Predictors of Subsidence and Its Clinical Impact Following Expandable Cage Insertion in Minimally Invasive Transforaminal Interbody Fusion.

Abstract

Retrospective review of prospectively collected multi-surgeon data. Examine the rate, clinical impact, predictors of subsidence after expandable MI-TLIF cage. Expandable cage technology has been adopted in minimally invasive transforaminal lumbar interbody fusion (MI-TLIF) to reduce the risks and optimize outcomes. Although subsidence is of particular concern when using expandable technology as force required to expand the cage can weaken the endplates, its rates, predictors, and outcomes lack evidence. Patients who underwent 1 or 2 level MI-TLIF using expandable cages for degenerative lumbar conditions and had a follow-up of>1 year were included. Preop and immediate, early, and late postoperative radiographs were reviewed. Subsidence was determined if the average anterior/posterior disc height decreased by>25% compared to the immediate postoperative value. Patient reported outcomes were collected and analyzed for differences at the early (<6mo) and late (>6mo) timepoints. Fusion was assessed by 1-year postop CT. 148 patients were included (mean age 61y, 86% 1-level, 14% 2-level). 22 (14.9%) demonstrated subsidence. Although statistically not significant, patients with subsidence were older, lower bone mineral density, and had higher BMI and comorbidity burden. Operative time was significantly higher (P=0.02) and implant width was lower (P<0.01) for subsided patients. VAS-Leg was significantly lower for subsided patients compared to non-subsided patients at a>6m time point. Long-term (>6m) patient acceptable symptom state (PASS) achievement rate was lower for subsided patients (53% vs. 77%), although statistically not significant (P=0.065). No differences existed in complication, reoperation, or fusion rates. 14.9% of patients experienced subsidence predicted by narrower implants. Although subsidence did not have a significant impact on most PROMs and complication, reoperation, or fusion rates, patients had lower VAS-Leg and PASS achievement rate at the>6-month timepoint. 4.