Static vs Expandable Interbody Cages for Minimally Invasive Transforaminal Interbody Fusion

Degenerative lumbar pathologies are common among the aging population and can often lead to pain, disability, and poor quality of life. Specifically, loss of lumbar lordosis (LL) is a common feature, and the literature suggests that restoration of sagittal alignment is associated with the best clinical outcomes. While decompression and stabilization of the spine have served as great surgical options, minimally invasive transforaminal lumbar interbody fusion (MI-TLIF) has proven to be a safe and effective treatment for optimizing sagittal alignment.1 MI-TLIF requires a relatively small surgical corridor to access the disc space posteriorly and, traditionally, large static cages were inserted in the space to correct the sagittal alignment. However, the size of these static cages through the small opening posed a significant risk to injuring nearby nerve roots and violating the endplates.[1] Thus, many spine surgeons hesitated to utilize static TLIF cages because of their limited ability to induce proper lordosis without complications.

Expandable TLIF cages were later developed and launched with the hope of optimizing the sagittal alignment dilemma and minimizing risks.[2] Expandable cages were designed to be inserted in a low-profile, nonexpanded state into the relatively small disc space, and then expanded at the right time to the desired height.[2] The theoretical advantage of an expandable cage was to allow improved restoration of alignment through a small incision without injuring nearby nerve roots. However, it is not entirely clear how effective the cage expansion results are in disc space lordosis and distraction. In this article, we compare the literature regarding reported outcomes of expandable and static TLIF cages.

The first static cages implemented in MI-TLIF were inserted obliquely into the interbody space and have been shown to slightly decrease segmental lordosis (SL) through increased posterior disc height and unchanged anterior disc height. Thus, MI-TLIF with static cages has historically been considered a “kyphogenic” procedure.[3] More recently, static cages have provided additional lordotic correction at the index level through increased anterior disc height at the potential cost of decreased foraminal height.[4] However, static cages have exhibited shortcomings, including an increased risk of pseudoarthrosis, subsidence, nerve root injuries, and lacerations of the dura.[5] Moreover, in a systematic review and meta-analysis by Calvachi-Prieto et al, patients who underwent minimally invasive lumbar fusion with static cages exhibited significantly longer hospital stays and smaller increases in postoperative disc height compared to patients with expandable cages.[6] The use of static cages in MI-TLIF necessitates extensive endplate preparation and testing, which may undermine biomechanical stability.[4]

To account for the barriers presented by the narrow access corridor and the need to individualize lordotic correction for patients, cages with the capacity for multidirectional expansion were later developed. These cages are inserted into the interbody space in a collapsed, low-profile state prior to in-situ expansion. This design alleviates the possibility of retraction-induced nerve root injury and iatrogenic endplate damage due to decreased trialing and impaction during implantation. The capacity for in-situ expansion enhances the fit of the cage and therefore maximizes disc height, allowing for greater indirect decompression and lordotic correction, all while decreasing operative time.[7] However, attempts to achieve optimal disc height via increased expansion may also induce endplate damage that has the potential to compromise fusion and augment the risk of subsidence.[8] So, while expandable cages may have more power to induce lordosis, restore disc height, and save expenses by decreasing operative time, subsidence and endplate violation may negate any significant gains compared to static cages. Yet, one of the biggest drawbacks of expandable interbody fusion cages may be related to the increased implant cost. Further research will have to show whether the higher implant cost on the front end of a clinical treatment cycle are justified by perioperative cost savings on the back end due to shorter operative time, fewer complications, and fewer reoperations.[9]

Many studies in the literature have retrospectively assessed direct comparisons between the two types of cages. One study found that there was a significant reduction in visual analog scale scores, numeric pain rating scale scores, and Oswestry Disability Index scores for both static and expandable cohorts, but the reductions were more pronounced in the expandable cohort.[3] Yet another study did the same assessments and found no statistical difference in the same scores for both cohorts.[6] Further studies found significantly greater restoration of foraminal height in expandable cohorts and found that although both anterior/posterior disc height was achieved in both the static and expandable cohorts, the change was greater in the expandable cage cohort.[3,6] Expandable cages offered similar improvements in segmental lordosis at 6 weeks (static: 1.69° vs expandable: 2.81°; P = 0.243), but segmental lordosis was better maintained with expandable cages leading to significant differences at 1-year follow-up (static: 0.86° vs expandable: 2.45°, P = 0.001).[3] However, one study in patients undergoing single-level TLIF reported no difference in SL or LL improvement at 1 month or 1 year postoperatively between static and expandable cages.[2] A separate study found that compared to static cages, expandable cages improve and maintain foraminal dimensions and disc height.[8] While expandable cages have shown increased subsidence rates compared to static cages in some studies, others have also reported no difference or a decreased rate.[6,7,10]

The literature has shown expandable cages to induce good segmental lordosis, lumbar lordosis, disc height, and foraminal height comparable to static cages. Expandable cages have also been shown to reduce operative time and length of stay, but might not be overall cost effective, with rates of subsidence and endplate violations. While the use of expandable interbody devices leads to beneficial changes in the spine, this may not necessarily correlate with superior patient outcomes. As such, patients undergoing MITLIF can expect similar improvements in all patient-reported outcomes whether receiving static or expandable interbody devices.

1. Chang CC, Chou D, Pennicooke B, et al. Long-term radiographic outcomes of expandable versus static cages in transforaminal lumbar interbody fusion [advance online publication November 13, 2020]. J Neurosurg Spine. https:// doi.org/10.3171/2020.6.SPINE191378

2. Yee TJ, Joseph JR, Terman SW, Park P. Expandable vs static cages in transforaminal lumbar interbody fusion: radiographic comparison of segmental and lumbar sagittal angles. Neurosurgery. 2017;81(1):69-74.

3. Woodward J, Koro L, Richards D, Keegan C, Fessler RD, Fessler RG. Expandable versus static transforaminal lumbar interbody fusion cages: 1-year radiographic parameters and patient-reported outcomes. World Neurosurg. 2022;159:e1-e7.

4. Gelfand Y, Benton J, De la Garza-Ramos R, Yanamadala V, Yassari R, Kinon MD. Effect of cage type on short-term radiographic outcomes in transforaminal lumbar interbody fusion. World Neurosurg. 2020;141:e953-e958.

5. Lin GX, Quillo-Olvera J, Jo HJ, et al. Minimally invasive transforaminal lumbar interbody fusion: a comparison study based on end plate subsidence and cystic change in individuals older and younger than 65 years. World Neurosurg. 2017;106:174-184.

6. Calvachi-Prieto P, McAvoy MB, Cerecedo-Lopez CD, et al. Expandable versus static cages in minimally invasive lumbar interbody fusion: a systematic review and meta-analysis. World Neurosurg. 2021;151:e607-e614.

7. Lin GX, Kim JS, Kotheeranurak V, Chen CM, Hu BS, Rui G. Does the application of expandable cages in TLIF provide improved clinical and radiological results compared to static cages? A meta-analysis. Front Surg. 2022;9:949938.

8. Boktor JG, Pockett RD, Verghese N. The expandable transforaminal lumbar interbody fusion—two years follow-up. J Craniovertebr Junction Spine . 2018;9(1):50-55.

9. Lewandrowski KU, Ferrara L, Cheng B. Expandable interbody fusion cages: an editorial on the surgeon’s perspective on recent technological advances and their biomechanical implications. Int J Spine Surg. 2020;14(s3):S56-S62.

10. Armocida D, Pesce A, Cimatti M, Proietti L, Santoro A, Frati A. Minimally invasive transforaminal lumbar interbody fusion using expandable cages: increased risk of late postoperative subsidence without a real improvement of perioperative outcomes: a clinical monocentric study. World Neurosurg. 2021;156:e57-e63.