A novel apparatus to assess the mechanical properties of Ankle-Foot Orthoses: Stiffness analysis of the Codivilla spring.

Ankle-Foot Orthoses (AFOs) are the most common devices prescribed to support the ankle and restore a quasi-normal gait pattern in drop-foot patients. AFO stiffness is possibly the main mechanical property affecting foot and ankle biomechanics. A variety of methods to evaluate this property have been reported, however no standard procedure has been validated and widely used. This study is reporting the repeatability of a novel apparatus to measure AFO stiffness in ideal frictionless conditions. The apparatus is based on a servo-hydraulic testing machine and allows to apply a displacement-controlled rotation of the AFO shell, simulating the physiological ankle dorsi/plantarflexion movement. The repeatability of the apparatus in measuring AFO stiffness in dorsiflexion and plantarflexion was assessed intra- and inter-session in a sample of standard polypropylene AFOs of different sizes (Codivilla spring). The repeatability of the apparatus in measuring the AFO stiffness was high. The Intra- and Inter-session Coefficient of Variation ranged between 0.02 ÷ 1.3 % and 1.3 ÷ 5 %, respectively. The Intra Class Correlation Coefficient ranged between 0.999 ÷ 1 intra- and 0.993 ÷ 0.997 inter-session. AFOs stiffness was observed to increase with the AFO size. The setup is easy to replicate and can be implemented with any torsion-controlled servo-hydraulic testing machine and has resulted simple to use and flexible enough to adapt to AFOs with different sizes. The frictionless contacts characterizing the apparatus make it possible to measure the ideal AFO stiffness by excluding the effect of the fixation methods to the leg and help to improve the repeatability of measurements.

Microindentation on cortical human bone: effects of tissue condition and indentation location on hardness values.

The hardness of cortical human bone has been measured on osteons in different conditions. However, no data are reported in the literature regarding the effect of cortical tissue condition and indentation location on the measured hardness values. This study aimed to investigate whether the hardness of the human cortical bone evaluated by micro-indentation is influenced, first, by the tissue condition and, second, by the distance of the indentation from the edge of the Haversian canal. Two femura were collected from a subject without musculoskeletal disease. The Vickers hardness was measured by means of microindentation (applied load, 100 gf) on osteons with a cross-section greater than 200 microm. The tests were performed on wet and embedded tissue at different distances from the Haversian canal edge (30-150 microm). No significant differences were found in hardness values between the two contralateral femura. Embedded tissue was significantly harder (12 per cent) than wet tissue. No significant differences were found in hardness values measured at different distances from the Haversian canal edge except for those closer than 60 microm. Therefore, indentations cannot be performed on osteons small in cross-section, since the distance from the closer pore has to be controlled. They should be performed on wet tissue, to avoid an offset in the measured hardness.

Effect of stem preheating on the fatigue behaviour of bone cement around hip prostheses.

Tensile fatigue behaviour of bone cement specimens obtained from cement mantles moulded in vitro, simulating the surgical scenario, was investigated. The effect of stem preheating, before its insertion into the cement dough, on specimen fatigue life was studied. A commercial bone cement was selected for this study. Bone cement mixing was conducted in air, following the manufacturer's instructions, and injected simulating the clinical practice. Two conditions were considered: stem maintained at the surgical room temperature (23 degrees C), and stem preheated to 45 degrees C. Four repetitions of the whole procedure were performed for each condition obtaining a total of 32 specimens. All specimens underwent fatigue testing (stress ratio, 0; maximum tensile stress, 15 MPa) until failure. Both two-parameter and three-parameter Weibull distributions were initially used to analyse the fatigue life data set. However, the two-parameter distribution was chosen for both groups on the basis of the coefficient of determination used to test the goodness of fit. Stem preheating seems to have a negligible effect on fatigue behaviour of the studied bone cement in the low range of fatigue lives (up to 10(4)). However, above this number of cycles, stem preheating seems to reduce the probability of failure. These findings are discussed in the text.

Dependence of mechanical compressive strength on local variations in microarchitecture in cancellous bone of proximal human femur.

Human cancellous bone is a heterogeneous material. Despite this, most of the published studies report correlations between mechanical properties and morphometric parameters averaged on the whole specimen. This work investigated whether local variations in morphometric parameters were linked to the localized failure regions of cancellous bone. Additionally, it was examined whether local values of morphometric parameters can predict the ultimate stress better than the average bone volume fraction (BV/TV). Cylindrical cancellous bone specimens extracted along the primary compressive group of human femoral heads were studied. These were microCT-imaged to assess the morphometric parameters, compressed to determine the ultimate stress, and rescanned by microCT to visualize the failure region. Failure involved slightly less than half of the free height of the specimens. Significant differences were found in the morphometric parameters calculated in the failure and in the non-failure regions. The cross-sections containing minimum BV/TV values were those most often located inside the failure region (83%, p<0.001). Regression analysis confirmed that variations in BV/TV best describe variations in ultimate stress (R2=0.84) out of the averaged morphometric parameters. The prediction of ultimate stress increased when minimum or maximum values of the morphometric parameters were taken, with the highest prediction found by considering the minimum BV/TV (R2=0.95). In conclusion, due to the heterogeneity of cancellous bone, there may exist regions characterized by a different microarchitecture, where the bone is weaker and consequently is more likely to fail. These regions mostly contain minimum values in BV/TV, which were found to predict ultimate stress better than average BV/TV.

Structural parameters and mechanical strength of cancellous bone in the femoral head in osteoarthritis do not depend on age.

For normal bone, aging has been associated with a decrease of both density and failure strength, and with the development of pathologies such as osteoporosis. Conversely, it has been reported that another common disease, osteoarthritis, may alter these age-related changes in cancellous bone, suggesting that it may have a protective role against osteoporosis and the correspondent fracture risk. It was reported that in the principal compressive region of the femoral head in osteoarthritis the bone density does not depend on age. However, it is not clear if this independence on age of the cancellous bone density corresponds also to a reduced dependence on age of the strength to failure. The present work examined cancellous bone from the principal compressive region of the femoral head of 37 patients having severe osteoarthritis. The aim was (1) to investigate the dependence on age of both the structural parameters and the ultimate stress and (2) to investigate the relationships between the ultimate stress and the structural parameters. Using X-ray microcomputed tomography, three-dimensional structural parameters, such as bone volume fraction, direct trabecular thickness and structure model index were calculated. Then the specimens were compressed to failure to determine the ultimate stress. It was found that none of the investigated structural parameters did depend on age, and also the ultimate stress did not depend on age (p>0.05 for all regressions on age). In addition, the ultimate stress was significantly correlated with the structural parameters, primary with the minimum bone volume fraction and the average bone volume fraction (R(2)=0.95 and R(2)=0.84, respectively). These findings show that severe osteoarthritis or a related factor may change the age dependences of both the structural parameters and the mechanical properties usually reported for normal cancellous bone. These results suggest for this pathology to have a protective role against the age-related decrease in density, the age-related deterioration of the microarchitecture and the age-related decrease of the failure strength for the cancellous bone in the principal compressive region of the human femoral head.

Radiopacity of tantalum-loaded acrylic bone cement.

Radiopacifying agents are commonly added to bone cements to enhance the visibility of the cement in radiography. The radiopacifiers usually employed may, however, have undesired effects on the mechanical properties of the cement. A potentially new radiopacifier is tantalum, which in the present work was evaluated in terms of radiopacity. Bone cements containing different percentages of tantalum were compared with plain bone cement as well as with formulations containing different percentages of the commonly used radiopacifier barium sulphate. The radiopacity was assessed quantitatively and qualitatively, by measuring with a digital densitometer the optical density of the cement on X-ray films, and consulting the expertise of ten orthopaedic surgeons. It was found that tantalum does present radiopacity, but not as high as barium sulphate under the specific conditions applied to this study.

Comparative and experimental study on different tendinous grasping techniques in rotator cuff repair: a new reinforced stitch.

Arthroscopic repair of wide rotator cuff ruptures is burdened by a percentage of recurrences that is greater than the repair carried out when an open technique is used. One of the main reasons for this difference can be searched for in the minor hold of stitching on the tendinous aspect obtained with arthroscopic repair. In fact, when an open technique is used, good hold can be guaranteed by using reinforced stitches such as the modified Mason-Allen. Thus, arthroscopic repair technique on the tendinous aspect, particularly in wide and massive injuries, must be improved. It was the purpose of this study to compare a new reinforced stitch that can easily be obtained in arthroscopy (simple stitch that orthogonally crosses a horizontal stitch previously knotted on the tendon: SS-HL), with traditional stitches (simple stitch, mattress-stitch and modified Mason-Allen stitch). Tests were carried out on sheep infraspinatus tendons in order to evaluate the resistance of pull-out. The SS-HL stitch showed resistance to loading that was similar to that when the modified Mason-Allen was used, but it was greater than that shown by the simple stitch (+48%) and the mattress stitch (+35%).

Pre-clinical validation of a new partially cemented femoral prosthesis by synergetic use of numerical and experimental methods.

The present work reports the pre-clinical validation of an innovative partially cemented femoral prosthesis called cement-locked uncemented (CLU) prosthesis. The inventors of the device under investigation claimed that, when compared to a comparable fully cemented stem, the new stem would present various advantages. Two previous experimental studies confirmed that primary stability and stress shielding were comparable to those of cemented stems. Aim of the present study was to investigate if the remaining claims were confirmed as well. A complete finite element model of the bone-implant complex was created from CT data. The model was validated against in vitro measurements of bone surface strains as well as against primary stability measurements. The peak stresses predicted in the CLU cement mantle were not found significantly lower than those reported in other studies on fully cemented stems. However, once the cement inlet geometry is optimised and the associated stress risers are eliminated, the CLU cement mantle should be subjected to much lower stresses. The stress induced in the stems by both load cases was well below the fatigue limit of the Ti6Al4V alloy. Finite element models predicted for all load cases relative motion between cement and metal lower than 60 microm. This amplitude may be fully accommodated by elastic deformations of the cement micro-ridges. The experimental and numerical results showed the validity of the new fixation concept, although a further optimisation of the geometry of the cement pockets is needed in order to further reduce the stresses in the cement.

Large-sliding contact elements accurately predict levels of bone-implant micromotion relevant to osseointegration.

Primary stability is recognised as an important determinant in the aseptic loosening failure process of cementless implants. An accurate evaluation of the bone-implant relative micromotion is becoming important both in pre-clinical and clinical studies. If the biological threshold for micro-movements is in the range 100-200 micrometer then, in order to be discriminative, any method used to evaluate the primary stability should have an accuracy of 10-20 micrometer or better. Additionally, such method should also be able to report the relative micromotion at each point of the interface. None of the available experimental methods satisfies both requirements. Aim of the present study is to verify if any of the current finite element modelling techniques is sufficiently accurate in predicting the primary stability of a cementless prosthesis to be used to decide whether the micromotion may or may not jeopardise the implant osseointegration. The primary stability of an anatomic cementless stem, as measured in vitro, was used as a benchmark problem to comparatively evaluate different contact modelling techniques. Frictionless contact, frictional contact and press-fitted frictional contact conditions were modelled using alternatively node-to-node, node-to-face and face-to-face contact elements. The model based on face-to-face contact elements accounting for frictional contact and initial press-fit was able to predict the micromotion measured experimentally with an average (RMS) error of 10 micrometer and a peak error of 14 micrometer. All the other models presented errors higher than 20 micrometer assumed in the present study as an accuracy threshold.

Endurance testing of hip prostheses: a comparison between the load fixed in ISO 7206 standard and the physiological loads.

BACKGROUND: Pre-clinical endurance validation of innovative hip prostheses femoral components are carried out following the ISO 7206 standard. The in vitro fatigue test must simulate accurately the physiological load to correctly define the section of the stem. OBJECTIVE: This study investigates if the loads defined in the ISO 7206 standard simulate correctly the physiological load that occur in vivo. DESIGN: Simulation of the damage induced by in vitro test and in vivo loads is performed. Different designs of the stem are examined. Materials with different fatigue limit are considered. For the in vivo loads, different body weights are modelled. METHODS: The minimal stem dimensions required to stand 20years of patient use and to pass the fatigue test are calculated based on linear damage accumulation. RESULTS: The results show that the ISO load simulates reasonably well the physiological load that is likely to occur in active patients with a low or normal body weight. Conversely, the ISO test underestimates the physiological load that heavy and active patients are likely to apply to the implant. CONCLUSIONS: Different minimum requirements for endurance strength should be fixed for hip prostheses designed for patients of different weight. This solution would reduce the risk of undersizing or oversizing the stem. RELEVANCE: The introduction of a classification of the hip prostheses, fixing different minimum requirements for endurance strength, will lead to correctly size stems made of new materials or coated using new surface treatments, reducing the risk of implant failure of innovative prostheses.

The influence of stem insertion rate on the porosity of the cement mantle of hip joint replacements.

This study investigates the effect of stem insertion rate on the porosity of the cement mantle. An experimental protocol was developed to simulate the surgical technique of cementing a prosthetic stem into the medullary canal of the femur. Cement mantle specimens were produced for three different stem insertion rates. The presence of porosity in the cement mantle was investigated. Additionally, the mechanical strength of the bone cement was assessed. Increasing the stem insertion rate did not have a significant effect on the porosity distribution within the bulk cement mantle. However, for all stem insertion rates investigated, the porosity concentration increased significantly moving from the cement/pseudofemur interface through to the stem/cement interface. In all cases, the presence of porosity significantly decreased the mechanical behaviour of the bone cement. High porosity concentration at the stem/cement interface seems to be attributed also to the rheology of the cement during implant insertion. Nevertheless, the surgeon cannot influence the formation of porosity by changing the stem insertion rate.

A procedure and criterion for bone cement fracture toughness tests.

Nowadays, two procedures, based on the recommendation of two American standards (ASTM E399 and ASTM D5045), are used to determine the fracture toughness, KIc, of bone cement. However, there is a lack of knowledge about the equivalence of the two testing methods applied to bone cement. Additionally, in spite of the recommendation of several authors to introduce a rejection criterion for specimens based on the size of defects found in the fracture surface, no data are available about the effect of porosity within the material on the KIc of bone cement. The aims of this study were to verify whether the KIc values calculated for bone cement using the two procedures are comparable and whether macroporosity within the tested samples affects the KIc value of bone cement, and, if so, to establish a rejection criterion for specimen selection. Samples of pure polymethyl methacrylate (PMMA) were tested by both procedures. Additionally, samples showing defects (macroporosity) of different sizes and located in different positions within the specimen were tested. The KIc value determined following the ASTM E399 procedure was 13 per cent lower than that calculated following the ASTM D5045 procedure. In the first series a lower data scatter was observed. Also, the presence of macroporosity on the fracture surface of the specimen affected the KIc value of bone cement. Therefore, the mechanical behaviour of samples was affected by defects within the material. Since it is possible to mould specimens without macroporosity, it seems recommendable to reject specimens with macroporosity on the fracture surface before calculating the KIc value of bone cement.

Temperature and ageing condition effects on the characterization of acrylic bone cement.

This study investigates the effect of the environmental temperature and ageing condition on the characterization of acrylic bone cement. The tests were performed according to ISO 5833. The testing parameters were allowed to vary within the limits defined by the standard, in order to assess their effect on the results of the test. In certain cases the tests were also performed under conditions which the standard does not provide for but which are likely to occur clinically. This investigation showed that the cement behaviour may also change in the temperature range specified in the standard. Therefore, it is deemed appropriate to correlate the curing parameters of the bone cement to the environmental temperature, performing the test at different temperatures. In this way the effect of temperature on the duration of the phases in the cement curing could be assessed. The resultant graphical representation of the effect of temperature on the duration of the phases in cement curing has direct clinical relevance. Furthermore, this study showed that the ageing conditions of the mechanically tested specimens affected the results. Hence, it is deemed advisable to modify the ageing conditions of the specimens, fixing them closer to the in vivo conditions.

Fatigue strength of PMMA bone cement mixed with gentamicin and barium sulphate vs pure PMMA.

Barium sulphate is added to polymethylmethacrylate (PMMA) bone cement as a radiopacifier. Gentamicin is an antibiotic added to bone cement to treat or prevent infection in arthroplasty. This study investigated the combined effect of barium sulphate and gentamicin sulphate on the fatigue strength of PMMA bone cement. Three different formulations were studied: pure PMMA, PMMA with barium sulphate added and PMMA with barium sulphate and gentamicin sulphate added. Before testing all specimens were stored in water at 37 degrees C for at least 15 days to season the PMMA and to elute the antibiotic. Fatigue tests were performed following a previously validated procedure. The slope part of the Wöhler diagram was obtained and a rough endurance limit was estimated for all three formulations. The experimental data showed that the addition of barium sulphate to PMMA bone cement affected the fatigue strength of the material, whereas addition of gentamicin sulphate to the radiopaque PMMA had no effect on the fatigue properties of the bone cement. While PMMA with barium sulphate added was confirmed to have a reduced fatigue strength when compared with plain PMMA, no detrimental effect was found for the addition of gentamicin sulphate to radiopaque PMMA.

The muscle standardized femur: a step forward in the replication of numerical studies in biomechanics.

The standardized femur is the computer aided design (CAD) solid model of a synthetic human femur, commonly used in experiments in vitro, available in the public domain through the International Society of Biomechanics Finite Element Mesh Repository. Currently used by hundreds of researchers, it was made available to simplify the experimental cross-validation of numerical studies as well as their replication by other researchers. One aspect that the standardized femur left uncovered is the definition of muscles and ligaments. In particular, for a variety of simulations it would be extremely useful to map on to the femoral surface the insertion of the principal muscles. The aim of the present study was to create a new solid model, called the muscle standardized femur, where the femoral insertion of each muscle is mapped on to the surface of the femur. Published data on muscle insertion morphometry were registered to the model by applying an affine scaling defined on bone landmarks. Good agreement was found with another similar study in which only the insertion centres were defined. The new model will be made available in the public domain for no-profit uses. When combined with published data on the direction and intensity of muscular forces this model is expected to make a useful contribution to the steadily growing library of models and data sets made available to the biomechanical community.

The effect on the fatigue strength of bone cement of adding sodium fluoride.

New bone cements that include several additives are currently being investigated and tested. One such additive is sodium fluoride (NaF), which promotes bone formation, facilitating implant integration and success. The influence of NaF on the fatigue performance of the cement as used in biomedical applications was tested in this paper. In fact fatigue failure of the cement mantle is a major factor limiting the longevity of a cemented implant. An experimental bone cement with added NaF (12 wt%) was investigated. The fatigue strength of the novel bone cement was evaluated in comparison with the cement without additives; fatigue tests were conducted according to current standards. The load levels were arranged based on a validated, statistically based optimization algorithm. The curve of stress against number of load cycles and the endurance limit were obtained and compared for both formulations. The results showed that the addition of NaF (12 wt%) to polymethylmethacrylate (PMMA) bone cement does not affect the fatigue resistance of the material. Sodium fluoride can safely be added to the bone cement without altering the fatigue performance of the PMMA bone cement.

Selection of the best element type in the finite element analysis of hip prostheses.

Hip prostheses have to fulfil biomedical, anatomical and strength requirements. In the initial design phase it is convenient to carry out contact analyses that simulate standardized fatigue tests using automatically generated finite element meshes of the implant design. While parabolic tetrahedrons are known to be more accurate, they usually cannot be used in contact analyses. In the present study, a prototype design was analysed using automatically generated tetrahedral meshes under linearly equivalent boundary conditions. Results of convergence analyses show that linear tetrahedral elements, even with rotational degrees of freedom, should be avoided, especially when modelling regions with a high stress gradient.

An FEA-based protocol for the pre-clinical validation of custom-made hip implants.

Custom-made prostheses are successfully used to treat particular pathologies such as congenital hip dysplasia. The new EC rules on medical devices require a complete technical dossier for each produced custom-made device counter signed by the surgeon who 'prescribes' the custom-made device. Thus, a specific pre-clinical validation protocol must be developed, considering the economical and temporal constraints imposed by the device type. As a first step, in the present study a protocol based on finite element analysis (FEA) was developed and validated, to verify each custom-made hip stem in terms of mechanical strength. The study was carried out on 12 custom-made cementless hip stem designs already produced and implanted, for which the 3D solid model was available. Two of the selected designs were used for the method validation, comparing strain gauges measurements with the stresses predicted by the finite element (FE) model. Once the proposed methodology was verified, all the remaining stem designs were analysed. The developed protocol made possible a complete analysis in less than 4 h; its accuracy (7-8% on the strain gauge measurements) was considered acceptable for the specific application.

Experimental study on the cement mantle in hip arthroplasty: effect of defects on the property of the materials used.

The study simulated implantation of a hip prosthesis stem in the femur. The cement mantle produced in vitro was observed under an optic microscope. A higher concentration of porosity in the cement mantle at the stem-cement interface was observed. By heating the stem to 45 degrees C and 55 degrees C the authors observed a reduction in porosity in the three surfaces examined: stem-cement interface, internal surface, and cement-pseudofemur interface. Heating of the stem causes a reduction in polymerization time and an increase in maximum temperature achieved during the polymerization process. A reduction in porosity at the stem-cement interface influenced bending strength of the specimens extracted from the mantle. A significant difference between resistance to flexion in the specimens produced with the stem at 55 degrees, and in those with the stem at 23 degrees C was observed.

Metallic wear debris in dual modular hip arthroplasty.

Two cementless hip arthroplasty models with a modular neck made titanium alloy were submitted to cyclical loading in air, in physiological solution, and in a solution for accelerated corrosion tests in order to study the damage process of modular couplings. Thereafter, on one of the two models a series of tests were conducted in order to evaluate the quality of debris produced by the couplings. The parameters of testing adopted allow us to estimate the production of metallic debris observable in vivo with this type of prosthetic stem. The results show that the couplings of modular prostheses may be the source of metallic debris. The amount of debris produced depends on the geometry of the coupling and of the entire implant. Good planning and realization of the couplings, however, reduce the phenomenon that is negligible as compared to other potential sources of debris such as the surface of the stem or of the prosthetic head.