The primary goal of adult spinal deformity (ASD) surgery is to restore sagittal balance, which is closely associated with pain and disability [1,2]. Disk degeneration is the main cause of ASD, and a correction at the interbody space is one of the most appropriate surgical methods [3,4]. Lateral lumbar interbody fusion (LIF) is widely used for ASD [5-7]. This technique, which was recently introduced, uses a lateral trans-psoas approach to the disk, which involves less invasive surgical exposure, avoids the major vessels, and permits the placement of large interbody cages that allow the correction of the deformity [6-8].

LIF is effective for the correction of moderate sagittal plane deformities when used in combination with traditional open posterior techniques that use facet osteotomies and a rod cantilever technique. Storm et al. [9] reported that LIF using an open posterior approach for ASD surgery with facet osteotomies and pedicle subtraction osteotomy could restore lumbar lordosis (LL) by 11°±7° and 29°±10°, respectively.

Various factors influence this degree of correction in sagittal plane deformity. One of the most important is posterior osteotomy, with the degree of correction increasing with a higher grade of osteotomy. However, osteotomy brings risks of intraoperative massive bleeding and neurological deficit, so cases and levels that need osteotomy should undergo thorough consideration preoperatively. As yet, no detailed data have been reported regarding how the degree of sagittal correction in each intervertebral disc space relates to whether or not osteotomy was performed [10], and the information needed for surgical planning is limited.

In addition to posterior osteotomy, the presence of endplate injury in LIF and anterior longitudinal ligament (ALL) rupture can also affect the degree of sagittal correction [11-18]. Endplate injury is an occasional intraoperative complication. It has been reported to occur at 0.3%–22% of LIF levels, and it can diminish sagittal plane correction [11-14]. If endplate injury is confirmed, the predicted degree of correction should be revised and posterior osteotomy or other procedures reconsidered, depending on the specifics of the case. ALL rupture can also occur accidentally during LIF and posterior correction, which can greatly affect sagittal correction. Anterior column reconstruction has introduced been introduced, and a greater lordotic angle can be obtained by severing the ALL and using a larger angle of cages [15-18].

The purpose of the current study was to investigate the degree of sagittal correction in the segmental intervertebral disc space after posterior osteotomy, endplate injury, or ALL rupture in patients with ASD, and to elucidate the correction angle data needed for surgical planning and modification.

LIF was performed at a total of 108 interbody levels. The mean fusion level after posterior instrumentation was 7.4±1.7 levels. Table 1 summarizes the radiological parameters preoperatively, after LIF, and after posterior fixation. The preoperative mean values of the radiographical parameters were as follows: LL, 16.5°±16.7°; SVA, 94.5±57.5 mm; PI minus LL (PI–LL), 34.4°±16.5°; and PT, 31.1°±10.3°. After LIF but prior to posterior fixation, all the radiographical parameters showed significant improvement: LL, 33.4°±13.8° (p<0.001); SVA, 39.2±44.7 mm (p<0.001); PI–LL, 10.2°±17.5° (p<0.001); and PT, 16.9°±15.9° (p<0.001). In the posterior correction and fixation, Schwab grade 2 osteotomies were performed at seven levels (6.5%) [10]. After posterior fixation, all the radiographical parameters showed significant improvement compared with before the posterior fixation: LL, 52.1°±7.9° (p<0.001); SVA, 18.2±36.6 mm (p<0.001); PI–LL, −9.1°±16.0° (p<0.001); and PT, 9.1°±17.9° (p<0.001). After LIF but before posterior fixation, LL and PT had already been corrected to 47.5% and 64.5% of their final total extent of correction, respectively.

After LIF, endplate injury was identified at 21 levels on CT (19.4%). The segmental lordotic angle was 5.3°±8.4° at levels without endplate injury, but 1.9°±5.9° at levels with endplate injury, with a smaller acquired angle (p=0.08). The mean loss in the acquired segmental lordotic angle due to endplate injury was 3.4°.

ALL rupture was identified at seven levels (6.5%) and grade 2 osteotomy was performed at seven levels (6.5%). The acquired segmental lordotic angle at levels with ALL rupture from after LIF to after posterior fixation and from the preoperative values to after posterior fixation were 14.1°±6.2° and 26.4°±9.2°, respectively (Table 2). At levels without ALL rupture or grade 2 osteotomy, the acquired segmental lordotic angle from after LIF to after posterior fixation and from the preoperative values to after posterior fixation were 3.1°±5.7° and 7.1°±9.1°, respectively. Of note, these acquired segmental lordotic angles were significantly larger at levels with ALL rupture than at those without ALL rupture (p<0.0001); and when there was accidental ALL rupture, an average of 19.3° more was added from the preoperative to the postoperative stage than in cases without ALL rupture.

At seven levels with complete facet joint osteotomy (Schwab grade 2) [10], the mean acquired segmental lordotic angles from after LIF to after posterior fixation and from the preoperative values to after posterior fixation were 9.9°±3.9° and 19.5°±9.2°, respectively (Table 2). These acquired angles were significantly larger at levels that underwent grade 2 osteotomy than at levels without ALL rupture or grade 2 osteotomy (p<0.0001): by adding grade 2 osteotomy, an average of 12.4° more was added between the preoperative and postoperative stages than in cases without osteotomy.

This study investigated the factors that affect postoperative sagittal alignment after LIF in patients with ASD. These were found to be grade 2 osteotomy, endplate injury, and ALL rupture. Our results indicated that LIF resulted in a mean increase of 5° in the segmental lordotic angle compared with the preoperative angle, and that there was a total increase of 7° by the postoperative stage, even when grade 2 osteotomy was not performed. In cases where grade 2 osteotomy was performed, the mean acquired lordotic angle was approximately 20°, with 12° was added to the segmental lordotic angle. In cases where ALL ruptured intraoperatively, the total acquired segmental lordotic angle was 26°, and the acquired lordotic angle increased. Thus, LIF provided adequate sagittal alignment restoration in ASD, but the degree of correction was greatly affected by grade 2 osteotomy, intraoperative endplate injury, and the occurrence of ALL rupture.

LIF was developed in the late 1990s as a minimally invasive fusion technique [5]. The LIF approach enables sufficient disk space preparation and the placement of a large interbody cage from the lateral side of the spine [5,20]. This cage is useful for the correction of ASD to restore disk height in cases where degeneration is the main cause of the deformity [3]. In previous studies, LIF was shown to have improved segmental sagittal alignment [21], with an increase in disk height of 41.9%–83% and an increase in foraminal height of 13.5%–83% [22,23]. Anand et al. [23] reported an 8.1° increase in segmental lordosis in ASD cases treated with LIF using a 10° cage. In an investigation of 80 patients (mean fusion level, 1.5 levels), Shiga et al. [24] found that the acquired lordotic angle was 3.8° after LIF with 6° lordotic cages. In the present study, LIF was found to result in mean increase in segmental lordotic angle of 5° compared with the preoperative angle, with a total increase of 7° by the postoperative stage; these results were largely consistent with those reported previously.

There is currently no consensus regarding the segmental lordotic angle in cases with additional grade 2 osteotomy, ALL rupture, or intraoperative endplate injury. Grade 2 osteotomy was introduced by Smith-Petersen et al. [25] and elaborated by Geck et al. [26] in their discussions of the use of wide segmental osteotomies and posterior compression along unfused regions of the kyphotic deformity. Grade 2 osteotomy involves the removal of the posterior ligaments, spinous process, and facets to achieve sagittal plane realignment. The degree of kyphotic correction achievable with this technique has been reported to be in the range 9.3°–10.7° per level [26,27]. In the present study, grade 2 osteotomy added 12° to the segmental lordosis at levels; this was slightly greater than that achieved previously with LIF surgery for ASD. This was because we were able to place a large cage in the anterior part of the vertebral body with LIF, which may have resulted in a slightly larger acquired lordotic angle when closing the posterior part of the spine using osteotomy. Grade 2 osteotomy is a relatively safe and rapid procedure for the correction of gradual kyphosis or hypolordosis [26,27], with less blood loss, a shorter operative time, and fewer neurological complications than with grade 3 osteotomy (pedicle subtraction osteotomy) [28]. However, the blood loss is greater than with grade 1 osteotomy, so the levels at which grade 2 osteotomy is performed should be carefully selected. Although LIF with grade 2 osteotomy can result in a segmental lordotic angle of approximately 20° in total, the procedures are not suitable in cases with sharp, angular kyphosis; grade 3 osteotomy should be considered in such cases.

Postoperative cage subsidence in LIF surgery has been reported, with a reported incidence of 0.3%–22% of cases [12-14]. Two types of cage subsidence have been identified: one as a result of intraoperative endplate injury, the other appearing gradually during the postoperative period. Intraoperative endplate injury may be highly associated with the progressive settling of cages and with less clinical improvement. In a study that focused specifically on intraoperative endplate injury, Satake et al. [11] reported that 10.4% of 102 consecutive cases showed signs of intraoperative endplate injury. In that study, the risk factors for endplate injury included female sex, lower bone mineral density, a higher rate of polyetheretherketone used as cage material, and greater cage height [11]. Age was also a factor; ASD is often performed for elderly patients, so the possibility of endplate injury is high. However, there have been no reports that have investigated the incidence of endplate injury in ASD surgery or the loss of acquired lordosis in cases with endplate injury. In the present study, the incidence of endplate injury was 19.4%. The acquired segmental lordotic angle was approximately 2° in levels with endplate injury, with a mean loss of 3° in the acquired segmental lordotic angle. Sufficient anterior disc height restoration could not be obtained at levels with endplate injury, making it difficult to achieve sufficient segmental lordotic angle without performing posterior column shortening.

Anterior column reconstruction has recently been introduced to address intraoperative ALL rupture [15-18], and the relationship between severing the ALL and the acquired lordotic angle is attracting considerable attention. Anterior column reconstruction surgery achieves greater segmental correction by severing the ALL, allowing the placement of hyperlordotic cages [18,29,30]. A mean segmental lordosis correction of 10°–27° has been reported with anterior column reconstruction using hyperlordotic cages [15-18]. In the present study, the acquired segmental lordotic angle after accidental ALL rupture without hyperlordotic cages was 26°, similar to the previously reported values. The ALL ruptures in the present study occurred during posterior surgery while using a rod cantilever technique for kyphosis correction; they were not due to directly severing the ALL during anterior surgery. This complication might be the result of performing the kyphosis correction by anterior stretching using the rod cantilever technique without posterior shortening. If LIF cages placed in the anterior intervertebral space become unstable after ALL rupture, revision surgery may be needed; spine surgeons should therefore exercise considerable caution.

This study had two limitations. First, the number of cases was relatively small. A prospective large-scale study is therefore needed to validate the results. Second, the diagnostic accuracy of ALL rupture on MRI was unclear. There may have been inaccuracy due to surrounding acute postoperative changes.

LIF resulted, on average, in a segmental lordotic angle of 5°, with this angle increasing to 12° when grade 2 osteotomy was also performed. Endplate injury and ALL rupture are not infrequent complications in ASD surgery using LIF; spine surgeons should take care to prevent these complications, which can seriously affect the postoperative sagittal alignment.