Correction of marked sagittal deformity with circumferential minimally invasive surgery using oblique lateral interbody fusion in adult spinal deformity

Conventional deformity surgery, including open posterior or combined anterior-posterior approaches, is considered the standard technique for adult spinal deformity (ASD), with reliable clinical and radiological outcomes [1]. However, conventional deformity surgery is known to have a high risk of surgical complications, especially in patients with advanced age [2].

Minimally invasive surgery (MIS) can be an alternative surgical treatment with comparable surgical outcomes and lower complication rates [3‐8]. Among various MIS techniques, transforaminal lumbar interbody fusion (TLIF), extreme lateral interbody fusion, or oblique lateral interbody fusion (OLIF) combined with percutaneous posterior spine fixation (PPSF) without any posterior osteotomy are important techniques for minimally invasive deformity [3‐6, 8‐10]. Deformity correction using MIS techniques without any conventional open surgery is referred to as circumferential MIS (cMIS) [5, 11, 12]. Recently, OLIF at L5–S1 was introduced as a minimally invasive anterior approach and is expected to yield a greater lordotic angle at the L5–S1 level [4, 10]. Although OLIF at L5–S1 is known to effectively improve the lordotic angle, no study has evaluated its effect on deformity correction [4]. Compared with conventional deformity surgery, cMIS is known to be effective for coronal but not for sagittal correction [5‐7, 13, 14]. cMIS has been advanced continuously in sagittal correction of ASD [5, 6, 13]. Despite recent advancements, cMIS remains limited by the insufficient correction of marked sagittal deformity [5, 15, 16].

Thus, this study aimed to investigate the effectiveness of cMIS using OLIF for ASD patients with marked sagittal deformity and analyzed the usefulness of OLIF at L5–S1.

The recent concept of cMIS is a combination of MIS lumbar interbody fusion and percutaneous fixation without osteotomy [3, 5‐8]. Previously, cMIS was not indicated for sagittal correction of ASD [7, 14, 25, 26]. Recent studies reported that cMIS could be indicated only for mild deformity, but osteotomies were required for marked deformity [4, 5]. However, according to our study, cMIS also seems to be effective for sagittal correction even in patients with marked deformity.

A ceiling effect is referred to as the maximum correction of sagittal parameters allowed by deformity surgery [16, 26]. Anand et al. reported that the ceiling effect of dSVA was 120 mm for cMIS [16]. In our study, the maximum dSVA was 141.5 mm, which was greater than that previously reported. Other studies reported the maximum dLL and corrected LL by cMIS were 23° and 54°, respectively [25, 26]. In our study, the maximum dLL and corrected LL were 67.5° and 62.4°, respectively, which were also greater than those of previous reports. Due to the increased angle correction, the postoperative sagittal parameters satisfied the surgical goals in our study. The greater LL correction in our study seemed to be related to the combination of OLIF with high-angle cages placed anteriorly in the disc space, PPSF with aggressively contoured rods, and intraoperative lumbar extension with a motorized operation table followed by percutaneous rod compression. Our study is the first to show successful correction of marked sagittal deformity with cMIS in ASD.

The mean postoperative LL was 51.7° in the whole group, which might suffice for most of the Korean population whose mean PI is relatively lower (47.8°) than the western population (50.2°–52.0°) [27‐30]. The mean postoperative LL was greater in the OLIF51 than in the TLIF51 group, while the mean postoperative LL in the TLIF51 group was similar to those of other studies using DLIF at lumbar levels and TLIF or AxiaLIF at L5–S1 [12, 25]. The greater mean postoperative LL in the OLIF51 group than in the TLIF51 group seemed to be due to the significant increase in disc angle at the L5–S1 level because the L5–S1 level contributes largely to the lumbar lordosis [31]. During OLIF at L5–S1, the anterior disc space was opened widely as in anterior lumbar interbody fusion (ALIF), which helped to create a larger disc angle than that with TLIF [32‐34]. Since TLIF was introduced in 1982, it was developed as a MIS technique [12, 33]. We had been mainly used TLIF at L5–S1 level before the introduction of OLIF51. Bilateral facetectomy and TLIF with banana-shaped cages were previously reported to increase segmental angles [35, 36]. However, there may be some debates because other studies observed no significant improvement in segmental angles after TLIF and no difference according to the TLIF cage type [32, 33].

Hybrid surgery is known to be more effective than cMIS in sagittal correction, with the drawbacks of increased operating time and EBL [7, 12, 25, 37]. However, the sagittal correction, operating time, and EBL in our study were better than those in other cMIS and hybrid surgery studies even though our patients had worse sagittal deformity preoperatively (Table 4). These better results seem to be related to OLIF use at L5–S1, given the improved results in the OLIF51 than in the TLIF51 group. TLIF and axial lumbar interbody fusion (AxiaLIF) for cMIS or ALIF for the hybrid surgery were the fusion techniques used at L5–S1 previously. Our study is also the first report evaluating the effects of OLIF at L5–S1 on sagittal deformity correction.

Significantly earlier PJK onset was observed in the OLIF51 group (8.6 months) than in the TLIF51 group (26.3 months), which was also earlier than that of previous reports (18.6–34.8 months) [38, 39]. Other reports found that the incidence of PJK could be reduced by minimally invasive lumbar interbody fusion and percutaneous fixation, which suggests that other factors may have accelerated PJK development after cMIS with OLIF at L5–S1 in our study [40, 41]. Faundez et al. reported an exaggerated upper lumbar lordosis resulting in junctional failure [39]. Previous studies reported that the ideal LLL/LL was 50–80% [30, 42]. According to our data, the LLL/LL of the OLIF51 group was within the ideal range. However, there was a lordotic curve at the thoracolumbar junction, which showed similar effect of exaggerated upper lumbar lordosis. The lordotic angle at the thoracolumbar junction related with the lordotic curve of the proximal rod (Fig. 1). The lordotic curve of the proximal rod was necessary to pass a long rod percutaneously through the lumbosacral curvature. However, it resulted in increased lordosis at the thoracolumbar junction, which might lead to early PJK. Postoperatively smaller SVA, higher LL correction, and smaller PI-LL in the OLIF51 group also seemed to contribute to earlier PJK onset [38, 43].

The retrospective design is the major limitation of this study. Although the follow-up period was at least 2 years, a larger number of patients and longer follow-up is necessary because correcting marked sagittal deformity using cMIS is a relatively new trial. Multicenter studies performing similar cMIS are therefore needed. We suggested several factors responsible for the improved correction of marked sagittal deformity, but other mechanisms should be further investigated. Rigid deformity itself was not evaluated in this study, but we could not rule out completely the effect of rigid joints on the sagittal correction by cMIS. Considering that early PJK in the OLIF51 group is associated with exaggerated proximal lordosis, we tried to reduce it by making a kyphotic angle at the proximal rods (data not presented), but still do not know the exact underlying cause because the preventive effects can only be observed after a longer period. Our results cannot be applied for adolescent idiopathic scoliosis because we studied only for ASD.

According to our data, the use of OLIF with cMIS seemed to help to overcome the main limitation of cMIS in correcting marked sagittal deformity of ASD. Although OLIF at L5–S1 showed a synergistic effect on the sagittal correction with cMIS, early PJK remained an unsolved issue of cMIS using OLIF at L5–S1.