Radiological outcomes of static and expandable cage placement in minimally invasive oblique lumbar interbody fusion: a retrospective study

Article information

Asian Spine J. 2025;19(5):745-754

Publication date (electronic) : 2025 August 11

doi : https://doi.org/10.31616/asj.2025.0032

Margaret Patricia Calder Seaton¹, Brian Robert Hirshman², Timothy Yushin Kim², Martin Huy Pham²

¹School of Medicine, University of California, San Diego, La Jolla, CA, USA

²Department of Neurological Surgery, University of California, San Diego, San Diego, CA, USA

Corresponding author: Margaret Patricia Calder Seaton, School of Medicine, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093, USA, Tel: +1-619-543-5540, Fax: +1-619-543-6645, E-mail: mseaton@health.ucsd.edu

Received 2025 January 24; Revised 2025 March 24; Accepted 2025 April 14.

Abstract

GRAPHICAL ABSTRACT

Introduction

Degenerative disk disease causes significant back and leg pain as well as activity limitation and reduced quality of life in affected patients. Minimally invasive techniques in the surgical treatment of refractory disease enable the placement of larger profile implants with minimal paraspinal tissue dissection, which allow for quicker discharge [1], and fewer complications [2], with demonstrated clinical and radiographic effectiveness [3] when compared with open approaches. Minimally invasive oblique lumbar interbody fusion (OLIF) allows access to the spine while avoiding the anterior vessels and psoas muscles via a retroperitoneal corridor at the anterior border of the psoas muscle, thereby enabling access to the disk space for indirect foraminal decompression for rapid and thorough fusion [4,5]. OLIF is an alternative treatment solution to the known approach-related disadvantages of anterior lumbar interbody fusion or lateral lumbar interbody fusion (LLIF), including significant iliac vessel retraction associated with the anterior approach [6], and psoas muscle splitting [7] with the lateral approach, which causes lumbar plexus injury.

Static cages have been traditionally applied in this technique. However, static cages present potential limitations, including repeated trialing and excessive impaction causing endplate violations resulting in implant subsidence [8]. Expandable cages are inserted in the disk space in a collapsed configuration and opened vertically in situ, enabling a smaller surgical window while preserving the height and lordotic angle. A downside of expandable cages is that the expansion mechanism limits the initial graft window, which requires the surgeon to backfill the cage with the graft once expanded, raising the theoretical risk of nonunion [9]. Previous studies have revealed that expandable cages are associated with lower rates of radiographic subsidence, greater short-term (6-month) foraminal height (FH) expansion, decreased intraoperative blood loss, and similar fusion rates to static cages in non-OLIF approaches [2,10–13]. However, to the best of our knowledge, no prior studies have compared outcomes in the use of static and expandable cages in OLIF. This current retrospective case series aimed to compare radiological outcomes, focusing on long-term radiographic subsidence, FH, posterior disk height (DH), and segmental lordosis (SL) after using expandable and static cages in OLIF.

Materials and Methods

Study design

This retrospective single-center series included 86 consecutive cases of OLIF performed by a single attending surgeon (senior author M.H.P.). Demographic and perioperative data were recorded, including age, sex, body mass index (BMI) categories, smoking status, previous abdominal surgery, estimated blood loss, operative level, number of levels, spacer dimensions, hospital length of stay, bone graft type, and reoperation (Table 1). Radiographic parameters, including bone mineral density (Hounsfield units), posterior DH, disk angle (DA), FH, SL, lumbar lordosis (LL), and implant subsidence were then analyzed preoperatively and at three postoperative time points, including immediate postoperatively, 3 months, and the most recent or longest follow-up (mean 387 days). The 3-month follow-up data point included radiographs collected closest to 90 days postoperatively, and the most recent follow-up time point was gathered using the latest radiograph obtained. The minimum follow-up for study inclusion was 100 days or at least two available postoperative radiographs. Hounsfield units were measured at L1 on sagittal preoperative computed tomography (CT) scans within the L1 vertebral body boundaries without including the cortical wall. The intervertebral posterior DH was measured between the endplates in the posterior third on the lateral plane. The DA was measured from the inferior endplate of the cranial vertebral body to the superior endplate of the caudal vertebral body. The FH was measured from the inferior pedicle wall of the superior level to the superior pedicle wall of the inferior level. SL was measured from the superior endplate of the cranial level to the inferior endplate of the caudal level, whereas LL was measured from the superior endplate of L1 to the superior endplate of S1 (Fig. 1). Implant subsidence was evaluated based on the Marchi criteria as the presence of vertebral body collapses around the disk space from standing neutral lateral radiographs. In this study, any level with grade ≥I subsidence, defined as >25% loss of postoperative DH, or clear endplate collapse postoperatively was counted as an incidence of subsidence (Fig. 2) [14].

Table 1

Characteristics of included patients

Fig. 1

Radiographic measures. (A) Lateral preoperative radiograph measurements including ^a)foraminal height, ^b)disc height, ^c)disc angle, ^d)segmental lordosis, and ^e)lumbar lordosis. Postoperative lateral radiographs demonstrating static (B) and expandable (C) cage placement.

Fig. 2

(A, B) Subsidence assessment. Cases of subsidence in static (left) and expandable (right) cage groups per level.

Results

Patient demographic and operative data

In this study, 86 patients who underwent OLIF at 128 levels were divided into expandable (39 patients, 63 levels) and static (47 patients, 66 levels) cage groups. The enrolled patients included 49 women and 37 men with a mean age of 63.8±11.5 years (range, 31–88 years). Most patients underwent single-level fusion (68.6%, 59/86), whereas an additional 19.8% (17/86) underwent two-level fusion, and 11.6% (10/86) had >3 levels of interbody fusion. L4–L5 level fusion was the most predominant for both the static and expandable groups (51.5% and 47.6%, respectively). No significant differences in any demographic variables or recorded baseline radiographic measurements except for the mean length of follow-up at the most recent time point were found between the static and expandable groups (Tables 1, 2).

Table 2

Static versus expandable cage baseline radiographic measurements

The incidence of radiographic subsidence was significantly higher in the static group than in the expandable group (Fig. 3). Cage type predicted the incidence of subsidence. Expandable cages were associated with 4.00 and 2.43 fewer subsidence incidences compared with static cages at the postoperative and most recent time points (p<0.05).

Fig. 3

Instance of subsidence by grade in static and expandable groups. Instance of subsidence and expandable groups by level. The expandable group had significantly fewer instances of subsidence at the postoperative and most recent follow-up dates (4.00 and 2.43) in the multivariable logistic regression model (p<0.05).

Patients were assessed for reoperation. Each patient who underwent subsequent revision surgery at the same level was included. The static and expandable groups involved five and two cases of reoperation, respectively (p>0.05). Refractory pain after the procedure was the most cited cause for reoperation, which was present in 4/5 patients in the static group and none in the expandable group. Other reasons for reoperation included wound revision in two patients, one in both the static and expandable groups, as well as persistent radiculopathy in one patient in the expandable group. Subsidence was present on radiographs before reoperation in 3/5 patients in the static group and none in the expandable group.

We then analyzed the additional radiographic measures within the cage groups and observed significant increases in posterior DH, FH, and DA within the static group and posterior DH, FH, SL, LL, and DA within the expandable group (Supplement 1). To assess between-group differences, we used a linear regression model to predict the change in each radiographic measure compared with the baseline measurement for each time point. In both the posterior disk and FH models, cage type was a significant predictor of the change in height. Expandable cages were associated with improved posterior DH expansion at all three time points when compared with the static cage type (1.24 mm, 0.88 mm, and 1.85 mm; p<0.01). Expandable cages in the FH model were associated with larger FH increases at 3 months and the most recent follow-up (1.12 mm, 0.40 mm, and 1.28 mm; p=0.096, 0.016, and 0.030). The expandable cage type was associated with significant improvement (3.46°, 3.12°, and 3.36°; p<0.01, 0.05, and 0.08) at the immediate postoperative and 3-month time points compared with the static group when predicting the change in SL (Figs. 4, 5). No significant differences in terms of changes in DA and LL were found between the cage groups (Fig. 6).

Fig. 4

Change in posterior disc and foraminal height. Mean overall changes in posterior disc (A) and foraminal height (B). Mean posterior disc height in the static (C) and expandable (D) groups with a significant difference between cage groups at each postoperative time point (p=0.01, 0.05, 0.01). Foraminal height change in the static (E) and expandable (F) cage groups. At the 3 month and most recent time points, there is a significant difference between the cage groups (p<0.05).

Fig. 5

Change in segmental lordosis. (A) Mean changes in segmental lordosis for the entire cohort. Static (B) and expandable (C) segmental lordosis changes with significant differences between groups at the postoperative and 3-month time points (p<0.01, 0.05).

Fig. 6

Lumbar lordosis and disc angle. Mean changes in static and expandable group in lumbar lordosis (A, B) and disc angle (C, D) with no differences between groups.

Discussion

Minimally invasive OLIF is an effective surgical intervention for treating degenerative disk disease. The antero–oblique trajectory allows an approach to the spine between the anterior vessels and psoas muscles, avoiding both to enable the efficient application of large interbody devices for effective foraminal decompression while decreasing the chance of major vessel or nerve injury [5]. The advent of expandable cage technology can further decrease the invasiveness of minimally invasive OLIF by allowing the placement of a large spacer through a narrower surgical corridor as well as in smaller disk spaces. Further, the use of expandable cages reduces soft tissue disruption, decreases recovery time, and improves patient outcomes with reductions in back and leg pain and enhancements in quality of life [20,21]. The expansion mechanism may reduce the risk of iatrogenic endplate damage while enabling customized expansion to achieve individualized optimal anterior column support and lordosis [22]. Expandable cages are not without potential downsides, as the expansion mechanism may leave a gap in the bone graft space [2,3], thereby increasing the chance of nonunion.

As expandable cage technology becomes increasingly popular, outcomes need to be continuously compared with those in patients with static cages to ensure that the theoretical benefits result in improved clinical and radiologic outcomes. A previous study comparing static and expandable cages has been completed in patients undergoing LLIF, where the spine is accessed through a lateral retroperitoneal transpsoas corridor for interbody placement [4]. A 2021 retrospective case review by Li et al. [13] revealed that patients undergoing LLIF with expandable cage placement had significant improvements in SL, Visual Analog Scale (VAS), and Oswestry Disability Index scores in patients with expandable interbody spacers at the 12-month follow-up, demonstrating both clinical and radiographic benefits. Moreover, patients with static cages experienced an increased frequency of subsidence (32.4% vs. 2.2%) [13]. Li et al. [12] revealed that SL significantly increased at all time points in the expandable group, whereas an improvement was only found at 24 months in the static group, with no differences between groups. Similarly, Huo et al. [11] demonstrated significant VAS scores as well as functional status improvements (using 12-item Short Form Survey) in the expandable group when measured at 12 months. Subsidence as accessed by CT was higher in the static group than in the expandable group [11]. Frisch et al. [2] included 56 patients undergoing LLIF and revealed increased subsidence rates in the static cage group (16.1% vs. 0%).

A study by Hiyama et al. [10] examining immediate postoperative results after using static versus expandable cages in LLIF conducted a retrospective review of 67 patients that compared pre- and postoperative CT and magnetic resonance imaging scans. At 2 weeks postoperation, the authors revealed no significant difference in the posterior DH, SL, or foraminal area between the static and expandable cages. The authors could not measure the subsidence or fusion rates due to the short follow-up times.

In this study, we compared radiological outcomes focusing on disk and FH, subsidence, and long-term SL in 86 patients who underwent OLIF at our institution with static or expandable interbody cage placement. Expandable cages were associated with fewer instances of severe (Marchi class ≥I) subsidence and reoperation at the long-term follow-up. Congruent with prior work in LLIF, implant subsidence at the postoperative and most recent follow-up was significantly higher in the static group (12.1%, 24.2%) than in the expandable group (1.6%, 3.2%). Endplate collapse with subsequent implant subsidence, particularly severe subsidence, can be detrimental to mechanical stability, potentially causing impact failure due to poor indirect neural decompression. Further, subsidence can be an early sign of nonunion, frequently causing complications, comorbidity, and revision procedures [19,23]. We did not directly measure fusion in this study, whereas previous research demonstrates a correlation between severe subsidence and fusion rate, and mild subsidence has similar fusion rates to non-subsided levels [19,24]. In this study, three of four patients in the static group who underwent reoperation due to a non-wound revision-related event had subsidence present on immediate postoperative radiographs, and all three patients reported refractory pain after the procedure. These results support other work that indicates that the repeated intraoperative trialing associated with static cage placement may result in immediate endplate compromise and subsidence as recorded from postoperative radiographs [2,11–13]. These results are among a small subset of this cohort and challenging to generalize; however, early implant radiographic subsidence in static cage placement may be associated with eventual implant failure, thereby requiring reoperation.

Expandable cage placement was associated with both improved posterior disk and FH up to the most recent time point. Both preoperative FH and posterior DH were similar between the cage groups, whereas subsequent measurements postoperatively indicated that the expandable group achieved greater postoperative FH and posterior DH expansion. Further, the expandable group experienced greater absolute foraminal and posterior DH, which was maintained at all postoperative time points. A hallmark of degenerative disk disease is persistent radicular pain, which is associated with foraminal narrowing caused by disk herniation and chronic degeneration. Hence, FH is an important variable associated with symptomatic degenerative disk disease. Individuals with normal FH have a space measured between 20 mm and 23 mm, and individuals with an FH of <15 mm combined with a posterior DH of <4 mm are substantially more likely to experience symptomatic nerve root compression [25,26]. In this study, both groups had degraded baseline FH and posterior DH of 17 mm and 5 mm, respectively. Postoperatively, we observed significant between-group differences in FH, with more significant changes in the expandable group.

Indirect foraminal decompression as measured by radiographic disk and FH is an essential goal in lumbar fusion procedures; however, another benefit of implant placement lordosis correction. Sagittal alignment maintenance or correction is crucial to achieve favorable radiographic and patient-reported outcomes. Previous studies have demonstrated the potential of minimally invasive lumbar fusion techniques to produce sustained correction of lordosis [27,28], with an increased risk of subsequent adjacent segmental disease and reoperation with suboptimal lordosis correction. Functionally, sustained SL correction is associated with improved outcomes in long cohort studies, including reduced revision rates and favorable patient-reported outcomes. In our cohort, expandable cages were associated with improved SL correction at all three time points, a result supported in other groups comparing static and expandable cages [12,13]. With significant improvements in three key radiographic measures, including posterior DH, FH, and SL, and a reduction in subsidence and reoperation, these results indicate that both static and expandable cages produce significant improvements in radiographic parameters, whereas expandable cages may be superior to static cages in OLIF.

This study has several limitations that are predominant in retrospective case series, including electronic charting flaws, measurement errors, selection biases, single-center study, single-surgeon study, and lack of a control group. Further, we observed differences in the baseline characteristics of the static and expandable groups. First, the longest follow-up time point significantly differed between the two groups, with a shorter follow-up time in the expandable group. Hence, we included the number of follow-up days in the linear regression. Second, several different combinations of bone grafts used in this study were recorded, and thus drawing meaningful conclusions from these differences is challenging (Supplement 2). With such a heterogenous distribution of graft types used within both groups, planning a similar study design is challenging. This study failed to assess radiographic fusion because these patients do not typically receive follow-up CTs. An additional important outcome after lumbar fusion procedures is the patient-reported outcome measures (PROMs), which were not included in this study due to limited preoperative data. Hence, this paper lacks a discussion on PROMs and is structured as a radiographic outcome report. To this end, we rely on data exhibiting the correlation of radiographic parameter realignment with improved PROMs [29].

However, the results of this study indicate that both static and expandable cages result in radiographic improvement in posterior DH, SL, and FH when used in OLIF. Expandable cages may present certain advantages over static cages due to the significantly lower instances of implant subsidence and the potential to generate greater posterior DH increases with substantial FH expansion. Future studies with long follow-up periods, patient randomization, and PROMs are required to further investigate and compare static with expandable cases for use in OLIF procedures.

Conclusions

Both static and expandable cages provide significant disk expansion and lasting improvements across several important radiographic measures in OLIF. However, expandable cages demonstrated a significantly decreased implant subsidence and reoperation rate, as well as increased SL, posterior disk, and FH, compared with static cages, thereby providing preliminary evidence to support the potential superiority of expandable cages in OLIF.

Supplementary Materials

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