Optimization of rib surgery parameters for the correction of scoliotic deformities using approximation models.

BACKGROUND: As opposed to thoracoplasty (a cosmetic surgical intervention used to reduce the rib hump associated with scoliosis), experimental scoliosis has been produced or reversed on animals by rib shortening or lengthening. In a prior work (J. Orthop. Res., 20, pp. 1121-1128), a finite element modeling (FEM) of rib surgeries was developed to study the biomechanics of their correction mechanisms. Our aims in the present study were to investigate the influence of the rib surgery parameters and to identify optimal configurations. Hence, a specific objective of this study was to develop a method to find surgical parameters maximizing the correction by addressing the issue of high computational cost associated with FEM. METHOD OF APPROACH: Different configurations of rib shortening or lengthening were simulated using a FEM of the complete torso adapted to the geometry of six patients. Each configuration was assessed using objective functions that represent different correction objectives. Their value was evaluated using the rib surgery simulation for sample locations in the design space specified by an experimental design. Dual kriging (interpolation technique) was used to fit the data from the computer experiment. The resulting approximation model was used to locate parameters minimizing the objective function. RESULTS: The overall coverage of the design space and the use of an approximation model ensured that the optimization algorithm had not found a local minimum but a global optimal correction. The interventions generally produced slight immediate modifications with final geometry presenting between 95-120% of the initial deformation in about 50% of the tested cases. But in optimal cases, important loads (500-2000 N mm) were generated on vertebral endplates in the apical region, which could potentially produce the long-term correction of vertebral wedging by modulating growth. Optimal parameters varied among patients and for different correction objectives. CONCLUSIONS: Approximation models make it possible to study and find optimal rib surgery parameters while reducing computational cost.

Nerve cuff electrode with shape memory alloy armature: design and fabrication.

In this paper, a new nerve cuff electrode with shape memory alloy armature is presented. The proposed electrode is dedicated either to peripheral nerve stimulation or recording and its manufacturing does not require any expensive or complex technique. Shape Memory Alloy (SMA) armature ensures the complete and firm closing of the electrode, so that the complexity of the installation procedure is considerably reduced. A preliminary analysis of the electrode mechanical behavior prior, during and after installation has been done through numerical simulations and in vitro testing. It was proved theoretically and experimentally that the SMA electrode closes completely with an appropriate few second delay after its installation. No external fixation such as sutures is needed to secure permanent electrode-nerve contact. Furthermore, theoretical analysis has shown that the design of SMA electrode can be adapted for safe close-fitting installation, thanks to the device partial opening in case of nerve swelling.

New easy to install nerve cuff electrode using shape memory alloy armature.

This paper presents an easy to install nerve cuff electrode dedicated to functional electrical stimulation (FES). In this new device, a shape memory alloy (SMA) armature is used to perform the closing of the electrode. This technique makes the electrode installation around the nerve much easier, quicker, and safer. Both remarkable mechanical properties of SMA materials, namely, shape memory effect and superelasticity, can be used to obtain the desired mode of electrode closing. The fabrication procedure of the new electrode is described. It does not require any expensive or complex techniques. Bipolar and tripolar electrodes have been manufactured with an inner diameter of 1.6 mm and a cuff wall thickness of 0.8 mm. These electrodes are to be used for FES of the bladder in spinal cord injured patients. Acute studies in dogs are being carried out to validate the device and the implantation procedure.

Approach for the smoothing of three-dimensional reconstructions of the human spine using dual Kriging interpolation.

In numerous situations, 3-D reconstructions of the spine are represented as curves in space, with the vertebral centroids as control points. Interpolation functions such as splines, polynomials or Fourier series have been used to minimise measurement errors and to perform specific calculations. A more general approach, dual Kriging, is presented which incorporates in a single formulation several methods, such as piece-wise linear interpolation, splines and least square functions as a limit case. To minimise user interaction and to control the different Kriging parameters, a computer program is developed allowing efficient use of these interpolation techniques in a clinical environment. Given different drift and covariance functions, the program determines the most suitable Kriging model for specific spine geometries and controls the amount of smoothing performed on raw data. Validation of the technique is with analytical 3-D curves, where random noise is added to represent reconstruction errors. A maximum absolute mean difference of 1.85 +/- 0.50 mm is found between the analytical and noisy curves smoothed with the Kriging technique for 200 points. Results obtained on actual 3-D reconstructions of scoliotic patients are very promising.

Review of shape memory alloys medical applications in Russia.

In the last twenty-five years a large variety of research has been carried out in Russia using Shape Memory Alloys (SMA), particularly nearly equiatomic NiTi alloys, for medical applications. In this field of activity, Russian research centers have been quite successful in treating different kinds of diseases, from bone fractures to dental implants. This review is intended to give a panorama of SMA medical applications in Russia in order to illustrate the remarkable possibilities offered by SMA materials in the medical field.

Thermomechanical analysis of shape memory devices.

Shape memory alloys (SMA) are being increasingly used in various industrial applications as actuators, connectors, or damping materials. In the medical field, superelastic devices such as eyeglass frames, stents or guide catheters have come to market in the recent years. The design of SMA devices has usually been based on trial and error, since until recently no general simulation model was available to assist application engineers. The purpose of this article is to describe the computational methodology developed, validated and used for several industrial projects at Ecole Polytechnique of Montréal to simulate the thermomechanical behavior of shape memory materials. This new approach includes three main stages: experimental characterization, construction of a nonlinear material law based on dual kriging interpolation and finally, calculation of the thermomechanical response of SMA devices. For complex geometry, finite element analysis is used, but for simple devices such as springs or electrically activated SMA wires, simplified calculation methods are satisfactory. Validation results recently obtained will also be presented, and examples of industrial applications briefly reviewed.