Computed tomography study of posterior S2 alar-iliac screw placement in adults

English Keywords:The second sacrum  Fixation  Sacral alar-iliac screw  Three-dimensional CT reconstruction  CT measurement

English Abstract:

  【Abstract】 Objective：To investigate the feasibility and safety of the posterior S2 alar-iliac（S2AI） screw placement in adult pelvises by computed tomography（CT）. Method：50 adults with normal pelvises，including 25 males and 25 females，with the age ranging from 20 to 60 were chosen.Based on three-dimensional（3D） CT reconstructions of these specimens，virtual S2AI screw channels were identified and measured.The anchoring point was defined as 1mm inferior and 1mm lateral to the S1 dorsal foramen.Then virtual S2AI screw channel holding the greatest length and width of osseous channel was meassured by rotating the 3D pelvis.The parameters of the determined channels including caudal angulation on the sagittal plane（sagittal angle，SA），lateral angulation on the transverse plane（transverse angle，TA），the maximal length of the channel（maximal length，ML），the intrasacral length of the channel（sacral length，SL），the narrowest width of iliac medullar cavity（iliac width，IW） and the vertical distance from the insertion point to skin（skin distance，SD） were measured respectively.Result：There existed virtual S2AI screw channels going through the sacral ala as well as sacroiliac joint and into the ilium in each specimen.The SA，TA，ML，SL，IW and SD was 29.56°±8.38°，36.11°±3.38°，120.94±7.89mm，26.56±4.08mm，17.00±3.16mm and 44.01±12.10mm respectively in males，and 35.11°±7.03°，36.69°±3.21°，115.21±8.80mm，27.85±5.56mm，14.85±2.51mm and 47.97±13.39mm respectively in females，which showed significant sex-related difference with respect to these indexes of SA，ML and IW（P＜0.001）.However，no significant sex-related difference was found with respect to TA，SL and SD（P＞0.05）.Conclusion：It is feasible and reliable to insert the S2AI screw when performing sacropelvic fixation.Preoperative CT imaging and 3D reconstruction may help to choose correct anchoring point and screw channel individually.