Previous studies have reported the differences between standing and sitting positions in asymptomatic subjects. Endo et al. performed an imaging analysis of sagittal alignment in the standing and sitting positions in 50 healthy people with an average age of 31.5 years1. The results indicated that in the sitting position, the curvature of lumbar lordosis decreased by 50% and PT increased by 25%. Other studies yielded similar results, which can be summarized as a straightened curve in the lumbar region, pelvic retroversion, and forward displacement of sagittal balance2–5, 11. Hey et al. compared the spinal sagittal alignment in three weightbearing positions (standing, erect sitting, and natural sitting) in healthy subjects and found that LL decreased by approximately 80% in the natural sitting position; furthermore, the curvature of the trunk was vaguely C-shaped3. The current study also measured the spinopelvic sagittal alignment in three common weightbearing positions (standing, erect sitting, and natural sitting). We found that the sagittal alignment noticeably changed among positions, particularly with regard to LL and unfused adjacent segment lordosis. When changing to a natural sitting position, the entire thoracic region, unfused adjacent segments, and fusion segments were also vaguely C-shaped. Currently, the gold-standard target for sagittal realignment surgery is to restore natural spinal curvature in a standing position12,13. Almost all of the published studies on related mechanical complications were conducted in the standing position for reference14 ,15. This was based on the premise that the standing position was the most desired posture for the spine. However, it was also suggested that for a substantial number of people, a more functional position may be sitting rather than standing. From a surgical perspective, the observations of this study in the natural sitting position could explain some of the problems observed after sagittal realignment surgery. Correction of LL based solely on the standing spinal alignment would prevent the spine from settling into its natural shape in the natural sitting position. Instead, the spine would invariably be forced to conform to an abnormal posture that would result in undue stress on the implant construct and its adjacent vertebrae16. If combined with osteoporosis, this presumably could lead to complications such as implant loosening and proximal or distal junctional kyphosis and failure. These considerations were ignored in previous investigations of the etiological factors of junctional complications.
Spinal fusion renders the spine immobile in a fixed curvature; therefore, the spinal surgeon should pay close attention to the characteristics of changing alignment and the influencing factors in different weightbearing positions. Several studies have examined standing spinal alignment variability across age with the aim of discovering a more individually tailored strategy for sagittal realignment surgeries and the possible causes of junctional complications. In a previous study, for asymptomatic subjects, it was demonstrated that age significantly affected the change in LL when the position was changed from standing to sitting. With increased age, the reduction in LL was smaller11, 17. In this study, we also found that with increased age, ΔLL and ΔURL were both smaller after lumbar fusion. These findings indicated that the ability to compensate for the changes in lumbar alignment was reduced in elderly people. In terms of age, it was often accepted that attempts to achieve a “normal” or “young” spinal alignment in an elderly patient was not only unnatural, but also resulted in overcorrection. We speculate that if LL is overcorrected, not only can this result in imbalance in standing, but imbalance in sitting position can easily occur, indicating that the patient would be unable to control the trunk when sitting down, and the trunk can easily tilt backward uncontrollably. To achieve balance, the demand on the paraspinal muscles to maintain spinal stability naturally increases, and it becomes reasonable to assume that these patients will have greater back pain or fatigue in the sitting position. Proposed age-adjusted and sitting values for sagittal realignment surgeries may further influence future surgical strategies18–20.
Maekawa et al. performed an imaging analysis of sagittal alignment in the standing and sitting positions in 253 subjects with an average age of 53.6 years21. The results revealed that when changing to a sitting position, the changes in sagittal lumbo-pelvic alignment (ΔPT and ΔLL) were found to be regulated by the degree of PI. An asymptomatic subject with low PI in the standing position would be less capable of performing changes in alignment between the sitting and standing positions. However, in the current study, with the exception of ΔPT, the changes in LL and unfused adjacent segment lordosis were not found to be regulated by PI. We speculate that the fused segments resulted in the loss of regulation between PI and ΔLL. Although PI is an anatomical parameter and plays an important role in sagittal realignment, age is a more important influencing factor than PI when we consider the changes in LL and unfused adjacent segment lordosis in the sitting position.
The current study had some limitations. First, only patients who had undergone short lumbar fusions were included. Thus, the features of sagittal alignment in the sitting position after long fusions will be discussed in future studies, particularly in the natural sitting position. Secondly, as we only investigated imaging changes without clinical scores and a biomechanical basis in this study, we cannot simply apply the conclusions drawn from these changes to junctional complications. This will be studied in the future. These previously ignored considerations may provide a new theoretical basis for the surgical strategy and the etiology of junction complications.