Chinese Normative Values of C1 Sagittal Canal Diameter and Definition of C1 Hypoplasia
The cervical spine plays a critical role in protecting the spinal cord, and structural abnormalities at the C1 (atlas) level can predispose individuals to myelopathy. While stenosis at lower cervical levels is well-documented, C1-level stenosis remains less common and understudied, particularly in Chinese populations. This study establishes normative values for the C1 sagittal canal diameter (SCD) in Chinese adults and defines C1 hypoplasia, a condition associated with congenital narrowing of the atlas that may increase the risk of spinal cord compression.
Methodology and Study Design
This cross-sectional observational study utilized computed tomography (CT) imaging to measure C1 SCD in a cohort of 567 Chinese adults (345 men, 222 women; mean age 29.6 ± 6.9 years). Participants were selected from emergency patients who underwent maxillofacial CT scans for suspected nasal fractures between 2005 and 2019. Exclusion criteria included a history of neck pain, neurologic deficits, prior cervical surgery, or syndromic diagnoses. High-resolution CT scans (0.625-mm slices) were analyzed using a picture archiving and communication system (PACS) by two independent researchers to ensure accuracy.
Key anatomical measurements included:
- C1 SCD: Distance from the posterior aspect of the fovea dentis to the anterior edge of the C1 posterior arch.
- Dens sagittal diameter: Widest outer diameter of the odontoid process.
- Space available for the cord (SAC) at C1 and C2: Measured from the posterior edge of the axis to the posterior aspect of the ventral lamina.
Statistical analyses determined sex-based differences and normative ranges. C1 hypoplasia was defined as a C1 SCD ≤2.5th percentile (1.96 standard deviations below the mean).
Key Findings
Normative Values of C1 SCD
The mean C1 SCD was 29.96 ± 1.99 mm (range: 24.60–35.28 mm). Men exhibited significantly larger C1 SCD values (30.34 ± 1.90 mm) compared to women (29.36 ± 1.98 mm; P < 0.001). The 95% confidence interval for C1 SCD ranged from 26.07 mm to 33.85 mm, with a normal distribution confirmed by the Shapiro-Wilk test (P = 0.059).
Definition of C1 Hypoplasia
C1 hypoplasia was defined as a C1 SCD ≤26.07 mm, corresponding to the lowest 2.5% of measurements. In the hypoplasia group, the mean C1 SCD was 25.28 ± 0.78 mm (range: 24.60–26.07 mm), significantly smaller than the non-hypoplasia group (30.07 ± 1.76 mm; P < 0.001).
Associated Anatomical Parameters
- Dens Sagittal Diameter: The mean dens diameter was 10.94 ± 0.53 mm (range: 10.01–11.87 mm), with larger values in men (11.07 ± 0.51 mm vs. 10.74 ± 0.51 mm; P < 0.001).
- C1 and C2 SAC: The mean SAC at C1 was 18.08 ± 1.49 mm (range: 13.38–22.56 mm) and at C2 was 18.22 ± 1.20 mm (range: 14.35–22.18 mm). No significant difference existed between C1 and C2 SAC values (P > 0.05). However, C1 SAC was markedly reduced in the hypoplasia group (14.50 ± 0.88 mm vs. 18.16 ± 1.40 mm; P < 0.001).
Clinical Implications of C1 Hypoplasia
Individuals with C1 hypoplasia demonstrated a higher dens-to-canal ratio (36.5% vs. 41.4%; P < 0.001), indicating disproportionate occupation of the spinal canal by the odontoid process. This deviation from Steel’s "rule of thirds" (which posits equal partitioning of the canal by the dens, spinal cord, and free space) suggests heightened susceptibility to cord compression, particularly when SAC falls below 12 mm. Despite this, many individuals with C1 hypoplasia remain asymptomatic unless compounded by conditions like atlantoaxial instability.
Discussion and Contextualization
This study aligns with prior research defining C1 hypoplasia thresholds. Kelly et al. (2014) reported a similar cutoff of 26.1 mm in a cadaveric study of Western populations, validating the universality of this threshold. The consistency between CT-based and cadaveric measurements underscores the reliability of in vivo imaging for clinical assessments.
Sex differences in C1 SCD and related parameters highlight the importance of population-specific normative data. The larger canal dimensions in men may reflect broader anthropometric variations, necessitating sex-adjusted diagnostic criteria.
Limitations and Future Directions
While this study provides critical normative data, several limitations exist:
- Imaging Modality: Magnetic resonance imaging (MRI) is superior for evaluating spinal cord compression but was not used due to the challenges of recruiting asymptomatic volunteers.
- Longitudinal Data: The absence of follow-up limits insights into the progression of myelopathy or spinal injury in hypoplasia patients.
- Symptomatic Validation: The threshold for C1 hypoplasia was derived from asymptomatic individuals; further studies are needed to correlate this threshold with clinical symptoms.
Conclusion
This study establishes the first CT-based normative values for C1 SCD in Chinese adults, defining C1 hypoplasia as a canal diameter ≤26.07 mm. These findings provide a critical reference for diagnosing congenital cervical stenosis and assessing the risk of myelopathy. Clinicians should consider C1 hypoplasia in patients with unexplained spinal cord compression, particularly in the absence of degenerative changes or instability. Future research should focus on longitudinal outcomes and the integration of MRI for comprehensive cord evaluation.
doi.org/10.1097/CM9.0000000000001497
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