Adult spinal deformity (ASD) is a complex spine disease. It is generally believed that the balance of the coronal and sagittal planes has a clear correlation with the patient’s quality of life. Among them, sagittal balance has a greater impact on the quality of life of patients with ASD [8, 9]. At present, the evaluation of sagittal balance is mainly accomplished by measuring SVA. It is generally believed that SVA less than 5 cm can be considered as sagittal balance . However, in patients with ASD, pelvis often rotate backward to compensate the imbalance trend. When a healthy adult stand in a natural standing position, pelvis is in the neutral position. For patients with ASD, due to lumbar lordosis decreased and thoracic kyphosis increased, their body shifts forward. However, to regain trunk balance and reduce energy consumption, the pelvis will rotate backward to compensate. In the case, SVA is a compensated value, which cannot objectively reflect the real sagittal imbalance and the real function. Therefore, it is important to find a new parameter to evaluate both sagittal balance and pelvis compensation——the modified sagittal vertical axis(MSVA).
MSVA is defined as the distance from C7 to PNP-LINE, which can reflect the real sagittal imbalance. (Fig.2). PNP-LINE is the parallel line of TPT-LINE drawn from the posterior superior corner of the sacrum (Fig.2). In the process of compensation, PT increases. The PT value at this time is also a value after pelvic compensation. Vialle et al. used the formula PT=0.37PI-7 to get theoretical PT value (TPT) through linear regression analysis to evaluate compensation of pelvis . The TPT value means the real PT value without pelvic compensation. The difference between the TPT value and PT value is defined as ∆PT ,which can reflect the pelvic rotation compensation. According to the TPT value, the line drawn from the midpoint of the hip axis is defined as TPT-LINE. Therefore, the author proposes a hypothesis: the MSVA can better reflect the sagittal balance.
In this study, the spine-pelvic parameters LL, TK, TLK, SS, PI, and quality of life scores were significantly improved postoperatively, indicating that orthopedic surgery can significantly improve spine-pelvic alignment and the patient’s quality of life of patients with ASD. Meanwhile, we found that postoperative quality of life score was correlated with postoperative SVA and MSVA (Table 4). And MSVA had the stronger correlation with postoperative quality of life score. Therefore, we thought MSVA can be used as a new parameter to assess sagittal balance in patients with ASD. For patients with spinal deformity, satisfactory sagittal alignment is closely related to satisfactory function postoperatively. MSVA has important clinical significance for assessing the postoperative sagittal balance and the quality of life.
Traditional SVA can reflect the overall sagittal balance, but cannot describe the compensation of the pelvis. Pelvis compensation is not only related to a satisfactory spinal alignment, but also related to the patient's quality of life. MSVA is the parameter which can reflect the rotation of the pelvis and the overall sagittal balance of the spine simultaneously. Although, SVA is better than MSVA in ODI scores, MSVA has stronger correlation and higher significance than SVA in SRS22 scores. Especially for patients with severe pelvic compensation, SVA value may be satisfactory, but the quality of life is poor. In the case, the MSVA is huge which may imply the poor quality of life. In Fig 3, we can find that the SVA is satisfactory but MSVA is huge. In the case, the quality of life is not satisfactory. And orthopedic surgery can reduce MSVA and improve the quality of life of patients. However, we can't simply draw a conclusion that MSVA is better than SVA. MSVA can be regarded as a powerful supplement to assess the sagittal balance of spine and pelvis.
There are still many deficiencies in this study. There were no meaningful results regarding preoperative MSVA and SVA. The reason may be that the sample size of this retrospective study is limited, and the inclusion criteria did not exclude patients with lumbar spinal stenosis which can interfere with the research results. As an innovative spine-pelvic parameter, MSVA still need further study. In the next step, we hope to get the best range of MSVA by large samples to guide the clinical treatment.