Abstract
Objective
Pedicle screw implantation is the most common technique to achieve stability during spinal surgeries. Current methods for locating the entry point do not have a quantified criteria and highly rely on the surgeons' experience. Therefore, we aim to propose a quantified pedicle screw placement technique in the lumbar spine and to investigate its accuracy and safety in clinical practice.
Methods
We conducted a retrospective study involving 110 patients who received spinal surgery in our hospital from August 2018 to August 2021. All patients included had herniation of a single lumbar disc and were consistently treated with posterior discectomy, inter‐body fusion, and transpedicular internal fixation. For 54 patients in the observation group, the pedicle screws were placed with our technique, which is located at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process. For 56 patients in the control group, pedicle screws were placed according to the traditional crista lambdoidalis method. Comparisons were made in terms of the operation time, blood loss, time for exposure, the accuracy of placement, and postoperative complications. Furthermore, we applied our method to 64 patients with indistinguishable crista lambdoidalis and evaluated the accuracy of screw placement and clinical outcomes according to the visual analogue scale (VAS) and the Japanese Orthopaedic Association (JOA) score.
Results
There was no significant difference in intraoperative bleeding, accuracy of placement, and postoperative complications between our technique and the traditional crista lambdoidalis method (P > 0.05). However, the exposure time before screw placement (12.8 ± 0.3 vs. 17.4 ± 0.3, P = 0.001) and the total surgery time (97.2 ± 1.9 vs 102.3 ± 0.9, P = 0.020) were significantly shortened with our method. Additionally, in cases with indistinguishable crista lambdoidalis, our technique showed satisfying accuracy, with 97.6% screws placed in appropriate trajectory on the first attempt and all screws eventually positioned in the safe zone according to the Gertzbein–Robbins grading. All patients experienced steady improvement after surgery.
Conclusion
Placing pedicle screws at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process is a viable pedicle screw placement method. With this method, we observed a higher success rate and shorter operation time. In addition, this method can be applied in cases with indistinguishable crista lambdoidalis, and have satisfied success rate and clinical outcome.
Keywords: Crista lambdoidalis technique, Lumbar disc herniation, Lumbar fusion, Vertebral pedicle screws
The entry point is positioned at 4 mm below the superior edge of the transverse process, in line with the lateral margin of the superior articular process in the observation group. B. In the control group, the entry point is positioned at the apex where the isthmus crest converses the accessory process crest, which is also known as the “∧” shape crest.
Introduction
Pedicle screw implantation is the most commonly used technique to achieve three‐column stability for the majority of spinal surgery procedures. 1 Indications for transpedicular fixation include congenital disease, deformities such as scoliosis and hyperkyphosis, vertebral fracture, spinal tumor, infection, and degenerative disease. 2 Given the vicinity of vital neurovascular structures, malposition of pedicle screws can cause serious complications and internal fixation failure. 3 Advances have been made in intraoperative navigation systems over the past decades. Computer tomography (CT)‐based, fluoroscopy‐assisted, and robot‐assisted systems are reported to increase the accuracy and decrease the comorbidities of screw implantations. 4 Nevertheless, the navigation system can be cost‐consuming with limited application in hospitals of small scales. So far, free‐hand screwing is still the most commonly used technique, especially in emergent and limited situations.
The establishment of a proper insertion point is the first and crucial step for accurate pedicle screw implantation. 5 Various methods have been advocated, including the Roy–Camille method, Magerl's technique, Weinstein's method, and Crista lambdoidalis technique, etc. 6 , 7 , 8 , 9 The Crista lambdoidalis method describes the entry point as the apex where the isthmus crest converses the accessory process crest, which is also known as the “∧” shape crest. 9 , 10 However, tactile feedback and experience‐based judgment are of utmost importance in present techniques. The misplacement rate in the lumbar spine was reported to range from 8.3% to 50.6%. 2 , 11 Additionally, in cases with lumbar spondylolysis, vertebral fracture, and severe hyperplasia or degeneration of the facet joint, the crista lambdoidalis can be difficult to distinguish. Therefore, to propose a safe and quantified method for locating the insertion point is of vital importance.
According to a study on 30 Chinese cadavers, the average width of the transverse process at base from L1 to L5 is 8.2 ± 2.4 mm, 8.9 ± 2.0 mm, 10.0 ± 1.8 mm, 8.6 ± 1.7 mm and 12.7 ± 2.8 mm, respectively. 12 Therefore, entry point determined at 4 mm below the superior edge of the transverse process tends to be above the midline of the transverse process, which is used as a reference in the Magerl's method. Since nerve root is usually adjacent to the inferior wall of the pedicle, a relatively superior entry point can reduce the risk of causing nerve root injury. Therefore, we propose to place the pedicle screws at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process. The aims of this study include: (i) to evaluate the accuracy and safety of this technique as compared to the traditional crista lambdoidalis method; and (ii) to apply our technique in patients with indistinguishable crista lambdoidalis and assess its safety and validity.
Method
Inclusion and Exclusion Criteria
Inclusion criteria were as follows: (i) patients with typical symptoms and radiological manifestations indicating disc herniation of a single segment; (ii) patients with no alleviation of symptoms after 3 months' conservative treatment; (iii) patients who underwent posterior discectomy, interbody fusion and transpedicular fixation by the same surgical team in our hospital; and (iv) patients who were followed for at least 12 months after surgery. Exclusion criteria were as follows: (i) patients who could not tolerate surgery; (ii) patients with vertebral infection or fracture involving articular process and/or transverse process; and (iii) patients with history of lumbar spine surgery. This research was approved by the ethics committee of our hospital, and written informed consent was obtained from all patients or their legal guardians (Y(2022)154).
Patient Information
From August 2018 to August 2021, 110 patients who met the inclusion and exclusion criteria were retrospectively enrolled in our study. Patient demographics, including patients' age, gender, duration of symptoms and visual analogue scale (VAS) score were evaluated to determine the baseline difference. Surgical information was also recorded, including the operation time, blood loss, time for exposure, the accuracy of screw placement, and postoperative complications (including surgical site infection, neurovascular injury, and internal fixation failure). Postoperative CT scans were assessed according to the Gertzbein–Robbins grading system. 13 Screws completely contained within the pedicles (Grade A) and breaches within 2 mm (Grade B) were considered in the safe zone with a low risk of neural damage. 13
Afterwards, we applied our technique to 64 patients with indistinguishable crista lambdoidalis, where the traditional crista lambdoidalis technique is unfeasible. The accuracy of screw placement and times of intra‐operative fluoroscopy were recorded. The clinical efficacy was evaluated by the Japanese Orthopaedic Association (JOA) score and VAS score before operation and 1, 3, and 6 months after surgery. 14 , 15 The Bridwell grading was adopted for assessment of interbody fusion at the last follow‐up, where “fused with remodeling and trabeculae” was defined as Grade I, while “intact graft without fully remodeling and incorporation, but no present lucency” was defined as Grade II. 16
Surgical Procedures
Patients were placed in the prone position after anesthesia. A mid‐line incision was made, followed by blunt separation of the subcutaneous tissues and subperiosteal dissection of the muscles until the superior articular process and, in the case of the control group, bilateral lamina was exposed. In the observation group, an awl was used to detect the superior edge of the transverse process, afterwards, a nerve root dissector of 4 mm’ width was used to locate the entry point at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process (Figs 1 and 2C). While in the control group, screws were placed at the apex of the crista lambdoidalis 10 (Fig. 1). After identification of the entry point, a blunt opener was used to enter the vertebral pedicle. Afterwards, a blunt probe was applied to ensure the intactness of the trajectory before a screw of appropriate diameter and length was implanted. Once all screws were placed, the C‐arm fluoroscopy was used to examine the position of all screws. If the pedicle was breached, the trajectory was re‐built and fluoroscopy was performed afterward until satisfactory implantation was achieved. After screw fixation, the posterior discectomy, and interbody fusion were subsequently performed. Finally, screws were connected with two longitudinal link rods before the wound was washed and sutured.
Fig. 1.
Schematic diagram of entry point (A). The entry point is positioned at 4 mm below the superior edge of the transverse process, in line with the lateral margin of the superior articular process in the observation group. (B) In the control group, the entry point is positioned at the apex where the isthmus crest converses the accessory process crest, which is also known as the “∧” shape crest.
Fig. 2.
A 53‐year‐old male patient with a complaint of severe lower back pain and radiating pain in the left leg was administered in our hospital. (A, B) An MRI examination was ordered, which indicated L4/5 intervertebral disc herniation. The patient was scheduled for pedicle screw fixation, posterior discectomy, and interbody fusion. (C) After exposing the superior articular processes of L4 and L5, pedicle screws were positioned at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process. The nerve root dissector was used to determine the 4 mm distance. (D, E) Postoperative x‐ray indicated a satisfying position of all pedicle screws. (F) CT scan was used for the Gertzbein–Robbins grading, which was determined as Grade A since all the screws were completely contained within the pedicles.
Statistical Analysis
The normal distribution of quantitative data was determined by the Shapiro–Wilk test, and the homogeneity of variance was tested by Levene's test. Student t‐test, Mann–Whitney U test, or χ 2 test was performed to compare the mean value or data distribution among different groups. P values less than 0.05 are considered statistically significant. All statistical analyses were conducted using SPSS version 26.0 (SPSS Inc., Chicago, IL, USA).
Results
Patient Demographics
Baseline information among the two groups was compared and listed in Table 1. There were 31 male patients and 23 female patients in the observation group, with an average age of 53.6 years. Also, 27 male patients and 29 female patients were included in the observation group, with an average age of 52.9 years. On average, patients from both groups suffered from herniation‐related symptoms for an average of 12 months before surgery. Symptoms evaluated by the VAS was similar between the two groups (7.3 ± 0.1 vs 7.2 ± 0.1, P = 0.691). There is not statistical difference between two groups in surgical segment and body mass index (BMI).
TABLE 1.
Comparison of related indexes between two groups
| Variables | Observation Group | Control group | Effect size (t/χ 2/Z value) | P value |
|---|---|---|---|---|
| Cases (No.) | 54 | 56 | ||
| Age (Mean ± SD, years) | 53.6 ± 2.0 | 52.9 ± 1.6 | Z = −0.446 | 0.656 |
| Gender (Male/Female) | 31/23 | 27/29 | χ 2 = 0.932 | 0.334 |
| Duration of symptoms (months) | 12.1 ± 0.5 | 12.1 ± 0.9 | Z = −1.001 | 0.317 |
| Visual analogue scale | ||||
| Pre‐operation | 7.3 ± 0.1 | 7.2 ± 0.1 | Z = −0.397 | 0.691 |
| Post‐operation | 2.0 ± 0.2 | 2.0 ± 0.1 | Z = −0.217 | 0.828 |
| Operation time (min) | 97.2 ± 1.9 | 102.3 ± 0.9 | t = −2.382 | 0.020 |
| Blood loss (mL) | 307.4 ± 7.8 | 309.5 ± 10.5 | t = −0.157 | 0.876 |
| Time for exposure (min) | 12.8 ± 0.3 | 17.4 ± 0.3 | Z = −7.633 | 0.001 |
| Accuracy of screw implantation (No., %) | ||||
| Success on the first attempt | 204 (94.5%) | 208 (92.9%) | χ 2 = 0.465 | 0.495 |
| Re‐implantation | 12 (5.5%) | 16 (7.1%) | ||
| The Gertzbein–Robbins grading | ||||
| Grade A & B | 213 (98.6%) | 217 (96.9%) | χ 2 = 1.492 | 0.222 |
| Grade C & D &E | 3 (1.4%) | 7 (3.1%) | ||
| Screw Location (No., %) | ||||
| L1 | 18 (8.3%) | 20 (8.9%) | χ 2 = 2.183 | 0.791 |
| L2 | 44 (20.4%) | 46 (20.5%) | ||
| L3 | 54 (25.0%) | 56 (25.0%) | ||
| L4 | 64 (29.6%) | 66 (29.5%) | ||
| L5 | 36 (16.7%) | 36 (16.1%) | ||
| BMI (kg/m2) | 22.3 ± 2.2 | 23.7 ± 1.7 | t = −0.175 | 0.983 |
Abbreviation: BMI, body mass index.
Screw Implantation
In the observation group, a total of 216 screws were placed, 204 (94.5%) of them were placed in appropriate trajectory on the first attempt based on the intraoperative fluoroscopy, while 12 (5.5%) of them were considered inappropriate and re‐implantation was performed. By comparison, 224 screws were placed in the control group and the success rate on the first attempt was 92.9%. Although the success rate on the first attempt was higher in the observation group, no statistical significance was reached (P = 0.495). In the meanwhile, the accuracy of screw placement was also evaluated on postoperative CT images according to the Gertzbein–Robbins grading. One hundred eighty‐four in the observation group were rated as grade A, 29 as grade B, and three as grade C. The accuracy was calculated as 98.6% (213/216). In comparison, of the 224 screws in the control group, 189 were rated as grade A, 28 as grade B, and seven as grade C. The overall accuracy was 96.8%, showing no significant difference from the 98.6% in the observation group (P = 0.222). Of note, it took shorter time to expose relevant bony landmarks in the observation group to prepare patients for screw implantation (12.8 ± 0.3 min vs 17.4 ± 0.3, P = 0.001), and the whole operation time was also significantly shortened with our technique (97.2 ± 1.9 min vs 102.3 ± 0.9 min, P = 0.020). During the follow‐up, patients in both groups showed significant pain alleviation and function restoration, with no significant difference noted as evaluated with the VAS and muscle strength grading. In addition, we performed subgroup analysis for each variable, which shows that the accuracy rate of pedicle screw implantation between the two groups is not affected by the patients' age, gender, BMI, duration of symptoms, preoperative VAS score or lumbar section (Table S1).
Application in Patients with Indistinguishable Crista Lambdoidalis
Furthermore, 35 male patients and 29 female patients with indistinguishable crista lambdoidalis were prospectively identified (Table 2). Pedicle screws were placed at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process according to our technique. The number of pedicle screws placed from L1 to L5 was 20, 52, 64, 76, 44 respectively. The average number of intraoperative fluoroscopies was 3.9 ± 0.7, and 279 out of 286 (97.6%) screws were placed accurately on the first attempt. All screws were placed in the safe zone, as evaluated with the Gertzbein–Robbins grading on postoperative CT scans. After an average follow‐up of 12.5 months, no complications were noticed in all patients. The Bridwell grading showed satisfied bone fusion, while the JOA and VAS grading indicated steady improvement of neural function in all patients (Table 3).
TABLE 2.
Basic information for patients receiving surgeries with indistinguishable crista lambdoidalis
| Parameter | Value |
|---|---|
| Age (Mean ± SD, years) | 49.9 ± 16.2 |
| Gender (Male/Female) | 35/29 |
| Follow‐up (months) | 11.2 ± 4.7 |
| Intraoperative fluoroscopy (times) | 3.9 ± 0.7 |
| Accuracy of screw implantation (No., %) | |
| Success on the first attempt | 279/286 (97.6%) |
| Re‐implantation | 7/286 (2.4%) |
| The Gertzbein–Robbins grading | |
| Grade A | 204 (79.7%) |
| Grade B | 52 (20.3%) |
| Grade C & D &E | 0 |
| The Bridwell grading | |
| I | 60/64 (93.8%) |
| II | 4/64 (6.2%) |
| III | 0 |
| Screw Location (No., %) | |
| L1 | 20 (7.8%) |
| L2 | 52 (20.3%) |
| L3 | 64 (25.0%) |
| L4 | 76 (29.7%) |
| L5 | 44 (17.2%) |
| BMI (kg/m2) | 21.5 ± 1.5 |
Abbreviation: BMI, body mass index.
TABLE 3.
Outcome evaluation for patients receiving surgeries with indistinguishable crista lambdoidalis
| Time | Visual analog scale | JOA |
|---|---|---|
| Preoperative | 6.859 ± 1.193 | 4.984 ± 1.339 |
| Post‐operation at 1 month | 2.016 ± 1.016 a | 20.094 ± 1.455 a |
| Post‐operation at 3 months | 1.031 ± 0.755 a , b | 24.469 ± 1.112 a , b |
| Post‐operation at 6 months | 0.250 ± 0.471 a , b | 25.203 ± 1.535 a , b |
comparison with preoperative, p < 0.05.
comparison with the previous review, p < 0.05.
Abbreviation: JOA, Japanese Orthopaedic Association.
Discussion
The implantation of pedicle screw is a fundamental technique in spinal surgery. In this study, we proposed a quantified method for locating the entry point at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process. This study demonstrated that our method has a higher success rate and shorter operation time compared to the Crista lambdoidalis method. In addition, our method can be applied in patients with indistinguishable crista lambdoidalis, where the traditional method may be unfeasible.
Previous Methods and Limitations
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 1 mm 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 et al. prefer the entry point as the lateral and inferior corner of the superior articular facet, what Weinstein et al. call 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 is difficult to be accurately determined during surgery and requires thorough exposure of the operative field. 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., 50 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, 2.8 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 and Lei, 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 is especially lower in L5. 9 Also, crista lambdoidalis can be indistinguishable in cases with 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 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process.
Novel Method and Advantages
Compared with traditional methods, the entry point located with our method is more superior and lateral, which decreases the risk of violating the spinal canal and the nerve roots, therefore improves the success rate and decreases iatrogenic complications. In the meanwhile, our method provides a qualified strategy to locate the entry point, which may benefit young surgeons with less experience. 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 2 mm, and three screws were placed with breaches between 2 to 4 mm. 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. Also, 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, reduce postoperative pain and decrease the risk for developing postoperative infection. Additionally, in cases with 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. Of the screws, 97.6% were placed in proper trajectory at 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.
Limitations
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.
Conclusion
This study highlights a valid and quantified technique for pedicle screw placement in the lumbar spine by locating the entry point at 4 mm below the superior edge of the transverse process in line with the lateral margin of the superior articular process, which may help inexperienced spinal surgeons to better locate the entry point of pedicle screws. In addition, we demonstrated that our method has higher success rate and shorter operation time compared to the crista lambdoidalis method. Moreover, our method can be applied in cases with indistinguishable crista lambdoidalis, where the traditional method may be unfeasible.
Author Contributions
Conceptualization, visualization and material preparation were performed by Weiqi Jiang. Data collection and formal analysis were performed by Weiqi Jiang and Ao Leng. The first draft of the manuscript was written by Weiqi Jiang and Ao Leng. Qi Wang and Lingzhi Meng reviewed and edited the original draft and supervised over the study. All authors read and approved the final manuscript.
Fundings Information
This work was generously supported by the Natural Science Foundation of Liaoning Province (Grant numbers [2019‐ZD‐1034] and [2022‐YGJC‐09]).
Conflict of Interest Statement
The authors have no relevant financial or non‐financial interests to disclose.
Ethics Approval
This work was approved by the Ethics Committee of General Hospital of Northern Theater Command.
Consent to Participate
Informed consent was obtained from all individual participants or their legal guardians.
Consent to Publish
Patients signed informed consent regarding publishing their data and photographs.
Supporting information
TABLE S1: Supporting Information.
Acknowledgments
We are grateful to the patients for their cooperation and to all of the clinicians and support staff who were involved in caring for the patients enrolled in this study.
First Author: Weiqi Jiang. Weiqi Jiang and Ao Leng contributed equally to this work.
Data Availability Statement
Original data is submitted as supplementary material.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
TABLE S1: Supporting Information.
Data Availability Statement
Original data is submitted as supplementary material.