[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.