Spinal instability refers to a condition where the structural components of the spine have become weakened, allowing for excessive motion and risk of spinal cord or nerve compression. This is a rare situation, but can occur as a result of trauma, infections, tumors, or simply progressive degenerative changes. When the instability threatens the spinal cord or nerves, or causes severe pain unresponsive to conservative treatment, surgery may be indicated. In these cases, a fusion of the spine is usually required to achieve adequate stability of the spine to resolve symptoms or prevent progression. When weakness is rapidly progressive, these cases can be urgent.