Percutaneous pedicle screw (PPS) placement is gradually being adopted for lumbar and thoracic spine surgeries, requiring minimally invasive fixation [123]. Most studies are designed to emphasize the favorable advantages of minimally invasive surgery (MIS), such as lowered invasiveness for posterior spinal muscles, and shorter hospital stay and clinical outcomes similar to those of the conventional open procedure. In contrast, PPS placement is seemingly more technically demanding because of a lower number of landmarks that can be palpitated during surgery, which can lead to potentially significant radiation exposure and inaccurate screw placement, causing potential neurological deficits. Reports have indicated 4.7%–23% inaccuracy in PPS placement, and many of these inaccurate placements resulted in pathological consequences after MIS [4567]. Some authors reported the accuracy of percutaneous pedicle screws (PS) placement using computer-assisted navigation systems and complex guidance devices, resulting in clinical outcomes similar to those of conventional open procedures [78910]. However, these systems and devices have not achieved widespread use because of the associated increased costs, longer set-up times, and technical difficulty.

Only a few studies have described the methods for accurate insertion percutaneous lumbar PSs using fluoroscopy, similar to conventional open procedures [1112]. Furthermore, no studies have compared the differences in mechanism and trends in resultant inaccuracies between open procedures and MIS, or the difference in radiation exposure in MIS among different fluoroscopic techniques. This study compared the tendency of screw malposition during the percutaneous procedure and the conventional open procedure, and investigated the efficacy of the oblique fluoroscopic view based on simple computed tomography (CT) measurements for accurate placement of lumbosacral PPS and reducing radiation exposure.

In total, 1046 PSs were placed in 230 consecutive patients. Groups M-1, M-2 and controls (group O) comprised 40, 42 and 148 patients with 174, 184 and 688 inserted screws, respectively. Malposition rates of MIS (groups M-1 plus M-2) and controls were 8.1% and 4.7%, respectively. The degree of perforation in controls gradually decreased as follows: grades B, C, and D were 21, 7, and 4, respectively. In contrast, the degree of perforation of the pedicle in group M-1 were as follows: grades B, C, and D were 8, 6, and 6 screws, respectively, and the inaccuracy was 11.5%. Group M-2 used the oblique view of fluoroscopy, and the degree of perforation of the pedicle gradually decreased as follows: grades B, C, and D were 6, 3, and 0, respectively.

A statistically significant difference was observed in accuracy between M-1 and controls, and between M-1 and M-2. Three of 6 grade Ds incorrectly placed PSs in group M-1 caused a neurological deficit; they were re-inserted during another surgery (Fig. 3). No anterior vertebral perforation was observed for any screw in the control or MIS group. A summary of the accuracy data comparisons between the groups is presented in Table 2. The level and direction of inaccuracies are listed in Table 3. Malpositioned screws were inclined towards L5 in group M-1, whereas they showed relatively equal distribution throughout the multi-levels in M-2 and controls. Furthermore, in group M-1, inaccuracy tended to gather at a horizontal plane as medial or lateral perforations of the pedicle.

The first percutaneous placement PS, described by Magerl [17], was subsequently used for only temporary external fixation, but was further developed for percutaneous internal fixation. Since then, several studies have described percutaneous techniques for posterior lumbar fusion to avoid iatrogenic injury to the dorsal musculoligamentous unit [123]. However, the limitations of MIS using percutaneous PSs include the learning curve required, exposure to radiation, and implant-associated complications. In particular, PS malpositioning may cause neurovascular injury, pedicle fracture, pseudoarthosis, and other problems. In MIS, surgeons lack the ability to visualize and palpate bony landmarks and the decompressed spinal canal itself; they must therefore rely heavily on fluoroscopic assistance. With the absence of true anatomical vision, the incidence of pedicle breach would be higher than in conventional open procedures. Few studies have reported on the differences in accuracy of PS placement between open procedures and MIS using conventional fluoroscopy.

This study reported that the open procedure was accurate in 95.3% of 688 screws, similar to previous studies. In contrast, MIS was accurate in 91.9% of cases, and the malpositioned screws in group M-1 tended to be placed far from the pedicle. In this study, there were no significant differences in complication rates, including infection, screw pull-out or pseudoarthrosis, except for the neurological deficits between group M-1 and M-2. Furthermore total OP time was almost the same for both procedures. However, the inaccuracy and radiation exposure in group M-2 were less than that observed in group M-1. We suspected that the high inaccuracy of percutaneous screw placement may be because of the entry points and pathways used for this procedure are different from those in the open procedure, and because the pedicle morphology of L5 is quite varied [1819]. The entry point of the percutaneous lumbar PS is more lateral than that of the open procedure, and its pathway is steep on the axial plane. The axis of the lumbar pedicles on the sagittal midline plane varies with the spinal level; the angle of the L5 pedicle axis is the largest. This steep angle makes it difficult to insert PSs accurately using the conventional fluoroscopic technique. Furthermore, the highest pedicle breach rate in L5 is considered to be due to the different shapes of pedicle, depending on the height [20]. L5 pedicular width and direction in the upper and lower parts are different. In the lower parts, L5 pedicle has a narrow shape and more convergent angle than the upper part. Subsequently, if the L5 PS is introduced at the same convergent angle as the higher part, it may cause lateral recess perforation in the lower part. The same phenomenon may occur at the S1 level.

Controversy exists over what is the best technique to achieve accurate percutaneous PS insertion using the conventional fluoroscopic technique. One example is the Magerl technique, termed the owl's-eye technique; another is a modified technique using AP and lateral fluoroscopic views. In a cadaveric study, Wiesner et al. [21] reported that no significant difference exists between the 2 techniques, but they found a higher misplacement rate in the upper lumbar spine using the Magerl technique. They also reported a 6.6% inaccuracy with 2 nerve injuries (L4 and L5), and that S1 showed the highest malposition rate (12%) using the modified technique in an in vivo lumbar spine [22]. Power et al. [12] recommended the owl's-eye technique for sighting in line with the pedicle. This technique helps surgeons to be constantly aware of the medial pedicle wall and, when performing percutaneous screw placement at S-1, without being readily aware of the initial entry point. In this previous research, they explained that connecting the medial aspect of the pedicle with the levels above helps to identify the medial aspect of S-1. Idler et al. [13] also recommended the owl's-eye technique using a novel guidance technology; they noted that the true pedicle axis enables the cephalad facet complex to be clearly visualized and avoided.

Crucially, however, the owl's eye technique requires manoeuvring by a skilful operator to achieve a clear radiographic demonstration. In this study, we used the oblique view of fluoroscopy, an angle that was easily adjusted on simple preoperative CTs. Consequently, it is easy to handle the fluoroscope, even if the nurse or resident using it chooses obliquity. Furthermore, this technique allows reduction of inadvertent labour hours to adjust the obliquity of the fluoroscope during surgery as well as radiation exposure, because the angle is measured on the preoperative CTs. Exact obliquity is beneficial for preventing adjacent joint disorder, because the facet joint is clearly visible. Most importantly, this is the only fluoroscopic technique to safely and accurately insert percutaneous PSs without the aid of other devices.

This study has several limitations; its retrospective design and the accuracy of percutaneous PS placement depends upon a learning curve. In general, the surgeon's experience improves the outcome of various procedures. The association between higher volumes and better outcomes has been established for some spinal surgeries, such as cervical PS placement, and lumbar endoscopic discectomy [232425]. The same holds true for MIS-TLIF using percutaneous PS placement. Operating time and blood loss were decreased as surgeons overcame the learning curve for the technical difficulty in this series. However, the accuracy of PS placement using the percutaneous procedure did not clearly depend on the surgeon's experience, because of the pedicle morphology and the optical illusion of the PS pathway using AP and lateral fluoroscopic views. Other limitations included the use of different types of implants, the rate of screw placement at the S1 level being relatively low, data on fluoroscopy time did not accurately reflecting the true radiation exposure because the time estimated included the time of interbody fusion, and the amount of radiation exposure was different from the incidence angle of the fluoroscope.

Many complicated devices and computed navigation systems for accurately inserting percutaneous PSs in MIS are available and widespread use is much expected in the near future. However, the best way to safely and accurately place percutaneous PSs is to study the multitude of clinical experiences and anatomical knowledge of open spinal procedures. We consider that the conventional oblique fluoroscopic technique is freely available and it that can be easily introduced and performed, even by surgeons with limited experience. Acceptable results may be achieved even by spinal surgeons who are accustomed to the open procedure with the conventional fluoroscopic technique.

The placement of percutaneous lumbosacral PSs using the conventional fluoroscopic technique carries an increased risk of inaccurate screw placement and resultant neurological deficits, compared with the conventional open procedure. Inaccuracy in our study tended to be distributed between the medial and lateral perforations of the L5 pedicle because of pedicle morphology and the percutaneous PS pathway. Besides reducing radiation exposure, being easily introduced without other devices and manoeuvrable even by inexperienced surgeons, the oblique fluoroscopic technique is advantageous because it is based on simple CT measurements that can improve accuracy and safety in percutaneous PS placement.