Which motion unit is affected by an atlas posterior?

Which motion unit is affected by an atlas posterior?

• A. C0-C1
• B. C1-C2
• C. C2-C3
• D. C3-C4

Answer: (B)

When we think about atlas posterior, the important idea is that the atlas (the first cervical vertebra, C1) sits atop the axis (C2) and forms the atlantoaxial joint, which is the primary pivot for head rotation. Movement described as atlas posterior typically involves how C1 moves relative to C2, so the motion unit most affected is the joint between C1 and C2. The joint between the skull and the atlas (occipito-atlantal, C0-C1) mainly handles nodding (flexion/extension), not rotation, so it isn’t the primary site influenced by atlas posterior. The lower cervical joints (C2-C3, C3-C4) aren’t the key players in atlas-posterior movement either. So, atlas posterior most directly affects the C1-C2 motion unit.

When we think about atlas posterior, the important idea is that the atlas (the first cervical vertebra, C1) sits atop the axis (C2) and forms the atlantoaxial joint, which is the primary pivot for head rotation. Movement described as atlas posterior typically involves how C1 moves relative to C2, so the motion unit most affected is the joint between C1 and C2. The joint between the skull and the atlas (occipito-atlantal, C0-C1) mainly handles nodding (flexion/extension), not rotation, so it isn’t the primary site influenced by atlas posterior. The lower cervical joints (C2-C3, C3-C4) aren’t the key players in atlas-posterior movement either. So, atlas posterior most directly affects the C1-C2 motion unit.