Ex vivo experiment on vertebral body with defect representing bone metastasis

BackgroundMetastatic diseases and infections frequently involve the spine. This is the result of seeding of the vertebral body by tumor cells or bacteria delivered by venous blood from Batson's plexus, which is hypothesized to enter the vertebral body via the epidural veins. Isolated spinal segments deform significantly at the bony end plate when under compression. This deformation could cause a volume change of the vertebral body and may be accompanied by retrograde flow of venous blood. To date, this process has not been investigated quantitatively. The purpose of this study was to determine the volume changes of the vertebral body and basivertebral vein for a vertebral body under compression.MethodsA three-dimensional finite element mesh model of the L4 segment with both adjacent discs was modified from a 3-D computed tomography scan image. An octagon representing the basivertebral vein was introduced into the center of the vertebral body in the model. Four compressive orientations (1500 N) were applied on the top disc. The volume change of the vertebral body model and the basivertebral vein were then computed.ResultsThe volume change of the vertebral body was about 0.1 cm3 (16.3% of the basivertebral vein) for the four loading conditions. The maximum cross-sectional area reductions of the basivertebral vein and volume reduction were 1.54% and 1.02%, for uniform compression.ConclusionOur study quantified the small but significant volume change of a modeled vertebral body and cross-sectional areas and that of the basivertebral vein, due to the inward bulging of the end plate under compression. This volume change could initiate the reverse flow of blood from the epidural venous system and cause seeding of tumors or bacterial cells.

Consensus Summary on the Diagnosis and Treatment of Osteoporosis

The strength and compression resistance of vertebral bodies have important clinical implications with regard to susceptibility to impact trauma. This study compares compression resistance limits between the thoracic and lumbar centrums from cadaveric material to evaluate structural strength and fracture risk. The samples were radiographed to ensure healthy specimens with no evident pathologies and to document density properties (e.g., osteoporotic characteristics). Vertebral bodies were mechanically tested in compression, using the Instron 8874 (Norwood, MA) Servohydraulic Test System. Results show thoracic and lumbar strengths ranging from 0.91 N/mm 2 to 1.57 N/mm 2 and 0.67 N/mm 2 to 1.05 N/mm 2 , respectively. Average maximum load to failure for the thoracic samples was 1011 N and 1620 N for lumbar samples. This study showed that while lower vertebral bodies exhibited higher load to failure values, thoracic centrums demonstrated greater maximum stress during compressive testing. Our preliminary results may shed light on the clinical understanding of compression fractures, common in the elderly, for improved outcomes. Grant Funding Source : SUNY Downstate College of Medicine

Posterior reduction and pedicle screw fixation is a widely used procedure for thoracic and lumbar vertebrae fractures. Usually, the pedicle screws would be removed after the fracture healing and screw tunnels would be left. The aim of this study is to evaluate the effect of screw tunnels on the biomechanical stability of the lumbar vertebral body after pedicle screws removal by finite element analysis (FEA). First, the CT values of the screw tunnels wall in the fractured vertebral bodies were measured in patients whose pedicle screws were removed, and they were then compared with the values of vertebral cortical bone. Second, an adult patient was included and the CT images of the lumbar spine were harvested. Three dimensional finite element models of the L1 vertebra with unilateral or bilateral screw tunnels were created based on the CT images. Different compressive loads were vertically acted on the models. The maximum loads which the models sustained and the distribution of the force in the different parts of the models were recorded and compared with each other. The CT values of the tunnels wall and vertebral cortical bone were 387.126±62.342 and 399.204±53.612, which were not statistically different (P=0.149). The models of three dimensional tetrahedral mesh finite element of normal lumbar 1 vertebra were established with good geometric similarity and realistic appearance. After given the compressive loads, the cortical bone was the first one to reach its ultimate stress. The maximum loads which the bilateral screw tunnels model, unilateral screw tunnel model, and normal vertebral model can sustain were 3.97 Mpa, 3.83 Mpa, and 3.78 Mpa, respectively. For the diameter of the screw tunnels, the model with a diameter of 6.5 mm could sustain the largest load. In addition, the stress distributing on the outside of the cortical bone gradually decreased as the thickness of the tunnel wall increased. Based on the FEA, pedicle screw tunnels would not decrease the biomechanical stability and strength of the vertebral body. A large diameter of screw tunnel and thick tunnel wall were helpful for the biomechanical stability of the vertebral body.

Single-fraction image-guided intensity-modulated radiation therapy (IG-IMRT) allows for tumoricidal treatment of traditionally radioresistant cancers while sparing critical adjacent structures. Risk of vertebral fracture after IG-IMRT for spinal metastases has not been defined. We evaluated 62 consecutive patients undergoing single fraction IG-IMRT at 71 sites for solid organ metastases. A neuroradiologist and three spine surgeons evaluated prospectively obtained magnetic resonance/computed tomography (CT) imaging studies for post-treatment fracture development and tumor recurrence. Fracture progression was noted in 27 vertebrae (39%). Multivariate logistic regression analysis showed that CT appearance, lesion location, and percent vertebral body involvement independently predicted fracture progression. Lesions located between T10 and the sacrum were 4.6 times more likely to fracture than were lesions above T10 (95% CI, 1.1 to 19.7). Lytic lesions were 6.8 times more likely to fracture than were sclerotic and mixed lesions (95% CI, 1.4 to 33.3). As percent vertebral body involvement increased, odds of fracture also increased. Patients with fracture progression had significantly higher narcotic use, change in Karnofsky performance score, and a strong trend toward higher pain scores. Local tumor progression occurred in seven patients and contributed to one fracture. Obesity, posterior element involvement, bisphosphonate use, and local kyphosis did not confer increased risk. Vertebral fracture is common after single fraction IG-IMRT for metastatic spine lesions. Lytic disease involving more than 40% of the vertebral body and location at or below T10 confer a high risk of fracture, the presence of which yields significantly poorer clinical outcomes. These results may help clinicians identify high-risk patients who would benefit from prophylactic vertebro- or kyphoplasty.

A Structural Approach to Renal Bone Disease

In vitro and in vivo assessment of the accuracy of devices proposed for tracking spine motion during surgery; in vivo assessment of vertebral motion during spine surgery. 1) To quantify the accuracy of newly designed vertebral body trackers; 2) to demonstrate the feasibility of tracking vertebral motion in a cadaveric model; and 3) to quantify the vertebral motion that occurs during spinal surgery. Computer techniques are beginning to be applied to spine surgery. Validation of accuracy of methods of spinal tracking has not been reported. No information exists on the amount of vertebral motion that occurs during surgery. Because the new techniques require accurate positional information for the vertebral body, it is important to understand and evaluate methods of tracking vertebrae. An optical tracking system (Northern Digital, Waterloo, Ontario, Canada) was used to track custom-designed trackers. The reliability and accuracy of the trackers were evaluated in vitro. The proposed tracking methodology for human testing was performed using a cadaveric model, and after successful completion, human testing was done in the operating room to evaluate the motion of two vertebral bodies during exposure for instrumentation of the lumbar spine. This technique was used to evaluate the custom designed trackers effectiveness for tracking vertebral bodies for pedicle screw insertion. The trackers developed were accurate and capable of tracking the motion of the spine. Measured motion of L3 and L4 during breathing was 1.3 mm, peak to peak. Maximal intraoperative motion of the vertebral bodies was 12.3 mm during maneuvers simulating dissection of soft tissue and targeting of spinal pedicles. Significant motion occurs in lumbar vertebral bodies during surgery. Breathing motion alone is up to 1.3 mm, and surgeon-induced motion up to 10 times greater. Vertebral body trackers for use with an optical position sensor were capable of measuring this motion.

Estimation of vertebral body strength by dual photon absorptiometry in elderly individuals: Comparison between measurements of total vertebral and vertebral body bone mineral

Trabecular bone score (TBS) rests on the textural analysis of dual-energy X-ray absorptiometry (DXA) to reflect the decay in trabecular structure characterizing osteoporosis. Yet, its discriminative power in fracture studies remains incomprehensible because prior biomechanical tests found no correlation with vertebral strength. To verify this result possibly owing to an unrealistic setup and to cover a wide range of loading scenarios, the data from three previous biomechanical studies using different experimental settings were used. They involved the compressive failure of 62 human lumbar vertebrae loaded 1) via intervertebral discs to mimic the in vivo situation ("full vertebra"); 2) via the classical endplate embedding ("vertebral body"); or 3) via a ball joint to induce anterior wedge failure ("vertebral section"). High-resolution peripheral quantitative computed tomography (HR-pQCT) scans acquired from prior testing were used to simulate anterior-posterior DXA from which areal bone mineral density (aBMD) and the initial slope of the variogram (ISV), the early definition of TBS, were evaluated. Finally, the relation of aBMD and ISV with failure load (F(exp)) and apparent failure stress (σexp) was assessed, and their relative contribution to a multilinear model was quantified via ANOVA. We found that, unlike aBMD, ISV did not significantly correlate with F(exp) and σexp , except for the "vertebral body" case (r(2) = 0.396, p = 0.028). Aside from the "vertebra section" setup where it explained only 6.4% of σexp (p = 0.037), it brought no significant improvement to aBMD. These results indicate that ISV, a replica of TBS, is a poor surrogate for vertebral strength no matter the testing setup, which supports the prior observations and raises a fortiori the question of the deterministic factors underlying the statistical relationship between TBS and vertebral fracture risk.

BackgroundPercutaneous vertebral augmentation (PVA) is an established treatment for selected patients with osteoporotic vertebral compression fractures (VCFs) but carries a substantial risk of subsequent vertebral fractures (SVFs). Opportunistic bone mineral density (BMD) assessment using CT-derived Hounsfield units (HU) enables site-specific vertebral measurements, which may provide additional insight into fracture risk beyond whole-vertebra or dual-energy X-ray absorptiometry (DXA)-based averages. This study aims to determine the distribution of vertebral body density along the spinal column, and to evaluate the predictive value of low site-specific preoperative HU measurements for SVF risk following cement augmentation.MethodsWe conducted a retrospective case-control study of patients who underwent PVA for osteoporotic VCFs between 2007 and 2017. HU values were measured in three locations on preoperative computed tomography (CT) vertebrae within thoracic, thoracolumbar, and lower lumbar spine. HU attenuation values were compared between the SVF and non-SVF groups, by univariant Cox regression and General Estimating Equation (GEE) model, for SVF risk factor prediction.ResultsSixty-six patients (50 women, mean 77.9 years) underwent single-level thoraco-lumbar percutaneous cement augmentation (kyphoplasty/vertebroplasty) for low-energy osteoporotic compression fracture. Twenty-two patients (33.3%) had SVF, within 31.2 months. Across the cohort, mean HU values were significantly different between locations (P<0.001): below superior endplate (57 HU), middle vertebral body (85.9 HU) and superior to inferior endplate (81.9 HU). Significant differences were also noted between mean HU values at thoracic spine (T5–T10, 100.6 HU), thoracolumbar junction (T11–L2, 80 HU) and lower lumbar levels (L3–L5, 75.7 HU) (P<0.001). Lower mean HU values were observed in the SVF group (47.22 vs. 53.86 HU, P=0.63 and 74.74 vs. 85.11 HU, P=0.36, respectively). Fractures of the L2 vertebra correlated significantly with predicted risk of SVF (P=0.012). In GEE analyses, no clinical variables correlated with SVF risk.ConclusionsSignificant novel site-specific findings include: lowest bone density at superior aspect of cancellous vertebral body in thoraco-lumbar junction; L2 vertebral fractures, a specific risk factor for SVF. No significant correlation between low site-specific bone-density and the risk of SVF was identified.

Parathyroid hormone (PTH) may be anabolic at trabecular bone and catabolic in cortical bone. As many regions of the skeleton contain both types of bone, the effects of PTH deficiency or excess may be difficult to evaluate using bone densitometry, a technique that integrates the cortical and trabecular compartments of bone. We asked the following questions: 1) Is the higher bone mineral density (BMD) in postsurgical hypoparathyroidism due to higher cortical, not trabecular, bone? 2) Is age-related bone loss slowed in patients with postsurgical hypoparathyroidism? 3) Is lower BMD in primary hyperparathyroidism the result of deficits in cortical, not trabecular, bone? BMD of the lumbar spine, proximal femur, distal radius, and femoral midshaft was measured by postero-anterior (PA) scanning, while bone mineral content (BMC) of the third lumbar vertebra was measured by lateral scanning using dual x-ray absorptiometry in 10 women, ages 64.6 +/- 3.2 yr, with postsurgical hypoparathyroidism and in 25 women, ages 68.7 +/- 1.6 yr, with primary hyperparathyroidism. Measurements were repeated 4.7 +/- 0.6 yr later in 8 patients with hypoparathyroidism and 4.0 +/- 0.4 yr later in 20 age-matched controls. Data were expressed as z scores (SD, mean +/- sem) derived from 405 postmenopausal women. In patients with hypoparathyroidism, bone mass z score of the third lumbar vertebra (vertebral body plus posterior processes) was higher than zero by PA scanning (1.26 +/- 0.58 SD, P < 0.05) and lateral scanning (1.04 +/- 0.60 SD, P = 0.1), and higher at the trabecular-rich vertebral body (1.02 +/- 0.47 SD, P = 0.07) and predominantly cortical posterior processes (0.98 +/- 0.66 SD, P = 0.1) determined by lateral scanning. The BMD z scores were higher than zero at the femoral neck (0.89 +/- 0.48 SD, P = 0.09), but not at the femoral midshaft (0.45 +/- 0.60, NS) and distal radius (0.04 +/- 0.51, NS). During follow-up, femoral neck BMD decreased in controls but not in patients with hypoparathyroidism (slope, -0.00818 +/- 0.00496 g/cm2/year vs. 0.00907 +/- 0.00583 g/cm2/year, respectively, P = 0.06). There was no change in lumbar spine BMD in either group. In 25 women with primary hyperparathyroidism, there were no deficits in BMD at the third lumbar vertebra (vertebral body plus posterior processes) by PA or lateral scanning. By lateral scanning, BMC was increased at the vertebral body (0.64 +/- 0.31 SD, P < 0.01) and reduced at the posterior processes (-0.65 +/- 0.26 SD, P < 0.05). BMD was lower at the midshaft of the femur (-0.82 +/- 0.37 SD, P < 0.05) and at the distal radius (-0.68 +/- 0.20 SD, P < 0.01), but not at the femoral neck (-0.08 +/- 0.20 SD, NS). Longitudinal data were unavailable in hyperparathyroid patients. In summary, trabecular bone is increased by both PTH deficiency and excess. Cortical bone loss is slowed by PTH deficiency and accelerated by PTH excess so that suppression of PTH may reduce age-related bone loss and the risk of fracture. Assessment of BMD in PTH deficiency and excess requires the separate study of cortical and trabecular bone.

Assessment of Spine Bone Mineral Density in Juvenile Idiopathic Arthritis: Impact of Scan Projection

Ex vivo biomechanical study. To compare the biomechanical properties of isolated, fractured, osteoporotic vertebral bodies after treatment by kyphoplasty with one of two cements: alpha-tri-calcium phosphate cement (Biopex-R; Mitsubishi Materials Corp., Tokyo, Japan) or polymethylmethacrylate (Simplex P; Stryker-Howmedica-Osteonics, Mahwah, NJ). Kyphoplasty and vertebroplasty typically use polymethylmethacrylate cements for the treatment of osteoporotic compression fractures. Scant information exists regarding the use of alternative cements in kyphoplasty. Simulated compression fractures were created in 24 vertebral bodies (T6-T9, L2-L5) harvested from three female cadavers. Vertebral bodies were assigned to one of two groups: kyphoplasty with Biopex-R or kyphoplasty with Simplex P. The kyphoplasty treatment consisted of inserting bone tamps bipedicularly into each vertebral body, inflating the tamp, and filling the created void with Biopex-R or Simplex P. Pretreatment and post-treatment heights were measured, and the repaired vertebral bodies were recompressed to determine posttreatment strength and stiffness values. Differences were checked for significance (P < 0.05) using a repeated-measures analysis of variance followed by Tukey's test. Kyphoplasty with Biopex-R restored strength in the lumbar and thoracic vertebral bodies. Kyphoplasty with Simplex P displayed significantly greater posttreatment strength than initial strength in the thoracic region. Vertebral bodies augmented with either cement were significantly less stiff than their initial conditions, except for the thoracic vertebrae treated with Simplex P, in which stiffness was restored. There was no significant difference in percentage of height restored between the cement treatments. Kyphoplasty with either cement restored initial strength. In general, stiffness was not restored.

Properties of PMMA end cap holders affect FE stiffness predictions of vertebral specimens

Using a retrograde neurotracing method with Fluoro-Gold (FG), the level at which dorsal root ganglions (DRGs) innervate the L2 and L5 vertebral bodies and the innervation pathways were investigated in rats. To clarify the levels at which DRGs innervate the lumbar vertebral bodies and to determine the pathways from the L2 and L5 vertebral bodies to DRGs. Elderly patients with osteoporosis sometimes experience lumbar vertebral fracture and may also feel diffuse nonlocalized pain in the back, lateral portion of the trunk, and area surrounding the iliac crest. However, the pattern of sensory innervation of vertebral bodies remains unclear. Forty female Sprague-Dawley rats were used. FG crystals were applied to the L2 (L2 vertebra group) or L5 (L5 vertebra group) vertebral bodies via an anterior approach, and numbers of labeled neurons in DRGs from T10 to L6 were counted. To determine sensory pathways, bilateral sympathectomy was performed. In nonsympathectomy animals, FG-labeled neurons were present in DRGs from T11 through L3 in the L2 vertebra group and from T13 through L6 in the L5 vertebra group. The number of labeled neurons following sympathectomy was not significantly different in L1, L2, and L3 DRGs in the L2 vertebra group or in L3, L4, L5, and L6 DRGs in the L5 vertebra group from those in nonsympathectomy animals. In contrast, fewer labeled DRG neurons were present in sympathectomy animals at T11, T12, and T13 in the L2 vertebra group, and at T13, L1, and L2 in the L5 vertebra group than in nonsympathectomy animals (P < 0.01). Sensory nerve fibers in the L2 and L5 vertebral bodies are derived from the T11-L3 and T13-L6 DRGs, respectively. Some sensory nerves from the L2 and L5 vertebral bodies enter the paravertebral sympathetic trunks and reach the DRGs at multisegmental levels. The present findings regarding multisegmental innervation to vertebral bodies may explain the diffuse pain that originates within osteoporotic vertebral fractures in elderly patients.