Mathematical relationships between bone density and mechanical properties: a literature review

FEM-Based Compression Fracture Risk Assessment in Osteoporotic Lumbar Vertebra L1

This paper presents a finite element method (FEM)-based fracture risk assessment in patient-specific osteoporotic lumbar vertebra L1. The influence of osteoporosis is defined by variation of parameters such as thickness of the cortical shell, the bone volume–total volume ratio (BV/TV), and the trabecular bone score (TBS). The mechanical behaviour of bone is defined using the Ramberg–Osgood material model. This study involves the static and nonlinear dynamic calculations of von Mises stresses and follows statistical processing of the obtained results in order to develop the patient-specific vertebra reliability. In addition, different scenarios of parameters show that the reliability of the proposed model of human vertebra highly decreases with low levels of BV/TV and is critical due to the thinner cortical bone, suggesting high trauma risk by reason of osteoporosis.

Material Mapping of QCT-Derived Scapular Models: A Comparison with Micro-CT Loaded Specimens Using Digital Volume Correlation

Subject- and site-specific modeling techniques greatly improve finite element models (FEMs) derived from clinical-resolution CT data. A variety of density-modulus relationships are used in scapula FEMs, but the sensitivity to selection of relationships has yet to be experimentally evaluated. The objectives of this study were to compare quantitative-CT (QCT) derived FEMs mapped with different density-modulus relationships and material mapping strategies to experimentally loaded cadaveric scapular specimens. Six specimens were loaded within a micro-CT (33.5 μm isotropic voxels) using a custom-hexapod loading device. Digital volume correlation (DVC) was used to estimate full-field displacements by registering images in pre- and post-loaded states. Experimental loads were measured using a 6-DOF load cell. QCT-FEMs replicated the experimental setup using DVC-driven boundary conditions (BCs) and were mapped with one of fifteen density-modulus relationships using elemental or nodal material mapping strategies. Models were compared based on predicted QCT-FEM nodal reaction forces compared to experimental load cell measurements and linear regression of the full-field nodal displacements compared to the DVC full-field displacements. Comparing full-field displacements, linear regression showed slopes ranging from 0.86 to 1.06, r-squared values of 0.82–1.00, and max errors of 0.039 mm for all three Cartesian directions. Nearly identical linear regression results occurred for both elemental and nodal material mapping strategies. Comparing QCT-FEM to experimental reaction forces, errors ranged from − 46 to 965% for all specimens, with specimen-specific errors as low as 3%. This study utilized volumetric imaging combined with mechanical loading to derive full-field experimental measurements to evaluate various density-modulus relationships required for QCT-FEMs applied to whole-bone scapular loading. The results suggest that elemental and nodal material mapping strategies are both able to simultaneously replicate experimental full-field displacements and reactions forces dependent on the density-modulus relationship used.

Automated segmentation of cortical and trabecular bone to generate finite element models for femoral bone mechanics

Finite element (FE) models based on quantitative computed tomography (CT) images are better predictors of bone strength than conventional areal bone mineral density measurements. However, FE models require manual segmentation of the femur, which is not clinically applicable. This study developed a method for automated FE analyses from clinical CT images. Clinical in-vivo CT images of 13 elderly female subjects were collected to evaluate the method. Secondly, proximal cadaver femurs were harvested and imaged with clinical CT (N = 17). Of these femurs, 14 were imaged with µCT and three had earlier been tested experimentally in stance-loading, while collecting surface deformations with digital image correlation. Femurs were segmented from clinical CT images using an automated method, based on the segmentation tool Stradwin. The method automatically distinguishes trabecular and cortical bone, corrects partial volume effect and generates input for FE analysis. The manual and automatic segmentations agreed within about one voxel for in-vivo subjects (0.99 ± 0.23 mm) and cadaver femurs (0.21 ± 0.07 mm). The strains from the FE predictions closely matched with the experimentally measured strains (R² = 0.89). The method can automatically generate meshes suitable for FE analysis. The method may bring us one step closer to enable clinical usage of patient-specific FE analyses.

Individual Differences in Women During Walking Affect Tibial Response to Load Carriage: The Importance of Individualized Musculoskeletal Finite-Element Models

Subject-specific features can contribute to the susceptibility of an individual to stress fracture. Here, we incorporated tibial morphology and material properties into a standard musculoskeletal finite-element (M/FE) model and investigated how load carriage influences joint kinetics and tibial mechanics in women. We obtained the morphology and material properties of the tibia from computed tomography images for women of three distinctly different heights, 1.51 m (short), 1.63 m (medium), and 1.75 m (tall), and developed individualized M/FE models for each. Then, we calculated joint and muscle forces, and subsequently, tibial stress/strain for each woman walking at 1.3 m/s under various load conditions (0, 11.3, or 22.7 kg). Among the subjects investigated, using individualized and standard M/FE models, the joint reaction forces (JRFs) differed by up to 4 (hip), 22 (knee), and 26% (ankle), and the 90th percentile von Mises stress by up to 30% (tall woman). Load carriage evoked distinct biomechanical responses, with a 22.7-kg load decreasing the peak hip JRF during late stance by ∼18% in the short woman, while increasing it by ∼39% in the other two women. It also increased peak knee and ankle JRFs by up to ∼48 (tall woman) and ∼36% (short woman). The same load increased the 90th percentile von Mises stress (and corresponding cumulative stress) by 31 (28), 22 (30), and 27% (32%) in the short, medium, and tall woman, respectively. Our findings highlight the critical role of individualized M/FE models to assess mechanical loading in different individuals performing the same physical activity.

Evaluation of the capability of the simulated dual energy X-ray absorptiometry-based two-dimensional finite element models for predicting vertebral failure loads

Prediction of the vertebral failure load is of great importance for the prevention and early treatment of bone fracture. However, an efficient and effective method for accurately predicting the failure load of vertebral bones is still lacking. The aim of the present study was to evaluate the capability of the simulated dual energy X-ray absorptiometry (DXA)-based finite element (FE) model for predicting vertebral failure loads. Thirteen dissected spinal segments (T11/T12/L1) were scanned using a HR-pQCT scanner and then were mechanically tested until failure. The subject-specific three-dimensional (3D) and two-dimensional (2D) FE models of T12 were generated from the HR-pQCT scanner and the simulated DXA images, respectively. Additionally, the areal bone mineral density (aBMD) and areal bone mineral content (aBMC) of T12 were calculated. The failure loads predicted by the simulated DXA-based 2D FE models were more moderately correlated with the experimental failure loads (R² = 0.66) than the aBMC (R² = 0.61) and aBMD (R² = 0.56). The 2D FE models were slightly outperformed by the HR-pQCT-based 3D FE models (R² = 0.71). The present study demonstrated that the simulated DXA-based 2D FE model has better capability for predicting the vertebral failure loads than the densitometric measurements but is outperformed by the 3D FE model. The 2D FE model is more suitable for clinical use due to the low radiation dose and low cost, but it remains to be validated by further in vitro and in vivo studies.

A round-robin finite element analysis of human femur mechanics between seven participating laboratories with experimental validation

Finite element analysis is a common tool that has been used for the past few decades to predict the mechanical behavior of bone. However, to our knowledge, there are no round-robin finite element analyses of long human bones with more than two participating biomechanics laboratories published yet, where the results of the experimental tests were not known in advance. We prepared a fresh-frozen human femur for a compression test in a universal testing machine measuring the strains at 10 bone locations as well as the deformation of the bone in terms of the displacement of the loading point at a load of 2 kN. The computed tomography data of the bone with a calibration phantom as well as the orientation of the bone in the testing machine with the according boundary conditions were delivered to seven participating laboratories. These were asked to perform a finite element analysis simulating the experimental setup and deliver their results to the coordinator without knowing the experimental results. Resultantly, four laboratories had deviations from the experimentally measured strains of less than 40%, and three laboratories had deviations of their numerically determined values compared to the experimental data of more than 120%. These deviations are thought to be based on different material laws and material data, as well as different material mapping methods. Investigations will be conducted to clarify and assess the reasons for the large deviations in the numerical data. It was shown that the precision of finite element models of the human femur is not yet as developed as desired by the biomechanics community.

Assessment of finite element models for prediction of osteoporotic fracture

With increasing life expectancy and mortality rates, the burden of osteoporotic hip fractures is continually on an upward trend. In terms of prevention, there are several osteoporosis treatment strategies such as anti-resorptive drug treatments, which attempt to retard the rate of bone resorption, while promoting the rate of formation. With respect to prediction, several studies have provided insights into obtaining bone strength by non-invasive means through the application of FE analysis. However, what valuable information can we obtain from FE studies that have focused on osteoporosis research, with respect to the prediction of osteoporotic fractures? This paper aims to fine studies that have used FE analysis to predict fractures in the proximal femur through a systematic search of literature using PUBMED, with the main objective of supporting the diagnosis of osteoporosis. The focus of these FE studies is first discussed, and the methodological aspects are summarized, by mainly comparing and contrasting their meshing properties, material properties, and boundary conditions. The implications of these methodological differences in FE modelling processes and propositions with the aim of consolidating or minimalizing these differences are further discussed. We proved that studies need to start converging in terms of their input parameters to make the FE method applicable to clinical settings. This, in turn, will decrease the time needed for in vitro tests. Current advancements in FE analysis need to be consolidated before any further steps can be taken to implement engineering analysis into the clinical scenario.

3D patient-specific finite element models of the proximal femur based on DXA towards the classification of fracture and non-fracture cases

Osteoporotic bone fractures reduce quality of life and drastically increase mortality. Minimally irradiating imaging techniques such as dual-energy X-ray absorptiometry (DXA) allow assessment of bone loss through the use of bone mineral density (BMD) as descriptor. Yet, the accuracy of fracture risk predictions remains limited. Recently, DXA-based 3D modelling algorithms were proposed to analyse the geometry and BMD spatial distribution of the proximal femur. This study hypothesizes that such approaches can benefit from finite element (FE)-based biomechanical analyses to improve fracture risk prediction. One hundred and eleven subjects were included in this study and stratified in two groups: (a) 62 fracture cases, and (b) 49 non-fracture controls. Side fall was simulated using a static peak load that depended on patient mass and height. Local mechanical fields were calculated based on relationships between tissue stiffness and BMD. The area under the curve (AUC) of the receiver operating characteristic method evaluated the ability of calculated biomechanical descriptors to discriminate fracture and control cases. The results showed that the major principal stress was better discriminator (AUC > 0.80) than the volumetric BMD (AUC ≤ 0.70). High discrimination capacity was achieved when the analysis was performed by bone type, zone of fracture and gender/sex (AUC of 0.91 for women, trabecular bone and trochanter area), and outcomes suggested that the trabecular bone is critical for fracture discrimination. In conclusion, 3D FE models derived from DXA scans might significantly improve the prediction of hip fracture risk; providing a new insight for clinicians to use FE simulations in clinical practice for osteoporosis management.

Ageing Effects on 3-Dimensional Femoral Neck Cross-Sectional Asymmetry: Implications for Age-Related Bone Fragility in Falling

This paper explores the effects of aging on femoral neck (FN) anatomy in a study of women aged 20-90years in relation to implications for FN fracture propensity in buckling. Five hundred and four participants were scanned by Quantitative Computed Tomography and analyzed using Quantitative Computed Tomography Pro BIT (Mindways). FN cross-section was split through geometric center into superior and inferior sectors. Bone mass, structural measurements, and bone mineral density were analyzed. Buckling ratio was calculated as ratio of buckling radius to cortical thickness. Between 2nd decade and 8th decade, age-related integral bone mass reduction in superior sector was substantially larger than in inferior sector (33% compared to 21%), especially in cortical bone superiorly compared to inferiorly (53% vs 21%; p < 0.001), principally due to reduction in cortical thickness, averaged cortical thickness (56%) with little difference in density. Superior and inferior sector trabecular bone mineral density reduction was similar at 41% and 43% respectively. Differential cortical bone loss in superior sector resulted in a 59% inferior displacement (δ) of center-of-mass from geometric center. Differences in δ and averaged cortical thickness with age accounted for a 151% increase in mean superior buckling ratio from 9 to 23. Analysis confirms significant progressive age-related superior cortical bone loss as the major age effect on FN structure with relative preservation of inferior cortex probably related to maintenance of inferior sector by regular loading as a result of standing and walking. Computation of buckling ratio may allow prediction of fracture propensity in a sideways fall.

Optimizing Accuracy of Proximal Femur Elastic Modulus Equations

Quantitative computed tomography-based finite element analysis (QCT/FEA) is a promising tool to predict femoral properties. One of the modeling parameters required as input for QCT/FEA is the elastic modulus, which varies with the location-dependent bone mineral density (ash density). The aim of this study was to develop optimized equations for the femoral elastic modulus. An inverse QCT/FEA method was employed, using an optimization process to minimize the error between the predicted femoral stiffness values and experimental values. We determined optimal coefficients of an elastic modulus equation that was a function of ash density only, and also optimal coefficients for several other equations that included along with ash density combinations of the variables sex and age. All of the optimized models were found to be more accurate than models from the literature. It was found that the addition of the variables sex and age to ash density made very minor improvements in stiffness predictions compared to the model with ash density alone. Even though the addition of age did not remarkably improve the statistical metrics, the effect of age was reflected in the elastic modulus equations as a decline of about 9% over a 60-year interval.

A comparison of density-modulus relationships used in finite element modeling of the shoulder

Subject- and site-specific modeling techniques greatly improve the accuracy of computational models derived from clinical-resolution quantitative computed tomography (QCT) data. The majority of shoulder finite element (FE) studies use density-modulus relationships developed for alternative anatomical locations. As such, the objectives of this study were to compare the six most commonly used density-modulus relationships in shoulder finite element (FE) studies. To achieve this, ninety-eight (98) virtual trabecular bone cores were extracted from uCT scans of scapulae from 14 cadaveric specimens (7 male; 7 female). Homogeneous tissue moduli of 20 GPa, and heterogeneous tissue moduli scaled by CT-intensity were considered. Micro finite element models (µ-FEMs) of each virtual core were compressively loaded to 0.5% apparent strain and apparent strain energy density (SED_app) was collected. Each uCT virtual core was then co-registered to clinical QCT images, QCT-FEMs created, and each of the 6 density-modulus relationships applied (6 × 98 = 588 QCT-FEMs). The loading and boundary conditions were replicated and SED_app was collected and compared to µ-FEM SED_app. When a homogeneous tissue modulus was considered in the µ-FEMs, SED_app was best predicted in QCT-FEMs with the density-modulus relationship developed from pooled anatomical locations (QCT-FEM SED_app = 0.979µ-FEM SED_app + 0.0066, r² = 0.933). A different density-modulus relationship best predicted SED_app (QCT-FEM SED_app = 1.014µ-FEM SED_app + 0.0034, r² = 0.935) when a heterogeneous tissue modulus was considered. This study compared density-modulus relationships used in shoulder FE studies using an independent computational methodology for comparing these relationships.

Comparison of femur stiffness measured from DXA and QCT for assessment of hip fracture risk

Femur stiffness, for example axial and bending stiffness, integrates both geometric and material information of the bone, and thus can be an effective indicator of bone strength and hip fracture risk. Femur stiffness is ideally measured from quantitative computed tomography (QCT), but QCT is not recommended for routine clinical use due to the public concern about exposure to high-dosage radiation. Dual energy X-ray absorptiometry (DXA) is currently the primary imaging modality in clinic. However, DXA is two-dimensional and it is not clear whether DXA-estimated stiffness has adequate accuracy to replace its QCT counterpart for clinical application. This study investigated the accuracy of femur stiffness (axial and bending) estimated from CTXA (computed tomography X-ray absorptiometry) and DXA against those directly measured from QCT. Proximal-femur QCT and DXA from 67 subjects were acquired. For each femur, the QCT dataset was projected into CTXA using CTXA-Hip (Mindways Software, Inc., USA). Femur stiffness at the femoral neck and intertrochanter were then calculated from QCT, CTXA and DXA, respectively, and different elasticity-density relationships were considered in the calculation. Pearson correlations between QCT and CTXA/DXA measured stiffness were studied. The results showed that there were strong correlations between QCT and CTXA derived stiffness, although the correlations were affected by the adopted elasticity-density relationship. Correlations between QCT and DXA derived stiffness were much less strong, mainly caused by the inconsistence of femur orientation in QCT projection and in DXA positioning. Our preliminary clinical study showed that femur stiffness had slightly better performance than femur geometry in discrimination of hip fracture cases from controls.

The Effect of Inhomogeneous Trabecular Stiffness Relationship Selection on Finite Element Outcomes for Shoulder Arthroplasty

An important feature of humeral orthopedic finite element (FE) models is the trabecular stiffness relationship. These relationships depend on the anatomic site from which they are derived; but have not been developed for the humerus. As a consequence, humeral FE modeling relies on relationships for other anatomic sites. The variation in humeral FE outcomes due to the trabecular stiffness relationship is assessed. Stemless arthroplasty FE models were constructed from CT scans of eight humeri. Models were loaded corresponding to 45 deg and 75 deg abduction. Each bone was modeled five times with the only variable being the trabecular stiffness relationship: four derived from different anatomic-sites and one pooled across sites. The FE outcome measures assessed were implant-bone contact percentage, von Mises of the change in stress, and bone response potential. The variance attributed to the selection of the trabecular stiffness relationship was quantified as the standard deviation existing between models of different trabecular stiffness. Overall, variability due to changing the trabecular stiffness relationship was low for all humeral FE outcome measures assessed. The variability was highest within the stress and bone formation potential outcome measures of the trabecular region. Variability only exceeded 10% in the trabecular stress change within two of the eight slices evaluated. In conclusion, the low variations attributable to the selection of a trabecular stiffness relationship based on anatomic-site suggest that FE models constructed for shoulder arthroplasty can utilize an inhomogeneous site-pooled trabecular relationship without inducing marked variability in the assessed outcome measures.

Patient-Specific Phantomless Estimation of Bone Mineral Density and Its Effects on Finite Element Analysis Results: A Feasibility Study

Objectives

This study proposes a regression model for the phantomless Hounsfield units (HU) to bone mineral density (BMD) conversion including patient physical factors and analyzes the accuracy of the estimated BMD values.

Methods

The HU values, BMDs, circumferences of the body, and cross-sectional areas of bone were measured from 39 quantitative computed tomography images of L2 vertebrae and hips. Then, the phantomless HU-to-BMD conversion was derived using a multiple linear regression model. For the statistical analysis, the correlation between the estimated BMD values and the reference BMD values was evaluated using Pearson's correlation test. Voxelwise BMD and finite element analysis (FEA) results were analyzed in terms of root-mean-square error (RMSE) and strain energy density, respectively.

Results

The HU values and circumferences were statistically significant (p < 0.05) for the lumbar spine, whereas only the HU values were statistically significant (p < 0.05) for the proximal femur. The BMD values estimated using the proposed HU-to-BMD conversion were significantly correlated with those measured using the reference phantom: Pearson's correlation coefficients of 0.998 and 0.984 for the lumbar spine and proximal femur, respectively. The RMSEs of the estimated BMD values for the lumbar spine and hip were 4.26 ± 0.60 (mg/cc) and 8.35 ± 0.57 (mg/cc), respectively. The errors of total strain energy were 1.06% and 0.91%, respectively.

Conclusions

The proposed phantomless HU-to-BMD conversion demonstrates the potential of precisely estimating BMD values from CT images without the reference phantom and being utilized as a viable tool for FEA-based quantitative assessment using routine CT images.

Knotless Anchors in Acetabular Labral Repair: A Biomechanical Comparison

PURPOSE

To analyze the failure mechanism, stiffness, and pullout strength of acetabular knotless suture anchors.

METHODS

Seven suture anchors were tested in high-density (0.48 g/cc) synthetic blocks. The anchors were implanted perpendicular to the bone block. The anchor's suture(s) were tied around a loop of 8 high-strength nonabsorbable sutures and pulled in line with the anchor at a rate of 1 mm/s until failure. The following knotless anchors were tested: Stryker Knotilus 3.5, Arthrex Pushlock 2.9, Linvatec PopLok 2.8, Linvatec PopLok 3.3, ArthroCare SpeedLock HIP (3.4-mm), and Smith & Nephew Bioraptor Knotless 2.9. The standard knot tying Smith & Nephew Bioraptor 2.9 mm served as a baseline for comparison.

RESULTS

Stiffness was highest in the Pushlock, the SpeedLock HIP, and Knotilus. At 1 mm displacement, the SpeedLock HIP exhibited significantly higher load than all other anchors, excluding the Pushlock and PopLok 3.3 (P ≤ .012 for all comparisons). Excluding the SpeedLock HIP and Knotilus, the Pushlock displayed significantly higher load than all other anchors at 2-mm displacement (P ≤ .015 for all comparisons). Maximum load was the highest for the Knotilus and Bioraptor knotted anchor (P < .001 compared with all other anchors).

CONCLUSIONS

All knotless suture anchors used in hip arthroscopy, except for the Knotilus 3.5, failed by suture pullout from the anchor. The 2 anchors with the highest maximum load, the Knotilus 3.5 and knotted Bioraptor 2.9, failed by suture failure; however, these anchors displayed the lowest stiffness and load at 1 mm displacement among all anchors tested. Stiffness and loads at clinically relevant displacements, not maximum load alone, may be most important in predicting anchor clinical performance during the early phases of labral healing.

CLINICAL RELEVANCE

Knotless suture anchors tend to fail by suture pullout from the anchor, yet the stiffness of these constructs suggests that minimal displacement of the repair will occur under physiologic loads.

"Peroperative estimation of bone quality and primary dental implant stability"

OBJECTIVES

Dental implants are widely used to restore function and appearance. It may be essential to choose the appropriate drilling protocol and implant design in order to optimise primary stability. This could be achieved based on an assessment of the implantation site with respect to bone quality and objective biomechanical descriptors such as stiffness and strength of the bone-implant system. The aim of this ex vivo study is to relate these descriptors with bone quality, with a pre-implantation indicator of implant stability: pilot-hole drilling force (F_drilling), and with two post-implantation indicators: maximal implantation torque (T_implantation) and resonance frequency analysis (RFA).

METHODS

Eighty trabecular bone specimens were cored from human vertebrae and bovine tibiae. Bone volume fraction (BV/TV), a representative for bone quality, was obtained through micro-computed tomography scans. Implants were kept in controlled laboratory conditions following standard surgical procedures. Forces and torques were recorded and RFA was assessed after implantation. Off-axis compression tests were conducted on the implants until failure. Implant stability was identified by stiffness and ultimate force (F_ultimate). The relationships between BV/TV, Stiffness, F_ultimate and F_drilling, T_implantation, RFA were established.

RESULTS

F_drilling correlated well with BV/TV of the implantation site (r² = 0.81), stiffness (r² = 0.75) and F_ultimate (r² = 0.80). T_implantation correlated better with stiffness (r² = 0.86) and F_ultimate (r² = 0.94) than RFA (r² = 0.77 and r² = 0.74, respectively).

CONCLUSION

Our results indicate that BV/TV and bone-implant stability can be directly estimated by the force needed for the pilot drilling that occurs during the site preparation before implantation. Moreover, implantation torque outperforms RFA for evaluating the mechanical competence of the bone-implant system.

Case-specific non-linear finite element models to predict failure behavior in two functional spinal units

Current finite element (FE) models predicting failure behavior comprise single vertebrae, thereby neglecting the role of the posterior elements and intervertebral discs. Therefore, this study aimed to develop a more clinically relevant, case-specific non-linear FE model of two functional spinal units able to predict failure behavior in terms of (i) the vertebra predicted to fail; (ii) deformation of the specimens; (iii) stiffness; and (iv) load to failure. For this purpose, we also studied the effect of different bone density-mechanical properties relationships (material models) on the prediction of failure behavior. Twelve two functional spinal units (T6-T8, T9-T11, T12-L2, and L3-L5) with and without artificial metastases were destructively tested in axial compression. These experiments were simulated using CT-based case-specific non-linear FE models. Bone mechanical properties were assigned using four commonly used material models. In 10 of the 11 specimens our FE model was able to correctly indicate which vertebrae failed during the experiments. However, predictions of the three-dimensional deformations of the specimens were less promising. Whereas stiffness of the whole construct could be strongly predicted (R²  = 0.637-0.688, p < 0.01), we obtained weak correlations between FE predicted and experimentally determined load to failure, as defined by the total reaction force exhibiting a drop in force (R²  = 0.219-0.247, p > 0.05). Additionally, we found that the correlation between predicted and experimental fracture loads did not strongly depend on the material model implemented, but the stiffness predictions did. In conclusion, this work showed that, in its current state, our FE models may be used to identify the weakest vertebra, but that substantial improvements are required in order to quantify in vivo failure loads. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodical, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 36:3208-3218, 2018.

Stochastic analysis of a heterogeneous micro-finite element model of a mouse tibia

Finite element (FE) analysis can be used to predict bone mechanical environments that can be used for many important applications, such as the understanding of bone mechano-regulation mechanisms. However, when defining the FE models, uncertainty in bone material properties may lead to marked variations in the predicted mechanical environment. The aim of this study is to investigate the influence of uncertainty in bone material property on the mechanical environment of bone. A heterogeneous FE model of a mouse tibia was created from micro computed tomography images. Axial compression loading was applied, and all possible bone density-modulus relationships were considered through stochastic analysis. The 1st and 3rd principal strains (ε₁ and ε₃) and the strain energy density (SED) were quantified in the tibial volume of interest (VOI). The bounds of ε₁, ε₃, and SED were determined by the bounds of the density-modulus relationship; the bone mechanical environment (ε₁, ε₃, and SED) and the bone density-modulus relationship exhibit the same trend of change; the relative percentage differences caused by bone material uncertainty are up to 28%, 28%, and 21% for ε₁, ε₃, and SED, respectively. These data provide guidelines on the adoption of bone density-modulus relationship in heterogeneous FE models.

Mechanical behavior of metastatic vertebrae are influenced by tissue architecture, mineral content, and organic feature alterations

Diminished vertebral mechanical behavior with metastatic involvement is typically attributed to modified architecture and trabecular bone content. Previous work has identified organic and mineral phase bone quality changes in the presence of metastases, yet limited work exists on the potential influence of such tissue level modifications on vertebral mechanical characteristics. This work seeks to determine correlations between features of bone (structural and tissue level) and mechanical behavior in metastatically involved vertebral bone. It is hypothesized that tissue level properties (mineral and organic) will improve these correlations beyond architectural properties and BMD alone. Twenty-four female athymic rats were inoculated with HeLa or Ace-1 cancer cells lines producing osteolytic (N = 8) or mixed (osteolytic/osteoblastic, N = 7) metastases, respectively. Twenty-one days post-inoculation L1-L3 pathologic vertebral motion segments were excised and μCT imaged. 3D morphometric parameters and axial rigidity of the L2 vertebrae were quantified. Sequential loading and μCT imaging measured progression of failure, stiffness and peak force. Relationships between mechanical testing (whole bone and tissue-level) and tissue-level material property modifications with metastatic involvement were evaluated utilizing linear regression models. Osteolytic involvement reduced vertebral trabecular bone volume, structure, CT-derived axial rigidity, stiffness and failure force compared to healthy controls (N = 9). Mixed metastases demonstrated similar trends. Previously assessed collagen cross-linking and proline-based residues were correlated to mechanical behavior and improved the predictive ability of the regression models. Similarly, collagen organization improved predictive regression models for metastatic bone hardness. This work highlights the importance of both bone content/architecture and organic tissue-level features in characterizing metastatic vertebral mechanics. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3013-3022, 2018.

Towards a patient-specific estimation of intra-operative femoral fracture risk

Total Hip Arthroplasty requires pre-surgical evaluation between un-cemented and cemented prostheses. A Patient with intra-operative periprosthetic fracture and another with a successful outcome were recruited, and their finite element models were constructed by processing CT data, assuming elastic-plastic behavior of the bone as function of the local density. To resemble the insertion of the prosthesis into the femur, a fictitious thermal dilatation is applied to the broach volume. Strain-based fracture risk factor is estimated, depicting results in terms of the total mechanical strain expressed using a simple "traffic lights" color code to provide immediate, concise, and intelligible pre-operative information to surgeons.

The Effect of Material Heterogeneity, Element Type, and Down-Sampling on Trabecular Stiffness in Micro Finite Element Models

Preclinical and clinical bone strength predictions can be elucidated by understanding bone mechanics at a variety of hierarchical levels. As such, down-sampled micro-CT images are often used to make comparisons across image resolutions or used to reduce computational resources in micro finite element models (µFEMs). Therefore, the objectives of this study were to compare trabecular apparent modulus among (i) hexahedral and tetrahedral µFEMs, (ii) µFEMs generated from 32, 64, and 64 µm down-sampled from 32 µm µCT scans, and (iii) µFEMs with homogeneous and heterogeneous tissue moduli. Trabecular µFEMs were generated from scans at the three spatial resolutions taken from the glenoid vault of 14 cadaveric specimens. Simulated unconstrained compression was performed and used to calculate and compare the apparent modulus of each µFEM. It was found that models derived from high-resolution images that account for material heterogeneity are nearly equivalent whether hexahedral or tetrahedral elements are used. However, translation of stiffness from down-sampled scans are not equivalent to scans performed at the down-sampled resolution, or that account for trabecular material heterogeneity. Material heterogeneity is most representative of in vivo trabecular bone and to accurately model trabecular mechanical properties, material heterogeneity should be considered in future µFEM development.

On the Relation of Bone Mineral Density and the Elastic Modulus in Healthy and Pathologic Bone

PURPOSE OF REVIEW

Osteoporosis could lead to the bone mechanical failure. To examine the bone health, mechanical properties are often estimated from the images of the bone density. Here, we review the relationships that have been experimentally determined between mineral density and the elastic modulus and factors that affect these relationships.

RECENT FINDINGS

Studies, which have investigated the relation between the elastic modulus and bone mineral at the bulk scale, have shown that approximately 70% of variations in the elastic modulus can be explained based on the amount of mineral in bone. At the tissue level, however, higher resolution techniques are used to characterize the density and modulus more locally, and this leads to the correlation of mineral with modulus to be not as strong as that of the bulk level and often times, insignificant. This observation indicates the importance of structural hierarchy and mineral crystal organization in determining the local stiffness of the bone tissue. At the bulk level in bone (cm scale), modulus (E) is related to density (ρ) through a power law relationship (E ∝ ρ^α). At the tissue level (μm-mm scale), the relationship between the modulus and density is weak, likely due to the effect of microstructural features at small length scales.

Fracture Prediction by Computed Tomography and Finite Element Analysis: Current and Future Perspectives

PURPOSE OF REVIEW

This review critiques the ability of CT-based methods to predict incident hip and vertebral fractures.

RECENT FINDINGS

CT-based techniques with concurrent calibration all show strong associations with incident hip and vertebral fracture, predicting hip and vertebral fractures as well as, and sometimes better than, dual-energy X-ray absorptiometry areal biomass density (DXA aBMD). There is growing evidence for use of routine CT scans for bone health assessment. CT-based techniques provide a robust approach for osteoporosis diagnosis and fracture prediction. It remains to be seen if further technical advances will improve fracture prediction compared to DXA aBMD. Future work should include more standardization in CT analyses, establishment of treatment intervention thresholds, and more studies to determine whether routine CT scans can be efficiently used to expand the number of individuals who undergo evaluation for fracture risk.

Measuring bone stiffness using spherical indentation

Objectives

Bone material properties are a major determinant of bone health in older age, both in terms of fracture risk and implant fixation, in orthopaedics and dentistry. Bone is an anisotropic and hierarchical material so its measured material properties depend upon the scale of metric used. The scale used should reflect the clinical problem, whether it is fracture risk, a whole bone problem, or implant stability, at the millimetre-scale. Indentation, an engineering technique involving pressing a hard-tipped material into another material with a known force, may be able to assess bone stiffness at the millimetre-scale (the apparent elastic modulus). We aimed to investigate whether spherical-tip indentation could reliably measure the apparent elastic modulus of human cortical bone.

Materials and methods

Cortical bone samples were retrieved from the femoral necks of nineteen patients undergoing total hip replacement surgery (10 females, 9 males, mean age: 69 years). The samples underwent indentation using a 1.5 mm diameter, ruby, spherical indenter tip, with sixty indentations per patient sample, across six locations on the bone surfaces, with ten repeated indentations at each of the six locations. The samples then underwent mechanical compression testing. The repeatability of indentation measurements of elastic modulus was assessed using the co-efficient of repeatability and the correlation between the bone elastic modulus measured by indentation and compression testing was analysed by least-squares regression.

Results

In total, 1140 indentations in total were performed. Indentation was found to be repeatable for indentations performed at the same locations on the bone samples with a mean co-efficient of repeatability of 0.4 GigaPascals (GPa), confidence interval (C.I): 0.33–0.42 GPa. There was variation in the indentation modulus results between different locations on the bone samples (mean co-efficient of repeatability: 3.1 GPa, C.I: 2.2–3.90 GPa). No clear correlation was observed between indentation and compression values of bone elastic modulus (r = 0.33, p = 0.17). The mean apparent elastic modulus obtained by spherical indentation was 9.9 GPa, the standard deviation for each indent cycle was 0.11 GPa, and the standard deviation between locations on the same sample was 1.01 GPa. The mean compression apparent elastic modulus was 4.42 GPa, standard deviation 1.02 GPa.

Discussion

Spherical-tip indentation was found to be a repeatable test for measuring the elastic modulus of human cortical bone, demonstrated by a low co-efficient of repeatability in this study. It could not, however, reliably predict cortical bone elastic modulus determined by platens compression testing in this study. This may be due to indentation only probing mechanical properties at the micro-scale while platens compression testing assesses millimetre length-scale properties. Improvements to the testing technique, including the use of a larger diameter spherical indenter tip, may improve the measurement of bone stiffness at the millimetre scale and should be investigated further.

Utility of cement injection to stabilize split-depression tibial plateau fracture by minimally invasive methods: A finite element analysis

BACKGROUND

Treatment for fractures of the tibial plateau is in most cases carried out by stable fixation in order to allow early mobilization. Minimally invasive technologies such as tibioplasty or stabilization by locking plate, bone augmentation and cement filling (CF) have recently been used to treat this type of fracture. The aim of this paper was to determine the mechanical behavior of the tibial plateau by numerically modeling and by quantifying the mechanical effects on the tibia mechanical properties from injury healing.

METHODS

A personalized Finite Element (FE) model of the tibial plateau from a clinical case has been developed to analyze stress distribution in the tibial plateau stabilized by balloon osteoplasty and to determine the influence of the cement injected. Stress analysis was performed for different stages after surgery.

FINDINGS

Just after surgery, the maximum von Mises stresses obtained for the fractured tibia treated with and without CF were 134.9 MPa and 289.9 MPa respectively on the plate. Stress distribution showed an increase of values in the trabecular bone in the treated model with locking plate and CF and stress reduction in the cortical bone in the model treated with locking plate only.

INTERPRETATION

The computed results of stresses or displacements of the fractured models show that the cement filling of the tibial depression fracture may increase implant stability, and decrease the loss of depression reduction, while the presence of the cement in the healed model renders the load distribution uniform.

Using Non-linear Homogenization to Improve the Performance of Macroscopic Damage Models of Trabecular Bone

Realistic macro-level finite element simulations of the mechanical behavior of trabecular bone, a cellular anisotropic material, require a suitable constitutive model; a model that incorporates the mechanical response of bone for complex loading scenarios and includes post-elastic phenomena, such as plasticity (permanent deformations) and damage (permanent stiffness reduction), which bone is likely to experience. Some such models have been developed by conducting homogenization-based multiscale finite element simulations on bone micro-structure. While homogenization has been fairly successful in the elastic regime and, to some extent, in modeling the macroscopic plastic response, it has remained a challenge with respect to modeling damage. This study uses a homogenization scheme to upscale the damage behavior from the tissue level (microscale) to the organ level (macroscale) and assesses the suitability of different damage constitutive laws. Ten cubic specimens were each subjected to 21 strain-controlled load cases for a small range of macroscopic post-elastic strains. Isotropic and anisotropic criteria were considered, density and fabric relationships were used in the formulation of the damage law, and a combined isotropic/anisotropic law with tension/compression asymmetry was formulated, based on the homogenized results, as a possible alternative to the currently used single scalar damage criterion. This computational study enhances the current knowledge on the macroscopic damage behavior of trabecular bone. By developing relationships of damage progression with bone's micro-architectural indices (density and fabric) the study also provides an aid for the creation of more precise macroscale continuum models, which are likely to improve clinical predictions.

Distal radius microstructure and finite element bone strain are related to site-specific mechanical loading and areal bone mineral density in premenopausal women

While weight-bearing and resistive exercise modestly increases aBMD, the precise relationship between physical activity and bone microstructure, and strain in humans is not known. Previously, we established a voluntary upper-extremity loading model that assigns a person's target force based on their subject-specific, continuum FE-estimated radius bone strain. Here, our purpose was to quantify the inter-individual variability in radius microstructure and FE-estimated strain explained by site-specific mechanical loading history, and to determine whether variability in strain is captured by aBMD, a clinically relevant measure of bone density and fracture risk. Seventy-two women aged 21–40 were included in this cross-sectional analysis. High resolution peripheral quantitative computed tomography (HRpQCT) was used to measure macro- and micro-structure in the distal radius. Mean energy equivalent strain in the distal radius was calculated from continuum finite element models generated from clinical resolution CT images of the forearm. Areal BMD was used in a nonlinear regression model to predict FE strain. Hierarchical linear regression models were used to assess the predictive capability of intrinsic (age, height) and modifiable (body mass, grip strength, physical activity) predictors. Fifty-one percent of the variability in FE bone strain was explained by its relationship with aBMD, with higher density predicting lower strains. Age and height explained up to 31.6% of the variance in microstructural parameters. Body mass explained 9.1% and 10.0% of the variance in aBMD and bone strain, respectively, with higher body mass indicative of greater density. Overall, results suggest that meaningful differences in bone structure and strain can be predicted by subject characteristics.

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Areal bone mineral density (aBMD) explains 51% of the variability in bone strain.

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Adult bone loading predicts greater cortical porosity and trabecular density.

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Greater body mass predicts greater aBMD and lower bone strain.

A REVIEW OF FINITE ELEMENT APPLICATIONS IN ORAL AND MAXILLOFACIAL BIOMECHANICS

Finite element method (FEM) is preferred to carry out mechanical analyses for many complex biomechanical structures. For most of the biomechanical models such as oral and maxillofacial structures or patient-specific dental instruments, including nonlinearities, complicated geometries, complex material properties, or loading/boundary conditions, it is not possible to accomplish an analytical solution. The FEM is the most widely used numerical approach for such cases and found a wide range of application fields for investigating the biomechanical characteristics of oral and maxillofacial structures that are exposed to external forces or torques. The numerical results such as stress or strain distributions obtained from finite element analysis (FEA) enable dental researchers to evaluate the bone tissues subjected to the implant or prosthesis fixation from the viewpoint of (i) mechanical strength, (ii) material properties, (iii) geometry and dimensions, (iv) structural properties, (v) loading or boundary conditions, and (vi) quantity of implants or prostheses. This review paper evaluates the process of the FEA of the oral and maxillofacial structures step by step as followings: (i) a general perspective on the techniques for creating oral and maxillofacial models, (ii) definitions of material properties assigned to oral and maxillofacial tissues and related dental materials, (iii) definitions of contact types between tissue and dental instruments, (iv) details on loading and boundary conditions, and (v) meshing process.

Automation of a DXA-based finite element tool for clinical assessment of hip fracture risk

Finite element analysis of medical images is a promising tool for assessing hip fracture risk. Although a number of finite element models have been developed for this purpose, none of them have been routinely used in clinic. The main reason is that the computer programs that implement the finite element models have not been completely automated, and heavy training is required before clinicians can effectively use them. By using information embedded in clinical dual energy X-ray absorptiometry (DXA), we completely automated a DXA-based finite element (FE) model that we previously developed for predicting hip fracture risk. The automated FE tool can be run as a standalone computer program with the subject's raw hip DXA image as input. The automated FE tool had greatly improved short-term precision compared with the semi-automated version. To validate the automated FE tool, a clinical cohort consisting of 100 prior hip fracture cases and 300 matched controls was obtained from a local community clinical center. Both the automated FE tool and femoral bone mineral density (BMD) were applied to discriminate the fracture cases from the controls. Femoral BMD is the gold standard reference recommended by the World Health Organization for screening osteoporosis and for assessing hip fracture risk. The accuracy was measured by the area under ROC curve (AUC) and odds ratio (OR). Compared with femoral BMD (AUC = 0.71, OR = 2.07), the automated FE tool had a considerably improved accuracy (AUC = 0.78, OR = 2.61 at the trochanter). This work made a large step toward applying our DXA-based FE model as a routine clinical tool for the assessment of hip fracture risk. Furthermore, the automated computer program can be embedded into a web-site as an internet application.

Young's Modulus and Load Complexity: Modeling Their Effects on Proximal Femur Strain

Finite element analysis (FEA) is a powerful tool for evaluating questions of functional morphology, but the application of FEA to extant or extinct creatures is a non-trivial task. Three categories of input data are needed to appropriately implement FEA: geometry, material properties, and boundary conditions. Geometric data are relatively easily obtained from imaging techniques, but often material properties and boundary conditions must be estimated. Here we conduct sensitivity analyses of the effect of the choice of Young's Modulus for elements representing trabecular bone and muscle loading complexity on the proximal femur using a finite element mesh of a modern human femur. We found that finite element meshes that used a Young's Modulus between 500 and 1,500 MPa best matched experimental strains. Loading scenarios that approximated the insertion sites of hip musculature produced strain patterns in the region of the greater trochanter that were different from scenarios that grouped muscle forces to the superior greater trochanter, with changes in strain values of 40% or more for 20% of elements. The femoral head, neck, and proximal shaft were less affected (e.g. approximately 50% of elements changed by 10% or less) by changes in the location of application of muscle forces. From our sensitivity analysis, we recommend the use of a Young's Modulus for the trabecular elements of 1,000 MPa for the proximal femur (range 500-1,500 MPa) and that the muscular loading complexity be dependent on whether or not strains in the greater trochanter are the focus of the analytical question. Anat Rec, 301:1189-1202, 2018. © 2018 Wiley Periodicals, Inc.

Automated DXA-based finite element analysis for hip fracture risk stratification: a cross-sectional study

Fracture risk indices (FRIs) generated from DXA-based finite element analysis were associated with hip fracture independent of FRAX score computed with femoral neck bone mineral density (BMD). Prospective studies are warranted to determine whether FRIs represent an improvement over BMD for predicting incident hip fractures.

INTRODUCTION

The study aims to examine the association between prior hip fracture and FRIs derived from automated finite element analysis (FEA) of DXA hip scans. Femoral neck, intertrochanteric, and subtrochanteric FRIs were calculated as the von Mises stress induced by a sideways fall divided by the bone yield stress over the specified region of interest (ROI).

METHODS

Using the Manitoba Bone Mineral Density Database, we selected women age ≥ 65 years with femoral neck T-scores below - 1 and no osteoporosis treatment. From this population, we identified 324 older women with hip fracture before DXA testing and a random sample of 658 non-fracture controls. FRIs were derived from the anonymized DXA scans. Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (95% CIs) for the associations between FRIs (per SD increase) and hip fracture.

RESULTS

After adjusting for FRAX score (hip fracture with BMD), femoral neck FRI (OR 1.36, 95% CI 1.13, 1.64), intertrochanteric FRI (OR 1.81, 95% CI 1.44, 2.27), and subtrochanteric FRI (OR 2.09, 95% CI 1.68, 2.60) were associated with hip fracture. Intertrochanteric and subtrochanteric FRIs gave significantly higher c-statistics (all P ≤ 0.05) than femoral neck BMD. Subgroup analyses showed that all FRIs were more strongly associated with hip fracture in women who were younger and had higher body mass index (BMI) or non-osteoporotic BMD (all P _interaction < 0.1).

CONCLUSIONS

FRIs derived from DXA-based FEA were independently associated with prior hip fracture, suggesting that they could potentially improve hip fracture risk assessment.

The Lattice Boltzmann method and computational analysis of bone dynamics-I

Bone is comprised of an enormously hierarchical construction that promotes transportation of necessary fluids and solids, guaranteeing accurate function and growth. Bone remodeling is a combined process of bone creation and destruction. A number of mathematical models have been developed for the balanced and imbalanced bone remodeling. A brief overview regarding mathematical modeling of bone remodeling is provided. The Lattice Boltzmann method (LBM) has widely been implemented in CFD simulations, and it is becoming more suitable in the application of image processing amongst several others. Mainly, the LBM simulates the communication between synthetic particles dispersed in a lattice. Canaliculi and tortuous channels that have more or less roughly circular structure link among oval bodies identified as lacunae, and are vital to the function of bone. As there is a lack of equipment to inspect flow in channels on the order of measure of canaliculi, so the use of computational methods are more advantageous to give perceptivities into the nature of the flows. In this article, the computational fluid dynamics analysis is descried, using the Lattice Boltzmann method, to examine the result of the microscopic surface roughness of the canalicular wall, which is formed by collagen fibrils, on the flow profiles in the pericellular space.

Biomechanics of the classic metaphyseal lesion: finite element analysis

BACKGROUND

The classic metaphyseal lesion (CML) is strongly associated with infant abuse, but the biomechanics responsible for this injury have not been rigorously studied. Radiologic and CT-pathological correlates show that the distal tibial CML always involves the cortex near the subperiosteal bone collar, with variable extension of the fracture into the medullary cavity. Therefore, it is reasonable to assume that the primary site of bone failure is cortical, rather than intramedullary.

OBJECTIVE

This study focuses on the strain patterns generated from finite element modeling to identify loading scenarios and regions of the cortex that are susceptible to bone failure.

MATERIALS AND METHODS

A geometric model was constructed from a normal 3-month-old infant's distal tibia and fibula. The model's boundary conditions were set to mimic forceful manipulation of the ankle with eight load modalities (tension, compression, internal rotation, external rotation, dorsiflexion, plantar flexion, valgus bending and varus bending).

RESULTS

For all modalities except internal and external rotation, simulations showed increased cortical strains near the subperiosteal bone collar. Tension generated the largest magnitude of cortical strain (24%) that was uniformly distributed near the subperiosteal bone collar. Compression generated the same distribution of strain but to a lesser magnitude overall (15%). Dorsiflexion and plantar flexion generated high (22%) and moderate (14%) localized cortical strains, respectively, near the subperiosteal bone collar. Lower cortical strains resulted from valgus bending, varus bending, internal rotation and external rotation (8-10%). The highest valgus and varus bending cortical strains occurred medially.

CONCLUSION

These simulations suggest that the likelihood of the initial cortical bone failure of the CML is higher along the margin of the subperiosteal bone collar when the ankle is under tension, compression, valgus bending, varus bending, dorsiflexion and plantar flexion, but not under internal and external rotation. Focal cortical strains along the medial margins of the subperiosteal bone collar with varus and valgus bending may explain the known tendency for focal distal tibial CMLs to occur medially. Further research is needed to determine the threshold of applied forces required to produce this strong indicator of infant abuse.

Strain shielding inspired re-design of proximal femoral stems for total hip arthroplasty

A large number of hip prosthesis with different designs have been developed. However, the influence of hip implant design changes on the strains induced in the bone remains unclear. The purpose of this study is to better understand the mechanics of short stem total hip arthroplasty. Specifically, it investigates whether strain shielding can be avoided by changing implant shape and/or material properties. It is hypothesized that the re-design of existing implant designs can result in further reduction of strain shielding and thus keep bone loss minimal following total hip replacement. Finite element methods were used to compare healthy and implanted models. The local mechanics strains/stresses in the intact and implanted femurs were determined under patient-specific muscle and joint contact forces. Results suggest that small changes in implant geometry and material properties have no major effect on strain shielding. Furthermore, it was found that improvement depends on a dramatic re-design of the original implant design. Whereas the benefit of this strategy of modification of the original geometry of a given short-stemmed hip consists in reduced bone remodeling, care should be taken with regard to long-term bone anchorage and implant fatigue strength. It is also shown that geometrical and material changes have a limited potential in avoiding strain shielding even in short-stemmed implants. Finally, it is suggested that an understanding of the influence of these changes on the strain distribution within the bone can guide in the process of optimizing the current stem designs toward minimal strain shielding effects. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2534-2544, 2017.

Femoral head necrosis: A finite element analysis of common and novel surgical techniques

BACKGROUND

Femoral head necrosis is a common cause of secondary osteoarthritis. At the early stages, treatment strategies are normally based on core decompression techniques, where the number, location and diameter of the drilling holes varies depending on the selected approach. The purpose of this study was to investigate the risk of femoral head, neck and subtrochanteric fracture following six different core decompression techniques.

MATERIALS

Five common and a newly proposed techniques were analyzed in respect to their biomechanical consequences using finite element analysis. The geometry of a femur was reconstructed from computed-tomography images. Thereafter, the drilling configurations were simulated. The strains in the intact and drilled femurs were determined under physiological, patient-specific, muscle and joint contact forces.

FINDINGS

The following results were observed: i) - an increase in collapse and fracture risk of the femur head by disease progression ii) - for a single hole approach at the subtrochanteric region, the fracture risk increases with the diameter iii) - the highest fracture risks occur for an 8mm single hole drilling at the subtrochanteric region and approaches with multiple drilling at various entry points iv) - the proposed novel approach resulted in the most physiological strains (closer to the experienced by the healthy bone).

INTERPRETATION

Our results suggest that all common core decompression methods have a significant impact on the biomechanical competence of the proximal femur and impact its mechanical potential. Fracture risk increases with drilling diameter and multiple drilling with smaller diameter. We recommend the anterior approach due to its reduced soft tissue trauma and its biomechanical performance.

Modal analysis for the assessment of cementless hip stem primary stability in preoperative THA planning

This numerical vibration finite element (FE) study introduces resonance three-dimensional planning (RP3D) to assess preoperatively the primary stability of a cementless stem for total hip arthroplasty. Based on a patient's CT-scan and a numerical model of a stem, RP3D aims at providing mechanical criteria indicative of the achievable primary stability. We investigate variations of the modal response of the stem to changes of area and apparent stiffness of the bone-implant interface. The model is computationally cheap as it does not include a mesh of the bone. The apparent stiffness of the bone is modeled by springs attached to the nodes of the stem's mesh. We investigate an extended range of stiffness values while, in future works, patient's specific Hounsfield values could be used to define stiffness. We report modal frequencies, shapes, and a ratio of elastic potential energies (rEPE) that quantifies the proximal motion that should be minimum for a stable stem. The modal response exhibits a clear transition between loose and tight contact as area and stiffness of the interface increase. rEPE thresholds that could potentially discriminate preoperatively between stable and unstable stems are given for a Symbios SPS^® size C stem.

Machine learning techniques for the optimization of joint replacements: Application to a short-stem hip implant

Today, different implant designs exist in the market; however, there is not a clear understanding of which are the best implant design parameters to achieve mechanical optimal conditions. Therefore, the aim of this project was to investigate if the geometry of a commercial short stem hip prosthesis can be further optimized to reduce stress shielding effects and achieve better short-stemmed implant performance. To reach this aim, the potential of machine learning techniques combined with parametric Finite Element analysis was used. The selected implant geometrical parameters were: total stem length (L), thickness in the lateral (R1) and medial (R2) and the distance between the implant neck and the central stem surface (D). The results show that the total stem length was not the only parameter playing a role in stress shielding. An optimized implant should aim for a decreased stem length and a reduced length of the surface in contact with the bone. The two radiuses that characterize the stem width at the distal cross-section in contact with the bone were less influential in the reduction of stress shielding compared with the other two parameters; but they also play a role where thinner stems present better results.