Applied anatomy of the pedicle screw placement on 2nd sacrum vertebra

English Keywords:Ｓacrum 2  Ｐedicle  Ｓcrew  Ａnatomic

English Abstract:

  【Abstract】　Objective：To explore the relationship between the 2nd sacrum pedicle and the first posterior sacral foramen and to establish anatomic landmark and technique of 2nd sacrum pedicle screw placement based on the first posterior sacral 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 paralleling to plane of sacrum bottom.Sacrum 2 pedicle shape and there surrounding structure were observed.Sacrum 2 pedicle was cut from the midline paralleling to the bottom of sacrum.The relations between pedicle and first posterior sacral foramen，lateral sacral crest were investigated.In the anterior edge of S2 vertebral pedicle（pelvic surface of sacrum），the intersection of cutting plane with medial marginal line of anterior foramen was defined as point a，the sagittal plane′s center was point b，and the lateral sacral crest point was s，the length of sa and sb were measured，the former represented the screw smallest length and the latter represented the maximum length.The angle between sa and sb at sagittal plane were defined as A and B respectively，A and B representing the smallest and biggest inclined angle in the horizontal plane respectively.The perpendicular line from neurocanal most lateral（point P） to the sa line and sb line were defined as h1 and h2 respectively，the length of Ph1 and Ph2 were measured，the diameter of screw smaller than 2 times of Ph2 lengths was regarded as safe.The line from the first posterior foramen lower end（W） to the second posterior foramen upper end（Y） was obtained.The intersection of equal division of S2 pedicle plane and the WY line was O，WO was measured.10 cadavers′ sacral anterior structure were observed，pedicle screws placement in 2nd sacrum was performed based on data acquired.Result：The S2 pedicle was fan-shaped，going from posterolaterally to anteromedially.The central axis plane of pedicle was rightly through the lowest point of the S1 posterior foramen.The pedicle of S2 sited at the point of intersection between lateral sacral crest and the line through lowest point of the bilateral S1 posterior foramen.The length and angle of screw was 27.53-35.46mm and 30.23°-45.73° respectively.The front of the sacrum in which the area between sympathetic stem and sacrum middle artery was a safe zone，where screws thread had no neurovascular risk.Conclusion：The anatomic relations between S2 pedicle and S1 posterior foramen is constant，which can be used as a landmark for anchoring S2 pedicle.The thread of screw is in safe zone.