Spondylosis or Spondylolisthesis: This condition involves a defect in the pars interarticularis, which can lead to a bony protrusion and slippage of one vertebra over another. It's a common cause of back pain and can be degenerative or congenital.

Herniated Disc: Although more commonly associated with soft tissue protrusion, a herniated disc at L4-L5 can cause bony changes and protrusions due to chronic inflammation and degeneration.
Osteophyte Formation: Bony spurs or osteophytes can form on the vertebrae due to degenerative changes, leading to a bony protrusion.
Degenerative Disc Disease: This condition can lead to bony changes, including protrusions, as the disc space narrows and the vertebrae adapt.

Metastatic Disease: Although less common, metastatic cancer to the spine can cause bony destruction and protrusion. It's crucial to consider this diagnosis, especially in patients with a known history of cancer.
Infection (Osteomyelitis or Discitis): Infections of the spine can cause bony destruction and protrusion, and are medical emergencies requiring prompt treatment.
Multiple Myeloma: This plasma cell malignancy can cause lytic lesions in the bone, leading to bony protrusions and other spinal abnormalities.

Aneurysmal Bone Cyst: A rare, benign tumor that can cause bony expansion and protrusion.
Giant Cell Tumor: Although typically found in the extremities, giant cell tumors can rarely occur in the spine, causing bony destruction and protrusion.
Osteoblastoma: A rare, benign tumor that can cause bony growth and protrusion in the spine.