Which vertebrae is firmly attached to the ilium?

The sacral vertebrae are firmly attached to the ilium, forming a critical connection between the spine and the pelvis. This attachment occurs at a structure known as the sacroiliac joint, where the sacrum, composed of fused sacral vertebrae, connects to the ilium of the pelvis. This joint plays a significant role in transferring weight and forces from the upper body to the lower limbs during standing, walking, and other movements.

The lumbar, thoracic, and cervical vertebrae do not have this attachment to the ilium; instead, they are part of the vertebral column that supports the back and protects the spinal cord. The lumbar vertebrae are located above the sacral region and are responsible for bearing weight and facilitating movements like bending and twisting. Thoracic vertebrae are connected to the rib cage and are situated above the lumbar region. Cervical vertebrae make up the neck region and support the head but do not connect to the pelvis at all. Thus, the sacral vertebrae's unique location and structure directly facilitate their firm attachment to the ilium, making them integral to the pelvic support structure.