EOS System: Overall Assessment of Balance of the Vertebral Column and Its Movements

Fig. 1

The Chain of Balance, the cephalic vertebra, the intercalary pelvic vertebra, and the “Cone of Economy”

In this space, joint work had become obligatory between computer engineers and biomechanic engineers, in particular at the LBM (Laboratoire de Biomécanique) of ENSAM (École Nationale Supérieure D’arts Et Métiers) in Paris. This allowed to obtain, from two radiographic planar projections, surface modelling and three-dimensional measurements of scoliotic deformities, until then only confined to 2D planar projections .

Still in the space where the genius of Georges Charpak, rewarded by his Nobel Prize in 1992, allowed, thanks to his invention (multi wires proportionate chamber) with a significant reduction in radiation of each radiograph.

In time, finally, where all these elements were together in the radiography department of Gabriel Kalifa, at Saint Vincent de Paul Hospital, to inspire the team of Georges Charpak to decide to build the prototype of the EOS device, and experimented in 2000 at the same hospital. Jacques Deguise, of LIO Montréal, helped to automate 3D reconstruction [2].

The device that gives simultaneous anterior-posterior and lateral view radiographs, without distortion or enlargement of the entire standing skeleton, sitting or squatting, gives 2D precise information on the alignment of the different skeletal parts, particularly the lower limbs and trunk. Numerous 3D reconstruction software programmes have since developed based on these raw radiographic data. This allowed at the level of the spine, thorax, pelvis, lower and upper limbs and also the global skeleton to have an exact idea, not only the alignment but also the 3D static morphology and in particular the stacking of skeletal parts in the horizontal planes, with considerably lower radiation compared to the same reconstructions obtained from CT sections whilst maintaining precision.

Despite availability of the first EOS devices for more than 10 years, orthopaedic surgeons, especially spinal, have largely remained using 2D imaging, very centred on the sagittal plane, oblivious to the rotatory aspect, often coupled elsewhere, which happens in the horizontal plane. This suggests a familiarity that has occurred with the various classifications of Lenke for scoliosis or Roussouly for sagittal morphology, which have their merits, but who have neglected this third horizontal dimension that is nevertheless fundamental for function especially the movement despite being “hidden” at first glance.

Information Provided for 2D Alignment of the Body (Fig. 2)

It is understood that with EOS, the X-rays exiting the emitter have the advantage of not being deformed as they are collimated and are always perpendicular to the target. They require a perfect position of the upper limbs, fingers of each hand placed on the corresponding cheekbone of each cheek (so as not to interfere with the posture) and immobility of the patient during the duration of the scan—which may be a possible difficulty in the young child. It takes about 10 s for an adult, less of course for a purely spinal simple sweep, but then we would lose a lot of postural information. These 2D images, obtained with a single shot, thus allow, from a line perpendicular to starting point from the centre of the “Polygon of Support” to give precise measurements on frontal and sagittal body alignment. By measuring the reference points, various bone parts with respect to this axis of reference have been shown to be even more precise and reliable than the no less famous plumb line. They are commonly used for preoperative and post-operative static measurements of orthopaedic surgical procedures involving lower limbs or trunk, and especially the spine.

../images/451372_1_En_8_Chapter/451372_1_En_8_Fig2_HTML.png — Fig. 2

Two-dimensional (2D) measurements of alignment to the vertical axis drawn from the centre of the Polygon of Support, made from simultaneous images both of the coronal and sagittal profiles of the entire standing skeleton

Information Provided by 3D Volumic Surface Reconstructions Obtained Through Computer Software

They give the same static information as before (Fig. 3), with better reliability because the measurements are generated from well-defined points or reference lines (e.g. mechanical axes of the lower limbs, leg length inequalities and Cobb angle).

../images/451372_1_En_8_Chapter/451372_1_En_8_Fig3_HTML.jpg — Fig. 3

3D Reconstruction of the entire skeleton while standing

Above all, this makes it possible to understand the spatial position of the skeletal elements, especially in the horizontal plane: thus the measurement of the vertebral rotation of each vertebra, and also intervertebral relationships within a scoliotic curvature, with this unique vision of the vertebral column seen from above or below (which demonstrates and visualizes the true basal torsional phenomenon of scoliotic deformity) (Fig. 4).