The intervertebral discs make up one-fourth of your spinal column and act as the spine's shock absorbing system, protecting the vertebrae, spinal cord, and other structures. However, sometimes these discs degenerate and thin, causing the vertebrae they support to get closer together and pinch the nerves that exit between them. In severe cases, these discs are removed and replaced with bone harvested from the pelvis. This is called spinal fusion. Many doctors choose to approach the diseased discs from the front by first retracting the intestines and other organs to reveal the spinal column. The damaged disc is then removed. Openings slightly wider than the removed disc are drilled into the surrounding vertebrae. Titanium casings, or cages, are filled with the harvested pelvis bone and placed into the holes. Specialized cells called osteocytes within this bone produce new bone and aid in the site's healing. The openings in these cages allow the bone to grow around it. In addition, these casings provide support and structure while the bone is healing. Repeat x-rays of the spine should be done at 6 weeks, 3 months, 6 months, 1 year and 2 years to ensure that the new bone is healing properly.