Since Boucher et al. initially reported use of the posterior transpedicular screw for lumbosacral fusion fixation in 1959 , various pedicle screw fixation systems have been used in spinal surgical procedures. They have provided superior postoperative spinal stability and promoted advances in spine surgery . Various studies have researched detailed PPS procedures, including projection parameters, the trajectory angle and depth of the screw [11, 12], and the regularity of pedicle screw insertion , each of which is important if PPS is to be performed with high accuracy. Limited studies, however, have reported on the details of procedures using the APS. As an effective means of pedicle screw fixation, APS not only provides superior postoperative spinal stability for ALIF, but allows a shorter operating time, less blood loss, and minimal posterior muscle damage [4, 5].
We conducted the present study to determine the anatomic and radiographic parameters of L5 and S1, including projection, the screw’s trajectory angle, and the depth of the APS. We found limited information in the literature on the location of the anterior projection, which is important for APS performance. Poor placement of the projection may cause the guide needle to be misplaced and result in complications.
The anterior projection in our study was determined based on the method of Ebraheim . The parameters addressed to determine its location included the distance of the APS to the upper endplate, lower endplate, and midline. The screw trajectory angle was also determined based on the method of Ebraheim , and the transverse screw and sagittal screw angles were measured. The screw trajectory angles for APS and PPS should be the same in the same patient. Ebraheim  reported that the transverse angle of L5 was 40.6 ± 2.6 in men and 39.6 ± 3.2 in women, and the sagittal angle of L5 was 2.7 ± 1.1 in men and 2.6 ± 0.9 in women. To determine the screw trajectory depth, the length of the bone screw passageway was measured. Ebraheim  reported that the pedicle length of L5 was 48.3 ± 2.3 mm in men and 48.3 ± 2.4 mm in women. In the present study, anatomic measurements revealed that the bone screw passageway length was 48.6 ± 3.5 mm for L5 and 48.0 ± 3.5 mm for S1.
The BSPL is defined by the length of the screw. Because the vertebra is formed as an irregular cylinder, with the front of vertebral body exhibiting the most anterior border in the lateral view, the needle may perforate the anterolateral cortex, although the lateral view suggests that the needle is still within the vertebra . Therefore, it is important to obtain the ideal needle depth/vertebral width ratio on the lateral view. Weinstein et al. considered the suitable ratio to be 50%–80% . Du et al. suggested that the ratio should be 85%–90% in lumbar vertebrae , and Acikbas and Tuncer reported that the suitable ratio was 60% ± 9% in lumbar vertebrae . Wang et al. found that the ratio was not the same on different lateral projection angle views even if the real length of the needle in the vertebra remained unchanged . They suggested that the suitable ratio of the needle depth/vertebral width ratio on a standard lateral view varied from 71.53% ± 5.72% to 93.28% ± 3.72% and that the ratio for L5 was 88.20% ± 6.72%. More work is obviously needed to establish a suitable ratio for the APS needle depth.
We also evaluated the regularity of APS insertion in this study, which was guided with high accuracy. A previous study  reported that the regularity of PPS insertion was apparent when, progressively, the needle reached the posterior projection in the lateral view, was on the outer edge of the pedicle on the anteroposterior view, reached the middle of the pedicle (lateral view), was on the middle of the pedicle (anteroposterior view), reached the posterior vertebral edge (lateral view), and was on the inner edge of the pedicle (anteroposterior view). Wang et al. reported that the posterior projection of PPS was at the 9 o’clock to 11 o’clock position of the left pedicle and at the 1 o’clock to 3 o’clock position of the right pedicle . This regularity also could be applied to the APS when the needle reaches the posterior projection.
Compared with PPS, APS had higher risk to damage lumbar vessels. There is great variability of vascular anatomy in front of L5–S1 disc space. The left common iliac vein is at greater risk than the common iliac arteries . Ebraheim et al. found a triangular safety zone averaging 60 mm in width and 40 mm in height between the left common iliac vein and the right common iliac artery in 40 human cadavers . In our study, we found that the distance from the abdominal aortic bifurcation to the L5 lower edge was 40.50 ± 9.40 mm, the distance from the common iliac vein confluence to the L5 lower edge was 27.80 ± 8.60 mm, and the horizontal distance from the inner edge of the common iliac vein to the L5 lower edge was 37.50 ± 1.30 mm, and the safe operating area was 2058.20 ± 84.30 mm2.
Lumbar fusion has been shown to accelerate adjacent segments degeneration (ASD). It was reported that ALIF may have an advantage over PLIF in preventing the developing ASD . Compared with PLIF, ALIF reduced the damage to the integrity of the posterior complex, which may be helpful in preventing accelerated adjacent segment degeneration after spinal fusion . Although percutaneous PPS was applied during ALIF procedure in these studies , we could believe that APS had comparable results to PPS with the potential benefit of preventing accelerated ASD, and more studies should be performed on this topic.