This was an attempt to investigate the decisive influence of PI on sagittal changes during standing-to-sitting transformation, and to figure out the difference in postural changes among different Roussouly types. Our results revealed that PI was correlated significantly with ΔPT, ΔSS, ΔLL, ΔUpper LL and ΔLower LL, and when standing-to-sitting, type 4 patients performed greatest ΔPT and ΔLL than other types.

When standing-to-sitting, we found that in patients with lumbar degenerative disease, SVA, OD-HA and PT increased significantly while SS, LL, TK, upper LL and lower LL had a significant decreasing trend. The OD-HA (odontoid hip axis) is a reliable parameter to evaluate the overall sagittal balance, and its normal range is + 2° to − 5° in asymptomatic patients [15]. Overall, when shifting from standing to sitting, the sagittal balance moved forward, the pelvis retroverted and the whole spine got flattened, which was consistent with previous studies [2, 12]. Compared with standing position, it was noted that PT/PI increased in sitting position. Mac-Thiong et al. [16] once reported the change of PT/PI was important to detect the early spinal pathology, such as spondylolisthesis. Thus, the increase of PT/PI might link to the decreased compensatory capacity of the pelvis in sitting position. In this study, we found that lower LL was larger than upper LL, regardless of posture. It is suggested that the contribution of the lower lumbar region was greater to global LL than the upper lumbar region, and the flexibility of L4-S1 was higher than L1-L4 during standing-to-sitting.

PI is a vital predicting factor among lumbo-pelvic parameters and represents the compensatory capacity of lumbo-pelvic sagittal alignment in standing position [17, 18]. In a study of asymptomatic populations, Pesenti et al. [19] found that PI affected the distribution of lumbar lordosis, and the percentage of L4-S1 lordosis and L1-L4 lordosis in global LL varied with PI. In our study, we found that PI was correlated to ΔPT, ΔSS and ΔLL significantly, which meant patients with a larger PI had a higher capacity of performing changes on sagittal alignment. Several studies have posed an opinion that lumbo-sacral fixation might limit pelvic retroversion and lumbar mobility [20, 21]. If the sagittal profile did not match the patient’s own PI value after lumbar fusion surgery, especially multisegmental lumbosacral fusion, patients could not increase their PT to the optimal value during sitting, which might affect the completion of some daily activities. The deficiency of pelvic retroversion also increased the risk of hip dislocation [9] and had a correlation to post-operation pain [22]. During preoperative planning, it is indispensable to evaluate PI and the capacity of pelvic retroversion in both standing and sitting position, which has a value in determining an appropriate restoration plan to minimize the risk of pelvic stiffness or anterior hip joint impingement.

Notably, ΔLower LL correlated more strongly with PI compared to ΔUpper LL, indicating that patients with a large PI had more loss of lumbar lordosis in L4-S1 during postural transition. Sitting required pelvic retroversion and spinal straightening, while the changes did not occur in fused segments after lumbar fusion surgery. The adjacent unfused segments tended to overcompensate, which increased the risk of the adjacent segment degeneration [23, 24]. Considering that the lower lumbar spine had greater mobility during sitting-to-standing motion, the possibility of adjacent segment disease in the upper lumbar spine would greatly increase if LL was not properly reconstructed in L4-S1 fusion surgery. Before restoring lumbar lordosis, the surgeons ought to make personalized analysis based on PI and spinal-pelvic anatomy, and select the surgical segments carefully, to prevent postoperative lumbar stiffness or junctional complications in sitting position.

Our results revealed that Roussouly type 4 patients had the most pronounced ΔPT and ΔSS. On the one hand, it might be related with higher PI values in type 4 patients. On the other hand, it suggested that compared with the other three types, type 4 patients had greater pelvic retroversion during postural changes. From standing to sitting, due to the loss of LL in type 4 patients, the overall lumbar segments tended to be flattened, resulting in forward displacement of the sagittal balance. To maintain spinal stability, type 4 patients had postural pelvic retroversion. Barrey et al. [17] once reported that PT had a certain reference value for evaluating spinal-pelvic sagittal compensation, and the pelvis would gradually retroverted as the sagittal imbalance developed. Earlier studies found that in order to retain balanced sagittal profile, pelvis retroverted further in sitting position as a compensatory mechanism [25, 26]. Considering that type 4 patients would have excessive pelvic rotation in sitting position, the ability of pelvic retroversion should be properly preserved in preoperative planning.

As shown in our study, ΔLL of type 4 patients was significantly greater than that of any other type. In types 1 and 2 patients, the lumbar curve was relatively flat and the stress mainly concentrated on the anterior and middle column when the posture changed. While in type 4 patients, the lumbar spine appeared in the shape of “C” when standing. Owing to remarkable ΔLL, the gravity produced forward shear force in sitting position which concentrated on the posterior facet joints and isthmus originally [27]. Thus, type 4 patients should pay attention to the occurrence of lumbar instability owing to greater range of motion. It may also explain the reason why sitting radiographs brought out a higher slip percentage in previous studies [3]. The ΔUpper LL and ΔLower LL of type 4 patients were the greatest, which meant that type 4 patients presented greater flexibility of the global lumbar spine than the other three types. After lumbar fusion surgery, surgeons ought to take precautions against potential challenges in adjacent segments when patients are seated.