With advances in surgical techniques, minimally invasive approaches have yielded favorable clinical and radiological outcomes for ASD, with the introduction of LLIF combined with PCO as an alternative to more traditional PSO [9, 25, 26]. However, research focusing on correction loss in ASD population that has undergone LLIF with PCO is scarce, leaving questions about the impact of correction loss in these patients unanswered.
Oba et al. [14] reported significant PI correction loss within a year following deformity correction for ASD, while Oe et al. [15] suggested preoperative T1 slope > 40° as a risk factor for correction loss. Banno et al. [16] reported a relatively high rate of correction loss in sagittal fixed segment alignment and suggested higher body mass index, high-grade osteotomies, pure titanium rods, screw loosening, and sagittal malalignment as risk factors for correction loss. However, these studies did not specify the exact locations where correction loss occurred. This study addresses this gap by analyzing the angles of each segment during the postoperative follow-up using CT scans, shedding light on where correction loss occurs and what factors influence it.
Correlation between correction loss and RF
The RF incidence was significantly higher in the correction loss group than in the non-correction loss group (10/66; 15.2% vs 13/23; 56.5%, p < .05), and correction loss was more pronounced in the lower lumbar region, including L4-5 and L5-S1 (Table 2). In particular, when comparing the two groups based on the presence or absence of RF, the angular difference was most pronounced at L5-S1 (Table 4), indicating the loss of correction in the lower lumbar constantly affects the implant, which in turn affects the occurrence of RF.
However, the RFs were most prevalent at L3-4 (n = 8) and L4-5 (n = 7), rather than L5-S1 (n = 4). In a subgroup analysis comparing changes in L5-S1 IVD angle by dividing it into the ALIF and PLIF groups, ALIF tended to have less L5-S1 IVD angle loss than PLIF (Table 5). In an analysis conducted separately based on the presence or absence of correction loss (Table 3, 21/59; 35.6% vs. 2/30; 6.7%) and RF (Table 4, 19/59; 32.2% vs. 4/30; 13.3%), considering the results of a favorable trend for ALIF, differences in LS fusion methods may have influenced the occurrence of RF. Moreover, since patients in this study have relatively high PI, and the apex of the LL was located at a relatively high level, it is thought that stress concentration occurred at the apex, resulting in a large number of RFs at L3-4, 4–5 [27–29].
Table 5
Comparison of L5-S1 intervertebral disc angle of ALIF and PLIF group. ALIF: anterior lumbar interbody fusion; PLIF: posterior lumbar interbody fusion; IMPO: immediate post-operative; PO1Y: post-operative 1 year; PO2Y: post-operative 2 year. * Statistically significant (p < .05), ** Statistically significant (p < .01)
| | ALIF group (n = 30) | PLIF group (n = 59) | p |
| L5-S1 (°) |
| IMPO | 19.0 ± 6.2 | 12.8 ± 6.4 | < .01** |
| | .016* | <.01** | |
| PO1Y | 18.7 ± 6.2 | 11.7 ± 5.9 | < .01** |
| | .011* | <.01** | |
| PO2Y | 17.8 ± 6.6 | 11.2 ± 5.7 | < .01** |
Lertudomphonwanit et al. [30] reported that among 97 patients who developed RF after fixation of 5 or more levels including the sacrum, RF developed most commonly at L5-S1 (28%), follow by L3-4 (23.8%). Godzik et al. [31] reported that RF occurred in 4 of 90 (4.4%) patients with ASD undergoing ACR surgery, with all cases occurring at the ACR location. El Dafrwy et al. [32] reported that RF occurred in 35 of 230 (15.2%) ASD patients undergoing long segmental fusion, among which, RF occurred most commonly at the upper L4 level in patients undergoing interbody fusion (17/20; 85%) and at L4-S1 in patients who did not undergo interbody fusion (11/15; 73%). Given these variations in RF occurrence rates and locations among previous studies, more extensive research with larger patient populations is necessary for a more precise understanding of these patterns. (Fig. 2)
Correlation between correction loss and PJK
In this study, we also performed additional analysis on a new parameter, the FSPA [22], to investigate its association with PJK. Although the difference in FSPA between the two groups was not statistically significant, it tended to be smaller in the non-correction loss group than in the correction loss group (0.8° vs 3.8°, p = .1). Additionally, PJK occurred more frequently in the non-correction loss group (23/66; 34.8% vs 3/23; 13.0%, p = .048); this is likely due to the more rearward positioning of the UIV relative to the pelvis in the non-correction loss group with a smaller FSPA, which requires a reciprocal change in the TK, resulting in a higher incidence of PJK.
Many previous studies have reported reciprocal changes in unfused segments after deformity correction surgery in ASD patients [33–39]. Interestingly, TK was greater in the non-correction loss group in this study, both immediately after surgery and at the 2-year follow-up. When the corrected LL remains more robust after surgery, there is more progressive adaptation over time, resulting in a greater reciprocal change in TK.
Moreover, the multilevel LLIF with posterior implantation may have increased the stiffness of the construct [40], possibly contributing to a higher incidence of PJK. In the non-correction loss group, the restored LL remained rigid after surgery, which may have increased the interbody fusion rate, resulting in a more rigid construct [41]. Consistent with our study, other studies have reported that increasing rod stiffness decreases rod breakage but increases PJK [42], and using low-density pedicle screws decreases the risk of PJK [43]. While increased rod stiffness or screw population might lead to reduced correction loss, additional research is required to verify this.
Factors contributing to correction loss
In the non-correction loss group, ALIF was used more than PLIF as the LS fusion method (28/66; 42.4% vs 2/23; 8.7%, p = .003). Maintaining LS arthrodesis after long fusion to the sacrum is an important issue, serving as the anchor in long constructs [44]. Therefore, sacropelvic fixation and anterior column support at L5-S1 were introduced to reduce LS pseudarthrosis [17, 45]. Lee et al. [46] reported that ALIF with a metal cage was superior when trying to achieve solid fusion at L5-S1 in ASD patients. ALIF has several advantages over PLIF, including the ability to easily produce a larger LL correction by using a more hyperlordotic cage, which is especially advantageous for sagittal imbalance correction in patients with high PI [47, 48], and a larger contact area than the PLIF cage, which can provide a more robust fusion [49], especially when using a metal cage [46].
Meanwhile, Varshneya et al. [50] reported that among 2,564 patients who underwent deformity correction surgery, the reoperation rate at 2 years was significantly higher in patients with osteoporosis (30.4% vs 22.9%). Khalid et al. [51] reported that among 1,044 ASD patients undergoing multilevel TL fusion, the risk of revision rate was higher in patients with osteoporosis (OR 1.57, 95% CI 1.05–2.35) and the risk of PJK was almost double (OR 1.88, 95% CI 1.34–2.64). They also found that correction loss was greater in patients with osteoporosis (11/66; 16.7% vs 10/23; 43.5%, p = .009) and RF was also higher although not statistically significant (13/66; 19.7% vs 8/23; 34.8%, p = .142). Therefore, since there is a high risk of correction loss when performing long-segment fixation, including multilevel LLIF with PCO, ALIF with metal cage seems essential when performing LS fusion in ASD patients with osteoporosis.
Age-adjusted LL
Lafage et al. [52] reported that the ideal spinopelvic value increased with patients’ age. As such, McCarthy et al. [24] provided a guideline of the amount of LL needed to reach a given alignment based on age, PI, and TK.
The comparison between the non-correction loss and correction loss groups was statistically significant, with the correction loss group correcting closer to age-adjusted LL (44.7° vs 39.0°, p = .036), suggesting that using more sufficient LL correction than McCarthy's equation is more favorable in correction loss. The difference between the results of McCarthy's study and this study is that the patients involved in this study were all elderly with a single etiology diagnosed as DLK and DBS [18, 19], as well as continued to show progression of TK and pelvic retroversion after surgery due to severe degeneration of the paravertebral muscles. Besides, all patients have PI of 40–70°, which corresponds Roussouly classification type 4, which requires more sufficient correction [27, 53]. This is a case that does not fit McCarthy’s formula in that a poor prognosis is expected if undercorrection is performed on patients with high PI and PT as pelvic non-responder suggested by Passias et al. [54].
Limitations
First, being a retrospective comparative study, it involves multiple variables. Second, data analysis might be limited because of relatively small number of patients. However, this study is significant, as it focuses on a single disease and a single surgical method. Third, we evaluated sagittal images on CT scans obtained from patients with supine position. Although we minimized measurement errors and ensured accurate angle measurements by using CT scans, they do not precisely represent the upright posture. However, since the angle of the disc was measured in the segment fixed with LLIF and rod, changes in angle due to patient position could be negligible. Fourth, because patients in this study were operated on by a single surgeon at a single institution, the results may have limited implications. Fifth, our results of relatively high PI (> 50°) indicate that an LL correction of > 50° is feasible. However in cases of patients with low PI of type 1 and 2 as in the study by Roussouly and Pinheiro-Franco [53] or relatively small LL correction due to mild sagittal imbalance, the results may be different.