265. Lumbar vertebral body Hounsfield units are associated with cage subsidence after transforaminal lumbar interbody fusion

Abstract

BACKGROUND CONTEXT Postoperative subsidence of transforaminal lumbar interbody fusion (TLIF) interbody cages can result in loss of lordosis and foraminal height, and in some cases recurrence of nerve root impingement and radicular symptoms. Hounsfield unit (HU) values as measured on computed tomography (CT) can be used in the assessment of bone quality and bone mineral density. PURPOSE Determine the association between HU measurement of the first lumbar vertebral body and cage subsidence after TLIF. We hypothesized that the HU measured in the vertebral body of L1 would be lower in patients with postoperative TLIF cage subsidence. STUDY DESIGN/SETTING Retrospective case-control study. PATIENT SAMPLE NA OUTCOME MEASURES NA METHODS We retrospectively reviewed all patients undergoing instrumented TLIF from two institutions between July 2004 and June 2014. Postoperative imaging studies (radiographs and CT) were evaluated for evidence of implant subsidence. Hounsfield unit values were obtained from the first lumbar vertebral body on postoperative CT scan. Primary outcomes of this study were presence of cage subsidence and measured HU value of first lumbar vertebral body. RESULTS We identified 89 patients with complete imaging who met our inclusion criteria, with a mean clinical follow up of 27 months. Forty-five patients (50.5%) had evidence of interbody cage subsidence. Average cage subsidence was 5.5mm, with a range of 2.2 to 10.8mm. We found implant height was significantly higher in the subsidence group (12.6mm) when compared to the nonsubsidence group (11.2mm). First lumbar vertebral body coronal (170.69±15.61 vs 138.64±14.21; P=0.003), sagittal (175.14±17.26 vs 140.16±13.71; P=0.002), axial (157.57±12.26 vs 134.35±12.30.83; P=0.009), and average HU (167.8±14.04 vs 137.71±12.83; P=0.002) were significantly higher in the non-subsided group vs subsided group (167.8±14.04 vs 137.71±12.83; P=0.002). The subsidence group had a fusion rate of 82% compared to 93% in the nonsubsidence group, although this finding did not reach statistical significance (p=0.08). CONCLUSIONS We found that HU measurements of the first lumbar vertebra are significantly higher in patients without evidence of TLIF cage subsidence when compared to patients with cage subsidence. We identified a trend toward a decreased fusion rate in the subsidence group, yet this finding was not statistically significant. Our findings suggest that HU measurements may function as a surrogate measurement for bone mineral density, and as a predictor for cage subsidence following TLIF. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.