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Journal of Craniovertebral Junction and Spine
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Year : 2016  |  Volume : 7  |  Issue : 2  |  Page : 75-81  

"Two-step" technique with OsiriXTM to evaluate feasibility of C2 pedicle for surgical fixation

Department of Neurosurgery, Egas Moniz Hospital, Lisbon, Portugal

Date of Web Publication5-May-2016

Correspondence Address:
Luis Miguel Sousa Marques
Department of Neurosurgery, Egas Moniz Hospital, Rua da Junqueira, Nș126, Lisbon - 1349-019
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0974-8237.181826

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Background: Surgical treatment of craniovertebral junction pathology has evolved considerably in recent decades with the implementation of short atlanto-axial fixation techniques, notwhithstanding increasing neurovascular risks. Also, there is strong evidence that fixation of C2 anatomical pedicle has the best biomechanical profile of the entire cervical spine. However, it is often difficult and misleading, to evaluate anatomical bony and vascular anomalies using the three orthogonal planes (axial, coronal, and sagittal) of CT. Objectives: The authors describe an innovative and simple technique to evaluate the feasibility of C2 pedicle for surgical screw fixation using preoperative planning with the free DICOM (Digital Imaging and Communications in Medicine) software OsiriX TM . Materials and Methods: The authors report the applicatin of this novel technique in 5 cases (3 traumatic, 1 Os Odontoideum, and 1 complex congenital malformation) collected from our general case series of the Department in the last 5 years. Results: In this "proof of concept" study, the pre-operative analysis with the "two-step" tecnique was detrimental for choosing the surgical tecnique. Detailed post-operative analysis confirmed correct position of C2 screws without cortical breach. There were no complications or mortality reported. Conclusion: This "two-step" technique is an easy and reliable way to determine the feasibility of C2 pedicle for surgical fixation. The detailed tridimensional radiological preoperative evaluation of craniovertebral junction anatomy is critical to the sucess and safety of this surgeries, and can avoid, to certain degree, expensive intra-operative tridimensional imaging facilities.

Keywords: C2 pedicle, multi-planar reformatting (MPR), OsiriX TM

How to cite this article:
Marques LM, d Almeida GN, Cabral J. "Two-step" technique with OsiriXTM to evaluate feasibility of C2 pedicle for surgical fixation. J Craniovert Jun Spine 2016;7:75-81

How to cite this URL:
Marques LM, d Almeida GN, Cabral J. "Two-step" technique with OsiriXTM to evaluate feasibility of C2 pedicle for surgical fixation. J Craniovert Jun Spine [serial online] 2016 [cited 2023 Mar 21];7:75-81. Available from:

   Introduction Top

In the last decades, the development of surgical techniques, such as posterior atlanto-axial short fixations, [1] have gradually replaced the occipital-cervical (subaxial) long fixations. Moreover, there is strong evidence that fixation of C2 vertebra, and particularly its anatomical pedicle, has the best biomechanical profile of the entire cervical spine, either in short (C1-C2) or longer constructions (C0-C1-C2 or C2-subaxial). [2],[3] However, quite often, it is difficult, and sometimes misleading, to evaluate anatomical bony variations (i.e., C2 pars or pedicle morphology) and vascular anomalies [i.e., high riding vertebral artery (VA)] using only the conventional three orthogonal planes of CT (axial, coronal, and sagittal). [4] For example, in [Figure 1] (a type II odontoid fracture in a 92-year-old patient [case 2]), only by changing the gantry angle of the axial plane (from the plane of the inter-vertebral disc to the possible pedicle angle) is enough to clarify the position of VA. Additionally, frequently the "true" plane is biased either because of suboptimal acquisition technique, or because of patient-related factors, such as scoliosis, rotation, or dislocation of vertebral bodies.
Figure 1: Effect of variation of gantry angle on perception of highriding vertebral artery: upper pictures depicts axial plane parallel to vertebral disc with bilateral high-riding vertebral artery; below, just by tilting to the interpolation angle (parallel to pedicle axis) it is clear the change in perception on anomalous vertebral artery

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There is also debate related to the best intraoperative imaging facility, ranging from free-hand technique to three-dimensional (3D) fluoroscopy or intraoperative computed tomography (CT). [5],[6],[7]

   Materials and methods Top

The authors propose a new and easy technique to rapidly evaluate the feasibility to surgical screwing of C2 pedicle using the multi-planar reformatting (MPR) function of OsiriX TM , a free-source DICOM ("Digital Imaging and Communications in Medicine") software. We use this function of OsiriX TM (version 5.5.2, Pixmeo, Geneva, Switzerland) to determine the anatomical variations (i.e., C2 pedicle/VA route) of this region in only two simple steps and additionally to calculate the position, length and angle of screws, as others have described. [4]

All patient had a thin cut cervical spine CT (1.25mm) either noncontrast or CT angiography (CTA), that was loaded in the DICOM software. Using the MPR tool, we perform the first step of reconstruction to the true axial and sagittal plane of C2 vertebrae, aiming to correct possible deformity or inaccurate acquisition technique. In a second step of reconstruction, we try to find the true anatomical pedicle of C2, with the "parasagittal" (yellow in figures) and "paraaxial" (purple in figures) planes showing the longitudinal axis of the "true" pedicle and the "paracoronal" (blue in figures) plane showing the cross-section of the pedicle. The narrowest point of this plane is the most important value to judge if the pedicle is feasible for screwing.

Usually, we prefer to have a margin, of at least 1 mm in each side, which means that for a 3.5 or 4.0 mm screw we ought to have a minimum of 5.5 or 6.0 mm pedicle, respectively.

Additionally it is possible to calculate the lateral-to-medial and caudal-cranial angles, as well as the entry point from the midline and the possible length of the screw, as adapted from others [Figure 2]. [8]
Figure 2: Exemplification of measurements possible after the "two-step" reconstruction technique: Screw length, entry point, lateral-tomedial angle and caudal-cranial angle

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The authors exemplify this technique in five cases out of the general series of craniovertebral junction (CVJ) pathology operated in the last 5 years in our department. All patients in our series were operated with 2D fluoroscopy.

Case 1

A 86-year-old female with a C2 body fracture (coronal split) sustained after ground-level fall. The initial CT without reconstruction (first column to the left in the picture) reveals a probable high-riding left VA with narrow pedicle. However, it is possible to see that there is a huge bias because of the incorrect position of the axes of gantry planes. After the first step of reconstruction [to the true axial (purple in [Figure 3]) and sagittal planes (yellow)] the pedicle seems favorable. After the second step of MPR (correction for the true pedicle axis), we found the pedicle to be reasonable (7.1 × 8.6 mm) for a pedicle screw (blue). The patient was treated with C1-C2 posterior fixation (Goel's technique); [9],[10] the last column shows the postoperative scan with adequate position of the screws [Figure 3].
Figure 3: Case 1, C2 coronal split of C2: Effect of 3D reconstruction (see text for details)

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Case 2

A 92-year-old male with a type II odontoid fracture from low-energy trauma (hyperextension mechanism). The patient had no neurologic deficits and the dynamic x-ray and CT showed atlantoaxial instability. In first column to the left, the CT without any reconstruction depicting a probable high-riding left VA with narrow pedicle. However, after two steps of MPR we found the pedicle to be reasonable (7.9 × 6.4 mm) for a pedicle screw. This patient was treated with C1-C2 posterior fixation (Goel's technique); the last column shows the postoperative scan. The postoperative period was uneventful, without any morbidity [Figure 4].
Figure 4: Case 2, type II odontoid fracture: Effect of 3-D reconstruction in evaluation of vertebral artery anatomy (see text for details)

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Case 3

A 41-year-old male with a late diagnosis of an Os Odontoideum with severe atlanto-axial instability, due to persistent mechanical axial cervical pain without neurologic deficit, but with Lhermitte's sign and constant episode of cervical "blockage." In the first column to the left, the CT without any reconstruction depicting a probable high-riding left VA (even with a reasonable "orthogonal" gantry plane); after two steps of MPR we found the pedicle to be favorable (7.6 × 8.6 mm) for a pedicle screw, although with a mild high-riding vertebral on left; This patient was treated with C1-C2 (pedicle bilaterally) posterior fixation (Goel's technique), but on the left, purposely, it was done a more convergent (lateral-to-medial) and superior (caudal-cranial) trajectory. The last column on the right shows the postoperative scan with a very good result, namely the left pedicle screw without cortical breach very close to the prominent VA (arrow on the picture) [Figure 5].
Figure 5: Case 3, Os Odontoideum: "Two-step" 3-D reconstruction (see text for details)

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Case 4

A 63-year-old male presented with a diagnosis of an acute shallow type III odontoid fracture with dislocation, due to high-energy trauma (fall from height). The first CT revealed probable bilateral high-riding vertebral arteries. However the two steps 3D reconstruction confirms adequate pedicle for screwing (8.2 × 6.2 mm on the left and 7.9 × 6.5 mm on the right). The patient was treated with a C1-C2 posterior fixation (pedicle bilaterally), with excellent result, with normal alignment of the fracture and correct position of the screws. Clinically, the patient had no neurological deficit and no complications [Figure 6].
Figure 6: Case 4, shallow type III fracture: Effect of 3-D reconstruction (see text for details)

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Case 5

In the last case, we report a 62-year-old female with a late diagnosis of a very rare CVJ malformation with atlantoaxial instability due to bipartite atlas (fusion defect of C1 anterior and posterior arches [11],[12] ) and an odontoid hypoplasia. [13] The patient had a long-term mild tetraparesis that began to deteriorate one and a half years prior to referral to our department. Again, the first CT depicts a probable high-riding left VA, but after the first step of reconstruction it seemed to be favorable for screwing. However, after the second step the pedicle was found to be in the borderline zone (5.3 × 5 mm) for a pedicle screw and it was decided to perform crossed translaminar technique in C2 [14],[15] associated with a standard lateral mass screw in C1 (last column the postoperative scan). The patient reported an impressive neurologic recovery, without postoperative morbidity [Figure 7].
Figure 7: Case 5, rare CVJ malformation (bifi d atlas and odontoid hypoplasia): Effect of 3D reconstruction (see text for details)

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   Results Top

The authors present this "proof of concept" study based on their general case series of CVJ pathology treated in the department in the last 5 years. In all 5 cases presented in this paper, the first pre-operative analysis showed anatomical variations precluding safe pedicle screwing (high-riding vertebral artery, for example). However the 3-D multiplanar reconstruction was able to correctly determine the exact measurements of the true anatomical pedicle of C2 vertebra, that was within the safe limits for surgical fixation in 4 cases. Only in one case, due to borderline dimensions of the pedicle (5,3x5mm), it was considered safer to perform an alternative technique (translaminar crossed screwing). Therefore, using the pre-operative analysis with the "two-step" reconstruction was detrimental for choosing the surgical technique for posterior C2 screwing in every case. Detailed post-operative analysis with "thin-cut" CT confirmed correct position of C2 screws without cortical breach or vascular complications. There was no other complications or mortality reported.

   Discussion Top

The short posterior cervical fixation techniques (such as Goel's technique) presents potential increased risks of neurovascular complications, but shows high effectiveness in the consolidation of fusion, maintaining great part of cervical range of motion (theoretically reducing around 50% of cervical rotation, but maintaining flexion and lateral bending). There is also some evidence that fixation of C2 vertebra and particularly its anatomical pedicle is biomechanically the strongest screwing modality in C2 vertebra. The incorrect acquisition of the CT scan or the proper deformity of each patient renders very difficult and misleading to judge feasibility of this surgical technique only by the original orthogonal three planes.

One critical step is to obtain a preoperative thin-cut CT, as "thicker" CT scans may hinder the reconstruction image quality and therefore impar correct reading of measurements. There is also some debate whether to perform CTA in every patient to study the anatomy of this region. As shown by Tomasino et al., [16] there is enormous variation in VA occupancy in transverse foramen throughout entire cervical spine. At the same time, O'Donnell et al. [17] evaluated 975 patients and found only 0.42% with extraosseous VA anomalies. Therefore, we agree with the opinion that there is no recommendation for routine preoperative CTA in upper cervical spine surgery. Either because of relatively rare extraosseous VA anomalies or because the C2 pedicle screw technique in purely intraosseous, meaning that, avoiding cortical breach at any mean, will prevent VA injury, independent of the real intraosseous trajectory. In our department, in general, we order a CTA only in certain cases such as C2 body fracture reaching transverse foramen, (with potential VA stenosis or dissection) or in some congenital CVJ malformations in syndromic patients (i.e., Down's syndrome), [18] or inflammatory conditions, both with evidence of increased number of VA anomalies.

This simple and practical "two-step" reconstruction technique described by the authors, allows for a rapid determination of the narrowest point of pedicle of C2. This is, in our experience, the main factor to determine the feasibility for preforming C2 pedicle screwing with safety and effectiveness. Furthermore, after obtaining the "true" anatomical pedicle, it is possible to quickly measure the correct angle and entry point for each screw, as this may vary from patient to patient. Also, there is no consensus in the cutoff size that enables safe C2 pedicle screwing. Burke et al.[19] report an analysis on 47 patients (94 samples) in which they measure the length and width of each C2 pedicle with OsiriX TM . They found an overall average width of C2 pedicle of 8.272 +/-1.364 mm and, with the same criteria of 1 mm free for each side, they calculate that 98% of pedicles tolerate a 3.5 mm screw and 97% a 4.0 mm screw.

   Conclusion Top

The authors propose a simple and effective "two-step" technique to access the feasibility of C2 pedicle for surgical screwing. The preoperative detailed study with OsiriX TM is, in our experience, crucial to study the vascular and osseous variations of CVJ and especially to study the feasibility to screw the true anatomical pedicle of C2 and, therefore, can change the approach or surgical technique. Additionally it provides valuable information concerning the angles and trajectories (lateral to medial and caudal to cranial, as well as proper length and correct entry point).

This technique is, in author's opinion, an invaluable tool to improve safety and effectiveness in complex CVJ surgery. It also proves that, with detailed tridimensional radiological evaluation of this complex anatomical region, this type of surgeries can be performed even without advanced intraoperative technology, such as 3-D neuronavigation, only available in a limited number of departments.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

Huang DG, Hao DJ, He BR, Wu QN, Liu TJ, Wang XD, et al. Posterior atlantoaxial fixation: A review of all techniques. Spine J 2015;15:2271-81.  Back to cited text no. 1
Lapsiwala S, Anderson P, Oza A, Resnick DK. Biomechanical comparison of four C1 to C2 rigid fixative techniques: Anterior transarticular, posterior transarticular, C1 to C2 pedicle, and C1 to C2 intralaminar screws. Neurosurgery 2006;58:516-21.  Back to cited text no. 2
Du JY, Aichmair A, Kueper J, Wright T, Lebl DR. Biomechanical analysis of screw constructs for atlantoaxial fixation in cadavers: A systematic review and meta-analysis. J Neurosurg Spine 2015;22:151-61.  Back to cited text no. 3
Ould-Slimane M, Le Pape S, Leroux J, Foulongne E, Damade C, Dujardin F, et al. CT analysis of C2 pedicles morphology and considerations of useful parameters for screwing. Surg Radiol Anat 2014;36:537-42.  Back to cited text no. 4
Gelalis ID, Paschos NK, Pakos EE, Politis AN, Arnaoutoglou CM, Karageorgos AC, et al. Accuracy of pedicle screw placement: A systematic review of prospective in vivo studies comparing free hand, fluoroscopy guidance and navigation techniques. Eur Spine J 2012;21:247-55.  Back to cited text no. 5
Sciubba DM, Noggle JC, Vellimana AK, Alosh H, McGirt MJ, Gokaslan ZL, et al. Radiographic and clinical evaluation of free-hand placement of C-2 pedicle screws. Clinical article. J Neurosurg Spine 2009;11:15-22.  Back to cited text no. 6
Liu G, Buchowski JM, Shen H, Yeom JS, Riew KD. The feasibility of microscope-assisted "free-hand" C1 lateral mass screw insertion without fluoroscopy. Spine (Phila Pa 1976) 2008;33:1042-9.  Back to cited text no. 7
Chin KR, Mills MV, Seale J, Cumming V. Ideal starting point and trajectory for C2 pedicle screw placement: A 3D computed tomography analysis using perioperative measurements. Spine J 2014;14:615-8.  Back to cited text no. 8
Goel A, Laheri V. Plate and screw fixation for atlanto-axial subluxation. Acta Neurochir (Wien) 1994;129:47-53.  Back to cited text no. 9
Harms J, Melcher RP. Posterior C1-C2 fusion with polyaxial screws and rod fixation. Spine (Phila Pa 1976) 2001;26:2467-71.  Back to cited text no. 10
Goel A, Nadkarni T, Shah A, Ramdasi R, Patni N. Bifid anterior and posterior arches of Atlas: Surgical implication and analysis of 70 cases. Neurosurgery 2015;77:296-306.  Back to cited text no. 11
Hummel E, de Groot JC. Three cases of bipartition of the atlas. Spine J 2013;13:e1-5.  Back to cited text no. 12
Osti M, Philipp H, Meusburger B, Benedetto KP. Os odontoideum with bipartite atlas and segmental instability: A case report. Eur Spine J 2006;(Suppl 5):564-7.  Back to cited text no. 13
Wright NM. Posterior C2 fixation using bilateral, crossing C2 laminar screws: Case series and technical note. J Spinal Disord Tech 2004;17:158-62.  Back to cited text no. 14
Dorward IG, Wright NM. Seven years of experience with C2 translaminar screw fixation: Clincal series and review of the literature. Neurosurgery 2011;68:1491-9.  Back to cited text no. 15
Tomasino A, Parikh K, Koller H, Zink W, Tsiouris A, Steinberger J, et al. The vertebral artery and the cervical pedicle: Morphometric analysis of a critical neighborhood. J Neurosurg Spine 2010;13:52-60.  Back to cited text no. 16
O′Donnell CM, Child ZA, Nguyen Q, Anderson PA, Lee MJ. Vertebral artery anomalies at the craniovertebral junction in the US population. Spine (Phila Pa 1976) 2014;39:E1053-7.  Back to cited text no. 17
Yamazaki M, Okawa A, Hashimoto M, Aiba A, Someya Y, Koda M. Abnormal course of the vertebral artery at the craniovertebral junction in patients with Down syndrome visualized by three-dimensional CT angiography. Neuroradiology 2008;50:485-90.  Back to cited text no. 18
Burke LM, Yu WD, Ho A, Wagner T, O′Brien JR. Anatomical feasibility of C-2 pedicle screw fixation: The effect of variable angle interpolation of axial CT scans. J Neurosurg Spine 2013;18:564-7.  Back to cited text no. 19


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

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