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Most commonly, flat back/fixed sagittal imbalance is related to previous surgery on the spine. Historically, Harrington-rod instrumentation was used to treat scoliosis and sometimes resulted in loss of lumbar lordosis. Occasionally, procedures which remove bone from the back of the spine to relieve pressure on nerves can result in instability and a forward-pitched position of the spine. Severe spondylolisthesis can also result in a fixed sagittal imbalance.