Is Lamina the Densest Region of the Lumbar Spine? An In Vivo Study Based on Asian Postmenopausal Women

Objective: Pedicle screw fixation is frequently used in spinal orthopedic surgery. It is widely known that postmenopausal women suffer from a significant decrease their bone mineral density (BMD), which may significantly induce a higher risk of fixator failure. Therefore, this study mainly investigated the distribution of lumbar spine BMD at different anatomical regions among postmenopausal women of Asian. Methods: 30 healthy Chinese postmenopausal female volunteers were recruited. Quantitative computed tomography (QCT) scan images were used, based on a 3D threshold segmentation algorithm, to separate the lumbar spine from the surrounding soft tissues. Based on the anatomy of the lumbar spine, each spine segments was divided into 7 anatomic regions based on the boundary lines of different anatomic regions in a 3-dimensional view, cortical and cancellous bone were segmented based on the threshold method. In addition, the average Hounsfield units (HU) for each region were acquired. Finally, the conversion between HU and BMD was performed based on individualized density asynchronous calibration. Results: The L1 segment of the lumbar spine had the lowest areal bone mineral density (aBMD) of all segments at 0.84 g/cm2 and volumetric bone mineral density (vBMD) of 341.06 mg/cm3 (p < 0.001), and the BMD distribution among different segments shows a trend of L1 < L2 < L3 and L4. The lamina (L) of the lumbar spine had the highest vBMD at 540 mg/cm3, whereas the vBMD of the vertebral body (VB) was only about half that of the L at 278.81 mg/cm3. The transverse process (TP) had a vBMD of 293.52 mg/cm3 which is slightly higher than the VB region. To summarize, the distribution of vBMD among different anatomical regions of a single vertebral was presented as L > inferior articular process (IAP) > pedicle (P) > superior articular process (SAP) > spinous process (SP) > TP and VB (p < 0.001). Conclusion: The distribution of BMD in different segments of the lumbar spine showed an increasing trend of L1 < L2 < L3 and L4. The vBMD distribution in each region of the lumbar spine decreased sequentially with increasing distance from the center of intervertebral foramen. The L is the densest area of the lumbar spine, with the IAP being the second densest after the L, followed by the P and SAP. The BMD of SP, TP, and VB was found to be the lowest in the three regions further away from the center of intervertebral foramen. Compared with the P, the L has a higher vBMD, which may be more beneficial for the fixation of implants such as pedicle screws.