4.1 Classic SIS fixation
The existing method of SIS fixation is placed percutaneously under fluoroscopy, the insertion point is located at the posterior- lateral side of the ilium, the insertion direction is inward, forward and upward, and enters the lateral mass and pedicle of the sacrum vertically through the sacroiliac joint surface, and into the vertebrae body. However, the L5 or S1 nerve root may be damaged if the screw deviates too much upward or downward; the iliac vessel and organ may be damaged if the screw deviates too much foreward; the sacral nerve in the spinal canal may also be damaged if the screw deviates too much backward. The screw placement can not be accurately determined by single AP or lateral view [9,10]. Therefore, repeated intraoperative fluoroscopic images must be used by examining the inlet and outlet views, especially the standard lateral view with overlap of the anatomic landmark of the iliac cortical density (ICD) for safe insertion of a sacroiliac screw. However, the presence of contrast, intestinal gas, and increased soft tissue density from obesity can cause difficulty in obtaining and interpreting appropriate intraoperative fluoroscopic images. This may lead to malpositioned implants and increased risk of neurovascular injuries. As a result, existing sacroiliac fixation technique has a learning curve and some incidence of screw penetration and nerve injury [4,5,7,8].
4.2 TFSIS fixation
The insertion point of TFSIS is located at the center of the facet, which is between the L5 and S1 nerve roots. Because it is placed either under direct vision after a spinal posterior incision or through a minimally invasive method with a aiming device, the position of the insertion point can be controlled; the insertion direction is outward, where is a safe area. Therefore, injury of nerve, iliac vessel and viscera organ can be avoided, i.e. this method is safe.
The medial wall of the screw channel is arcuate line, the lateral wall of the screw channel is the posterolateral wall of the spinal canal of S1 and the outer layer of the ilium. The inferior wall of the screw channel is the line between the sacral foramen of S1 and the ischial notch. The upper wall of the screw channel is the sacral wing slope, the surface of the sacroiliac joint and the bottom of the inner wall of the great pelvis. With naked eyes, the range of screw placement is large.
D2 was (7.75 ± 0.89) smaller than D1(11.91±1.47) mm; A and B were (11.91 ± 1.47) ° and (8.57±1.63) °, respectively. Therefore, it is not allowed to use larger diameter screws, we think φ 5.0 to 6.0 mm screws are suitable.
L averaged 10.84 cm. However, if the insertion direction deviates inward and forward, the screw channel is longer; if the insertion direction deviates outward and laterally, the screw channel is shorter. Considering that too long screws may penetrate the ilium or the acetabulum, 90 mm long screw is recommended.
The key to the safe placement of TFSIS is the insertion point position and insertion direction. The insertion point is close to the center of the superior facet of S1 (M1=1.14 mm). E averaged 53.96 °, F averaged 47.47 °. According to the insertion point and insertion direction of each patient, within screw channel accounted for 100% for φ 5 mm and φ 6 mm screws, which indicated that this method was feasible and safe.
The iliac side of TFSIS fixation is almost all cortical bone, and the screw channel in the iliac side is longer, the anti-pulling force of screw is stronger than that of the classic SIS. Therefore, TFSIS is suitable for patients with severe osteoporosis. The iliac side of classic SIS fixation is about 50% cancellous bone and 50% cortical bone, and the stability of screw in patients with severe osteoporosis is poor [11,12]. Therefore, the classic placement of SIS is not indicated in patients with severe osteoporosis and local fractures or infection at the insertion position of ilium. However, these situations are suitable for TFSIS, i.e. TFSIS can be used as an alternative to the classic placement of SIS.
In a word, larger diameter and longer screws should not be used; there were individual differences in the anatomical parameters, so the preoperative 3D CT reconstruction of pelvic should be performed to measure the relevant parameters, individualized screw placement is needed. On the premise of being within the channel, the screw should be placed as close as possible to the medial-anterior of the central axis, so as to maximize the stability and safety effect.
The disadvantages of TFSIS fixation: the diameter of the screw channel in this method is small, only one screw with φ 5.0-6.0 mm can be used. In classic SIS fixation, the width and height of the screw pathway were (27.7 ± 1.9) mm and (20.2±2.3) mm, respectively [4,6], and 1-2 φ 6 mm screws can be placed. Therefore, its anti-bending strength of TFSIS is not as good as that of the classic method.