Applied Anatomic Study Of The Parameters Of Screw Fixation For The Sacroiliac Joint

Objective: To provide anthropometric measurement of the pedicle of the S₁ and S₂, and the anatomy around the sacroiliac joint. To measure the diameter of screw path.And to determine the entry point, orientations and the length of the screw.To provide anatomic basis for iliosacral screw fixation at the level of the pedicle of the first sacral vertebra (S₁).To provide anatomic basis for iliosacral screw fixation at the level of the pedicle of the second sacral vertebra (S₂).Methods:Slide gaud of electron(precision 0.01mm) and the universal bevel protractors were used in this laboratory. 13 pelvis specimens of native adults were studied, for a total of 26 sacroiliac joint (provided by Dalian Medical University).The shape and occurrence rate of posterior gluteal line was counted. Anthropometric measurement were performed on 13 cadaveric pelvises to determine the width and height of the pedicle of S₁ and S₂; the distance from entry point to contrary anterior cortex of S₁ vertebral body; the distance from entry point to the center of S₂ vertebral body; the distance from entry point to the center of S₁ and S₂ pedicle; the relation between entry point with greater sciatic notch and posterior inferior iliac spine, and the orientations of hollow screw; the angle of screw to the coronal and horizontal plane .The same measurement were performed by CT on 13 cadaveric pelvises.StatisticsThe data was adopted by SPSS₁1. 0 statistical analysis system and expressed by mean square deviation ( x±s). Pars ossea of pelvis measurement and CT measurement were analyzed by paired T-test.αnumerical value was 0. 05. If P<0. 05, the difference wound get distinctiveness. Results: There is no statistic difference between pars ossea of pelvis measurement and CT measurement (by paired T-test). So the two measure result can be analyzed together.The measure result of pedicle of S₁:14 in 26 cadaveric pelvic could be found the posterior gluteal line which could be used as the landmark of the entry point of S₁.The width of pedicle of S₁: 27.71±1.55mm by anthropometric measurement(by CT measurement was 27.71±1.53mm ). And the height of S₁: 21.22±1.72 mm by anthropometric measurement (by CT measurement was 21.22±1.58mm). The distance from entry point to contrary anterior cortex of S₁ vertebral body was 85.11士1.91mm by anthropometric measurement(by CT measurement was 85.16士1.69mm ). And the distance from entry point to the center of S₁ pedicle was43.44士2.26mm by anthropometric measurement(by CT measurement was 43.46士1.58mm ). The entry point of S₁ was at 27.38士1.61mm(CT:27.44士1.3mm )affront of posterior inferior iliac spine and 42.67士1.68mm( CT:42.7士1.61mm) above of greater sciatic notch. The direction of S₁ screw was 7.7°±1.2 (by CT measurement was 7.7°±0.48 )to the coronal plane and 7.1°±0.57, (by CT measurement was 7.1°±0.31 ) to the horizontal plane.The measure result of pedicle of S₂The width of pedicle of S₂: 13.63士1.26mm by anthropometric measurement, and 13.6士1.16mm by CT measurement. The height of S₂: 18.71士2.21mm by anthropometric measurement and 18.71士2.34mm by CT measurement. The distance from entry point to the center of vertebral body was 64.34士2.24mm(CT: 64.0士2.25mm). The distance from entry point to the center of S₂ pedicle was 53. 11士1.32mm by anthropometric measurement and 53.11士1.24mm by CT measurement. The entry point of S₂ was at 26.48士2.46mm(CT: 26.46士2.15mm) affront of posterior inferior iliac spine and15.73士1.16mm(CT: 15.73士1.18mm )above of greater sciatic notch. The direction of S₂ screw was 65.1°±1.7 (CT:63.4°±0.76)ConclusionThe entry point of sacroiliac screws( the length is 75 mm) at the level of the pedicle of S₁ was at 27mm affront of posterior inferior iliac spine and 42mm above of greater sciatic notch. The direction of S₁ screw was 5-10°to the coronal plane and 5-10°to the horizontal plane on the outer table of the posterolateral ilium. The entry point of S₂ was at 26mm affront of posterior inferior iliac spine and 15mm above of greater sciatic notch. The angle between the direction of S₂ screw and the outer table of ilium was 65°. The result shows that the diameter of pedicle of S₂ was less than that of S₁, so the sacroiliac screws through the pedicle of S₂ should be thin and short.