Relationship between mechanical properties and bone mineral density of human femoral bone retrieved from patients with osteoarthritis

Effects of Development on Bone Mineral Density and Mechanical Properties in the Aquatic Frog, Xenopus Laevis, and a Terrestrial Frog, Lithobates Catesbianus

The limb bones of vertebrates have a critical role in supporting the weight of the body and transmitting forces that power locomotion. The loads that limb bones experience can vary in association with a range of factors, including locomotor environment or developmental stage. Limbed vertebrates that are habitually found in environments with low locomotor loads (e.g., water) might be predicted to also exhibit limb bones with less elevated mechanical properties, such as yield stiffness and yield stress. Frogs provide a distinctive case, in which these ideas can be tested as they experience changes in both locomotor style and habitat as they develop. However, while many frog taxa shift from aquatic to terrestrial habitats as they metamorphose, some lineages, such as pipids, maintain an aquatic lifestyle even after metamorphosis, providing a comparative framework for the effects of habitat shifts on developing limbs in vertebrates. This study compares the material composition and mechanical properties of the femur between frog species that are aquatic specialists (Xenopus laevis) vs generalists that spend considerable time both on land and in water (Lithobates catesbeianus) as they transition from metamorphic tadpoles to fully grown adults. MicroCT scanning was used to determine changes in bone density related to developmental stage and hindlimb use during swimming. Microindentation was then used to collect hardness values from the cortical bone of each femur, which was used to evaluate bone material properties. We found that aquatic frogs had less overall bone mineral density (BMD) than terrestrial frogs and that BMD was more elevated in the cortical region of the diaphysis than trabeculae and distal and proximal epiphyses. Despite its less elevated BMD, bone mechanical properties were not significantly different in aquatic specialist X. laevis than in more terrestrial L. catesbeianus. Our results suggest that the limb bones of aquatic frogs may experience compensatory effects through development to offset their lower BMD. Furthermore, changes in bone density and material properties across development may help to explain some of the differences in locomotor performance found between aquatic and terrestrial metamorphic frogs, providing insight into how environmental factors might correlate with bone ossification.

Effect of stress on the dissolution/crystallization of apatite in aqueous solution: a thermochemical equilibrium study

Bone mineralization is critical to maintaining tissue mechanical function. The application of mechanical stress via exercise promotes bone mineralization via cellular mechanotransduction and increased fluid transport through the collagen matrix. However, due to its complex composition and ability to exchange ions with the surrounding body fluids, bone mineral composition and crystallization is also expected to respond to stress. Here, a combination of data from materials simulations, namely density functional theory and molecular dynamics, and experimental studies were input into an equilibrium thermodynamic model of bone apatite under stress in an aqueous solution based on the theory of thermochemical equilibrium of stressed solids. The model indicated that increasing uniaxial stress induced mineral crystallization. This was accompanied by a decrease in calcium and carbonate integration into the apatite solid. These results suggest that weight-bearing exercises can increase tissue mineralization via interactions between bone mineral and body fluid independent of cell and matrix behaviours, thus providing another mechanism by which exercise can improve bone health. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Biomechanical analysis analyzing association between bone mineral density and lag screw migration

A proximal femoral nail using a helical blade (HB) is commonly utilized to treat proximal femoral fracture but cut through failure of the lag screws is one of the devastating complications following the surgery. While controversial, one of the potential risk factors for cut through failure is poor bone strength which can be predicted by measuring bone mineral density (BMD). In this study, we performed a biomechanical test on the fractured femoral head to validate whether the indirectly measured BMD from the contralateral hip or that measured directly from the retrieved femoral head can elucidate the structural strength of the fractured femoral head and thereby can be used to predict migration of lag screws. Our result showed that directly measured BMD has a significant correlation with the HB migration on the osteoporotic femoral head. However, while the BMDs measured from the contralateral femoral neck or total hip is the most widely used parameter to predict the bone strength of the fractured femur, this may have limited usability to predict HB migration.

Ultimate Load Measuring System for Fixation of Soft Tissue to Bone

BACKGROUND

The initial ultimate load for graft fixation is one of the essential factors in the reconstruction of lateral ankle ligaments. Several anchoring devices have been developed to fix the substitute ligament into the bone. A fair comparison of these fixation methods warrant a reproducible examination system. The purpose of this study was to make an experimental animal model and to compare the initial ultimate loads of 3 graft fixation methods, including the use of EndoButton (EB), interference screw (IFS), and a novel socket anchoring (SA) technique.

METHODS

Porcine calcaneus bones and 5-mm-wide split bovine Achilles tendons were used as fixation bases and graft materials, respectively. Both ends were firmly sutured side-by-side, using the circumferential ligation technique as a double-strand substitute that was 45 mm in length. Porcine calcanei with similar characteristics to adult human calcanei were mounted on a tensile testing machine, and substitutes were fixed into bones using the 3 fixation methods. A polyester tape was passed through the tendon loop and connected to a crosshead jig of the testing machine. The initial ultimate loads were measured in 15 specimens for each fixation method to simulate a lateral ankle ligament (LAL) injury.

RESULTS

The ultimate loads (ULs) were 223.6 ± 52.7 N for EB, 229.7 ± 39.7 N for SA, and 208.8 ± 65.3 N for IFS. No statistically significant difference was observed among the 3 groups (P = .571). All failures occurred at the bone-ligament substitute interface.

CONCLUSION

The initial ULs in all 3 fixation methods were sufficient for clinical usage. These values were larger than the UL of the anterior talofibular ligament; however, these were smaller than the UL of the calcaneofibular ligament.

CLINICAL RELEVANCE

In an experimental animal model, ULs for SA, EB, and IFS techniques showed no significant difference. All failures were observed in the fixation site of the calcaneus and were overwhelmingly related to suture fixation failure.

Bone Density Micro-CT Assessment during Embedding of the Innovative Multi-Spiked Connecting Scaffold in Periarticular Bone to Elaborate a Validated Numerical Model for Designing Biomimetic Fixation of Resurfacing Endoprostheses

Our team has been working for some time on designing a new kind of biomimetic fixation of resurfacing endoprostheses, in which the innovative multi-spiked connecting scaffold (MSC-Scaffold) that mimics the natural interface between articular cartilage and periarticular trabecular bone in human joints is the crucial element. This work aimed to develop a numerical model enabling the design of the considered joint replacement implant that would reflect the mechanics of interacting biomaterials. Thus, quantitative micro-CT analysis of density distribution in bone material during the embedding of MSC-Scaffold in periarticular bone was applied. The performed numerical studies and corresponding mechanical tests revealed, under the embedded MSC-Scaffold, the bone material densification affecting its mechanical properties. On the basis of these findings, the built numerical model was modified by applying a simulated insert of densified bone material. This modification led to a strong correlation between the re-simulation and experimental results (FVU = 0.02). The biomimetism of the MSC-Scaffold prototype that provided physiological load transfer from implant to bone was confirmed based on the Huber–von Mises–Hencky (HMH) stress maps obtained with the validated finite element (FE) model of the problem. The micro-CT bone density assessment performed during the embedding of the MSC-Scaffold prototype in periarticular bone provides insight into the mechanical behaviour of the investigated implant-bone system and validates the numerical model that can be used for the design of material and geometric features of a new kind of resurfacing endoprostheses fixation.

Influence of the fixation technique on the mechanical properties of human cancellous bone of the femoral head

BACKGROUND

The femoral head is of central importance for the force transmission from the suprapelvic body mass to the lower extremity. However, the condition of the subcortical bone and its mechanical properties in case of pathological changes due to coxarthrosis or femoral head necrosis differ from the healthy condition.

METHODS

Fresh femoral heads were gathered during hip total endoprosthesis surgeries and cylindrical cancellous bone samples were extracted with a hollow drill. By means of a uniaxial tensile-compression test system, the compressive strength was determined for two different specimen types (fresh and 24 h storage in acetone). Exemplary tests on an exceptionally large femoral head were performed to compare properties of fresh, fresh-deep-frozen and acetone-stored samples.

FINDINGS

The deformation behaviour and the material parameters determined were very heterogeneous. For most of the specimens, a destructive material test was successfully carried out, i.e. the compressive strength was determined. The average strength of fresh specimens was slightly higher than that of acetone specimens. On the other hand, the average Young's modulus of the acetone specimens was higher than that of the fresh specimens.

INTERPRETATION

The lower Young's moodulus of the fresh samples compared to the acetone samples could indicate a causal effect of the degreasing influence of the acetone. The partly considerable individual differences in compressive strength and failure compression can have patient-specific influencing factors such as constitution and physical fitness as well as causes in the initial pathological condition.

Design of dental implant using design of experiment and topology optimization: A finite element analysis study

Ever since the introduction of topology optimization into the industrial and manufacturing fields, it has been a top priority to maximize the performance of any system by optimizing its geometrical parameters to save material while keeping its functionality unaltered. The purpose of this study is to design a dental implant macro-geometry by removing expendable material using topology optimization and to evaluate its biomechanical function. Three-dimensional finite element models were created of an implant embedded in cortical and cancellous bone. Parameters like the length and diameter of the implant and the bone quality (±20% variation in Young’s modulus, Poisson’s ratio and density for both cortical and cancellous bone) were varied to evaluate their effect on the principal stresses induced on the peri-implant bone tissues and the micromotion of the implant at 150 N applied load. Design optimization is used to select one suitable implant for each material property combination with optimum parameters that experiences the least von Mises stress and axial deformation, out of twenty implants with different length and diameter for each material property combination. Topology optimization was then used on the selected implants to remove the redundant material. The biomechanical functions of the implants with optimized parameter and volume were then evaluated. The finite element analyses estimated that a reduction of 32% to 45% in the implant volume is possible with the implant still retaining all of its functionality.

Heterogeneous Strain Distribution in the Subchondral Bone of Human Osteoarthritic Femoral Heads, Measured with Digital Volume Correlation

Osteoarthritis (OA) is a chronic disease, affecting approximately one third of people over the age of 45. Whilst the etiology and pathogenesis of the disease are still not well understood, mechanics play an important role in both the initiation and progression of osteoarthritis. In this study, we demonstrate the application of stepwise compression, combined with microCT imaging and digital volume correlation (DVC) to measure and evaluate full-field strain distributions within osteoarthritic femoral heads under uniaxial compression. A comprehensive analysis showed that the microstructural features inherent in OA bone did not affect the level of uncertainties associated with the applied methods. The results illustrate the localization of strains at the loading surface as well as in areas of low bone volume fraction and subchondral cysts. Trabecular thickness and connectivity density were identified as the only microstructural parameters with any association to the magnitude of local strain measured at apparent yield strain or the volume of bone exceeding yield strain. This work demonstrates a novel approach to evaluating the mechanical properties of the whole human femoral head in case of severe OA.

Femoral bone mineral density distribution is dominantly regulated by strain energy density in remodeling

BACKGROUND

It is well known that there is a relationship between bone strength and the forces that are daily applied to the bone. However, bone is a highly heterogeneous material and it is still not clear how mechanical variables regulate the distribution of bone mass in a femur.

METHODS

We studied the role of four mechanical variables, i.e. principal tensile/compressive stress, von Mises stress, and strain energy density (SED), in the regulation of bone mineral density (BMD) distribution in the human femur. The actual BMD in a femur was extracted from quantitative computed tomography (QCT) and used as a reference for comparison. A finite element model of the femur was constructed from the same set of QCT scans and then used in iterative simulations of femur remodeling under stance and walking loading. The finite element model was initially assigned a homogeneous BMD distribution. During the remodeling, femur BMD was locally modified according to one of the four mechanical variables. The simulations were stopped when BMD change in two consecutive iterations was adequately small. The four simulated BMD patterns were then compared with the actual BMD.

RESULTS

It was found that the BMD pattern regulated by SED had the best similarity with the actual BMD. The medullary canal was successfully reproduced by simulated remodeling, indicating that in addition to its biological functions, the medullary canal has important biomechanical functions.

CONCLUSIONS

Both the actual and simulated BMD distributions showed that the proximal femur has much lower BMD than the femur shaft, which may explain why hip fractures most often occur at the proximal femur.

Comparing cellular bone matrices for posterolateral spinal fusion in a rat model

INTRODUCTION

Cellular bone matrices (CBM) are allograft products that provide three components essential to new bone formation: an osteoconductive scaffold, extracellular growth factors for cell proliferation and differentiation, and viable cells with osteogenic potential. This is an emerging technology being applied to augment spinal fusion procedures as an alternative to autografts.

METHODS

We aim to compare the ability of six commercially-available human CBMs (Trinity ELITE®, ViviGen®, Cellentra®, Osteocel® Pro, Bio4® and Map3®) to form a stable spinal fusion using an athymic rat model of posterolateral fusion. Iliac crest bone from syngeneic rats was used as a control to approximate the human gold standard. The allografts were implanted at L4-5 according to vendor specifications in male athymic rats, with 15 rats in each group. MicroCT scans were performed at 48 hours and 6 weeks post-implantation. The rats were euthanized 6 weeks after surgery and the lumbar spines were harvested for X-ray, manual palpation and histology analysis by blinded reviewers.

RESULTS

By manual palpation, five of 15 rats of the syngeneic bone group were fused at 6 weeks. While Trinity ELITE had eight of 15 and Cellentra 11 of 15 rats with stable fusion, only 2 of 15 of ViviGen-implanted spines were fused and zero of 15 of the Osteocel Pro, Bio4 and Map3 produced stable fusion. MicroCT analysis indicated that total bone volume increased from day 0 to week 6 for all groups except syngeneic bone group. Trinity ELITE (65%) and Cellentra (73%) had significantly greater bone volume increases over all other implants, which was consistent with the histological analysis.

CONCLUSION

Trinity ELITE and Cellentra were significantly better than other implants at forming new bone and achieving spinal fusion in this rat model at week 6. These results suggest that there may be large differences in the ability of different CBMs to elicit a successful fusion in the posterolateral spine.

The distribution of bone mineral density in the femoral heads of unstable intertrochanteric fractures

PURPOSE

Intertrochanteric fractures are usually treated with open reduction and internal fixation, but controversy still remains regarding the proper placement of the lag screw on the anteroposterior view. The stability of the lag screw has been shown to correlate with the bone quality around the screw, but the three-dimensional distribution of the bone mineral density (BMD) in the femoral head has not been studied in detail. Herein, the BMD along the femoral neck axis was measured to clarify the recommended position of the lag screw.

METHODS

Ten femoral heads acquired from intertrochanteric fractures were evaluated in this study. Each femoral head was scanned with micro computed tomography and the BMD along the femoral neck axis was measured in five regions: center, anterior, posterior, superior, and inferior. The BMD on the anteroposterior view (superior, center, and inferior) and the BMD on the lateral view (anterior, center, and posterior) were compared.

RESULTS

The BMD of the center region (173.0 ± 50.6 mg/cm³) was significantly higher than that of the inferior region (139.7 ± 50.1 mg/cm³) on the anteroposterior view ( p < 0.01). On the lateral view, the BMD was lower than the center region in the anterior region (165.7 ± 52.8 mg/cm³) and in the posterior region (157.5 ± 42.3 mg/cm³), but the difference was not significant.

CONCLUSION

The BMD was higher in the center region of the femoral head than in the inferior region. Therefore, lag screws are recommended to be inserted into the center of the femoral head.

Characterization of human cancellous and subchondral bone with respect to electro physical properties and bone mineral density by means of impedance spectroscopy

Computational simulation of electrical bone stimulation of the electrical and dielectric parameters of osteoarthritic bone tissue is useful for an exact patient-individual adaptation of the bone models. Therefore, we investigated electrical and dielectric parameters at a frequency of 20Hz of cancellous and subchondral human femoral head bone samples. Furthermore, the mechanical properties and the bone mineral density (BMD) were determined. Finally, these data were compared with the electrical and dielectric parameters. The bone samples were taken from patients with hip osteoarthritis. Electrical conductivity and dielectric permittivity of cancellous bone amounted to 0.043S/m and 8.1⋅10⁶. BMD of the bone samples determined by dual-x-ray-absorptiometry (DXA) and ashing resulted in 193 ± 70mg/cm² and 286 ± 59mg/cm³ respectively. Structural modulus (E_S) and ultimate compression strength (σ_max) were measured with 227 ± 94N/mm² and 6.5 ± 3.4N/mm². No linear correlation of the electrical and dielectric parameters compared with BMD and mechanical properties of cancellous bone samples was found. Electrical conductivity and dielectric permittivity of subchondral bone resulted in 0.029S/m and 8.97×10⁶.