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Biomedical subjects

L Cristofolini

Publications and source records attributed to L Cristofolini.

At least 37 records · Page 2Linked to original sources
PubMed2001

Design revision of a partially cemented hip stem.

In a previous preclinical study the prototype version of a partially cemented hip stem, cement-locked uncemented (CLU) prosthesis, showed optimal primary stability and moderate stress shielding. However, numerical analysis suggested that the prototype design would induce relatively high stresses in the cement and a significant relative motion between cement and metal. The present study aimed to verify if these problems could be eliminated once the CLU design is improved. The revised design was analysed using a complete finite element model of an implanted human femur. To further strengthen the predictions of the finite element analysis, the cement damage induced by a severe load history was assessed experimentally in synthetic femurs implanted with the improved CLU stem or with a clinically successful fully cemented stem. The modifications made to the CLU stem design did not reduce its good primary stability but decreased the metal-cement relative micromotion. The same load induced stresses in the cement mantle of the improved CLU stem that were significantly lower than those predicted for the prototype design. Although the presence of modelling artefacts produced a highly localized stress peak of 13 MPa. 99 per cent of the cement volume was subjected to a principal tensile stress lower then 4 MPa. These levels of stress compare favourably with the tensile fatigue limit of the acrylic cement used in this study (9.7 MPa). The experimental results further supported these findings. The cemented stem showed a number of cracks per volume unit approximately ten times higher than the partially cemented stem under investigation.

Bone Cements↗
PubMed2001

Interface biomechanics of the Anca Dual fit hip stem: an in vitro experimental study.

The Anca Dual Fit hip stem (Cremascoli Wright, Milan, Italy) is a partially cemented stem developed to overcome the drawbacks of both cemented and uncemented fixations. Its design was based on the hypothesis that partial cementing would ensure the primary stability necessary to allow bone ingrowth on the cement-free stem surfaces. At the same time, the limitation of the cement to the proximal regions would prevent stress-shielding by increasing proximal load transfer. After finite element (FE) simulations and in vitro primary stability assessment, an analysis of the long-term stability of the Anca Dual Fit stem was necessary to conclude the preclinical testing. Three stems were implanted in composite femurs and subjected to testing for 1 x 10(6) cycles, each cycle reproducing the activity of stair climbing. The simulation was designed so as to replicate the physiological loading in a simplified, yet relevant way for this test. Various measurements were collected before, during and after the test in order to give exhaustive information on the response of the implant to long-term, cyclic loading. The present study confirmed the positive results of previous investigations, and proved that the Anca Dual Fit stem has excellent long-term stability; therefore successful clinical outcomes are predicted.

Biomechanical Phenomena↗
PubMed2000

Glass transition and relaxation following PhotoPerturbation in thin polymeric films

In this letter we employ null ellipsometry on Langmuir Blodgett multilayers of a glass forming photosensitive side chain polymer to investigate both the structural changes and the related time relaxation as a function of temperature and sample thickness following isothermal optical perturbation. Below a thickness of a few layers the glassy multilayer system collapses to a smecticlike crystal. This effect is discussed in the context of the glass transition in restricted dimensionality.

Journal Article↗
PubMed2000

Initial stability of a new hybrid fixation hip stem: experimental measurement of implant-bone micromotion under torsional load in comparison with cemented and cementless stems.

A new hybrid fixation stem, named cemented-locked uncemented (CLU), for total hip arthroplasty was developed to achieve good initial stability. Primary stability is guaranteed by the cement which is injected into two pockets in the lateral area. This leaves a large surface available for long-term biologic fixation (direct bone attachment on implant). This study evaluates in vitro the initial stability of the CLU prototype under torsional load, in comparison with cemented and cementless stems. The results show that the CLU stem is very stable in simulated stair climbing. Its micromotions are comparable to those of a cemented prosthesis, and significantly less (80-90% lower) than those for a cementless stem. These findings confirm the optimal initial stability expected from the CLU prototype. This new design, which employs hybrid fixation, should improve bone formation on the implant and reduce the risk of stem loosening.

Arthroplasty, Replacement, Hip↗
PubMed2000

Kinetic and structural study of the interaction of myelin basic protein with dipalmitoylphosphatidylglycerol layers.

The interaction of myelin basic protein (MBP) with dipalmitoylphosphatidylglycerol films has been investigated by means of a microgravimetric gauge sensitive to the changes in load and structural modifications of the layer deposited onto its surface. Fourier transform infrared spectroscopy, circular dichroism, and x-ray diffraction have confirmed protein uptake by the lipid phase along with a global disordering effect onto the lipid alkyl chains and have shown a temporal evolution of the structure of water penetrating the lipid phase together with the protein. These effects are clearly related to the temporal variation of the microgravimetric gauge signal. Finally, measurements carried out on pre-annealed samples point out the role of mesoscopic morphology in determining the pathways through which MBP penetrates the lipid multilayer. The results obtained in our model system could be useful in clarifying the mechanisms of the myelinating and demyelinating processes that take place in the natural membrane.

Animals↗
PubMed2000

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.

Bone and Bones↗
PubMed2000

Mechanical validation of whole bone composite tibia models.

Composite synthetic models of the human tibia have recently become commercially available as substitutes for cadaveric specimens. Their use is justified by the advantages they offer as a substitute for real tibias. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human specimens for different loading conditions. The stiffness of the tibias was measured with a torsional load applied along the long axis, and with a bending load applied both in the latero-medial and in the antero-posterior direction. The bending stiffness of the composite tibias matched well with that of the cadaveric specimens. This was not true for the torsional stiffness. In fact, the composite tibias were much stiffer than the cadaveric specimens, possibly due to the structure of the reinforcement material. The inter-specimen variability for the composite tibias was much lower than that for the cadaveric specimens. Thus, it seems that the composite tibias are suitable to replace cadaveric specimens for certain types of test, whereas they might be unsuitable for others, depending on the loading regimen.

Biomechanical Phenomena↗
PubMed2000

A novel transducer for the measurement of cement-prosthesis interface forces in cemented orthopaedic devices.

When a cemented orthopaedic device is being investigated there is a need to estimate the forces at the cement-prosthesis interface. For this reason a miniature transducer was developed that could be included inside the surface of most prostheses. A load cell (based on a piezoelectric sensor) and the required accessories and amplification were custom designed and built. The present work describes the validation that was performed on the piezo sensors alone, when mounted on a simplified structure, and when applied to a hip stem.Linearity, repeatability, reproducibility, and sensibility to shear and axial eccentric loads were tested, yielding satisfactory results. The repeatability on the same sensor was found to be good while reproducibility between sensors was lower. Thus, each sensor was calibrated separately with a second order relationship. Sensitivity to shear and eccentric loads was very low. The overall accuracy of the load cell (including non-linearity, and signal drift) was of the order of about 1%.A hip stem instrumented with four such sensors was successfully implanted in a composite femur, yielding meaningful readouts.Thus, this type of sensor can readily be used to assess the cement-prosthesis interface forces in cemented devices.

Biomechanical Phenomena↗
PubMed2000

Radiopacity and fatigue characterization of a novel acrylic bone cement with sodium fluoride.

Acrylic bone cement must provide good radiographic visibility and good long-term mechanical resistance in joint replacements. A new formulation of cement with 6% barium sulfate and 6% sodium fluoride was developed (Fluoride Bone Cement). Barium sulfate is a necessary addition to allow radiographic visibility although it reduces the mechanical strength of the material. Sodium fluoride promotes bone formation. However, its effect on the mechanical behavior is currently unknown while its influence on radiopacity can only be roughly estimated. The aim of this investigation was to establish if the new formulation would be suitable for clinical trials. In this respect, a mechanical (fatigue test) and radiographic (optical density measurements on x-ray films) characterization was performed on a typical commercially available cement with barium sulfate added and on the Fluoride Bone Cement. It was demonstrated that the fluoride cement has a (marginally) superior fatigue strength and comparable radiopacity to commercial radiopaque cements.

Arthroplasty, Replacement↗
PubMed1999

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.

Hip Prosthesis↗
PubMed1999

In vitro stress shielding measurements can be affected by large errors.

Hip prostheses and other implantable devices for the proximal femur are tested experimentally to study their effects on load transfer. We report on some experimental errors (related to the load simulation) that can undermine the reliability of strain measurements. A first source of error is that of overconstraining the setup. This situation makes it impossible to control or even determine the force values. The second source of error is related to geometric alterations induced by surgery, which modify the lever arms and thus the loading system. Two options are available to compensate for a geometric alteration: either applying the same system of forces or the same resultant bending moment to the implanted femur. The errors that arise if these parameters are not controlled can make it impossible to determine if one device performs better or worse than another.

Biomechanical Phenomena↗
PubMed1999

Methods for quantitative analysis of the primary stability in uncemented hip prostheses.

Torsional loads of daily activities contribute to the failure of the primary fixation of hip prostheses. Implant torsional stability must be evaluated prior to in vivo clinical trials. Whereas previous work has investigated this phenomenon, descriptions of physiologically accurate and reproducible in vitro methodologies are rare. The present study aimed to detect and control the sources of error and variability that influence in vitro methods. A typical set-up for the analysis of primary stability of hip stems was studied. Load cycles included proximal-to-distal axial force, torque, and bending moment. The effects of loading frequency and strain distribution across the cortical bone were investigated in order to optimize testing conditions and measurement set-up. The relative shear motion at the bone-stem interface was measured transcortically using linear variable displacement transducers. The procedures developed for mounting specimens on the testing machine and for positioning sensors on the specimen were standardized and tested for reproducibility. The protocol was finally tested for repeatability and accuracy. Measurement errors were 2.3 microm between load cycles and 4.9 microm for repeated set-ups, comparing favorably with the literature.

Activities of Daily Living↗
PubMed1998

A comparative study on different methods of automatic mesh generation of human femurs.

The aim of this study was to evaluate comparatively five methods for automating mesh generation (AMG) when used to mesh a human femur. The five AMG methods considered were: mapped mesh, which provides hexahedral elements through a direct mapping of the element onto the geometry; tetra mesh, which generates tetrahedral elements from a solid model of the object geometry; voxel mesh which builds cubic 8-node elements directly from CT images; and hexa mesh that automatically generated hexahedral elements from a surface definition of the femur geometry. The various methods were tested against two reference models: a simplified geometric model and a proximal femur model. The first model was useful to assess the inherent accuracy of the meshes created by the AMG methods, since an analytical solution was available for the elastic problem of the simplified geometric model. The femur model was used to test the AMG methods in a more realistic condition. The femoral geometry was derived from a reference model (the "standardized femur") and the finite element analyses predictions were compared to experimental measurements. All methods were evaluated in terms of human and computer effort needed to carry out the complete analysis, and in terms of accuracy. The comparison demonstrated that each tested method deserves attention and may be the best for specific situations. The mapped AMG method requires a significant human effort but is very accurate and it allows a tight control of the mesh structure. The tetra AMG method requires a solid model of the object to be analysed but is widely available and accurate. The hexa AMG method requires a significant computer effort but can also be used on polygonal models and is very accurate. The voxel AMG method requires a huge number of elements to reach an accuracy comparable to that of the other methods, but it does not require any pre-processing of the CT dataset to extract the geometry and in some cases may be the only viable solution.

Algorithms↗
PubMed1997

Relationship between bone-prosthesis bonding and load transfer in total hip reconstruction.

The effect of bone-prosthesis bonding on proximal load transfer is investigated using a coupled experimental and finite element analysis on a synthetic femur. Three-dimensional finite element models for an intact femur and a femur implanted with a cementless prosthesis were constructed from the experimental models used, and the proximal femoral strains recorded for two loading conditions approximating a one-legged stance. The approach was used to investigate a press-fitted and a fully bonded bone-prosthesis structure to identify the stem-bone behaviour for both interface conditions and their implications for proximal bone load transfer. Regression slopes close to unity indicated that the finite element predictions were an accurate estimate of the experimental measurements. Physiological surface strains were recorded only when the abductor force was included in the loading. Meanwhile, experimental measurements and numerical predictions showed that a different load transfer pattern is to be expected for normally press-fitted and glued press-fitted stems. The finite element model for the treated femur, modelling both interface conditions correlated very well with the experimental model. These finite element models subsequently modified and used to analyse the effect of different interface conditions predicted a significant increase in the load transfer to the proximal calcar bone when only proximal bonding is achieved. This study suggests that information obtained for the assessment and prediction of total hip arthroplasty longevity by numerical and experimental techniques used together and in parallel is of greater value than either technique used alone. The employment of a femur analogue as featured in this study is also shown to be a suitable alternative to cadaveric specimens in such an analysis.

Biomechanical Phenomena↗
PubMed1997

A critical analysis of stress shielding evaluation of hip prostheses.

In vitro evaluation of the load transfer of hip prostheses has been performed in recent years for the purpose of understanding the stress shielding phenomena. Over 200 papers were reviewed to determine if a standard exists to evaluate and compare the performance of hip stems. Surprisingly, it was found that little agreement exists in the testing protocol. This makes it very difficult to compare the results reported in different investigations. In several cases very incomplete data are reported about the testing conditions, thus making it impossible to compare the results. This article focuses on: (1) how the loading conditions should be chosen based on physiological loading in a way to give a reproducible setup; (2) how the femur should be constrained; (3) how to generate the same system of loads in the intact and the implanted femur; (4) how to define a reference system; (5) how the specimen type and sample size are chosen; (6) the advantages and limitations of the different strain measurement techniques; (7) how the testing parameters have been chosen in the literature; and (8) how the accuracy of the results has been reported in the literature.

Bone Resorption↗
PubMed1996

Mechanical validation of whole bone composite femur models.

Composite synthetic models of the human femur have recently become commercially available as substitutes for cadaveric specimens. Their quick diffusion was justified by the advantages they offer as a substitute for real femurs. The present investigation concentrated on an extensive experimental validation of the mechanical behaviour of the whole bone composite model, compared to human fresh-frozen and dried-rehydrated specimens for different loading conditions. First, the viscoelastic behaviour of the models was investigated under simulated single leg stance loading, showing that the little time dependent phenomena observed tend to extinguish within a few minutes of the load application. The behaviour under axial loading was then studied by comparing the vertical displacement of the head as well as the axial strains, by application of a parametric descriptive model of the strain distribution. Finally, a four point bending test and a torsional test were performed to characterize the whole bone stiffness of the femur. In all these tests, the composite femurs were shown to fall well within the range for cadaveric specimens, with no significant differences being detected between the synthetic femurs and the two groups of cadaveric femurs. Moreover, the interfemur variability for the composite femurs was 20-200 times lower than that for the cadaveric specimens, thus allowing smaller differences to be characterized as significant using the same simple size, if the composite femurs are employed.

Biomechanical Phenomena↗
PubMed1996

A minimal parametric model of the femur to describe axial elastic strain in response to loads.

Evaluating the state of stress/strain for a given geometry and load in femurs can be done both experimentally, measuring strain at a limited number of locations, and theoretically with finite element models. Another approach is to describe the state of strain with a few synthetic indices. For this purpose the reverse elastic problem (i.e. bone parameters are estimated given the strain distribution and loads) needs to be solved as opposed to the finite element direct problem. Such reverse models can be then used: (1) to describe simply the strain distribution by means of few synthetic indices; (2) to explain the state of strain; and (3) to predict the strain distribution under different loading conditions. Various linear models, characterized by two to five bone related parameters, were tested on (1) 12 femurs, (2) a finite element model, and (3) data taken from the literature, for a total of 43 loading cases. Three and four-parameter models were able to fit the experimental strain distributions with mean squared residuals smaller than 5% of the strain range. The consistency of the model was proved by the repeatability of the parameters estimate for identical femurs. Furthermore, the bone-related coefficients were able to detect the stiffening effect of the implantation of an uncemented stem. Finally, the model can be used for predictive purposes if the parameter estimates are used with different loading conditions.

Biomechanical Phenomena↗