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Biomedical subjects

L Cristofolini

Publications and source records attributed to L Cristofolini.

At least 19 recordsLinked to original sources
PubMed2007

Predicting the subject-specific primary stability of cementless implants during pre-operative planning: preliminary validation of subject-specific finite-element models.

Pre-operative planning help the surgeon in taking the proper clinical decision. The ultimate goal of this work is to develop numerical models that allow the surgeon to estimate the primary stability during the pre-operative planning session. The present study was aimed to validate finite-element (FE) models accounting for patient and prosthetic size and position as planned by the surgeon. For this purpose, the FE model of a cadaveric femur was generated starting from the CT scan and the anatomical position of a cementless stem derived by a skilled surgeon using a pre-operative CT-based planning simulation software. In-vitro experimental measurements were used as benchmark problem to validate the bone-implant relative micromotions predicted by the patient-specific FE model. A maximum torque in internal rotation of 11.4 Nm was applied to the proximal part of the hip stem. The error on the maximum predicted micromotion was 12% of the peak micromotion measured experimentally. The average error over the entire range of applied torques was only 7% of peak measurement. Hence, the present study confirms that it is possible to accurately predict the level of primary stability achieved for cementless stems using numerical models that account for patient specificity and surgical variability.

Biomechanical Phenomena↗
PubMed2006

A physical phantom for the calibration of three-dimensional X-ray microtomography examination.

X-ray microtomography is rapidly gaining importance as a non-destructive investigation technique, especially in the three-dimensional examination of trabecular bone. Appropriate quantitative three-dimensional parameters describing the investigated structure were introduced, such as the model-independent thickness and the structure model index. The first parameter calculates a volume-based thickness of the structure in three dimensions independent of an assumed structure type. The second parameter estimates the characteristic form of which the structure is composed, i.e. whether it is more plate-like, rod-like or even sphere-like. These parameters are now experiencing a great diffusion and are rapidly growing in importance. To measure the accuracy of these three-dimensional parameters, a physical three-dimensional phantom containing different known geometries and thicknesses, resembling those of the examined structures, is needed. Unfortunately, such particular phantoms are not commonly available and neither does a consolidated standard exist. This work describes the realization of a calibration phantom for three-dimensional X-ray microtomography examination and reports an application example using an X-ray microtomography system. The calibration phantom (external size 13 mm diameter, 23 mm height) was based on various aluminium inserts embedded in a cylinder of polymethylmethacrylate. The inserts had known geometries (wires, foils, meshes and spheres) and thicknesses (ranging from 20 microm to 1 mm). The phantom was successfully applied to an X-ray microtomography device, providing imaging of the inserted structures and calculation of three-dimensional parameters such as the model-independent thickness and the structure model index. With the indications given in the present work it is possible to design a similar phantom in a histology laboratory and to adapt it to the requested applications.

Calibration↗
PubMed2006

On the biomechanical stability of cementless straight conical hip stems.

The aim of the present study was to investigation in vitro the effect of deficient bone-implant contact on the primary stability of a straight conical stem. Various possible deficient contact patterns were derived from surgical simulations. The effect of stair climbing loads on the bone-implant micromotion was firstly investigated using a finite element model and then an in vitro test aimed at assessing primary stability. It was found that if the surface features are prevented from biting dense bone in a few small but critical regions, stem primary stability is completely lost. These results suggest that the surface features used in the axisymmetric stem under investigation can be too sensitive to deficient contact conditions, and thus should be augmented with additional antirotational fins. Preliminary tests showed that a stem with the addition of such fins presents good primary stability in all tested conditions.

Arthroplasty, Replacement, Hip↗
PubMed2005

Device to measure intra-operatively the primary stability of cementless hip stems.

The primary stability of cementless prostheses is critical for the long-term outcome of the operation. Surgeons are currently driven only by their experience in evaluating the extent of stem stability achieved. The aim of the present work was to develop a new device that enables the stability of a cementless stem to be quantitatively assessed intraoperatively. The angle of the stem/femur rotation under torsion and the torque are acquired and compared in real-time to a pre-set threshold inferred from the literature. The device indicates whether the stem is stable or not. It was extensively tested and finally validated in vitro on cadaveric and composite femurs hosting different sizes of the same kind of prostheses, implanted with different levels of press-fitting. The overall accuracy (23%) takes into account not only the overall measurement error but also the variability due to differences in bone quality and stem press-fitting. This error was deemed sufficient to discriminate between stable and unstable implants.

Arthroplasty, Replacement, Hip↗
PubMed2005

Microstructural analysis of the effects of incorporation of myelin basic protein in phospholipid layers.

We report an X-ray reflectivity study on the effects of adsorption of myelin basic protein (MBP) on Langmuir monolayers and on deposited Langmuir-Schaefer multilayers of the phospholipid dipalmitoyl phosphatidylglycerol (DPPG). We provide for the first time, direct microscopic evidence on the destructuring effects of MBP leading to plasticity of the DPPG layers supporting commonly accepted models of the stabilizing role of MBP in the myelin membrane. We also show how protein adsorption onto the layer is determined both by electrostatic and nonspecific hydrophobic interactions.

Adsorption↗
PubMed2004

Effect of the initial implant fitting on the predicted secondary stability of a cementless stem.

A numerical model able to investigate the influence of biomechanical factors on the long-term secondary stability of implants would be extremely useful for the design of new cementless prosthetic devices. A purely biomechanical model of osseo-integration has been developed, formulated as a rule-based adaptation scheme. Due to its complexity, the problem was divided into three steps: preliminary implementation of the model (proof of concept); implementation of the complete model and investigation of the model solution; and model validation. The paper describes the first of these three steps. The model was implemented as a discrete-states machine, and the few parameters required were derived from the literature. It was then applied to a real clinical case. The study was conducted using the frictional contact finite element model of a human femur implanted with a cementless anatomical stem. A stable solution was achieved after between three and 15 iterations for all initial positions considered. Similar initial conditions yielded similar final configurations. The model predicted all initial configurations, with the exception of a partial osseo-integration, ranging between 62% (distal fit) and 78% (proximal fit) of the viable interface. This is in good agreement with the values reported in the literature that never exceed 75%, even in the best conditions, and report better clinical results for proximal fit. For the varus configuration, which lacks cortical support, the algorithm predicted a completed loosening.

Arthroplasty, Replacement, Hip↗
PubMed2004

Modelling the fibrous tissue layer in cemented hip replacements: experimental and finite element methods.

The long-term fixation of cemented femoral components may be jeopardised by the presence of a fibrous tissue layer at the bone-cement interface. This study used both experimental and finite element (FE) methods to investigate the load transfer characteristics of two types of cemented hip replacements (Lubinus SPII and Müller-Curved) with a fibrous tissue layer. The experimental part investigated six stems of each type, where these were implanted in composite femurs with a specially selected silicone elastomer modelling the soft interfacial layer. Two fibrous tissue conditions were examined: a layer covering the full cement mantle, representing a revision condition; and a layer covering the proximal portion of the cement mantle, representing a non-revised implant with partial debonding and fibrous tissue formation. The FE method was used to model the full fibrous tissue layer condition, for both implants. The layer was modelled as a homogeneous, linearly isotropic material. A cross-comparison was performed of the experimental and FE findings. Agreement between experimental and FE models was verified to be within 15%. Varying the stiffness parameter of the FE soft tissue layer had little influence on the cortical bone strains, though had considerable effect on the cement strains. Stress shielding occurred for both stems under both fibrous tissue conditions, with the greatest reduction around the calcar. However, the cortical bone strains were generally larger than those for the equivalent well-fixed stems. The fibrous tissue layer was not found to increase the general strain pattern of the cement mantle, though localised regions of high stress were detected.

Arthroplasty, Replacement, Hip↗
PubMed2003

Experimental investigation of bone remodelling using composite femurs.

OBJECTIVE: To determine the load transfer patterns of femurs in the intact, immediate post-operative and long-term (remodelled) post-operative implanted conditions for Lubinus SPII and Müller-Curved cemented hip prostheses, and to examine to what extent remodelling may influence the long-term outcome. DESIGN: Experimental and finite element (FE) methods were applied to composite femurs under loads representing the heel-strike phase of gait, determining cortical bone and cement strains for the different femur conditions. BACKGROUND: The authors previously developed protocols to measure bone and cement strains, and to produce remodelled femur specimens, yet these have not been applied together to compare strain patterns of different femur conditions. The Lubinus SPII is clinically more successful than the Müller-Curved stem, with failure mainly due to aseptic loosening. METHODS: Cortical bone strains were determined in intact femurs. Six femurs each were implanted with the two stem types and cortical bone and cement strains were measured. Bone remodelling was recreated using a validated CAD-CAM procedure to remove a layer of proximal cortical bone, replicating a typical scenario found in stable clinical retrievals. Strains were remeasured. FE methods were used to compliment the experiments. RESULTS: Stress shielding was reduced with remodelling, though bone strains did not return to their intact values, particularly around the calcar. Cement strains increased with remodelling. Differences occurred between the two stems; the Müller-Curved produced a more severe strain transition. CONCLUSIONS: Procedures were successfully combined together to investigate in vitro the performance of two cemented stems, in immediate and long-term post-operative conditions. The increase of cement strains with remodelling is a potential indicator for in vivo cement failure. RELEVANCE: The consequences of femoral bone remodelling on the long-term success of joint replacements are not well understood, where remodelling may lead to increased bone and cement stresses.

Bone Remodeling↗
PubMed2003

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.

Anti-Bacterial Agents↗
PubMed2002

Molecular relaxation and microscopic structure of multilayers and superlattices of a photosensitive liquid-crystalline polymer.

We report a detailed study of photoinduced changes in the microscopic structure of monolayers, multilayers, and superlattices of a photosensitive side chain liquid crystalline polymer, deposited by the Langmuir-Schaefer technique. We probe both out-of-plane and in-plane ordering and its changes due to optical pumping of the trans-cis photoisomerization transition of the azobenzene side chain in an azopolyacrylate. Microscopic structure was studied mainly by synchrotron radiation x-ray reflectometry and grazing incidence diffraction; we also used null-ellipsometry and atomic force microscopy. Our results provide a quantitative modeling of the structural changes and corresponding relaxation times taking place as a function of confinement, temperature and optical pumping, and in particular confirm previously reported ellipsometric results on such changes as a function of sample thickness. This allows a quantitative description of the effects of reduced dimensionality on the structural transitions in this glass-forming system.

Journal Article↗
PubMed2002

Finite element and experimental models of cemented hip joint reconstructions can produce similar bone and cement strains in pre-clinical tests.

Finite element (FE) models could be used for pre-clinical testing of cemented hip replacement implants against the damage accumulation failure scenario. To accurately predict mechanical failure, the models should accurately predict stresses and strains. This should be the case for various implants. In the current study, two FE models of composite hip reconstructions with two different implants were validated relative to experimental bone and cement strains. The objective was an overall agreement within 10% between experimental and FE strains. Two stem types with different clinical results were analyzed: the Lubinus SPII and the Mueller Curved with loosening rates of 4% and 16% after 10 yr, respectively (Prognosis of total hip replacement. 63rd Annual Meeting of the American Academy of orthopaedic surgeons, Atlanta, USA). For both implant types, six stems were implanted in composite femurs. All specimens were subjected to bending. The Mueller Curved specimens were additionally subjected to torsion. Bone strains were recorded at 10 locations on the cortex and cement strains at three locations within the cement mantle. An FE model was built for both stem types and the experiments were simulated. Bone and cement strains were calculated at the experimental gauge locations. Most FE bone strains corresponded to the mean experimental strains within two standard deviations; most FE cement strains within one standard deviation. Linear regression between the FE and mean experimental strains produced slopes between 0.82 and 1.03, and R(2) values above 0.98. Particularly for the Mueller Curved, agreement improved considerably when FE strains were compared to the strains from the experimental specimen used to build the FE model. The objective of overall agreement within 10% was achieved, indicating that both FE models were successfully validated. This prerequisite for accurately predicting long-term failure has been satisfied.

Bone Cements↗
PubMed2001

Photoinduced dynamics in a photosensitive side chain polymeric liquid crystal by quasielastic and inelastic neutron scattering.

We report the first study by inelastic (INS) and quasielastic neutron scattering of photoinduced changes in the reorientational and vibrational dynamics of a liquid crystalline side chain polymer. We use the cis-trans photoisomerization transition to take the system out of equilibrium and determine the quasielastic (QE) and inelastic scattering laws on two distinct time windows, corresponding to the time-of-flight (IN6) and backscattering (IN16) spectrometers at Institut Laue Langevin (ILL) (Grenoble). Our investigation was focused on the dynamics of the coupling between the mesogenic side chains and the polymeric main chain, which is connected to the extensive optical writing and memory effects that have been demonstrated in this complex material. We report data on the QE broadening and Debye-Waller factors, as a function of temperature across the glass transition (T(g)=293 K). We also studied the dynamical coupling of side and main chains. We report photoinduced changes on the static structure factor, on the purely elastic scattering fraction, on the low frequency vibrational dynamics (around the boson peak region). In particular, we find that on the space-time scales accessible to the INS techniques there is a time structure in the coupling, and that over longer times and distances the two dynamics are decoupled.

Journal Article↗
PubMed2001

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.

Alloys↗
PubMed2001

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.

Arthroplasty, Replacement↗
PubMed2001

In vitro testing of the primary stability of the VerSys enhanced taper stem: a comparative study in intact and intraoperatively cracked femora.

The Zimmer VerSys enhanced taper uncemented stem was tested in vitro for primary stability. Six stems were implanted in six composite femora. Three femora were unintentionally damaged by cracking of the bone during stem insertion and press-fit. A previously validated protocol was used to load the specimens cyclically and to record locally the rotational and axial bone/implant relative motions in terms of elastic motion and permanent migration. For the undamaged femora, the VerSys stem showed excellent primary stability, in terms of both elastic motions (less than 9 microns) and permanent migrations (less than 6 microns), and in both axial and rotational directions, comparing favorably with the literature. Intraoperatively-induced proximal cracks did influence the primary stability of the stem in terms of permanent migration. As cracks may easily be produced in the diaphyseal canal during insertion of cementless stems, which rely for primary stability on conical fitting into the canal, great care must be taken in preparation of the canal and insertion of the stem.

Arthroplasty, Replacement, Hip↗
PubMed2001

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.

Compressive Strength↗
PubMed2001

A CAD-CAM methodology to produce bone-remodelled composite femurs for preclinical investigations.

Femoral bone remodelling, following total hip arthroplasty, is a clinically observed phenomenon attributed to the changed stress environment of the postoperative implanted hip. While this process cannot be avoided, there is concern as to its consequences on the long-term survival of hip joint replacements. Previous methods of studying remodelling, such as clinical or animal-based studies, or finite element analyses, have their limitations. The aim of this study is to develop experimental specimens incorporating bone resorption features typical of clinically successful implants. This work describes the use of computer aided design/manufacturing methods (CAD-CAM) to produce these specimens, based on modifying commercially available composite femurs. The procedures are investigated and verified for two different designs of cemented prostheses (Lubinus SPII and Muller Curved). Quantitative clinical data is used to define the remodelled geometry of a CAD model of the femur for each stem design. Composite femur specimens are machined using a three-axis milling machine, where each specimen can be accurately positioned using a custom-designed jig and a digitizer system. The accuracy of the process is assessed by analysing the deviation of the digitized premachined and postmachined surfaces of each specimen in relation to the CAD model. The results demonstrate that the procedure can be used for developing in vitro specimens with bone resorption features. These specimens are proposed as a useful tool for performing preclinical trials, such as load transfer or longevity/stability testing, with the advantage of modelling a long-term clinical situation, rather than solely analysing implanted femurs in an immediate postoperative state.

Arthroplasty, Replacement, Hip↗