Anatomic study of the posterior sacral 1-2 translateral mass screw fixation

English Keywords:Sacrum  Lateral mass  Anatomy  Screw fixation

English Abstract:

  【Abstract】 Objectives: To investigate the anatomic characteristics and feasibility of the posterior sacral 1-2 translateral mass screw(STMS) fixation.　Methods: Three-dimensional CT reconstructions of sacrums from 27 cases(14 males and 13 females) were obtained. The Advantage Workstation(AW) 4.2 software was used to process the anatomic data of STMS. The screw entry point located at 5mm above the superior edge of the second sacral foramen. The projection of the screw was determined as from the entry point to the top corner of the sacral ala. The parameters used for statistical analysis included the extraversion angle(the angle between the line from the entry point to the top corner of the sacral ala and the median line crossing sacrum at the horizontal plane), the inclined angle(the angle between the line from the entry point to the top corner of the sacral ala and the line parallel to the upper endplate of S1 vertebra at the sagittal plane) and the length(the distance from the entry point to the top corner of the sacral ala). The channel of STMS was observed on three-dimensional reconstruction. STMS placement was performed on 15 cadavers′ sacrums based on the above-mentioned parameters, then the relationships between STMS and the spinal canal, facet, the anterior and posterior sacral foramen were investigated. Results: There was significant difference as for the extraversion angle of STMS between males and females(P＜0.05), the average extraversion angle of STMS was 30.39°±5.01° for males, 31.14°±5.25° and 29.64°±4.86° for left and right side, while 35.81°±4.45° for females, 36.46°±4.94° and 35.15°±4.00° for left and right side. No sex-related or side-related significant difference was noted as for extraversion angle(P＞0.05) as well as inclined angle and the length of STMS. The average inclined angle of STMS was 26.71°±16.50° for males and 19.48°±9.09° for females, and 22.22°±12.89° for the right and 23.67°±14.81° for the left. The average length of STMS was 54.48±3.01mm for males and 53.02±2.89mm for females, and 53.56±3.28mm for right and 53.99±2.78mm for left. The channel of STMS located in the lateral mass on three-dimensional reconstruction completely with no penetration into the spinal canal, facet, and anterior or posterior sacral foramen.　Conclusions:　By obtaining the parameters of screw fixation for STMS in three-dimensional reconstruction of sacrum before having operation,　the technique of STMS can be performed safely during the fixation of sacrum,　especially when the S1 screw fixation can not be installed effectively.