In this study, the correction loss in SKA was 10.4°, which agrees with previous reports (ranging from 1° to 13°) [1, 2, 4–9, 20]. Several risk factors associated with correction loss following SSPF have been reported. Aono et al. reported a mean correction loss of 9.1° in SKA for 76 patients. They identified a preoperative SKA greater than 15.4° and canal compromise greater than 52.8% as risk factors [4]. Jang et al. noted that an age greater than 43 years and a preoperative AVBHR less than 54% were risk factors [6]. In this study, the preoperative AVBHR was significantly smaller in patients with ΔSKA ≥ 10° and an older age was a significant risk for correction loss. These results indicate that severely collapsed vertebrae cannot remodel and that the repair capacity of the vertebral body and intervertebral discs is likely to decrease with age.
Previous report indicates that postoperative correction loss occurs at the disc level [4]. Schömig et al. reported that the vacuum disc phenomenon of the adjacent disc often occurred in burst fractures and found a significant correlation between AO A3 fractures and the vacuum disc phenomenon; this may lead to disc degeneration due to nutritional supply disturbances via the vertebral endplate [21]. Other reports have described the importance of reducing the damaged vertebral endplates to prevent progressive kyphotic deformity [22, 23]. Therefore, a preoperative assessment of the endplate and the adjacent disc may be crucial for choosing a treatment strategy and predicting correction loss. Several authors have reported methods for assessing traumatic intervertebral disc injuries [10–13]. Sander et al. classified TIDL according to MRI (Table 1) and reported that 40.7% of their cases were classified as grade 3 at injury [10, 13]. In this series, grade 3 TIDL was observed in 42% of patients, and all patients had cranial injuries. The presence of severe cranial endplate and disc injury (grade 3 TIDL) and an older age were significant risk factors for correction loss. Moreover, cases with AO type A3, A4, and B2 fractures, which accompany the destruction of the vertebral endplate, showed significantly worse correction loss than cases with an AO type B1, which has no endplate injury. Wang et al. reported that endplate fractures were strongly associated with disc degeneration [12]. Therefore, it is presumed that significant damage to the endplates will also cause considerable damage to the intervertebral discs, resulting in disc degeneration and correction loss at the disc level. Lee et al. showed an association between an intervertebral disc and endplate complex injury and postoperative correction loss [24]. However, their study only included patients younger than 45 years and did not assess the correction loss associated with both endplate injury and adjacent disc degeneration in older patients.
To our knowledge, this is the first study to investigate the association between the severity of an endplate and adjacent disc injury and correction loss following SPFF at all ages. Therefore, based on our findings, it can be presumed that the damage to either the vertebral endplate, adjacent disc, or both may cause disc degeneration leading to failure of the anterior support mechanism and severe kyphotic deformity. In addition, older patients are at a higher risk because of a reduced ability to repair the discs and endplates.
There is no consensus on whether residual kyphotic deformity is associated with worse clinical outcomes and back pain [6. 7, 20]. Our study demonstrates that severe postoperative back pain was significantly associated with grade 3 TIDL and ΔSKA ≥ 10°. Xu et al. noted that postoperative pain was associated with kyphotic deformity greater than 20° but not with the narrowing or degeneration of intervertebral disc [20]. Similarly, McLain et al. found that a progressive kyphotic deformity of more than 10° was associated with significant postoperative pain [7]. Together, these results demonstrate the association between residual kyphotic deformity and back pain; therefore, it is desirable to prevent excessive correction loss after surgery.
In younger patients or patients without severe TIDL and endplate injury, SSPF is an effective treatment because it can be expected to yield satisfactory results. In contrast, older patients or patients with severe TIDL and endplate injuries are at risk of progressive kyphotic deformity. They can be treated initially with SSPF; however, they require careful observation for correction loss. If severe correction loss occurs, additional surgery should be considered. In older patients with severe TIDL and endplate injury, LSPF or anterior reconstructive surgery may be an effective alternative for initial surgery.
This study had some limitations. First, the retrospective study design may have decreased the level of evidence. Second, the small number of patients evaluated limits the applicability of the study. Despite these limitations, we believe that this study is important. It demonstrated that severe damage to the vertebral endplates and adjacent discs are risk factors for postoperative correction loss and delayed kyphotic deformity.