The Pedicles Are Not the Densest Regions of the Lumbar Vertebrae: Implications for Bone Quality Assessment and Surgical Treatment Strategy

Abstract

Study Design:Cadaver study.Objective:To determine the bone density of lumbar vertebral anatomic subregions. Bone mineral density (BMD) is a major factor in osseous fixation construct strength. The standard region for implant fixation of the spine is the pedicle; however, other regions may be more viable options with higher bone quality.Methods:Using computed tomography images, the spine was digitally isolated by applying a filter for adult bone. The spine model was separated into 5 lumbar vertebrae, followed by segmentation of each vertebra into 7 regions and determination of average Hounsfield units (HU). HU was converted to BMD with calibration phantoms of known BMD.Results:Overall mean BMD in vertebral regions ranged from 172 to 393 mg/cm3 with the highest and lowest BMD in the lamina and vertebral body, respectively. Vertebral regions formed 3 distinct groups (P < .03). The vertebral body and transverse processes represent one group with significantly lower BMD than other regions. Spinous process, pedicles, and superior articular processes represent a second group with moderate BMD. Finally, inferior articular process (IAP) and lamina represent a third group with significantly higher BMD than other regions.Conclusions:Standard lumbar fusion currently uses the vertebral body and pedicles as primary locations for fixation despite their relatively low BMD. Utilization of posterior elements, especially the lamina and IAP, may be advantageous as a supplement to modern constructs or the primary site for fixation, possibly mitigating construct failures due to loosening or pullout.

Comparison of the Pull‐Out Strength between a Novel Micro‐Dynamic Pedicle Screw and a Traditional Pedicle Screw in Lumbar Spine

“…The reason may be that DEXA, a two-dimensional technique, is unable to capture the threedimensional structure and true volumetric BMD (g/cm 3 ). Hohn et al 29 reported that the effectiveness of the standard procedure for determining spine BMD through DEXA scans may be limited in surgical planning due to its inability to assess the variation of BMD within the vertebrae. In addition, Wang et al 30 demonstrated that the vertebral body contributes on average two-thirds of the vertebral volume in the lumbar spine and only one-third of the BMD.…”

Pedicle Screws Challenged: Lumbar Cortical Density and Thickness Are Greater in the Posterior Elements Than in the Pedicles

“…A previous study using this method of segmentation produced an average difference in BMD of 1.5% between the first and second measurements of the same anatomical region (SD ±0.7%). 14 The largest percent discrepancy between the repeated measures was 3.0%.…”

“…While Hohn et al 14 found the posterior elements to exhibit high total BMD, they do not report other indicators of bone quality such as cortical volume, cortical thickness, and cortical-cancellous bone ratios. BMD may only explain 60% to 70% of the variability in bone strength, while the remaining bone strength is determined by other factors such as the bone geometry, cortical thickness and porosity, and trabecular bone morphology.…”

“…The difference in bone density between the 2 trajectories was even greater for patients with osteoporosis than for those without osteoporosis. In a similar study comparing 7 predefined regions of the lumbar spine in nonosteoporotic cadaveric specimen, Hohn et al 14 reported that the lamina and inferior articular processes have significantly higher BMD than the pedicles and other vertebral regions.…”

“…BMD may only explain 60% to 70% of the variability in bone strength, while the remaining bone strength is determined by other factors such as the bone geometry, cortical thickness and porosity, and trabecular bone morphology. 15 In addition, Hohn et al 14 were unable to detect any differences between the extrapedicular regions requiring a more granular comparison. Since cancellous bone is more affected by osteoporosis, 17 anatomical regions of the lumbar spine with a high cortical-cancellous bone ratio may represent ideal sites for extrapedicular fixation in osteoporotic patients.…”