Anatomy study of the pedicle screw placement anterolaterally on S2 vertebra

English Keywords:S2  Pedicle  Internal fixation  Anatomy

English Abstract:

  【Abstract】 Objective：To investigate the S2 pedicle anatomy and determine the anatomic landmark and technique of S2 pedicle screw anterolateral placement associated with S1 foramen and lateral sacral crest.Method：15 dry sacrum specimens from human being were used and the sacrum segments between S1 and S2，S2 and S3 vestige were separated with the line parallel to plane of sacrum bottom.The profile of S2 pedicle was observed.S2 pedicle was cut from the first posterior foramen lower edge parallel to the bottom of sacrum.In the anterior edge of S2 pedicle（pelvic surface of sacrum），the intersection between cutting plane and lateral marginal line of anterior foramen was defined as point A，and the ear-shaped front edge（the sacroiliac joints） of the intersection was defined as point B.The lateral sacral crest point was defined as X，and the length of XA and XB representing the minimum and maximum length of the screw was measured respectively.The angles between XA and sagittal plane，ＸＢ　ａｎｄ　ｓａｇｉｔｔａｌ　ｐｌａｎｅ were defined as α and β respectively，which represented the minimum and maximum inclined angle in the horizontal plane respectively.In this plane，the middle sacral crest was defined as point Y，which was the same ａｓ X，the YA，YB，γ and θ angles wｅｒｅ measured.Pedicle screws were placed anterolaterally in 5 dry specimens，with the sagittal planｅ parallel to the sacral bottom and with the angle between α and β.CT scan was used to detect the site of screws.In 10 adult cadavers，the relationship between pre-sacral blood vessel and nerve and pre-sacral foramen，sacroiliac joint and pre-sacral sympathetic trunk，the iliac artery and vein were investigated.According to the above-mentioned results，S2 pedicle screws were placed in X and Y point.The tip of the screw and its relationship with pre-sacral neurovascular were also investigated.Result：Space was available for S2 screw placement，and XA was 26.0±1.7mm，XB was 33.1±1.1mm，α angle（22.1±2.0）°，β angle（36.0±3.7）°；YA was 30.１±0.8mm，YB was 35.5±１．０mm，γ angle was 36.7°±1.５°，θ angle was 43.８°±1.3°。In front of the sacroiliac joint，from the lateral border of the anterior sacral foramen，there sited the internal iliac vein，internal iliac artery，external iliac vein，external iliac artery respectively.In this region，S1 nerve root passed through.Pre-sacral sympathetic trunk located ｉｎ　the inside edge of pre-sacral foramen，and the tip of screws located behind the iliac artery and vein，and also laterally in anterior sacral foramen.Conclusion：S2 pedicle screw placement anterolaterally is feasible and safe，the front cortical bone should not be penetrated to avoid the injury to sacral vｅｓｓｅｌ and nerve root.X point can be used as entrance point.