Note that the lumbar spine is straight (Image A). It usually has at least 40 degrees of lordosis (swayback). Therefore, when it is straight as in the lower example, there is in essence 40 degrees of excess kyphosis.
This patient underwent multiple lumbar fusion operations. She developed a lumbar flatback deformity (decreased lordosis). Consequently, she developed a stooped forward posture and debilitating back and buttock pain. She was noted to have severe spinal stenosis adjacent to her fusion.
It is important not only to do a decompression to relieve the stenosis, but also to realign the spine otherwise there was risk of continued problems above the fusion. It is also important to realign the spine to improve the back pain and correct the stooped forward posture.
Note that a large wedge was cut out of the spine (Image B). The screws and rods are used to hold the spine in the corrected position.