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Basilar invagination (BI) is a common deformity. This study aimed to quantitatively evaluate the height of clivus and atlanto-occipital lateral mass (LM) in patients with BI with or without atlas occipitalization (AOZ). We evaluated 166 images of patients with BI and of controls. Seventy-one participants were control subjects (group A), 68 had BI with AOZ (group B), and 27 had BI without AOZ (group C). Parameters were defined and measured for comparisons across the groups. Multiple linear regression analysis was used to test the relationship between Chamberlain's line violation (CLV) and the clivus height ratio or atlanto-occipital LM height. Based on the degree of AOZ, the lateral masses in group B were classified as follows: segmentation, incomplete AOZ, complete AOZ. From groups A to C, there was a decreasing trend in the clivus height and clivus height ratio. There was a linear negative correlation between the clivus height ratio and CLV in the three groups. Generally, the atlanto-occipital LM height followed the order of group B < group C < group A. The atlanto-occipital LM height was included only in the equations of groups B. There were no cases of atlantoaxial dislocation (AAD) in group C. There was a decreasing trend in LM height from the segmentation type to the complete AOZ type in group B. BI can be divided into three categories: AOZ causes LM height loss; Clivus height loss; Both clivus and LM height loss. The clivus height ratio was found to play a decisive role in both controls and BI group, while the atlanto-occipital LM height loss caused by AOZ could be a secondary factor in patients with BI and AOZ. AOZ may be a necessary factor for AAD in patients with congenital BI. The degree of AOZ is associated with LM height in group B.

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The atlas (C1) and occipital bone at the base of the skull fuse together in atlas occipitalization, an uncommon congenital abnormality. Because it can result in cervical spine instability, nerve impingement, and related symptoms including stiffness, pain, and neurological impairments, it poses a challenging therapeutic problem. We describe the case of a female patient, 27 years old, who had gradually deteriorating neck discomfort, stiffness, and limited cervical mobility for six years prior to presentation. Her symptoms worsened over time despite conservative treatment, so more testing was necessary. Atlas occipitalization, congenital fusion at the C7 and D1 vertebrae, and other related cervical spine pathologies were identified by imaging examinations. The intricacies of atlas occipitalization and related cervical spine pathologies are highlighted in this case study, along with the diagnostic difficulties and interdisciplinary therapeutic strategy needed to address them. To improve cervical range of motion (ROM), lessen discomfort, and improve functional results, the patient underwent a thorough musculoskeletal examination and was given a customized physiotherapeutic intervention.

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BACKGROUND: Patients with basilar invagination and atlas occipitalization usually present abnormal anatomy of the vertebral arteries (VAs) at the craniovertebral junction (CVJ). OBJECTIVE: To describe and further classify different types of VA variations at the CVJ with 3D visualization technology. METHODS: One hundred twenty patients with basilar invagination and atlas occipitalization who had undergone 3-dimensional computed tomographic angiography (3D-CTA) were retrospectively studied. Imaging data were processed via the separating, fusing, opacifying, and false-coloring-volume rendering technique. Abnormal anatomy of the VA at the CVJ was categorized and related anatomic parameters were measured. RESULTS: Seven different types were classified. Type I, VAs enter the cranium after leaving VA groove on the posterior arch of atlas (26.7% of 240 sides); Type II, VAs enter an extraosseous canal created in the assimilated atlas lateral mass-occipital condyle complex before reaching the cranium (53.3%); Type III, VA courses above the axis facet or curves below the atlas lateral mass then enter the cranium (11.7%); Type IV, VAs enter the spinal canal under the axis lamina (1.3%); Type V, high-riding VA (31.3%); Type VI, fenestrated VA (2.9%); Type VII, absent VA (4.2%). Distance from the canal of Type II VA to the posterior facet surface of atlas lateral mass (5.51 ± 2.17 mm) means a 3.5-mm screw can be safely inserted usually. Shorter distance from the midline (13.50 ± 4.35) illustrates potential Type III VA injury during exposure. Decreased height and width of axis isthmus in Type V indicate increased VA injury risks. CONCLUSION: Seven types of VA variations were described, together with valuable information helpful to minimize VA injury risk intraoperatively.

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Klippel-Feil syndrome is an uncommon anomaly that may be asymptomatic. Early clinical signs such as restricted neck motion or short neck can be subtle and incorrectly treated as spasms. High incidence of associated craniovertebral junction (CVJ) anomalies such as occipitalized atlas predisposes them to serious neurologic complications requiring invasive procedures and surgeries. However, these often have anomalous vertebral artery course which is more prone to injury during CVJ procedures, and also sparsely known in radiology literature. We demonstrate the importance of computed tomography angiography in preprocedural planning to avoid catastrophic injury to anomalous vertebral artery at CVJ in such case.

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Patients with craniovertebral junction anomalies are more likely to undergo craniovertebral junction procedures because of neurologic complications. Vertebral arteries (VAs) are more prone to injury in these cases because of an often anomalous course, with potentially disabling or grave sequel. In our patient with partially occipitalized atlas, CT angiogram revealed that 1 of the VAs passed through the bony canal in the occipitalized and/or fused bone, whereas the VA on unfused side was also unexpectedly anomalous and of C2 segmental type. To the best of our knowledge, such anomaly has never been reported in studies of patients with occipitalized atlas in the literature.

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PURPOSE: Our study highlights the morphometry of the partial and complete atlas occipitalization (AOZ), its coexistence with fusions of the 2nd and 3rd cervical vertebrae and morphological and morphometric abnormalities of the posterior cranial fossa that are of paramount neurological importance. METHODS: One hundred and eighty adult dry skulls, the atlas and axis vertebrae were examined. RESULTS: Four skulls (2.2 %) showed AOZ. Two of them (1.1 %) presented a partial AOZ, one male skull (0.6 %) a complete AOZ and a female skull (0.6 %) had a fused left hemiatlas with the occipital bone and a fusion of the 2nd and 3rd cervical vertebrae. The inner anteroposterior and transverse diameters of the foramen magnum (FM) in the control group were 34.6 ± 3.46 and 29.3 ± 3.47 mm. Only the skull with the complete AOZ had a reduced outer anteroposterior diameter of the FM (29.8 mm), while no specimen was found with a reduced transverse diameter. A wide total decrease (range 13.1-50.9 %) in the surface area of the FM in skulls with AOZ was detected. Extracranial, the clivus length in two skulls with AOZ was smaller than the normal range. No skull was detected with a reduction in the intracranial length of the clivus. All skulls with the AOZ had a vermian fossa. CONCLUSIONS: The study adds important morphometric details about the partial and complete AOZ and correlates the phenomenon of synostosis with the narrowing of the FM, particularly in the case of complete AOZ. Awareness of the AOZ and other fusions of the upper cervical vertebrae and their topographical relations and attendant problems are of paramount importance to surgeons, when operate to the craniocervical junction, or interpret imaging studies to plan a safe surgery for nerve or spinal tissue decompression.

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Cranio-vertebral junction is a pivot which holds the globe of the head. Bony anomalies at this point are particularly significant because they lodge the spinal cord and lower part of the brain stem. Clinically fusion of the atlas with the lower part of the occiput is known as Atlanto-occipital assimilation or atlas occipitalization, which can be either partial or complete depending upon the extent of fusion. It can present as totally asymptomatic accidental finding or can be a cause behind major neuro-vascular compression. The present case study is an endeavor to explain occipitalization of atlas bone on the basis of embryology and explain its clinical relevance.

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