Atlantoaxial Transarticular Screw Research Articles

Characteristics of Screw Perforation and Screw Loosening in Atlantoaxial Transarticular Fixation Using a Preoperative Computed Tomography-Based Navigation System.

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. 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. 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. 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. Spine surgeons should follow up with patients with caution because screws with unilateral insertion are prone to loosening.

Efficacy of Atlantoaxial Transarticular Screw Fixation Using Navigation-Guided Drill: Technical Note

Atlantoaxial transarticular screw fixation (TASF) is a procedure that involves inserting screws vertically into the articular processes of C1 and C2. However, this procedure is associated with a risk of injury to surrounding structures including the vertebral artery, carotid artery, pharynx, and spinal cord by misinserting K-wires or screws. This study was performed to evaluate the risk of TASF using 3-dimensional navigation-guided drilling and screw insertion tract creation. Three patients underwent the surgical procedure using a navigation system guided by intraoperative computed tomography. The insertion tract of the screw was created using the navigation system to avoid penetration of the C1 anterior arch or damage to the vertebral artery. A blunt-tipped guide wire was used, which was safe to advance to the cortex of the anterior arch of C1. There were no complications or instrument failures in any of the surgeries. In each case, the total radiation dose delivered was 5.31-7.02 mGy, and total radiation exposure time was 55.6-106.8 seconds. Bone fusion was achieved in all cases. TASF using a navigation system for drilling is useful for accurate placement of K-wire and preventing damage of the vital structures, lowering the risk of the procedure.

Anatomic Considerations of Anterior Transarticular Screw Fixation for Atlantoaxial Instability.

Study DesignCadaveric, observational study.PurposeAtlantoaxial instability (AAI) is characterized by excessive movement at the C1–C2 junction between the atlas and axis. An anterior surgical approach to expose the upper cervical spine for internal fixation and bone grafting has been developed to fix AAI. Currently, no anatomic information exists on the anterior transarticular atlantoaxial screw or screw and plate fixation between C1 and C2 in the Indian population. The objective of this study is to assess the anatomic landmarks of C1–C2 vertebrae: entry point, trajectory, screw length, and safety of the procedure.Overview of LiteratureMethods outlined by Magerl and Harms are the optimal approaches among the dorsal techniques. Contraindications for these techniques include aberrant location of vertebral arteries, fractures of C1–C2 posterior structures. In these cases, anterior transarticular fixation is an alternative. Several available screw insertion trajectories have been reported. Biomechanical studies have demonstrated that adequate rigidity of this fixation is comparable with posterior fusion techniques.MethodsDirect measurements using Vernier calipers and a goniometer were recorded from 30 embalmed human cadavers. The primary parameters measured were the minimum and maximum lateral and posterior angulations of the screw in the sagittal and coronal planes, respectively, and optimum screw length, if it was placed accurately.Results The posterior and lateral angles of screw placement in the coronal and sagittal planes ranged from 16° to 30° (mean±standard deviation [SD], 23.93°±3.93°) and 8° to 17° (mean±SD, 13.3°±2.26°), respectively. The optimum screw length was 25–38 mm (mean±SD, 28.76±3.69 mm).ConclusionsIf the screw was inserted without lateral angulation, the spinal canal or cord could be violated. If a longer screw was inserted with greater posterior angulation, the vertebral artery at the posterior or posterolateral aspect of the C1 superior facet could be violated. Thus, 26° and 30° of lateral and posterior angulations, respectively, are the maximum angles permissible to avoid injury of the vertebral artery and violations of the spinal canal or atlanto-occipital joint.

Guideline for Posterior Atlantoaxial Internal Fixation Assisted by Orthopaedic Surgical Robot.

Atlantoaxial transarticular facet screw fixation (Magerl technique) and C1 lateral mass screws combined with C2 pedicle screws fixation (Harms technique) are the most commonly used techniques for posterior internal fixation in the upper cervical spine. Upper cervical spinal surgery is a technically demanding and challenging procedure because of complicated anatomical structures and frequent occurrence of anomalies. Accurate insertion of screws allows for stable and secure internal fixation, which is necessary for both techniques. Traditional methods under fluoroscopic assistance in this region cannot meet the requirements of high levels of accuracy and security during the procedure. Robot‐assisted spinal surgery can provide accurate and reliable guidance during the screw insertion, which is evidenced in the literature. As a recently developed technique, robot‐assisted surgery is supposed to be performed by skilled surgeons who have received standard training for robotic surgery. The standardized upper cervical spinal surgery assisted by the robot system needs to be introduced to these surgeons. Based on the consensus of consultant specialists, the literature review, and our local experience, this guideline included the introduction of the robotic system, the workflow of robot‐assisted procedures, and the precautions to take during procedures. This guideline aims to provide a standardization of the robotic surgery for posterior atlantoaxial internal fixation.

Evaluation of the efficiency of the C-ring aiming guide for atlantoaxial transarticular screw fixation in toy breed dogs

The goal of the present study was to evaluate the efficiency and safety of the C-ring aiming guide for the atlantoaxial transarticular screw fixation technique in toy breed dogs. Twenty-one adult canine cadavers of toy breed dogs were used in this study. The left and right sides of the cervical vertebrae were randomly assigned to two implant insertion groups: a C-ring aiming guide group and a drill guide group. A 1.2-mm Kirschner wire was inserted into each side by using either a C-ring aiming guide or a drill guide. CT scans were performed before and after surgery. The optimal safe implantation corridor angle and length, the implant insertion angle and length, the implant insertion time and the proportion of the insertion corridor to the optimal corridor were evaluated. Violations to the alar foramen and the vertebral canal also were evaluated. The implant insertion time was twice as long as that observed in the aiming guide group (P < 0.05). The proportion of the insertion angle and length to the optimal angle were not significantly different between groups (P > 0.05). With respect to precision, there was a trend toward less variability in the aiming guide group; however, this difference was not significant (P = 0.09). The violation of the alar foramen was significantly lower in the aiming guide group than in the drill guide group (P < 0.05). Violation to the vertebral canal was detected in one cadaver in the drill guide group but did not occur in the aiming guide group. The use of a C-ring aiming guide was associated with less damage to the alar foramen and the vertebral canal during atlantoaxial transarticular screw fixation in toy breed dogs.

Designing patient-specific 3D printed devices for posterior atlantoaxial transarticular fixation surgery

Atlantoaxial transarticular screw fixation is an effective technique for arthrodesis. Surgical accuracy is critical due to the unique anatomy of the atlantoaxial region. Intraoperative aids such as computer-assisted navigation and drilling templates offer trajectory guidance but do not eliminate screw malposition. This study reports the operative and clinical performance of a novel process utilising biomodelling and 3D printing to develop patient specific solutions for posterior transarticular atlantoaxial fixation surgery. Software models and 3D printed 1:1 scale biomodels of the patient's bony atlantoaxial spine were developed from computed tomography data for surgical planning. The surgeon collaborated with a local medical device manufacturer using AnatomicsC3D to design patient specific titanium posterior atlantoaxial fixation implants using transarticular and posterior C1 arch screws. Software enabled the surgeon to specify screw trajectories, screw sizes, and simulate corrected atlantoaxial alignment allowing patient specific stereotactic drill guides and titanium posterior fixation implants to be manufactured using 3D printing. Three female patients with unilateral atlantoaxial osteoarthritis were treated using patient specific implants. Transarticular screws were placed using a percutaneous technique with fluoroscopy and neural monitoring. No screw malposition and no neural or vascular injuries were observed. Average operating and fluoroscopy times were 126.0 ± 4.1 min and 36.7 ± 11.5 s respectively. Blood loss was <50 ml per patient and length of stay was 4–6 days. Clinical and radiographic follow up data indicate satisfactory outcomes in all patients. This study demonstrates a safe, accurate, efficient, and relatively inexpensive process to stabilise the atlantoaxial spine using transarticular screws.

Percutaneous Posterior Transarticular Atlantoaxial Fixation for the Treatment of Odontoid Fractures in the Elderly: A Prospective Study.

A prospective study of 20 multimorbid patients older than 65 years undergoing minimally invasive surgical treatment for odontoid fracture. To analyze the results of percutaneous transarticular atlantoaxial screw fixation as a new minimally invasive treatment modality in this high risk group of patients. Odontoid fractures are a common injury pattern in the elderly. These fractures typically present significant challenges as geriatric patients often have multiple comorbidities that may adversely affect fracture management. Despite numerous publications on this subject, with a trend toward primary operative stabilization, the appropriate treatment for this frequent and potentially life threatening injury remains controversial. Between January 2013 and December 2015, 20 consecutive patients underwent posterior percutaneous transarticular atlantoaxial screw fixation for odontoid fracture type II. The two main inclusion criteria were age 65 years or older and ASA score of III or IV. The screws were inserted percutaneously with the help of two fluoroscopy devices. Clinical and radiological examinations were regularly performed for a minimum of 18 months postoperatively. The mean age was 81 years, all of them with multiple comorbidities. Reduction of the fracture and screw insertion was possible in all cases. The mean operative time was 51.75 minutes and mean blood loss was 41.7 mL. Three patients died in the first 3 months after surgery. Healing of the fracture occurred in 15 patients (88.2%). Revision surgery was not necessary in any of the patients. Mean visual analogue scale (VAS) at the final follow-up was 2.4, and mean patient satisfaction score was 7.1. Percutaneous transarticular atlantoaxial fixation in elderly patients offers a good minimally invasive operative treatment in this multimorbid group of patients. This new technique with short operative time is well tolerated by the geriatric patients leading to a healing rate up to 88%. 4.

Robot-Assisted Posterior C1-2 Transarticular Screw Fixation for Atlantoaxial Instability: A Case Report.

Case report. The aim of this study was to assess the feasibility, safety, and accuracy of a new robotic system for performing posterior C1-2 transarticular screw fixation for atlantoaxial instability. C1-2 transarticular fixation is a reliable procedure. However, the procedure is high-risk because of the important structures and frequent anatomical variation around the atlantoaxial region. Navigation improves the accuracy, but it might require repeated adjustments of the trajectories, which is inconvenient. Robot-assisted surgery can make this process easier and has the potential to improve the safety and accuracy. A 43-year-old man with atlantoaxial deformity and instability underwent posterior C1-2 transarticular screw fixation under the guidance of the new robotic system (TiRobot, co-designed by Beijing Jishuitan hospital and TINAVI Medical Technologies Co., Ltd.). We performed a surgery of posterior decompression and C0-2 fusion. Based on the intraoperative 3D images, we planned the trajectory on the robot system. Then, the robotic arm spontaneously moved to guide the screw insertion. A unilateral C1-2 transarticular screw was safely placed with good accuracy. The calculated deviation of the planned position and actual position was 0.8798 mm. There were no intraoperative complications. Postoperative computed tomography showed there were no perforations and loosening of the screw. We report the first case of atlantoaxial transarticular screw fixation using the TiRobot system. Robotic guidance for complex upper cervical deformity based on intraoperative 3D images could be feasible, safe, and accurate, and has a significant clinical potential in spine surgery. 4.

Prevalence of High-Riding Vertebral Artery and Morphometry of C2 Pedicles Using a Novel Computed Tomography Reconstruction Technique.

Study DesignCross-sectional, matched-pair comparative study.PurposeTo determine whether a thin-sliced pedicular-oriented computed tomography (TPCT) scan reconstructed from an existing conventional computed tomography (CCT) scan is more accurate for identifying vertebral artery groove (VAG) anomalies than CCT.Overview of LiteraturePosterior atlantoaxial transarticular screw fixation and C2 pedicle screws can cause vertebral artery (VA) injury. Two anatomic variations of VAG anomalies are associated with VA injury: a high-riding VA (HRVA) and a narrow pedicle of the C2 vertebra. CCT scan is a reliable method used to evaluate VAG anomalies; however, its accuracy level remains debatable. Literature comparing the prevalence of C2 VAG anomalies between CCT and TPCT is limited.MethodsA total of 200 computed tomography scans of the upper cervical spine obtained between January 2008 and December 2011 were evaluated for C2 VAG anomalies (HRVA and narrow pedicular width) using CCT and TPCT. The prevalence of C2 VAG anomalies was compared using these two different measurement methods via a McNemar's test.ResultsOf the 200 patients studied, 23 HRVA (6.01%; 95% confidence interval [CI], 3.61%–8.39%) were detected with CCT, whereas 66 HRVA (16.54%; 95% CI, 12.85%–20.23%) were detected with TPCT (p<0.001). Sixty-two narrow pedicles (15.58%; 95% CI, 11.99%–19.15%) were detected with CCT, whereas 90 narrow pedicles (22.83%; 95% CI, 18.58%–26.87%) were detected with TPCT (p<0.001).ConclusionsVAG anomalies are commonly observed. A preoperative evaluation using TPCT reconstructed from an existing CCT revealed a significantly higher prevalence of C2 VAG anomalies than did CCT and showed comparable prevalence to previously published studies using more sophisticated and higher risk techniques. Therefore, we propose TPCT as an alternative preoperative evaluation for C2 screw placement and trajectory planning.

Deviation factors of posterior atlantoaxial transarticular screw placement assisted by rapid prototyping drill templates

Objective To validate the safety and accuracy of a rapid prototyping drill template (RPDT) for posterior atlantoaxial transarticular screw placement and analyze factors for screw deviation.Methods Twelve normal cadaveric cervical spines were examined using 64 slice CT with 1-mm thick scan and data in Dicom format were recorded.After data was processed using software Mimics 10.01 for three-dimensional (3-D) model reconstruction,computer-assisted design of optimum trajactory for atlantoaxial transarticular screw placement was worked out and made into a drill template,where the surface was created as the inverse of axial posterior surface.The drill template was materialized in a rapid prototyping machine and used to place the screws.After surgery,the position of posterior atlantoaxial transarticular screw was evaluated by X-ray and CT scan.Screw entry point,angle and orientation of the optimal and actual trajactory were determined after fitting the position of the pre-operative and post-operative specimen in computer software and the redefining the 3-D coordinate axis.Results Twenty-four screws were implanted with no cortex perforation.Depth of the optimum save screw trajectory for atlantoaxial transarticular fixation was (37.34 ± 2.31) mm on the left side and (37.11 ± 2.21) mm on the right side.Introversion angles of the optimum save screw trajectory was 0° in both sides,but the actual angle was (0.15 ±0.58)°on the left side and (0.11 ±0.46)°on the right side.Elevation angle of the optimum save screw trajectory was (49.35 ± 1.62) °on the left side and (48.83 ± 1.83) ° on the right side,but the actual angle was (49.29 ± 1.68) °and (49.10 ± 1.45) °respectively.Average displacement of screw entry point in the x,y and z axis was respective (0.21 ±0.65) mm,(0.69 ± 1.48) mm and (0.39 ±0.11) mm on the right side,while (0.19 ± 0.66) mm,(0.53 ± 1.45) mm and (0.38 ± 0.13) mm on the left side.There were no statistically significant differences in deviation levels of entry point and orientation between the optimum and actual screw trajectory (P ＞ 0.05).Conclusions Causes for deviation in RPDT-assisted placement of atlantoaxial transarticular screw are mainly intrinsic factors of the hardware and software and human factors in the operation.RPDT is easy in operation and has individualized design,which greatly improves the accuracy of screw placement and reduces screw deviation.RPDT can be widely used in clinical practice. Key words: Spinal injuries; Cervical vertebrae; Transarticular screws

CT scan based determination of optimal bone corridor for atlantoaxial ventral screw fixation in miniature breed dogs

To describe the most reliable insertion angle, corridor length and width to place a ventral transarticular atlantoaxial screw in miniature breed dogs. Retrospective CT imaging study. Cervical CT scans of toy breed dogs (n = 21). Dogs were divided into 2 groups--group 1: no atlantoaxial abnormalities; group 2: atlantoaxial instability. Insertion angle in medial to lateral and ventral to dorsal direction was measured in group 1. Corridor length and width were measured in groups 1 and 2. Corridor width was measured at 3 points of the corridor. Each variable was measured 3 times and the mean used for statistical analysis. Mean ± SD optimal transarticular atlantoaxial insertion angle was determined to be 40 ± 1° in medial to lateral direction from the midline and 20 ± 1° in ventral to dorsal direction from the floor of the neural canal of C2. Mean corridor length was 7 mm (range, 4.5-8.0 mm). Significant correlation was found between corridor length, body weight, and age. Mean bone corridor width ranged from 3 to 5 mm. Statistically significant differences were found between individuals, gender and measured side. Optimal placement of a transarticular screw for atlantoaxial joint stabilization is very demanding because the screw path corridor is very narrow.

Evaluation of biomechanical properties of anterior atlantoaxial transarticular locking plate system using three-dimensional finite element analysis

To evaluate a new anterior atlantoaxial transarticular locking plate system using finite element analysis. Thin-section spiral computed tomography was performed from occiput to C2 region. A finite element model of an unstable atlantoaxial joint, treated with an anterior atlantoaxial transarticular locking plate system, was compared with the simple anterior atlantoaxial transarticular screw system. Flexion, extension, lateral bending, and axial rotation were imposed on the model. Displacement of the atlantoaxial transarticular screw and stress at the screw-bone interface were observed for the two internal fixation systems. Screw displacement was less using the anterior atlantoaxial transarticular locking plate system compared to simple anterior atlantoaxial transarticular screw fixation under various conditions, and stability increased especially during flexion and extension. The anterior atlantoaxial transarticular locking plate system not only provided stronger fixation, but also decreased screw-bearing stress and screw-bone interface stress compared to simple anterior atlantoaxial transarticular screw fixation.

Atlantoaxial transarticular screw fixation and posterior fusion using polyester cable: a 10-year experience

To retrospective review the clinical outcomes of the modified operative technique using a polyester suture material (Ethibond* Excel) for atlantoaxial transarticular screw fixation and posterior fusion. The retrospective reviews were conducted from 2002 to 2012. The patient's medical record reviews included demographic data, cause of atlantoaxial instability, orthopedic and surgical history, clinical presentation, radiographic finding including plain radiography, complications, operative detail, and outcome of treatment. Fusion of C1-C2 was defined as either graft consolidation or absence of C1-C2 movement on lateral flexion-extension radiograph. Twenty-three patients demonstrated clinical and radiographic evidence of atlantoaxial instability (13 men and 10 women, with a mean age of 42 years). Majority of atlantoaxial instability was caused by trauma. Most common clinical symptom was neck pain with or without cervical myelopathy. Bilateral screws were placed in 18 of the 23 patients. Five patients underwent placement of unilateral screws. The 13 patients were inserted by screws with diameter 4.0 mm. The means screw length was 40.33 mm. The means of operative time and estimated blood loss were 3.6 h and 234 ml, respectively. The mean of follow-up duration was 18 months. All 41 screws were positioned satisfactorily in C1 lateral mass. All 23 patients achieved fusion (100% fusion rate). After a period of follow-up, 9 of the 10 neurological deficit patients had completely recovered. We concluded that the atlantoaxial transarticular screw fixation and posterior fusion using polyester cable can be used for C1-2 fusion with a high fusion rate and less complications in various cases.

Anterior atlantoaxial transarticular screw fixation for treatment of atlantoaxial instability

Objective To analyze clinical outcome of anterior atlantoaxial transarticular screw fixation in treatment of atlantoaxial instability.Methods Thirty-two patients with atlantoaxial instability treated between March 2004 and June 2009 were enrolled in the study.The patients consisted of 21 males and 11 females,at age of 22-64 years (mean 49 years).Atlantoaxial instability was attributed to old odontoid fracture in 10 patients,free odontoid malformation in 16,transverse ligament rupture in two,and rheumatoid arthritis in four.Anterior atlantoaxial transarticular screw fixation under monitoring of cortical somatosensory evoked potential (CSEP) was performed for all patients.Operation time,intraoperative blood loss,and complications were recorded.Japanese Orthopedic Association (JOA) scoring system was used to evaluate neurologic function preoperatively and at one year postoperatively.Results Operation lasted for average 98 minutes and intraoperative blood loss averaged 110 ml.Injuries on esophagus,nerve and vertebral arteries as well as leakage of cerebrospinal fluid were not observed in operation.All patients received a follow-up of 12-31 months.JOA score was increased from preoperative 9.8 points to 15.8 points at one year postoperatively,with improvement rate of 83％.Bone fusion and satisfactory internal fgxation were achieved in all patients.Hypoglossal nerve injury symptom was found in two patients postoperatively and was recovered two months later.Conclusion Anterior atlantoaxial transarticular screw fixation is an effective treatment for atlantoaxial instability. Key words: Spinal injuries; Atlanto-axial joint; Fracture fixation, internal; Anterior approach

Atlantoaxial transarticular screw fixation: a technique never reported from (sub-saharan) Africa?

Background: Atlantoaxial transarticular screw fixation is considered to be the gold standard for treating atlantoaxial instabilities. This challenging surgical procedure requires high skills and appropriate surgical armamentarium. Such conditions are rarely met in sub-Saharan Africa. After a thorough review of medical literature, we did not find a single report on atlantoaxial transarticular screwing from Africa. For this reason, we decided to report on a case we operated on at our institution. Methods: We retrospectively reviewed a case of post-traumatic acute atlantoaxial rotatory fixation (AARF) associated with C5-C6 left facet joint dislocation that we managed in our hospital with posterior atlantoaxial/transarticular screw fixation and C5-C6 lateral masses plating. Results: A 56 years old female was admitted to our service for head and cervical spine trauma following a road traffic accident. She had asymmetrical quadriplegia and urinary retention. Computed tomography (CT) scans revealed a type II atlantoaxial rotatory fixation of left atlantoaxial joint along with stage II distractive-flexion injury of left C5-C6 facet joint. Transcranial traction with Gardner-Wells tongs was performed and patient subsequently underwent surgical stabilization with modified Magerl's technique and C5-C6 lateral masses plating. Fourthy-five days after surgery, the patient was able to walk alone without any help.

Commentary: Anterior atlantoaxial transarticular screws: should this be the preferred atlantoaxial fixation technique?

Commentary on Xu H, Chi YL, Wang XY, et al. Comparison of the anatomic risk for vertebral artery injury associated with percutaneous atlantoaxial anterior and posterior transarticular screws. Spine J 2012;12:656–62 ( in this issue ).

Dominant vertebral artery injury during posterior atlantoaxial transarticular screw fixation in a juvenile rheumatoid arthritis patient with atlantoaxial subluxation.

Many authors have reported on iatrogenic vertebral artery (VA) injury, but, to our knowledge, this is the first report of a dominant VA injury with compensatory blood flow from the hypoplastic VA. A 23-year-old woman with juvenile rheumatoid arthritis and atlantoaxial subluxation sustained injury to her dominant VA after occipitocervical fusion using transarticular screws. This did not result in lethal consequences due to compensation from her hypoplastic contralateral VA. Postoperative angiography, however, illustrated occlusion of the dominant left side, while the hypoplastic VA of the right side was enlarged. The patient experienced vertigo and loss of consciousness several times during rehabilitation. At the 4-year follow-up exam, bony fusion was observed, with no neurological deficits or correction loss. She had had no episodes of unconsciousness and no recurrence of any symptoms over the previous 3 years.

Study of the Anatomical Variations of Vertebral Artery in C2 Vertebra With Magnetic Resonance Imaging and Its Application in the C1–C2 Transarticular Screw Fixation

Use of magnetic resonance imaging (MRI) with Constructive Interference in Steady State (CISS) sequence and isometric voxels to demonstrate the anatomic variations of vertebral artery in C2 vertebra. To determine the transarticular screw trajectory on CISS MRI and to identify patients with anatomic variations of vertebral artery in C2 vertebra. Atlantoaxial transarticular screw fixation has been reported to be biomechanically superior to other posterior techniques for atlantoaxial arthrodesis. Vertebral artery injury can be associated with catastrophic sequelae. Anatomic variation of vertebral artery is well recognized and computed tomography scan is the traditional preoperative assessment. However, no report has evaluated the use of MRI in preoperative assessment for the screw trajectories and the anatomic variation of vertebral artery. The 3-dimensional (3D) CISS MRI with isometric voxels was performed in 30 local Chinese patients. The 3D reconstruction images were created to determine the proposed screw trajectories and their relationship with the vertebral arteries. In 12 patients (40%), the vertebral arteries were lying within the screw trajectories prohibiting transarticular screw fixation on at least one side. Bilateral variations with high risk of vertebral artery injuries were found in 6 patients. The remaining 6 patients had unilateral variations prohibiting the insertion of transarticular screws on one side. The 3D CISS MRI with isometric voxels is a safe and simple imaging technique to outline the vertebral arteries in C2. Reconstruction images are easily created and undistorted. It is one of the useful imaging in preoperative planning of transarticular screw fixation and determination of anatomy of vertebral artery.

173. Unilateral Posterior Atlantoaxial Transarticular Screw Fixation in Patients with Atlantoaxial Instability: Comparison with Bilateral Method

BACKGROUND CONTEXT: Simultaneous bilateral posterior transarticular screw fixation (TAF) and bone grafting with wire loop provides immediate, rigid internal fixation for atlantoaxial instability (AAI). It is the most effective biomechanical technique to attain C1-2 stability, and is superior to conventional wiring methods by virtue of more rigid fixation. Solid atlantoaxial fusion has been reported in 80 to 100% of patients who were treated with this method. However, bilateral TAF across the C1-2 may be contraindicated or impossible in up to 20% of patients because of its structural disfigurements such as anomalous course of the vertebral artery (VA), comminuted fractures of C-1 or C-2, or other pathological processes.

Posterior atlantoaxial dislocation without fracture and neurologic deficit: a case report and the review of literature

Traumatic posterior atlantoaxial dislocation without related fracture of the odontoid process is very rare, and only ten cases have been previously reported. The objective of this paper was to describe a case of traumatic posterior atlantoaxial dislocation without related fracture of the odontoid process, and its management with atlantoaxial transarticular screw fixation and bony fusion through an anterior retropharyngeal approach, and to review the relevant literature. The patient's medical and radiographic history is reviewed as well as the relevant medical literature. Posterior atlantoaxial dislocation was confirmed in a 48-year-old male struck by an automobile through conventional radiography, computed tomography and magnetic resonance imaging. No related fracture of the odontoid process or neurological deficit was found in this patient. Transarticular screw fixation of the atlantoaxial articulation through anterior retropharyngeal approach was performed after several unsuccessful attempts of closed reduction. At the latest follow-up, the lateral cervical spine radiography in flexion and extension demonstrated no instability of the atlantoaxial complex 21 months after the operation. In conclusion, patients with posterior atlantoaxial dislocation without fracture may survive with few or no-long term neurological deficit. Routine CT and MRI of the cervical spine should be carried out in patients with head or neck trauma to prevent missing of this rare clinical entity. Transarticular screw fixation of the atlantoaxial articulation through anterior retropharyngeal approach is safe and useful in case the management of dislocation is unsuccessful under closed reduction.