Biomedical subjects

M Viceconti

Publications and source records attributed to M Viceconti.

At least 73 records · Page 4Linked to original sources

PubMed1995

Effects of some technological aspects on the fatigue strength of a cementless hip stem.

Four prototype cementless hip stems were tested following the ISO 7206 protocol for the assessment of the endurance properties and compared with a cast Cr-Co-Mo (ASTM F75) commercially available stem which was used as reference design. All the tested stems were similar in shape and size, but with some substantial differences. The first was made of forged Ti6A14V alloy (ASTM F136). The second, made of the same material, featured a central hole intended to reduce the bending stiffness of the stem itself. The third was identical to the second but for a small tooling notch in one of the fillets of the hole. The fourth was similar to the first but had a coating of sintered titanium beads in the proximal part. All of these modifications were made to evaluate the effect of fatigue strength of intentional or unintentional features commonly found in commercial stems. The forged Ti6A14V allow was found to be substantially stronger than the cast ASTM F75 Cr-Co-Mo alloy. However, tooling notches or sintered coatings were found to dramatically reduce this strength. Thus, the Ti6A14V alloy calls for an accurate design process, especially when complex shapes or sintered structures are required.

Alloys↗

PubMed1995

Influence of thigh muscles on the axial strains in a proximal femur during early stance in gait.

This work is focused on the in vitro simulation of the loads occurring in the femur during early stance in gait, for hip prosthesis stress shielding test purposes. Ten thigh muscles (the three gluteal muscles, the three vasti, rectus femoris, adductor longus and magnus, biceps femoris), simulated by nylon straps, were tested in order to establish their influence on the strains in the proximal femur. Axial and hoop strains were recorded from 16 strain gauges for the effect of each muscle and compared to the strains recorded as a result of the hip joint reaction force only (i.e. without muscle simulation). It appears that the three glutei are the principal muscles in determining the vertical strains, however the rectus femoris, biceps femoris and the adductors were also seen to significantly affect the strain pattern. The inadequacy of increasing the adduction angle and applying the resultant force at the hip joint to simulate the abductors was also confirmed.

Femur↗

PubMed1995

Initial stability of uncemented hip stems: an in-vitro protocol to measure torsional interface motion.

The difficulty in quantitatively assessing the inherent variables of surgical stem insertion and interfemur differences continues to be a problem in experimental methodologies which assess hip stem stability. An in-vitro torsional stability protocol was developed which limited the mechanical testing variability and provided a reproducible micromotion measurement of an uncemented stem in synthetic composite femurs. Using a controlled mechanical stem insertion resulted in less interfemur variability within each group with the coefficient of variation being reduced from 35% overall to less than 20%. Femurs with shallow stem insertion depths had significantly larger micromotion than femurs having deep stem insertion depths. The sensitivity of the experimental protocol and the synthetic composite femurs to the varied functional behaviour of three different stem designs was demonstrated. The stem with a hollowed anterior-to-posterior proximal section experienced significantly more motion than the two stems with full proximal sections, reinforcing the need for proximal contact to ensure minimal micromotion in torsional loading.

Biocompatible Materials↗

PubMed1994

Automatic fracture reduction with a computer-controlled external fixator.

The reduction of fractures by means of an Ilizarov's fixator is obtained by successively shortening or lengthening the rods. This entails that all reduction operations of the fracture stumps be performed with a series of empirical attempts, requiring great experience and manual dexterity in the surgeon. Moreover this process involves a long exposure of both physician and patient to potentially harmful radiation due to the continuous checking of the intermediate positions on the X-ray image intensifier. In order to overcome these limits a new device has been conceived, based on the application of three stepper-motors on three-rods. Its basic principle is functionally very similar to Ilizarov's prototype. The relative motions between the two frames are carried out by controlling the three actuators with a computer, which processes the number of required steps on the basis of an algorithm, starting from a few inputs supplied by the surgeon. This article illustrates the functional kinematic study necessary for the complete automation of the reduction process. Also considered is the complex problem of the reduction trajectory definition, intended as a sequence of configurations of partial correction, obtained by formalizing in geometrical terms the empirical criteria followed by the orthopaedic surgeon in reducing fractures. Such a sequence is intended to be a suggestion for the surgeon who can visualize and possibly interact with the system to determine a trajectory harmless for the soft tissues surrounding the bone.

External Fixators↗

PubMed1993

Digital dynamic range expansion applied to X-ray densitometric analysis of total hip replacement.

Due to the presence of the stiff prosthetic stem fitted in the medullary canal during total hip replacement, the surrounding cortex of the femur changes its density over time. This bone remodelling takes place with every type of total hip prosthesis; however, its intensity may vary between prostheses and patients. In the worst cases this process can lead to the late failure of the implant. To monitor such bone density evolution, we are developing a tailored Computer-aided Densitometric Image Analysis system (the major part of this our system uses an 8-bit commercial hardware with 256 levels of grey). The equivalent dynamic range of an X-ray picture is about 10 bits. In this paper we present a method to overcome these hardware limitations by improving the software. Using a double-exposure acquisition it is possible to build a 9-bit image that is good enough for most applications involving bone density measurement.

Absorptiometry, Photon↗

PubMed1993

A software simulation of tibial fracture reduction with external fixator.

In modern orthopaedic practice, circular external fixators are frequently preferred to plaster cast for various reasons. The realignment of the two fractured bone segments is usually performed under a continuous fluoroscopy checking which involves a long radiation exposure time for both patients and surgeon. In order to overcome this problem, a computer controlled external fixator is under development. One relevant problem which this project faced in its initial stage was the difficulty to define in geometrical terms the manual reduction trajectory normally adopted by the surgeon. Basically it was a typical problem of empirical knowledge transfer from the surgeon to the engineer. Thus, it was necessary to create a common ground where the two experts could carry out the necessary analyses in order to define an empirical algorithm capable of suggesting a correction trajectory. The problem was solved by developing a simulation program called S.E.R.F. (Simulation Environment of a Robotic Fixator) provided with an extremely powerful graphic output, able to visualize the whole reduction trajectory from any viewpoint in space. The authors suggest the use of this kind of tool every time the techno-clinical information exchange is a critical issue.

Algorithms↗

PubMed1990

A generalized procedure for predicting bone mass regulation by mechanical strain.

Understanding of the mechanisms that control the modifications of the bone weight-bearing attitude in response to external load conditions attracted considerable attention from researchers in the biological, medical, and radiological fields. This study presents a general approach for predicting the reaction of the bone tissue to cyclic loads with different intensity and temporal distribution. Empirical relationships are generated that incorporate the wealth of published experimental data, obtained from in vivo, ex vivo, and in vitro studies, into an integrated analysis. The developed procedure was guided by and is in close agreement with the published experimental data. The approach provides a general framework for predicting the effect of mechanical strain deviation from the physiological strain environment only, without consideration of the influence of any other changes in the biochemical, physiological, or psychological mechanisms controlling bone growth and damage. Further clinical investigations with controlled exercise and systematic bone scanning are necessary to check the applicability of the coefficients generated in the proposed method for general use on human subjects.

Animals↗

PubMedDate not supplied

Analysis of titanium induced CT artifacts in the development of biomechanical finite element models.

X-ray computerized tomography (CT) is capable of providing detailed information about the geometry and mineral density of skeletal structures. Such accurate data are of great interest in studying the effects of orthopaedic implants on bone adaptive behaviour in vivo. Metallic implants, however, generate artifacts, typically seen as starburst streaking. These artifacts can degrade the capabilities of CT images to provide accurate information about the geometry and mineral density of bone structures. The aim of this work was to investigate the possibility of developing finite element models (FEM) of the human femur after hip joint arthroplasty using CT images acquired directly after surgery. The capability of modern CT scanners to accurately reconstruct the cross-section geometry of titanium alloy hip joint prosthetic stems was primarily investigated. A new measuring procedure dealing with the geometry of real stems was developed and its accuracy assessed. Secondly, the artifacts generated by a prosthetic stem on the radiological density of the bone were analysed, and their effects on the assessment of FEM material properties were evaluated. Results showed that CT images provide accurate information on metal stem geometry. An average error of 0.45 mm was estimated in the reconstruction of stem cross-section geometry. Concerning bone density estimation around the implant, it was observed that the effect of metal artifacts on tissue density becomes zero at a distance of 2 mm from the implant.

Alloys↗

PubMedDate not supplied

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.

Biomechanical Phenomena↗

PubMedDate not supplied

A new method for the automatic mesh generation of bone segments from CT data.

A new procedure for the automatic generation of finite element meshes of bone segments from computed tomography (CT) data sets is described. The new method allows a direct automatic generation from the CT data and produces a very accurate unstructured hexahedral mesh. The accuracy of the method was established using the CT images of an artificial femur showing range of attenuation values comparable to those of a human femur. To establish the optimal values for the parameters controlling the mesh a sensitivity analysis was carried out using mesh-conditioning indicators. Some of the best meshes, with increasing levels of refinement, were used to analyse the stresses induced in the proximal femur by single leg stance posture. The accuracy of the meshes was evaluated using an implicit a posteriori residual-based error estimates. The number of elements with stress residuals larger than 10% of the peak stress was 7.8% using the coarsest mesh and only 1.8% with the finest mesh. The proposed method has been proved able to conjugate full automation with high-quality finite element meshes. The stress predictions obtained using these hexahedral-only meshes have been more accurate than those obtained by any other automatic mesh generation algorithm. Once properly integrated in an easy-to-use application, the described method could finally make feasible many clinical applications of finite element analysis.

Bone and Bones↗

PubMedDate not supplied

Development of a software for the design of custom-made hip prostheses using an open-source rapid application development environment.

The present work describes a technology transfer project called HIPCOM devoted to the re-engineering of the process used by a medical devices manufacturer to design custom-made hip prostheses. Although it started with insufficient support from the end-user management, a very tight scheduling and a moderate budget, the project developed into what is considered by all partners a success story. In particular, the development of the design software, called HIPCOM Interactive Design Environment (HIDE) was completed in a time shorter than any optimistic expectation. The software was quite stable since its first beta version, and once introduced at the user site it fully replaced the original procedure in less than two months. One year after the early adoption, more than 80 custom-made prostheses had been designed with HIDE and the user had reported only two bugs, both cosmetics. The scope of the present work was to report the development experience and to investigate the reasons for these positive results, with particular reference to the development procedure and the software architecture. The choice of TCL/TK as development language and the adoption of well-defined software architecture were found to be the success key factors. Other important determinants were found to be the adoption of an incremental software engineering strategy, well suited for small to medium projects and the presence in the development staff of a technology transfer expert.

Computer-Aided Design↗

PubMedDate not supplied

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.

Alloys↗

PubMedDate not supplied

Fracture prediction for the femoral neck using finite element models.

Fracture of the femoral neck is an important clinical, social and economic topic. Prediction in subjects who are at risk for this type of fracture has been the object of numerous studies. Nonetheless, the methods of classification based on densitometric indicators alone have shown poor accuracy. It is the purpose of this study to propose a method to obtain an estimate of the resistance of the proximal femur, and to improve accuracy in prediction in subjects who are at risk for fracture. Based on the densitometric dataset alone, a bidimensional finite element model was developed that takes into account the distribution of density together with the femoral anatomy and the typical conditions of trauma. The model was applied to a group of preliminary osteoporotic patients. The statistical classification showed an increase in accuracy by 13%, as compared to a classification based on densitometric indicators alone.

Adult↗

PubMedDate not supplied

Radiographic evaluation of HDPE cemented and cementless Lord and An.C.A. screwed acetabular models.

A total of 187 alumina screwed porous-ceramic coated sockets (An.C.A.), 48 screwed smooth-surfaced Lord sockets, and 251 cemented polyethylene sockets were radiographically evaluated at an average follow-up of 30, 51 and 96 months respectively. After 6 years the Lord prostheses revealed a 38% incidence of loosening, similar to that observed for cemented sockets 10-12 years after surgery. The An.C.A. prostheses revealed radiographic loosening equal to 12% (6 cases) in the first 50 implants, and only 0.7% in the remaining 137 cases: overall, the An.C.A. acetabular prosthesis revealed an index of radiographic loosening equal to 3.3% (7/187). To guarantee "osteointegration" of the porous coating of An.C.A. sockets optimal stability must be obtained when the prosthesis is screwed in. Because the mid-term follow-up for this clinical experience is relatively short (30 months), an opinion on the reliability of the screwed "porous" sockets must await confirmation.

Acetabulum↗

PubMedDate not supplied

A computer assisted approach for monitoring and planning biological reconstructions in orthopaedic oncology: preliminary results and future perspectives.

The evaluation of the mechanical strength of a reconstructed bone, and its evolution during the follow-up is, at present, almost impossible. This information, however, may prove extremely useful in clinical practice, both in the surgical planning and in the management of the rehabilitation therapy. In this work, a non-invasive technique, based on the use of finite element modelling, is presented that allows the simulation of the mechanical behaviour of a skeletal reconstructions starting from Computed Tomography data. This method was applied to study the evolution, during the first year of follow-up, of the strength of the reconstructed femur of a ten year-old child if a short, slow, level walk were allowed. The preliminary results indicate that the window sculpted by the surgeon to allow the anastomosis of the vascular pedicle acts as a stress concentrator and that the reduced bone mineral density, induced in the child's bone by the absence of load during the post-operative period, increases the stress level in the proximal femur.

Bone Neoplasms↗

PubMedDate not supplied

Computer-assisted tridimensional preoperative planning in hip revision surgery.

It is often difficult to obtain excellent clinical results in complex cases of hip replacement surgery. Over recent years, in order to improve the success rate of this type of surgery, prosthetic implants that are more ductile and more reliable have been developed. At the same time, important progress has been made in improving the accuracy of surgical method. A great deal of effort has been made to improve methods of preoperative planning. The world over, computerized systems that aid the surgeon in his or her clinical practice (CAOS systems) have been developed. The authors present Hip-op, a new CAOS type system, for preoperative planning. In particular, the use of Hip-op in some very complex cases of hip revision surgery is reported. Based on clinical experience, it is believed that Hip-op is a useful system, one that is easy to use, and that it is capable of improving the accuracy of surgery.

Adult↗

PubMedDate not supplied

The effect of autoclaving on the mechanical properties of bank bovine bone.

Xenologous bone grafts are frequently used in modern orthopaedic surgery. There is a risk of infection associated with the transmission of diseases among different species for these types of grafts. As for bovine bone grafts in recent times attention has been paid to the diffusion in various countries of the European Economic Community of "bovine spongiform encephalopathy"; this pertains to the same group of encephalopathies as Creutzfeldt-Jakob disease which strikes humans and has a lethal outcome. Thus, in the absence of clinical evidence, there is a potential source of risk of infection associated with specimens of bovine origin in relation to which a document of the European community suggests proceeding by sterilization by autoclaving at 132 degrees C. for at least 1 hour. It is the purpose of this study to verify the effect of this treatment on the mechanical properties of bone graft. The results show how this procedure reduces the graft's resistance to compression by approximately 70%. Thus, it is not acceptable for all bone grafts for which good mechanical resistance is required. In cases such as these, bovine bone grafts may be used only when the immunity of the farm of origin has been certified; then sterilization may be conducted using methods that are less destructive for the mechanical integrity of the bone tissue.

Animals↗

PubMedDate not supplied

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.

Bone Substitutes↗