The lateral masses of the atlas (C1) are its thickest and most solid portions, bearing the entire weight of the skull through their superior articular facets. Each lateral mass has a concave superior articular facet for the occipital condyle and a flat or slightly concave inferior facet for the C2 lateral mass. The transverse processes project laterally from each mass and contain the transverse foramina.
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| Landmarks |
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| Ossification | Three ossification centres, two lateral centres for the lateral masses and a posterior centre for the posterior arch. The anterior arch ossifies from a separate centre or by extension from the lateral masses, fusing by age 3-6. |
The atlas lateral mass is fractured in Jefferson burst fractures from axial loading (diving injuries), where bilateral fractures allow the lateral masses to spread outward. CT with coronal reconstruction is required to assess lateral mass displacement. A combined spread of more than 6.9 mm on open-mouth odontoid view indicates transverse ligament rupture and instability. Lateral mass screws are placed through the posterior arch into the lateral mass for atlantoaxial fusion.
Axial loading drives the occipital condyles into the lateral masses, spreading them apart; overhang of more than about 7 mm combined suggests transverse ligament rupture.
Locking of the lateral masses in a rotated position, mainly in children after minor trauma or throat infection, causes a fixed torticollis.
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