Axial CT Measurement And Clinical Practice Of Atlantoaxial Pedicle

Objective:The atlas is special with no vertebral body and no spinous process, which is constituted by anterior arch, posterior arch, two lateral masses and transverse process. No structure of pedicle of vertebral arch can be found in classic anatomy, but massive anatomical observations reveal that the connecting part between the posterior arch of atlas and lateral mass resemble pedicle of vertebral arch of other spines in structure and mechanics. Anatomical and imaging measurements demonstrate that atlas lateral mass screw can be fixed through posterior arch and pedicle of vertebral arch, which is called atlas pedicle screw. Measurement data present that the data in the anatomy and image are consistent, therefore, imaging measurement can guide the placement of pedicle screw in surgery. One of the key points in successful placement of pedicle screw lies in taking control of the entry point and the angle of pedicle screw. Through axial CT measurement of atlas, the study is to confirm the safe placement area and corresponding entry angle of atlas pedicle screw, and to guide the clinical operations.Methods:With random selection of spiral CT thin-layer scanning and three-dimensional reconstruction of atlas of38patients, the space available for the screw (SAS) is defined as the distance between two paralleled lines, one of which is the tangent line of the inner edge of transverse foramen, the other of which is the tangent line of outer edge of vertebral canal. The SAS should be at least5mm with successful placement of the atlas pedicle screw, the diameter of which is3.5mm. The minimum distance between the outer edge of vertebral canal and the inner edge of transverse foramen is the width of pedicle of vertebral arch, and the intersection of the perpendicular bisector of the width and posterior arch of atlas is the lateral boundary of safe placement area. If the intersection is beyond that intersection of tangent line and the posterior arch, the latter intersection was the lateral boundary. The tangent line, in parallel with atlas midline, is of the inner edge of transverse foramen. The intersection of central axis line of screw channel and posterior arch of atlas is the inner boundary when the space available for the screw is5mm. If the intersection is in the inside of that intersection of tangent line and the posterior arch, the latter intersection is the inner boundary. The tangent line, in parallel with atlas midline, is of the outer edge of vertebral canal. Measure the distance between the posterior tubercle of atlas and the inner and the lateral boundary of the safe placement area of screw, corresponding entry angle and depth of pedicle screw, and the width of pedicle of vertebral arch. Guided by preoperative spiral CT measurement,7patients were performed pedicle screw internal fixation in atlas with the application of dredging pipeline method.Results:CT measurement:the width of pedicle of vertebral arch was (9.15±2.57) mm, which was safe to fix the3.5mm screw. The safe placement area of screw:the entry point was (18.35±2.86) mm～(25.26±1.76) mm away from posterior tubercle of atlas, the entry angle of screw ranges from leaning outwards (9.09±7.45)°to leaning inwards (18.72±17.42)°. The length of screw channel was (26.20±2.69) mm～(27.04±2.51) mm. The width of the safe placement area of screw was6.91±7.66) mm. The transverse angle was (27.81±10.32)°. But there were individual differences. The duration of operation of the7patients was90～120min, and intraoperative hemorrhage was60～300ml, with no vertebral artery injury, no nerve root injury. or no spinal cord injury. After surgery the7patients were examined by X-film and spiral CT of cervical spine, which revealed that the14pedicle screw positions were ideal within the safe placement area of screw measured before surgery, with no break-through of bone cortex of pedicle of vertebral arch. The7patients were followed up from6to12months, which showed no loosening and breakage of internal fixation and bone fusion were observed in all7patients.Conclusion:There exists safe placement area in fixing pedicle screw. The fixation of pedicle screw in the safe placement area can lean inwards or lean outwards according to the entry point of screw. It is critical for successful surgery to have exhaustive preoperative imaging measurement, to confirm safe placement area of screw, and to fix pedicle screw individually.