The Maximal Length Of The Channel Of Anterolateral S1 Sacrum Alae Screws In Relation To The Bone Density: A Computed Tomography Study

Objective:To investigate the relation between the channel of anterolateral S1 sacrum alae screw and the bone density by computed tomography(CT). statistical analysis various parameters of the channel of the anterolateral S1 sacrum alae screw of were measured, and statistical analysis were performed, which provided theoretical support for the maximal length of the channel of anterolateral S1 sacrum alae screw.Method:CT images of the sacrum were collected from 40 patients(lumbar disc herniation), including 20 females and 20 males, with the age ranging from 40 to 65 (The average age was 52.75)years old, who did not have any congenital anomaly or pathological deformity around sacrum.A three-dimensional (3D) image of the sacrum was reconstructed on the computer using the program MMWP. Based on three-dimensional (3D) CT reconstructions, virtual channels of anterolateral S1 sacrum alae screw were identified and measured. S1 screw entry point was set as the intersection of lines where extension of the L5/S1 facet joint 1mm inferior margin and lmm lateral margin cross over. The maximal length of the channel (channel A) of anterolateral S1 sacrum alae screw was measured by rotating the 3D sacrum, which was in the study plane (plane O). And caudal angulation on the sagittal plane was sagittal angle(SA).For example, in the right side of the sacrum, S1 screw entry point was set as the center,then we got channel B after 10°clockwise rotation,channel C after another 10°. Lateral angulation on the transverse plane was transverse angle (TA),and we got TAA, TAB, TAC. So the length was respectively LA, LB,LC, and the bone average CT values was CTA, CTB, CTC(Standardized the average CT value, we got SCTA, SCTB, SCTC). Using SPSS 19.0 software for statistical analysis, we made comparisons with the parameters between females and males, left and right side.Result:There were no significant difference (P>0.05) between the left sides and right sides of the parameters, counted the two sides together.The parameters we got showed significant sex-related difference with respect to these indexes of TAA, TAB and TAC (P<0.05).While no significant sex-related difference was found with respect to these indexes of LA, LB, LC, SA, CTA,CTB and CTC (P>0.05). However, Standardized the average CT value, we got SCTA, SCTB and SCTC. We found SCTA had significant difference with SCTB and SCTC respectively (P<0.05), and SCTB had no significant difference with SCTC (P>0.05).Conclusion:The channel of anterolateral S1 sacrum alae screw not only had a great advantage of length over other channels, but also a great advantage of bone average CT values over other channels, which provided a strong theoretical support for the placement of anterolateral S1 sacrum alae screw. Preoperative three-dimensional (3D) reconstructions and virtual channels of anterolateral S1 sacrum alae screw can be a routine preoperative preparation work, which helped to choose accurate screw channel.