Prediction of vertebral strength under loading conditions occurring in activities of daily living using a computed tomography-based nonlinear finite element method

Correlation analysis between morphologic characteristics of the thoracolumbar basivertebral foramen and Kummell's disease in patients with osteoporosis using imaging techniques

BACKGROUND

The aging of the population is a social problem faced by many countries in the world. With the increase in the elderly population, the number of patients with Kummell's disease is also gradually increasing. No study has demonstrated that Kummell's disease has a clear correlation with the foramen of a vertebrobasilar vein.

OBJECTIVES

The research was conducted to describe and evaluate the morphological characteristics of a basivertebral foramen in patients with osteoporosis and Kummell's disease by CT; to infer whether the specific morphological characteristics of basivertebral foramen may be one of the risk factors of Kummell's disease; to provide clinical suggestions for the treatment of Kummell's disease.

DESIGN

Retrospective analysis from January 2020 to December 2021 on 83 patients with 83 vertebral bodies (T8-L5) diagnosed with senile osteoporosis and Kummell's disease hospitalized in our hospital due to chronic low back pain, including 57 women and 23 men. Group A was assigned for the following patients: the age ranged from 59 to 86 years old, with the average age of 67.30 ± 7.32 years old; the body mass index ranged from 20.01 to 29.46 kg/m², with the average body mass index of 23.51 ± 3.03 kg/m².Group B was assigned for the following patients: 83 patients diagnosed with senile osteoporosis in our outpatient department from January 2020 to December 2021, including 41 males and 42 females; the age ranged from 60 to 85 years, with an average age of 68.52 ± 4.68 years old; the height to weight ratio met the normal reference standard (except 20% above or 10% below the standard weight). Through the lanwon PACS imaging system, the related parameters of the vertebrobasilar foramen in patients with osteoporosis and Kummell's disease were measured to evaluate and analyze the correlation between the morphological characteristics of the vertebrobasilar foramen in patients with osteoporosis and Kummell's disease.

RESULTS

In patients with osteoporosis, the distribution of incidence rate of Kummell's disease in the spine was consistent with that of osteoporotic compression fractures. Sagittal view of the vertebral body on CT scan and the triangular-shaped, trapezoidal-shaped, and irregular-shaped basivertebral foramen in group A accounted for 18%,57%,and 36%,respectively. In group B, triangular-shaped, trapezoidal-shaped, and irregular-shaped foramen accounted for 51%,17%,and 26%,respectively.The distribution of triangular-shaped, trapezoidal-shaped, and irregular-shaped foramen was compared between groups A and B, and the difference was recorded as statistically significant (P < 0.05). Additionally, the difference in the distribution of triangular-shaped, trapezoidal-shaped, and irregular-shaped foramen in group A was found statistically significant (P < 0.05),while that of Group B was found statistically insignificant (P > 0.05).On a horizontal CT scan of the vertebra of group A, triangles, trapezoids, and irregularities accounted for 28%, 26%, and 47%, respectively. In group B, triangles,trapezoids,and irregularities accounted for 31%, 37%, and 30%, respectively. The difference in the distribution of the triangular-shaped and trapezoidal-shaped foramen in groups A and B was statistically insignificant (P > 0.05), while that of irregular-shaped was statistically significant (P < 0.05). Additionally, there was no statistical significance (P > 0.05) in the difference in the morphological distribution of triangular-shaped and trapezoidal-shaped foramen in group A, while that of irregular-shaped was found to be statistically significant (P < 0.05). Further, the difference in the morphological distribution of triangular-shaped, trapezoidal-shaped, and irregular-shaped foramen in group B was not statistically significant (P > 0.05).In general, about 8% of the vertebral body of BF has an osseous septum. In group A, 97% are single-holed while the remaining 3% are porous; in group B, those with single holes accounted for 76%, while the remaining 24% are porous. In groups A and B, the difference in the morphological distribution of single-holed and multi-holed T8, T11, T12, L1, L2, L4, and L5 vertebral bodies was statistically significant (P < 0.05). In group A, the difference in the distribution of single-holed and multi-holed L1 and L5 vertebral bodies was statistically significant (P < 0.05). Similarly, the difference in the distribution of single-holed and multi-holed T8, T11, T12, L1, L2, and L4 basivertebral foramen was statistically significant (P < 0.05).

CONCLUSIONS

In patients with osteoporosis, the incidence of vertebral Kummell's disease can be associated with the morphological characteristics of the basivertebral foramen, as observed in the CT scan. Furthermore, the vertebral body with trapezoidal-shaped and irregular-shaped basivertebral foramen and boneless septum in the foramen is highly susceptible to Kummell's disease.

Biomechanical CT-computed bone strength predicts the risk of subsequent vertebral fracture

Following primary fractures and percutaneous kyphoplasty (PKP), patients have a high risk of incurring a subsequent vertebral fracture (SVF). Given that SVF is a consequence of mechanical deterioration of the vertebra, we sought to examine whether vertebral strength derived from QCT-based finite element analysis (i.e., BCT) can predict the risk of SVF. Sixty-six patients who underwent PKP were categorized into two groups: control or non-SVF group (age: 70 ± 7 years; n = 40) and SVF group (age: 69 ± 8 years; n = 26). BCT was performed on L4 or L3 vertebrae to noninvasively measure vertebral strength. Vertebral strength was also estimated based upon the geometry and material properties of the vertebra. Additionally, trabecular volumetric bone mineral density (vBMD) and L1 Hounsfield unit (HU) were measured. t-Test, χ² test or Mann Whitney U test were used to compare differences in these parameters between the two groups. The predictive abilities of BCT strength and other measured parameters were evaluated using the receiver operating characteristic (ROC) analysis. Results showed no significant difference in either vBMD or L1 HU between the control and SVF groups (p > 0.05), whereas BCT-computed and estimated vertebral strength values were significantly reduced by 33 % and 24 % for the SVF group relative to the non-SVF group, respectively. ROC curve indicated that BCT strength had the largest area under the curve, compared to other parameters. These results suggest that BCT-computed vertebral strength may serve as a surrogate for assessing risk of SVF.

Experimental testing and biomechanical CT analysis of Chinese cadaveric vertebrae with different modeling approaches

Osteoporosis is characterized by reduced bone strength predisposing to an increased risk of fracture. Biomechanical computed tomography (BCT), predicting bone strength via CT-based finite element analysis (FEA), is now clinically available in the USA for diagnosing osteoporosis or assessing fracture risk. However, it has not been previously validated using a cohort of only Chinese subjects. Additionally, the effect of various modeling approaches on BCT outcomes remains elusive. To address these issues, we performed DXA and QCT scanning, compression testing, and BCT analyses on thirteen vertebrae derived from Chinese donors. Three BCT models were created (voxBCT and tetBCT: voxel-based and tetrahedral element-based FE models generated by a commercial software; matBCT: tetrahedral element-based FE model generated by a custom MATLAB program). BCT-computed outcomes were compared with experimental measures or between different BCT models. Results showed that, DXA-measured areal bone mineral density (aBMD) showed weak correlations with experimentally-measured vertebral stiffness (R² = 0.28) and strength (R² = 0.34). Compared to DXA-aBMD, BCT-computed stiffness provided improved correlations with experimentally-measured stiffness (voxBCT: R² = 0.82; tetBCT: R² = 0.77; matBCT: R² = 0.76) and strength (voxBCT: R² = 0.55; tetBCT: R² = 0.57; matBCT: R² = 0.53); BCT-computed mechanical parameters (stiffness, stress and strain) of the three different models were highly correlated with each other, with coefficient of determination (R²) values of 0.89-0.98. These results, based on a cohort of Chinese vertebral cadavers, suggest that BCT is superior over aBMD to consistently predict vertebral mechanical characteristics, regardless of the modeling approaches of choice.

CNN-based qualitative detection of bone mineral density via diagnostic CT slices for osteoporosis screening

The features extracted from diagnostic computed tomography (CT) slices were used to qualitatively detect bone mineral density (BMD) through neural network models, and the evaluation results indicated that it may be a promising approach to perform osteoporosis screening in clinical practice.

INTRODUCTION

The purpose of this study is to design a novelty diagnostic method for osteoporosis screening by using the convolutional neural network (CNN), which can be incorporated into the procedure of routine CT diagnostic in medical examination thereby improving the osteoporosis diagnosis and reducing the patient burden.

METHODS

The proposed CNN-based method mainly comprises two functional modules to perform qualitative detection of BMD by analyzing the diagnostic 2D CT slice. The first functional module aims to locate and segment the ROI of diagnostic 2D CT slice, called Mark-Segmentation-Network (MS-Net). The second functional module is used to determine the category of BMD by the features of ROI, called BMD-Classification-Network (BMDC-Net). The diagnostic 2D CT slice of pedicle level in lumbar vertebrae (L1) was selected from 3D CT image in our experiments firstly. Then, the trained MS-Net can get the mark image of input original 2D CT slice, thereby obtain the segmentation image. Finally, the trained BMDC-Net can obtain the probability value of normal bone mass, low bone mass, and osteoporosis by inputting the segmentation image. On the basis of network results, the radiologists can provide preliminary qualitative diagnosis results of BMD.

RESULTS

Training of the network was performed on diagnostic 2D CT slices of 150 patients. The network was tested on 63 patients. Each patient corresponds to a 2D CT slice. The proposed MS-Net has an excellent segmentation precision on the shape preservation of different lumbar vertebra. The dice index (DI), pixel accuracy (PA), and intersection over union (IOU) of segmentation results are greater than 0.8. The proposed BMDC-Net achieved an accuracy of 76.65% and an area under the receiver operating characteristic curve of 0.9167.

CONCLUSIONS

This study proposed a novel method for qualitative detection of BMD via diagnostic CT slices and it has great potential in clinical applications for osteoporosis screening. The method can potentially reduce the manual burden to radiologists and diagnostic cost to patients.

Radiographic osteoporotic vertebral fractures in elderly men: a brief review focusing on differences between the sexes

Bone loss occurs in both sexes as a result of ageing but is exacerbated in women by the hormonal changes associated with menopause. Unlike in women, secondary osteoporosis occurs in almost half of men diagnosed with osteoporosis. Moreover, vertebral fractures (VFs) seen in elderly men may more likely be the result of high energy trauma. The osteoporotic vertebral fracture (OVF) radiograph diagnosis criteria for women may not be directly applicable for men. Particular attention should be paid to the mid-thoracic level where over-diagnosis commonly occurs. For wedge-shaped vertebral deformities (VDs) or VDs with anterior height reduction only, a diagnosis of OVF requires great caution, as they are poorly correlated to bone mineral density (BMD). For age-matched subjects, it is likely that elderly men's prevalent radiographic OVFs are approximately half of the elderly women's. This male-female ratio is very similar to other clinical fractures such as those occurring at the hip. Even so, the clinical relevance of OVF in elderly men may be less than that of elderly women. On the other hand, for elderly men with hip BMD-based osteoporosis, the OVF risk is as high as that of osteoporotic women. Elderly Chinese men have a lower OVF prevalence than age-matched Caucasian men.

Density-Dependent Material and Failure Criteria Equations Highly Affect the Accuracy and Precision of QCT/FEA-Based Predictions of Osteoporotic Vertebral Fracture Properties

About 700,000 vertebral fractures occur in the US as a result of bone loss. Quantitative computed tomography (QCT)-based finite element analysis (FEA) is a promising tool for fracture risk prediction that is becoming attractive in the clinical setting. The goals of this study were (1) to perform individual and pooled specimen optimization using inverse QCT/FEA modeling to obtain ash density-elastic modulus equations incorporating the whole vertebral body and accounting for all variables used during FE modeling, and (2) to determine the effect of material equations and failure criteria on the accuracy and precision of mechanical properties. Fifty-four (54) human vertebrae were used to optimize material equations based on experimental outcomes and, together with a previously proposed material equation, were implemented in our models using three different failure criteria to obtain fracture loads. Our robust QCT/FEA approach predicted 78% of the failure loads. Material equations resulted in poor accuracy in the predicted stiffness, yet yielded good precision and, more importantly, strong correlations with fracture loads. Both material and fracture criterion equations are equally important in estimating accurate and precise QCT/FEA predictions. Results suggest that both elastic modulus and fracture criterion equations should be validated against experimental outcomes to better explain the response of the tissue under various conditions.

Comparison study of bone strength of the proximal femur with and without hip osteoarthritis by computed tomography-based finite element analysis

Proximal femoral fractures are common in elderly osteoporosis patients; however, its prevalence is clinically rare in hip fracture patients with hip osteoarthritis (OA). This study aimed to evaluate bone strength of the proximal femur with or without hip OA and proximal femoral fracture risk using computed tomography (CT)-based finite element analysis (FEA). A retrospective analysis was done on CT data of 20 patients who underwent total hip arthroplasty for unilateral hip OA. Furthermore, the fracture load between OA and contralateral sides was compared and the association between fracture load and risk factor was analyzed. The fracture load was significantly higher in the OA side than that in the contralateral side (3819.5 ± 1557.8 N vs. 3224.5 ± 943.7 N, respectively; P = 0.0405). There was no significant difference in fracture load between OA and the contralateral side in Kellgren-Lawrence (KL)-3 and KL-4 grade, but the KL-4 fracture load tends to be high (P = 0.2461 and P = 0.0527, respectively). Moreover, there was no significant association between fracture load and OA severity or age. The study findings may assist in predicting bone strength and proximal femur fracture risk. The results of this FEA study indicate the bone strength of the proximal femur was affected by the severity of the osteoarthritis.

Heterogeneity and Spatial Distribution of Intravertebral Trabecular Bone Mineral Density in the Lumbar Spine Is Associated With Prevalent Vertebral Fracture

The spatial heterogeneity in trabecular bone density within the vertebral centrum is associated with vertebral strength and could explain why volumetric bone mineral density (vBMD) exhibits low sensitivity in identifying fracture risk. This study evaluated whether the heterogeneity and spatial distribution of trabecular vBMD are associated with prevalent vertebral fracture. We examined the volumetric quantitative computed tomography (QCT) scans of the L3 vertebra in 148 participants in the Framingham Heart Study Multidetector CT study. Of these individuals, 37 were identified as cases of prevalent fracture, and 111 were controls, matched on sex and age with three controls per case. vBMD was calculated within 5-mm contiguous cubic regions of the centrum. Two measures of heterogeneity were calculated: (i) interquartile range (IQR); and (ii) quartile coefficient of variation (QCV). Ratios in the spatial distributions of the trabecular vBMD were also calculated: anterior/posterior, central/outer, superior/mid-transverse, and inferior/mid-transverse. Heterogeneity and spatial distributions were compared between cases and controls using Wilcoxon rank sum tests and t tests and tested for association with prevalent fractures with conditional logistic regressions independent of integral vBMD. Prevalent fracture cases had lower mean ± SD integral vBMD (134 ± 38 versus165 ± 42 mg/cm3 , p < .001), higher QCV (0.22 ± 0.13 versus 0.17 ± 0.09, p = .003), and lower anterior/posterior rBMD (0.65 ± 0.13 versus 0.78 ± 0.16, p < .001) than controls. QCV was positively associated with increased odds of prevalent fracture (OR 1.61; 95% CI, 1.04 to 2.49; p = .034), but this association was not independent of integral vBMD (p = .598). Increased anterior/posterior trabecular vBMD ratio was associated with decreased odds of prevalent fracture independent of integral vBMD (OR 0.38; 95% CI, 0.20 to 0.71; p = .003). In conclusion, increased trabecular vBMD in the anterior versus posterior centrum, but not trabecular vBMD heterogeneity, was associated with decreased risk of prevalent fracture independent of integral vBMD. Regional measurements of trabecular vBMD could aid in determining the risk and underlying mechanisms of vertebral fracture. © 2019 American Society for Bone and Mineral Research.

Prediction of lumbar vertebral strength of elderly men based on quantitative computed tomography images using machine learning

The parameters extracted from quantitative computed tomography (QCT) images were used to predict vertebral strength through machine learning models, and the highly accurate prediction indicated that it may be a promising approach to assess fracture risk in clinics.

INTRODUCTION

Vertebral fracture is common in elderly populations. The main factor contributing to vertebral fracture is the reduced vertebral strength. This study aimed to predict vertebral strength based on clinical QCT images by using machine learning.

METHODS

Eighty subjects with QCT data of lumbar spine were randomly selected from the MrOS cohorts. L1 vertebral strengths were computed by QCT-based finite element analysis. A total of 58 features of each L1 vertebral body were extracted from QCT images, including grayscale distribution, grayscale values of 39 partitioned regions, BMD_QCT, structural rigidity, axial rigidity, and BMD_QCTA_min. Feature selection and dimensionality reduction were used to simplify the 58 features. General regression neural network and support vector regression models were developed to predict vertebral strength. Performance of prediction models was quantified by the mean squared error, the coefficient of determination, the mean bias, and the SD of bias.

RESULTS

The 58 parameters were simplified to five features (grayscale value of the 60% percentile, grayscale values of three specific partitioned regions, and BMD_QCTA_min) and nine principal components (PCs). High accuracy was achieved by using the five features or the nine PCs to predict vertebral strength.

CONCLUSIONS

This study provided an effective approach to predict vertebral strength and showed that it may have great potential in clinical applications for noninvasive assessment of vertebral fracture risk.

Opportunistic osteoporosis screening in multi-detector CT images via local classification of textures

Our study proposed an automatic pipeline for opportunistic osteoporosis screening using 3D texture features and regional vBMD using multi-detector CT images. A combination of different local and global texture features outperformed the global vBMD and showed high discriminative power to identify patients with vertebral fractures.

INTRODUCTION

Many patients at risk for osteoporosis undergo computed tomography (CT) scans, usable for opportunistic (non-dedicated) screening. We compared the performance of global volumetric bone mineral density (vBMD) with a random forest classifier based on regional vBMD and 3D texture features to separate patients with and without osteoporotic fractures.

METHODS

In total, 154 patients (mean age 64 ± 8.5, male; n = 103) were included in this retrospective single-center analysis, who underwent contrast-enhanced CT for other reasons than osteoporosis screening. Patients were dichotomized regarding prevalent vertebral osteoporotic fractures (noFX, n = 101; FX, n = 53). Vertebral bodies were automatically segmented, and trabecular vBMD was calculated with a dedicated phantom. For 3D texture analysis, we extracted gray-level co-occurrence matrix Haralick features (HAR), histogram of gradients (HoG), local binary patterns (LBP), and wavelets (WL). Fractured vertebrae were excluded for texture-feature and vBMD data extraction. The performance to identify patients with prevalent osteoporotic vertebral fractures was evaluated in a fourfold cross-validation.

RESULTS

The random forest classifier showed a high discriminatory power (AUC = 0.88). Parameters of all vertebral levels significantly contributed to this classification. Importantly, the AUC of the proposed algorithm was significantly higher than that of volumetric global BMD alone (AUC = 0.64).

CONCLUSION

The presented classifier combining 3D texture features and regional vBMD including the complete thoracolumbar spine showed high discriminatory power to identify patients with vertebral fractures and had a better diagnostic performance than vBMD alone.

Prediction of lumbar vertebral body compressive strength of overweight and obese older adults using morphed subject-specific finite-element models to evaluate the effects of weight loss

BACKGROUND

Diet and exercise can promote weight loss in older adults; however, there is potential to increase fracture risk due to loss of bone mineral density (BMD) known to accompany weight loss. Weight loss effects on measures of bone quality and strength are currently unknown.

AIMS

The purpose of this study is to develop subject-specific finite-element (FE) models of the lumbar spine and study the effect of intentional weight loss on bone strength in a pilot data set.

METHODS

Computed tomography (CT) scans of the lumbar spine of 30 overweight and obese (mean BMI = 29.7 ± 3.9 kg/m²), older adults (mean age = 65.9 ± 4.6 years) undergoing an 18-month intentional weight loss intervention were obtained at baseline and post-intervention. Measures of volumetric BMD (vBMD) and variable cortical thickness were derived from each subject CT scan. Development of the subject-specific FE models of the lumbar spine involved model morphing techniques to accelerate the development of the models. vBMD-derived material properties and cortical thickness measures were directly mapped to baseline and post-intervention models. Bone strength was estimated through simulation of a quasi-static uniaxial compression test.

RESULTS

From baseline to 18-month post-weight loss intervention, there were statistically significant decreases in estimated bone strength (6.5% decrease; p < 0.05). Adjusting for baseline bone measures and gender revealed no statistically significant correlations between weight change and change in vBMD, cortical thickness, or bone strength.

CONCLUSION

Integration of CT-based measures and FE models with conventional areal BMD can improve the understanding of the effects of intentional weight loss on bone health.

Effects of Low Bone Mineral Status on Biomechanical Characteristics in Idiopathic Scoliotic Spinal Deformity

BACKGROUND

Low bone mass in patients with adolescent idiopathic scoliosis has been well reported. Poor bone quality was regarded as a new and unique prognostic factor in aggravating curve progression. However, the potential biomechanical correlation between them remains unclear.

METHODS

Three-dimensional finite element models of idiopathic scoliotic spine with different bone mineral status were created for axial loading simulation. An axial load of 3 different body weights was applied on different bone mineral mass models. The mechanical responses of the vertebral cortical and cancellous bone, facet joints, end plate, and intervertebral disc were analyzed.

RESULTS

Accompanied with the low bone mineral status, thoracic scoliosis produced asymmetric and higher stress in the cortical bone, lumbar facet joints, and end plate at the concave side of the thoracic structure curve. Stress increased in the disc at the apex of the scoliosis, whereas it mildly decreased in the L4-5 and L5-S1 disc. Body weight gain increased the stress in scoliotic spine structures in all bone mineral statues.

CONCLUSIONS

Biomechanical simulations indicated that low bone mineral mass might aggravate curve progression and induce more serious lumbar compensatory scoliosis in patients with adolescent idiopathic scoliosis. Weight gain was also a risk factor for curve progression.

Effects of Scan Resolutions and Element Sizes on Bovine Vertebral Mechanical Parameters from Quantitative Computed Tomography-Based Finite Element Analysis

Quantitative computed tomography-based finite element analysis (QCT/FEA) has been developed to predict vertebral strength. However, QCT/FEA models may be different with scan resolutions and element sizes. The aim of this study was to explore the effects of scan resolutions and element sizes on QCT/FEA outcomes. Nine bovine vertebral bodies were scanned using the clinical CT scanner and reconstructed from datasets with the two-slice thickness, that is, 0.6 mm (PA resolution) and 1 mm (PB resolution). There were significantly linear correlations between the predicted and measured principal strains (R² > 0.7, P < 0.0001), and the predicted vertebral strength and stiffness were modestly correlated with the experimental values (R² > 0.6, P < 0.05). Two different resolutions and six different element sizes were combined in pairs, and finite element (FE) models of bovine vertebral cancellous bones in the 12 cases were obtained. It showed that the mechanical parameters of FE models with the PB resolution were similar to those with the PA resolution. The computational accuracy of FE models with the element sizes of 0.41 × 0.41 × 0.6 mm³ and 0.41 × 0.41 × 1 mm³ was higher by comparing the apparent elastic modulus and yield strength. Therefore, scan resolution and element size should be chosen optimally to improve the accuracy of QCT/FEA.

The use of subject-specific Finite Element analysis of L1-L4 vertebra to screening osteoporosis in postmenopausal women

The aim of this work is to study Computed Tomography (CT)-based Finite Element (FE) modelling of lumbar vertebra to classify osteoporotic patients using dual-energy x-ray absorptiometry (DXA) as reference. Cohort comprised 15 postmenopausal female patients with CT-DXA pairs. Sixty subject-specific CT-based FE models were constructed of L1 to L4 vertebrae. Load-displacement in each node of the models was analyzed. Mean kurtosis values of displacement distributions between osteoporosis and healthy groups at L1 level showed a statistically significant difference (p<;0.005). The CT-based FE model of L1 vertebra with patient specific material properties can be used to identify osteoporosis patients.

Finite Element-Based Mechanical Assessment of Bone Quality on the Basis of In Vivo Images

Beyond bone mineral density (BMD), bone quality designates the mechanical integrity of bone tissue. In vivo images based on X-ray attenuation, such as CT reconstructions, provide size, shape, and local BMD distribution and may be exploited as input for finite element analysis (FEA) to assess bone fragility. Further key input parameters of FEA are the material properties of bone tissue. This review discusses the main determinants of bone mechanical properties and emphasizes the added value, as well as the important assumptions underlying finite element analysis. Bone tissue is a sophisticated, multiscale composite material that undergoes remodeling but exhibits a rather narrow band of tissue mineralization. Mechanically, bone tissue behaves elastically under physiologic loads and yields by cracking beyond critical strain levels. Through adequate cell-orchestrated modeling, trabecular bone tunes its mechanical properties by volume fraction and fabric. With proper calibration, these mechanical properties may be incorporated in quantitative CT-based finite element analysis that has been validated extensively with ex vivo experiments and has been applied increasingly in clinical trials to assess treatment efficacy against osteoporosis.

The Effect of Quantitative Computed Tomography Acquisition Protocols on Bone Mineral Density Estimation

Osteoporosis is characterized by bony material loss and decreased bone strength leading to a significant increase in fracture risk. Patient-specific quantitative computed tomography (QCT) finite element (FE) models may be used to predict fracture under physiological loading. Material properties for the FE models used to predict fracture are obtained by converting grayscale values from the CT into volumetric bone mineral density (vBMD) using calibration phantoms. If there are any variations arising from the CT acquisition protocol, vBMD estimation and material property assignment could be affected, thus, affecting fracture risk prediction. We hypothesized that material property assignments may be dependent on scanning and postprocessing settings including voltage, current, and reconstruction kernel, thus potentially having an effect in fracture risk prediction. A rabbit femur and a standard calibration phantom were imaged by QCT using different protocols. Cortical and cancellous regions were segmented, their average Hounsfield unit (HU) values obtained and converted to vBMD. Estimated vBMD for the cortical and cancellous regions were affected by voltage and kernel but not by current. Our study demonstrated that there exists a significant variation in the estimated vBMD values obtained with different scanning acquisitions. In addition, the large noise differences observed utilizing different scanning parameters could have an important negative effect on small subregions containing fewer voxels.

Quantitative, 3D Visualization of the Initiation and Progression of Vertebral Fractures Under Compression and Anterior Flexion

The biomechanical mechanisms leading to vertebral fractures are not well understood. Clinical and laboratory evidence suggests that the vertebral endplate plays a key role in failure of the vertebra as a whole, but how this role differs for different types of vertebral loading is not known. Mechanical testing of human thoracic spine segments, in conjunction with time-lapsed micro-computed tomography, enabled quantitative assessment of deformations occurring throughout the entire vertebral body under axial compression combined with anterior flexion ("combined loading") and under axial compression only ("compression loading"). The resulting deformation maps indicated that endplate deflection was a principal feature of vertebral failure for both loading modes. Specifically, the onset of endplate deflection was temporally coincident with a pronounced drop in the vertebra's ability to support loads. The location of endplate deflection, and also vertebral strength, were associated with the porosity of the endplate and the microstructure of the underlying trabecular bone. However, the location of endplate deflection and the involvement of the cortex differed between the two types of loading. Under the combined loading, deflection initiated, and remained the largest, at the anterior central endplate or the anterior ring apophysis, depending in part on health of the adjacent intervertebral disc. This deflection was accompanied by outward bulging of the anterior cortex. In contrast, the location of endplate deflection was more varied in compression loading. For both loading types, the earliest progression to a mild fracture according to a quantitative morphometric criterion occurred only after much of the failure process had occurred. The outcomes of this work indicate that for two physiological loading modes, the vertebral endplate and underlying trabecular bone are critically involved in vertebral fracture. These outcomes provide a strong biomechanical rationale for clinical methods, such as algorithm-based qualitative (ABQ) assessment, that diagnose vertebral fracture on the basis of endplate depression. © 2015 American Society for Bone and Mineral Research.

A biomechanical sorting of clinical risk factors affecting osteoporotic hip fracture

Osteoporotic fracture has been found associated with many clinical risk factors, and the associations have been explored dominantly by evidence-based and case-control approaches. The major challenges emerging from the studies are the large number of the risk factors, the difficulty in quantification, the incomplete list, and the interdependence of the risk factors. A biomechanical sorting of the risk factors may shed lights on resolving the above issues. Based on the definition of load-strength ratio (LSR), we first identified the four biomechanical variables determining fracture risk, i.e., the risk of fall, impact force, bone quality, and bone geometry. Then, we explored the links between the FRAX clinical risk factors and the biomechanical variables by looking for evidences in the literature. To accurately assess fracture risk, none of the four biomechanical variables can be ignored and their values must be subject-specific. A clinical risk factor contributes to osteoporotic fracture by affecting one or more of the biomechanical variables. A biomechanical variable represents the integral effect from all the clinical risk factors linked to the variable. The clinical risk factors in FRAX mostly stand for bone quality. The other three biomechanical variables are not adequately represented by the clinical risk factors. From the biomechanical viewpoint, most clinical risk factors are interdependent to each other as they affect the same biomechanical variable(s). As biomechanical variables must be expressed in numbers before their use in calculating LSR, the numerical value of a biomechanical variable can be used as a gauge of the linked clinical risk factors to measure their integral effect on fracture risk, which may be more efficient than to study each individual risk factor.

Computed tomography-based finite element analysis to assess fracture risk and osteoporosis treatment

Finite element analysis (FEA) is a computer technique of structural stress analysis and developed in engineering mechanics. FEA has developed to investigate structural behavior of human bones over the past 40 years. When the faster computers have acquired, better FEA, using 3-dimensional computed tomography (CT) has been developed. This CT-based finite element analysis (CT/FEA) has provided clinicians with useful data. In this review, the mechanism of CT/FEA, validation studies of CT/FEA to evaluate accuracy and reliability in human bones, and clinical application studies to assess fracture risk and effects of osteoporosis medication are overviewed.

Specimen-specific vertebral fracture modeling: a feasibility study using the extended finite element method

Osteoporotic vertebral body fractures are an increasing clinical problem among the aging population. Specimen-specific finite element models, derived from quantitative computed tomography (QCT), have the potential to more accurately predict failure loads in the vertebra. Additionally, the use of extended finite element modeling (X-FEM) allows for a detailed analysis of crack initiation and propagation in various materials. Our aim was to study the feasibility of QCT/X-FEM analysis to predict fracture properties of vertebral bodies. Three cadaveric specimens were obtained, and the L3 vertebrae were excised. The vertebrae were CT scanned to develop computational models and mechanically tested in compression to measure failure load, stiffness and to observe crack location. One vertebra was used for calibration of the material properties from experimental results and CT gray-scale values. The two additional specimens were used to assess the model prediction. The resulting QCT/X-FEM model of the specimen used for calibration had 2 and 4% errors in stiffness and failure load, respectively, compared with the experiment. The predicted failure loads of the additional two vertebrae were larger by about 41-44% when compared to the measured values, while the stiffness differed by 129 and 40%. The predicted fracture patterns matched fairly well with the visually observed experimental cracks. Our feasibility study indicated that the QCT/X-FEM method used to predict vertebral compression fractures is a promising tool to consider in future applications for improving vertebral fracture risk prediction in the elderly.

Analysis of vertebral bone strength, fracture pattern, and fracture location: a validation study using a computed tomography-based nonlinear finite element analysis

Finite element analysis (FEA) is an advanced computer technique of structural stress analysis developed in engineering mechanics. Because the compressive behavior of vertebral bone shows nonlinear behavior, a nonlinear FEA should be utilized to analyze the clinical vertebral fracture. In this article, a computed tomography-based nonlinear FEA (CT/FEA) to analyze the vertebral bone strength, fracture pattern, and fracture location is introduced. The accuracy of the CT/FEA was validated by performing experimental mechanical testing with human cadaveric specimens. Vertebral bone strength and the minimum principal strain at the vertebral surface were accurately analyzed using the CT/FEA. The experimental fracture pattern and fracture location were also accurately simulated. Optimization of the element size was performed by assessing the accuracy of the CT/FEA, and the optimum element size was assumed to be 2 mm. It is expected that the CT/FEA will be valuable in analyzing vertebral fracture risk and assessing therapeutic effects on osteoporosis.

Finite element analysis predicts experimental failure patterns in vertebral bodies loaded via intervertebral discs up to large deformation

Vertebral compression fractures are becoming increasingly common. Patient-specific nonlinear finite element (FE) models have shown promise in predicting yield strength and damage pattern but have not been experimentally validated for clinically relevant vertebral fractures, which involve loading through intervertebral discs with varying degrees of degeneration up to large compressive strains. Therefore, stepwise axial compression was applied in vitro on segments and performed in silico on their FE equivalents using a nonlocal damage-plastic model including densification at large compression for bone and a time-independent hyperelastic model for the disc. The ability of the nonlinear FE models to predict the failure pattern in large compression was evaluated for three boundary conditions: healthy and degenerated intervertebral discs and embedded endplates. Bone compaction and fracture patterns were predicted using the local volume change as an indicator and the best correspondence was obtained for the healthy intervertebral discs. These preliminary results show that nonlinear finite element models enable prediction of bone localisation and compaction. To the best of our knowledge, this is the first study to predict the collapse of osteoporotic vertebral bodies up to large compression using realistic loading via the intervertebral discs.

Accuracy of specimen-specific nonlinear finite element analysis for evaluation of radial diaphysis strength in cadaver material

The feasibility of a user-specific finite element model for predicting the in situ strength of the radius after implantation of bone plates for open fracture reduction was established. The effect of metal artifact in CT imaging was characterized. The results were verified against biomechanical test data. Fourteen cadaveric radii were divided into two groups: (1) intact radii for evaluating the accuracy of radial diaphysis strength predictions with finite element analysis and (2) radii with a locking plate affixed for evaluating metal artifact. All bones were imaged with CT. In the plated group, radii were first imaged with the plates affixed (for simulating digital plate removal). They were then subsequently imaged with the locking plates and screws removed (actual plate removal). Fracture strength of the radius diaphysis under axial compression was predicted with a three-dimensional, specimen-specific, nonlinear finite element analysis for both the intact and plated bones (bones with and without the plate captured in the scan). Specimens were then loaded to failure using a universal testing machine to verify the actual fracture load. In the intact group, the physical and predicted fracture loads were strongly correlated. For radii with plates affixed, the physical and predicted (simulated plate removal and actual plate removal) fracture loads were strongly correlated. This study demonstrates that our specimen-specific finite element analysis can accurately predict the strength of the radial diaphysis. The metal artifact from CT imaging was shown to produce an overestimate of strength.

Specimen-specific nonlinear finite element modeling to predict vertebrae fracture loads after vertebroplasty

STUDY DESIGN

Vertebral fracture load and stiffness from a metastatic vertebral defect model were predicted using nonlinear finite element models (FEM) and validated experimentally.

OBJECTIVE

The study objective was to develop and validate an FEM-based tool for predicting polymer-augmented lytic vertebral fracture load and stiffness and the influence of metastatic filling materials.

SUMMARY OF BACKGROUND DATA

Percutaneous vertebroplasty has the potential to reduce vertebral fracture risk affected with lytic metastases by providing mechanical stabilization. However, it has been shown that the mismatch in mechanical properties between poly(methyl-methacrylate) (PMMA) and bone induces secondary fractures and intervertebral disc degeneration. A biodegradable copolymer, poly(propylene fumarate-co-caprolactone) (P(PF-co-CL)), has been shown to possess the appropriate mechanical properties for bone defect repair.

METHODS

Simulated metastatic lytic defects were created in 40 cadaveric vertebral bodies, which were randomized into 4 groups: intact vertebral body (intact), simulated defect without treatment (negative), defect treated with P(PF-co-CL) (copolymer), and defect treated with PMMA (PMMA). Spines were imaged with quantitative computed tomography (QCT), and QCT/FEM-subject-specific, nonlinear models were created. Predicted fracture loads and stiffness were identified and compared with experimentally measured values using Pearson correlation analysis and paired t test.

RESULTS

There was no significant difference between the measured and predicted fracture loads and stiffness for each group. Predicted fracture loads were larger for PMMA augmentation (3960 N [1371 N]) than that for the copolymer, negative and intact groups (3484 N [1497 N], 3237 N [1744 N], and 1747 N [702 N]). A similar trend was observed in the predicted stiffness. Moreover, predicted and experimental fracture loads were strongly correlated (R=0.78), whereas stiffness showed moderate correlation (R=0.39).

CONCLUSION

QCT/FEM was successful for predicting fracture loads of metastatic, polymer-augmented vertebral bodies. Overall, we have demonstrated that QCT/FEM may be a useful tool for predicting in situ vertebral fracture load resulting from vertebroplasty.

LEVEL OF EVIDENCE

N/A.

Biomechanical comparison of mono-segment transpedicular fixation with short-segment fixation for treatment of thoracolumbar fractures: A finite element analysis

Mono-segment transpedicular fixation is a method for the treatment of certain types of thoracolumbar spinal fracture. Finite element models were constructed to evaluate the biomechanics of mono-segment transpedicular fixation of thoracolumbar fracture. Spinal motion (T10–L2) was scanned and used to establish the models. The superior half of the cortical bone of T12 was removed and the superior half of the cancellous bone of the T12 body was assigned the material properties of injured bone to mimic vertebral fracture. Transpedicular fixation of T11 and T12 was performed to produce a mono-segment fixation model; T11 and L1 were fixed to produce a short-segment fixation model. Motion differences between functional units and von Mises stress on the spine and implants were measured under axial compression, anterior bending, extensional bending, lateral bending and axial rotation. We found no significant difference between mono- and short-segment fixations in the motion of any functional unit. Stress on the T10/T11 nucleus pulposus and T10/T11 and L1/L2 annulus fibrosus increased significantly by about 75% on anterior bending, extensional bending and lateral bending. In the fracture model, stress was increased by 24% at the inferior endplate of T10 and by 43% at the superior endplate of L2. All increased stresses were reduced after fixation and lower stress was observed with mono-segment fixation. In summary, the biomechanics of mono-segment pedicle screw instrumentation was similar to that of conventional short-segment fixation. As a minimally invasive treatment, mono-segment fixation would be appropriate for the treatment of selected thoracolumbar spinal fractures.

Locally measured microstructural parameters are better associated with vertebral strength than whole bone density

Whole vertebrae areal and volumetric bone mineral density (BMD) measurements are not ideal predictors of vertebral fractures. We introduce a technique which enables quantification of bone microstructural parameters at precisely defined anatomical locations. Results show that local assessment of bone volume fraction at the optimal location can substantially improve the prediction of vertebral strength.

INTRODUCTION

Whole vertebrae areal and volumetric BMD measurements are not ideal predictors of vertebral osteoporotic fractures. Recent studies have shown that sampling bone microstructural parameters in smaller regions may permit better predictions. In such studies, however, the sampling location is described only in general anatomical terms. Here, we introduce a technique that enables the quantification of bone volume fraction and microstructural parameters at precisely defined anatomical locations. Specific goals of this study were to investigate at what anatomical location within the vertebrae local bone volume fraction best predicts vertebral-body strength, whether this prediction can be improved by adding microstructural parameters and to explore if this approach could better predict vertebral-body strength than whole bone volume fraction and finite element (FE) analyses.

METHODS

Eighteen T12 vertebrae were scanned in a micro-computed tomography (CT) system and FE meshes were made using a mesh-morphing tool. For each element, bone microstructural parameters were measured and correlated with vertebral compressive strength as measured experimentally. Whole bone volume fraction and FE-predicted vertebral strength were also compared to the experimental measurements.

RESULTS

A significant association between local bone volume fraction measured at a specific central region and vertebral-body strength was found that could explain up to 90% of the variation. When including all microstructural parameters in the regression, the predictive value of local measurements could be increased to 98%. Whole bone volume fraction could explain only 64% and FE analyses 76% of the variation in bone strength.

CONCLUSIONS

A local assessment of volume fraction at the optimal location can substantially improve the prediction of bone strength. Local assessment of other microstructural parameters may further improve this prediction but is not clinically feasible using current technology.

Prediction of new clinical vertebral fractures in elderly men using finite element analysis of CT scans

Vertebral strength, as estimated by finite element analysis of computed tomography (CT) scans, has not yet been compared against areal bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) for prospectively assessing the risk of new clinical vertebral fractures. To do so, we conducted a case-cohort analysis of 306 men aged 65 years and older, which included 63 men who developed new clinically-identified vertebral fractures and 243 men who did not, all observed over an average of 6.5 years. Nonlinear finite element analysis was performed on the baseline CT scans, blinded to fracture status, to estimate L1 vertebral compressive strength and a load-to-strength ratio. Volumetric BMD by quantitative CT and areal BMD by DXA were also evaluated. We found that, for the risk of new clinical vertebral fracture, the age-adjusted hazard ratio per standard deviation change for areal BMD (3.2; 95% confidence interval [CI], 2.0-5.2) was significantly lower (p < 0.005) than for strength (7.2; 95% CI, 3.6-14.1), numerically lower than for volumetric BMD (5.7; 95% CI, 3.1-10.3), and similar for the load-to-strength ratio (3.0; 95% CI, 2.1-4.3). After also adjusting for race, body mass index (BMI), clinical center, and areal BMD, all these hazard ratios remained highly statistically significant, particularly those for strength (8.5; 95% CI, 3.6-20.1) and volumetric BMD (9.4; 95% CI, 4.1-21.6). The area-under-the-curve for areal BMD (AUC = 0.76) was significantly lower than for strength (AUC = 0.83, p = 0.02), volumetric BMD (AUC = 0.82, p = 0.05), and the load-to-strength ratio (AUC = 0.82, p = 0.05). We conclude that, compared to areal BMD by DXA, vertebral compressive strength and volumetric BMD consistently improved vertebral fracture risk assessment in this cohort of elderly men.

Vertebral fracture risk and alendronate effects on osteoporosis assessed by a computed tomography-based nonlinear finite element method

Computed tomography-based nonlinear finite element method (CT/FEM) can accurately predict vertebral compressive strength ex vivo and this method is clinically available in vivo. This study aimed to assess vertebral fracture risk and alendronate effects on osteoporosis in vivo using CT/FEM. Vertebral strength in 123 postmenopausal women was analyzed and the discriminatory power for vertebral fracture was assessed cross-sectionally. Alendronate effects were also prospectively assessed in 33 patients with postmenopausal osteoporosis who were treated with alendronate at a dose of 5 mg/day for 18 months. CT/FEM had higher discriminatory power for vertebral fracture than areal bone mineral density (BMD) and volumetric BMD and detected alendronate effects at 3 months. Marked bone density increases were noted in juxtacortical areas compared to inner trabecular areas. CT/FEM was useful for assessing vertebral fracture risk and therapeutic effects on osteoporosis.

A new method to include the gravitational forces in a finite element model of the scoliotic spine

The distribution of stresses in the scoliotic spine is still not well known despite its biomechanical importance in the pathomechanisms and treatment of scoliosis. Gravitational forces are one of the sources of these stresses. Existing finite element models (FEMs), when considering gravity, applied these forces on a geometry acquired from radiographs while the patient was already subjected to gravity, which resulted in a deformed spine different from the actual one. A new method to include gravitational forces on a scoliotic trunk FEM and compute the stresses in the spine was consequently developed. The 3D geometry of three scoliotic patients was acquired using a multi-view X-ray 3D reconstruction technique and surface topography. The FEM of the patients' trunk was created using this geometry. A simulation process was developed to apply the gravitational forces at the centers of gravity of each vertebra level. First the "zero-gravity" geometry was determined by applying adequate upwards forces on the initial geometry. The stresses were reset to zero and then the gravity forces were applied to compute the geometry of the spine subjected to gravity. An optimization process was necessary to find the appropriate zero-gravity and gravity geometries. The design variables were the forces applied on the model to find the zero-gravity geometry. After optimization the difference between the vertebral positions acquired from radiographs and the vertebral positions simulated with the model was inferior to 3 mm. The forces and compressive stresses in the scoliotic spine were then computed. There was an asymmetrical load in the coronal plane, particularly, at the apices of the scoliotic curves. Difference of mean compressive stresses between concavity and convexity of the scoliotic curves ranged between 0.1 and 0.2 MPa. In conclusion, a realistic way of integrating gravity in a scoliotic trunk FEM was developed and stresses due to gravity were explicitly computed. This is a valuable improvement for further biomechanical modeling studies of scoliosis.

Vulnerability of healthy vertebrae in patients with and without previous vertebral fracture

Vertebral deformities are associated with a marked increase in morbidity, mortality, and burden in terms of sanitary expenditures. Patients with vertebral fractures have a negative impact in their health, less quality of life, and loss of functional capacity and independence. The purpose of this study was to explore the vulnerability of healthy vertebrae in patients who have sustained already a compression fracture and in patients who do not have prevalent fractures in the thoracic spine; and to explore the association of the deformity in healthy vertebrae with different variables, such as bone mineral density (BMD), body mass index, age, loss of height, presence of clinical kyphosis, history of other osteoporotic fractures, and falls occurring during the last year. Clinical data and complementary studies from 175 postmenopausal outpatients were analyzed. These women (age: 69.7±11.1 years) had not received any treatment for osteoporosis. Anteroposterior and lateral radiographs of the thoracic spine and bone densitometry of the hip were obtained; morphometry was performed in 1575 thoracic vertebrae from T4 to T12. The angle of wedging of each vertebral body was calculated using a trigonometric formula. Then, the sum of wedge angles of vertebral bodies (SWA) was determined, and Cobb angle was measured. In patients with vertebral fractures, after excluding the angles of fractured vertebral bodies, the mean wedge angle of the remaining vertebrae (MWAhealthy) was calculated. The same procedure was followed in patients without vertebral fractures. MWAhealthy was considered as an indicator of the structural vulnerability of non-fractured vertebrae. Patients with prevalent fractures had lower BMD, wider Cobb angle, and higher sum of wedge angles than patients without vertebral fractures. The proportion of patients with accentuation of clinical kyphosis was higher in the group with prevalent vertebral fractures. A highly significant difference was found in the MWAhealthy, which was higher in patients with prevalent fractures (4.1±1.3° vs. 3.0±1.1°; p<0.001). Patients showing vertebral fractures had 7.1±4.2 cm height loss in average, significantly superior than that found among non-fractured women (3.6±3.2 cm; p<0.01). In multivariate analysis, the increase of MWAhealthy was associated with advancing age (p<0.02), lower femoral neck BMD (p<0.005), presence of clinical kyphosis (p<0.01) and vertebral fractures (p<0.02). This study presents evidence that a series of factors independently influence the increase in wedging deformity of vertebral bodies that are not fractured yet. These factors could contribute to an increased vulnerability of the vertebrae, making them more susceptible to fracture.

Biomechanics of vertebral fractures and the vertebral fracture cascade

Vertebral fractures (VFxs) are the most common osteoporotic fracture, and are a strong risk factor for future fracture. The presence of a VFx greatly increases the risk of sustaining subsequent VFxs-a phenomenon often referred to as the "vertebral fracture cascade." VFxs do not occur uniformly along the spine, but occur more often at the mid-thoracic and thoracolumbar regions than elsewhere. It is likely that both the vertebral fracture cascade and the bimodal distribution of VFx along the spine are attributable to biomechanical factors. VFxs occur when the forces applied to the vertebral body exceed its strength. Loading on the spine is primarily determined by a person's height, weight, muscle forces, and the task or movement performed, but can also be affected by other factors, such as spinal curvature and invertebral disk deterioration. Vertebral strength is determined mainly by bone size, shape, and bone mineral density, and secondarily by bone microarchitecture, collagen characteristics, and microdamage. Better understanding of VFx etiology is hampered by the fact that most VFxs do not come to clinical attention; therefore, the factors and activities that cause VFxs remain ill defined, including possible differences in the etiology of acute fractures versus those of slow onset. Additional research is needed to elucidate the precise mechanical, morphologic, and biological mechanisms that underlie VFx to improve strategies for assessing VFx risk and preventing the vertebral fracture cascade.