The Feasibility Research of Posterior Screw Fixation on the Lateral Mass of Atlas

Objective:To study the feasibility of posterior screw fixation on the lateral mass of atlas in the aspects of anatomy and biomechanics.Methods:30 atlas specimens of native adults were used to measure the significant clinical data.6 fresh specimens of upper cervical were used to observe the relationship of C 2 nerve and the lateral mass of atlas.The pull out force of screw on the lateral mass of atlas was measured and compared with that of screw on the occiptal and axial pedicle screw respectively.The ROMs (Range of motion)of upper cervical spine were also measured.Results:The distances from the middle line to the midpoint of lateral mass,to the inner wall of transverse foramen and to the inner edge of pedicle were 17.6±1.2mm?23.0±1.7mm and 14.2±1.1mm respectively.The width and height of lateral mass in the middle of atlas were 11.6±1.4mm and 12.7±1.0mm respectively.The distance between the midpoint in the cross line of posterior arch and the lateral mass to the edge of inferior joint was 4.1±0.7mm.The width of posterior arch in the point of vertebra artery overpass was 4.7±1.0mm.The observation on the fresh specimens showed that the C 2 nerve went down lateral beneath the C 1/2 joint and it could moved easily.The pull out force of screw on the lateral mass of atlas was 1718.2±422.7N.It is significantly smaller than that of screw on the occiptal and there was no different with that of axial pedicle screw.The ROMs of C 0～1 and C 1～2 after screw fixation in the lateral mass were improved significantly in all directions than that of pre fixation,and almost have no difference with that of normal control except the ROM of rotation in C 1～2 motive segment( P 0.05).Conclusions:The width and height of the atlas lateral mass are larger than that of the axial pedicle and there is enough space to insert the screw in the atlas lateral mass over the C 2 nerve.The pull out force of screw on the lateral mass of atlas is the same as that of axial pedicle screw.It is quite possible to insert a screw in the atlas lateral mass.The direction of screw should be about 20° anterosuperior in the vertical plane and 15°inward in the horizontal plane.The suitable length of screw should be about the one with 20～23mm inside the lateral mass.