Proceedings of the Institution of Mechanical Engineers,Part H: Journal of Engineering in Medicine

In vivo experiments have been performed to test the effectiveness of a torso-cooling pad to reduce the temperature in the spinal cord and brain in rats. Coolant was circulated through the cooling pad to provide either mild or moderate cooling. Temperatures in the brain tissue, on the head surface, and on the spine and back surfaces were measured. During mild cooling, the temperature on the back surface was 22.82 ± 2.43 °C compared to 29.34 ± 1.94 °C on the spine surface. The temperature on the back surface during moderate cooling was 13.66 ± 1.28 °C compared to 24.12 ± 5.7 °C on the spine surface. Although the temperature in the brain tissue did not drastically deviate from its baseline value during cooling, there was a difference between the rectal and brain temperatures during cooling, which suggests mild hypothermia in the brain tissue. Using experimental data, theoretical models of the rat head and torso were developed to predict the regional temperatures and to validate the rat models. There was good agreement between the theoretical and experimental temperatures in the torso region. Differences between the predicted and measured temperatures in the brain are likely to be the result of imperfect mixing between the cold spinal fluid and the warm cerebrospinal fluid that surrounds the brain.

One of the major causes of aseptic loosening in an uncemented implant is the lack of any attachment between the implant and the bone. The implant’s stability depends on a combination of primary stability (mechanical stability) and secondary stability (biological stability). The primary stability may affect the implant–bone interface condition and thus influence the load transfer and mechanical stimuli for bone remodelling in the resurfaced femur. This paper reports the results of a study into the affect of primary stability on load transfer and bone adaptation for an uncemented resurfaced femur. Three-dimensional finite element models were used to simulate the intact and resurfaced femurs and the bone remodelling. As a first step towards assessing the immediate post-operative condition, a debonded interfacial contact condition with varying levels of the friction coefficient (0.4, 0.5, and 0.6) was simulated at the implant–bone interface. Then, using a threshold value of micromotion of 50 µm, the implant–bone interfacial condition was varied along the implant–bone boundary to mechanically represent non-osseointegrated or osseointegrated regions of the interface. The considered applied loading conditions included normal walking and stair climbing. Resurfacing leads to strain shielding in the femoral head (20–75 per cent strain reductions). In immediate post-operative conditions, there was no occurrence of elevated strains in the cancellous bone around the proximal femoral neck–component junction resulting in a lower risk of neck fracture. Predominantly, the micromotions were observed to remain below 50 µm at the implant–bone interface, which represents 97–99 per cent of the interfacial surface area. The predicted micromotions at the implant–bone interface strongly suggest the likelihood of bone ingrowth onto the coated surface of the implant, thereby enhancing implant fixation. For the osseointegrated implant–bone interface, the effect of strain shielding was observed in a considerably greater bone volume in the femoral head as compared to the initial debonded interfacial condition. A 50–80 per cent periprosthetic bone density reduction was predicted as compared to the value of the intact femur, indicating bone resorption within the superior resurfaced head. Although primary fixation of the resurfacing component may be achieved, the presence of high strain shielding and peri-prosthetic bone resorption are a major concern.

Bone void fillers that can enhance biological function to augment skeletal repair have significant therapeutic potential in bone replacement surgery. This work focuses on the development of a unique microporous (0.5–10 µm) marine-derived calcium phosphate bioceramic granule. It was prepared from Corallina officinalis, a mineralized red alga, using a novel manufacturing process. This involved thermal processing, followed by a low pressure-temperature chemical synthesis reaction. The study found that the ability to maintain the unique algal morphology was dependent on the thermal processing conditions. This study investigates the effect of thermal heat treatment on the physiochemical properties of the alga. Thermogravi-metric analysis was used to monitor its thermal decomposition. The resultant thermograms indicated the presence of a residual organic phase at temperatures below 500°C and an irreversible solid-state phase transition from mg-rich-calcite to calcium oxide at temperatures over 850°C. Algae and synthetic calcite were evaluated following heat treatment in an air-circulating furnace at temperatures ranging from 400 to 800°C. The highest levels of mass loss occurred between 400–500°C and 700–800°C, which were attributed to the organic and carbonate decomposition respectively. The changes in mechanical strength were quantified using a simple mechanical test, which measured the bulk compressive strength of the algae. The mechanical test used may provide a useful evaluation of the compressive properties of similar bone void fillers that are in granular form. The study concluded that soak temperatures in the range of 600 to 700°C provided the optimum physiochemical properties as a precursor to conversion to hydroxyapatite (HA). At these temperatures, a partial phase transition to calcium oxide occurred and the original skeletal morphology of the alga remained intact.

Background: The blood pressure (BP) waveform is suggested to reflect the whole-body blood supply distribution, but its non-invasive assessment is not sufficiently user friendly for practical applications. The present authors studied the correlation between BP and photo-plethysmography (PPG) waveforms, with the aim of determining the optimal range for contact pressure stimulation (PS) to produce a reliable transfer function in their harmonic parameters. Methods: Finger PPG and radial-artery BP signals were measured simultaneously and non-invasively on healthy volunteers (n = 45). PS of 0–200 mmHg was applied to the finger, and 1 min data sequences were recorded. In frequency-domain analysis, linear regression was applied to the calculated amplitude ratios or the first five harmonics between BP and PPG waveforms. Results: In the 60 mmHg-PS group, the BP–PPG regression of amplitude ratios was highest, and the agreements between them were also the best verified by Bland–Altman analysis. Conclusion: In the present study, frequency-domain analysis was performed to study the correlation between BP and PPG waveforms. Differences in pressure-induced-vasodilation responses underlie the different BP-PPG waveform correlations obtained by applying different PSs. The non-invasively derived PPG parameters might help to provide an easier method to acquire the radial-artery BP waveform, and hence broaden the application of BP waveform analysis.

Homogenized elastic properties are often assumed for macro-finite element (FE) models used in orthopaedic biomechanics. The accuracy of material property assignments may have a strong effect on the ability of these models to make accurate predictions. For cortical bone, most macro-scale FE models assume isotropic elastic material behaviour and do not include variation of material properties due to bone micro-architecture. The first aim of the present study was to evaluate the variation of apparent-level (homogenized) orthotropic elastic constants of cortical bone with age and indices of bone micro-architecture. Considerable age-dependent differences in porosity were noted across the cortical thickness in previous research. The second aim of the study was to quantify the resulting differences in elastic constants between the periosteum and endosteum. Specimens were taken from the anterior femoral midshaft of 27 female donors (age 53.4 ± 23.6 years) and micro-FE (µFE) analysis was used to derive orthotropic elastic constants. The variation of orthotropic elastic constants (Young’s moduli, shear moduli, and Poisson’s ratios) with various cortical bone micro-architectural indices was investigated. The ratio of canal volume to tissue volume, Ca.V/TV, analogous to porosity, was found to be the strongest predictor (r _ave ² = 0.958) of the elastic constants. Age was less predictive (r _ave ² = 0.385) than Ca.V/TV. Elastic anisotropy increased with increasing Ca.V/TV, leading to lower elastic moduli in the transverse, typically less frequently loaded, directions. Increased Ca.V/TV led to a more substantial reduction in elastic constants at the endosteal aspect than at the periosteal aspect. The results are expected to be most applicable in similar midshaft locations of long bones; specific analysis of other sites would be necessary to evaluate elastic properties elsewhere. It was concluded that Ca.V/TV was the most predictive of cortical bone elastic constants and that considerable periosteal–endosteal variations in these constants can develop with bone loss.

Three models of different stent designs implanted in a cerebral aneurysm, originating from the Virtual Intracranial Stenting Challenge’07, are meshed and the flow characteristics simulated using commercial computational fluid dynamics (CFD) software in order to investigate the effects of non-Newtonian viscosity and pulsatile flow. Conventional mass inflow and wall shear stress (WSS) output are used as a means of comparing the CFD simulations. In addition, a WSS distribution is presented, which clearly discriminates in favour of the stent design identified by other groups. It is concluded that non-Newtonian and pulsatile effects are important to include in order to avoid underestimating WSS, to understand dynamic flow effects, and to discriminate more effectively between stent designs.

A small-scale biomechanical disc culture system was designed to stimulate intervertebral disc (IVD) ‘motion segment’ in culture environment with load-controlled compression and combined load (compression + shear). After 7 days of diurnal mechanical loading, cell viability of discs stimulated with static compression load (0.25 MPa) and static combined load (compression (0.25 MPa) + shear (1.5 N)) were similar (> 90 per cent) to unloaded controls. Mechanically stimulated discs showed decrease in static/dynamic moduli, early stress relaxation, and loss of disc height after 7 days of diurnal loading. Histological data of discs indicated load-induced transformations that were not apparent in controls. The feasibility of studying the mechanobiology of intact IVD as a motion segment was demonstrated. Media conditioning (improve tissue stability in long-term culture) and application of biochemical gene expression assays (differential tissue response to types of mechanical stimulation) are proposed as future improvements. The study suggests that the limitations in studying mechanobiology of IVD pathology in vitro can be overcome and it is possible to understand the physiologically relevant mechanism of IVD pathology.

Previous research has demonstrated that the number of degrees of freedom (DOF) modelled at a given joint affects the antagonistic muscle activity predicted by inverse dynamics optimization techniques. This higher level of muscle activity in turn results in greater joint contact forces. For instance, modelling the knee as a 3 DOF joint has been shown to result in higher hip and knee joint forces commensurate with a higher level of muscular activity than when the knee is modelled with 1 DOF. In this study, a previously described musculoskeletal model of the lower limb was used to evaluate the sensitivity of the knee and hip joint contact forces to the DOF at the knee during vertical jumping in both a 1 and a 3 DOF knee model. The 3 DOF knee was found to predict higher tibiofemoral and hip joint contact forces and lower patellofemoral joint contact forces. The magnitude of the difference in hip contact force was at least as significant as that found in previous research exploring the effect of subject-specific hip geometry on hip contact force. This study therefore demonstrates a key sensitivity of knee and hip joint contact force calculations to the DOF at the knee. Finally, it is argued that the results of this study highlight an important physiological question with practical implications for the loading of the structures of the knee; that is, the relative interaction of muscular, ligamentous, and articular structures in creating moment equilibrium at the knee.