Anatomical And Biomechanical Study Related To Hangman's Fracture And Invention Of Aiming Device Of Lateral Arch Of Axis

Objective The purpose of the present study is to find the optimal operative approach and internal fixation method to treat Hangman's fracture by comparing anterior approach following C2-3 plate fixation with posterior approach following lateral arch screw fixation of axis based on anatomic and biomechanical study. An adjustable bilateral aiming device of lateral arch of axis was devised to guarantee safe lateral arch screw insertion of axis by accurately following the trajectory which was measured and calculated with computer-assisted simulating techniques, and the effect of the device's preliminary application in guiding and correcting bit direction was assessed. To compensate the fixation strength lack of the present prevalent posterior short-segment screw-rob fixation system in treatment of Type-Ill Hangman's fracture because the screw and rob interface with each other through point-to-plane connection or line-to-plane connection mechanism. An affiliating connection component was devised to change the interface mechanism to plane-to-plane connection to increase the interface strength. Background Being transitional vertebra between atlas-cranium compound and the remaining subjacent typical cervical vertebra , the structure of axis is atypical, with its superior surface of central part of anterior structure uniting with the odontoid, which is originated from the body of atlas, the superior articular process moving forwards and finally occupying the lateral region of upper aspect of anterior structure, following the pars interarticularis extending forward and appearing as lateral ring of axis, which is the location of pedicle in typical cervical vertebra. All these changes made the use of the anatomic terminology of the pedicle or pars interarticularis (isthmus) in axis con/using. Some stated that the true anatomic pedicle of the axis was the narrower portion joining the complex vertebral body-odontoid base to the superior articular process, and the isthmus was the portion located between the superior and inferior facet, while some named the portion beneath and posterior to the superior facet as pedicle. The unique structure and shape of axis also made load transmitting through it peculiar. The weight of the skull is transmitted caudally to the superior facets of the atlas, then to the inferior facets of the atlas, and then to the superior facets of the axis. The axis, which has a vertebral body-like structure anteriorly and a disc space linking it to the third cervical vertebra below, will receive most of the vertical load. At this segment most of the weight is therefore transmitted in an anterior direction, towards the disc space. Force vectors, passing downwards, diverge at the level of the axis and part of this force, is transmitted to the posterior articular facets through lateral arch of axis. These vertical compressive forces aremainly secondary to violent hyperextension of the cervical spine and act against the intact anterior stabilizing structures; the force vectors therefore divide at the ring of the axis and account for the location of the fracture within the bony ring, and for the anterior displacement of the body of the axis. Forceful hyperextension accentuates the divergence of these vectors of force. Both cervical hyperextension and axial loading are the cause of fracture at the lateral ring of axis, which is referred as Type 1 Hangman's fracture. The posterior portion of lateral arch, parted by posterior nodule of transverse foramen, is weaker than the anterior portion, and is more likely to fracture. If a fracture occurs at this portion, it is named typical Hangman's fracture while a fracture occurred at the anterior portion is called an atypical Hangman's fracture. Effendi and his advocators claimed that further hyperextension force will rupture the anterior longitudinal ligament and the anterior annulus of the disc space, producing an unstable Type II lesion (injury), and the anterior dislocation of inferior facet in Type III lesion preceded the fracture of the ring of axis and w...