Previous studies showed that the postoperative complication rates (8.4–42%), revision rates (9–17.6%) in ADS were still high, which could increase after long-term follow-up [14, 15]. Increased junctional stress concentration ADS to the collapse of the implant, or vertebra; this could cause mechanical complications such as PJK, distal junction kyphosis (DJK), pseudoarthrosis, rod breakage or vertebral fracture [5, 9, 11, 16]. The patients included in this study were all elderly, and therefore had poor tolerance to spinal stress concentration. The most common mechanical complication in this study was screw loosening. This is due to a decrease in bone density in older patients. Therefore, the stress concentration on the contact surface between screw and bone structure can be alleviated by screw loosening .
Several parameters were reported to be associated with mechanical complications, including TK, SVA, and SS [16–19]. The PI-LL in SRS-Schwab classification can be used to quantify spinopelvic harmony, which is also thought to be an important vital factor for predicting mechanical complication and is usually used to predict better postoperative health-related quality of life [20, 21]. However, this kind of classification is based on linear absolute numerical parameters, which cannot include the whole spectrum of PI. The realignment targets in SRS-Schwab classification can be misleading when the PI values are near the ends of normality (too high or too low) . Therefore, previous studies used the degree of compensation and present PI to estimate the ideal spinal alignment according to Roussouly-types [22, 23].
Roussouly defined four basic shapes of normal sagittal spine alignment in a healthy population based on SS . However, lumbar degeneration and thoracolumbar coronal deformity could modify lumbar lordosis, which could consequently influence SS . Therefore, SS becomes an inadequate parameter to classify sagittal types in pathologic patients. In addition, the Roussouly classification relies on PI which is considered not to vary with age, pathology, or compensation . However, Roussouly classification is mainly based on the classification of normal spine; most of the studies related to the compensatory mechanism of spinal degeneration were cross-sectional studies [12, 22, 25]. In this study, more cases without mechanical complications were Roussouly-type 1 compared to those with mechanical complications. This is because Roussouly-type 1 is a combination of long kyphosis and short lordosis at the lower arc of the spine; the inflexion point, which always represents the region with the highest junctional stress concentration, has already been fixed in the central structure of the long-segment internal fixation system. Our study showed: there were more patients who matched Roussouly-type in the no mechanical complication group compared with that in mechanical complication groups; compared to cases with mechanical complications, there were more patients without mechanical complications matching ideal LA. These results suggested that the difference in Roussouly type matching between the two groups was mainly due to the ideal LA matching rather than the ideal IP matching. Changing the original IP of the spine can easily lead to overcorrection of spinal deformities, thus increasing the stress on the internal fixation system and then the risk of mechanical complications. Therefore, it is more important to adjust LA of ADS patients during surgery. Our study showed: there was no significant correlation between Roussouly-type matching and mechanical complications; the ROC analysis implied that Roussouly-type matching could not accurately predict the risk of mechanical complications. Roussouly-type only morphologically described the sagittal characteristics of ADS patients, which lacked three-dimensional analysis and quantitative indicators of the spinal deformity in ADS patients. Therefore, the usage of Roussouly classification in the realignment of ADS is difficult.
In contrast, the GAP score quantifies the imaging parameters as well as the age of the patients, thus intuitively predicting the risk of postoperative mechanical complications in ADS patients . However, there is no study comparing Roussouly classification with GAP score in their effectiveness of predicting mechanical complications following ADS surgery. In our study, GAP was better than Roussouly classification in predicting mechanical complications; GAP score was most effective in predicting PJK and PJF; however, the prediction accuracy of GAP for implant breakage and DJK or DJF is low. This is because implant breakage is closely related to the material properties of the internal fixation system itself, the living habits of patients, and the overall structure of the internal fixation. The occurrence of DJK is affected by many factors, such as the distal fixation method, the severity of ADS, and the levels of internal fixation; these factors are not fully reflected in the GAP score, so the accuracy of prediction is also low .
There are some limitations to this study. First, because older patients are more sensitive to spinal sagittal orthodontics, the patients included in this study were older than those in previous studies. This may make the results of this study differ from those of previous studies. However, the age span of patients included in this study was small, so the conclusion of this study could be more accurate when applied to elderly patients. Second, this study only analyzed the parameters involved in Roussouly classification and GAP score, while did not discuss the conditions of paraspinal muscles and lower limb compensations. This will prevent the results of this study from explaining all the causes of postoperative mechanical complications. For example, if the paravertebral muscles of the thoracic vertebrae are weak and the PT is large (pelvic compensation is poor), the compensatory capability of the patient without lower limb compensation will be poor; then the sagittal imbalance of the spine will develop quickly. Despite the deficiencies mentioned above, this study compared the Roussouly classification with the GAP score through careful statistical analysis, and the results of this study were still of a high reference value. A new classification method, considering the compensation of patient-specific spinal alignment and spinal balance, is still needed.