C2 Vertebra: An Enigma for Young Spine/Neurosurgeons

Abstract

Background  Instrumentation of C2 vertebra is considered the most difficult for young neurosurgeons and trainees due to its complex anatomical structures, variety of surgical approaches and techniques, and proximity to important neurovascular structures. Key points from a surgical perspective for midline posterior approach is described in the era of neuroradiological advancements.

Method  Computed tomography angiographies (CTAs) of a total of 92 patients were evaluated with special attention to the key findings for insertion of screws for craniovertebral junction (CVJ) fixations. All these patients were operated though midline posterior approach in past 4 years.

Results  CTAs included various CVJ disorders, which included traumatic ( n  = 14), congenital ( n  = 55), and rheumatoid arthritis ( n  = 2) patients. Established landmarks for screw insertion sites do not prove safe for congenital anomalous CVJ conditions. Instead of highlighting screw insertion entry points, part of the corridor, which is relevant, should be stressed up on.

Conclusion  Midpoint of portion of bone segment medial to vertebral artery foramen should be the focus, which is important for pars interarticularis (and transarticular) and pedicle screws. A laminar screw should cross the midpoint of the lamina on each side.

Introduction

The C2 vertebral body (VB) is the most important vertebra in craniovertebral junction (CVJ) regarding surgical alignment, fusion, and fixation through the midline posterior approach. It is the one element incorporated in all CVJ surgeries for diseases leading to or due to unstable CVJ. CVJ surgery is considered difficult due to its complex anatomy and its proximity to important neurovascular structures, the most feared is, injury to the vertebral arteries. ^{1 2 3 4 5 6} It leads to lots of referrals to tertiary care hospitals in India from peripheral hospitals. In the beginning of this century, Goel et al changed the philosophy of CVJ leading to shift of focus from midline structures (basion, tip of dens, atlantodental interval, opisthion, etc.) to the sides to C1–2 joint. ^{1 2 3 4 5} Current management strategies are based on this philosophy and C1–2 facet joint alignment and fusion are the mainstay of the treatment. ^{7 8 9 10}

Authors have been using digital software to plan CVJ surgeries using three-dimensional (3D) images of computed tomography angiography (CTA) for all CVJ surgeries and report their findings to revisit screw insertion into C2, especially in anomalous CVJ. ^{11 12} This study may help increase access to surgical treatments for patients suffering from diseases due to traumatic and congenital CVJ instabilities.

Materials and Methods

CTA records of patients who were operated for CVJ in past 4 years (2018–2023), who were subjected to CVJ surgery through midline posterior approach, were analyzed to evaluate morphologies of C2 vertebra. Technique of using the software and its application have CVJ have been described in our earlier reports. ^{11 12} Deviations from the normal anatomy, which pose difficulty in safe C2 pedicle and pars interarticularis (PI) screw insertions, especially for the young neurosurgeons, were noted. Anatomical features of C2 were examined using 3D multiplanar reformation (MPR) and 3D volume rendering technique (VRT) images of CTA to specifically to see pedicle, PI, and laminae of both the sides and their relation to the vertebral arteries for safe screw insertion. Special points, with regard to safety measures against injury to vertebral arteries, were noted.

Results

A total of 92 CTA studies were available for the analysis, which included traumatic Atlantoaxial dislocation (AAD) ( n  = 14), basilar invagination (BI) with or without Chiari malformation (CM) and/or platybasia ( n  = 47), os–odontoideum (Os–Od) ( n  = 8), rheumatoid arthritis with AAD ( n  = 2) and CM ( n  = 21; Table 1 ). C2 morphological (in 3D VRT) and anatomical features (in 3D MPR) of both the sides were examined. Observations were noted in relation to the normal anatomy ( Fig. 1A ) of C2 and were as follows:

Table 1. Types of craniovertebral junction conditions based of computed tomography angiography.

S. no.	Diagnosis	Number
1.	Traumatic	14
2.	BI	47
3.	Os–Od	8
4.	RA + AAD	2
5.	CM	21
Total		92

Abbreviations: AAD, atlantoaxial dislocation; BI, basilar invagination; CM, Chiari malformation; Os–Od, os–odontoideum; RA, rheumatoid arthritis.

Fig. 1.

Normal C2 ( A ) and various landmarks marked simulating concepts of rooms (yellow areas) and connecting corridors (red for lamina, green for PI, and purple for pedicle on both the sides) ( B ). C2 of where both SF are dislocated completely behind the C1 lateral masses ( C ) and over it markings of various areas designating rooms (yellow) and corridors in red, green, and purple for corridors for lamina, PI, and pedicle, respectively ( D ). PI, pars interarticularis; SF, superior facet.

• C2 had normal anatomy in all traumatic cases and Os–Od cases, except separation of dens from C2 vertebra in cases of Os–Od.

• C2 was anomalous asymmetrically in all patients with BI.

• In congenitally anomalous CVJ with BI, there was rotational dislocation of C1–2 in addition to posterior dislocation of C2 along with cranial migration (telescoping) leading to BI.

• Complete unilateral and bilateral ( Fig. 1B ) facet dislocation (C2 superior facet [SF] dislocated posterior to C1 inferior facet [IF]) was observed in 12 patients and 3 patients respectively, where it was lying against part of the occiput just posterior to the ipsilateral fused C1 lateral mass-occipital condyle. Identification of superior surface of superior C2 facets in these cases was difficult to identify due to change in the articular surfaces due to long-standing dislocations ( Fig. 1C ).

• Small segment of C2, which joins to the ipsilateral pedicle with PI was very thin (less than 5.5 mm, which is the minimum width to accommodate 3.5-mm screw) due to the course of the vertebral artery in 17 cases bilaterally and in 26 cases unilaterally ( Fig. 1A, 1C ).

• Vertebral artery was over the posterior surface of C1–2 joint bilaterally in 7 patients and on one side only ( Fig. 1B ) in 12 patients. All these were cases of BI where C1 was occipitalized.

• C1 was occipitalized in all patients with BI and Arnold Chiari Malformation (ACM).

• C2 had morphological features like C1 in two patients; out of which, one had posterior arch like C1 and another had no posterior arch. C3 spines of both the cases looked like that of C2 ( Fig. 2 ).

• Part of PI between medial margin of exit foramen of vertebral artery over C2 lateral mass and medial margin of lamina was the main corridor, which communicated the parts above (pedicle and SF) and below (lamina and IF). This part continued anteroinferiorly as pedicle ( Fig. 1A–D ).

• Superomedial margin of this part of the PI continued anterosuperiorly along with the pedicle and joined at the base of the dens ( Fig. 1 ).

Fig. 2.

Three dimesional CT VRT of CVJ left lateral (A), posterior (B) and anterior (C) views of anomalous C2 showing its posterior arch like C1 and C3 having posterior arch like C2. In another case, 3D CT VRT of CVJ posterior (D), edited posterior view with removed posterior parts of foramen magnum and occiput (E) and left anteriolateral oblique (F) views show another C2 with absent posterior arch with C3 like C2.

Overall, we can consider C2, as the depicted in Fig. 3 ( Fig. 1B, D ):

Fig. 3.

 C2 showing trajectories for pedicle ( A ), PI ( B ) and laminar ( C ) screws where CP is shown as red, blue, and yellow stars as CP of pedicle, PI and laminar screws. Trajectory for any particular screw may be more than one but it must cross through CP for that particular screw. CP, central point; PI, pars interarticularis.

• − Two midline rooms, spinous process (SP) and VB (yellow encircled areas in midline in Fig. 1B, D )

• − Two lateral rooms, SF and IF (yellow encircled areas on both the sides of Fig. 1B, D )

• − And three corridors on each side: PI, pedicle, and lamina (purple, green, and red areas on both the sides of Fig. 1B, D ).

Table 2 and Fig. 3 show starting point, central point (CP) in the center of the corridor to be crossed and end point where it should reach. Any of the three types of screws (laminar, PI, or pedicle) starts from either SP, IF, or PI to cross through lamina, PI, or pedicle, respectively. The laminar screw should start from SP, cross through CP of lamina (yellow star in Fig. 3 ) and should reach up to or just short of IF. Similarly, pars screw should start from IF or lamina, should cross through CP of PI (blue star in Fig. 3 ), and should reach up to SF or cross through SF to C1 lateral mass for a transarticular screw. A pedicle screw should enter PI, should cross CP of the pedicle (red star in Fig. 3 ), and should reach VB.

Table 2. Course and trajectories of various C2 screw insertions for fixation ( Fig. 4 ) .

S. no.	Screw type	Starting room	Corridor	End point/target room
1.	Laminar screw	SP	Lamina	IF
2.	PI screw or transarticular screw	IF	PI	SF
3.	Pedicle screw	PI	Pd	VB

Abbreviations: IF, inferior facet of C2; Pd, pedicle; PI, pars interarticularis; SF, superior facet; SP, spinous process; VB, vertebral body.

Discussion

CVJ surgery is considered difficult due to its complex neurovascular relationship and fear of vertebral artery injury. This becomes more complex when there is a congenital anomaly of CVJ leading to variations in C1–2 anatomical features mentioned in the literature. In low- or middle-income countries like India, these surgeries are done at only a few tertiary care hospitals mostly at tier 1 cities. BI and ACM are not uncommon in this part of the world, and many of these patients present late with advanced disease conditions with significant neurological deteriorations along with respiratory involvement, which are negative prognostic factors.

Plenty of literature is available about safe screw insertion techniques of C2 for the fixation of CVJ. However, in congenital anomalous C2, these landmarks cannot be considered safe for C2 pedicle or PI screw insertions and if followed may enhance injury risk to vertebral arteries. For long, fear of vertebral artery injury has kept young neurosurgeons avoid surgeries at CVJ, especially where they need to put in C2 pedicle or PI screws for fixation. Newer surgical approaches and techniques have been suggested to prevent it through both anterior and posterior approaches. ^{10 11} However, easy access of midline posterior approach to CVJ is the simplest, direct, and easy for even less experienced neurosurgeons. Length of the screw purchase in the bone is associated with greater pullout strength, so will lead to better fixations. Techniques of C2 pedicle and PI screws suggested in the literature add to the list of many other vertebral levels' screw insertion landmarks. ^{13 14 15} Vertical and horizontal angles of screw insertions are difficult to memorize and may not be suited to patients with normal or congenital variations. It is important that all these landmarks were suggested in the pre-computed tomography (CT) and pre-magnetic resonance imaging era when only X- rays were available. Based on X-rays only, it was not possible to visualize patient-specific 3D anatomy of the spine, and therefore, these were of great help in the earlier times. ^{14 16} However, authors believe that these landmarks should be revisited in the light of significant advancements in neuroradiological technologies available nowadays. Virtual images and real 3D-printed models are being used nowadays where preoperative planning is done, which are patient specific and accurate. Various intraoperative localizing equipment like neuronavigation, 3D fluoroscopy, O-Arm, and intraoperative CT too are being or will be replaced by virtual images in near future. ^{17 18} Most cost-effective among these are virtual 3D images, which can be viewed on console of the CT scanner or using various software on laptop or desktop. ^{12 19}

Patient-specific bony anatomical/morphological details, including in 3D format, needs only a plain CT scan, which is available even at remote places nowadays. CTA is capable of revealing all the patient-specific bony and vertebral artery details. VRT images of CTA display real-like images of bones and vertebral arteries that can be viewed in any projection and even virtual surgeries (editing by removing parts of the image) can be done on these software. Interestingly, color of bone is more or less similar to what is actually seen during surgery. In authors' opinion, morphology (instead of anatomy) is more helpful for selecting screw insertion sites and their trajectories. Various surface features over posterior surface of C2 seen on these images can be used as a guide to accurately identify screw insertion sites and trajectories too. However, for selecting required lengths and diameters of the screws, one should take help of MPR images in right axis/projection. ¹³

Authors find that screw insertions in C2 lamina and PI can always be done under direct vision and does not need fluoroscopy, because entire length of both the screws are directly under the bony cortices and lengths can be measured directly. Authors suggest identifying CP for each side on laminar, PI, and pedicle screws ( Fig. 4 ). The CP is the point that must be crossed by that particular screw (laminar, PI, or pedicle). For PI or transarticular screws insertions, CP is to be selected at the center of the bony segment medial to the vertebral artery exit foramen and medial margin of lamina at that level. CP for laminar screws are simple and trajectories of the screws of both sides should cross without obstructing each other. ²⁰ Course of PI screws within C2, only CP is between medial edge of exit foremen of C2 lateral mass and medial edge of the lamina at that level. One should select the entry point that should pass through the CP right under the surface cortex and should go grossly toward midpoint of superior C2 articular facet. One should align the trajectory of the screw to cross these two points for C1–2 PI or transarticular screws. Screw insertion point may be anywhere from lamina–facet junction to lamina–spine junction depending on the shape and dimensions of the bony segment medial to the exit foramen of the vertebral artery. Another important point is to position the patient in such a way that shoulder or upper back doesn't hinder to the screw insertion, because trajectory is much parallel to the floor ( Fig. 4A, B ). If required, one may use stab incision in the midline below the main incision for inserting PI screws. The same segment is also used for C2 pedicle screw insertion. CP for pedicle insertion is a point just under the cortices of superomedial and medial surfaces of lamina–SF junction. The pedicle screw should pass just under these two cortices, because this corner continues till the junction of dens with VB. Both these cortices are visible under direct vision. Fluoroscopy is not needed for selecting the site of screw entry but may be used to check the depth, which too can be decided preoperatively based on CT scan findings. The concept of CP is similar to crossing through a corridor for going from one room to another room in a straight line. One should keep the corridor in the center of the planning instead of starting point in either of the rooms that were suggested earlier and are still being followed. ^{13 14 15} It is important that laminar and PI screws should be just under the surface cortices of lamina and pars, respectively, whereas pedicle screw too should be under lamina–SF junction cortex in its early course and trajectory. Authors need to highlight that rigorous preoperative evaluation of the patient-specific anatomy is must to see widths, course of the vertebral artery, which may pass with a high arch under the lamina–facet junction. Lengths of the screw planned too should be decided based on the CT findings. Authors find that minimum width of the bony element for safe passage of screw should be 2 mm more than the screw diameter (1 mm on each side). It is also important to remember that fixation is only to keep the bony segment stable for some time (few weeks to couple of months), and unless bony fusion occurs, screws will fail. Therefore, all steps described by the masters in field, must be followed, most importantly preparing bony surfaces planned for fusion and sufficient amount of bone grafts at denuded bony surfaces (C1–2 facet joints decorticated and graft packing in CVJ). ^{1 5}

Fig. 4.

( A ) Ideal position of the patient on table which should be used for PI screws, where cervicodorsal extension and mild CVJ flexion will help in accessing the screw trajectory by instruments. ( B ) Flexion of cervicodorsal spine will make the trajectory of PI screw difficult to access by instruments. VRT images of CT showing soft tissue ( C ) and bony ( D ) elements by adjusting window settings, where trajectories (long, blue arrows) can be accessed easily when patient is positioned ideally or with difficulty when patient is positioned with cervicodorsal spine flexed resulting in trajectory through mid or upper dorsal regions (E shows soft tissue and F shows bone window settings). CT, computed tomography; CVJ, craniovertebral junction; PI, pars interarticularis; VRT, volume rendering technique.

The findings and techniques described are mainly for the young neurosurgeons, and after some maturity and experience, one is capable of visualizing the CVJ of the patient during surgery for safely contemplating the surgery. This study highlights that the part of PI medial to the vertebral artery exit from C2 lateral mass is the most important part, the dimension of which help in selecting a safe screw insertion point and trajectory. If this segment has a width of less than 5.5 mm, 3.5-mm transarticular screw placement should not be attempted and instead of pedicle or PI, laminar screw should be used for fixation in early part of the career.

Recently, C1–2 distraction with fusion and fixation have been reported from different parts of the world including India. ^{9 21} Anterior approach definitely has additional benefit of feasibility of anterior decompression of bony elements by drilling, if the deformity is not reducible posteriorly. Authors don't have enough experience of anterior approach currently and posterior approach is the technique of choice for them.

We conclude that traditional concepts, which were given in pre-CT scan era, of screw insertions of C2 screws need to be revisited in view of significant advancements in concepts, philosophy, and neuroradiological technology. Laminar and PI screws can always be inserted under direct vision without need of fluoroscopy. Pedicle screws too can be inserted at least to half of its depth in its early part under direct vision.