Fusion of the lumbar spine is sometimes performed by implanting titanium cages packed with bone, in the interspace between two vertebral bodies. The aim of this surgery is to have the metal cages packed with bone act as a supportive structure initially, while the bone slowly grows through the openings in the cage, and eventually the vertebral body above should fuse to the vertebral body below. Pictured here is a translucent model of the spine, with two titanium cages situated between the vertebral bodies.