Associations between the hounsfield unit values of different trajectories and bone mineral density of vertebrae: cortical bone and traditional trajectories

Cortical bone trajectory (CBT) is widely used in orthopedic surgery to improve fixation while minimizing soft tissue dissection. This study used radiological methods to assess the correlation between the bone mineral density (BMD) of vertebrae and Hounsfield unit (HU) values of CBT and traditional trajectory (TT). A total of 240 thoracic and lumbar (T9-L5) vertebrae from 40 cadaveric spines were obtained. The specimens were measured using computed tomography (CT). The axial CT images of TT were sliced in a plane horizontal to the pedicle, whereas those of CBT were sliced in a caudocranial plane. The regions of interest of TT and CBT were selected to calculate an average HU value within the area, wherein the screws were inserted and fixed at 6.0 mm x 40 mm and 4.0 mm x 30 mm, respectively. The BMD of vertebrae was measured by dual-energy X-ray absorptiometry (DEXA) and quantitative CT (QCT). The HU value of CBT (286.74 +/- 120.80) was almost twice higher than that of TT (165.61 +/- 92.38). The average lateral and anteroposterior BMDs of 240 vertebrae determined using DEXA were 0.540 +/- 0.193 and 0.651 +/- 0.180 g/cm(2), respectively. The average cortical and cancellous BMDs of 240 vertebrae determined using QCT were 245.63 +/- 80.09 and 88.24 +/- 61.78 mg/cm(3), respectively. The BMD determined using DEXA and QCT was significantly and positively associated with the HU values of CBT and TT. The ratio of the HU values of CBT and TT was significantly and negatively associated with the lateral BMD determined using DEXA and the cancellous BMD determined using QCT. However, it was significantly and positively associated with segments but not with the anteroposterior BMD determined using DEXA and the cortical BMD determined using QCT. Collectively, the HU values of CBT and TT significantly decreased with decreasing BMD. However, the CBT HU values significantly decreased less than the TT HU values, especially in low-BMD vertebrae and cauda lumbar segments.