Abstract
Background Atlantoaxial transarticular fixation, also called the Magerl technique, is said to be the most robust biomechanical method of fixation of the atlantoaxial vertebrae. However, the procedure carries a risk of spinal cord and vertebral artery injury during the insertion process, especially in patients with a high-riding vertebral artery. In this study, a computed tomography (CT)-based navigation system was used for preoperative planning and insertion. This investigation sought to determine the rate and direction of screw perforation as well as the incidence of screw loosening in computer-assisted atlantoaxial transarticular fixation.
Methods Sixty patients (31 men and 29 women; mean ± SD age: 65.3 ± 19.6 years) who received atlantoaxial transarticular screw insertion with preoperative CT navigation were analyzed. We investigated screw position and loosening by CT at the final follow-up.
Results Of the 108 screws inserted, the rate of Grade 2 or higher perforation was 4.6% (5/108). Nine of 81 (11.1%) screws inserted into the 44 patients who were followed for at least 6 months showed loosening. Logistic regression analysis revealed that unilateral insertion (odds ratio: 8.50, 95% confidence interval: 1.53–47.2, P = 0.014) was significantly associated with the incidence of screw loosening.
Conclusions The screw perforation rate of Grade 2 or higher in computer-assisted atlantoaxial transarticular screw fixation was 4.6%, with comparable frequencies of perforation direction. Unilateral insertion was a significant independent factor associated with screw loosening, which occurred in 11.1% of insertions.
Clinical Relevance Spine surgeons should follow up with patients with caution because screws with unilateral insertion are prone to loosening.
Level of Evidence 4.
Introduction
Posterior atlantoaxial transarticular fixation, also known as the Magerl technique, was first reported by Magerl and Seeman in 1979.1 Although this procedure is reportedly the most robust biomechanical method of atlantoaxial fixation,2–4 it also carries a high risk of damaging significant structures, including the vertebral artery (VA), spinal cord, and anatomical variations such as a high-riding VA.5
Surgical medicine has undergone tremendous progress owing to a variety of factors,6 with technique advances widening the scope of spinal surgeries. As normal C1-2 articulations are responsible for approximately 50% of cervical spine rotation cases,7 ensuring proper stabilization is an important issue.8 However, fusion of atlantoaxial joint instability in certain circumstances, such as those involving VA anomalies, ponticulus posticus, or arcuate foramen anomalies, can be challenging to carry out.
Navigation has become widely used to enhance safety in spine surgery, with accuracy improving with the increased use of technology and navigation-based instruments.9 Accordingly, our group routinely employs a computed tomography (CT)-based navigation system for preoperative planning in atlantoaxial transarticular screw insertion.10 However, few reports exist on the accuracy of the procedure by preoperative CT-based navigation.
The present study investigated the rate and direction of screw perforation as well as the presence of screw loosening in computer-assisted atlantoaxial transarticular fixation.
Materials and Methods
Sixty patients (31 men and 29 women; mean ± SD age: 65.3 ± 19.6 years) undergoing Magerl screw insertion with preoperative CT-based navigation between October 1998 and March 2022 at our University Hospital were analyzed. Lateral fluoroscopy was also used as an adjunct for all cases. Indications included idiopathic atlantoaxial subluxation in 21 cases, trauma in 19 cases, rheumatoid cervical spine in 14 cases, and posterior pseudotumor of the odontoid process and odontoid bone in 3 cases each. The mean follow-up period was 24.4 ± 26.0 (1–96) months. Details of the surgical procedure are presented in a previous study.10 Briefly, a skin incision is made from the posterior cranial fossa to the third cervical vertebra to prepare the operation field. The reference frame is then positioned at the spinous process of the second cervical vertebra, and registration is carried out. A 3-mm speed drill is used to create an insertion hole at each screw insertion point. Next, a guide pin is inserted under navigation system guidance, and drilling is performed. Finally, an atlantoaxial transarticular screw is inserted under lateral view fluoroscopy. All patients underwent iliac bone grafting using the Brooks or McGraw procedure. In all cases, CT was performed within 2 weeks postoperative for evaluating screw position. In addition, 44 patients underwent CT 6 months after surgery, at which time we examined patients for screw loosening. We classified screw position as Grade 0 (no perforation), Grade 1 (perforation <2 mm), Grade 2 (perforation ≥2 but <4 mm), or Grade 3 (perforation ≥4 mm;11 (Figure), with the direction of each perforation recorded.
Screw loosening was judged as the appearance of radiolucency around pedicle screws.12,13 To determine the risk factors of screw loosening, we divided the cohort according to the presence or absence of loosening for multivariate analysis. Logistic regression models were employed to identify factors associated with screw loosening, with the presence of loosening as a response variable and such patient-related factors as age, sex, screw perforation (Grade 3 or Grade 2 or higher), and unilateral screw insertion as explanatory variables. The selection of factors included in the multivariate analysis was performed by stepwise model testing based on the Akaike information criterion. All statistical analyses were conducted using EZR software (Saitama Medical Center, Jichi Medical University, Saitama, Japan), a graphical user interface for R (The Foundation for Statistical Computing, Vienna, Austria). EZR is a modified version of R commander designed to add statistical functions frequently used in biostatistics. The level of significance was set at P < 0.05.
Screw perforation grading.
Results
The mean (SD) operation time was 186 ± 65 (81–383) minutes, and the mean (SD) blood loss volume was 242 ± 168 (20–800) mL. Fifteen of 60 (25%) patients underwent additional laminoplasty, laminectomy, or another level of posterior fusion. A total of 108 screws were inserted, with a Grade 3 perforation rate of 0.9% (1/108) and a Grade 2 or higher perforation rate of 4.6% (5/108). Regarding direction, 1 lateral, 1 medial, 2 cranial, and 1 caudal screw were inserted into patients with Grade 2 or higher perforation (Table 1). One lateral and 1 caudal screw protruded anteriorly into the anterior arch of the atlas. In the 44 patients who were followed for at least 6 months, 9 of 81 (11.1%) screws showed loosening. Seven of the 44 patients received a unilateral atlantoaxial transarticular screw, of which 3 (42.9%) had loosened. All patients who exhibited loosening had no symptoms and did not require reoperation. Logistic regression analysis revealed that unilateral insertion (OR: 8.50, 95 % CI: 1.53–47.2, P = 0.014) was significantly associated with the incidence of screw loosening, while age (OR: 0.99, 95% CI: 0.95–1.03, P = 0.75), gender (OR: 0.67, 95% CI: 0.16–2.73, P = 0.58), rheumatoid arthritis (OR: 4.11e-08, 95% CI: 0.00–infinity, P = 0.99), and Grade 2 or higher perforation (OR: 1.78e-07, 95% CI: 0.00–infinity, P = 0.99) were not. According to multivariate analysis, unilateral insertion was an independent factor associated with screw loosening (OR: 8.25, 95% CI: 1.49–45.8, P = 0.015; Table 2).
Perforation rates by direction (N = 60).
Effects of patient-related factors on screw loosening.
Discussion
This study identified the rate and direction of screw perforation as well as the presence of screw loosening in computer-assisted atlantoaxial transarticular fixation using CT. Of the 108 inserted atlantoaxial transarticular screws, 5 (4.6%) had a Grade 2 or higher perforation, although no VA injury or other serious complications were observed. Screw loosening was detected in 11.1% of screws. Unilateral transarticular screw insertion was a significant independent factor associated with screw loosening.
Posterior atlantoaxial fixation with transarticular screws provides biomechanically strong fixation, with reported fusion rates from 87% to nearly 100%.14–16 The most serious complication associated with the procedure is VA injury due to inferior or lateral perforation during screw placement.15,17 Screw insertion into the upper cervical spine is technically difficult, especially in patients with high-riding VA, which increases the risk of complications. High-riding VA ranges between 10.0% and 23% of individuals.18–21 VA injury rates during atlantoaxial transarticular instrumentation are reportedly 1.3% to 4.1%.15,16 A meta-analysis of atlantoaxial fusion with transarticular screws estimated a screw deviation rate of 7.1% and a VA injury rate of 3.1%.22
Various navigation systems have enhanced the safety and accuracy of spine surgery.9 Wada et al described that O-arm–assisted transarticular screw fixation eliminated screw deviation and reduced blood loss and operation time compared with C1-2 screw-rod constructs to demonstrate the usefulness of intraoperative navigation.23 However, the financial outlay required to purchase and maintain such equipment is likely why it has not yet become commonplace in many hospitals.24 In the present study, using a lower-cost preoperative CT-based navigation system, the transarticular screw deviation rate was relatively low at 4.6%, with no serious complications, making it a good option for transarticular screw insertion as well.
Screw loosening after spinal implant surgery is a complication that leads to bone fusion failure and poor outcomes.25,26 A study on scoliosis surgery revealed a significant association between screw deviation and screw loosening.27 The screw loosening rate in our cohort was 11.1%. Although there was a significant association between unilateral screw insertion and screw loosening, no relationship for screw deviation with screw loosening was found. Since transarticular screws can be relatively long and impart excellent biomechanical stability, their strength may be assured even if the screw perforates slightly. Sutipornpalangkul et al reported 5 cases (22%) having only unilateral transarticular screw fixation but were able to achieve C1-2 biomechanical stability, which was comparable to the results of bilateral transarticular screw fixation.28 In our series, 7 of 44 patients underwent unilateral fixation. Multivariate analysis revealed unilateral transarticular screw insertion as an independently associated factor for screw loosening, although no revision surgery was needed.
The present study had several limitations, including its small sample size, relatively short follow-up period, and retrospective design. Nevertheless, our analysis of the results of 60 patients, which was a relatively large number for an atlantoaxial fixation cohort, demonstrated the safety and low screw loosening rate of insertion with preoperative CT-based navigation along with identifying an independent associated risk factor in fixation with atlantoaxial transarticular screws. We believe that the fact that this report evaluates not only the frequency of screw deviation but also the frequency of clinically problematic screw loosening and its associated factors offers a novel perspective on this surgical technique.
Conclusion
The screw perforation rate of Grade 2 or higher in computer-assisted atlantoaxial transarticular screw fixation was 4.6%, with a similar frequency of screw perforation in all directions. Unilateral insertion, but not screw perforation, was a significant independent factor associated with screw loosening in atlantoaxial transarticular screw fixation with preoperative CT navigation.
Footnotes
Disclosures The authors have no disclosures related to the content of the present study.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2024 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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