The protrusion of the center of an intervertebral disc into the soft, spongy material of an adjacent vertebra. When this protrusion is substantial, it can lead to vertebrae taking on a wedge-shaped appearance and result in significant curvature of the spine. These are known as siderotic nodules, which are nodules found in certain organs. They develop due to a localized hemorrhage, with the nodule forming from a blood clot that gradually transforms into fibrous tissue.