Left-right differences in the proximal femur's strength of post-menopausal women: a multicentric finite element study

Evaluation of bone-related mechanical properties in female patients with long-term remission of Cushing's syndrome using quantitative computed tomography-based finite element analysis

BACKGROUND

Hypercortisolism in Cushing's syndrome (CS) is associated with bone loss, skeletal fragility, and altered bone quality. No studies evaluated bone geometric and strain-stress values in CS patients after remission thus far.

PATIENTS AND METHODS

Thirty-two women with CS in remission (mean age [±SD] 51 ± 11; body mass index [BMI], 27 ± 4 kg/m2; mean time of remission, 120 ± 90 months) and 32 age-, BMI-, and gonadal status-matched female controls. Quantitative computed tomography (QCT) was used to assess volumetric bone mineral density (vBMD) and buckling ratio, cross-sectional area, and average cortical thickness at the level of the proximal femur. Finite element (FE) models were generated from QCT to calculate strain and stress values (maximum principal strain [MPE], maximum strain energy density [SED], maximum Von Mises [VM], and maximum principal stress [MPS]). Areal BMD (aBMD) and trabecular bone score (TBS) were assessed by dual-energy X-ray absorptiometry (2D DXA).

RESULTS

Trabecular vBMD at total hip and trochanter were lower in CS as compared with controls (P < .05). Average cortical thickness was lower, and buckling ratio was greater in CS vs controls (P < .01). All strain and stress values were higher in CS patients vs controls (P < .05). 2D DXA-derived measures were similar between patients and controls (P > .05). Prior hypercortisolism predicted both VM (β .30, P = .014) and MPS (β .30, P = .015), after adjusting for age, BMI, menopause, delay to diagnosis, and duration of remission.

CONCLUSIONS

Women with prior hypercortisolism have reduced trabecular vBMD and impaired bone geometrical and mechanical properties, which may contribute to an elevated fracture risk despite long-term remission.

Comparison of the Right and Left Femur Bone Mineral Densities in Postmenopausal Women

OBJECTIVES

Bone mineral density (BMD) is measured in the hip and posteroanterior spine; moreover, according to the 2019 International Society for Clinical Densitometry guidelines, unilateral hip can be used. This study aimed to determine whether there is a difference between the BMD of both the femurs in postmenopausal women.

METHODS

A total of 343 postmenopausal women were enrolled in this study from January 1, 2010, to December 31, 2019 at a single tertiary hospital. By using the Hologic® Horizon W DXA System, the femur and spine BMD was measured; BMD was recorded in g/cm². Following regions were analyzed in both the femurs: the femur neck, the trochanter area, and total femur.

RESULTS

Mean age at imaging was 62 ± 9.7 years, and significant difference in the total BMD of both the femurs (P = 0.003) was observed. In secondary analysis, patients with osteoporosis showed significant contralateral BMD discrepancies in trochanter and total proximal femur BMD (P = 0.041 and P = 0.011, respectively). However, in women with normal BMD, no significant difference between the right and left femur BMD was observed. Furthermore, measurement of solely the unilateral hip can lead to a 16.9% of underdiagnosis in postmenopausal women.

CONCLUSIONS

In conclusion, it is necessary to check BMD in both hips, particularly in patients suspected of osteoporosis.

A taper-fit junction to improve long bone reconstruction: A parametric In Silico model

PURPOSE

Critical size long bone defects represent a clinical challenge in orthopaedic surgery. Various grafting techniques have been developed through the years, but they all present several downsides. A key requirement of all grafting techniques is the achievement of a continuous interface between host bone and graft to enhance both biological processes and mechanical stability. This study used a parametric in silico model to quantify the biomechanical effect of the inaccuracies inherent to current osteotomy techniques, and to test a new concept of accurate taper-fit junction that may improve the biomechanical parameters of the reconstruction under load.

METHODS

A population-based in-silico 3D model of the reconstruction of a long bone defect was built to represent a defect of the femoral mid-diaphysis. To fix the reconstruction a titanium plate was placed on the lateral aspect of the reconstruction. The model was modified to (i) quantify the biomechanical consequences of actual inaccuracies in the realization of a flat host-graft interface, (ii) compare the contact behaviour and bone strains among different taper angles of the new design and the current host-graft flat interface, (iii) evaluate the robustness of the taper-fit design to inter-subject variability in bone geometry and defect length.

RESULTS

The influence of 2° single-plane misalignments of the host-graft interface is highly dependent on the misalignment orientation with respect to the metal plate. For some misalignment orientations, tangential micromotions of contact interfaces exceeded alert thresholds. When the angle of the taper-fit host-graft junction is changed from 10° to 30° and the results obtained are compared with the planar case, the overall stiffness is almost preserved, the bone strains are almost unchanged with safety factors higher than five, and full contact closure around the host-graft junction is achieved at 20°. Similarly, contact pressures decrease almost linearly with a 20% decrease at 30°. The host-graft micro motions are almost unchanged in both value and distribution up to 20° and never exceed the warning threshold of 50 μm.

CONCLUSIONS

The present in silico study developed quantitative biomechanical evidence that an osteotomy performed with attention to the perpendicularity of the cut planes is needed to reduce the risk of mismatch and possible complications of long bone reconstructions, and that a new concept of a taper-fit junction may improve the biomechanical environment of the interface between the graft and the host bone. The optimal taper-fit configuration is suggested to be around a 20° taper angle. These results will serve as an input to conduct exvivo experiments to further corroborate the proposed taper-fit junction concept and to refine its surgical implementation.

Virtual mechanical tests out-perform morphometric measures for assessment of mechanical stability of fracture healing in vivo

Finite element analysis with models derived from computed tomography (CT) scans is potentially powerful as a translational research tool because it can achieve what animal studies and cadaver biomechanics cannot-low-risk, noninvasive, objective assessment of outcomes in living humans who have actually experienced the injury, or treatment being studied. The purpose of this study was to assess the validity of CT-based virtual mechanical testing with respect to physical biomechanical tests in a large animal model. Three different tibial osteotomy models were performed on 44 sheep. Data from 33 operated limbs and 20 intact limbs was retrospectively analyzed. Radiographic union scoring was performed on the operated limbs and physical torsional tests were performed on all limbs. Morphometric measures and finite element models were developed from CT scans and virtual torsional tests were performed to assess healing with four material assignment techniques. In correlation analysis, morphometric measures and radiographic scores were unreliable predictors of biomechanical rigidity, while the virtual torsion test results were strongly and significantly correlated with measured biomechanical test data, with high absolute agreement. Overall, the results validated the use of virtual mechanical testing as a reliable in vivo assessment of structural bone healing. This method is readily translatable to clinical evaluation for noninvasive assessment of the healing progress of fractures with minimal risk. Clinical significance: virtual mechanical testing can be used to reliably and noninvasively assess the rigidity of a healing fracture using clinical-resolution CT scans and that this measure is superior to morphometric and radiographic measures.

Elementwise material assignment in reconstructed or transformed patient-specific FEA models developed from CT scans

In patient-specific finite element modeling, elementwise material assignment calculates local mechanical properties from the underlying CT data. If meshes must be transformed, for example to reconstruct broken bones, this elementwise material mapping is not possible using commercial software. Accordingly, we developed an algorithm to transform and reconstruct CT scans and fill gaps at discontinuities. Virtual mechanical testing showed that iterative reconstruction retains material heterogeneity with minimal strain artifacts and achieves whole-bone mechanics clinically equivalent (within 5%) to homogeneous models. This approach may expand the range of clinical CT scans that are viable for virtual biomechanics by allowing defect repair.

Potential bone fragility of mid-shaft atypical femoral fracture: Biomechanical analysis by a CT-based nonlinear finite element method

INTRODUCTION

The authors previously reported a CT-based nonlinear finite element analysis (nonlinear CT/FEA) model to investigate loading stress distribution in the femoral shaft of patients with atypical femoral fractures (AFFs). This showed that stress distribution, influenced primarily by femoral bowing, may determine the location of AFF. Here, we demonstrate the locational characteristics associated with AFFs in an Asian, specifically Japanese, population regarding bone strength. This is the second report from our multicentre research project suggesting a possible new concept of diagnostic criteria or treatment according to AFF subtype.

PATIENTS AND METHODS

A multicentre prospective study was conducted at 12 hospitals in Japan from August 2015 through June 2018. We recruited three study groups composed of elderly females over the age of 60 years-the mid-shaft AFF group (n = 14; 80.0 ± 6.5 years), the subtrochanteric AFF group (n = 15; 73.9 ± 6.8 years), and the control group who had sustained unilateral hip fracture (n = 21; 82.1 ± 7.1 years)-and analysed femoral neck bone density and strength. Bone strength of the femoral neck was predicted with an evaluation method using nonlinear CT/FEA in both standing and falling configuration.

RESULTS

Femoral neck bone density and strength were significantly higher in the subtrochanteric AFF group compared with the mid-shaft AFF and control groups (p <  0.0001). No significant difference was seen in bone strength between the mid-shaft AFF and control groups (standing, p =  0.7616; falling, p =  0.3803).

CONCLUSIONS

AFF has different features, in terms of bone strength, depending on fracture location. At the very least, Japanese patients with mid-shaft AFF could be at high risk of hip fracture because of bone fragility, in contrast to the firm bone of subtrochanteric AFF. For internal fixation of mid-shaft AFF using an IM nail, cervical screw insertion toward the femoral head might be recommended to prevent possible hip fracture.

Mechanical behavior of metastatic femurs through patient-specific computational models accounting for bone-metastasis interaction

This paper proposes a computational model based on a finite-element formulation for describing the mechanical behavior of femurs affected by metastatic lesions. A novel geometric/constitutive description is introduced by modelling healthy bone and metastases via a linearly poroelastic constitutive approach. A Gaussian-shaped graded transition of material properties between healthy and metastatic tissues is prescribed, in order to account for the bone-metastasis interaction. Loading-induced failure processes are simulated by implementing a progressive damage procedure, formulated via a quasi-static displacement-driven incremental approach, and considering both a stress- and a strain-based failure criterion. By addressing a real clinical case, left and right patient-specific femur models are geometrically reconstructed via an ad-hoc imaging procedure and embedding multiple distributions of metastatic lesions along femurs. Significant differences in fracture loads, fracture mechanisms, and damage patterns, are highlighted by comparing the proposed constitutive description with a purely elastic formulation, where the metastasis is treated as a pseudo-healthy tissue or as a void region. Proposed constitutive description allows to capture stress/strain localization mechanisms within the metastatic tissue, revealing the model capability in describing possible strain-induced mechano-biological stimuli driving onset and evolution of the lesion. The proposed approach opens towards the definition of effective computational strategies for supporting clinical decision and treatments regarding metastatic femurs, contributing also to overcome some limitations of actual standards and procedures.

Are CT-Based Finite Element Model Predictions of Femoral Bone Strength Clinically Useful?

PURPOSE OF REVIEW

This study reviews the available literature to compare the accuracy of areal bone mineral density derived from dual X-ray absorptiometry (DXA-aBMD) and of subject-specific finite element models derived from quantitative computed tomography (QCT-SSFE) in predicting bone strength measured experimentally on cadaver bones, as well as their clinical accuracy both in terms of discrimination and prediction. Based on this information, some basic cost-effectiveness calculations are performed to explore the use of QCT-SSFE instead of DXA-aBMD in (a) clinical studies with femoral strength as endpoint, (b) predictor of the risk of hip fracture in low bone mass patients.

RECENT FINDINGS

Recent improvements involving the use of smooth-boundary meshes, better anatomical referencing for proximal-only scans, multiple side-fall directions, and refined boundary conditions increase the predictive accuracy of QCT-SSFE. If these improvements are adopted, QCT-SSFE is always preferable over DXA-aBMD in clinical studies with femoral strength as the endpoint, while it is not yet cost-effective as a hip fracture risk predictor, although pathways that combine both QCT-SSFE and DXA-aBMD are promising.