The traditional posterior lumbar surgery is from the posterior with a skin entry that is a short width distance from the midline, which can directly reach the target with the shortest distance. In this case, the CACAP is large and the CA is small. For example, the trajectory angulations of percutaneous kyphoplasty and percutaneous pedicle screw fixation are mainly determined by the targeted vertebral pedicle[9]. In the PETD technique, the working channel enters the spinal canal through the intervertebral foramen; thus, the trajectory of PETD requires a longer width skin entry distance from the midline (i.e., a small CACAP) and a suitable CA. The requirements of angulations differ greatly among PETD techniques[4, 10, 11]. In the Yeung endoscopic spine system (YESS) technique, in order to puncture into the targeted disc and achieve indirect neural decompression, the CA of the trajectory is determined by the targeted disc bisecting the inclination line, and the CACAP of the trajectory is 25° to 30°[10]. In the transforaminal endoscopic spine system (TESSYS) technique, in order to achieve direct neural decompression inside the spinal canal, the CA of the trajectory is determined with a metal rod that was projected with image guidance toward the isthmus of the upper lamina of each level, and the CACAP differs according to different width skin entry distance from the midline at different levels[4, 11]. The CA in the TESSYS technique is larger than that in the YESS technique. As the entrance point of PETD is far from the midline, where there is a lack of bony landmarks such as spinous process, coupled with the different requirements for the different disc levels and the different PETD techniques, more precise control of the trajectory’s angulation is required in the puncture and localization procedures of PETD.
Achieving an optimal CA of the trajectory is important during PETD surgery. Our previous work has shown that with an overlarge CA, the trajectory of PETD may be blocked by some atypical structures of the upper vertebral body, such as the pedicle, the transverse process, and the accessory process. With an insufficient CA, the trajectory may be blocked by the pedicle and transverse process of the lower upper vertebral body[12]; as for the L5-S1 level, the trajectory may also be blocked by the high iliac crest[13].
The control of CACAP is mainly determined by the distance from the entrance point to the midline. With an insufficient CACAP, the trajectory may be blocked by the liver, the spleen, and the kidneys at L1/2 and L2/3, and the intestines at L3/4 and L4/5[14]. With an overly large CACAP, the trajectory may be blocked by the whole facet joint rather than a ventral part of the superior articular process. Although we can use instruments such as burrs and trephines to enlarge the intervertebral foramen, an optimal angulation of the trajectory may reduce damage to the bony structures.
AP and lateral fluoroscopies are widely used in the puncture and localization procedures during surgery. Although new technologies such as the navigation technique[15] and the surgical robot[16] have been applied recently, fluoroscopy is still the most commonly used technique because of its convenience, good economical performance, and considerable effectiveness[17]. AP and lateral fluoroscopy are also commonly used in PETD to verify the location of the trajectory. From the AP view, it can be verified whether the tip of the trajectory enters the spinal canal; from the lateral view, the depth of the trajectory can be evaluated[18]. The location of the trajectory shown in fluoroscopy is absolutely accurate; however, the angulation shown in fluoroscopy is not always reliable.
As is shown in this study, there is a great difference between the angulations shown in the AP and lateral views; nevertheless, according to the regularity, we can still evaluate the angulation of the trajectory through intraoperative fluoroscopy. As the trajectory of PETD requires a longer width skin entry distance from the midline, the CACAP of the PETD trajectory ranges from approximately 10° to 30°. Under such conditions, the coronal CA is more reliable than the sagittal CA. The coronal CA is approximately equal to the real CA, with a small angle difference and percentage error, whereas the sagittal CA shows a rather great angle difference and percentage error. Because of the high reliability of coronal CA, we can judge the CA according only to the AP view during the localization procedure in PETD.
Do the results in the study prove the CA shown in the lateral view meaningless? Not really. The sagittal CA still makes sense. First, it was found that the sagittal CA and coronal CA are always larger than the real CA (Fig. 6), so the real CA can be judged to be small when the CA shown in the lateral view is small. Second, because the sagittal CA is sensitive to the change of CACAP when CACAP is small (Fig. 6), it can be used to detect the change in the patient’s position. As PETD is performed under local anesthesia, patients may turn their bodies in case of intraoperative pain caused by inadequate anesthesia[19], especially in the lateral position, which would change the CACAP (by changing the patient’s anatomic planes) and influence the judgment of the operator regarding the trajectory’s angulation. During surgery, if the trajectory is unaltered while the sagittal CA shows a large change, it is necessary for the operator to reevaluate the patient’s position. Third, CACAP can be estimated roughly from the difference between the sagittal CA and the coronal CA. When the sagittal CA is obviously larger than the coronal CA, it can be determined that the CACAP is small; as CACAP increases to 45°, according to the angular relation formula, the sagittal CA is equal to the coronal CA; when the sagittal CA is smaller than the coronal CA, it can be determined that the CACAP is greater than 45° (Fig. 6). In the latter two cases, it can be interpreted that the entrance point of the trajectory is too close to the midline and should be moved outward. Moreover, once the intraoperative sagittal CA and the coronal CA are measured, the real CA and CACAP can be calculated precisely by the following formulae (derived from the previous formulae):
If α = CA, β = CACAP, γ = sagittal CA, δ = coronal CA, then:
These formulae may be significant to verify the angulation of the intraoperative trajectory, especially for the applications of the advanced technologies such as surgical navigation, surgical robotics, virtual reality and augmented reality surgery systems, which require advance preoperative trajectory design, intraoperative matching, and registration[20]. The estimation process can provide an important test to compare the intraoperative trajectory with the preoperative designed trajectory, reducing the system error, and improving the system accuracy.
These formulae and regularities are also suitable for the traditional operative approach with a short distance from the midline. It should be noted that the CACAP is large (approximately 70°-90°); in this situation, the sagittal CA is more reliable than the coronal CA.