Origin of the median and lateral sacral crests

The dorsal surface of the sacrum has three types of five rows of longitudinal crests.
The median sacral crest is a bony fusion of the spinous processes and the interspinous ligaments of the five sacral vertebrae.
Both lateral sides of the median sacral crest are weak intermediate sacral crests. These are fused articular processes.
In addition, both lateral sides to the intermediate sacral crests are the lateral sacral crests. These are caused by the fusion of the transverse processes.