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We present a deep Cousins RI photometric survey of the open cluster NGC 1960, complete to R_C \simeq 22, I_C \simeq 21, that is used to select a sample of very low-mass cluster candidates. Gemini spectroscopy of a subset of these is used to confirm membership and locate the age-dependent "lithium depletion boundary" (LDB) --the luminosity at which lithium remains unburned in its low-mass stars. The LDB implies a cluster age of 22 +/-4 Myr and is quite insensitive to choice of evolutionary model. NGC 1960 is the youngest cluster for which a LDB age has been estimated and possesses a well populated upper main sequence and a rich low-mass pre-main sequence. The LDB age determined here agrees well with precise age estimates made for the same cluster based on isochrone fits to its high- and low-mass populations. The concordance between these three age estimation techniques, that rely on different facets of stellar astrophysics at very different masses, is an important step towards calibrating the absolute ages of young open clusters and lends confidence to ages determined using any one of them., Comment: Accepted for publication in MNRAS

We have used medium resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion Nebula Cluster for strong lithium depletion through analysis of veiling corrected 6708 angstrom lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M_dot > 5 x 10-4 M_sol yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 < M < 1.9 M_sol stars. We also find that the dispersion in the strength of the 6708 angstrom lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour magnitude diagrams. Evidence for weak lithium depletion (< 10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 x 10-4 M_sol yr-1 are responsible., Comment: 13 pages, 10 figures; Accepted for publication in Monthly Notices of the Royal Astronomical Society, 2013 June 03

We have derived ages for 13 young (<30 Myr) star-forming regions and find they are up to a factor two older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (~10-12 Myr) and that the average Class I lifetime is greater (~1 Myr) than currently believed. For each star-forming region we derived two ages from colour-magnitude diagrams. First we fitted models of the evolution between the zero-age main-sequence and terminal-age main-sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50 Myr) we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us confidence in the new age scale. This agreement is largely due to our adoption of empirical colour-Teff relations and bolometric corrections for pre-main-sequence stars cooler than 4000 K. The revised ages for the star-forming regions in our sample are: ~2 Myr for NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon Nebula; M 8), and NGC 2244 (Rosette Nebula); ~6 Myr for {\sigma} Ori, Cep OB3b, and IC 348; ~10 Myr for {\lambda} Ori (Collinder 69); ~11 Myr for NGC 2169; ~12 Myr for NGC 2362; ~13 Myr for NGC 7160; ~14 Myr for {\chi} Per (NGC 884); and ~20 Myr for NGC 1960 (M 36)., Comment: 28 pages, 18 figures, 34 tables, accepted for publication in MNRAS. All photometric catalogues presented in this paper are available online at the Cluster Collaboration homepage http://www.astro.ex.ac.uk/people/timn/Catalogues

We present a critical assessment of commonly used pre-main-sequence isochrones by comparing their predictions to a set of well-calibrated colour-magnitude diagrams of the Pleiades in the wavelength range 0.4 to 2.5 microns. Our analysis shows that for temperatures less than 4000 K the models systematically overestimate the flux by a factor two at 0.5 microns, though this decreases with wavelength, becoming negligible at 2.2 microns. In optical colours this will result in the ages for stars younger than 10 Myr being underestimated by factors between two and three. We show that using observations of standard stars to transform the data into a standard system can introduce significant errors in the positioning of pre-main-sequences in colour-magnitude diagrams. Therefore we have compared the models to the data in the natural photometric system in which the observations were taken. Thus we have constructed and tested a model of the system responses for the Wide-Field Camera on the Isaac Newton Telescope. As a benchmark test for the development of pre-main-sequence models we provide both our system responses and the Pleiades sequence., Comment: 15 pages, 12 figures, 7 tables, accepted for publication in MNRAS. All tables are available online at the Cluster Collaboration homepage http://www.astro.ex.ac.uk/people/timn/Catalogues

Blade design can have a large effect on the surgical wear debris produced during TKA, and has quantified the weight of debris lost from the blades and cutting blocks. In the effort to address this metal debris generation, an oscillating-tip blade such as the Precision blade can significantly reduce the amount of debris produced

Although cement-within-cement revision arthroplasty minimizes the complications associated with removal of secure PMMA, failure at the interfacial region between new and old cement mantles remains a theoretical concern. This article assesses the variability in shear properties of bilaminar cement mantles related to duration of postcure and the use of antibiotic cements. Bilaminar cement mantles were 15% to 20% weaker than uniform mantles (P < .001) and demonstrated variability in shear strength related to duration of postcure of the freshly applied cement (P < .001). The use of Antibiotic Simplex did not significantly influence interfacial cement adhesion (P = .52). Interfacial adhesion by mechanisms other than mechanical interlock plays a significant role in the bond formed between new and old PMMA cements, with an important contribution by diffusion-based molecular interdigitation. In the presence of a secure cement-bone interface, we recommend cement-within-cement revision techniques in suitable patients.

The mechanical properties of the short glass fibre reinforced (SGFR) epoxy resin used as the cortical bone analogue in the third-generation Sawbones femurs were investigated for use within orthopaedic benchtop tests. Tensile and four-point bending tests were used to assess the material properties of the SGFR epoxy at both room (22° C) and body temperatures (37° C). The 20 standardized specimens used for the materials testing were machined from third-generation Sawbones femurs. The flexural properties of the specimens were determined using ASTM D6272-02 and the tensile properties were obtained using ASTM D638-02. The mean (and standard deviation, or SD) values of the modulus of elasticity in four-point bending for room and body temperature specimens of 7.8 (0.64) GPa and 2.8 (0.66) GPa respectively were significantly different (P<0.001). The mean (and SD) values of the modulus of elasticity in tension for the room and body temperature specimens of 9.4 (0.8) GPa and 5.4 (1.3) GPa respectively were also significantly different (P=0.02). The modulus of elasticity of SGFR epoxy is highly temperature dependent. A reduction in the modulus of elasticity of up to 63 per cent was observed when increasing the temperature of the specimens from room to body temperature. SGFR epoxy Sawbones do not accurately represent the material properties of bone at body temperature.

The effect of cyclic loading on the torsional stiffness of a polished double tapered femoral stem was investigated in vitro. Initial torsional stability was compared to torsional stability following cyclic loading. Stems were removed from the cement mantle and reinserted without the use of additional cement. Torsional stability was measured following reinsertion and following further cyclic loading. Subsidence of the stem was observed. No difference in torsional stiffness was observed during loading. No difference between the stiffness prior to extraction and following reinsertion was observed. Torsional stiffness of an Exeter stem does not decrease following axial subsidence under cyclic loading. Stability is retained following reinsertion into the original cement mantle. Debonding of the Exeter stem is not associated with rotational instability of the implant.

This research demonstrates that the torsional stiffness of a cemented highly polished double tapered femoral stem does not decrease following axial subsidence under cyclic loading. Additionally the Exeter stem regains high torsional stiffness following explantation/ re-implantation as it was found that debonding did not have a detrimental effect on the performance of the stem under cyclic and torsional loads.

Total hip arthroplasty (THA) is a successful surgical technique that can be used for the effective treatment of fractured neck of femur, osteoarthritis, tumours, avascular necrosis, failed internal fixation, developmental dysplasia and rheumatoid arthritis. Revision surgery is necessary if loosening allows relative motion between the femoral stem and femur, causing pain and mechanical instability of the THA. The large number of revision operations undertaken each year as a result of implant failure emphasises the need for better biomechanical understanding of the femoral implant system. During 2001-02 in Australia 26,689 hip replacement operations were performed, with 3,710 of these being revision operations. The Exeter stem is the most commonly used cemented stem for primary and revision hip replacement in Australia. It is therefore very important to understand the mechanics of this clinically successful implant. Few studies have presented a through investigation into the mechanics of the Exeter stem from a fundamental perspective. To address these issues, mechanical and finite element (FE) methods were used to conduct experiments and numerical investigations into the mechanics of the Exeter stem. The femur geometry, for both the experimental and FE studies, was based upon the Sawbones model 3303 medium left third generation femur. The stem orientation for all specimens of the study was replicated from the orientation achieved by the senior surgeon implanting into the Sawbones femur. Test rigs were designed specifically to constrain the femur for the purposes of loading and stability measurements. The experimental investigation was used to investigate the torsional mechanical stability of the stem and to monitor this stability following periods of cyclic loading, using a resultant hip contact force, while monitoring the distal migration of the stem. The experimental investigation was also able to provide data for the validation of the finite element model. The resu

Pilot study and critical appraisal of the prototype Exeter PMMA Cement Reamer conducted in order to determine potential applications and improvements to design.

Tensile and four point bending tests were used to assess the material properties of the short glass fibre reinforced (SGFR) epoxy resin that are used as the cortical analogue in third generation Sawbones, at both room (22 degrees Celsius) and body temperatures (37 degrees Celsius). This research demonstrates that the modulus of elasticity of SGFR epoxy used in third generation Sawbones is highly temperature dependent. A reduction in modulus of elasticity of up to 63 percent was observed when increasing the temperature of the specimens from room to body temperature. When performing in vitro total hip modelling ideally the specimens should be at 37°C due to the properties of PMMA cement, however SGFR epoxy Sawbones do not accurately represent bone at this temperature. Hence, this material is not an appropriate model for testing the mechanics of implants that use bone cement.

This investigation has been designed to determine the strain patterns in the proximal femur prior to THA stem implantation and to compare this to the strain of the proximal femur implanted with an Exeter stem in both neutral and varus stem positions.