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Biomedical subjects

F Lavaste

Publications and source records attributed to F Lavaste.

53 records · Page 3Linked to original sources
PubMed1995

Quantification of three-dimensional vertebral rotations in scoliosis: what are the true values?

STUDY DESIGN: The aim of this study is to quantify differences between three-dimensional rotations in space and their calculated values, either on two-dimensional projections (radiographs or computed tomographic scans) or three-dimensional calculations using various mathematical procedures. OBJECTIVE: To use a vertebral model to quantify differences between three-dimensional rotations and their calculated values, using two-dimensional projections or various three-dimensional mathematical procedures. METHODS: A specific program allowed us to move a geometric vertebral model in space using given values and sequences of lateral, sagittal, and axial rotations. Differences in positions due to different sequences were visualized and quantified. Differences due to rotation around global or vertebral axes were considered. RESULTS: For rotations of about 10 degrees, differences are about 2 degrees between three-dimensional and projected angles. Differences increase when combined rotations are large, as generally occurs in a scoliotic spine. They reach 16 degrees for lateral and sagittal rotations of 30 degrees. CONCLUSION: Axial rotation measured on transverse projection is misleading for vertebrae rotated in space. Moreover, dealing with large three-dimensional rotations is meaningful only if the used mathematical convention is given.

Computer Simulation↗
PubMed1995

[Cinematic in vivo analysis of the knee: a comparative study of 4 types of total knee prostheses].

PURPOSE OF THE STUDY: The goal of the study was to assess in vivo kinematics for four designs of knee prosthesis during level walking, stair climbing and non weight-bearing flexion-extension. PATIENT AND METHODS: 19 patients with unilateral total knee arthroplasty (TKA) were included [5 bicruciate sparing prosthesis (BI), 5 posterior cruciate sparing prosthesis with flat tibial polyethylene (PP), 5 posterior cruciate sparing prosthesis with congruent tibial polyethylene (PC), 4 postero stabilised (PS)]. These 19 patients had no prosthesis nor pathological situation in any other joint of the lower limbs. Each of these 19 prosthesis had an HSS score greater than 80 and no radiographic signs of loosening. Magnitudes of the knee rotations (flexion-extension axial rotation valgus-varus) were evaluated with a 6 degrees freedom of motion electromagnetic-goniometer during level walking, stair climbing and non weight-bearing flexion-extension. The magnitudes of the three rotations were recorded for the 19 prosthetic knees and for the 19 controlateral non prosthetic knees of the patients. Reproducibility of the method was also evaluated on 12 healthy subjects by comparison of magnitudes observed during two different recordings. RESULTS: Reproducibility was excellent for magnitudes of flexion (r = 0.95/p = 0.0001) and axial rotation (r = 0.55/p = 0.002) but less satisfactory for valgus-varus movements (r = 0.46/p = 0.005). The magnitudes of the three rotations were inferior for TKA in comparison with healthy knees for any activities. By comparison of the 19 prosthetic and non prosthetic knees we recorded smaller magnitudes of axial rotation during swing phase for level and stair climbing and during non weight-bearing flexion extension movements. Between the four kind of prosthesis we observed: greater magnitudes of flexion for BI and PC prosthesis during stair climbing (p < 0.05) and greater magnitudes of flexion for BI PC and PS prosthesis during stair descending (p < 0.05). PC prosthesis instead of a congruent polyethylene tibial plateau had greater magnitudes of axial rotation than non constrained BI prosthesis during stair climbing (p = 0.009). In spite of a high femoro-tibial congruency we recorded axial rotation in PS prosthesis during each activities. DISCUSSION AND CONCLUSION: Our method evaluating in vivo knee kinematics was reproducible. These four knee prosthetic designs in spite of a good functional results were unable to reproduce magnitudes of movements recorded in healthy subjects. The small number of prosthesis included in the study prompt us to consider as no definitive the differences observed between the 4 designs. Anyway the influence of design on kinematics should be considered as relative since we recorded axial rotation for all four cruciate substituting prostheses although they had high femoro-tibial congruency. Influence of femoro-tibial congruency and cruciate ligament sparing could be assessed in vivo by means of this reproducible method on a larger population.

Adult↗
PubMed1995

[In vitro study of the properties of bioresorbable lactic acid polymer materials].

PURPOSE OF THE STUDY: The potential applications of biodegradable osteosynthesis implants present many advantages over conventional metallic devices. Polyesters of the poly and hydroxy-acid type were recognized early as serious candidates. These polymers have demonstrated a very good biocompatibility and are biodegradable in vivo. After biological and chemical testing poly L. lactic acid 98 (PLA 98) was selected as a candidate. We used a static and dynamic investigation in vitro to assess firstly the material properties of PLA 98 and secondly how its characteristics could be modified within a physiological environment. MATERIAL: Michel Vert and colleagues have shown that polymers of lactic acid have a similar time to resorption providing they contain 98 per cent of the "L" form of the polymer. In vitro studies were assessed on bars made in PLA 98. METHODS: In a first time in vitro studies in traction and flexion on bars allowed an assessment of mechanical properties of PLA 98. In a second time stresses were applied on bars using a physiological environment (Haemacel - 37 degrees C). In a third time we assessed the mechanical properties at the temperature of 37 degrees C with dynamic tests on bars in traction and flexion. RESULTS: The stress-strain curves on bars showed that the material is fragile. Sterilisation with ethylene-oxide did not affect the mechanical properties. When bars were placed in a thermostatically controlled (37 degrees C) physiological environment, the stress-strain curve showed that the material became ductile. With a temperature of 37 degrees C and with a frequency better than one hertz, the dynamic tests on bars showed that the material endurance is good up to 20,000 cycles. At 37 degrees C and at the end of one month, the Young modulus and the maximal strain before breaking lose 50 per cent of their initial value. DISCUSSION: All things considered and as the digital value showed, the PLA 98 appear to be ten times less strong than steel. In a physiological environment the mechanical properties improved due to hydratation of the polymer. The material become quickly ductile or malleable. This allowed transient loading without causing breakage. CONCLUSION: The mechanical properties of bioresorbable materials are very different from those of stainless steel and there is a learning curve in their utilisation. The PLA 98 polymer has demonstrated a very good biocompatibility and is totally biodegradable in vivo. With these results we think that PLA 98 can be used in clinical practice. Indications and clinical use should remain limited to bones regions with low applied stresses.

Biocompatible Materials↗
PubMed1995

[Geometrical modeling of the spine and the thorax for the biomechanical analysis of scoliotic deformities using the finite element method].

In order to study the biomechanical behavior of the whole human spine and thorax, as well as orthopaedic treatment effects, a new generation model is proposed, which includes a precise functional representation of the posterior part of the spine, while respecting computational capabilities. This paper presents the geometrical aspects of this model. The latter is built using an hybrid method which combines steroradiographic 3-D reconstructions of the spine and thorax [1] to serial CT scan 3-D reconstructions of typical human vertebrae and sternum [4] and published morphometric data of ribs [2, 3]. These anatomical structures were deformed in order to fit as well as possible the personalized data of scoliotic patients using geometrical transformations as well as interpolation or extrapolation techniques. In the posterior part, articular facets are modelled and parameterized as elementary surface shapes (plane, cylinder, sphere). For the articular facet geometry of a given normal subject, results revealed that the zygapophyseal facets are better represented by planes for T1 to T11 and by portions of cylinders for T12 to L5, which is in concordance with the literature [5, 6]. Evaluation of this modelling approach was done on 2 cadaveric vertebral segments. Parametric data obtained from the model were compared to precise measurements done on the vertebrae using a 3-D digitizer, and concordance was found. These personalized geometric informations were then used to build a finite element model [7], which will be useful to study scoliotic deformities as well as personalized orthopaedic treatments.

Humans↗
PubMed1994

Experimental model of posterolateral spinal arthrodesis in sheep. Part 1. Experimental procedures and results with autologous bone graft.

OBJECTIVES: The authors evaluated the reliability in obtaining a posterolateral spinal arthrodesis (PSA) with autologous bone graft. SUMMARY OF BACKGROUND DATA: Posterolateral spinal arthrodesis using autogenous cancellous bone graft is the most simple and efficient technique to get a spinal graft. No extensive biomechanical study of PSA is available. Thus, an experimental model of PSA is needed. METHODS: Eleven sheep underwent lumbar autologous bone grafts and Cotrel-Dubousset instrumentations, and four sheep were used as controls. Sacrifice and biomechanical evaluation of the lumbar spines were performed after 1 year. RESULTS: All grafts appeared continuous. A large decrease of flexibility (in rotation and in translation) was found in grafted spines in every direction. Failure in extension occurred at a mean value of 35.26 +/- 3.71 Nm. CONCLUSION: A constant and homogenous PSA appears to be obtained in sheep under conditions close to the human surgery.

Animals↗
PubMed1994

Experimental model of posterolateral spinal arthrodesis in sheep. Part 2. Application of the model: evaluation of vertebral fusion obtained with coral (Porites) or with a biphasic ceramic (Triosite).

OBJECTIVES: The authors evaluated two bone substitutes in a posterolateral spinal arthrodesis (PSA) model in sheep: coral porites (99% calcium carbonate, Biocoral, Inoteb, France) and a biphasic ceramic (BCP) (65% hydroxyapatite and 35% B tricalcium phosphate, Triosite, Zimmer International). SUMMARY OF BACKGROUND DATA: Bone substitutes would be of great interest for PSA. Previous trials began with two kinds of biomaterials: natural coralline calcium carbonate, and phosphate calcium ceramic. METHODS: A lumbar PSA was performed in 11 sheep (coral group) and in 9 sheep (BCP group). Sacrifice and biomechanical tests were performed after 1 year. RESULTS: A large decrease of flexibility in all directions was obtained with both coral PSA and BCP PSA similarly to autologous graft. No nonfusion case was observed. CONCLUSION: In conditions close to the human surgery, a PSA can be obtained using either coral porites or BCP as bone substitutes.

Animals↗
PubMed1994

Effects of glucocorticoids on skeletal growth in rabbits evaluated by dual-photon absorptiometry, microscopic connectivity and vertebral compressive strength.

The effects of corticosteroid on bone were examined in female growing rabbits treated with 0.7 mg/kg per day prednisolone for 5 months. The evolution of whole-body total bone mineral measured by dual-photon absorptiometry showed a significant difference between the prednisolone-treated group and the control group from the first to the fifth month. The histomorphometric profile of corticosteroid-induced osteoporosis was observed, in particular the lower bone volume and thinner and fewer trabecular plates. Mechanical tests are possible on rabbit vertebrae and showed a very significant difference in bone strength between the prednisolone-treated and control groups, and a good correlation between mechanical tests and histomorphometric or densitometric results. This bone corticosteroid model shows that vertebral compression tests are possible on rabbit lumbar vertebrae. It may contribute to a better evaluation of corticosteroid treatments.

Absorptiometry, Photon↗
PubMed1994

Influence of geometrical factors on the behavior of lumbar spine segments: a finite element analysis.

The main objective of this study was the assessment of the influence of geometrical factors on the behavior of lumbar segments. To this end, a three-dimensional, parameterized, finite element model of the lumbar spine was used, and the results were compared with inhouse experimental results and with the few published experimental results available concerning either the geometry of the tested samples or the differences observed at different vertebral levels. Furthermore, in order to appreciate the relative importance of the geometry, the influence of the variation of some other parameters was studied, such as the orientation of the facet joints, the gap between the articular processes, and the Young's modulus of the disk fibers. As a first approach, a series of computations was carried out in order to evaluate the role of geometry in the mechanical behavior differences observed at different levels. It has been found that geometrical factors do exert a noticeable influence on the behavior of the spine, especially those which interfere with the dimensions of the intervertebral disk.

Biomechanical Phenomena↗
PubMed1993

A biomechanical analysis of short segment spinal fixation using a three-dimensional geometric and mechanical model.

Vertebral stabilization using spinal fixation devices is a widely used technique. A three-dimensional geometric and mechanical finite element model has been used as a simulation tool for the evaluation of the mechanical behavior of spinal devices. The geometry of lumbar vertebrae was parameterized, which allows the construction of the geometric model for a given lumbar segment from the digitization of two roentgenographs. This procedure was used to construct a finite element model for a three-vertebra segment with simulation of fractures in the middle vertebra, and with simulation of a restoration using an osteosynthesis device, implemented in a frame fashion with four screws and two rods linked by two transverse rods, and/or an anterior bone graft. Compression force and torsion moment were considered, and different cases were investigated, by varying the severity of the fracture, the geometric characteristics of the device, and the mechanical characteristics of the material joining the two intact vertebral bodies. Results were analyzed considering the mobility of the vertebral segment, which indicates the ability of the restoration system to stabilize the vertebral segment, and considering the forces and moments distribution in the device, which gives information on part of the forces that pass through the device in each situation. Results show that maximum values of forces and moments in the device are more important in compression than in torsion. Adding an anterior bone graft has an effect mainly for compression, whereas in torsion its effect is negligible. For a rigid fixation device, no significant difference was found between different fracture models, indicating that the posterior arch does not play an important role for an instrumented segment. For compression, a rigid posterior wall, or the presence of a bone graft, reduces greatly the mobility of the instrumented segment. For torsion, suppressing the two transverse rods in the device greatly increases the mobility of the instrumented segment. Using a finite element model of a lumbar vertebral segment appears to be an interesting tool to analyze the behavior of an instrumented spine and to compare between different stabilization systems.

Biomechanical Phenomena↗
PubMed1992

Three-dimensional geometrical and mechanical modelling of the lumbar spine.

The main objective of this study is to design a three-dimensional geometrical and mechanical finite element model of the lumbar spine. The model's geometry is constructed using six parameters per vertebra. These parameters are digitized from two X-rays (anterio-posterior and lateral), thus yielding an individualized model which can be arrived at from the radiographs of a tested specimen. This procedure makes the model validation easier, as geometry is generally a factor of dispersion in experimental results. The geometrical reconstruction, in the form of a finite elements mesh, was effected for the whole lumbar spine. The global coherence of the model was verified.

Humans↗
PubMed1991

[Three-dimensional rigidity of the Ilizarov external fixator (original and modified) implanted at the femur. Experimental study and clinical deductions].

The application of the Ilizarov device to the femur creates several problems: anatomical (transfixion of the thigh), mechanical (asymetrical assembly) and clinical (patient acceptability and duration of treatment). Geometric modifications of the original fixator are proposed. These comprise the use of threaded pins proximally and special connecting pieces. Two large fixators were tested under load and after the introduction of certain variables led the authors to experiment with 18 different assemblies. Four loads were used (compression, flexion in the sagittal and coronal planes and torsion) and stiffness calculated in three dimensions. Three linear and three angular displacements were defined for each type and magnitude of external load. The following sequence of analysis was followed to select the best type of assembly; increased ridigity in torsion; moderate displacement in flexion and axial elasticity. These considerations also apply to the mechanics of fracture healing. After testing under load it was concluded that the original assembly gave the best compromise. The modified assemblies gave a slightly inferior mechanical performance but their geometrical configuration should give better patient tolerance if the following items are used: a proximal arc of 120 degrees, 5 mm diameter threaded pins for the adult and 4 mm diameter for the child.

Biomechanical Phenomena↗
PubMed1988

[Restatement and conception of an experimental battery of measurements of displacements in the three planes of a vertebral unit].

The study of motions of a vertebral unit is difficult because of the great number of participating structures. Analysis in the three planes is required to obtain optimal visualization of the entire spectrum of criteria involved. Initially, we used comparators to achieve these measurements. However, the complexity and number of parameters quickly led us to switch to electronic sensors. At present, a completely automated test bench has been developed using an Orion data processing chain. Decoding and interpretating results nevertheless remains difficult in such tridimensional analyses.

Automation↗
PubMed1984

[Epiphyseal fixation with an external fixator. Biomechanical and clinical study].

Fifty one fractures of an epiphysis were treated by fixation using a new external fixator. The stability of the fixation was compared with that obtained by other types of fixator. The basis of the technique was the insertion of five pins into the epiphysis and of a series of pins in the diaphysis. Fixation between the epiphyseal and diaphyseal pins was achieved by three bars. The results obtained in 38 cases, half of them being at the proximal end of the tibia were analysed and were found to be encouraging. Only three had to be grafted secondarily. There was a low number of mal-unions. The main advantage of this method was to allow early mobilisation of the joint. The risk of sepsis did not give rise to concern provided that the rods do not cross the joint.

Biomechanical Phenomena↗
PubMedDate not supplied

[Multi-segment ventral stabilization of the lumbar spine: a comparative biomechanical study].

The stability provided by three anterior spinal fixation devices (VDS, TSRH, CDH) designed for multisegmental instrumentation have been studied in an in-vitro model using L1-L5 sections of six human cadaveric spines for each instrumentation. Three-dimensional measurement of rotation and translation for the intact and instrumented spine under physiological loads in flexion/extension, lateral bending, and axial rotation were determined. After measuring the intact spine the destabilization was performed by complete intersection of the intervertebral disc, all spines were instrumented for 3 segments. TSRH was found to be significantly (p < 0.05) higher in stiffness in flexion/extension and torsion than VDS. In bending there was no statistical difference. In all tests CDH was significantly more stable than VDS. In flexion/extension it proved higher in stiffness than TSRH, no statistical difference was observed in bending and rotation.

Biomechanical Phenomena↗