Independent Risk Factors of Postoperative Coronal Imbalance after Adult Spinal Deformity Surgery

Maintaining or restoring global alignment is a fundamental goal of Adult Spinal Deformity (ASD) surgery [1,2,3]. Ideal balance after surgery produces favorable outcomes and avoids mechanical complications [4,5]. In fact, both sagittal and coronal imbalance (CIB) negatively influence functional outcomes and patient satisfaction [6,7,8]. However, while great attention has been devoted to sagittal imbalance [9,10], less has been paid to understanding the coronal plane. Recent literature indicates that greater postoperative CIB is associated with pain, loss of function, and decreased quality of life [11,12,13]. Moreover, adult patients suffering from CIB do not have many compensatory mechanisms due to the rigidity of the spine and frequent long fusions, even to the pelvis; therefore, immediate restoration of coronal balance (CB) is of the utmost importance. Recently, some authors have addressed the problem. Bao et al. [14] proposed a classification for coronal alignment and reported the preoperative direction of the trunk shift relative to the major curve to be a risk factor for persistent postoperative imbalance. This result was confirmed by other authors, who explained it with an inadequate correction of the lumbosacral fractional curve [5,15]. However, while some authors have analyzed the influence of the preoperative trunk shift on the postoperative balance [5,15], and others studied the L4 and L5 tilt [16,17,18], as far as the authors know, there is no study evaluating the correlation of all these preoperative variables (trunk shift, L4 tilt, L5 tilt, and sagittal parameters) on the postoperative balance, providing a comprehensive understanding of the preoperative radiographical risk factors for CIB. The aim of the present study was to elucidate preoperative risk factors for inadequate correction of coronal imbalance and/or creation of new postoperative coronal imbalance (iatrogenic CIB) in patients who undergo surgery for Adult Spinal Deformity (ASD).

The aim of the present study was to elucidate preoperative risk factors for inadequate correction of coronal imbalance and/or creation of new postoperative coronal imbalance (iatrogenic CIB) in patients who undergo surgery for Adult Spinal Deformity (ASD). Our main findings are twofold: first, patients with a preoperative trunk shift towards the convexity of the main curve (type C) are at higher risk of postoperative CIB; second, preoperative L5 tilt is an independent risk factor for both coronal imbalance and iatrogenic coronal imbalance. In the present study, preoperatively balanced (type A) and imbalanced (type B and C) patients did not differ significantly on the coronal plane (lumbosacral fractional curve severity, L4, and L5 tilt). However, postoperatively, type C patients showed the highest postoperative CVA (p = 0.04, η² 0.3) and the highest L4 tilt angle (15.7 ± 5.2, p = 0.005), while the major curve Cobb angle was similar between the groups: this might indicate that, in type C patients, correction of the major curve has been excessive and not tailored to match correction of the LSF curve, failing to restore a good balance [14,16,19]. Our result is in line with the existing literature [5,14]. An explanation for the high risk of CIB in patients with a preoperative type C imbalance can be found in the surgical strategy: in fact, traditional approaches aiming to correct coronal deformities consist of achieving distraction of the concave side and compression of the convex side (for example, by placing cages in concavity or by performing asymmetric tricolumn osteotomies); however, while these maneuvers can correct the coronal plane in type A and B patients, they may aggravate the inclination of the trunk in type Cs [14,16,17]. As for independent risk factors, the multivariate analysis demonstrated that preoperative L5 tilt is an independent risk factor for both postoperative coronal imbalance (CIB, p = 0.007) and iatrogenic imbalance (iCIB, p = 0.01). These results are in line with the findings of other authors [17,20,21]. Even if the direction of the tilt (consistent or opposite to the direction of the CVA) was not considered [17], a high value indicates that the preoperative imbalance is partly driven by the lumbar/lumbosacral compensatory curve and that the surgical strategy should aim at restoring a horizontal foundation, thus avoiding the takeoff phenomenon [14]. Regarding surgical strategies for preventing CIB, some suggestions were published by Bao et al. and Obeid et al. [22], sharing common concepts: in fact, both consider addressing the lumbosacral fractional curve of utmost importance and report that leveling the L5 coronal tilt is an appropriate intraoperative landmark for good balance. This is even more important when a fusion to the pelvis is planned: in fact, the chance of spontaneous correction of postoperative CIB is lower when the Lower Instrumented Vertebra (LIV) is below L5 [14,15]. Nevertheless, whether postoperative coronal malalignment had an impact on postoperative clinical outcomes appears to be less clear. In fact, no difference was seen in the rate of complications and revision surgery, and the existing literature reports mixed results. While Obeid et al. stated that the correction of CIB was an important factor for improving surgical outcomes, other authors concluded that the magnitude of coronal deformity and the extent of coronal correction are less critical parameters in adult scoliosis [23,24]. Therefore, the impact of postoperative CIB on patients remains a wide area of study. Moreover, our results show a significant improvement of CVA at last follow-up (p = 0.004). This finding, which is in line with Zuckerman et al. [5] makes it even more difficult to draw conclusions on the significance of postoperative coronal malalignment. The results of this study should be considered in the context of its limitations. First, data collection was subject to the limitations of retrospective data extraction from electronic records. Then, this was a single center and single surgeon study, and all surgeries were performed with all-posterior strategies; moreover, the vast majority of included patients were female; these considerations may represent biases and limit the generalizability of the results. Not employing anterior surgery in this cohort of patients may represent a selection bias that should not be ignored. Another potential limitation is the threshold of coronal balance; in fact, its definition in the literature ranges from 2 [17] to 4 cm [23]. In our study, 3 cm was chosen as a threshold because it is the most frequently used [4,5,16,17], in line with the Nanjing classification [14]. Moreover, measures were taken manually on simple anteroposterior and lateral radiographs: this may limit the reproducibility of the results, especially those about the spinopelvic parameters. Furthermore, we did not comment on clinical outcomes for the three subgroups (type A, B, and C) because the focus of the study was the assessment of preoperative radiographic risk factors for postoperative coronal imbalance; then, intraoperative and postoperative parameters (such as fusion levels) that may influence postoperative balance were not taken into account because they were not in line with the aim of the study. Finally, longer follow-up data are needed to enforce our results, because spontaneous resolution (or at least improvement) of CIB might be possible over time. Despite these limitations, this manuscript has a relatively large cohort and a long follow-up and reports new findings. Therefore, the authors believe it contributes to the knowledge on coronal alignment in ASD.

In conclusion, our results suggest that patients with a preoperative trunk shift towards the convexity of the main curve (type C imbalance) are more prone to postoperative CIB and that leveling the L4 and L5 vertebrae is the key to achieve coronal alignment, preventing the “takeoff phenomenon” [14]: this is extremely useful information. In fact, while determining intraoperative global coronal balance can be difficult, an intraoperative posteroanterior lumbar X-ray can easily be performed offering the necessary visualization of L4 and L5 tilt.