Computed tomography-based morphometric analysis of cervical pedicles in Indian population: A pilot study to assess feasibility of transpedicular screw fixationAR Patwardhan, PS Nemade, SK Bhosale, SK Srivastava
Department of Orthopedics, Seth G. S. Medical College and K.E.M. Hospital, Acharya Donde Marg, Parel, Mumbai, Maharashtra, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.97173
Source of Support: None, Conflict of Interest: None
Background: Cervical transpedicular screw fixation is safe and is probably going to be the gold standard for cervical spine fixation. However, cervical transpedicular screw use in the Asian population can be limited as the transverse diameter in this group of patients may not be adequate to accommodate the 3.5-mm pedicular screw thus injuring the vital structures located in the close proximity of the pedicles. Thus lateral mass fixation remains the mainstay of treatment. The present study evaluated the transverse cervical pedicle diameter of C2-C7 vertebrae in a pilot study in 27 Indian subjects using computed tomography (CT) imaging and evaluated the feasibility of transpedicular screw fixation in them. Aims: To evaluate the feasibility of transpedicular screw fixation in the Indian population. Settings and Design: The cervical pedicle diameter size differs between the Asian and non-Asian population. The authors studied the transverse pedicle diameter of the C2-C7 of the cervical spine in the Indian population using CT measurements. This cross-sectional study was carried out at a tertiary care centre for a period of four months from October 2010 to December 2010. Material and Methods: Measurements of cervical pedicles in the subjects were performed on the CT workstation from the CT images taken at 2.5-mm interval. The transverse pedicle diameter was defined as the outermost diameter of the pedicle, taken perpendicular to the axis of the pedicle at the narrowest point and measured in millimeters±0.1 mm. Statistical Analysis: Descriptive statistics was used to represent percentage of transverse diameter of cervical pedicles less than 5 mm in male and female subjects at C2-C7 levels. Since there is no previous study done in India, we initiated the study with sample size of 27 as a pilot study. The statistical analysis was performed using SPSS software. Results: The mean transverse diameters of the cervical pedicles of C2, C3, C4, C5, C6 and C7 in males were 5.3, 5.3, 5.3, 5.6, 5.6 and 6.1 mm respectively and ranged between 5.3 to 6.1 mm. The mean transverse diameters of the cervical pedicles of C2, C3, C4, C5, C6 and C7 in females were 5.1, 4.6, 4.7, 4.7, 5.3 and 5.6 mm respectively and ranged between 4.6 to 5.6 mm. Between 2.1% and 55.7% of pedicles in our male population and between 5.5% and 74.3% pedicles in our female population was smaller than 5.0 mm in transverse diameter and thus cannot have fixation with a 3.5 mm screw using this technique. Conclusions: We found that the transverse pedicle diameter of cervical pedicles in the Indian subjects is smaller compared to the Western population. Although transpedicular screw fixation has stronger pullout strength compared to lateral mass fixation, its use must be considered carefully and individually. Preoperative CT evaluation is a must before transpedicular fixation in the cervical spine, especially in the Indian female population. As an option 2.7-mm screws can be devised for the Indian population giving a wider safety margin.
Keywords: Cervical pedicle, computed tomography evaluation, Indian population, transpedicular screw fixation
Contemporary spinal reconstruction surgery can employ a wide variety of surgical implants to impart immediate segmental stability. The major aspects of the treatment of cervical spine instability include neural decompression, spinal realignment, rigid fixation, solid fusion and early rehabilitation. The insertion of such instrumentation must respect the anatomic limits of the region. Transpedicular screw fixation provides the greatest stability for stabilizing the cervical spine. It is becoming a standard way to stabilize the spine in scoliosis surgery, degenerative conditions, trauma treatment, tuberculous spondylitis and tumor reconstruction. The results of comparative biomechanical study provided evidence of the greater pullout strength of cervical pedicle screws compared to lateral mass screws. 
Screw placement, however, can cause injuries to adjacent vital structures such as the spinal cord medially and the vertebral artery laterally. Studies performed upon Singaporean Chinese and Malaysian population have showed the cervical pedicle dimension to be smaller compared to the Western counterparts. , No such similar data is available for the Indian population. Thus the exact diameter of the pedicles of the Indian population must be determined before any transpedicular screw fixation can be performed and accepted as a standard procedure in this population. Hence, we conducted a pilot study to determine the same. The results of this study can be used as a baseline for carrying out further large exploratory studies.
The morphology of the cervical spine pedicles has been studied extensively by using both cadavers and computed tomography (CT) films. ,, Computed tomography scans provide the most accurate rendering of anatomy for assessing the accuracy of pedicle screw placement. 
In this study we particularly focused on the measurement of the transverse diameter of the cervical spine using CT imaging to assess the feasibility of transpedicular screw fixation in the Indian population.
This cross-sectional study, approved by the Institutional Review Board, involved evaluation of pedicle diameters of the cervical spine using CT imaging of 27 patients who presented to our institution between October 2010 and December 2010 with injuries other than cervical spine injuries. The patients chosen for this study were between 18-45 years of either gender with prior informed consent taken. Pre-study radiographic evaluation was done to rule out congenital malformation, previous signs of trauma and any signs of neoplastic or infective changes in the cervical spine.
Most studies have shown the saggital diameter of the cervical pedicles to be larger than the transverse diameter in all the vertebrae. , Hence we focused only on the transverse diameter of the cervical pedicles to avoid breech during transpedicular screw placement. The CT cuts were taken parallel to the upper endplate of the vertebral body using a helical CT scanner at 2.5-mm interval. CT images in which each pedicle appeared largest were selected for calculating the transverse diameter. The transverse diameter was measured from the CT images using measurement tools of the workstation [Siemens 3D Virtuoso Software, Siemens Medical Systems, Inc. ST: Illinois]. The transverse pedicle diameter was defined as the outermost diameter of the pedicle, taken perpendicular to the axis of the pedicle at the narrowest point and measured in millimeters ±0.1 mm [Figure 1]. The method of measurement was adapted from the technique reported by Jones et al.  The levels measured were C2-C7. Descriptive statistics was used to represent percentage of cervical pedicles with transverse diameter less than 5 mm in male and female subjects at C2-C7 levels. The statistical analysis was performed using SPSS software [Version 16.0; SPSS, Inc., Chicago, IL].
A total of 14 [51.8%] subjects were males, with a median age of 30.5 years. Thirteen [48.2%] subjects were females with a median age of 32 years. C2-C7 pedicles were measured bilaterally in each of the subjects with the total number of pedicles measured being 324. The analyses of male and female subjects were performed separately.
The mean transverse diameters of the cervical pedicles of C2, C3, C4, C5, C6 and C7 in males were 5.3, 5.3, 5.3, 5.6, 5.6 and 6.1 mm respectively. The diameter of the pedicles ranged between 5.3 to 6.1 mm [Table 1].
The mean transverse diameters of the cervical pedicles of C2, C3, C4, C5, C6 and C7 in females were 5.1, 4.6, 4.7, 4.7, 5.3 and 5.6 mm respectively. The diameter of the pedicles ranged between 4.6 to 5.6 mm [Table 2].
There was no significant difference in the transverse diameter between the right and left pedicle in both male and female subjects [P>0.05]. Between 2.1% and 55.7% of pedicles in our male population [Table 3] and between 5.5% and 74.3% pedicles in our female population [Table 4] were smaller than 5.0 mm in transverse diameter and thus inadequate for fixation with a 3.5-mm screw using this technique.
Pedicle screw fixation has become an established technique for the lumbar and thoracic regions. The cervical spinal pedicles are very strong structural elements of the vertebrae. Transpedicular screw fixation gives the best stability of all the fixation devices in the circumferential discoligamentous cervical injury model.  In addition, pedicle screw fixation may obviate the need for anterior surgery in some patients with metastatic vertebral tumor, who might otherwise require combined anterior and posterior surgery.
Heightened concern exists for the consequences of technical errors in this region due to the proximity of the vertebral arteries and neural elements. Neurological and vascular complications due to misplaced pedicle screw can result in profound and permanent physical impairment to patients undergoing spinal fusion and instrumentation. Morphological studies of the cervical spine have been reported as a way to evaluate the feasibility and safety of the technique.
Studies conducted upon the Western population have showed the diameter of the cervical spine to be adequate for screw fixation. ,, Various studies have shown a difference between the size of the lumbar and thoracic pedicles in the Western and Asian population. , Since the transverse diameter measurement is critical before screw insertion, a study of pedicles in the local population is imperative before any attempt for cervical fixation using a pedicle screw is initiated. Data extracted from studies on non-Asians should be used with caution because they might not be applicable for the Indian population.
Measurements of the transverse pedicle diameter in the cervical spine showed a characteristic trend which has been reported earlier by other investigators. , The C7 has the widest diameter of all cervical pedicles measured in both male and female subjects.
A study by Ludwig et al., demonstrated 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 pedicular 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 adjacent vital structure.
As per [Table 3], between 2.1% and 55.7% of pedicles in our male population and as per [Table 4], between 5.5% and 74.3% pedicles in our female population were smaller than 5.0 mm. Thus our data showed that a great number of pedicles in our female population cannot be fixed using 3.5-mm pedicular screws compared to our male population because of smaller transverse diameter.
Lateral mass fixation is another option available for cervical instrumentation. However, lateral mass screws have less pullout strength compared to transpedicular screw fixation.  As an option 2.7-mm screws can be devised for the Indian population giving a wider safety margin. Biomechanical studies have to be conducted upon the prospective use of these 2.7-mm screws comparing them with 3.5 mm lateral mass fixation.
Pedicle diameter was also smaller in females compared with males at all cervical levels, especially at C5 and C7 levels. Hence, transpedicular screw fixation is more risky in the Indian female population. Based on these findings, preoperative CT evaluation is mandatory especially in Indian female patients before any transpedicular screw fixation is considered in them. Wider database cutting across the geographical barriers is required in order to corroborate these findings.
[Table 1], [Table 2], [Table 3], [Table 4]