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Introduction: Minimally invasive surgery for transforaminal lumbar interbody fusion (MIS-TLIF) has become a mainstay procedure in the treatment of degenerative conditions of the lumbar spine. Expandable interbody devices have gained popularity in recent times due to several well-cited advantages including greater correction of disc height and lordosis and decreased intraoperative disruption of neurologic structures. However, the clinical benefits offered by these devices compared to traditional static implants remain unclear. This study seeks to investigate differences in the radiographic and clinical outcomes between traditional static versus expandable interbody devices used in MIS-TLIF.

Methods: Patients who underwent MIS-TLIF performed by three surgeons from 2014 to 2020 at a single institution high-volume center were retrospectively reviewed. Radiographic measurements were performed on lateral radiographs taken preoperatively, 3-weeks following, and at least 6 months following the date of surgery. Radiographic analysis included anterior and posterior disc height, segmental lordosis, endplate violation, and cage subsidence. Clinical outcomes were evaluated by assessing for the presence of radicular leg pain on the side of the TLIF following surgery at 3 months follow up. Statistical analysis included independent t tests for continuous variables and chi-square analysis for categorical values.

Results: Three-hundred and sixty-seven patients who underwent MIS-TLIF for degenerative diagnoses using either a static (229 patients) or expandable (138 patients) cage were included. The mean age was 62.9 ± 11.7 years in the static group and 67.4 ± 11.2 years in the expandable group. The mean body mass index (BMI) was 30.4 ± 6.4 in the static group and 30.73 ± 7.3 in the expandable group. Patients receiving expandable cages had significantly greater anterior disc height (11.5 mm static vs. 13.2 mm expandable, p < 0.001), posterior disc height (7.18 mm static vs 8.19 mm expandable, p < 0.001), and segmental lordosis (6.47 degrees static vs 7.49 degrees expandable, p = 0.001) at most recent follow up. No significant differences in cage subsidence rates were noted between static (19.7%) and expandable (22.9%) devices (OR =0.823, CI 0.513 – 1.321, p = 0.42). Patients who received expandable devices reported greater improvements in leg pain at 3 months follow up (p = 0.012).

Discussion: Patients who underwent MIS-TLIF with an expandable device demonstrated greater correction of anterior and posterior disc height, as well as greater and more sustained correction of segmental lordosis compared to static cages. No significant differences were noted in cage subsidence rate between static and expandable cages. Patients who received expandable devices were noted to have greater improvements in radicular leg pain.