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Biomedical subjects

L R Rakotomanana

Publications and source records attributed to L R Rakotomanana.

9 recordsLinked to original sources
PubMed2004

Peri-implant bone remodeling after total hip replacement combined with systemic alendronate treatment: a finite element analysis.

In order to decrease the peri-implant bone loss during the life-time of the implant, oral use of anti-osteoporosis drugs (like bisphosphonates) has been suggested. In this study, bone remodeling parameters identified from clinical trials of alendronate were used to simulate the effect of those drugs used after total hip arthroplasty on the peri-implant bone density. Results of the simulation show that the oral administrated drugs increase bone density around the implant and decreases, at the same time, the micromovements between the implant and the surrounding bone tissue. Incorporation of drug effect in numerical studies of bone remodeling is a promising tool especially to predetermine safe bisphosphonate doses that could be used with orthopedic implants.

Administration, Oral↗
PubMed2003

Biphasic constitutive laws for biological interface evolution.

A model of tissue differentiation at the bone-implant interface is proposed. The basic hypothesis of the model is that the mechanical environment determines the tissue differentiation. The stimulus chosen is related to the bone-implant micromotions. Equations governing the evolution of the interfacial tissue are proposed and combined with a finite element code to determine the evolution of the fibrous tissue around prostheses. The model is applied to the case of an idealized hip prosthesis.

Algorithms↗
PubMed2000

On the independence of time and strain effects in the stress relaxation of ligaments and tendons.

The hypothesis of variables separation, namely the time and the strain separation in the relaxation function, is widely used in soft tissue biomechanics. Although this hypothesis is central to several biomechanical models, only few experimental works have tried to verify it. From these studies, contradictory results have been found. Moreover, it has recently been noted that no such experimental verification has been performed for ligament tissues. In this paper, an experimental method is developed to test the hypothesis of variables separation. This method is then used with human cruciate ligaments and patellar tendons. It is shown that the use of the variables separation hypothesis is justified at least for strain values lower than 16% for anterior cruciate ligament, lower than 12% for posterior cruciate ligament and lower than 6% for patellar tendon. The method presented in this paper could be used to verify the validity of variables separation for other tissues.

Anterior Cruciate Ligament↗
PubMed2000

The fixation of the cemented femoral component. Effects of stem stiffness, cement thickness and roughness of the cement-bone surface.

After cemented total hip arthroplasty (THA) there may be failure at either the cement-stem or the cement-bone interface. This results from the occurrence of abnormally high shear and compressive stresses within the cement and excessive relative micromovement. We therefore evaluated micromovement and stress at the cement-bone and cement-stem interfaces for a titanium and a chromium-cobalt stem. The behaviour of both implants was similar and no substantial differences were found in the size and distribution of micromovement on either interface with respect to the stiffness of the stem. Micromovement was minimal with a cement mantle 3 to 4 mm thick but then increased with greater thickness of the cement. Abnormally high micromovement occurred when the cement was thinner than 2 mm and the stem was made of titanium. The relative decrease in surface roughness augmented slipping but decreased debonding at the cement-bone interface. Shear stress at this site did not vary significantly for the different coefficients of cement-bone friction while compressive and hoop stresses within the cement increased slightly.

Alloys↗
PubMed2000

[Noncemented total hip arthroplasty: influence of extramedullary parameters on initial implant stability and on bone-implant interface stresses].

PURPOSE OF THE STUDY: After total hip replacement, the initial stability of the cementless femoral stem is a prerequisite for ensuring bone ingrowth and therefore long term fixation of the stem. For custom made implants, long term success of the replacement has been associated with reconstruction of the offset, antero/retro version of the neck orientation and its varus/valgus orientation angle. The goals of this study were to analyze the effects of the extra-medullary parameters on the stability of a noncemented stem after a total hip replacement, and to evaluate the change of stress transfer. MATERIAL AND METHODS: The geometry of a femur was reconstructed from CT-scanner data to obtain a three-dimensional model with distribution of bone density. The intra-medullary shape of the stem was based on the CT-scanner. Seven extra-medullary stem designs were compared: 1) Anatomical case based on the reconstruction of the femoral head position from the CT data; 2) Retroverted case of - 15 degrees with respect to the anatomical reconstruction; 3) Anteverted case with an excessive anteversion angle of + 15 degrees with respect to the anatomical case; 4) Medial case: shortened femoral neck length (- 10 mm) inducing a medial shift of the femoral head offset; 5) Lateral case: elongated femoral neck length (+ 10 mm) inducing lateral shift of the femoral head offset 6) Varus case with CCD angle 127 degrees; 7) Valgus case with CCD angle 143 degrees. The plasma sprayed stem surface was modeled with a frictional contact between bone and implant (friction coefficient: 0.6). The loading condition corresponding to the single limb stance phase during the gait cycle was used for all cases. Applied loads included major muscular forces (gluteus maximus, gluteus medius, psoas). RESULTS: Micromotions (debonding and slipping) of the stems relative to the femur and interfacial stresses (pressure and friction) were different according to the extra-medullary parameters. However, the locations of peak stresses and micromotions were not modified. The highest micromotions and stresses corresponded to the lateral situation and to the anteverted case (micro-slipping and pressure were increased up to 35 p.100). High peak pressure was observed for all designs, ranging from anatomical case (34 MPa) to anteverted case (44 MPa). The peak stresses and micromotions were minimal for the anatomical case. The maximal micro-debonding was not significantly modified by the extra-medullary design of the femoral stem. DISCUSSION: The extra-medullary stem design has been shown to affect the primary stability of implant and the stress transfer after THR. Most interfacial regions present small micro-slipping which normally allows the occurrence of bone ingrowth. The anatomical design presents the lowest micromotions and the lowest interfacial stresses. The worst cases correspond to the anteverted and lateralized cases. Probably, the anteverted situation involves higher torsion torque, which in turn may induce high torsion shear micromotions and higher stress at the interface. Moreover, the lever arm of the weight bearing force on the femoral head is augmented for the augmented neck length situation. This increases the bending moment, and therefore may increase the stresses as well as the stem shear micromotions. In summary, the present results could be taken as biomechanical arguments for the requirement of anatomical reconstruction of not only the intra-medullary shape but also the extra-medullary parameters (reconstruction of the normal hip biomechanics).

Arthroplasty, Replacement, Hip↗
PubMed1999

Strain rate effect on the mechanical behavior of the anterior cruciate ligament-bone complex.

Traction tests were performed on the bovine anterior cruciate ligament-bone complex at seven strain rates (0.1, 1, 5, 10, 20, 30, 40%/s). Corresponding stress-strain curves showed that, for a given strain level, the stress increased with the augmentation of the strain rate. This phenomenon was important since the stress increased by a factor of three between the tests performed at the lowest and highest strain rates. The influence of the strain rate was quantified with a new variable called the "supplemental stress". This variable represented the percentage of total stress due to the effect of strain rate. It was observed that at a strain rate of 40%/s, more than 70% of the stress in the ligament was due to the strain rate effect. In fact, the strain rate strongly affected the toe region, but did not influence the linear part of the stress-strain curves. The use of the linear tangent moduli was then not adequate to describe the strain rate effect in the anterior cruciate ligament-bone complex. This study showed that the "supplemental stress" was a synthetic and convenient variable to quantify the effect of the strain rate on the entire stress-strain curves. This quantification is especially important when comparing the mechanical behavior between anterior cruciate ligament and tissues used as ligament graft.

Animals↗
PubMed1998

Viscoelastic constitutive law in large deformations: application to human knee ligaments and tendons.

Traction tests on soft tissues show that the shape of the stress strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -- two for the elasticity and one for the viscosity -- were enough to precisely fit the non-linear stress strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

Algorithms↗
PubMedDate not supplied

A finite element model of the shoulder: application to the comparison of normal and osteoarthritic joints.

OBJECTIVE: The objective of the present study was to develop a numerical model of the shoulder able to quantify the influence of the shape of the humeral head on the stress distribution in the scapula. The subsequent objective was to apply the model to the comparison of the biomechanics of a normal shoulder (free of pathologies) and an osteoarthritic shoulder presenting primary degenerative disease that changes its bone shape. DESIGN: Since the stability of the glenohumeral joint is mainly provided by soft tissues, the model includes the major rotator cuff muscles in addition to the bones. BACKGROUND: No existing numerical model of the shoulder is able to determine the modification of the stress distribution in the scapula due to a change of the shape of the humeral head or to a modification of the glenoid contact shape and orientation. METHODS: The finite element method was used. The model includes the three-dimensional computed tomography-reconstructed bone geometry and three-dimensional rotator cuff muscles. Large sliding contacts between the reconstructed muscles and the bone surfaces, which provide the joint stability, were considered. A non-homogenous constitutive law was used for the bone as well as non-linear hyperelastic laws for the muscles and for the cartilage. Muscles were considered as passive structures. Internal and external rotations of the shoulders were achieved by a displacement of the muscle active during the specific rotation (subscapularis for internal and infrapinatus for external rotation). RESULTS: The numerical model proposed is able to describe the biomechanics of the shoulder during rotations. The comparison of normal vs. osteoarthritic joints showed a posterior subluxation of the humeral head during external rotation for the osteoarthritic shoulder but no subluxation for the normal shoulder. This leads to important von Mises stress in the posterior part of the glenoid region of the pathologic shoulder while the stress distribution in the normal shoulder is fairly homogeneous. CONCLUSION: This study shows that the posterior subluxation observed in clinical situations for osteoarthritic shoulders may also be cause by the altered geometry of the pathological shoulder and not only by a rigidification of the subscapularis muscle as often postulated. This result is only possible with a model including the soft tissues provided stability of the shoulder. RELEVANCE: One possible cause of the glenoid loosening is the eccentric loading of the glenoid component due to the translation of the humeral head. The proposed model would be a useful tool for designing new shapes for a humeral head prosthesis that optimizes the glenoid loading, the bone stress around the implant, and the bone/implant micromotions in a way that limits the risks of loosening.

Computer Simulation↗