Jérôme Thevenot

Ct-based finite element models can be used to estimate experimentally measured failure loads in the proximal femur

The objective of this experimental finite element (FE) study was to assess the accuracy of a simulation model estimate of the experimentally measured fracture load of the proximal femur in a sideways fall. Sixty-one formalin-fixed cadaver femora (41 female and 20 male) aged 55-100 years (an average of 80 years) were scanned with a multi-detector CT scanner and were mechanically tested for failure in a sideways fall loading configuration. Twenty-one of these femurs were used for training purposes, and 40 femurs were used for validation purposes. The training set FE models were used to establish the strain threshold for the element failure criteria. Bi-linear elastoplastic FE analysis was performed based on the CT images. The validation set was used to estimate the fracture loads. The Drucker-Prager criterion was applied to determine the yielding and the maximum principal stress criteria and the minimum principal strain criteria for element failure in tension and in compression, respectively. The estimated fracture load values were highly correlated with the experimental data (r=0.931; p<0.001). The slope was 0.929, with an intercept of 258 N, which was not significantly different from 1 and 0, respectively. The study shows that it is possible to estimate the fracture load with relatively high accuracy in a sideways fall configuration by using the CT-based FE method. This method may therefore be applied for studying the biomechanical mechanisms of hip fractures.

Cortical bone finite element models in the estimation of experimentally measured failure loads in the proximal femur

Highly accurate nonlinear finite element (FE) models have been presented to estimate bone fracture load. However, these complex models require high computational capacity, which restricts their clinical applicability. The objective of this experimental FE study was to assess the predictive value of a more simple cortical bone simulation model in the estimation of experimentally measured fracture load of the proximal femur. The prediction was compared with that of DXA, and with the prediction of our previous, more complex FE model including trabecular bone. Sixty-one formalin-fixed cadaver femora (from 41 women and 20 men, age 55-100 years) were scanned using a multi-detector CT and were mechanically tested for failure in a sideways fall loading configuration. Trabecular bone was completely removed from the FE models and only cortical bone was analyzed. The training set FE models (N=21) was used to establish the stress and strain thresholds for the element failure criteria. Bi-linear elastoplastic FE analysis was performed based on the CT images. The validation set (N=40) was used to estimate the fracture load. The estimated fracture load values were highly correlated with the experimental data (r(2)=0.73; p<0.001). The slope was 1.128, with an intercept of -360 N, which was not significantly different from 1 and 0, respectively. DXA-based BMD and BMC correlated moderately with the fracture load (r(2)=0.41 and r(2)=0.40, respectively). The study shows that the proximal femoral failure load in a sideways fall configuration can be estimated with reasonable accuracy by using the CT-based bi-linear elastoplastic cortical bone FE model. This model was more predictive for fracture load than DXA and only slightly less accurate than a full bone FE model including trabecular bone. The accuracy and calculation time of the model give promises for clinical use.

Association between subchondral bone structure and osteoarthritis histopathological grade.

Despite increasing evidence that subchondral bone contributes to osteoarthritis (OA) pathogenesis, little is known about local changes in bone structure compared to cartilage degeneration. This study linked structural adaptation of subchondral bone with histological OA grade. Twenty‐five osteochondral samples of macroscopically different degeneration were prepared from tibiae of 14 patients. Samples were scanned with micro‐computed tomography (μCT) and both conventional structural parameters and novel 3D parameters based on local patterns were analyzed from the subchondral plate and trabecular bone. Subsequently, samples were processed for histology and evaluated for OARSI grade. Each bone parameter and OARSI grade was compared to assess structural adaptation of bone with OA severity. In addition, thicknesses of cartilage, calcified cartilage, and subchondral plate were analyzed from histological sections and compared with subchondral bone plate thickness from μCT. With increasing OARSI grade, the subchondral plate became thicker along with decreased specific bone surface, while there was no change in tissue mineral density. Histological analysis showed that subchondral plate thickness from μCT also includes calcified cartilage. Entropy of local patterns increased with OA severity, reflecting higher tissue heterogeneity. In the trabecular compartment, bone volume fraction and both trabecular thickness and number increased with OARSI grade while trabecular separation and structure model index decreased. Also, elevation of local patterns became longitudinal in the subchondral plate and axial transverse in trabecular bone with increasing OARSI grade. This study demonstrates the possibility of radiological assessment of OA severity by structural analysis of bone.

Assessment of Risk of Femoral Neck Fracture with Radiographic Texture Parameters: A Retrospective Study

PURPOSE To investigate whether femoral neck fracture can be predicted retrospectively on the basis of clinical radiographs by using the combined analysis of bone geometry, textural analysis of trabecular bone, and bone mineral density (BMD). MATERIALS AND METHODS Formal ethics committee approval was obtained for the study, and all participants gave informed written consent. Pelvic radiographs and proximal femur BMD measurements were obtained in 53 women aged 79-82 years in 2006. By 2012, 10 of these patients had experienced a low-impact femoral neck fracture. A Laplacian-based semiautomatic custom algorithm was applied to the radiographs to calculate the texture parameters along the trabecular fibers in the lower neck area for all subjects. Intra- and interobserver reproducibility was calculated by using the root mean square average coefficient of variation to evaluate the robustness of the method. RESULTS The best predictors of hip fracture were entropy (P = .007; reproducibility coefficient of variation < 1%), the neck-shaft angle (NSA) (P = .017), and the BMD (P = .13). For prediction of fracture, the area under the receiver operating characteristic curve was 0.753 for entropy, 0.608 for femoral neck BMD, and 0.698 for NSA. The area increased to 0.816 when entropy and NSA were combined and to 0.902 when entropy, NSA, and BMD were combined. CONCLUSION Textural analysis of pelvic radiographs enables discrimination of patients at risk for femoral neck fracture, and our results show the potential of this conventional imaging method to yield better prediction than that achieved with dual-energy x-ray absorptiometry-based BMD. The combination of the entropy parameter with NSA and BMD can further enhance predictive accuracy.

A novel methodology for generating 3D finite element models of the hip from 2D radiographs

Finite element (FE) modelling has been proposed as a tool for estimating fracture risk and patient-specific FE models are commonly based on computed tomography (CT). Here, we present a novel method to automatically create personalised 3D models from standard 2D hip radiographs. A set of geometrical parameters of the femur were determined from seven a-p hip radiographs and compared to the 3D femoral shape obtained from CT as training material; the error in reconstructing the 3D model from the 2D radiographs was assessed. Using the geometry parameters as the input, the 3D shape of another 21 femora was built and meshed, separating a cortical and trabecular compartment. The material properties were derived from the homogeneity index assessed by texture analysis of the radiographs, with focus on the principal tensile and compressive trabecular systems. The ability of these FE models to predict failure load as determined by experimental biomechanical testing was evaluated and compared to the predictive ability of DXA. The average reconstruction error of the 3D models was 1.77 mm (±1.17 mm), with the error being smallest in the femoral head and neck, and greatest in the trochanter. The correlation of the FE predicted failure load with the experimental failure load was r(2)=64% for the reconstruction FE model, which was significantly better (p<0.05) than that for DXA (r(2)=24%). This novel method for automatically constructing a patient-specific 3D finite element model from standard 2D radiographs shows encouraging results in estimating patient-specific failure loads.

Quantification of differences in bone texture from plain radiographs in knees with and without osteoarthritis

OBJECTIVE To quantify differences in bone texture between subjects with different stages of knee osteoarthritis (OA) and age- and gender-matched controls from plain radiographs using advanced image analysis methods. DESIGN Altogether 203 knees were imaged using constant X-ray parameters and graded according to Kellgren-Lawrence (KL) grading scale (KL0: n = 110, KL1: n = 28, KL2: n = 27, KL3: n = 31, KL4: n = 7). Bone density-related and structure-related parameters were calculated from medial and lateral tibial subchondral bone plate and trabecular bone and from femur. Density-related parameters were derived from grayscale values and structure-related parameters from Laplacian- and local binary patterns (LBP)-based images. RESULTS Reproducibilities of structure-related parameters were better than bone density-related parameters. Bone density-related parameters were significantly (P < 0.05) higher in KL2-4 groups than in control group (KL0) in medial tibial subchondral bone plate and trabecular bone. LBP-based structure parameters differed significantly between KL0 and KL2-4 groups in medial subchondral bone plate, between KL0 and KL1-4 groups in medial and lateral trabecular bone, and between KL0 and KL1-4/KL2-4 in medial and lateral femur. Laplacian-based parameters differed significantly between KL0 and KL2-4 groups in medial side regions-of-interest (ROIs). CONCLUSIONS Our results indicate that the changes in bone texture in knee OA can be quantitatively evaluated from plain radiographs using advanced image analysis. Based on the results, increased bone density can be directly estimated if the X-ray imaging conditions are constant between patients. However, structural analysis of bone was more reproducible than direct evaluation of grayscale values, and is therefore better suited for quantitative analysis when imaging conditions are variable.

Trabecular homogeneity index derived from plain radiograph to evaluate bone quality.

Radiographic texture analysis has been developed lately to improve the assessment of bone architecture as a determinant of bone quality. We validate here an algorithm for the evaluation of trabecular homogeneity index (HI) in the proximal femur from hip radiographs, with a focus on the impact of the principal compressive system of the trabecular bone, and evaluate its correlation with femoral strength, bone mineral density (BMD), and volumetric trabecular structure parameters. A semiautomatic custom‐made algorithm was applied to calculate the HI in the femoral neck and trochanteric areas from radiographs of 178 femoral bone specimens (mean age 79.3 ± 10.4 years). Corresponding neck region was selected in CT scans to calculate volumetric parameters of trabecular structure. The site‐specific BMDs were assessed from dual‐energy X‐ray absorptiometry (DXA), and the femoral strength was experimentally tested in side‐impact configuration. Regression analysis was performed between the HI and biomechanical femoral strength, BMD, and volumetric parameters. The correlation between HI and failure load was R2 = 0.50; this result was improved to R2 = 0.58 for cervical fractures alone. The discrimination of bones with high risk of fractures (load <3000 N) was similar for HI and BMD (AUC = 0.87). Regression analysis between the HIs versus site‐specific BMDs yielded R2 = 0.66 in neck area, R2 = 0.60 in trochanteric area, and an overall of R2 = 0.66 for the total hip. Neck HI and BMD correlated significantly with volumetric structure parameters. We present here a method to assess HI that can explain 50% of an experimental failure load and determines bones with high fracture risk with similar accuracy as BMD. The HI also had good correlation with DXA and computed tomography–derived data.

Discrimination of Cervical and Trochanteric Hip Fractures Using Radiography-Based Two-Dimensional Finite Element Models

Introduction: Predictors of fracture risk differ between cervical and trochanteric hip fractures. The aim of this experimental study was therefore to investigate whether two-dimensional (2D) finite element (FE) models, generated from standard radiographs, are able to predict and discriminate fracture types, originating from a simulated fall on the greater trochanter. Methods: A semi-automatic custom algorithm was applied to segment cortical and trabecular bone contours from radiographs of 49 female cadaver femora (mean age 80.7±10.3 years). Two types of 2D FE models were generated, either one or four material properties assigned to the trabecular bone. The cartilage and soft tissue were also simulated, and the boundary conditions were mimicking the experiment. VonMises stress distributions within the trabecular bone were evaluated and the regions of maximum continuous stress patterns were determined. Results: The best fracture type prediction was obtained with the criterion that a cervical fracture was predicted if the maximum stress in trabecular bone was located at the superior part of the femoral neck and the maximum continuous stress pattern through the neck region; and in all other cases a trochanteric fracture was predicted. The two different models predicted 79.6% and 85.7% of the fracture cases correctly, in comparison with the actual failure type. Conclusion: Our results suggest that the cervical and trochanteric hip fractures can be discriminated with a satisfactory level of accuracy, using a relatively simple radiography- based 2D model. Based on the current experimental findings, the predictive power of these models should now be tested in clinical studies.

Local Binary Patterns to Evaluate Trabecular Bone Structure from Micro-CT Data: Application to Studies of Human Osteoarthritis

Osteoarthritis (OA) causes progressive degeneration of articular cartilage and pathological changes in subchondral bone. These changes can be assessed volumetrically using micro-computed tomography (\(\mu \)CT) imaging. The local descriptor, i.e. local binary pattern (LBP), is a new alternative solution to perform analysis of local bone structures from \(\mu \)CT scans. In this study, different trabecular bone samples were prepared from patients diagnosed with OA and treated with total knee arthroplasty. The LBP descriptor was applied to correlate the distribution of local patterns with the severity of the disease. The results obtained suggest the appearance and disappearance of specific oriented patterns with OA, as an adaptation of the bone to the decrease of cartilage thickness. The experimental results suggest that the LBP descriptor can be used to assess the changes in the trabecular bone due to OA.

Subchondral bone histology and grading in osteoarthritis

Objective Osteoarthritis (OA) has often regarded as a disease of articular cartilage only. New evidence has shifted the paradigm towards a system biology approach, where also the surrounding tissue, especially bone is studied more vigorously. However, the histological features of subchondral bone are only poorly characterized in current histological grading scales of OA. The aim of this study is to specifically characterize histological changes occurring in subchondral bone at different stages of OA and propose a simple grading system for them. Design 20 patients undergoing total knee replacement surgery were randomly selected for the study and series of osteochondral samples were harvested from the tibial plateaus for histological analysis. Cartilage degeneration was assessed using the standardized OARSI grading system, while a novel four-stage grading system was developed to illustrate the changes in subchondral bone. Subchondral bone histology was further quantitatively analyzed by measuring the thickness of uncalcified and calcified cartilage as well as subchondral bone plate. Furthermore, internal structure of calcified cartilage-bone interface was characterized utilizing local binary patterns (LBP) based method. Results The histological appearance of subchondral bone changed drastically in correlation with the OARSI grading of cartilage degeneration. As the cartilage layer thickness decreases the subchondral plate thickness and disorientation, as measured with LBP, increases. Calcified cartilage thickness was highest in samples with moderate OA. Conclusion The proposed grading system for subchondral bone has significant relationship with the corresponding OARSI grading for cartilage. Our results suggest that subchondral bone remodeling is a fundamental factor already in early stages of cartilage degeneration.