Fig. 6

Measurement of the distance between insertion point of C1 and C2 screws. a1 C1 PS or LMS combined with C2 PS on simulation diagram. b1 C1 PS or LMS combined with modified C2 isthmus screw on simulation diagram. a2 C1 PS or LMS combined with C2 PS on lateral X-ray. b2 C1 PS or LMS combined with modified C2 isthmus screw on lateral X-ray. L1 indicates the base of odontoid process; L2 indicates inferior margin of C2; L3 indicates the line connecting the midpoint of L1 and L2. The measurement lines of HD are parallel to L3. APD and DD of C1 PS and C2 PS are d1 and h1; APD and DD of C1 LMS and C2 PS are d2 and h2; APD and DD of C1 PS and modified C2 isthmus screw are d3 and h3; APD and DD of C1 LMS and modified C2 isthmus screw are d4 and h4; with results showed d3 > d1, d4 > d2, h3 > h1, h4 > h2, which offer more operation space for reduction. LMS, lateral mass screw; PS pedicle screw; APD anteroposterior distance; DD direct distance