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A unique ring-shaped vertebra without a vertebral body and no adjacent disc, which has narrow anterior and posterior arches connected to stout wedge-shaped lateral masses; C1 transitions a rigid skull and a mobile cervical spine, acting like a “washer”, buffering various forces. Transverse atlantal ligament and alar ligaments stabilise C1 on C2; atlantoaxial joint ROM is 40º rotation, 10º flexion and 10º lateral bending.
A designation used in the UK for FNAC (fine needle aspiration cytology) specimens which cannot be interpreted due to:
(1) Hypocellularity—As a general rule, 6 to 10 epithelial clusters should be present, unless cells are not expected (e.g., fatty fragments) in a lesion that is clinically or radiologically a lipoma. Other situations in which hypocellular lesions should not be labelled C1 include fat necrosis and breast abscesses.
(2) Technical errors in aspiration, spreading or staining the specimen—Examples include blood staining, which makes it difficult to visualise epithelial clusters, crush artefact due to poor spreading of friable cells, over-/understaining, inadequate air drying and poor fixation.