrosion of Ti6Al4V

UNLABELLED The synergistic effect of albumin and H2O2 on corrosion of titanium alloy Ti6Al4V in physiological saline was investigated with long-term immersion tests and electrochemical methods. It was found that in the presence of both albumin and H2O2, the rate of metal release in immersion tests was far higher than in the presence of either species alone. Electrochemical polarisation curves and potentiostatic tests showed that H2O2 increased both the rates of the anodic and cathodic reactions, whilst albumin significantly decreased the rate of the cathodic reaction and slightly decreased the rate of the anodic reaction. The synergistic effect of albumin and H2O2 during immersion tests was attributed to the effect of adsorption of albumin in lowering the rate of the cathodic reaction and thus lowering the open circuit potential into the active region of titanium where complexation by H2O2 increased the corrosion rate. The corrosion attack was found to be greater in the β-phase of the alloy. The findings suggest that current standard tests in physiological or phosphate-buffered saline may underestimate the rate of corrosion in the peri-implant environment, in which albumin is the predominant protein, and reactive oxygen species such as H2O2 can occur as a result of inflammatory reactions in response to surgery, infection, or implant corrosion products. STATEMENT OF SIGNIFICANCE Corrosion of many biomedical implant materials occurs in the body leading to adverse biological responses. Several components of the environment into which a metal implant is placed including proteins and products of cellular physiology, been shown to modify corrosion resistance. Previously all studies on such components including the common protein albumin and the inflammatory product H2O2 have considered the effects of these species in isolation. For the first time we report a synergistic interaction between albumin and H2O2 significantly accelerating corrosion of Ti6Al4V at physiological pH and temperature. This is attributed to an increased rate of the anodic reaction caused by H2O2 complexation of Ti, suppression of cathodic reaction by albumin adsorption shifting OCP to the active region of Ti6Al4V.