The Impact of CT-based Subsidence Assessment on Radiographic and Clinical Outcomes After Transforaminal Lumbar Interbody Fusion

Introduction: Subsidence represents a well-known complication after interbody fusion that has been previously associated recurrence of preoperative symptoms and higher reoperation rates. Currently, there is incomplete evidence to characterize the effect of subsidence on postoperative alignment and patient reported outcomes measures (PROMs) after transforaminal lumbar interbody fusion (TLIF). The present investigation utilized CT-based subsidence assessment to determine if subsidence was an independent predictor of change in lumbar alignment, PROMs, and complications after TLIF.

Methods: All adult patients who underwent one-two level TLIF for lumbar degenerative conditions at a multi-institutional academic center between 2017-2019 were retrospectively identified. Interbody subsidence at the superior and inferior endplate of each TLIF level was directly measured on the endplate-facing surface of coronal and sagittal CT scans obtained at six months- one year postoperatively. Patients were grouped based on the maximum subsidence at each operative level: mild-moderate ≤4mm or severe ≥4mm. Preoperative and immediate ( < 3 months) and long-term (>6 months) postoperative radiographic outcomes (fusion status, local lumbar alignment, global alignment) and PROMs (VAS Back, Oswestry Disability Index, Short Form-12) were collected. Univariate and multivariate analysis compared patient demographics, surgical factors, change in radiographic measures, change in PROMs, and complication rates across subsidence groups.

Results: A total of 67 patients with 85 unique fusion levels met the inclusion/exclusion criteria (55 with mild-moderate subsidence, 30 with severe subsidence). Severe subsidence was associated with lower Hounsfield units at the L3 vertebral body (Severe [S]:126.4, not severe [NS]:177.3, p=0.046), greater preoperative disc height at the TLIF level (S:9.2mm, NS:6.9mm, p=0.013), and greater superior (S:33.6mm, NS:29.8mm, p< 0.001) and inferior (S:32.7mm, NS:28.5mm, p=0.002) vertebral body heights. Preoperative and postoperative PROMs and preoperative radiographic alignment did not differ significantly on the basis of moderate or severe subsidence. Severe subsidence was associated with reduced regional lordosis (S:7.2 degrees, NS:8.8 degrees, p= 0.05) at six months, reduced long-term change in regional lordosis (S:0.0 degrees, NS:3.6 degrees, p=0.007), reduced fusion rate (S: 50.0%, NS:92.7%, p< 0.001), and increased reoperation rate (S:23.3%, NS:5.5%, p=0.029).

Conclusion : Severe subsidence was identified as a significant risk factor for regional kyphosis, pseudoarthrosis, and need for reoperation. However, subsidence was not significantly associated with PROMs.