Morphometric Analysis of C2 Pedicle in 247 Patients and Proposal for Trajectory and Size of Pedicle Screw
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.325374
Source of Support: None, Conflict of Interest: None
Keywords: C2 pedicle morphometry, craniocaudal angulation, pedicle height, pedicle length, pedicle transverse angle, pedicle widthKey Message: Morphometric analysis of the C2 pedicle in 247 Indian patients (a total of 494 pedicles analyzed) including pedicle width, height, length, craniocaudal and mediolateral angulation.
Leconte was the first to describe the use of C2 pedicle screws in the management of traumatic spondylolisthesis in 1964. Borne, Roy-Camille and later Goel and Harms popularized the technique of C1-C2 fixation.
Transpedicular fixation of the cervical spine is not widely performed because of its complex morphologic features. Knowing and understanding the pedicle size, relationship between pedicle and nearby structures are crucial for safe and effective fixation.
As a transitional vertebra, the C2 has a unique structure. Most of the available literature has presented confusing definitions of the C2 pedicle and isthmus. Although the terms are used frequently and interchangeably, the pars and pedicle of the C2 are two distinct structures. Precise measurement of the pedicle angulation and morphometry is necessary to help manufacturers design screws which can accommodate the need of a wide variety of population that might need pedicle screw fixation. Previous studies are available documenting the diameter of the C2 transverse foramen and the distance of the vertebral artery and spinal cord to the foramen.,, We measure all the data pertaining to C2 pedicle, in a large number of normal population, in order to create a nomogram which can help surgeons keep in mind the large variation in sizes which one can encounter while placing CPSs.
This was a retrospective analysis of data at a single center. Data of the patients who underwent CT cervical spine between July 2015 to July 2017 was used for analysis. Approval was obtained from the institutional ethics committee. As this was a retrospective study, patient consent was not required. Patient anonymity was maintained while using their scans for study purposes.
Patient selection criteria
Two hundred and forty-seven patients who had undergone CT cervical spine performed on the Somatom Definition edge 128 slice 64 row detector CT Scanner of Seimens (Siemens Healthcare Pvt. Ltd., Malvern, Pennsylvania, USA) installed in the Department of Neuroimaging and Interventional Neuroradiology, were included. Any patient with a craniovertebral junction anomaly or any cervical spine disease, for example, of traumatic, infective, neoplastic or degenerative etiology, or previously operated cervical spine patients were excluded. Any study degraded due to movement artifacts or technically inadequate study was excluded. The decision to exclude was taken by two neuroradiologists.
The images were reconstructed into 0.5-mm slice thickness. All the scans were analyzed on the 'bone window' setting. Patient data were sourced from the medical record section of our hospital. CT images were retrieved from PACS of the Neurosciences Centre of the institute.
Source images were retrieved from PACS onto the Synovia (Siemens Healthcare Pvt. Ltd., Malvern, Pennsylvania, USA) image processing software which is a multimodality reading solution built on a client-server platform. Multiplanar reconstruction (sagittal, coronal, and axial reconstructions) were analyzed to select the best images for measurement. The images were magnified for accurate measurement.
The pathologies and variations were determined with the consensus of two neuroradiologists. All the measurements were performed independently by two observers; and the recordings were made upon agreement with a maximum permissible interobserver difference of 0.5 mm/degree for a particular measurement. In the case of dispute between observers, the opinion of an expert radiologist was considered final. Linear parameters were measured in millimeters +/- 0.1 mm. Averages and standard deviations were calculated for all dimensions. SPSS software (IBM Corp., IBM SPSS Statistics for Windows, Armonk, New York, USA) was used for determination of statistical significance. Unpaired t test was used to assess difference for continuous variables. The threshold of significance was set at P ≤ 0.05. The following parameters were analyzed:
The width and height were measured using the external cortical borders of the pedicle.
CT scans of 247 patients were analyzed, with age ranging from 13 to 86 years. Of them 152 were male patients (61.5%) [Table 1].
Mean PW in females was 5.3 mm and in males it was 5.9 mm [Table 2]. This difference was statistically significant (P = 0.003). Overall 43 pedicles out of 494 were less than 5.0 mm (8.7%). Out of these 15 were less than 3.5 mm (3.0%), while 7 were less than 3 mm. Mean PW on the right was 5.5 mm and on the left was 5.8 mm; there was no statistically significant difference.
The mean PH in the study population was 8.9 mm. It was 8.9 mm on the right while it was 8.8 mm on the left side. Only 3 pedicles studied were less than 3.5 mm.
The mean PL was 29.7 mm. There was no statistically significant difference between the right and left side, or between the male and female population.
The mean PTA was 40.0 degrees and the CCA was 28.4 degrees.
The C2 pedicle is the small portion of bone connecting the posterior elements with the vertebral body. It is located medially beneath the superior articular facet, anteromedial to the transverse foramen. On the other hand, pars interarticularis of C2 is defined by a short part of bone between the superior articular facet and the inferior articular process. General teaching states that the pathway of a C2 pedicle screw should start at the posterior portion of the inferior articular process and via the pars interarticularis and should continue into the pedicle and the vertebral body. To avoid violation of the transverse foramen, it is recommended that transpedicular screw be placed close to the medio superior cortex.
The entry point for a C2 pedicle screw is in the pars of C2. The screw is usually placed with 15-25 degrees of medial angulation and cranial angulation of 25 degrees. Screws of size 3.5 or 4 mm are generally available for fixation. This is based on the literature available from the morphometric analysis of pedicle dimensions. The length of the screw usually ranges from 18 to 26 mm, depending on the length of the pedicle as determined on preoperative CT scans. It has been noted by previous authors like Yoshida et al. that pedicle width on reconstructed CT studies is the limiting parameter in the placement of C2 pedicle screws. There is more than adequate pedicle height and length available. Ludwig et al. concluded that if the pedicle diameter was greater than 5.0 mm, 79% of pedicle screws were in, 19% had non-critical breeches, and only 2% had critical breeches. Thus, if the 3.5-mm screw is to be inserted into the cervical pedicle, the minimum transverse diameter desired is 5.0 mm to allow at least 0.75-mm bony bridge medially and laterally to avoid injury to the adjacent vital structures. Karaikovic et al. evaluated C2–C7 pedicular dimensions in 53 cadaveric specimens. They obtained their measurements manually as well as by evaluating CT scans. They reported a mean C2 outer pedicle width of 6.9 and 6.5 mm, in males and females respectively. Smith et al. studied axial CT scans of 93 patients and found that the overall mean pedicle width was 5.8 ± 1.2 mm. The mean pedicle width in males (6.0+/- 1.3 mm) was greater than that in the female subjects (5.6 ± 1.1 mm). However, this difference was not found to be statistically significant. Howington et al. observed the average pedicle width measured was 7.9 mm. Tomasino et al. found that males have significantly greater pedicle width on both the right and left sides as compared to the female population.
The studies conducted in the Asian population show a lesser average pedicle width than the Western population. Yusof and colleagues obtained CT measurements of the axis in a Malaysian population and reported an average pedicle width of 5.4 mm in 24 males and 5.0 mm in 16 females, with pedicle width ranging from 4.0 to 6.5 mm. Patwardhan et al. measured the C2 pedicle width in the Indian population. They enrolled 27 patients (14 male and 13 female) in their study and found the males to have a mean transverse diameter of 5.3 mm and females of 5.1 mm. Dash et al. studied 49 adult Indian patients and found a mean pedicle width of less than 4.5 mm.
Sciubba et al. reported 15 (15%) breaches among 100 consecutive free-hand screw placements, 12 (80%) of which were lateral. Alosh et al. reported the detection of 43 (25.3%) breaches on postoperative CT scans of 93 patients in which a total of 170 C2 pedicle screws were placed. A pedicle diameter <6 mm was associated with a nearly twofold increase in the risk of cortical breach.
The readily available screw of 3.5 mm diameter will be inadequate for about 8% of the population which might require C2 pedicle screw placement. Based on the above literature review, and our own observation in a large number of pedicles (494), we conclude that a screw diameter of 2.7 mm would be readily implantable in a majority of the population. This would give an adequate margin of 0.75 mm bridge of bone on either side, and also reduce the incidence of pedicle breach. Thus, this screw diameter could be used as the universal screw for almost all cases and age groups.
The lateral cortical wall of the pedicle has been found to be the thinnest. This and the extreme medial angulation of the cervical pedicle probably explain why cervical pedicle screw misplacement is more likely to happen laterally.
Detailed knowledge of pedicle is essential if transpedicular screw placement is desired to stabilize a fracture line through C2 pedicle. Pedicle angles represent the safe bounds for transpedicular screw fixation.
There are varying reports of pedicle length in world literature. Some anatomical reports defined the length as the distance from the posterior surface of the C2 inferior articulating process to the anterior surface of the body., Others tend to define it as the distance from the posterior surface of the inferior articulating process to the junction of the pedicle with the body. Such variation may account for shorter pedicle length in some studies. However, for the practical purpose of inserting screws, pedicle axis length is more relevant than the true pedicle length, so we have calculated that.
The present study may also assist with placement of screws into pedicle or lateral mass. For this reason, the posterior point of projection of the pedicle axis has been defined in terms of reference points visible in a posterior surgical approach to the cervical spine and the two angles represent the safe bounds for transpedicular screw placement.
El Hawary analyzed 18 pedicles inserted in his series of 9 patients. The mean pedicle screw length was 16.5 mm (range: 16 to 17 mm), and the mean medial and rostral inclinations measured were 34.7 degrees (range: 28 to 41 degrees) and 38.6 degrees (range: 28 to 41 degrees), respectively.
Gupta and Goel studied dried specimens of the axis vertebra in fifty patients and concluded that forty degrees of medial angulation and twenty degrees of angulation superior to the axial plane was safe for C2 pedicle screw insertion. In the anatomical study of Xu et al. the average medial angulation of the pedicle was 30 degrees and the rostrocaudal inclination was 20 degrees cephalad to the transverse plane. They also reported that the proper screw entry point based on these angles was located at a point 5 mm inferior to the superior border of the C2 lamina and 7 mm lateral to the lateral border of the spinal canal. Ebraheim et al. compared the screws placed using the above values with those placed using the superomedial border of the pedicle as a guide. They found a higher incidence of cortical breaching using the structured approach and recommended that C2 pedicle screws be placed using direct visualization of the pedicle.
In the anatomical study by Howington et al. screws were placed under direct visualization, and the angles as well as the distance to the entry point from structures routinely encountered at time of surgery were then measured. The average mediolateral angulation in their study measured 35.2 degrees, and the average rostrocaudal angulation measured 38.8 degrees.
Spinal CT scans of 32 Chinese adults. Average mediolateral angle of projection, 42.6° ±4.9°. Average height and width were 11.6 and 6.9 mm. Medial internal cortical bone was significantly thicker than lateral bone (P < 0.01).
In the study by Howington et al. in 10 cadavers, the average pedicle height, width, and length measured 9.1 mm, 7.9 mm, and 16.6 mm, respectively, and the medial inclination and rostrocaudal angulation averaged 35.2° and 38.8°, respectively.
In a study on 60 cadaveric specimens of C2 vertebrae, Lalit et al. found the mean length, height and width of the pedicle were 21.61 ± 2.37 mm, 8.82 ± 2.43 mm, and 5.63 ± 2.06 mm respectively. The mean pedicle superior angle (measured as the angle between the pedicle axis and a line drawn perpendicular to the body of the axis) and median angle (the angle between the pedicle axis and line passing through the midline of the vertebral body and the spinous process) were 23.3 and 32.2 degrees. In a review of literature, they compared their results with previous such reports and found no statistically significant gender or left/right difference.
In conclusion, various authors have used varying methods to measure the morphometry of the C2 pedicle. To the best of our knowledge, this has not been analyzed in such a large number of persons before.
A surgeon must possess detailed knowledge of the variability of bony anatomy, specifically related to the placement of C2 pedicle screws as detailed in this study, so as to reduce the risk of complications related to surgical approaches and procedures involving the posterior craniovertebral junction. Even in anatomically difficult cases with extremely thin pedicles or a high riding vertebral artery, C2 fixation has been described using subfacetal and subisthmic screws.
The values presented in this analysis are alone not sufficient to use as a surgical template applicable to all patients. Instead, it merely acts as a reference of general pedicle measurements and differences among populations. It is an adjunct when either direct visualization is not possible, or the regional anatomy is distorted. Another limitation is that CT measurements are often less than the actual dimensions. The biomechanical strength of the 2.7 mm screw proposed in this study needs to be tested in another study for pullout strength and screw breakage.
Accurate preoperative planning should include the best screw length needed for the procedure, depending on the size of the patient's axis vertebra, and the best angulations for the safety of the neighboring neurovascular structures.
The study may provide information for the surgeons to determine the safe site of entry and trajectory for the screw implantation and also to avoid injuries to vital structures while operating around axis. We conclude that a screw diameter of 2.7 mm can be safely implanted in a vast majority of the patient population.
Ref No. IECPG-498/29.08.2016, RT-30/26.04.2017.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]