Long-segment corrective surgery is associated with a high complication rate. Scheer et al.11 analyzed 138 patients aged 63.3±11.5 years with severe sagittal imbalance (PI-LL≥30°) and reported a postoperative complication rate of 74.6%, of which internal fixation-related complications constituted 30.4%. In our study, 10 patients (43.5%) presented with mechanical complications, and the revision rate was 17.4%. This could be associated with advanced age, severe preoperative sagittal imbalance, obesity, low bone density, and/or other factors. PJK was the most common complication, with an incidence of 21.7%. Previously reported risk factors for PJK include advanced age, damage to the posterior tension band, combined anterior and posterior surgery, fusion to the sacrum/pelvis, choice of upper fixed vertebrae, rod material, insufficient or excessive correction, and sarcopenia.12 Unoki et al.13 analyzed patients with long-segment corrective surgery and reported an incidence of PJK of 23.3%, 30.4%, and 29.2% for distal fixation to L5, the sacrum, and to the pelvis with S2AI screws, respectively. No significant difference between techniques was observed, suggesting that the S2AI screw technique did not increase the risk of PJK. At the last follow-up, two patients (8.7%) developed symptoms of S2AI screw loosening in our study, which manifested as sacroiliac joint pain. Both patients were treated conservatively. Their BMDs were -2.91 and -2.84, respectively, which were considered osteoporotic. In the literature, the breaking rate of S2AI screws is reported to be 0.0-1.0%, and 7.8-10.4% of patients with S2AI screws demonstrated visible translucent bands around the screws on imaging at more than 1 year follow-up14,15, which was consistent with our findings. Further, two patients presented with rod breakage. Because elderly patients often have osteoporosis, we used a titanium alloy rod with a 5.5-cm diameter rather than a 6.0-cm diameter in order to reduce the risk of PJK and stress at the junction of the bone and screw; nevertheless, this reduced rod strength. Therefore, we recommend adding satellite rods to the kyphotic apex area to reduce the risk of rod breakage.
Compensation in patients with sagittal imbalance may be achieved via various mechanisms, including pelvic retroversion, reduction of thoracic kyphosis, hip hyperextension, and knee flexion. In this study, PT was greater than SS preoperatively, confirming significant pelvic retroversion in both groups. At 1 month postoperatively, PT was decreased, SS was increased, and pelvic retroversion improved in both groups. No significant differences were observed in PI, PT, and SS between 1 month postoperatively and at the last follow-up, suggesting that the S2AI screw technique maintained the relative position of the pelvis effectively. Accordingly, for patients with sagittal imbalance and severe retroversion of the pelvis, especially those with lumbosacral muscle atrophy, we recommend using S2AI screws for sacral pelvic fixation if long-segment corrective surgery is performed.
We observed that the PI increased by 3.7±3.9° in group B at 1 month postoperatively. In this regard, S2AI screws may stabilize the sacroiliac joint and change the morphology of the pelvis, thus changing the PI. Ishida et al. 16 used S2AI screws for pelvic fixation in 46 adult patients aged 61.5±10.7 years with spinal deformity. In their study, the preoperative PI was 63.4±12.3°, and the postoperative PI decreased by 6.0±12.5°. Wei et al.17 reported that the use of S2AI screws for pelvic fixation reduced PI after surgery in patients with high PI (PI≥60°); conversely, PI increased after surgery in patients with normal or low PI (PI<60°). In this study, preoperative PI was 43.4±9.9° and the degree of kyphosis was severe in group B, which could partly explain the increase in postoperative PI.
Although postoperative VAS and ODI scores in both groups were significantly improved compared with preoperative values, VAS and ODI scores were significantly better in group A than in group B at the last follow-up. This indicates that the occurrence of mechanical complications significantly worsens surgical outcomes. Preoperatively, LL in group B was -0.9±10.2°, indicating a kyphotic curve, whereas that in group A was 12.8±11.6°, with a significant difference between groups. As patients in group B had more severe preoperative sagittal imbalance, greater correction was required during surgery. This suggests that the risk of postoperative mechanical complications is higher in patients with severe sagittal imbalance and lumbar kyphosis. Therefore, sagittal alignment should be carefully evaluated preoperatively, and a detailed surgical plan should be designed to correct this imbalance.
At 1 month postoperatively, SS, LL, and PI-LL in group B were 20.2±8.6°, 24.4±9.9°, and 22.7±12.5°, respectively, indicating that sagittal reconstruction was insufficient. In group A, SS, LL, and PI-LL were 28.8±8.5°, 39.6±6.7°, and 11.4±11.3°, respectively, indicating that the sagittal alignment was significantly better than that in group B. Pearson correlation analysis demonstrated that mechanical complications were significantly associated with SS, LL, and PL-LL at 1 month postoperatively, highlighting the importance of restoring lumbar lordosis. In our study, the best cutoff values of SS, LL, and PI-LL were 24.1°, 32.75°, and 12.0°, respectively. SS represents lower lumbar lordosis, and normal lower lumbar lordosis accounts for approximately two-thirds of the total lordosis curve. Therefore, the recovery of L4-S1 lordosis during operation is important. In the L4/5 and L5/S1 segments, we opted to release and distract the intervertebral space and subsequently placed a cage with a large lordosis angle to restore the local lordosis angle. Based on our experience, we propose that for the upper lumbar segments, lordosis can be reconstructed via intervertebral space release and posterior multi-segment Ponte osteotomy, whereas for areas with large local kyphosis, PSO should be considered.
At the last follow-up, LL was significantly reduced, and PI-LL, TPA, and SVA were significantly increased in both groups. Patients underwent long-segment fixation, and the pelvis was in a relatively stable position. Therefore, the loss of sagittal correction was predominantly caused by the reduction in lumbar lordosis during the follow-up period. LL decreased by 4.4±4.7° in group A and by 8.1±7.3° in group B, but the change in LL was not significantly different between the two groups (P>0.05). In group B, the restoration of LL was insufficient at 1 month postoperatively, and the further reduction in LL may have partly underpinned the occurrence of mechanical complications. Nakazawa et al.18 and Im et al.19 reported that insufficient restoration of the PI-LL could lead to progressive worsening of sagittal imbalances after long-segment orthopedic surgery with sacropelvic fixation. Therefore, we should also consider the reduction of lumbar lordosis in the preoperative planning.
The average age of the patients in our study was 68.0±6.5 years. As natural degeneration of the spine is accompanied by an increase in positive sagittal balance, age is a key factor affecting sagittal alignment. In elderly patients with sagittal imbalance, the intervertebral discs, articular processes, paravertebral ligaments, muscles, and other structures have adapted to a positive balance for a long time. Immediate correction to the "ideal" state after surgery did not seem to improve clinical outcomes, but increased the incidence of mechanical complications.20 Lafage et al.21 emphasized that for elderly patients aged over 75 years, the ideal sagittal parameters were PT=28.5°, PI-LL=16.7°, and SVA=7.8 cm. In our study, PT, PI-LL, and SVA in group A were 25.1±11.8°, 17.3±9.6°, and 5.8±3.1 cm at the last follow-up. According to the sagittal modifiers of SRS-Schwab classification,22 these parameters were classified as positive if 20°≤PT≤30°, 10°≤PI-LL≤20°, and 4 cm≤SVA≤9.5 cm. These results indicated that the sagittal alignment was in a moderate positive balance, and the VAS and ODI scores of the patients were significantly improved. Zhang et al.23 performed long-segment corrective surgery on 44 patients aged 65.1±2.8 years with spinal deformities and reported that postoperative PI-LL was between 10° and 20°, which was similar to the results of our study. As such, we did not consider it necessary to perform corrections to the state of LL = PI ± 9° in elderly patients.
The GAP score was used to predict mechanical complications in patients undergoing long-segment corrective surgery for adult spinal deformities. The GAP score includes five categories: relative pelvic version, relative lumbar lordosis, lordosis distribution index, relative spinopelvic alignment, and age. The GAP score enables the calculation of the ideal sagittal parameters based on PI and emphasizes the significance of lordosis distribution in the lumbar spine. The PI value influences the morphology of the spine and pelvis. Patients with a small PI have less sagittal compensation ability, and the requirements for reconstruction of lumbar lordosis are stricter.24 In our study, mechanical complications were significantly correlated with GAP scores. The average GAP scores were 6.9±2.0 and 10.9±2.4 in groups A and B, respectively. In a study by Yilgor et al.,10 the ideal GAP score after surgery was ≤4, and the incidence of mechanical complications at 7 points and 11 points or above was 72.7% and 100%, respectively. These results are discordant with our findings, which could be related to individual differences, such as age and degree of deformity. Ham et al.25 retrospectively evaluated 84 patients aged 70.5 ± 7.7 years old with severe sagittal imbalances and reported that the area under the curve for predicting mechanical complications based on the GAP score was 0.839, similar to our study. Moreover, 82% of patients with mechanical complications had a GAP score ≥ 7. For elderly patients with severe sagittal imbalance of the spine, a postoperative GAP score ≥ 10 indicated a high risk of mechanical complications, which should be carefully monitored during follow-up.
This study has several limitations. First, it was a retrospective study and lacked a randomized control group with distal fixation to the S1 or ilium. Second, the sample size was small, and selection bias may have occurred. Due to the lack of a unified standard for evaluating the strength of the lower back muscles, this study did not include muscle-associated factors. Further, full-length radiographs of the lower extremities were lacking; hence, the compensation status of the hip and knee joints could not be analyzed in detail. At present, there is a paucity of studies discussing the surgical treatment of elderly patients with severe kyphosis, and long-term follow-up with a large sample is warranted to confirm the effects of long-segment corrective surgery using the S2AI screw technique.