Anatomic Study Of The Safety And Restrictions Using Posterior Approch Screw Fixation Of The Atlas

Objective:(1)To study the anatomical safety and restrictions of posterior screw fixation of the atlas.(2)To discuss the location of the screw entry point to the "lateral mass screw fixation via the posterior arch of the atlas technique".(3) To discuss a safe exposure for this surgery.Method:20 specimens of skull and atlanto-axial vertebra(C0-C2)of native adults were used to measure the relative anatomical data.The atlas,axis, vertebrae artery,C2 nerve et al.were dissected.The entry path to the lateral mass for screw fixation via the posterior arch of the atlas was confirmed by the intersecting line of two sections one vertical through the middle of the atlas pedicle and the other transverse 2mm above the inferio-posterior arch of the atlas.The projection of this line on the posterior arch is the entry point(A). Relevant measurement of lines and angles made with the atlas were taken with a caliper(0.02mm)and goniometry(0.1°).Results:Measurements of the atlas on the left and right sides had no significant variation,so the average values between left and right was used in the statistical analysis.The distance from the entry point A to the middle line of the atlas(L1)is 18.53±1.41mm,the distance from the entry point A to the mid point of the anterior surface to the lateral mass(entry path)(L4)is 29.23±1.67mm,the distance from the entry point A to the mid point of the posterior surface of the lateral mass(L3)is 10.56±1.02mm,the width(L2)and height(L8)of the lateral mass at its middle are 13.56±1.14mm and 12.04±1.32mm respectively.The width of the atlas pedicle(L9)is 10.05±1.35mm.The height(L5)of the posterior arch at the entry point A is 5.78±1.52mm.Among the 40 sides measured,there were 5 sides(12.5%)with less than 4mm,there were 3 sides(7.5%)with less than 3.5mm.The height of the atlas's pedicle(L10)was 4.46±0.95mm for the medial 1/3 section, 4.71±1.05mm for the middle point and 4.62±0.97mm for the lateral 1/3.The height of middle point is the longest,the data varied significantly.But among the 40 sides measured,there were still 10 sides(25%)with less than 4mm and 6 sides(15%)with less than 3.5mm.The longitudinal direction angle of the lateral mass to the sagittal plane(the median inclination angle of screw)is 11.2±1.7°,the longitudinal direction angle of C1 posterior arch to the horizontal plane(the superior inclination angle of screw)is 6.1±2.2°.The distance from the mid point on the posterior surface of the lateral mass to the inferior articular surface of the atlas(L6)is 4.81±0.84mm,there were 4 sides(10%)with less than 4mm,2 sides(5%)with less than 3.5mm.The distance from the mid point of the posterior surface of the lateral mass to the superior of the C2 nerve(L7)is 5.34±0.54mm.L7 was greater than L6,the data varied significantly.There were still 9 sides(22.5%)with L7 less than L6.Conclusion:1.The main restriction of the "lateral mass screw fixation via the posterior arch of the atlas technique "is the height of the atlas's pedicle (L10).The second restriction of the "lateral mass screw fixation via the posterior arch of the atlas technique "is the height(L5)of posterior arch at the entry point A.We should measure them pre-operation using C-T scan.If either of these heights is less than 4.0mm,it is not fitting to use the" lateral mass screw fixation via the posterior arch of the atlas technique".But here we can use a smaller diameter of screw instead if necessary.There were 20%of cases with this situation.2.The location of the entry point of the "lateral mass screw fixation via the posterior arch of the atlas technique" is 18-19 mm away from the middle line of the atlas and 2mm above the atlas inferio-posterior arch.The medial inclination angle for the screw is 10°,the superior inclination angle for the screw is 5°,the length of the screw is 28mm.3.The main restriction of the "lateral mass screw fixation technique" is the distance from the mid point on the posterior surface of the lateral mass to the inferior articular surface of the atlas(L6).If the height of L6 is less than 4.0mm, it is not fitting to use the "lateral mass screw fixation technique".There were 10%of the cases with this situation.The second restriction of the lateral mass screw fixation technique is the distance from the mid point of the posterior surface of the lateral mass to the superior of the C2 nerve(L7).If the height of L7 is less than 4.0mm,it is not fitting to use the "lateral mass screw fixation technique",because the screw can compress the C2 nerve.There were 22.5%of cases with this situation.4.A range of 20mm away from the middle line of atlas is safe for exposure. But we should strictly detach below the periosteum to avoid injuring the vertebrae artery above.