What structures result from the failure of the laminae of S5 to form and fuse?

What structures result from the failure of the laminae of S5 to form and fuse?

• A. Median sacral crest
• B. Coccygeal joints
• C. Sacral promontory
• D. Sacral cornua

Answer: (D)

The laminae of the last sacral segment normally help form the posterior arch of the sacrum. If they fail to form and fuse, the posterior boundary of the sacrum at that level is incomplete, which leaves the inferior articular region of S5 more prominent as downward projections. These downward projections become the sacral cornua, the two horns that flank the sacral hiatus and articulate with the coccyx. So, when the laminae don’t form and fuse, the resulting structures are the sacral cornua.

The laminae of the last sacral segment normally help form the posterior arch of the sacrum. If they fail to form and fuse, the posterior boundary of the sacrum at that level is incomplete, which leaves the inferior articular region of S5 more prominent as downward projections. These downward projections become the sacral cornua, the two horns that flank the sacral hiatus and articulate with the coccyx. So, when the laminae don’t form and fuse, the resulting structures are the sacral cornua.