C1 Screw Placement Research Articles

Types of high-riding vertebral artery: a classification system for preoperative planning of C2 instrumentation based on 908 potential screw insertion sites

BACKGROUND CONTEXTOur recent studies indicated that a high-riding vertebral artery (HRVA) is a common variant posing a risk of injuring the vessel during C2 instrumentation. However, several different types fit in the current definition of HRVA, which may require a different strategy for C2 screw placement. PURPOSETo classify HRVA and provide a clinical aid for preoperative planning of C2 instrumentation. A secondary goal was to estimate coexistence of each HRVA type with the ipsilateral narrow C2 pedicle (NP). STUDY DESIGNA retrospective observational study involving radiologic measurements of the estimated number of anonymized cervical computed tomography (CT) scans. STROBE checklist was applied. PATIENT SAMPLEA total of 908 potential screw insertion sites (PSIS) of 454 consecutive cervical CT scans were analyzed. The sample size was estimated using ScalaR SP function in RStudio. OUTCOME MEASURESThree types of HRVA based on a series of C2 vertebral morphological parameters including the C2 isthmus height (C2IsH) and C2 internal height (C2InH). Also, the prevalences of each HRVA type and coexisting NP based on the C2 pedicle width (C2PW). METHODSHRVA was defined as C2IsH of ≤5 mm and/or C2InH of ≤2 mm measured 3 mm lateral to the lateral border of the spinal canal. A narrow pedicle was defined as C2PW of ≤4 mm. Measurements were done using Syn.govia software. Interobserver, intraobserver, and inter-software agreement coefficients for C2IsH, C2InH, and C2PW parameters were adopted from our previous study. K-means cluster analysis was applied. RESULTSPrevalence of at least 1 HRVA was 24.9% (n=113 subjects) and 16.2% of PSIS (n=147 sites). Based on the measurements and K-means clustering, the following 3 types of HRVA have been distinguished: type 1—isthmic with only C2IsH being reduced and normal C2InH; type 2—internal with only C2InH being reduced and C2IsH within normal limits; type 3—isthmo-internal with both C2IsH and C2InH being reduced. Kruskal-Wallis test followed by unadjusted and Bonferroni-adjusted post-hoc multiple comparison analysis detected significant differences across the types. The prevalences of the newly identified types were as follows: 78.2%, 8.8%, and 12.9% for type 1, type 2, and type 3, respectively. 73.9% of type 1 HRVA, 53.8% of type 2 HRVA, and 100% of type 3 HRVA had a concomitant ipsilateral NP. Prediction of the HRVA types by the K-means clustering has been evaluated. Screw placement techniques for each type are proposed and discussed. CONCLUSIONWe present the first classification system for the high-riding vertebral artery distinguishing 3 types based on the large homogenous cohort, which may serve as an adjunct to preoperative planning of C2 instrumentation. External validation of this classification scheme shall determine its further clinical utility.

A novel technique for C1-C2 posterior screw insertion using patient-specific guides created by CT-based 3D printing.

C1-C2 fixation has been developed for the rigid fusion of atlantoaxial instability. C1 lateral mass screw (C1 LMS)-C2 pedicle screw fixation is used more frequently due to its rigid fixation and high bone fusion rate. However, C1 screw placement is relatively unsafe even with recently developed image-based navigation systems. Patient-specific screw guide templates (PSGT) were developed to improve the accuracy and safety of C1 screw placement. Herein, we investigated the outcomes of the C1-C2 posterior fixation technique using PSGT. This was a retrospective study of six patients who underwent posterior cervical spinal fusion using the PSGT between January 2022 and April 2023. Operative time, estimated blood loss, intraoperative radiation dose, surgical cost, and screw placement accuracy were evaluated and compared with those achieved with preoperative CT-based navigation (navigation group, n = 15). Screw accuracy was assessed using Neo's classification. PSGT showed good results, although the differences were not statistically significant (operation time: 104.3 ± 9.7 min vs 116.4 ± 20.8 min; estimated blood loss: 56.7 ± 72.4 mL vs 123.2 ± 162.3 mL; and radiation dose: 1.8 ± 1.2 mSv vs 2.6 ± 0.8 mSv, respectively). PSGT was particularly better in terms of the accuracy of C1 LMS (PSGT: 100%, navigation: 83.3%). The deviation at the entry point was minimal, and the difference between the sagittal and transversal angles from the preoperative plan was small. We investigated the clinical efficacy of using the PSGT for C1-C2 posterior fixation. PSGT improved the accuracy of C1 LMS insertion.

Anatomical Evaluation of the Atlas and Analysis of Accuracy for Freehand C1 Screw Placement

Objective: Atlanto-axial fusion surgery for instability is a recognized method. However, the complex anatomical structures of the atlas and axis make freehand C1 screw placement technically challenging. This study investigated the accuracy and safety of C1 screw placement without fluoroscopic or other guidance.Methods: A retrospective analysis of radiological data was performed on patients who underwent C1 instrumentation between April 2019 and January 2023, with all procedures performed by a spine surgeon with 4 years of experience. Computed tomography was used to evaluate cortical breaches during C1 screw placement. Breach severity was categorized based on the percentage of screw diameter beyond the cortical edge. Morphometric measurements of 30 healthy individuals and 22 patients who underwent surgery were obtained from preoperative CT scans of the bilateral pedicles at the C1 level. Coronal, axial, and sagittal CT reconstructions were examined to ascertain the distance from the screw entry point to the midpoint of the C1 posterior tubercle, diameter of the central canal of the atlas, screw convergence angle, height of the C1 posterior arch at the screw entry point, and width of the canal. Results: The surgeon placed 43 C1 screws in 22 consecutive patients. Seven (16%) breaches were identified, all of which were medial C1 screws. The breaches were classified as grades I, III, and IV in 4 (57%), 2 (29%), and 1 (14%) cases, respectively. No clinical complications arose owing to the breaches.Conclusion: The freehand technique for C1 screw placement without guidance yielded accurate and safe results. However, a preoperative morphometric assessment and careful familiarization with the unique anatomy are imperative for improving screw placement precision.

Different C2 screw placement techniques with mobilization of the vertebral artery in high-riding vertebral artery cases: Cadaver dissection.

In neurosurgery, posterior approaches intended at the craniovertebral junction are frequently used. The most popular procedures for treating upper cervical instability are C1 lateral mass, C2 pedicle, and C1-C2 transarticular screw stabilization. Due to their proximity to neural structures and the presence of the high-riding vertebral artery (VA), these techniques are complicated. The risk of VA damage can be decreased by mobilizing the VA. Using cadaveric specimens in this study was aimed to demonstrate C2 pedicle and C1-C2 transarticular screw placement with VA mobilization and a novel C2 inferior corpus screw placement technique. In this study, twelve adult cadaveric specimens and two adult dry cadaveric C2 bones were used with the permission and decision of the University Research Ethics Committee. Colored silicone was injected into the arteries and veins of these twelve cadaveric specimens. Then, muscle dissection was performed stepwise, and the C2 vertebrae of the cadavers were revealed with a surgical microscope. Each specimen and entire stages of the dissections were recorded photographically. After cadaver dissections, screw placement was performed with three different techniques. Finally, radiological imaging was done with fluoroscopy. After dissection, the lateral mass of the C2 vertebra was observed, and lateral to it, the transverse process and foramen were detected with the help of a hook. Next, the posterior wall of the VA groove was removed using a 1 mm thin plate Kerrison rongeur until the VA loop could partially be observed the VA. This enables us to find the top of the loop of the VA and mobilize it inferiorly using a dissector. Following this step, the C1-2 transarticular, C2 pedicle, and the novel C2 inferior corpus screw placement can be performed safely by directly visualizing the artery. Due to the nearby neurologic and vascular structures, placing the C2 pedicle and C1-2 transarticular screw is a challenging procedure, especially in high-riding VA cases. However, it is possible to place the C2 pedicle, C1-2 transarticular, and novel C2 inferior corpus screw after the mobilization of the VA. This study aimed to show all of them together on a cadaver for the first time, to understand the anatomy of the C2 vertebra, and to use screw placement techniques to minimize the risk of complications.

Clinical application of spinal robot in cervical spine surgery: safety and accuracy of posterior pedicle screw placement in comparison with conventional freehand methods

The novel robot-assisted (RA) technique has been utilized increasingly to improve the accuracy of cervical pedicle screw placement. Although the clinical application of the RA technique has been investigated in several case series and comparative studies, the superiority and safety of RA over conventional freehand (FH) methods remain controversial. Meanwhile, the intra-pedicular accuracy of the two methods has not been compared for patients with cervical traumatic conditions. This study aimed to compare the rate and risk factors of intra-pedicular accuracy of RA versus the conventional FH approach for posterior pedicle screw placement in cervical traumatic diseases. A total of 52 patients with cervical traumatic diseases who received cervical screw placement using RA (26 patients) and FH (26 patients) techniques were retrospectively included. The primary outcome was the intra-pedicular accuracy of cervical pedicle screw placement according to the Gertzbin-Robbins scale. Secondary outcome parameters included surgical time, intraoperative blood loss, postoperative drainage, postoperative hospital stay, and complications. Moreover, the risk factors that possibly affected intra-pedicular accuracy were assessed using univariate analyses. Out of 52 screws inserted using the RA method, 43 screws (82.7%) were classified as grade A, with the remaining 7 (13.5%) and 2 (3.8%) screws classified as grades B and C. In the FH cohort, 60.8% of the 79 screws were graded A, with the remaining screws graded B (21, 26.6%), C (8, 10.1%), and D (2, 2.5%). The RA technique showed a significantly higher rate of optimal intra-pedicular accuracy than the FH method (P = 0.008), but there was no significant difference between the two groups in terms of clinically acceptable accuracy (P = 0.161). Besides, the RA technique showed remarkably longer surgery time, less postoperative drainage, shorter postoperative hospital stay, and equivalent intraoperative blood loss and complications than the FH technique. Furthermore, the univariate analyses showed that severe obliquity of the lateral atlantoaxial joint in the coronal plane (P = 0.003) and shorter width of the lateral mass at the inferior margin of the posterior arch (P = 0.014) were risk factors related to the inaccuracy of C1 screw placement. The diagnosis of HRVA (P < 0.001), severe obliquity of the lateral atlantoaxial joint in the coronal plane (P < 0.001), short pedicle width (P < 0.001), and short pedicle height (P < 0.001) were risk factors related to the inaccuracy of C2 screw placement. RA cervical pedicle screw placement was associated with a higher rate of optimal intra-pedicular accuracy to the FH technique for patients with cervical traumatic conditions. The severe obliquity of the lateral atlantoaxial joint in the coronal plane independently contributed to high rates of the inaccuracy of C1 and C2 screw placements. RA pedicle screw placement is safe and useful for cervical traumatic surgery.

Ideal entry point and trajectory for C2 pedicle screw placement in basilar invagination patients with high-riding vertebral artery based on 3D computed tomography

BACKGROUNDC2 pedicle screw placement in patients with basilar invagination (BI) is fraught with risks because of a high incidence of anatomical variations and high-riding vertebral artery (HRVA). However, no study can be found in the literature that attempted to identify the ideal entry point and trajectory through the C2 pedicle in BI patients with HRVA. PURPOSETo investigate the parameters of ideal entry point and trajectory for C2 pedicle screw placement in BI patients with HRVA and compare them with those in BI patients without HRVA and patients without BI as control. These parameters would serve as a guide to pedicle screw placement. STUDY DESIGNA retrospective comparative study. PATIENT SAMPLEA total of 396 patients (198 consecutive BI patients and 198 matched patients without BI as control) and 792 unilateral pedicles from April 2017 to October 2021 at two medical centers were included. OUTCOME MEASURESThe insertion parameters of mediolateral angle, surface distance, cephalad angle, and vertical distance from the superior border of the lamina were the primary outcome measures for the reference of C2 pedicle screw placement. Furthermore, factors that affect the primary insertion parameters were assessed via multiple linear regression analyses. METHODAccording to the diagnosis of BI and HRVA, the unilateral pedicles were assigned into HRVA of BI, non-HRVA of BI, HRVA of control, and non-HRVA of control groups. Subgroup analyses based on Goel types A and B were also performed. Moreover, vertebral artery (VA) anomalies that might result in potentially serious complications were identified and systematically compared. RESULTSThe measurements of insertion parameters in BI patients with HRVA indicated a mean mediolateral angle of 27.42°, a mean cephalad angle of 43.02°, a mean surface distance of 9.74 mm, and a mean vertical distance from the superior border of the lamina of 3.85 mm. Compared with that in BI patients without HRVA, the measurements suggested that the entry point in BI patients with HRVA should be shifted upward by 0.38 mm and the trajectory should be angled cephalad by 6.05° and medially by 4.78°. In the control group, changes in the insertion parameters between HRVA and non-HRVA showed a similar trend to the BI group. Multiple linear regression showed that mediolateral angle was significantly associated with the male gender (B=−0.930, p=.017) and the diagnoses of HRVA (B=6.964, p<.001), Goel type A (B=−1.656, p=.003), and Goel type B (B=0.981, p=.030). Moreover, cephalad angle was significantly associated with the length of lateral mass (B=−0.319, p=.001) and the diagnoses of HRVA (B=3.254, p<.001) and Goel type A (B=6.924, p<.001). The VA anomalies were significantly higher in the BI group than in the control group. CONCLUSIONSThe insertion parameters of the ideal entry point and trajectory for C2 screw placement in BI patients with HRVA were remarkably different from those of non-HRVA of BI, HRVA of control, and non-HRVA of control cohorts. Preoperative 3D computed tomography (CT) and CT angiography are highly recommended in such patients to improve intraoperative safety and reduce postoperative complications.

Letter: C2 Pedicle Screws Combined With C1 Laminar Hooks for Reducible Atlantoaxial Dislocation: An Ideal Salvage Technique for C1-C2 Pedicle Screws.

To the Editor: We read the article by Han et al1 with great interest in which the authors describe an innovative option to salvage C1-C2 fusion using laminar hooks instead of pars screws. We take this opportunity to share a similar experience and agree that it is not always possible to place screws in C1 or C2. C1-C2 fusion using C1 lateral mass and C2 pars or pedicle screws is a safe modality for the reduction and fusion of abnormal C1-C2 alignment and instability, including cases of post-traumatic C1-C2 subluxation.2-4 We managed a case of a 4-yr-old child who suffered odontoid tip fracture with rotatory atlantoaxial dislocation (Figure 1) following a fall from a height and presenting with torticollis and quadriparesis. The patient presented to us 2 wk after the fall. A 3-dimensional (3D) computed tomography (CT) reconstruction revealed a thin C2 pars and lamina. Based on CT measurements, a high chance of cortical breach of C2 while placing 3.5-mm pars or pedicle screws was anticipated during preoperative planning. Hence, that laminar screws over C2 could be an option during surgery was kept as an alternative salvaging procedure. The subluxation reduced under traction as soon as bilateral C1-C2 joints were opened and further distracted using dissectors. Then C1-C2 joint surfaces were curetted, and bone chips from iliac crest were inserted. The placement of C2 pedicle, pars, or laminar screws is considered to be more technically demanding than C1 lateral screw placement.5 Often, it is the C2 screw placement that more commonly leads to vascular or neural injury.6 This is particularly true in the case of children in which the small dimension of C2 makes it even more difficult to place screws of diameter 3.5 mm, which is the commonly available size.FIGURE 1.: Preoperative imaging of the patient showing the odontoid fracture and atlantoaxial dislocation A in the sagittal view. In the 3D CT angiogram, the right lateral mass of C1 is not visible B from the posterior view, which is seen subluxated anteriorly in the anterior view C.C1 lateral mass screws were placed according to the technique described by Goel et al.7 Because C2 dimensions were very thin for 3.5-mm screws, laminar hooks were placed bilaterally over C2 and minirods placed between C1 lateral mass screws and C2 laminar hooks. Finally, the C1 and C2 laminae were decorticated and iliac crest graft placed to achieve posterior bony fusion as well. A repeat scan at 3 mo showed good decompression of the craniovertebral junction and cord along with the implant in Situ (Figure 2). The patient's myelopathy has improved and is now mobilized independently. Therefore, in cases in which certain factors preclude the use of C2 pars/pedicle or lamina screws, C2 laminar hooks can be a useful rescue adjunct.FIGURE 2.: Postoperative CT showing a good reduction achieved with restoration of canal space in the sagittal view A. The 3D color rendering on maximum intensity projection B shows the C1 lateral mass screws and C2 laminar hooks in Situ.Funding This study did not receive any funding or financial support. Disclosures The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.

Evaluation of the Accuracy of Free-hand C1 and C2 Screw Placement

Objective Posterior atlantoaxial fixation is an effective treatment for various pathologies. The anatomies of the atlas and axis are highly complex, and thus, freehand placement of C1, 2 instrumentation is technically challenging. The objective of this study was to evaluate the accuracy and safety of C1, 2 screw placement without fluoroscopic or any other guidance method by a 2-year spine surgeon. Methods A retrospective analysis of radiologic data was performed on all patients that underwent C1 or C2 instrumentation from April 2019 to January 2020; all procedures were performed by a 2-year spine surgeon. Computed tomography was used to assess cortical breaches of screw placement at C1 or C2. Severities of breaches were quantified using percentages screw diameter beyond cortical edges (0=cortical touch, 1≤25%; II=26-50%; III=51-75%; IV=76-100%). Results The surgeon placed 16 C1 screws and 20 C2 screws in 10 consecutively treated patients. For C1 screws, there were 4 breaches (25%), of which 3 were medial (18%) and 1 was anterior (7%), and for C2 screws, there were 3 total breaches (15%), of which 2 were anterior (10%) and 1 was medial (5%). Breaches were classified as grade I in 3 cases (42.8%), grade III in 1 case (14.4%), and grade IV in 3 cases (42.8%). No clinical sequelae were attributed to these breaches. Conclusion Although 25% of C1 screws and 15% of C2 screws placed using a freehand technique without guidance may be achieved accurately and safely. Preoperative morphometric evaluation and controlled exposure to this unique anatomy are essential to improve screw placement. Key Words: Accuracy of screws; Free hand technique; Malposition; Posterior atlantoaxial fixation; VA injury

Intraoperative imaging and navigation for C1-C2 posterior fusion.

Background:Cervical axial spine fusion is challenging as the anatomy is extremely variable, and screw misplacement can lead to severe complications. C1 lateral mass screws and C2 pedicle screws are routinely placed under either fluoroscopic guidance or imaging-assisted navigation. Here, we compared the two for axial screw placement.Methods:We retrospectively evaluated patients’ treated from 2011–2016 utilizing the Harm’s procedure for C1-C2 screw fixation performed under either fluoroscopic guidance (nine patients) or image-assisted O-arm navigation (five patients). The groups had similar demographic and risk factors. Variables studied included operative time, estimated blood loss (EBL), accuracy of screw placement, screw reposition rates, and reoperation rates.Results:The mean EBL was 555CC and 260CC, respectively (not a significant difference) utilizing fluoroscopic versus O-arm navigation. Of interest, the mean surgical duration was 27 min longer in the O-arm versus fluoroscopy group (P = 0.03). Ten C2 pedicle screws were performed using O-arm navigation. Alternatively, as 9 of 18 C2 pedicles were considered “risky” for the placement of fluoroscopic-guided pedicle screws, laminar screws were utilized. Although the accuracy rate of C1 and C2 screw placement was higher for the navigated group, this finding was not significant. Similarly, despite complications involving two unacceptably placed screws from the fluoroscopic guidance group, there were no neurological sequelae.Conclusion:Axial cervical spine instrumentation is challenging. Utilization of Imaging-assisted navigation increases the accuracy and safety of screw placement.

Surgicoanatomical aspect in vascular variations of the V3 segment of vertebral artery as a risk factor for C1 instrumentation

ObjectAwareness of vascular anomalies in V3 segment of vertebral artery (VA) is crucial to avoid iatrogenic injuries during surgical procedure. This study aimed to analyze the incidence of V3 segment vascular variations and demonstrate the importance of deciding the surgical strategy for C1 screw placement. MethodsPrevalence of vascular variations and morphometric measurements of the VA in the region of the craniocervical junction in 200 cases based on three-dimensional computed tomographic angiography (3D-CTA) scans were studied. ResultsThe VA has a variable course through C2 before it passes above its groove on the posterior arch of C1. Following the vascular variations of V3 segments of VA were persistent including first intersegmental artery (FIA), fenestration (FEN) of the VA, high-riding (HRVA and the posterior inferior cerebellar artery (PICA) branch originating from the C1/2 part of VA. HRVA was observed in 10.1% of patients, FIA in 1.8%, FEN in 1.3%, and PICA in 1.3%. One hundred and twenty-three (24.1%) patients were identified to have HRVA, 6% present on both sides. ConclusionThe VA with FIA and FEN were rare in this study as many as a 10% the VA present over the starting point for C1 lateral screw. With respect to the vascular anatomy of V3 and more frequent left-sided VA dominancy, standard screw insertion should be started from the right side. Routine preoperative 3D-CTA evaluation is mandatory to prevent the VA injury when C1-C2 instrumentation is planned.

Change of Anatomical Location of the Internal Carotid Artery Relative to the Atlas with Congenital Occipitalization and the Relevant Clinical Implications

The occipitalization of the atlas (OA) is always associated with multiplanar dislocation and olisthy of the C1 over C2 facets, which may change the anatomical relationship between the internal carotid artery (ICA) and the atlas. The purpose of this current study is to identify the location of the ICA relative to the anterior aspect of the atlas in patients with OA and define the clinical implications for screw placement. We retrospectively reviewed the computed tomography angiography data of 86 patients with OA and 86 control subjects. Several parameters were also measured to quantitatively evaluate the mutual relationship. In the OA group, 25.6% of ICAs were located in area 3 and 74.4% in area 2, whereas the percentages were 57.4% and 42.6%, respectively, in the control group. There were 73 (42.4%) ICAs in which the shortest distance between the dorsal surface of the ICA and the ventral cortex of the atlas was less than 4 mm in the OA group and only 50 (29.1%) in the control group. The ideal angulation of C1 screw trajectory was about 5 degrees more medial in the OA group than that in the control group (P < 0.01). The risk of ICA injury is much higher in OA patients than in non-OA patients during the C1 screw placement. A mean medial angulation about 20 degrees will permit a long and safe screw purchase, but should be individualized. We recommend careful preoperative computed tomography angiography evaluation in all patients before surgery.

C1 Titanium Cables Combined with C2 Pedicle Screw-Rod Fixation for Atlantoaxial Instability Not Suitable for Placement of C1 Screws

We sought to develop a new posterior fusion technique composed of bilateral C1 titanium cables and C2 pedicle screw-rods for treatment of atlantoaxial instability not suitable for C1 screw placement. A study was conducted of 18 patients with atlantoaxial instability who had C1 broken screw trajectory or anatomic anomalies. All patients underwent posterior fixation with bilateral C1 titanium cables and C2 pedicle screws. The follow-up period was a minimum 1 year. Clinical outcomes measurements included visual analog scale score for neck pain assessment, the American Spinal Injury Association Impairment Scale and Japanese Orthopaedic Association score for neurologic status and function. According to preoperative computed tomography (CT) reconstruction and CT angiography, the patients selected in this study were not suitable for C1 screw placement. Postoperative plain radiographs and CT reconstruction were performed to evaluate the reduction, bony fusion, and implant position. All outcomes were evaluated at each follow-up. The average clinical follow-up period was 24 months (range 12-36 months). All patients had complete neck pain relief at postoperative 6 months. Their neurologic symptoms had improved significantly at 1-year follow-up. Radiologic outcomes indicated good bony fusion and construction stability in all patients without implant failure at the last follow-up. No neural or vascular complications related to this technique were observed. Posterior atlantoaxial fixation using C1 titanium cables and C2 pedicle screw-rod construct appears to be an effective and safe technique for treatment of atlantoaxial instability, which could be an alternative method for cases unsuitable for C1 screw placement when using C1-C2 screw fixation.

When does intraoperative 3D-imaging play a role in transpedicular C2 screw placement?

IntroductionThe stabilization of an atlantoaxial (C1-C2) instability is demanding due to a complex atlantoaxial anatomy with proximity to the spinal cord, a variable run of the vertebral artery (VA) and narrow C2 pedicles. We perfomed the Goel & Harms fusion in combination with an intraoperative 3D imaging to ensure correct screw placement in the C2 pedicle. We hypothesized, that narrow C2 pedicles lead to a higher malposition rate of screws by perforation of the pedicle wall. The purpose of this study was to describe a certain pedicle size, under which the perforation rate rises. Patients and methodsIn this retrospective study, all patients (n=30) were operated in the Goel & Harms technique. The isthmus height and pedicle diameter of C2 were measured. The achieved screw position in C2 was evaluated according to Gertzbein & Robbin classification (GRGr). ResultsA statistically significant correlation was found between the pedicles size (isthmus height/pedicle diameter) and the achieved GRGr for the right (p=0.002/p=0.03) and left side (p=0.018/p=0.008). The ROC analysis yielded a Cut Off value for the pedicle size to distinguish between an intact or perforated pedicle wall (GRGr 1 or ≥2). The Cut-Off value was identified for the isthmus height (right 6.1mm, left 5.4mm) and for the pedicle diameter (6.6mm both sides). ConclusionThe hypothesis, that narrow pedicles lead to a higher perforation rate of the pedicle wall, can be accepted. Pedicles of <6.6mm turned out to be a risk factor for a perforation of the pedicle wall (GRGr 2 or higher). Intraoperative 3D imaging is a feasible tool to confirm optimal screw position, which becomes even more important in cases with thin pedicles. The rising risk of VA injury in these cases support the additional use of navigation.

Detethering of the C2 Nerve Root and Avoidance of Transection and Injury During C1 Screw Placement: A Cadaveric Feasibility Study

Regional neurovascular structures must be avoided during invasive spine hardware placement. During C1 lateral mass screw placement, the C2 nerve root is put in harm's way. Therefore the current anatomic study was performed to identify techniques that might avoid such neural injury. On 10 cadaveric sides, dissection was carried down to the craniocervical junction. The C2 nerve root was identified, and its distal branches were traced out into the surrounding posterior cervical musculature. Once dissected, the nerve was displaced inferiorly away from the lateral mass of C1. On all sides, the C2 nerve root could be easily detethered from surrounding tissues. On all sides, this allowed lateral mass screw placement without compression of the nerve. On the basis of our cadaveric study, the C2 nerve root can be detethered enough at the level of the posterior lateral mass of C1 to avoid its injury during screw placement into this area.

Risk factor analysis and decision-making of surgical strategy for V3 segment anomaly: significance of preoperative CT angiography for posterior C1 instrumentation

Background ContextAwareness of vascular anomalies is crucial to avoid iatrogenic injuries during surgical procedure. Although V3 segment anomaly has been well described, the incidence of V3 segment anomaly has been rather variable in the literature, and there are few reports regarding the adequate surgical strategy for each type of V3 segment anomaly. PurposeThis study aimed to analyze the incidence of V3 segment anomaly and demonstrate the importance of recognizing vertebral artery (VA) anomaly in deciding the surgical strategy for C1 screw placement. Study DesignA retrospective cohort study was carried out. Patient SampleThe sample included 147 patients who underwent C1 posterior instrumentation and preoperative three-dimensional computed tomography angiography (3D CTA). Outcome MeasuresThe primary outcome of this study was the incidence of V3 segment anomaly using preoperative CTA, and the secondary outcome was the risk factor analysis of the V3 segment anomaly. MethodsThere were 147 patients who underwent C1 posterior instrumentation to treat various kinds of upper cervical disease. The 3D CTA of the patients were assessed preoperatively to identify the anomaly of the VA. Each surgical technique of C1 posterior instrumentation was decided upon the shape and the course of the VA around the atlas. ResultsDuring the study period, 11 cases of V3 segment anomaly (7.5%) were found on 3D CTA. Persistent intersegmental artery was found in nine cases and was the most common variant of VA anomaly. Early branch of posterior inferior cerebellar artery was found in three cases. Most of V3 segment anomaly was found unilaterally, but there were two cases with bilateral V3 anomaly. Seven cases (63.6%) were associated with congenital bony abnormality around craniovertebral junction (CVJ), such as occipital assimilation, Klippel-Feil syndrome, and os odontoideum. V3 segment anomaly was significantly common in the cases with bony abnormality (29.2% (7/24) vs. 3.6% (4/123), p<.05). Compared with patient without bony abnormality, the odds ratio was 10.78 (95% CI: 2.88–40.37) for those with congenital bony abnormalities. Rheumatoid arthritis was not a risk factor of V3 segment anomaly (p=.391). ConclusionsThe course of the VA is heterogenous, and the V3 segment anomaly of the VA is more common in the cases with congenital bony abnormalities around CVJ. Therefore, preoperative radiological studies should be performed to identify V3 segment variations and reduce the risk of VA injury. To avoid significant morbidities associated with VA, surgical technique of C1 posterior instrumentation should be decided depending upon the V3 segment anomaly. A more optimal entry point and trajectory for C1 fixation can be selected.

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.

Impact of Starting Point and C2 Nerve Status on the Safety and Accuracy of C1 Lateral Mass Screws

Literature review and meta-analysis. To compare clinical and radiographic outcomes of patients treated with C1 lateral mass screws (C1LMS), analyzing the impact of screw starting point and C2 nerve sectioning on malposition, vertebral artery injury (VAI), and C2 neuralgia and numbness. Various starting points have been suggested for C1LMS insertion. Some advocate sectioning the C2 nerve root to ease placement. Online databases were searched for English language articles between 1994 and 2012 reporting on C1LMS. Forty-two studies describing 1471 patients instrumented with 2905 C1LMS met inclusion criteria. Three surgical techniques included posterior arch starting point and center of lateral mass with nerve root preservation or sacrifice. All studies provided class III evidence. Three injuries to the vertebral artery occurred secondary to C1LMS insertion (0.1%) and 5 instances of clinically significant screw malpositions (0.2%). Postoperative imaging revealed 45 malpositioned screws (1.6%) without clinical consequences. Meta-analysis techniques demonstrated that sacrifice of the C2 nerve root caused greater postoperative numbness but less neuralgia and fewer screw malpositions. Similar rates of screw malposition and VAI arose with posterior arch screws and those starting below the arch with C2 nerve preservation, but the latter had greater numbness and pain. A thorough understanding of atlantoaxial anatomy and modern surgical techniques renders the insertion of C1LMS safe and accurate. The incidence of clinically significant malpositioned screws or VAI is <0.5%. Sacrifice of the C2 nerve root did result in fewer malpositioned screws. Numbness occurred in 11% of patients, an outcome that may be unacceptable to certain patient populations, but neuropathic pain was nearly absent with nerve sectioning. C2 nerve preservation and retraction for C1 screw placement may have higher incidence of neuropathic pain. Posterior arch screws are a viable option for patients with arches that are of adequate height.

Posterior C2 Instrumentation in the Management of Trauma: Accuracy and Risks Associated with Four Techniques

Introduction Posterior instrumentation of C2 is important in treating occipitocervical, atlanto-axial, and subaxial spinal injuries. The variable C2 anatomy can make instrumentation challenging and prone to potentially severe and even life-threatening complications. The goal of this study was to (1) evaluate a large series of posterior C2 screws to determine accuracy as assessed by computed tomography (CT) scan, (2) assess dimensions of “safe bony windows” with CT, and (3) assess the perioperative complication rate related to errant screw placement. We hypothesize that the evolution of a greater variety of options for C2 fixation and the ability to tailor these techniques to patients' specific anatomy has resulted in decreased complications and increased accuracy. Material and Methods A retrospective review of a single Level I trauma center spine database was assessed to identify all patients with operative management for traumatic conditions with C2 instrumentation between December 2002 and September 2008. Radiographic analysis included evaluation of pre- and postoperative CT scans to quantify the patients' bony anatomy as well as to classify the accuracy of C2 screw placement. All C2 screws were graded using the following definitions: Type I: Screw threads completely within the bony cortex. Type II: Less than half the diameter of the screw violates the surrounding cortex. Type III: Clear violation of the transverse foramen or spinal canal. Results A total of 169 patients (99 males and 70 females) underwent posterior C2 screw fixation for traumatic conditions of the spine, including 23 atlanto-occipital dissociations, 16 unstable C1 ring injuries, 79 dens fractures, 9 Hangman fractures, 8 other C2 fractures, 3 C2 hyperextension injuries, 4 C2–C3 flexion–distraction injuries, 14 subaxial injuries, and 19 odontoid nonunions. Average CT measurements of pedicle height, axial width, and laminar width were 8.1 mm (standard deviation 2.1 mm), 5.8 mm (1.9 mm), and 5.7 mm (1.5 mm), respectively, with males having larger pedicle height ( p &lt; 0.001), pedicle width ( p &lt; 0.001), and laminar width ( p &lt; 0.022). A total of 324 screws underwent CT analysis and chart review. A total of 169 pedicle, 85 trans-articular, 50 laminar, and 20 short pars screws were placed with 80.6, 84.4, 96, and 94.4% accuracy rates (Grade I vs. II and III), respectively. Overall 85% of the screws were rated as being in the ideal location. There were no cases of known vertebral artery injury at the time of surgery. Two pedicle screws were unacceptably placed (Grade III). Both patients had screws encroaching on the vertebral artery foramen: one patient had a vertebral artery occlusion and the other had a Grade I dissection documented on CT angiogram without clinical signs of stroke or emboli as measured on transcranial Doppler. No neurological injuries resulted from instrumentation. Conclusion Our findings demonstrate a lower than previously reported incidence of complications associated with posterior C2 screw placement. In the management of 169 patients with 324 screws, only 2 patients had vascular injuries, neither of whom had any sequelae. The multiple techniques of posterior C2 fixation that are currently available allow for unprecedented flexibility in determining which technique is best suited for a given patient's anatomy.

The development and evaluation of individualized templates to assist transoral C2 articular mass or transpedicular screw placement in TARP-IV procedures: adult cadaver specimen study

OBJECTIVES:The transoral atlantoaxial reduction plate system treats irreducible atlantoaxial dislocation from transoral atlantoaxial reduction plate-I to transoral atlantoaxial reduction plate-III. However, this system has demonstrated problems associated with screw loosening, atlantoaxial fixation and concealed or manifest neurovascular injuries. This study sought to design a set of individualized templates to improve the accuracy of anterior C2 screw placement in the transoral atlantoaxial reduction plate-IV procedure.METHODS:A set of individualized templates was designed according to thin-slice computed tomography data obtained from 10 human cadavers. The templates contained cubic modules and drill guides to facilitate transoral atlantoaxial reduction plate positioning and anterior C2 screw placement. We performed 2 stages of cadaveric experiments with 2 cadavers in stage one and 8 in stage two. Finally, guided C2 screw placement was evaluated by reading postoperative computed tomography images and comparing the planned and inserted screw trajectories.RESULTS:There were two cortical breaching screws in stage one and three in stage two, but only the cortical breaching screws in stage one were ranked critical. In stage two, the planned entry points and the transverse angles of the anterior C2 screws could be simulated, whereas the declination angles could not be simulated due to intraoperative blockage of the drill bit and screwdriver by the upper teeth.CONCLUSIONS:It was feasible to use individualized templates to guide transoral C2 screw placement. Thus, these drill templates combined with transoral atlantoaxial reduction plate-IV, may improve the accuracy of transoral C2 screw placement and reduce related neurovascular complications.

Deviation analysis of atlantoaxial pedicle screws assisted by a drill template.

Although C1-C2 pedicle screw fixation provides an excellent fusion rate and rigid fixation, this technique has a potential risk. It is essential to develop an accurate screwing method to avoid this neurovascular injury. To develop and validate the accuracy of a novel navigational template for C1-C2 pedicle screw placement in cadaveric specimens, computed tomography scans with 1-mm-wide cuts were obtained of 32 cadaveric cervical specimens. The authors developed 64 three-dimensional full-scale templates that were created by computer modeling with a rapid prototyping technique from the computed tomography data. Drill templates were constructed with a custom trajectory for each level and side. The drill templates were used to guide the establishment of a pilot hole for screw placement. The average distances between ideal and actual entry points of the C1 pedicle screws in the x, y, and z axes were 0.16±0.46 mm, 0.11±0.52 mm, and -0.01±0.54 mm, respectively, on the left side and 0.11±0.49 mm, 0.01±0.56 mm, and -0.09±0.59 mm, respectively, on the right side. The average distances between ideal and actual entry points of the C2 pedicle screws in the x, y, and z axes were 0.05±0.54 mm, 0.20±0.59 mm, and -0.06±0.58 mm, respectively, on the left side and 0.17±0.55 mm, 0.1±0.58 mm, and -0.01±0.49 mm, respectively, on the right side. Factors related to human error and imprecision are responsible for most malpositioning of instrumentation. The rapid prototyping drill template for C1-C2 screw placement is described to minimize human error, although it introduces error related to computer software and variation in manufacturing.