In the present study, we found that patients with ASD showed significantly poorer performance in four types of FMTs compared with those with LSS. While the prolonged time of FMTs in the LSS group can be explained by the neurogenic claudication, the poorer results in the ASD group were likely due to a different mechanism. Truncal deformity, which mostly manifests as a positive SVA, induces forward shifting of the center of gravity (CG), affecting the pelvis and eventually inducing deformation in the lower limbs. These deformities in the pelvis and lower limbs may directly induce a compensatory mechanism that manifests as a characteristic crouching gait. This suggests that reduced physical functionalities in ASD are directly reflected in FMTs.
In our study, all four FMTs showed strong correlations with the ODI scores in the ASD group (Fig. 3, Table 2). On the other hand, static radiologic parameters did not show significant correlations with the ODI scores (Fig. 4). Although various attempts have been made in terms of the radiographic evaluation of ASD, confirmed standards have not been established [3, 20–23]. In addition, patient-reported outcomes (PROs) can only be described within the limits of their subjective aspects . A recent multicenter, prospective study in ASD patients reported that an SVA of 4 cm or greater was associated with worse ODI, while no strong correlation was discovered between any radiographic parameters and the PROs . Our correlation results in ASD patients add further evidence that FMTs may be useful as an objective evaluation tool for assessing the physical function, trunk and lower limb muscle coordination, overall body balance, and risk of falling in patients with ASD.
Interestingly, only the STS result was correlated with the PROs of ASD patients and with the EQ-5D of LSS patients. Results of the STS are associated with postural stability, proprioception, and risk of falling [5, 25] as well as the vertical component of CG. Slow movements are required for controlling the CG and maintaining postural stability, especially in physically impaired patients . A forward shift in the CG can cause an individual to fall forward when rising from a seated position. Patients with LSS have difficulty standing in the upright position due to buckling of the ligamentum flavum with truncal extension. On the other hand, patients with ASD have a characteristic anterior stooping posture, which results in an anterior displacement of the CG. The correlation results of STS show that the STS is more sensitive than other FMTs in reflecting the patients’ quality of life.
The FMTs have been shown to be sensitive in reflecting the functional impairment in other diseases including hip or knee osteoarthritis and lumbar degenerative disease, and correlated with the PROs of patients with such diseases [16, 17, 27, 28]. Moreover, optimal cutoff points of FMTs have been suggested for identifying patients with a high risk of falling [14, 29]. The suggested cutoff points for the TUGT range from 10 to 16 seconds, and those for the SMT, STS, and AST have been reported to be 6, 12, and 10 seconds, respectively . In the current study, the mean values of FMTs of ASD were higher than these cutoff values [16–18], and also higher than those of LSS (Fig. 2). This suggests that the physical function, body balance, and coordination of muscles in ASD were severely deteriorated to the point of increasing the risk of falling. Patients with LSS were also shown to be at high risk of falling in previous studies [16–18]. Although ASD shares several characteristics with LSS in terms of stenosis and sagittal deformities, comparison in our study cohort (Table 1) showed significant differences in most of the regional and global sagittal parameters between ASD and LSS. The weak correlation between FMTs and ODI in the LSS group is in agreement with the results of a previous study . T The significant relationship of FMTs with ODI scores only in the ASD group suggests that the structural malalignment and deterioration of muscles in the pelvis and lower limbs were more robustly connected with disability in ASD.
Collectively speaking, our results on the FMTs in ASD may change the fashion for treating patients with ASD. A considerable portion of ASD patients with severely deteriorated radiological parameters report that their quality of life is satisfactory , which suggests that surgical decisions should not be made solely based on static radiologic parameters. Surgeons need to comprehensively consider multiple factors including subjective complaints of the patients including PROs. Therefore, our results in FMTs might play a bridging role between static radiographic parameters and subjective PROs, which are expected to give useful, adjunctive information to surgeons.
A valuable future study direction may include comparing the preoperative FMT results of ASD patients and the follow-up FMT after deformity correction surgery. This will determine how well the FMTs and PROs reflect the improvements in the structural alignment by corrective surgery. There are still many issues to overcome in ASD surgery including proximal junctional kyphosis, sarcopenia, personal care, and lifting [30, 31]. Hence, preoperative measure of FMTs is potentially useful for the confirmation of functional improvement and risk stratification in patients undergoing spinal surgery.
This study has the following limitations. First, we did not investigate the correlations between the results of FMTs and the actual incidence of falling in patients with ASD, because patients underwent surgery immediately after performing the FMTs. However, the actual risk of falling has shown strong correlations with the results of the FMTs [12, 18], and the mean values of these mobility tests were significantly higher in ASD than in LSS patients, in whom the risk of actual falling was demonstrated . Large cross-sectional or longitudinal population-based studies would be helpful for comparing the actual incidence of falling with the results of FMTs. Second, there is a potential for selection bias considering that the ASD group only included patients who were scheduled to undergo surgery at our institution, and not those who were treated with conservative care. Cross-sectional studies that include all patients with ASD are therefore needed, as well as studies assessing whether the mobility function improves after deformity correction surgery and whether improvements in mobility function result in the actual prevention of falls.