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Biomedical subjects

Marco Viceconti

Publications and source records attributed to Marco Viceconti.

30 records · Page 2Linked to original sources
PubMed2004

Validation of two algorithms to evaluate the interface between bone and orthopaedic implants.

The level of fit and fill of the prosthetic stem in the femoral canal is an important parameter when planning a cementless total hip arthroplasty. However, the standard templates used in combination with radiographs are not always effective in the pre-operative evaluation of the level of fitting. For this reason, two algorithms were developed able to provide clinically relevant three-dimensional indicators of the implant fit and fill in the host femur, based on the CT data of each specific patient as collected in vivo. In this study the computational methods were described and validated using digital phantom datasets. Then the algorithms were applied for in vivo datasets and the sensitivity of each indicator was evaluated. The validation showed that the two algorithms are accurate from a computational point of view. Moreover, the in vivo testing demonstrated that the developed methods provide reasonable quantitative indicators of the stem positioning in the femoral CT dataset.

Algorithms↗
PubMed2004

An improved method for the automatic mapping of computed tomography numbers onto finite element models.

The assignment of bone tissue material properties is a fundamental step in the generation of subject-specific finite element models from computed tomography data. Aim of the present work is to investigate the influence of the material mapping algorithm on the results predicted by the finite element analysis. Two models, a coarse and a refined one, of a human ileum, femur and tibia, were generated from CT data and used for the tests. In addition a convergence analysis was carried out for the femur model, using six refinement levels, to verify whether the inclusion of the material properties would significantly alter the convergence behaviour of the mesh. The results showed that the choice of the mapping algorithm influences the material distribution. However, this did not always propagate into the finite element results. The difference between the maximum Von Mises stress remained always lower than 10%, apart one case when it reached the 13%. However, the global behaviour of the meshes showed more marked differences between the two algorithms: in the finer meshes of the two long bones 20-30% of the bone volume showed differences in the predicted Von Mises stresses greater than 10%. The convergence behaviour of the model was not worsened by the introduction of inhomogeneous material properties. The software was made available in the public domain.

Absorptiometry, Photon↗
PubMed2003

The primary stability of a cementless stem varies between subjects as much as between activities.

The rehabilitation program adopted immediately after a cementless total hip replacement is a very important factor, because of the known relationship between osseointegration and implant micromotion. The present study was aimed to evaluate which type of task is the most critical in terms of bone-implant relative micromotion. Both inter-task and inter-subject variability were taken into account to verify if the movement strategy could be determinant on this assessment. A previously validated finite element model was used to predict the peak total micromovements over the entire bone-implant contact surface in four different patients, performing nine different tasks, using published data on joint forces recorded by instrumented hip prostheses. The results predicted by the various simulations suggest that while stair climbing is surely a critical task for primary stability, for some subjects other tasks may be as critical as stair climbing. From a variance analysis for simple crossover design on the predicted peak micromotion, the inter-subject variability had much more influence on the primary stability of cementless implant than the inter-task variability. Even if the results of Patient IBL, who was reported to have difficulties to perform any activities in a normal way, were excluded from the statistical analysis, the inter-subject variability remained still higher than the inter-task variability. The results obtained from simulations suggest that the strategy the hip replacement patient adopts to perform a given motor task, may be, for the implant stability, equally or even more critical than the type of motor task performed.

Cementation↗
PubMed2003

An automated method to position prosthetic components within multiple anatomical spaces.

The level of fit and fill of a stem in the host femur is the most critical factor for the mechanical stability and success of the prosthesis. It would be useful to have a simulation tool able to investigate the anatomical compatibility of a new implant in a large library of femoral anatomies in the early phases of the design process. In order to realise this tool, it is necessary to develop an automatic method for the positioning of the stem in a database of anatomies. The aim of this study was to develop and evaluate a method for the automatic positioning of the stem geometry in the anatomical CT dataset. Two different strategies were considered: a completely automatic registration technique and a semi-automatic method based on an anatomical referencing. The two procedures were compared to the manual positioning obtained by an expert surgeon in a set of nine CT datasets. For both methods in each femur the positioning and the orientation of the stem were good. The results showed a better level of fitting for the automatic method, while the shift of the hip joint centre was lower for the anatomical referencing technique. However, the anatomical referencing method requires a higher computational effort without being significantly better than the automatic method. For this reason, the automatic method should be chosen to develop the automatic positioning of a stem in a database of anatomies.

Arthroplasty, Replacement, Hip↗
PubMed2003

A new method to compare planned and achieved position of an orthopaedic implant.

The present study describes an automatic method to evaluate the efficacy of a computer aided orthopaedic surgery system by comparing the position of the joint implant, as derived from post-operative computed tomography (CT) scans, to that planned by the surgeon before the operation. The method relies on two spatial registrations, one to align the post-operative femur with the pre-operative femur, the second to compute the planned versus achieved (PVA) accuracy as the roto-translation that registers the pre-operative implant position with the post-operative position. Two surface registration algorithms (a generic average distance minimisation and the specialised iterative closest point (ICP) method) were comparatively evaluated first on a set of test cases to measure the absolute accuracy and robustness with respect to peculiar situations such as a distant starting point. The average distance method failed the registration of one test case and showed peak errors of 0.97 degrees on the rotations and 3.09 mm on the translations. The ICP method was found much more efficient and was able to register all test cases. The peak error was 0.44 degrees on the rotations and 0.67 mm on the translations. The ICP method was then used to compute the PVA accuracy on six clinical cases treated with a CT-based planning system in combination with conventional surgical procedures. The method successfully processed all cases demonstrating the efficacy of the proposed procedure in the specific application.

Orthopedics↗
PubMed2003

Accuracy and repeatability of cementless total hip replacement surgery in patients with deformed anatomies.

The present study is aimed to assess the repeatability of orthopaedic surgeons in planning total hip replacement surgery, and the Planned-vs.-Achieved accuracy obtainable with a conventional unassisted surgical procedure. A CT-based surgical planning system called Hip-Op was used for pre-operative planning the pose of the cementless components. The study group included only patients affected by severe deformities of the hip joint. In the repeatability study three surgeons were asked to repetitively plan the same three cases in a blind way. There was agreement among surgeons and also consistency for each surgeon in planning the implant position, while the most expert surgeon was more repeatable in planning the implant orientation. For all patients of the study group, the Planned-vs.-Achieved accuracy was computed as the difference between the spatial position of both prosthetic components derived from the post-operative CT scans and that achieved by the surgeon in the pre-operative planning. The average differences for the stem were lower than 5 mm for the position, and lower than 5 degrees for the orientation. For the socket the average differences increased to 8 mm and 10 degrees. The study shows the need for a more informative planning environment and for intra-operative supports, especially when deformed anatomies are involved.

Anatomy↗
PubMed2003

Automatic generation of finite element meshes from computed tomography data.

A major obstacle for a broader adoption of the finite element method (FEM) in clinical biomedical applications is the generation of the model, frequently too slow for the times imposed by the clinical practice. The algorithms for automatic mesh generation have greatly improved, but their adoption by the biomedical community is still limited. The aim of this work is to review the principal algorithms for automatic mesh generation and to critically discuss them with particular reference to their applicability in the biomedical field. Specialized literature on numerical methods was reviewed in order to identify the main theoretical approaches currently available for automatic mesh generation. Then, published methods for the automatic generation of finite element models of organs from computed tomography data were reviewed and classified with a proposed taxonomy. Each method was reconnected to a theoretical approach described in the specialized literature whenever possible. Last, each method was critically reviewed with respect to its applicability to the clinical practice. None of the methods described satisfy all the requirements in terms of automation, generality, accuracy, and robustness imposed by a clinical application. However, some of these methods can already be successfully used in various application contexts, and a few guidelines are drawn.

Algorithms↗
PubMed2002

The role of parameter identification in finite element contact analyses with reference to orthopaedic biomechanics applications.

A finite element model accounting for large sliding frictional contact requires, depending on the type of contact algorithm in use, the definition of many numerical parameters such as contact stiffness, convergence norm and tolerance, compenetration monitoring, over-relaxing factors, etc. All these parameters do not have a physical meaning and thus they cannot be measured experimentally. This makes their identification quite complex. The aim of this study was to investigate the role of parameter identification on the accuracy of results produced by finite element models accounting for bone-implant frictional contact, when the Penalty method is used. The sensitivity analysis of several numerical parameters that may govern the state of results was carried out. Two parameters, contact stiffness and convergence tolerance, were found to play a crucial role in establishing the accuracy of the finite element results. Based on the achieved results it was stated that any numerical-only study involving contact non-linearity and omitting careful qualification of the model limits should be rejected from any peer-reviewed journal.

Biomechanical Phenomena↗
PubMed2002

Effect of display modality on spatial accuracy of orthopaedic surgery pre-operative planning applications.

Graphical representation of a patient's anatomy is fairly similar in the majority of orthopaedic surgery planning programs. The position of implantable devices is usually established using a three-pane window showing 2D cross sections of the CT data set taken on three user-selectable orthogonal planes. In some cases this orthogonal-plane representation is replaced or extended by interactive 3D visualization, obtained using surface rendering techniques. These ways to represent the CT data come naturally and easily to the programmer. However, the efficacy of these display strategies is questionable. The present study aims to assess if the display modality used to visualize CT data affects the inherent spatial accuracy achievable in a surgical planning application. A group of users was asked to perform repeatedly a specific planning task using various display interfaces to the same underlying software application. The planning task was designed to allow an assessment of the accuracy with which each user was able to position the prosthetic component. A specialized interface, called multimodal display, presented a peak error of 0.45 mm and 0.95 deg, significantly lower than the 2.4 mm and 4 deg for the othogonal slices interface, and the 3.8 mm and 16.7 deg for the 3D-rendering interface. The results of this study indicate an important effect of the type of visualization modality used to represent CT data on final accuracy of the planning operation.

Computer Graphics↗
PubMed2002

Hip-Op: an innovative software to plan total hip replacement surgery.

This paper describes an innovative surgical simulation software environment for the pre-operative planning of total hip replacement surgery. The software is a CT-based three-dimensional planning environment, with a user-friendly graphic user interface based on the multimodal display visualization paradigm. Although it relies on a fully three dimensional internal representation, this approach represents the relevant anatomical objects by means of multiple views, each simulating a different medical imaging modality familiar to the medical professional. In the Hip-Op program the multimodal display interface integrates four different display modalities: orthogonal radiographs, Blended slices, CT slices, and arbitrary slices. A conventional surface rendering view is also available. The user 'navigates' the prosthetic components, which are dynamically selected from a library of available parts, within the CT volume while the implant and the patient anatomy are simultaneously rendered in each specialized view. Beside a consideration of anatomical compatibility, the surgeon may evaluate the planned implant type, size and position, also on the basis of two analysis modules that compute the achieved level of implant fitting and filling. After being evaluated in an internal clinical trial, the software is currently made available as freeware at http:// www.ior.it/hipop/.

Algorithms↗
PubMed2002

Prediction of hip fracture can be significantly improved by a single biomedical indicator.

Femoral neck fractures are a relevant clinical and social problem. The aim of this study was to improve the prediction of patients at-risk of femoral neck fracture with respect to the current densitometric-based methods. In particular, finite element models were used to assess the prediction accuracy obtained by combining together data from the bone density distribution, the proximal femur anatomy, and the fall-related loading conditions. Two-dimensional finite element models were developed based on dual energy x-ray absorptiometry data. A population of 93 elder Caucasian women (half of them reporting a femoral neck fracture) were retrospectively classified both using the standard clinical protocol and Bayes' linear classifiers. This study showed that the bone mineral density in the femoral neck region dominated the fracture event (65% accuracy). Adding the subject's height and the neck-shaft angle to the bone density increased the accuracy to 77%. The classification accuracy was further improved to 82% by including the peak principal tensile strain obtained from the finite element analyses. This research demonstrated that adding one single biomechanical indicator to the standard clinical measurements improves the identification of patients at-risk of femoral neck fracture.

Aged↗