S1 (Pelvis)
The first sacral vertebra is called S1, which is the first of the five fused vertebrae that make up the sacrum. The sacrum is a triangular bone located at the base of the spine, between the two hip bones, and forms the back wall of the pelvic cavity.
Along with the fifth lumbar (L5), the S1 vertebra bears all the weight of the upper body, making it more prone to injury.