Computed Tomography Study Of Posterior S1Alar-iliac Screw Placement In Adults

Objcetive:To investigate the feasibility and safety of the postenor S1alar-iliac(S1AI) screw placement in adult pelvises by computed tomography(CT).Method:After obtaining patient consent,60adults with normal pelvises,including30males and30females,with the age ranging from23to67were chosen(L5/S1disc herniation43cases,L5spondylolisthesis17cases).Based on three-dimensional (3D)CT reconstructions of these specimens,virtual S1AI screw channels were identified and measured. The anchoring point was defined as lmm inferior and lmm lateral to the L5/S1facet.Then virtual S1AI screw channel holding the greatest length and width of osseous channel was meassured by rotating the3D 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) were measured respectively.Result:There existed virtual S1AI screw channels going through the sacral ala as well as sacroiliac joint and into the ilium in each specimen.There were no significant difference (P>0.05) between the left and right sides of the parameters, counted all of them together.The SA,TA,ML,SL and IW was52.61±1.86°,40.45±1.43°,105.77±4.83mm,33.13±1.79mm and15.54±1.60mm respectively in males,and46.73±1.85°,40.21±1.31°,102.38±5.14mm,31.64±2.53mm and14.33±1.90mm respectively in females,which showed significant sex-related difference with respect to these indexes of SA,ML,SL and IW(P<0.05).However,no significant sex-related difference was found with respect to TA (P>0.05). Conclusion:It is feasible and reliable to insert the S1AI screw when performing sacropelvic fixation.The anchoring point is easy to reveal, but because of the large angle of screw placement, the operation is difficulty, need the accuracy of pedicle screw placement and higher proficiency of doctors.Preoperative CT imaging and3D reconstruction may help to choose correct anchoring point and screw channel individually.