Selective Densitometry of the Lumbar Spine

Introduction: Bone mineral density (BMD) has been identified as a major factor in spine construct strength, with failures resulting in pedicle screw loosening and pullout [2,3]. Computed tomography (CT) scans have been shown to effectively measure BMD [1,4]. Previous research has utilized this linear correlation of CT Hounsfield Units (HU) to BMD in order to determine BMD as a function of anatomic location within cervical vertebrae [1]; however, the lumbar spine has not yet been reported on. The goal of this study was to describe BMD of anatomical regions within lumbar vertebrae using the correlation between HU and BMD. It was hypothesized that posterior elements of the lumbar vertebra would exhibit significantly different BMD than the vertebral body. This was tested through means comparison of BMD for each anatomical region Methods: Three-dimensional models of the lumbar spine (L1-L5) were generated by high-resolution helical CT scans of three fresh-frozen cadaver specimens. The lumbar spine was digitally isolated from surrounding tissue by applying a preset thresholding filter for adult bone to the CT scans using medical image processing software. Using this thresholding as well as manual segmentation, the spine model was separated into five vertebrae, followed by segmentation of each vertebra into seven anatomic regions and determination of average HU (Figure 1). HU was then converted to BMD with calibration phantoms of known BMD. One-way ANOVA indicated any significant differences in BMD of the vertebral regions. Results: Overall mean BMD in vertebral regions ranged from 172-393 mg/cm^3, with the highest and lowest BMD found in the lamina and vertebral body, respectively (Figure 2). Vertebral regions demonstrated BMD values in three distinct groups (Figure 2, p < 0.03).The vertebral body and transverse processes represent one group, with significantly lower BMD (146 ± 27, 172 ± 62 mg/cm^3) than other regions. The spinous process, pedicles, and superior articular processes represent a second group, with moderate BMD (281 ± 74, 303 ± 77, 304 ± 79 mg/cm^3). Finally, the inferior articular process and lamina represent a third group, with significantly higher BMD (390 ± 98, 393 ± 116 mg/cm^3) than any other region. Discussion: This study determined relative BMD of clinically relevant anatomic regions of the lumbar vertebra. The lamina and inferior articular processes exhibited the highest BMD, while the BMD of the pedicles was significantly lower (p < 0.02). Standard lumbar fusion currently uses the vertebral body and pedicles as primary locations for osseous …