Lumbar interbody fusions have become common for treating radiculopathy and neurogenic claudication caused by spondylolisthesis and/or spinal stenosis [1]. The placement of an interbody cage allows for indirect neural element decompression via ligamentotaxis while also optimizing lumbar lordosis and endplate obliquity correction [2]. In addition to providing indirect neuroforaminal decompression and sagittal balance restoration, interbody fusions increase the likelihood of arthrodesis through anterior column stabilization [3]. Although transforaminal lumbar interbody fusion (TLIF) cages are commonly placed unilaterally, bilateral placement increases the bone graft surface area, optimizes the footprint of the graft, and may improve stability especially in the setting of bilateral facetectomies [4]. Therefore, bilateral placement of TLIF cages may increase the arthrodesis rate, improve lordosis, and optimize construct stability, although there is minimal literature supporting its use for each of these indications.

While unilateral cage placement likely confers less surgical risk to the patient than bilateral cage placement, there is limited and conflicting literature regarding surgical outcomes when comparing bilateral and unilateral cages [5,6]. A prospective randomized controlled trial found that bilateral interbody TLIF cages increased fusion rates but did not significantly influence postoperative patient-reported outcome measures (PROMs), including the Visual Analog Scale [5]. A separate retrospective study found that unilateral cages were less likely to achieve successful arthrodesis, but this had minimal impact on the postoperative PROMs [6]. However, there is little published literature comparing radiographic parameters following unilateral and bilateral cage placement, which may allow us to understand whether one technique is superior in its restoration of sagittal alignment, disc space distraction, or endplate obliquity [6].

Even when expanding the comparisons of bilateral and unilateral cage placement to patients undergoing posterior lumbar interbody fusions, most studies have been limited to small sample sizes [4,7]. Thus, there is no clear evidence to indicate whether bilateral or unilateral cages promote superior radiographic or surgical outcomes in posterior-based spinal fusions. Therefore, the objectives of our study were to (1) compare radiographic alignment between bilateral and unilateral cage placement in TLIF and to (2) determine whether radiographic fusion rates vary between bilateral versus unilateral cage placement.

The requirement for informed consent was omitted due to the low risk and retrospective nature of this study. Upon obtaining the institutional review board approval at Thomas Jefferson University (Control #19D.508), patients ≥18 years old who underwent primary or revision one- or two-level TLIF with placement of bilateral interbody cages between 2013–2021 were retrospectively identified. Unilateral and bilateral cages were defined as the use of a single or bilateral bullet cages, respectively, at a single level. For a unilateral cage, it is placed at the patient’s symptomatic side, whereas bilateral cages are placed at the lateral margins of the endplate. All bullet cages had similar dimensions (23 mm×9 mm×9 mm). The following Current Procedural Terminology codes for TLIF were included: 22630, ±22632. The exclusion criteria included any patient with a fusion of more than two levels or an indication of infection, malignancy, or trauma. Patients without adequate visualization of the entire lumbar spine on anteroposterior (AP) and lateral lumbar spine radiographs at the preoperative, 6-week postoperative, and 1-year postoperative visits were excluded. The use of unilateral or bilateral interbody cages was left to the discretion of the surgeon. However, soft indications for the use of bilateral interbody cages included possible continued postoperative instability (i.e., bilateral facetectomies especially in the setting of a revision procedure where the patient had bilateral radiculopathy) or when additional lordosis or a greater likelihood of fusion were deemed necessary. For all procedures, local autograft was used to supplement fusion. Bone grafts were placed within the cage, outside the cage in the disc space, and in the lateral gutter contralateral to the TLIF. If bilateral TLIF was performed, no graft was placed in the lateral gutter to minimize bone growth around the exiting nerve root. Determination of bilateral versus unilateral cage and cage material (titanium versus polyetheretherketone [PEEK]) was performed by assessing cage markers on radiographs. Titanium is completely radiopaque, while PEEK cages only have radiopaque markers. The number of cages utilized and their composition were verified by examining the implant records.

TLIFs are performed to restore neuroforaminal height and sagittal balance of the lumbar spine. However, few studies have analyzed the differences in radiographic alignment between bilateral and unilateral interbody TLIF cages. While some studies suggest that bilateral cages promote greater arthrodesis rates, the improved fusion rate does not affect clinical outcomes [5,6]. However, none of the previous studies have provided robust radiographic data describing whether a unilateral or bilateral cage promotes improved segmental lordosis or endplate obliquity angles. Given the paucity of evidence, additional analyses documenting the radiographic alignment of the lumbar spine following unilateral or bilateral cage placement in TLIF are warranted. The results of our study suggest that bilateral interbody cage placement is more likely to restore segmental lordosis compared to unilateral interbody cage placement, while endplate obliquity was more likely to be restored following unilateral interbody cage placement. Similar to previous studies, bilateral interbody cage placement significantly increased the likelihood of fusion. Surprisingly, we also observed a greater degree of subsidence in the posterior aspect of the interbody space in patients with bilateral interbody cages, which is likely due to the greater amount of disc space correction achieved intraoperatively [6,12,13].

Previous literature reported a positive correlation between improved lumbar sagittal balance and reduced postoperative pain [3,14–16]. Therefore, optimizing the restoration of lordosis during TLIF may be an important goal of surgery [3,17–19]. However, prior studies analyzing the effect of unilateral interbody TLIFs on lordosis improvement have obtained mixed results. While some studies indicate that TLIFs effectively restore segmental and lumbar lordosis, others have found that TLIFs produce net neutral or overall kyphotic segmental alignment [3,15,17,20–22]. While most of these studies are based on unilateral TLIF cage placement, our study found that segmental lordosis increased by 3.32° at the 1-year postoperative point in patients in whom a bilateral body cage was placed, whereas unilateral body cage placement resulted in kyphosis by an average of 1.62°. Given that bilateral interbody cages provide a larger surface area to bolster the vertebral column, bilateral cages may be more effective at achieving and maintaining the corrected lordosis [3,18,19].

Unilateral interbody cages are associated with improved endplate obliquity of the affected spinal segment [23]. For patients with degenerative lumbar scoliosis, Zhu et al. [23] found that the endplate obliquity was minimized after unilateral TLIFs. Similar results were obtained by Zhao et al. [24], who utilized sequential TLIF procedures to correct coronal imbalance in patients with degenerative scoliosis. Similarly, our study observed a significant decrease in endplate obliquity in patients who received unilateral TLIF cages.

For both endplate obliquity angles and segmental lordosis measurements, differences in the underlying surgical indications likely caused variance in the unilateral and bilateral preoperative radiographic measurements. Patients with a more pronounced endplate obliquity were more likely to receive a unilateral TLIF cage to correct the coronal lumbar curve. Additionally, bilateral interbody cages were more likely to be selected when there was a concern for spinal instability [25]. Therefore, our study suggests that patients requiring improved lordotic correction may receive greater benefit with bilateral interbody devices.

The average loss of disc height following interbody cage placement is believed to range between 0.5 mm and 0.75 mm [3,26,27]. Our study found similar anterior disc height loss regardless of unilateral or bilateral TLIF cage placement (approximately 1 mm), but there was slightly greater posterior disc height loss in patients with bilateral cages. Given the increased surface area provided by bilateral interbody cages, less disc height collapse was expected. It is hypothesized that the improved disc height correction obtained in the bilateral cage group resulted in greater compressive forces across the interbody device, possibly due to additional ligamentous tension or disc space over-distraction. Since the difference in posterior disc height loss between unilateral and bilateral cages was only 0.47 mm at the 6-week postoperative point, this be attributed to measurement error and is unlikely to be clinically relevant.

One purported benefit of bilateral TLIF is an increased likelihood of successful bony arthrodesis, which was observed in our retrospective study [5,6]. One theory behind the increased fusion rates is enhanced dynamic stabilization; however, previous studies have not identified any biomechanical differences between unilateral and bilateral TLIF cage placement [6,28]. Therefore, the improved arthrodesis rates may be due to the additional bone grafting surface area provided by the second cage, which optimizes the ability for the construct to fuse.

Limitations to our study include those inherent to any retrospective review. Although patients were systematically included in the study, they were identified based on the availability of preoperative, 6-week postoperative, and 1-year postoperative data following the index TLIF, which may have introduced a selection bias. Additionally, we did not consider the impact of interbody sagittal location on lordotic correction, which may have affected segmental lordosis measurements. Further, our analysis of radiographic fusion was limited to AP radiographs. While this is a less sensitive and specific assessment compared to computed tomography (CT), this technique has been validated in several previous studies, and we do not routinely obtain postoperative CT scans unless a potential symptomatic pseudarthrosis exists [10,29]. Additionally, TLIFs were performed by several different surgeons at our institution, introducing variability in surgical technique and postoperative care which may have influenced clinical and radiographic outcomes; however, this may ultimately increase the generalizability of the study. Lastly, it is important to note that previous studies have reported a margin of error of 8° for Cobb angle measurements made on AP radiographs [8,9,30]. All angular measurements in our study were made using the Cobb method, and all Δ were found to be <8°. As previously discussed, the differences in disc height loss were within human measurement errors, which may not be clinically relevant. Additionally, the significant differences in the Δ postoperative endplate obliquity angle and segmental lordosis may be due to margin of error measurements.

TLIFs with bilateral cages resulted in greater improvement in segmental lordosis and bony arthrodesis, while unilateral cages were more likely to affect endplate obliquity. Given that this is one of the few studies comparing radiographic outcomes between unilateral and bilateral cages in patients undergoing TLIF procedures, additional high-quality prospective studies are needed to verify the results of the present study and add further context to whether the improvements in segmental lordosis and endplate obliquity result in greater long-term clinical outcomes.