The implantation of pedicle screw is a fundamental technique in spinal surgery. The accuracy of pedicle screw placement strongly affects the outcome of spinal surgery and mainly relies on the surgeons’ experience. Inaccurate placement may incur devastating neural injuries and internal fixation failure[16]. Therefore, finding an accurate and quantified method of locating the entry point is of vital importance, especially for young surgeons with less experience.
In the past decades, various methods have been proposed for the localization of entry points. Roy-Camille suggests the entry point as the intersection of two lines: the vertical line is the prolongation of the facet joint while the horizontal line passes through the middle of the insertion of the transverse process or 1mm below the joint line[6]. Magerl’s technique describes the entry point as the intersection of the lateral border of the superior facet joint and the midline of the transverse process[7]. Weinstein prefers the entry point as the lateral and inferior corner of the superior articular facet, what Weinstein calls the ‘nape of the neck’ of the superior articular facet[8]. The crista lambdoidalis method describes the entry point as the vertex where the isthmus crest converses the accessory process crest[9]. Among these techniques, both the Roy-Camille method and the Magerl’s method use the midline of the transverse process as the reference, which requires thorough exposure of the operative field, therefore longer operation time and more blood loss. Additionally, the transverse process can be asymmetrical, and using the midline of the transverse process as the reference can be misleading. In a study conducted by Ebraheim et al., fifty dry lumbar specimens were obtained to measure the distance from the projection point of the lumbar pedicle axis to the midline of the transverse process for each level of the lumbar vertebrae[17]. From L1 to L3, the projection point of the pedicle axis lays 3.9 mm, 2.8 mm and 1.4 mm above the midline of the transverse process, respectively[17]. Therefore, using the midline of the transverse process as the reference tends to increase the risk of breaching the inferior wall of the pedicle and increase the risk of causing nerve root injury. In a study carried out by Yu et al., three methods (the Roy-Camille method, the Magerl’s method, and the crista lambdoidalis method) were simulated on 3D reconstructed images to compare the distance from the entry point to the axis of the pedicle[18]. In their study, the crista lambdoidalis method proved to be superior from L1 to L4, while the Magerl’s technique showed comparable accuracy in L3-L4 and has the highest accuracy at L5[18]. However, the overall existence of the crista lambdoidalis is reported to be around 94.5% and it’s especially lower in L5[9]. Besides, crista lambdoidalis can be indistinguishable in cases of deformities, lumbar spondylolysis, vertebral fracture, or severe hyperplasia of the lumbar facet joint. Therefore, to reduce the risk of screw malposition and to shorten the learning curve for young surgeons, we proposed a valid and quantified method, by locating the entry point at 4mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process.
Compared with traditional methods, the entry point located with our method tends to have a superior and lateral position, which decreases the risk of violating the spinal canal and the nerve roots. In the meanwhile, our method provides a qualified strategy to locate the entry point, which proves to be related with higher success rate and less complications. According to the Gertzbein Robbins grading, 184 screws in the observation group were completely placed within the pedicles, while 29 screws were placed with breaches less than 2mm, and 3 screws were placed with breaches between 2mm to 4mm. In total, 98.6% of the screws were considered to be contained in the safe zone with our method, compared to 96.9% in the control group. During the follow-up, all patients showed steady improvement in pain alleviation and neurologic restoration, with no serious neurovascular complications being reported. In the meanwhile, compared with traditional methods, our method requires less extensive exposure and paraspinal dissection. According to recent studies, erector spinae and multifidus muscle injury resulted from surgical dissection are associated with postoperative low back pain, functional disability, and proximal junctional kyphosis[19, 20]. With our method, both the exposure time before screw implantation and the total operation time is significantly shortened (p < 0.05), which can reduce intraoperative blood loss, improve postoperative pain and decrease the risk for developing postoperative infection. Additionally, in cases of lumbar spondylolysis, vertebral fracture, and severe hyperplasia or degeneration of the facet joint, the crista lambdoidalis can be difficult to distinguish. In these cases, using the crista lambdoidalis as the reference could be confusing. In our study, we prospectively applied our method to 64 patients with indistinguishable crista lambdoidalis. 97.6% of the screws were placed in proper trajectory on the first attempt, and all screws were placed within the ‘safe zone’ as assessed with the Gertzbein Robbins grading. Last but not the least, by proposing a quantified approach for screw implantation, the learning curve for inexperienced surgeons could be significantly shortened, meanwhile the incidence of malposition could also be decreased.
Despite the superiority shown in our study, there are several limitations to be addressed. The average follow-up was only 12 months in our study, longer follow-up is expected to examine the incidence of long-term complications, such as pedicle screw loosening, pull out, or internal fixation failure. In addition, this study demonstrates experience in our single center, multi-center validations are critical for the evaluation and amendment of our technique.