| Objective: Since Matta put forward in X-ray fluoroscopy applied pelvic inlet, outlet,AP and lateral view implant the sacroiliac screws, It has been widely applied in fixation about fracture dislocation of posterior pelvic ring. In recent years, many scholars have made extensive research on how to implant SI-screw securely, (for instance choice of the entry points, SI-screw implanted range, quantity, length and direction, various projection and path design of computer-assisted system, etc.) makes screw embedded accuracy has been greatly improved, but complications such as SI-screw malposition and nerve injury and so on are still exist. An ideal screw position is considered entirely within the cortical margins of the sacrum and parallel to the respective sacral end plate and the S1 neuroforamina. Unfortunately, the complex shape of the SI joint is extremely different from simple geometric structures, and screws often penetrate the sacram, damage structures like iliac blood vessels, lumbosacral cord and cauda equian, etc. We base on the study about sacrum anatomy, calculating the triangle area above the sacral ala slope which surrounded by S1 vertebral body and sacral ala slope, in lateral view. Screw implant this region means penetrates the sacrum. It is possible to damage vessels and nerves, and we will be defined this region as non-secure area. To provide anatomic basis for iliosacral screw fixation at the level of the first sacral vertebra (S1), the present study of non-secure area was carried out.Methods: 15 sacrum corrosion specimens of native adults were studied, including males 8 and 7 females, each of them eliminate fractures, tumor, deformity. Each specimen is selected left flank with observations. Soft tissue around pelvic remove carefully, linear measurement adopts vernier caliper which precision is 0.02mm, straightedge and set square which precision is 1mm,protractor which precision for 1°and compasses. The sacrum is fixed on experimental platform vertically. In lateral view, when superior border of the first sacral body in parallel with horizontal plane, bilateral sacral ala completely overlap, it can display the triangle area above the sacral ala slope which surrounded by S1 vertebral body and sacral ala slope, namely non-secure area. Use vernier caliper and straightedge measurement midline diameter length of the S1 vertebral called D, compasses measurement the length of sacral slope in projection of the vertebral of the S1named L , the height from anterior body of the S1 to the sacral slope in projection of the vertebral of the S1is H. Then, use vernier caliper to measurement it. We calculated area of the"non-secure area"by formula of triangle area. The angleαformed at the intersection of D and L, and computed the Sin∠α,then we consult table of trigonometric function and work out degree of it. Statistical analysis was performed with SPSS17.0 software. Results expressed as mean and standard deviation. Comparison between the men and women adopts two independent sample t-tests. A confidence interval of 95% was assumed and P < 0. 05 the significant difference was considered.Results: The sacrum is fixed on experimental platform vertically. In lateral view, when bilateral sacral alae overlap completely, it can display the non-secure area above the sacral ala slope. Use measuring tool measurement midline diameter length of the S1 vertebral, the length of D for male and female were 27.68±0.90mm and 26.30±2.38 mm;the length of sacral slope in projection of the vertebral of the S1(L) for male and female were 23.62±2.62 mm, 22.05±1.25 mm , the height from anterior body of the S1 to the sacral slope in projection of the vertebral of the S1(H) for male and female were 17.7±0.98 mm, 16.4±1.03mm. Then,"non-secure area"is calculated by formula of triangle area. The area of it for male and female were 209.38±28.71 mm~2, 80.79±15.21 mm~2, The Sin∠αis computed , we consult table and work out degree of it for male and female were 39.7°±2.3°, 39.2°±7.4°,respectively. According to the statistical analysis of male and female about the area ,there is no significant difference is found between both sexes in non-secure area (P>0.05). The area of it is relatively constant , but the angleαexist changes. Below the"non-secure area"inserted the sacroiliac screw which makes the surgery more precise and safer, and can not into the pelvic.Conclusion: In clinical, freehand SI-screw insertion under fluoroscopic control in the standard anterior-posterior (a.p.), inlet, outlet projections actually represents the state of the art. But because of traditional method in implant screw can not accurately response osseous structure, it makes safety reduce. In lateral view, instead of sacral ala slope projection, there is iliac cortical density. Many surgeons estimate sacral ala slope through the high-density line, consequently, there are some errors. Computer-assisted navigation technologies which can accurate positioning still in the development stage, the cost is high and hard to widely use at present. Therefore, to understand the sacrum anatomy is especially important. At anterosuperior of normal sacral ala there is an inclined plane, namely the sacrum ala slope. In this region, in front of sacral ala is L5 nerve root and iliac blood vessels. The sacrum ala slope cortical is inferior borders of non-secure area. In our study shows that non-secure area which exists anterosuperior of S1 vertebral body can not implant SI-screw, and the area of it is relatively constant. Through the understanding of this region, further compute first vertebral cross-sectional area, concluded that " non-secure area " accounted for S1 vertebral proportion. Comparison with iliac cortical density can improve security of percutaneous sacroiliac joint screws. Provide surgeons with simple, accurate judgment method, without additional technical requirements, which can effectively prevent screws penetrate the sacrum because of upward or forward.
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