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.

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.

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.

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.

The mesh-matching algorithm: an automatic 3D mesh generator for finite element structures.

Several authors have employed finite element analysis for stress and strain analysis in orthopaedic biomechanics. Unfortunately, the definition of three-dimensional models is time consuming (mainly because of the manual 3D meshing process) and consequently the number of analyses to be performed is limited. The authors have investigated a new patient-specific method allowing automatically 3D mesh generation for structures as complex as bone for example. This method, called the mesh-matching (M-M) algorithm, generated automatically customized 3D meshes of anatomical structures from an already existing model. The M-M algorithm has been used to generate FE models of 10 proximal human femora from an initial one which had been experimentally validated. The automatically generated meshes seemed to demonstrate satisfying results.

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.