The Imaging And Clinical Study On The Method Of Atlas Pedicle Screw Fixation With The Inner Wall Of The Axis Vertebral Isthmus As An Anatomic Landmark

Objective:To evaluate the feasibility,the security of the method of atlas pedicle screw fixation by free hand with the inner wall of the axis vertebral isthmus as an anatomic landmark and the validity of atlas pedicle screw fixation.Methods:1,Study of imaging measurement: Three dimensional CT reconstruction were performed in 30 cases of normal atlantoaxial and 25 cases of atlantoaxial with anterior dislocation of atlas. Measurements were executed on the cross-sectional images of atlas and upper edge of axis. Measurement items of the normal group: The distance from the median line to the medial border of atlas pedicle (L1) /the lateral border of atlas pedicle (D1) /the medial border of the axis vertebral isthmus (L2) /the lateral border of the axis vertebral isthmus (D2) was measured respectively. Analog entry point A and B were established in the posterior arch of atlas , the horizontal distance from the medial border of the axis vertebral isthmus to entry point A/B was (L1-L2+1.75mm) /(D1-L2-1.75mm). The theoretical maximum depth X/Y of screw projection in entry point A/B at 0°insertion angle was measured respectively. Analog entry point M met the condition that the difference between the maximum medial angleαand the maximum lateral angleβof screw projection was the smallest. The entry point M was found by measuring and the horizontal distance N from point M to the medial border of the axis vertebral isthmus (N take integer values when practical measuring)/the angleα/the angleβwere recorded respectively. The theoretical maximum depth Y of screw projection in entry point M at 0°insertion angle was measured. Measurement items of the group with anterior dislocation of atlas : The distance from the median line to the medial border of atlas pedicle (L1) /the lateral border of atlas pedicle (D1) /the medial border of the axis vertebral isthmus (L2) /the lateral border of the axis vertebral isthmus (D2) was measured respectively. All data above of the two groups were compared.2,Study of clinical application: 36 cases with operative indication in clinic were all treated by posterior internal fixation of atlantoaxial pedicle screw from March 2004 to March 2009 .Of these, 14 cases had old odontoid fracture, 17 cases had fresh odontoid fracture, 5 cases had ligamentum transversum injury. All patients had varying degrees of atlantoaxial dislocation. All the atlas pedicle screws were inserted by free hand with the inner wall of the axis vertebral isthmus as an anatomic landmark. Intraoperative and postoperative complications associated with atlas pedicle screw fixation were observed. Screw position was also observed by three dimensional CT reconstruction after operation. All of cases were followed up every 3 months. Effects of internal fixation and bone graft fusion were assessed by imaging examination.Results:1,results of radiological measurement:1) The normal group: The distance from the median line to the medial border of atlas pedicle/the medial border of the axis vertebral isthmus was (14.36±0.47mm) /(11.78±0.74mm) respectively and there was significantly different between them (p<0.05); the horizontal distance from the medial border of the axis vertebral isthmus to which of atlas pedicle was (2.57±0.72mm); the distance from the median line to the lateral border of atlas pedicle/the lateral border of the axis vertebral isthmus was (22.26±0.76mm) /(20.84±0.67mm) respectively and there was significantly different between them (p<0.05);The width of atlas pedicle was(7.91±0.57mm). The horizontal distance from the medial border of axis pedicle to entry point A/B was (4.32±0.72mm) /(8.72±0.96mm). The theoretical maximum depth of screw projection at 0°insertion angle in entry point A/M/B was (30.04±1.71mm)/(28.08±1.63mm) /(25.00±2.07mm) respectively and there was significantly different between them (p<0.05). The horizontal distance from the medial border of the axis vertebral isthmus to entry point M was (5.98±0.70mm) . The angleα? the angleβwas(7.3±1.4°)(?6.9±1.4°)and there was no significantly different between them (p>0.05). 2)The group with anterior dislocation of atlas: The distance from the median line to the medial border of atlas pedicle/the lateral border of atlas pedicle?the medial border of the axis vertebral isthmus ?the lateral border of the axis vertebral isthmus was (14.22±0.51mm) /(21.93±0.80mm) ?(11.70±0.76mm) ?(20.61±0.74mm) respectively; The horizontal distance from the medial border of the axis vertebral isthmus to which of atlas pedicle was (2.43±0.58mm). There was no significantly different between all the results of the two groups (p>0.05).2,results of clinical application:72 of atlas pedicle screw were inserted and the inner wall of the axis vertebral isthmus was well exposed in operation. There were no complications such as injury to blood vessels and nerves. The average blood loss was 350ml,with the average operative time of 150min. Patients with anterior dislocation of atlas preoperative were found to get a good reduction by C‐arm. The results of three dimensional CT reconstruction after operation showed that there was sufficient distance from the pedicle screw to the spinal and transverse foramen. No internal fixation loosening or breakage case was found during follow-up and all of the atlantoaxial got good bone graft fusion after average 6~8 months postoperative.Conclusion: There is relatively constant relationship of anatomical location between the atlas pedicle and the inner wall of the axis vertebral isthmus. The method of atlas pedicle screw fixation with the inner wall of the axis vertebral isthmus as an anatomic landmark is feasible,safe and effective. But this method must be carried out under the guidance of individual imaging measurement .