Spontaneous recovery of experimental valgus deformity in lambs.

INTRODUCTION: Angular deformity in the growing skeleton of animals, especially in the radius and ulna, is occasionally seen in clinical practice. The mechanism of spontaneous correction of these angular deformities however remains to be elucidated. The purpose of our experiment was to explore the ability of a growth plate to correct an induced valgus deformity, and to study the mechanism of correction. METHODS: Before beginning the study, valgus deformity of the distal radius had been induced in lambs by the application of a device that causes asymmetrical compression of the growth plate. The study began after removal of the device and spontaneous correction of the induced deformity was observed weekly for 20 weeks. The angles of the deformity and longitudinal growth on the medial and lateral portions of the growth plate were respectively measured on craniocaudal and mediolateral radiographs. RESULTS AND CONCLUSIONS: Spontaneous correction of the valgus deformity occurred during the first 16 weeks. It resulted from asymmetrical growth characterised by restricted activity of the medial portion of the growth plate (14.8%) in comparison to the lateral portion of the experimental radius, and also in comparison to the medial portion of the control radius.

2D calculation method based on composite beam theory for the determination of local homogenised stiffnesses of long bones.

A calculation method using the finite element technique is presented. Its main objective was to determine strains, stresses and more particularly stiffnesses in any cross section of a tibia, thus enabling the localisation of tibial torsion in vivo. Each tibial cross section was considered to be a non-uniform cross section of a composite beam with arbitrary orientation of fibres. The determination of stresses, strains and stiffnesses within a composite beam cross section has been defined by solving a variational problem. The validation of this method was performed on a tibial diaphysis of which each cross section was assumed to be the cross section of a composite beam made of orthotropic materials with orthotropic axes of any orientation with respect to the principal axis of the bone. The comparison of the results, from our model and that of a three-dimensional one, was performed on each nodal value (strains, stresses) of the meshed cross section as it was impossible to obtain local stiffnesses by experimentation. The good agreement between the results has validated our finite element program. Actually, this method has enabled to treat directly 2D geometric reconstructions from CT scan images with a good accuracy to determine locally the homogenised mechanical characteristics of human tibia in vivo, and particularly to quantify torsional tibial abnormalities of children without approximation of the shape of the cross section and by calculating the real moment of inertia J. The importance of the fibre orientation with regards to the stiffness values has been emphasised. This 2D method has also allowed to reduce CPU time of the 3D modelling and calculation.

Development of a three-dimensional finite element model of a human tibia using experimental modal analysis.

The modal analysis of a human tibia consisted of characterizing its dynamic behavior by determining natural frequency, damping ratio and mode shapes. Two methods were used to perform the modal analysis: (1) a finite element method (structural model); (2) an experimental modal analysis (modal model). The experimental modal model was used to optimize the structural model. After optimization, differences in results between the two models were found to be due only to mechanical properties and mass distribution. The influences of boundary conditions and geometric properties (such as inertia and length) were eliminated by the finite element model itself. The percent relative error between the two methods was approximately 3%, corresponding to the standard deviation of the measured frequencies. For the frequency range considered, the mode shapes were bending modes in two different vibration planes (latero-medial and sagittal), with a slight torsion effect due to the twisted geometry of the tibia.

Finite element modelling of the vibrational behaviour of the human femur using CT-based individualized geometrical and material properties.

Frequency analysis of long bones has been investigated as a tool to assess bone quality or integrity. The objective of the present paper was to develop a three-dimensional finite element model of a fresh human femur with geometrical and mechanical properties derived from quantitative computer tomography images. This model was then exercised and the results were compared to those obtained from a vibration analysis technique. The percent relative error between the numerically and experimentally derived results was found about 4%. Finally, the influence of mechanical properties on the resonant spectre was studied. The results exhibit the limitations of the vibrational technique to detect slight material changes.

Morphological and mechanical analysis of the glenoid by 3D geometric reconstruction using computed tomography.

OBJECTIVE: To provide a morphological and mechanical analysis of the glenoid by 3D geometric reconstruction using computed tomography. DESIGN: For patients with different pathologies (Group A=control group, Group B=primary osteoarthritis, Group C=rheumatoid arthritis), the variation in shape of the scapula was characterized by measuring the glenoid version (beta). METHODS: Mapping the computed tomography number and its 3D variation in the bone as a finite element structure. RESULTS: In Group A, the mean value of version was 17 degrees (range 12-22 degrees ). In Groups B and C the mean value of version were 27 degrees (range 4-48 degrees ) and 31 degrees (range 25-31 degrees ) of retroversion. At the center of the glenoid there was a homogeneous area of bony tissue with low computed tomography values and the subchondral bone could be clearly identified. For Group B patients, the computed tomography values were increased at the posterior margin of the glenoid, with a thickening of the posterior area acting as a strengthening column. For the Group C patients, the anatomical modifications were not reproducible between two cases examined. CONCLUSION: Results reveal a great difference between a healthy and a pathological glenoid. RelevanceThe method will be the basis for future study of the pathological characteristics of the joint. Results should provide a new pre-operative insight to help guide the surgeon.

Finite element analysis of the mechanical behavior of a scapula implanted with a glenoid prosthesis.

OBJECTIVE: The objective of the present study was to analyze the mechanical effect of some of the surgical variables encountered during shoulder arthroplasty using the finite element method. The effect of one eccentric load case, cement thickness and conformity has been investigated. DESIGN: A 3D finite element model of a healthy cadaveric scapula implanted with an anatomically shaped glenoid has been developed from computed tomography (CT) images. BACKGROUND: Glenoid component fixation can present the most difficult problem in total shoulder arthroplasty, loosening of this component remains one of the main complications. METHODS: The 3D finite element model was first validated by comparison with experimental measurements and by fitting of the mechanical properties of the cortical bone. Then the articular pressure location, the surface contact geometry and the cement thickness have been analyzed to observe their effect on stresses and displacements at the interfaces and within the scapular bone. RESULTS: The antero-posterior bending of the scapula was a notable feature and this was accentuated when an eccentric load was applied. The gleno-humeral contact area had a major role on the stress level in the supporting structures though but not on the global displacements. Varying the cement mantle modified stresses according to the load case and it essentially changed the latero-medial displacement of the cement relatively to the bone. CONCLUSIONS: This analysis provided an insight into the mechanical effects of an implanted scapula according to different parameters related to implantation technique. RELEVANCE: Results emphasized the role of some of the parameters a clinician may face. They demonstrated the importance of the humeral head centering in the horizontal plane. Conformity decreasing may involve drastic increase of stresses within structures and a thick cement mantle is not necessarily advantageous relatively to the stresses at the cement/bone interface.

[Experimental biomechanical study of the advancement of tibial tuberosity]. / Etude biomécanique expérimentale de l'avancement de la tubérosité tibiale.

In 1963 P. Maquet suggested tibial tuberosity advancement as a therapeutic solution for patellofemoral arthrosis, but the fiability of the operation remains controversial. An advancement of the tibial tuberosity by 1.5 centimeter, with and without bilateral retinacular release, was performed on non arthrosic knees of fresh cadavers and on knees presenting medial, lateral or bipolar patellofemoral arthrosis. The joints were tested with flexion ranging from 10 to 70 degrees. The study concentrated on contact areas, pressures and patellofemoral forces analysis using an original and efficient measuring system. The pressure sensitive film used was the Fuji Prescale Film. The study showed that, on non arthrosic knees, the patellofemoral contact was not modified to any significant extent by advancing the tibial tuberosity. In cases of bipolar patellofemoral arthrosis reduced pressures were noted. On the lateral patellofemoral arthrosis experimental model, the tibial tuberosity advancement was efficient as it allowed a 50 per cent reduction in pressure at 10 to 30 degrees of flexion and a 20 per cent reduction at 40 to 50 degrees of flexion and a 20 per cent reduction at 40 to 50 degrees of flexion. In all cases, the best results were obtained from tibial tuberosity advancement associated with bilateral retinacular release. These results indicate that tibial tuberosity advancement may have beneficial effects in cases of lateral and bipolar patellofemoral arthrosis.

[Anterior displacement of the tibial tuberosity. An experimental biomechanical study of its effects on the femoro-patellar joint]. / L'avancement de la tubérosité tibiale. Etude biomécanique expérimentale de ses effets sur l'articulation fémoro-patellaire.

The anterior displacement of the tibial condyle was proposed as early as 1963 by P. Maquet as a therapeutic solution to treat femoropatellar arthritis. However, the benefits of such a procedure remain controversial. In this work, the effects of a 1. cm anterior displacement of the tibial condyle, with and without section of the patellar wings, have been studied on the knees of fresh corpses without osteoarthritis. The knees were tested in flexion from 10 to 70 degrees. The study dealt with the contact surfaces, pressures, and femoropatellar forces, which were analyzed with an original and efficient measurement system, with the Fuji Prescale Film as a sensor. This study showed that the femoropatellar contact has not been significantly modified by the displacement of the tibial condyle. We have noted only a slight decrease in the pressure after this type of surgery. These results are in contradiction with Marquet's results. In all cases, the anterior displacement of the tibial condyle with resection of both patellar wings produced the best results. These results suggest that displacing the tibial condyle anteriorly does not have the expected effects. The indication of such a surgical procedure must therefore be established only cautiously and after ruling out other therapeutic methods.

In vivo characterization of glenoid with use of computed tomography.

Improvement in the treatment of the shoulder could be achieved by accurately describing the pathologic characteristics of the joint. The goal of this study was to characterize, in vivo, glenoids with 3 different diagnoses by using computed tomography (CT): rotator cuff pathology with a limited rupture and without bony changes (group A, n = 15), primary osteoarthritis (group B, n = 13), and rheumatoid arthritis (group C, n = 4). The bone density distribution was assessed by means of the CT value expressed in Hounsfield units. The version angle was also measured. The examination of the CT value showed different distributions according to the pathology. In group A, the cancellous bone presented a central area with a relatively homogeneous and low density. In group B, the reinforcement of the density along with the posterior region seemed to be correlated with the retroversion angle. In the rheumatoid arthritis group, the main characteristic was the loss of the subchondral bone margin. The cartography of the CT value was not reproducible among the 4 cases examined. These in vivo descriptions provide guidelines for the surgeon before total shoulder arthroplasty, helping preoperative planning as well as simulation of implantation.

Anatomic variation of the mechanical properties of the glenoid.

Finite element analysis modeling is an important tool in the design of total joint replacements. However, to use a finite element analysis the material properties of the studied bone must be known. The aim of the study was to measure the elastic properties of the glenoid bone in the axial, coronal, and sagittal planes with an ultrasound transmission technique. The relative density and Houndsfield computed tomography numbers were also assessed. Three pairs of scapulas were obtained from unembalmed human cadavers. Seventy-four cubic cancellous bone specimens of 6 mm were used for ultrasonic measurements. The study showed significant differences with anatomic location. Mechanical properties of cancellous bone were found to be higher near the direction of application of the resultant force, perpendicular to the articular surface of the glenoid. Mechanical properties were found to be significantly higher at the center and posterior edge of the glenoid (p < 0.01). Significant differences were also found in the three planes studied. The lateromedial Young's modulus (E1) was higher than the anteroposterior modulus (E2) and the superoinferior modulus (E3) (E1 = 372 +/- 164 MPa, E2 = 222 +/- 79 MPa, E3 = 198 +/- 75 MPa).

Bone cutting.

Bone cutting has always been a problem for surgeons because bone is a hard living material, and many osteotomes are still very crude tools. Technical improvement of these surgical tools has first been their motorization. Studies of the bone cutting process have indicated better features for conventional tools. Several non-conventional osteotomes, particularly ultrasonic osteotomes are described. Some studies on the possible use of lasers for bone cutting are also reported. Use of a pressurised water jet is also briefly examined. Despite their advantages, non-conventional tools still require improvement if they are to be used by surgeons.