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L Rakotomanana

Publications and source records attributed to L Rakotomanana.

6 recordsLinked to original sources
PubMed2002

Determination of orthotropic bone elastic constants using FEA and modal analysis.

Finite element models have been widely employed in an effort to quantify the stress and strain distribution around implanted prostheses and to explore the influence of these distributions on their long-term stability. In order to provide meaningful predictions, such models must contain an appropriate reflection of mechanical properties. Detailed geometrical and density information is now readily available from CT scanning. However, despite the use of phantoms, a method of determining mechanical properties (or elastic constants) from bone density has yet to be made available in a usable form. In this study, a cadaveric bone was CT scanned and its natural frequencies were measured using modal analysis. Using the geometry obtained from the CT scan data, a finite element mesh was created with the distribution of density established by matching the mass of the FE bone model with the mass of the cadaveric bone. The maximum values of the orthotropic elastic constants were then established by matching the predictions from FE modal analyses to the experimental natural frequencies, giving a maximum error of 7.8% over 4 modes of vibration. Finally, the elastic constants of the bone derived from the analyses were compared with those measured using ultrasound techniques. This produced a difference of <1% for both the maximum density and axial Young's Modulus. This study has thereby produced an orthotropic finite element model of a human femur. More importantly, however, is the implication that it is possible to create a valid FE model by simply comparing the FE results with the measured resonant frequency of the CT scanned bone.

Anisotropy↗
PubMed2002

Virtual power based algorithm for decoupling large motions from infinitesimal strains: application to shoulder joint biomechanics.

New trends of numerical models of human joints require more and more computation of both large amplitude joint motions and fine bone stress distribution. Together, these problems are difficult to solve and very CPU time consuming. The goal of this study is to develop a new method to diminish the calculation time for this kind of problems which include calculation of large amplitude motions and infinitesimal strains. Based on the Principle of Virtual Power, the present method decouples the problem into two parts. First, rigid body motion is calculated. The bone micro-deformations are then calculated in a second part by using the results of rigid body motions as boundary conditions. A finite element model of the shoulder was used to test this decoupling technique. The model was designed to determine the influence of humeral head shape on stress distribution in the scapula for different physiological motions of the joint. Two versions of the model were developed: a first version completely deformable and a second version based on the developed decoupling method. It was shown that biomechanical variables, as mean pressure and von Mises stress, calculated with the two versions were sensibly the same. On the other hand, CPU time needed for calculating with the new decoupled technique was more than 6 times less than with the completely deformable model.

Algorithms↗
PubMed1997

Displacements of the tibial tuberosity. Effects of the surgical parameters.

A three-dimensional computer model is used, based on the finite element method, to investigate the effects of 1-, 1.5-, and 2-cm tibial tubercle elevations and of 0.5- and 1-cm medial displacements of the tuberosity, performed with different bone shingles. Patellar kinematics and patellofemoral interface peak pressure, between 45 degrees and 135 degrees of passive knee flexion, are compared for these different surgical parameters with those of a normal knee not surgically treated. The shingle lengths of 3, 5, 7, and 10 cm have little influence on the results. Augmenting tubercle medializations decrease the lateral peak pressure but result in an overpressure of the medial facet that is 154% of the normal peak value. With knee flexion between 45 degrees and 60 degrees, increasing tubercle elevations decreases later and medial peak pressures. With flexion of more than 60 degrees, increasing elevations decrease the lateral peak pressure, but they augment and even cause overpressure on the medial facet. An overpressure on the lateral facet also is seen in midrange knee flexion (75 degrees-90 degrees) for all tubercle elevation values. Increasing tubercle elevations and medializations appear to be the predominant parameters from a biomechanical point of view.

Computer Simulation↗
PubMed1995

The biomechanics of the human patella during passive knee flexion.

The fundamental objectives of patello-femoral joint biomechanics include the determination of its kinematics and of its dynamics, as a function of given control parameters like knee flexion or applied muscle forces. On the one hand, patellar tracking provides quantitative information about the joint's stability under given loading conditions, whereas patellar force analyses can typically indicate pathological stress distributions associated for instance with abnormal tracking. The determination of this information becomes especially relevant when facing the problem of evaluating surgical procedures in terms of standard (i.e. non-pathological) knee functionality. Classical examples of such procedures include total knee replacement (TKR) and elevation of the tibial tubercle (Maquet's procedure). Following this perspective, the current study was oriented toward an accurate and reliable determination of the human patella biomechanics during passive knee flexion. To this end, a comprehensive three-dimensional computer model, based on the finite element method, was developed for analyzing articular biomechanics. Unlike previously published studies on patello-femoral biomechanics, this model simultaneously computed the joint's kinematics, associated tendinous and ligamentous forces, articular contact pressures and stresses occurring in the joint during its motion. The components constituting the joint (i.e. bone, cartilage, tendons) were modeled using objective forms of non-linear elastic materials laws. A unilateral contact law allowing for large slip between the patella and the femur was implemented using an augmented Lagrangian formulation. Patellar kinematics computed for two knee specimens were close to equivalent experimental ones (average deviations below 0.5 degrees for the rotations and below 0.5 mm for the translations) and provided validation of the model on a specimen by specimen basis. The ratio between the quadriceps pulling force and the patellar tendon force was less than unity throughout the considered knee flexion range (30-150 degrees), with a minimum near 90 degrees of flexion for both specimens. The contact patterns evolved from the distal part of the retropatellar articular surface to the proximal pole during progressive flexion. The lateral facet bore more pressure than the medial one, with corresponding higher stresses (hydrostatic) in the lateral compartment of the patella. The forces acting on the patella were part of the problem unknowns, thus leading to more realistic loadings for the stress analysis, which was especially important when considering the wide range of variations of the contact pressure acting on the patella during knee flexion.

Algorithms↗
PubMed1994

Influence of soft structures on patellar three-dimensional tracking.

During knee flexion, the human patella moves along a complex path resulting from the combined actions of articular contact and soft-tissue stabilization. The current study is an attempt to characterize the role of these soft structures on patellar kinematics. To this end, the three-dimensional patellar motion during full knee flexion was accurately measured before and after partial dissection of the joint. The guiding role of the femoral groove prevailed over soft-tissue action through most of the range of motion. At full extension, however, when the patella and the femur were not in contact, the influence of the retinaculi was most noticeable, highlighting the unstable behavior of the patella near extension. The differences between the intact and dissected knee kinematics suggested that control over patellar motion is ensured by the transverse soft-tissue structures near extension and by the patellofemoral joint geometry during further flexion.

Biomechanical Phenomena↗