Purpose
The biplanar whole body imaging system (EOS) is a new tool for measuring whole body sagittal alignment in a limited space. This tool may affect the sagittal balance of patients compared to conventional whole spine radiography (WSX). This study is to investigate the difference in sagittal alignment between WSX and EOS.
Materials and Methods
We compared spinal and pelvic sagittal parameters in 80 patients who underwent EOS and WSX within one month between July 2018 and September 2019.The patients were divided based on sagittally balanced and imbalanced groups according to pelvic tilt (PT) >20˚, pelvic incidence-lumbar lordosis >10°, C7-sagittal vertical axis (SVA) > 50 mm in WSX.
Results
In sagitally imbalanced group, for WSX versus EOS, the pelvic parameters demonstrated compensation in EOS with smaller PT (27.4±11.6° vs. 24.9±10.9°, p=0.003), greater sacral slope (SS), and patients tended to stand more upright with smaller C7-SVA (58.4±17 mm vs. 48.9±57.3 mm, p=0.003), T1-pelvic angle (TPA), T5-T12, and T2-T12. However, in sagitally balanced group, these differences were less pronounced only with smaller PT (10.8±6.9° vs. 9.4±4.7°, p=0.04), TPA and T2-T12 angle, but SS and C7-SVA were similar (p>0.05).
Conclusions
EOS shows a negative SVA shift and lesser pelvic tilt than WSX especially in patients with sagittal imbalance. When making a surgical plan, surgeon should consider these differences between EOS and WSX.

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.