The invention generally relates to the field of prostheses for the vertebral column, which are intended to correct and stabilize scoliosis or to reposition and stabilize vertebrae in the case of a trauma or a tumor.
Such prostheses, called spinal osteosynthesis devices, can be implanted either by the posterior route or by the anterior route. When a posterior route or approach is used, the surgeon is unable to directly access the vertebral bodies. In contrast, the anterior route has in particular the characteristic of permitting direct access to the vertebral bodies, as well as many other advantages which are listed in the document FR-A-2 697 744, which is hereby incorporated by reference in its entirety.
Such devices implanted by the anterior route and permitting fusion between two or more adjacent vertebrae can be classified in two categories:                those which use one or more components in the form of rigid plates or the like, the plates being fixed to the different vertebrae in the zone to be treated;        those which use one or two metal rods fixed along the vertebral column by means of suitable connection devices.        
To fix these devices on the vertebral column using the known techniques, the surgeon fixes, on each vertebra of the zone in question, a plate-like anchoring member, called an anchoring plate, which serves as an anchoring point for the component or components for regulating the relative positions of these vertebrae, namely the plates and/or the rods.
Generally, the anchoring plate is placed in an orientation that is substantially parallel to the vertebral discs, and its lower face must be quite strongly curved inwards to best match the surface of the vertebra in this orientation. In the case where plates are used, these are directly fixed on the anchoring plate. In the case where rods are used, these are generally connected to the anchoring plate via one or more connectors that can each connect a single rod to the anchoring plate (as in FR-A-2 761 256), or two rods simultaneously (as in FR-A-2 697 744).
In order for the existing devices to be implanted in the thoracic region, it is necessary for the surgeon to make an incision between two ribs. In some cases the surgeon is forced to section a rib. However, as the available space is relatively small, the surgeon then has few possibilities for fixing another anchoring plate on an adjacent vertebra. Therefore, in order to do this, the surgeon very often has to make a new incision, with all the disadvantages which this means for the patient.