Morphometric parameters of the odontoid process of C2 vertebrae, in Indian population, a CT evaluation.

BACKGROUND: Osteosynthesis of odontoid fractures, especially for type II odontoid fractures, is often achieved by the placement of screws. Here, utilizing CT, we evaluated the normal anatomy of the odontoid process in an Indian population to determine whether one or two screws could be anatomically accommodated to achieve fixation.
METHODS: CT-based morphometric parameters of the odontoid process were assessed in 200 normal Indian patients (2018-2020).
RESULTS: Of 200 patients, 127 were male, and 73 were female. The mean minimum external transverse diameter (METD) was 8.80 mm (range 6.1-11.9 mm). Six (3%) patients had a minimum internal transverse diameter (TD) of >8.0 mm that would allow for the insertion of two 3.5-mm cortical screws without tapping, while 10 (5%) patients had TDs of <7.4 mm; none had diameters of <5.5 mm. The mean length of the implant was 36.45 mm in females and 36.89 mm in males, and the mean angle of screw insertion was 60.34° in females and 60.53° in males.
CONCLUSION: About two-thirds (59%) of the 200 subjects in our study had a METD of <9 mm, indicating the impracticality for introducing second screws for odontoid fixation. Copyright:

INTRODUCTION

The accurate radiological evaluation and assessment of the morphometric parameters of the odontoid are critical in achieving stable fixation and fusion of odontoid fractures.[1] Fractures of the odontoid process account for 50–60% of all fractures of C2, 7–27% of all cervical vertebral column fractures and 1–2% of all vertebral column fractures.[6] The technique of internal fixation of the odontoid process is directly related to the dimensions of the odontoid process.[8] Fixation with two screws has high consolidation rates, better rigidity, and gives higher rotational stability.[3] Here, utilizing CT, we determined how many patients’ odontoid processes could accommodate 1 versus 2 screws for odontoid spinal fixation.

MATERIALS AND METHODS

This is a retrospective analysis of the CT studies of the odontoid process performed in 200 asymptomatic adults (2018–2020). The measurements were performed by a single observer.

The CT scans were randomly collected from our hospital radiology archives along with relevant variables; there were multiple inclusion and exclusion criteria [Tables 1 and 2].

Table 1:

Patient demographics, inclusion and exclusion criteria.

Table 2:

CT variables

Statistical analysis

Statistical comparisons between variables within gender were tested using the Student’s t-test after verifying that the distribution of the variables was normal (P < 0.05 is significant). The Fisher’s exact test was used to examine the significance of the association between sex and minimum external transverse diameter (METD) smaller than 9 mm (Statistical Package for Social Sciences – Version 11.0).

RESULTS

There were 200 CT studies of odontoid processes. Ten (5%) patients had transverse diameter (TD) <7.4 mm, while none of the patients had diameters of <5.5 mm. The mean minimal external antero-posterior (AP) diameter of the entire population was 10.17 mm (range 7.10– 13.05 mm, SD 1.09 mm), while the mean length of the implant was 36.45 mm. The mean angle of screw insertion was 60.34° [Table 3]. The two-tailed P value equals 0.446 and this difference was considered to not be statistically significant.

Table 3:

The dimensions of the odontoid by CT morphometry.

DISCUSSION

In this study, we evaluated the morphology of the odontoid process using an accurate and reliable CT-based measurement method. The external CT measurements correlated well with calliper-derived data.[4]

Several studies evaluated the size of the odontoid process

Kulkarni et al.,[5] found that AP and TD were 11.52 mm and 9.85 mm, respectively. Yusof et al.[12] found that 28%of odontoid processes had a cross-sectional diameter of <9.0 mm, making it impossible to perform fixation with two 3.5 mm screws. Daher et al. similarly found 39% of Brazilian subjects had similar measurements, while.[2] Nucci et al.[9] found in the American population, that only 5% of subjects had a minimum TD of less than 9.0 mm.[2,9] In our study 118 (59.0%) subjects had a METDs of <9 mm, precluding two screw fixation. Hence, two 2.7 mm screws could be used safely in 95% of patients, while a single 4.5 mm Herbert screw could be used safely in the entire population. We also found that 82 (41%) of patients required an implant screw length of < 36 mm [Table 4].

Table 4:

Comparing the odontoid dimensions from the study by Daher et al. 2011 and the C.S.

Angle of screw insertion

The mean angle of screw insertion was 60.34° (60° on average in females and 60.53° average in males). In Tun et al.,[11] the mean value of the angle of the odontoid screw was 62.4 ± 4.7° on CT and 64.2 ± 4.1° on X-rays (i.e., A statistically significant difference in the two measurements of the screw angle).

Estimated length of the implant (LI) (distance between the apex of the odontoid process and anterior border of the axis).

Angle of implant (AI): Angle between the line of the apex of the odontoid process to the anterior edge of the axis and tangent to the plateau below the axis).

(a) Minimum internal transverse diameter and (b) External transverse diameter of the odontoid process.

Pros for 2 screw fixation

There are considerably higher rates of union with dual screw osteosynthesis.[8] Two 3.5 mm screws versus one 4.0 mm screw give a greater surface area to penetrate the cortical bone of the odontoid tip, and increases the bone/screw interface, especially in osteoporotic patients.[10]

Pros for single Herbert screw placement

For TD measurements of the dens <5.5 mm, only a single 4.5 mm Herbert screw can be safely used. Notably, Lee and Sung reported a good union rate and no implant failures using single 4.5 mm Herbert screws.[7]

Minimum external (a) and internal (b) anteroposterior diameter of the odontoid process.

C2 Antero-posterior (AP) diameter at its base divided by the estimated length of the implant (C2 AP base/Length of implant).

Use of CT to calculate appropriate screw length and diameter

CT reconstructed images make it possible to calculate the diameter and length of the odontoid process and to estimate the quality of bone and size of the odontoid for the safety/ efficacy of anticipated one to two screw fixation.

CONCLUSION

About two-thirds (59%) of the subject in our Indian population had a METD of the odontoid process of <9 mm. This meant that only one screw could be safely placed in an odontoid process for these individuals.