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Asian Spine J > Volume 11(5); 2017 > Article
Srivastava, Mahajan, Nanda, Nanda, Mishra, Kanagaraju, Batra, and Chhabra: Morphometric Study of C1 Pedicle and Feasibility Evaluation of C1 Pedicle Screw Placement with a Novel Clinically Relevant Radiological Classification in an Indian Population

Abstract

Study Design

A retrospective computed tomography (CT)-based morphometric study of 84 C1pedicles in an Indian population focusing on critical morphometric dimensions vis-a-vis C1 pedicle screw placement.

Purpose

To determine the feasibility of C1 pedicle screw placement in an Indian population and propose a novel classification system for the same.

Overview of Literature

At present, C1 pedicle screws are rarely used, and very few studies have focused on the feasibility of pedicle screw placement in terms of racial, gender, and ethnic variations in anatomical structures. There are no CT-based data on C1 pedicles that assess the feasibility of pedicle screw placement in the Indian population.

Methods

We measured C1 pedicle diameter on CT coronal scan images of 42 adult patients. Extramedullary height (EMH) and intramedullary height (IMH) were measured. We examined the differences between the right and left atlas pedicles and compared measures between males and females. These data were analyzed using significance tests. Based on the results, we propose a novel classification system, which we believe will help in determining the feasibility of C1 pedicle screw placement.

Results

Forty-two adult patients (84 pedicles) were examined. Average EMH and IMH were 4.48±0.91 and 0.86±0.77, respectively. Approximately, 32% of the C1 pedicles had bone thicknesses of <4 mm, 49% had IMH of <1 mm, and 38% had no pedicles. The average thickness in women was 4.21±0.93 mm, which was significantly thinner than that in men (4.73±0.81 mm, p=0.004). Right and left pedicles were not significantly different.

Conclusions

Our data indicate that approximately one-third of the Indian population may not be suitable candidates for C1 pedicle screw placement. Caution should be exercised while placing type 1B and type 2 pedicles based on our proposed classification system.

Introduction

Posterior transarticular screws are widely used for rigid atlantoaxial fixation, with good results [1]. Nevertheless, because of anatomical limitations and technical difficulties, an alternative technique involving separate instrumentation in Atlas (C1) and Axis (C2) vertebra is increasingly being used. This technique was initially described by Goel and Laheri [2] in 1994 and further popularized by Harms and Melcher [3]. It involves placing lateral mass screws in C1 and pedicle/pars screws in C2 connected by a rod/plate. This is a versatile technique, although accessing the C1 lateral mass can be difficult because of excessive bleeding from the venous plexus surrounding the C1–C2 joint [3,4]. There is also a possibility of internal carotid artery and hypoglossal nerve injuries due to bicortical screw placement [5,6]. To minimize these risks, an alternative technique is suggested. This technique by Resnick and Benzel [7] uses the C1 pedicle as an entry to access the lateral mass. Because the C1 pedicle is just below the vertebral artery groove on the C1 arch, the dimensions of this area are the limiting factor in determining the feasibility and screw size [8].
Genetic and ethnic variations are characteristic of spinal structures, such as pedicles. These variations may require instrumental modification or render the technique unsuitable for use in some ethnic groups because of incompatible anatomic variations. Atlas morphometry is extensively studied and well-documented in the literature [9,10,11,12,13]. However, there are very few studies of C1 pedicle morphometry [14,15,16]. In spite of the fact that the Indian population constitutes one-sixth of the world's population, there have been no reports of computed tomography (CT)-based C1 pedicle morphometry in the literature till date. We conducted a morphometric study to examine potential pathways for C1 pedicle screw placement and determine the feasibility of the procedure in an Indian population. Despite apparently adequate anatomy for pedicle screw placement on preoperative imaging, it is sometimes difficult to place the screws. This is generally ascribed to variations in the intramedullary canal. We also classified C1 pedicles based on extramedullary height (EMH) and intramedullary height (IMH). Such a classification system may help surgeons preoperatively determine the feasibility of the technique and choose alternative methods in difficult cases.

Materials and Methods

After receiving appropriate approvals including the Institutional Review Board approval (ISIC/RP/2014/004) of Institutional Ethics committee (IEC) registered under central drug standard control organisation (CDSCO) is the national regulatory body, 50 random adult volumetric cervical spine CT scans, with a minimum slice thickness of <1 mm (0.5–1 mm), were obtained from the database of a single center based in New Delhi, India. The raw volumetric axial images were taken and used for CT reconstructions in the sagittal and coronal planes, using software (RadiAnt DIACOM viewer 4.0.2, Medixant, Poznan, Poland). Patients within complete cervical scans were excluded from the study. Patients with demonstrable atlas lesions in the form of fractures, tumors, infection, inflammatory disease, or congenital malformation were also excluded. Because the craniocaudal height of the atlas pedicle is the restrictive factor in pedicle screw placement [14,15], we measured EMH and IMH (Figs. 1, 2). These measurements were performed from coronal reconstructions, as described by Gebauer et al. [17], using the bone window setting of the Picture Archiving and Communication System (PACS) software. EMH and IMH were determined at the thinnest portion located underneath the vertebral artery groove. All EMH and IMH measures were agreed upon by two authors based on the criteria by Gebauer et al. [17].
Significance was calculated between right and left pedicle dimensions, along with gender variations, using the Student's t-test. EMH and IMH correlations were also determined. In addition, we aimed to distinguish the morphological patterns and thereby the feasibility to use instrumentation as per the morphology. This classification system, based on EMH and IMH, may assist physicians in preoperatively determining the feasibility of the procedure.

Results

Among the 50 randomly selected scans, one was of a pediatric patient (because of a clerical error) and seven were incomplete studies; hence, these were excluded. The final study sample consisted of 42 patients, with a total of 84 C1 pedicles. There were 21 males (age [mean±standard deviation, SD], 48.95±14.31 years) and 21 females (average age [mean±SD], 47.29±13.59 years). The left and right pedicles did not significantly differ; consequently, these data were merged.
Average EMH and IMH were 4.48±0.91 and 0.86±0.77, respectively. Approximately 32% (27/84) of the pedicles had bone thicknesses of <4 mm, 49% (41/84) had IMH of <1 mm, and 38% (32/84) had no pedicle (no intramedullary canal) (Fig. 3). The average pedicle thickness in women was significantly lesser (4.21±0.93 mm) than that in men (4.73±0.81 mm) (p=0.004). EMH and IMH were highly correlated (r=0.93).
Here, we propose a radiological classification system (Table 1, Fig. 4) for C1 pedicles based on observed data and already published literature. The goal of this system is to assist physicians in determining the feasibility of C1 pedicle screw placement in patients. We classified the pedicles into three main types depending on EMH and IMH (Table 1, Fig. 4). Representative sagittal CT sections are shown in Fig. 5.
This system, as used in the current study, is presented in Table 2 with the pedicle numbers indicating the various types and subtypes. C1 pedicle heights sampled from various ethnic populations are presented in Table 3 [14,15,16,18,19].

Discussion

Segmental C1-C2 fixation, using atlas lateral masses, was previously described by Goel and Laheri [2]. Since then, the technique is gaining popularity over transarticular fixation as described by Jeanneret and Magerl [20]. However, in order to expose the lateral masses of the atlas, dissection has to be performed around the C2 nerve, which is surrounded by Batson venous plexus. This dissection often leads to significant bleeding, making visualization of the entry point difficult. Moreover, the purchase obtained with lateral mass screws is primarily cancellous, of short length which is biomechanically a less stable construct than using a pedicle screw. Therefore, bicortical screw insertion is recommended, which puts Internal Carotid artery and Hypoglossal nerve [21] at a significant risk. Lee et al. [16] described a modified technique that included partial burring of the C1 arch, thereby reducing bleeding; however, this technique did not address all the shortcomings of lateral mass fixation. Considering these shortcomings, various authors explored screw placement in the C1 lateral massusing the pedicle, which is usually located just below the lateral aspect of the vertebral artery groove over the C1 arch. This allows for longer screw length with good circumferential cortical purchase in the initial third of the trajectory, avoiding the need for dissection around the venous plexus and bicortical screw placement. C1 pedicle screws are particularly useful in deformity correction surgery or in patients with severe osteoporosis in whom purchase of lateral mass screws may be questionable. However, because of high variations in the C1 pedicle morphology, preoperative CT is recommended to identify patients for whom the procedure is unsafe. Some authors have proposed a role of the intramedullary canal in determining the feasibility of C1 pedicle screw placement.
Despite an initial description of the atlas lateral mass fixation technique in India, studies on Indian patients are lacking. This study presents relevant CT-based C1 pedicle morphometry to help determine the safety of screw placement in this population.
The mean EMH in our study was 4.48 mm (standard deviation, ±0.91), which was higher than that observed in a Chinese CT-based morphometric study done by Qian et al. [18]. The EMH variability in our study was smaller compared with that in their study. A number of cadaver studies, as tabulated in Table 3, have reported wide EMH variance (3.95–4.80 mm) among various ethnic groups.
Pedicles with EMH of <4 mm are typically considered unsuitable for safe placement of 3.5 mm screws [22] without splitting the pedicle or injuring the vertebral artery, unless exploration and mobilization of the vertebral artery is done, increasing the operative time and complicating the procedure [23]. Based on this, 32% of the pedicles in the current study were unsuitable for safe placement, which is significantly higher than that reported in the Chinese study and most Western studies (Table 3). The only study that reported higher percentage of pedicles, with EMH of <4 mm, was Lee et al. [16]. This study was conducted on a large sample of 709 atlases of African American and Caucasian people.
The intramedullary diameter depicts the cancellous portion of the C1 pedicle channel. It forms a least resistant path, safely directing the cannulating instrument into the central rigid core of the pedicle, which is resistant to perforation. This reduces superior breaches and reduces the risk of injury to the vertebral artery. In this study, mean IMH was 0.86±0.77 mm in an Indian population, which was significantly less than that previously reported in a Chinese population (Table 3). Both the studies by Tan et al. [14] and Qian et al. [18] reported a mean IMH of >1 mm. In general, a1-mm burr/probe is used to create a groove in the pedicle. Consequently, dimensions less than this may impede tract creation by pedicle screws. In our study, 38% of the patients had cortical pedicles without an intramedullary canal. Some pedicles with EMH of >4 mm also had no intramedullary canals (9.5% of the total pedicles). Qian et al. [18] reported similar findings in their CT-based study. This has clinical implications as the drill/probe used to create a channel in cortical bone may slip and lose direction from the central core of the pedicle, injuring the surrounding structures.
EMH and IMH were highly positively correlated (r=0.93). However, as discussed above, pedicle size does not always indicate the presence of an intramedullary canal. There were significant differences between pedicles of males and females. The average thickness in women (44.21±0.93 mm) was significantly lesser than that in men (4.73±0.81 mm). This observation is in agreement with past studies and may relate to general body habitus differences between the sexes.
The above information, along with published literature (Table 3), guided the development of a system for classifying pedicles based on EMH and IMH. This system may serve as a communication tool to guide the surgeon to preoperatively assess feasibility/difficulties in C1 pedicle screw placement. Based on this classification system (Table 1, Fig. 1), type 3 pedicles are unsuitable for screw placement and type 2 pedicles require extreme caution as there is no intramedullary canal. Together, these account for almost 41.67% (35/84) of all pedicles in this small series. Given that nearly half of the patients cannot undergo safe placement of screws, this is an extremely challenging technique. Damaging the vertebral artery has grave consequences compared with bleeding from the plexus at the C1-C2 joint that is sometimes encountered while laterally placing C1 mass screws. These findings support consideration of alternatives such as C1 lateral mass screws, sublaminar wires, and occipital plates. Type 1 pedicles, which constitute nearly 58.33% (49/84) of the study sample, allow placement of pedicle screws without significant difficulties under appropriate precautions and techniques. Nevertheless, within the type 1 category, the 1B type requires more vigilance compared with 1A as the likelihood of injury/breach is higher in this type due to a small intramedullary canal.
The limitations of the study include a small, retrospective sample size derived from a limited geographical area. Our findings may therefore not address all regional variations. However, this study gives valuable insight into the anatomy and ethnic variability of the channel used for C1 pedicle screws. Moreover, our classification system requires validation in cadaveric and clinical settings. The measurements were performed by two authors using the PACS software. This may have introduced bias; however, studies using similar methodologies have shown high k values for correlation [24].

Conclusions

This study confirms prior observations of wide variability in pedicle dimensions between various ethnic groups. CT C1 morphometric data shows that approximately 40% of the Indian population may be unsuitable candidates for C1 pedicle screw placement. This justifies the need for proper preoperative CT scans before instrumentation is contemplated in this region. Based on our classification system, type 1A pedicles maybe safe for C1 pedicle screw placement. However, caution is required while contemplating the placement of pedicle screws in Type 1B and extreme caution is required for type 2.

Acknowledgments

The authors would like to acknowledge the efforts of Ms. Meenakshi Mohan (Research Co-ordinator) for her administrative support for this study.

Notes

Conflict of Interest: No potential conflict of interest relevant to this article was reported.

References

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Fig. 1

A representative coronal computed tomography scan section depicting the C1 pedicle.

asj-11-679-g001.jpg
Fig. 2

Representative scans with intramedullary height and extramedullary height measurements of both positions.

asj-11-679-g002.jpg
Fig. 3

A representative scan depicting C1 pedicles without any intramedullary canal.

asj-11-679-g003.jpg
Fig. 4

Pictorial representation of C1 pedicle types.

asj-11-679-g004.jpg
Fig. 5

Representative sagittal scans, depicting C1 pedicles of various types.

asj-11-679-g005.jpg
Table 1

Author's radiological classification of C1 pedicles

asj-11-679-i001.jpg

EMH, extramedullary height; IMH, intramedullary height.

Table 2

Pedicle numbers in various types and subtypes of author's classification

asj-11-679-i002.jpg
Table 3

C1 pedicle dimensions in various ethnic population

asj-11-679-i003.jpg

CT, computed tomography.

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