The Association Between Vertebral Body Hounsfield Units And Cage Subsidence After Transforaminal Lumbar Interbody Fusion

Objective: Cage subsidence after lumbar interbody fusion may lead to spinal deformity,recurrence or aggravation of neurologic symptoms.Bone mineral density is one of the factors that are associated with cage subsidence after lumbar interbody fusion.As a screening tool of osteoporosis,HU measurement has been used in clinical practice.Thus,this study was to assess the association between HU measurement and cage subsidence after lumbar interbody fusion.Methods: All the patients who received L4/5 lumbar interbody fusion with unilateral fixation in our department were reviewed from June 2013 to June 2014.The minimum follow up was 6 months.The postoperative CT was reconstructed to obtain sagittal plane to assess cage subsidence.Cage subsidence was defined as any subsidence into upper or lower endplate.Based on age and sex,patients with cage subsidence was 1:1 matched to controls in database.The mean HU value of whole lumbar vertebral body and regional vertebrae,lumbar lordosis,regional lumbar lordosis were measured.Then,the association between HU measurement and cage subsidence was analyzed.Results: 242 patients received L4/5 lumbar interbody fusion with unilateral internal fixation.Cage subsided in 18 patients and they were 1:1 matched to controls.HU measurement was a reliable method.The HU value in whole lumbar vertebral body in patients with cage subsidence was significantly lower than that in controls(112.4 ±10.08 vs.140.2 ± 10.17,p=0.0015).Meanwhile,the regional HU value in patients with cage subsidence was lower than that in controls(113.4 ± 10.47 vs.127.9 ± 8.13,p=0.0075).The areas under receiver operating characteristic cure were 0.715 and 0.636 for whole vertebral body HU value and regional HU value.The cut-off of whole vertebral body HU value and regional HU value is 132(sensitivity: 83.3%,specificity: 61.1%),122(sensitivity: 72.2%,specificity: 55.6%).Conclusion: Lower HU values in preoperative lumbar CT was associated with cage subsidence after lumbar interbody fusion with unilateral fixation.As a reliable method,HU measurement may be an index to predict cage subsidence.