The "lateral mass" is part of the anatomy of the cervical spine. Lateral mass screws are a type of hardware that can be used to stabilize the cervical spine.
When we look at the bony anatomy of the posterior aspect of the cervical spine, we can see there is a thickening of the bone to the right and left of midline. These areas of bone, which also form the facet joints of the cervical spine, are regions where screws could be placed when fixation in the anterior (front) of the cervical spine is not an option.
The top and bottom aspects of the lateral masses are covered with cartilage and they form the facet joints of the cervical spine, which are the joints on the back side to the right and left of midline, and as seen above, have the shape like shingles on a roof.
Using anatomic landmarks to avoid injuring any vital structures, holes can be drilled in the lateral masses, into which lateral mass screws can be placed.
After the screws are in place, rods can be contoured to fit the alignment of the screw heads, and locking caps can be applied.
Bone graft is then added to the backside of the cervical spine to help bring about the fusion.
