Vertebral bone density in Hounsfield units as a predictor of interbody non-union and implant subsidence in lumbar circumferential fusion

Abstract

Objective. To determine the values of Hounsfield units (HU) of the lumbar vertebrae predicting unsatisfactory radiological results of circumferential interbody fusion at the lumbar level.Material and Methods. The data of patients who underwent a single-level decompression and stabilization intervention at the L4–L5 or L5–S1 level for degenerative diseases of the spine were analyzed. The CT images of the lumbar spine were assessed before surgery with the measurement of HU values of the vertebral bodies at the intervention level, as well as CT images one year after surgery to evaluate the degree of interbody block formation and subsidence of the cage. Three groups of patients were distinguished: patients with a formed interbody bone block and without cage subsidence (control group), patients with failed fusion and patients with cage subsidence.Results. The study presents CT data of 257 patients. The incidence of non-union was 32.3 % (83/257), and of cage subsidence – 43.6 % (112/257). The proportion of patients with reduced bone mineral density (BMD) was 26.1 % (67/257). Patients with non-union and subsidence had higher ODI scores (p = 0.045 and p = 0.050, respectively) compared to controls. The presence of fusion failure and subsidence is associated with reduced BMD (p < 0.05), HU values of vertebrae (p < 0.05), and higher ODI score (p < 0.05). According to the ROC analysis, threshold HU values were determined equal to 127 HU, 136 HU and 142 HU for the L4, L5, S1 vertebral bodies, respectively. Upon reaching these values, the risk of a combination of fusion failure and subsidence increases significantly (p = 0.022).Conclusions. Patients with non-union and cage subsidence have less satisfactory clinical outcomes. The HU values of the vertebral bodies equal to 127 HU, 136 HU and 142 HU for the L4, L5, and S1, respectively, are advisable to use in practice to predict non-union and subsidence after a single-level decompression and stabilization intervention at the lower lumbar levels.