Unfused Posterior Arch of Atlas (C1): Radiological Features and Clinical Significance

The CT scan image displays an axial view of the first cervical vertebra (C1), also known as the atlas, with a notable unfused posterior arch highlighted by the red arrow. This congenital anomaly represents a failure of proper fusion during embryonic development of the posterior neural arch components. While often discovered incidentally during radiological examinations for unrelated conditions, unfused or bifid arch of C1 deserves clinical attention due to its potential implications for atlantoaxial stability, spinal cord protection, and surgical planning in the craniovertebral junction region. The prevalence of posterior arch defects of the atlas ranges from 3-5% in the general population, with complete absence of the posterior arch being considerably rarer than partial defects. Understanding the embryological basis, anatomical variations, and clinical significance of this finding is essential for radiologists, neurosurgeons, orthopedic surgeons, and other specialists involved in the management of upper cervical spine disorders.

Labeled Structure in the Image

Unfused posterior arch of C1 (red arrow): The red arrow in the image points to a discontinuity in the posterior arch of the first cervical vertebra (atlas). This represents a failure of fusion between the two hemiarches that normally develop from separate ossification centers and join during development. The gap in the posterior arch is clearly visible on this axial CT image, demonstrating the radiolucent space where bone fusion should have occurred.

Embryology and Classification of C1 Arch Anomalies

Developmental Basis

The atlas vertebra develops through a complex process of chondrification and ossification during embryonic development. Understanding this process helps explain the various anomalies that can occur in this critical vertebra.

• The normal C1 vertebra forms from three primary ossification centers: one for the anterior arch (body) and two for the lateral masses with their associated posterior arch components.
• Ossification begins in the lateral masses during the 7th week of gestation, proceeds posteriorly along the neural arches, and typically completes fusion of the posterior arch by the 3rd to 5th year of life.

Failure of proper chondrification or subsequent ossification can lead to various defects in the posterior arch. This embryological understanding provides the basis for the classification systems used to describe atlas anomalies.

• The posterior arch defects result from failure of the two hemiarches to fuse in the midline due to impaired chondrogenesis rather than failed ossification.
• Anterior arch defects, though less common, result from failure of fusion between the lateral masses and the body ossification center.

Classification Systems

Several classification systems have been proposed for posterior arch defects of C1, with Currarino’s being among the most widely used. This five-type classification helps standardize terminology and guide clinical management.

• Type A: Failure of posterior midline fusion with small defects or clefts (as seen in the image)
• Type B: Unilateral defects with partial posterior arch present
• Type C: Bilateral defects with preservation of the most posterior tubercle
• Type D: Complete absence of the posterior arch with preservation of the posterior tubercle
• Type E: Complete absence of the entire posterior arch including the tubercle

Understanding these variations is clinically relevant as the different types may be associated with different symptoms and complications. The stability of the atlantoaxial complex and potential for neurological complications varies across these types.

• Types A and B are typically stable and asymptomatic, often discovered incidentally.
• Types C, D, and E may be associated with greater instability and neurological symptoms, particularly with traumatic injury.

Radiological Evaluation of C1 Anomalies

Imaging Techniques and Findings

Computed tomography is the gold standard for evaluating bony abnormalities of the craniovertebral junction. The axial image provided offers excellent visualization of the atlas morphology and clearly demonstrates the unfused posterior arch.

• CT scanning allows precise measurement of the gap in the posterior arch and evaluation of associated anomalies in the lateral masses and anterior arch.
• Three-dimensional CT reconstruction can provide additional information about the overall architecture of C1 and its relationship to adjacent structures.

While CT excels at bone detail, magnetic resonance imaging (MRI) plays a complementary role in evaluating potential soft tissue and neurological implications of atlas anomalies.

• MRI can demonstrate the relationship between the unfused arch and the spinal cord, dural sac, and ligamentous structures.
• T2-weighted sequences may reveal high signal intensity within the spinal cord if compression or myelopathy is present.

Differential Considerations

When evaluating imaging of the upper cervical spine, several conditions may need to be distinguished from developmental anomalies of the atlas posterior arch. Accurate differentiation affects management decisions and prognostic considerations.

• Acute fractures of the posterior arch show irregular margins, potential displacement, and may be associated with soft tissue swelling.
• Os odontoideum and other congenital anomalies of the axis (C2) may coexist with atlas defects and require comprehensive evaluation.

The pattern of cortication along the edges of the unfused segments provides important diagnostic clues. Well-corticated margins suggest a congenital origin rather than traumatic or pathologic processes.

• Smooth, sclerotic margins of the unfused segments indicate long-standing developmental anomaly.
• Sharp, non-sclerotic edges with surrounding bone edema would suggest acute traumatic etiology.

Clinical Significance and Management

Symptomatic Presentations

Most cases of unfused posterior arch of C1 remain asymptomatic throughout life and are discovered incidentally. However, certain circumstances can lead to clinical manifestations requiring medical attention.

• Neck pain, particularly exacerbated by extension, may occur due to impingement of the unfused segments against the occipital bone.
• Neurological symptoms including paresthesias, weakness, or myelopathy can develop if significant atlantoaxial instability results in spinal cord compression.

Traumatic injury deserves special consideration in patients with pre-existing atlas anomalies. The absence of a complete posterior arch may reduce the stability of the atlantoaxial complex and increase vulnerability to displacement.

• Even minor trauma can potentially lead to significant neurological compromise in patients with extensive posterior arch defects.
• Sports participation, particularly contact sports, may need to be limited in cases with significant instability.

Management Approaches

The management of unfused posterior arch of C1 is guided by the presence of symptoms, degree of instability, and associated anomalies. A multidisciplinary approach involving neurosurgery, orthopedics, and rehabilitation specialists offers the most comprehensive care.

• Asymptomatic patients with incidentally discovered defects typically require no specific treatment beyond documentation and patient education.
• Dynamic imaging studies may be warranted to assess for occult instability even in asymptomatic cases.

Surgical intervention is reserved for cases with significant instability, progressive neurological deficits, or intractable pain. The surgical approach depends on the specific anatomical considerations and presenting symptoms.

• Posterior fusion techniques may be challenging due to the absence of normal posterior elements for anchoring instrumentation.
• Combined approaches utilizing lateral mass screws and occipital plate fixation may be necessary for extensive defects.

Associated Anomalies and Syndromes

Isolated posterior arch defects of C1 are most common, but awareness of potential associated anomalies is important for comprehensive evaluation and management.

• Anterior arch defects may coexist with posterior anomalies, potentially resulting in greater instability.
• Anomalies of the odontoid process, such as os odontoideum or hypoplasia, have been reported in association with C1 arch defects.

Several genetic syndromes include anomalies of the craniovertebral junction among their manifestations. Recognition of these associations can guide evaluation for other systemic manifestations.

• Down syndrome has an increased prevalence of atlantoaxial instability and C1 anomalies.
• Klippel-Feil syndrome, characterized by fusion of cervical vertebrae, may include atlas anomalies in its spectrum.
• Goldenhar syndrome and other first and second branchial arch disorders can involve craniocervical junction abnormalities.

Biomechanical Considerations

The posterior arch of C1 serves important biomechanical functions in stabilizing the atlantoaxial complex and protecting the spinal cord. Understanding these functions helps in appreciating the potential implications of arch defects.

• The intact posterior arch forms a ring with the anterior arch, providing resistance to lateral bending and rotation forces.
• The posterior arch serves as an attachment site for important ligamentous structures, including the posterior atlantooccipital membrane.

The transverse atlantal ligament, which maintains the relationship between the odontoid process and the anterior arch of C1, is not directly affected by posterior arch defects. However, overall stability may still be compromised.

• Flexion-extension radiographs or dynamic CT/MRI studies can help assess the degree of atlantoaxial motion.
• Measurement of the atlantodental interval (ADI) provides objective assessment of anterior stability, with values exceeding 3mm in adults suggesting instability.

Conclusion

The unfused posterior arch of the atlas, as demonstrated in this CT image, represents an important anatomical variant with potential clinical implications. While often asymptomatic and discovered incidentally, recognition of this finding is essential for proper interpretation of imaging studies and appropriate clinical decision-making. The smooth, well-corticated margins of the unfused segments suggest a congenital origin rather than acute trauma. Understanding the embryological basis, classification, and potential clinical consequences of atlas arch defects enables healthcare providers to appropriately counsel patients and plan management strategies. For the majority of patients with isolated, asymptomatic defects, reassurance and documentation are sufficient, while those with symptoms or evidence of instability may require more extensive evaluation and potential surgical intervention. As with many congenital anomalies, a personalized approach considering the unique anatomical and clinical features of each case yields the best outcomes.

1. Unfused Posterior Arch of Atlas: CT Imaging Features and Clinical Implications
2. C1 Vertebral Anomalies: Radiological Assessment of Unfused Atlas Posterior Arch
3. Congenital Defects of the Atlas: Diagnostic Imaging and Management of C1 Posterior Arch Non-fusion
4. Craniovertebral Junction Variations: Computed Tomography Evaluation of Unfused C1 Posterior Arch
5. Atlas Vertebra Developmental Anomalies: Radiographic Identification and Clinical Significance