Does lumbar lordosis minus thoracic kyphosis predict the clinical outcome of patients with adult degenerative scoliosis?

Owing to the degeneration of spinal motion segments, adult degenerative scoliosis (ADS) was a common type of adult spinal deformity (ASD) without history of adolescent idiopathic scoliosis (AIS) [1]. This kind of deformity could be found in approximately 30% of elderly people [2]. ADS could manifest various symptoms including low back pain, radiating pain in lower extremities, neurological deficits, and claudication [3], which significantly impacts patient’s health-related quality of life (HRQOL) [4]. Treatment of adult degenerative scoliosis remained a challenge for spine surgeons. Conservative therapy was initially recommended, but for patients with persistent symptoms, surgical treatment was needed.

In these patients, the key procedure of surgery was to achieve an ideal correction of deformity, especially on sagittal plane, since sagittal balance has been proven to be most closely associated with surgical outcome [5‐8]. Schwab et al. proposed a classification for patients with ASD based on three critical sagittal parameters including sagittal vertical axis (SVA), pelvic tilt (PT), and pelvic incidence minus lumbar lordosis (PI-LL), and they pointed out that corrective surgery should reach the objectives of SVA less than 50 mm, PT less than 25°, and PI-LL within 10° [9, 10], which had been widely used in clinical practice.

Apart from these classical sagittal modifiers, another study demonstrated that lumbar lordosis minus thoracic kyphosis (LL-TK) could be a good predictor for sagittal balance in Chinese elderly people [11], since LL-TK reflected the regional compensatory mechanism and significantly associated with SVA. Realizing the importance of this regional predictor, we speculated that LL-TK could also be a good parameter to guide the correction surgery for patients with ADS, which has not been addressed by the previous studies.

Thus, this study was aimed to evaluate the predictive effect of LL-TK on the surgical outcome of patients with ADS and define the suitable correction goal based on this parameter.

Since the sagittal balance was reported to be significantly related with patient’s health-related quality of life (HRQOL) [12‐14], the importance of sagittal alignment and sagittal balance has been gradually recognized. A suitable sagittal alignment could save the energy of maintaining erect position and reduce the load of muscle. Many spinal degenerative diseases were accompanied by a decrease in LL and increase in PT, which could usually lead to sagittal imbalance, especially in patients with ADS whose compensatory mechanisms failed to compensate for primary deformity [15].

To rebalance the forward moving center of gravity, the compensatory mechanism involved in the spine (decrease in TK and extension of adjacent segments), pelvis (pelvic retroversion, in other words, increase in PT), and lower extremities (flexion of knee and extension of ankle). As proposed by previous study, LL-TK accounted for the local compensatory mechanism, and LL-TK > 0° could predict a good sagittal balance in elderly population since TK could decrease to compensate the loss in LL [11]. In the long-fixed spine of patients after corrective surgery, the fused sagittal alignment, including lumbar and thoracic curve, as well as pelvic rotation, limited the regional compensatory mechanism: decreased thoracic kyphosis and increased pelvic tilt. So it was still unclear whether the regional parameters expressing as LL-TK could effectively predict the postoperative sagittal balance and HRQOL in patients with ADS.

In the present study, we evaluated the relationships between postoperative sagittal parameters and HRQOL and found that SVA, PT, PI-LL, and LL-TK were all significantly related with postoperative ODI. This result was consistent with previous studies [5, 16, 17], which demonstrated that the sagittal balance, rather than coronal balance, influenced patients’ HRQOL. Among these parameters, LL-TK reflecting the regional balance of lumbar and thoracic spine most highly correlated with ODI (r = − 0.300), and it was also associated with SVA (r = − 0.185). These results indicated that LL-TK could predict the postoperative HRQOL and sagittal balance. In the normal spine, TK could decrease to compensate the loss of LL and rebalance the spine and LL also increased with increased TK, so LL-TK could remain > 0°. However, this compensatory mechanism was limited in the spine of patient after correction surgery, so the decrease in LL or increase in TK could more directly influence LL-TK, then the sagittal balance. Therefore, the postoperative LL-TK could effectively predict the postoperative sagittal balance and HRQOL.

In addition, by using the ROC curve, we calculated the cut-off value of LL-TK at 10° to reach a good clinical outcome (ODI < 20) and sagittal balance (SVA < 50 mm). Then, we divided the patients into two groups based on LL-TK assessed soon after surgery, and compared the preoperative and postoperative data of the two groups. According to Table 4, group 1 (LL-TK > 10°) showed significantly better VAS back, ODI, and JOA at the last follow-up than group 2 (LL-TK < 10°), and there were more patients with sagittal imbalance in group 2 than group 1. These results demonstrated that LL-TK > 10° could more successfully predict a better clinical outcome in patients with ADS. With respect to the predictive effect, LL-TK > 10° rather than LL-TK > 0° might be more effective, since LL decreased and TK increased, corresponding to decrease in LL-TK from soon after surgery to the last follow-up as shown in Table 1. Interestingly, the decrease in LL-TK was approximately 10°. Lafage et al. [18] defined the ideal threshold of LL-TK ranging from 27.6° for ASD patients aged < 35 years to 2.1° for patients aged > 75 years. Considering the average age (63.6 years) of our patients’ group, our conclusion was generally consistent with their study.

There were still some limitations in the present study. First, this was only a single-center retrospective study which might bring the selection bias. Second, the sample size was relatively small. Therefore, a multicenter prospective study was needed to further test the effectiveness of LL-TK in predicting the outcome of correction surgery. Third, thoracic kyphosis was not easily determined without instrumentation of all thoracic vertebras. However, the lower thoracic vertebrae account for the major part of thoracic kyphosis, so TK could be controlled greatly in the correction surgery (19). Despite these limitations, in patients with ADS (elderly population), TK was larger and took more part in the condition of sagittal balance, so it should be considered when making correction strategy for these patients apart from the classical parameters, and this study proposed that LL-TK was a good predictor for clinical outcome for patients with ADS, and LL-TK > 10° could be an optimal corrective target.

LL-TK measured soon after surgery was strongly associated with patients’ HRQOL and SVA at last follow-up, so it was suitable for predicting the surgical outcome for patients after correction surgery. Based on ROC curve, the cutoff value of LL-TK to determine a good clinical outcome (ODI < 20) and sagittal balance (SVA < 50 mm) was set at 10°. Patients with LL-TK > 10° showed significantly better clinical outcome than patients with LL-TK < 10°, so LL-TK > 10° could be a suitable correction target for patients with adult degenerative scoliosis.