Spinal sagittal balance is influenced by thoracic kyphosis, lumbar lordosis, as well as the position and angle of the pelvis. The abnormal position of sagittal imbalance causes easy fatigue and pain on the back muscles, so that the body voluntarily takes action to compensate for the imbalance. Compensatory mechanism occurs over several steps. It begins primarily in the mobile lumbar segments. When sagittal imbalance occurs due to decreased lumbar lordosis, the disc space at the mobile segments is hyperextended, allowing the C7 plumb to be restored within near normal range. As patients are older, subsequent degenerative disc changes develops at the hyperlordotic segments leading to loss of lumbar lordosis, which results in sagittal imbalance again. If the compensation in the spine is no longer able to restore the sagittal imbalance, posterior rotation of pelvis can reestablishes sagittal balance. This finding shows that the rotation of pelvis is important in the compensatory mechanisms. Another thing to consider is that compensation gets involved in the actual lumbar lordosis. During the compensation process rotation of pelvis determines the sagittal imbalance. This is also very difficult to estimate how much the rotation of pelvis is, because the rotation is variable depending on the position. Pelvic incidence (PI) is a suitable index to determine how much correction of lumbar lordosis requires, as this does not change depending on the position of the pelvis. Patients with high PI are able to compensate the sagittal imbalance more than them with low PI. As PI is the sum of pelvic tilt and sacral slope, PI is closely related to lumbar lordosis. Patients with high PI needs more correction of lumbar lordosis to restore sagittal imbalance than the patients with low PI.

To understand the compensatory mechanisms of knee and hip joint for sagittal imbalance, it is very important to investigate the response of each joint to the abnormal condition of other joint. Most of studies so far were limited to the response of spine and hip joint to the sagittal imbalance and it is very rare to include the response of knee joint. We assume four conditions and investigate the response of each joint to the abnormal condition of other joint 1. The normal response of spine and pelvis to the knee flexion contracture 2. The response of spine and pelvis to the correction of knee flexion contracture 3. The response of knee and hip joint to the loss of lumbar lordosis 4. The response of knee and hip joint to the restoration of knee and hip joint The results show that the flextion contracture of knee joint makes hip jont flexed and loss of lumbar lordosis with shift the sagittal balance forward. When flexion contracure was corrected like TKRA, hip joint also extended and lumbar lordosis increase and sagittal balance moves backward. Loss of lumbar lordosis makes more positive sagittal balance and hip joint becomes extended and knee joint flexed. The restoration of lumbar lordosis, sagittal balance shift backward and knee joint extended. However, if restoration of lumbar lordosis is not complete, hip joint remains extended, while knee joint becomes normal position, the extended position.

In General, sagittal spinal imbalance is divided into dynamic and static type. Static sagittal imbalance is due to iatrogenic, post-traumatic causes and the major contributory factor of dynamic sagittal imbalance is degenerative change such as lumbar degenerative kyphosis. The operative technique and method of correction for static sagittal imbalance is established somewhat and dynamic sagittal imbalance has different pathophysiology from the static one. So the operative method for correction of dynamic sagittal imbalance should be different from the static one. The degenerative change and segmental instability of lumbar spine in dynamic sagittal imbalance and accordingly, loss of lumbar lordosis and sagittal imbalance causes lumbar kyphosis and forward bending of the body. But there are a few studies about dynamic sagittal imbalance and there is controversy about operative method for correction of dynamic sagittal imbalance.

In patient with loss of lumbar lordosis, lumbar lordosis restoration is very important to get a normal sagittal balance. It is difficult to know the ideal lumbar lordosis, because it is a individual-unique pattern and is influenced by numerous variables. To restore a more ideal lumbar lordosis, careful analysis and planning will be needed because incomplete restoration of lumbar lordosis may result in poor clinical outcomes and serious complications. Lumbar lordosis has strong relationship with pelvic orientation, so there are many reports about predictive equation of lumbar lordosis using pelvic parameters. Pelvic incidence is major determinant of pelvic orientation, therefore, influence on lumbar lordosis. So pelvic incidence is a useful value to determine the ideal lumbar lordosis, and we recommend that the degree of restoration of lumbar lordosis is just enough to pelvic incidence at least. And also, a close study and understanding of the numerous factors affecting lumbar lordosis was needed to effective lumbar lordosis restoration.

Purpose
There is an increasing recognition of the clinical importance of the sagittal balance after lumbar fusion surgery. The purpose of this study to review the etiology of sagittal imbalance after lumbar fusion surgery and report the radiographic and clinical results of surgical treatment of these patients.
Materials and Methods
Retrospective review of revision spine surgery due to sagittal imbalance in 35 patients. Various surgical methods such as posterior–anterior–posterior (PAP) sequential approach, Smith–Peterson osteotomy (SPO), pedicle subtraction osteotomy (PSO), and vertebral column resection (VCR) were performed to restore lumbar lordosis. The outcome variables included preoperative, postoperative, and follow-up radiographic films, and a clinical assessment using Oswestry Disability Index (ODI), SRS 22, and a review of postoperative complications.
Results
The mean age of the patients was 62 years (age range, 49–74), and mean follow-up duration was 31 months (range, 24–37) for clinical and radiographic outcome variables. The mean preoperative LL/PI (lumbar lordosis/ pelvic incidence) ratio was different from postoperative value (P< 0.0001). Twenty one out of 35 patients showed perioperative complications including proximal junctional kyphosis or infection. All functional outcomes measures improved postoperatively (P < 0.0001).
Conclusion
Most common causes of revision spine surgery due to sagittal imbalance include failure to enhance lumbar lordosis, proximal vertebral collapse, and junctional kyphosis. LL/PI ratio was considered as one of the valuable spinopelvic parameter for evaluation of sagittal imbalance. Following surgical treatment, sagittal balance was generally improved with good to excellent clinical outcomes and high patient satisfaction, although the perioperative complication rates are high.