Spinal injury: false-colour X-ray of the lumbar spine showing compression of the 4th and 5th lumbar vertebrae. The 5th lumbar vertebra is at bottom, resting on the sacrum; the 4th vertebra appears immediately on top of it, that is, without preservation of any intervertebral space. This type of injury is often referred to as a slipped or prolapsed disc. The bones comprising the spinal column are normally held apart by cushions of pulp-like material, the intervertebral discs. The lumbar discs are especially vulnerable to mechanical injury. A disc may become compressed & its contents extruded to possibly compress an adjacent nerve root.