Anatomical And Biomechanical Study Related To Hangman's Fracture And Screw Insertion Study By Virtue Of Navigator

ObjectiveTo find the optimal operative approach to treat Hangman's fracture by virtue of navigator used in research of axis anatomize .To resolve the problem of the point and direction of the fixation of Axis lateral arch screw, using navigator finds the safe trajectory.BackgroundBeing 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 inter articularis 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 inter articularis (isthmus) in axis confusing. 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 are mainly 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 I 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. As the development of spine surgery, reconstruction the spine in physiological way and retrieve its function in maximum have becoming a trend. it is feasible to treat some type of Hangman fracture by tensile force screw.MethodsFirst, we investigate some correlate subjects such as the working principle of navigator, the process of operation navigation, precision of operation navigation and advantage of operation navigator .Direct external observation on 30 dry axes was performed and observation of internal structures of axes was done by thiprecision-slice CT scanning and reconstructing on four fresh axes samples. We found that the triangle -like protrusion is the real shrunk body of axis, the articular process is the true pedicle of axis, while the structure occupying the lateral arch of axis is real pars interarticularis and can be divided into two portions by posterior nodule of transverse foramen: the posterior portion is the real isthmus of pars inter articulris , the anterior portion is lengthening of structure between posterior nodule and superior process, expressing as inner border of transverse foramen. Thus a typical Hangman's fracture should be termed as the fracture of isthmus of axis, and an atypical fracture should be called as the fracture happened between superior process and posterior nodule of transverse foramen. Based on consecutive CT scanning image" jf4 dry and 4 fresh axes samples interval scale of 1.25mm. Imaginary axis representing its physical ones were reconstructed by navigator. A 3.5mm-diameter imaginary cortical screw was then accepted to imitate insertion into the lateral arch of axis to find out the safe trajectory. And it was found that the projection of safe screw trajectory on horizontal plane is mainly restricted by the lateral boundary of median and inferior area of transverse foramen region but is not parallel to them, the projection of that on sagittal plane is restricted by the superior facet of axis. From bottom view, the safe trajectory passes through the intersection of medial-median division of posterior part and that of median-lateral division of anterior part of median-inferior area of foramen region, and from lateral view, it passes through the median area of posterior part and intersection of inferior edge of superior process and median area of anterior part of for amen region. These inferred points are found to be coincident between dry and fresh samples. To study the breakage force of lamina around or adjunct to head of screw and pull-out strength of screw (AO 3.5mm cortical screw) inserted into uni- or bi-lateral arch of axis to treat Hangman's fracture, Eight fresh cervical vertebra of health cadaver were classified into three groups(.Group A: unilateral arch screw fixation without a washer, four lateral arches in two axes ; Group B: bilateral arch screw fixation without affiliation of a washer, four axes ; Group C: unilateral arch screw fixation with the affiliation of a I washer, four lateral arches in two axes) randomly with the isthmus of axis fractured by an osteotom to establish a typical Hangman's fracture model after the entrance point and trajectory orientation of lateral arch screw of axis was determined with the help of navigator to reconstruction , After inserting appropriate 3.5-mm cortical screw into the lateral arch of axis with or without a washer according to group classification, the breakage force of lamina around the head of screw or pull-out strength of screw was tested. And the results showed fracture of lamina around the head of screw occurred before the lateral arch screw was pulled out in group A and B, with the breakage force of lamina in group A and Group B being 11.58±3.18kg (8.5 ~ 16. Okg ) and 25.75 ±2.41kg (22.5 ~ 28.3kg ), respectively, the pull-out force of screw ofthese being 29.65±3.89kg(25.0±33.6kg) and 30.90±3.99kg(26.8±38.0kg),respectivefy, no fracture of lamina was found in group C before and after the screw was pulled out, the pull-out strength of screw was 33.83±4.20kg (28.0 ~ 37.0kg ) .which was undifferentiated statistically compared with the pull-out force of Group A or B, and was superficially greater than the breakage force of lamina in Group B. It was then concluded that adding a washer underneath the head of screw provided a larger surface area of the cortex for the screw head and made breakage force limit of lamina increase obviously. Compared to bilateral arch screw fixation without affiliation of a washer, unilateral arch screw fixation with a washer was no less effective or might be more superior in the treatment of Hangman's fracture. When a Type HI lesion of Hangman's fracture is present, internal fixation with a posterior short-segment screw-rob system is indicated. But the fixation strength of the present prevalent screw-rob fixation system is limited because the screw and rod interface each other through point-to-plane connection or line-to-plane connection mechanism. An affiliating ^nnection component was advised to make the connection become plane-to-plane mechanism which aimed to increase the connection strength obviously.ConclusionsThe triangle-like protrusion locating at the anterior-inferior region of anterior structure of axis might be the real shrunk vertebral body of axis, the real axis pedicle be the narrow portion between the shrunk vertebral body and superior process, the structure occupying the lateral arch of axis is real pars inter articular is. which. had extended and moved forward from its original position, and could be divided into two portions by posterior nodule of transverse foramen: the posterior portion is the real isthmus of pars interarticulris , the anterior portion occupying the inner edge of transverse foramen is the lengthened structure between posterior nodule and superior process. Fracture locates at isthmus region is referred as a typical Hangman's fracture, and as an atypical Hangman's fracture if the fracture line involves inner edge of the transverse foramen. Posterior approach could reduce the dislocated facets directly and additional lateral arch screw fixation could maintain alignment of fracture. From bottom view, the safe screw trajectory of lateral arch passes through the intersection of medial-median division of posterior part and that of median-lateral division of anterior part of median-inferior area of foramen region, and from lateral view, it passes through the median area of posterior part and intersection of inferior edge of superior process and median area of anterior part of foramen region, but this is difficult to confirm intraoperatively.Adding a washer underneath the head of screw provided a larger surface area of the cortex for the screw head and made breakage force of lamina exceed the pullout strength of lateral arch screw, preventing the head of the screw from immersing into the bone. Unilateral arch screw fixation with a washer was no less effective or might be more superior to bilateral arch screw fixation without affiliation of bevel washer. This provides a choice to those sacrificers with the unilateral width of middle part of anterior portion of lateral arch of axis did not reach the safe limit 5.0mm.Adding an affiliating connection component to the connection components between the rod and screw to the existing posterior short segment rod-screw fixationsystem could make the point-to-plane or line-to-plane connection mechanism to plane-to-plane mechanism and make the connection strength increased remarkably.