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

An Anterior Spike Decreases Bone-Implant Micromotion in Cementless Tibial Baseplates for Total Knee Arthroplasty: A Biomechanical Study

BACKGROUND

Cementless tibial baseplates in total knee arthroplasty include fixation features (eg, pegs, spikes, and keels) to ensure sufficient primary bone-implant stability. While the design of these features plays a fundamental role in biologic fixation, the effectiveness of anterior spikes in reducing bone-implant micromotion remains unclear. Therefore, we asked: Can an anterior spike reduce the bone-implant micromotion of cementless tibial implants?

METHODS

We performed computational finite element analyses on 13 tibiae using the computed tomography scans of patients scheduled for primary total knee arthroplasty. The tibiae were virtually implanted with a cementless tibial baseplate with 2 designs of fixation of the baseplate: 2 pegs and 2 pegs with an anterior spike. We compared the bone-implant micromotion under the most demanding loads from stair ascent between both designs.

RESULTS

Both fixation designs had peak micromotion at the anterior-lateral edge of the baseplate. The design with 2 pegs and an anterior spike had up to 15% lower peak micromotion and up to 14% more baseplate area with micromotions below the most conservative threshold for ingrowth, 20 μm, than the design with only 2 pegs. The greatest benefit of adding an anterior spike occurred for subjects who had the smallest area of tibial bone below the 20 μm threshold (ie, most at risk for failure to achieve bone ingrowth).

CONCLUSIONS

An anteriorly placed spike for cementless tibial baseplates with 2 pegs can help decrease the bone-implant micromotion during stair ascent, especially for subjects with increased bone-implant micromotion and risk for bone ingrowth failure.

Tensile Yield Strain of Human Cortical Bone from the Femoral Diaphysis Is Constant among Healthy Adults and across the Anatomical Quadrants

The literature suggests that the yield strain of cortical bone is invariant to its stiffness (elastic modulus) and strength (yield stress). However, data about intra-individual variations, e.g., the influence of different collagen/mineral organisations observed in bone aspects withstanding different habitual loads, are lacking. The hypothesis that the yield strain of human cortical bone tissue, retrieved from femoral diaphyseal quadrants subjected to different habitual loads, is invariant was tested. Four flat dumbbell-shaped specimens were machined from each quadrant of the proximal femoral diaphysis of five adult donors for a total of 80 specimens. Two extensometers attached to the narrow specimen region were used to measure deformation during monotonic tensile testing. The elastic modulus (linear part of the stress-strain curve) and yield strain/stress at a 0.2% offset were obtained. Elastic modulus and yield stress values were, respectively, in the range of 12.2-20.5 GPa and 75.9-136.6 MPa and exhibited a positive linear correlation. All yield strain values were in the narrow range of 0.77-0.87%, regardless of the stiffness and strength of the tissue and the anatomical quadrant. In summary, the results corroborate the hypothesis that tensile yield strain in cortical bone is invariant, irrespective also of the anatomical quadrant. The mean yield strain value found in this study is similar to what was reported by inter-species and evolution studies but slightly higher than previous reports in humans, possibly because of the younger age of our subjects. Further investigations are needed to elucidate a possible dependence of yield strain on age.

A probabilistic approach for assessing the mechanical performance of intertrochanteric fracture stabilized with proximal femoral nail antirotation

Maintaining post-operative mechanical stability is crucial for successfully healing intertrochanteric fractures treated with the Proximal Femoral Nail Antirotation (PFNA) system. This stability is primarily dependent on the bone mineral density (BMD) and strain on the fracture. Current PFNA failure analyses often overlook the uncertainties related to BMD and body weight (BW). Therefore, this study aimed to develop a probabilistic model using finite element modeling and engineering reliability analysis to assess the post-operative performance of PFNA under various physiological loading conditions. The model predictions were validated through a series of experimental test. The results revealed a negative nonlinear relationship between the BMD and compressive strain. Conversely, the BW was positively and linearly correlated with the compressive strain. Importantly, the compressive strain was more sensitive to BW than to BMD when the BMD exceeded 0.6 g/cm3. Potential trabecular bone compression failure is also indicated if BMD is equal to or below 0.15 g/cm3 and BW increases to approximately 2.5 times the normal or higher. This study emphasizes that variations in the BMD significantly affect the probability of failure of a PFNA system. Thus, careful planning of post-operative physical therapy is essential. For patients aged > 50 years restrictions on high-intensity activities are advised, while limiting strenuous movements is recommended for those aged > 65 years.

FEA Comparison of the Mechanical Behavior of Three Dental Crown Materials: Enamel, Ceramic, and Zirconia

The restoration of endodontically treated teeth is one of the main challenges of restorative dentistry. The structure of the tooth is a complex assembly in which the materials that make it up, enamel and dentin, have very different mechanical behaviors. Therefore, finding alternative replacement materials for dental crowns in the area of restorative care isa highly significant challenge, since materials such as ceramic and zirconia have very different stress load resistance values. The aim of this study is to assess which material, either ceramic or zirconia, optimizes the behavior of a restored tooth under various typical clinical conditions and the masticatory load. A finite element analysis (FEA) framework is developed for this purpose. The 3D model of the restored tooth is input into the FEA software (Ansys Workbench R23)and meshed into tetrahedral elements. The presence of masticatory forces is considered: in particular, vertical, 45° inclined, and horizontal resultant forces of 280 N are applied on five contact points of the occlusal surface. The numerical results show that the maximum stress developed in the restored tooth including a ceramic crown and subject to axial load is about 39.381 MPa, which is rather close to the 62.32 MPa stress computed for the natural tooth; stresses of about 18 MPa are localized at the roots of both crown materials. In the case of the zirconia crown, the stresses are much higher than those in the ceramic crown, except for the 45° load direction, while, for the horizontal loads, the stress peak in the zirconia crown is almost three times as large as its counterpart in the ceramic crown (i.e., 163.24 MPa vs. 56.114 MPa, respectively). Therefore, the zirconia crown exhibits higher stresses than enamel and ceramic that could increase in the case of parafunctions, such as bruxism. The clinician's choice between the two materials should be evaluated based on the patient's medical condition.

Exploring Advanced Functionalities of Carbon Fiber-Graded PEEK Composites as Bone Fixation Plates Using Finite Element Analysis

This study aims to address the challenges associated with conventional metallic bone fixation plates in biomechanical applications, such as stainless steel and titanium alloys, including stress shielding, allergic reactions, corrosion resistance, and interference with medical imaging. The use of materials with a low elastic modulus is regarded as an effective approach to overcome these problems. In this study, the impact of different types of chopped carbon fiber-reinforced polyether ether ketone (CCF/PEEK) functionally graded material (FGM) bone plates on stress shielding under static and instantaneous dynamic loading was explored using finite element analysis (FEA). The FGM bone plate models were established using ABAQUS and the user's subroutine USDFLD and VUSDFLD, and each model was established with an equivalent overall elastic modulus and distinctive distributions. The results revealed that all FGM bone plates exhibited lower stress shielding effects compared to metal bone plates. Particularly, the FGM plate with an elastic modulus gradually increased from the centre to both sides and provided maximum stress stimulation and the most uniform stress distribution within the fractured area. These findings offer crucial insights for designing implantable medical devices that possess enhanced mechanical adaptability.

Point-of-Care Orthopedic Oncology Device Development

BACKGROUND

The triad of 3D design, 3D printing, and xReality technologies is explored and exploited to collaboratively realize patient-specific products in a timely manner with an emphasis on designs with meta-(bio)materials.

METHODS

A case study on pelvic reconstruction after oncological resection (osteosarcoma) was selected and conducted to evaluate the applicability and performance of an inter-epistemic workflow and the feasibility and potential of 3D technologies for modeling, optimizing, and materializing individualized orthopedic devices at the point of care (PoC).

RESULTS

Image-based diagnosis and treatment at the PoC can be readily deployed to develop orthopedic devices for pre-operative planning, training, intra-operative navigation, and bone substitution.

CONCLUSIONS

Inter-epistemic symbiosis between orthopedic surgeons and (bio)mechanical engineers at the PoC, fostered by appropriate quality management systems and end-to-end workflows under suitable scientifically amalgamated synergies, could maximize the potential benefits. However, increased awareness is recommended to explore and exploit the full potential of 3D technologies at the PoC to deliver medical devices with greater customization, innovation in design, cost-effectiveness, and high quality.

Anisotropy, Anatomical Region, and Additional Variables Influence Young's Modulus of Bone: A Systematic Review and Meta-Analysis

The importance of finite element analysis (FEA) is growing in orthopedic research, especially in implant design. However, Young's modulus (E) values, one of the most fundamental parameters, can range across a wide scale. Therefore, our study aimed to identify factors influencing E values in human bone specimens. We report our systematic review and meta-analysis based on the recommendation of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guideline. We conducted the analysis on November 21, 2021. We included studies investigating healthy human bone specimens and reported on E values regarding demographic data, specimen characteristics, and measurement specifics. In addition, we included study types reporting individual specimen measurements. From the acquired data, we created a cohort in which we performed an exploratory data analysis that included the explanatory variables selected by random forest and regression trees methods, and the comparison of groups using independent samples Welch's t test. A total of 756 entries were included from 48 articles. Eleven different bones of the human body were included in these articles. The range of E values is between 0.008 and 33.7 GPa. The E values were most heavily influenced by the cortical or cancellous type of bone tested. Measuring method (compression, tension, bending, and nanoindentation), the anatomical region within a bone, the position of the bone within the skeleton, and the bone specimen size had a decreasing impact on the E values. Bone anisotropy, specimen condition, patient age, and sex were selected as important variables considering the value of E. On the basis of our results, E values of a bone change with bone characteristics, measurement techniques, and demographic variables. Therefore, the evaluation of FEA should be performed after the standardization of in vitro measurement protocol. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Dual-zone material assignment method for correcting partial volume effects in image-based bone models

In image-based finite element analysis of bone, partial volume effects (PVEs) arise from image blur at tissue boundaries and as a byproduct of geometric reconstruction and meshing during model creation. In this study, we developed and validated a material assignment approach to mitigate partial volume effects. Our validation data consisted of physical torsion testing of intact tibiae from N = 20 Swiss alpine sheep. We created finite element models from micro-CT scans of these tibiae using three popular element types (10-node tetrahedral, 8-node hexahedral, and 20-node hexahedral). Without partial volume management, the models over-predicted the torsional rigidity compared to physical biomechanical tests. To address this problem, we implemented a dual-zone material model to treat elements that overlap low-density surface voxels as soft tissue rather than bone. After in situ inverse optimization, the dual-zone material model produced strong correlations and high absolute agreement between the virtual and physical tests. This suggests that with appropriate partial volume management, virtual mechanical testing can be a reliable surrogate for physical biomechanical testing. For maximum flexibility in partial volume management regardless of element type, we recommend the use of the following dual-zone material model for ovine tibiae: soft-tissue cutoff density of 665 mgHA/cm3 with a soft tissue modulus of 50 MPa (below cutoff) and a density-modulus conversion slope of 10,225 MPa-cm3/mgHA for bone (above cutoff).

Approach to standardized material characterization of the human lumbopelvic system-Specification, preparation and storage

The complexity of the osseo-ligamentous lumbopelvic system has made it difficult to perform both, the overall preparation as well as specimen harvesting and material testing with a reasonable amount of time and personnel. The logistics of such studies present a hurdle for reproducibility. A structured procedure was developed and proved, which allows all necessary steps to be carried out reproducibly and in a reasonable time. This enables the extraction of 26 soft tissue, 33 trabecular and 32 cortical bone specimens from this anatomical region per cadaver. The integrity of the specimens remains maintained while keeping requirements within manageable limits. The practicability of the intended five-day specimen harvesting and testing procedure could be demonstrated on five test and two pre-test sequences. The intended minimization of physical, biological, and chemical external influences on specimens could be achieved. All protocols, instructions and models of preparation and storage devices are included in the supporting information. The high grade of applicability and reproducibility will lead to better comparability between different biomechanical investigations. This procedure proven on the human pelvis is transferable to other anatomical regions.

Relation between mechanical and densimetric properties to fractal dimension in human rib cortical bone

BACKGROUND

Numerous prior studies hypothesized a power-law relationship (E∝ρ^α) between cortical bone Young's modulus (E) and density (ρ) with an exponent 2.3≤α≤3.0, that has not been previously justified in the literature on a theoretical level. Moreover, despite the fact microstructure have been extensively studied, the material correlate of Fractal Dimension (FD) as a descriptor of bone microstructure was not clear in previous studies.

METHODS

This study examined the effect of mineral content and density on the mechanical properties of a large number of human rib cortical bone samples. The mechanical properties were calculated using Digital Image Correlation and uniaxial tensile tests. CT scans were used to calculate the Fractal Dimension (FD) of each specimen. For each specimen, the mineral (f_min), organic (f_org) and water (f_wat) weight fractions were determined. In addition, density was measured after a drying-and-ashing process. Then, Regression Analysis was employed to investigate the relationship between anthropometric variables, weight fractions, density and FD, as well as its impact on the mechanical properties.

FINDINGS

Young's modulus exhibited a power-law relationship with an exponent of α>2.3 when using the conventional density (wet density), but α=2 when using dry density (desecated specimens). In addition, FD increases with decreasing cortical bone density. A significant relationship has been found between FD and density, whereby FD is correlated with the embedding of low density regions in cortical bone.

INTERPRETATION

This study provides a new insight in the exponent value of the power-law relation between Young's Modulus and density, and relates bone behavior with the fragile fracture theory in ceramic materials. Moreover, the results suggest that Fractal Dimension is related to presence of low-density regions.

Equivalent loads from the life-cycle of acetabular cages in relation to bone-graft transformation

BACKGROUND AND OBJECTIVES

Bone grafts placed behind acetabular cages change their structure in response to mechanical stimuli. The full consideration of lifestyle loads is extremely resource-intensive, so a method using substitutive loads was proposed to reduce the calculation cost. The aim of the study is to present and prove this method.

METHODS

By means of mechanical equations and using the force vectors from the literature which have the same initial point and their relative frequency, while applying a linear model, the average strain energy density distribution for all load cases can be calculated, compiling a matrix from the external loads. From the elements of this matrix, three substitutive load vectors can be calculated, which can be proven to produce the same strain energy density distribution by averaging their effects. The feasibility of using this to model the transformation of bone grafts placed behind acetabular cages is demonstrated with a finite element model, along with a reference calculation.

RESULTS

The substitutive load vectors could be calculated in closed form and the simulations showed that they produced a similar density distribution to the reference model with a numerical calculation error range. Accordingly, the density distribution calculated from bone graft transformation is almost the same.

CONCLUSIONS

In addition to the aforementioned linearity and the same initial point limitations, the applied method is able to produce the substitutive load vectors with which the calculation of the strain energy density distribution and the bone graft's new density distributions can be carried out faster.

Efficacy of percutaneous kyphoplasty on vertebral compression fractures with different bone mineral densities: a retrospective study

BACKGROUND

This study was performed to investigate the clinical efficacy of percutaneous kyphoplasty (PKP) for vertebral compression fractures with different bone mineral densities (BMD).

METHODS

We performed a retrospective analysis of 232 patients with single-segment vertebral compression fractures who underwent PKP. Patients were divided into the normal BMD, osteopenia, and osteoporosis groups according to their average lumbar BMD before surgery. The visual analog scale (VAS) was used to compare differences in pain relief before and after surgery in each group. Corrections of the wedge angle and kyphotic angle before and after surgery were observed using anteroposterior and lateral radiographs and compared among the groups, as was the incidence of bone cement leakage.

RESULTS

Patients were followed up for 6-12 months, with an average follow-up time of 9.12 ± 1.68 months. The VAS score, wedge angle, and kyphotic angle of the three groups of patients decreased significantly at the end of the follow-up (P < 0.05). The changes in VAS score and wedge angle correction in the osteoporosis group were significantly larger than those in the normal BMD and osteopenia groups (P < 0.05). There were no significant differences among the three groups in terms of kyphotic angle correction or bone cement leakage rates (P > 0.05).

CONCLUSIONS

PKP has a positive effect on vertebral compression fractures with different BMD, and is especially suitable for osteoporotic vertebral compression fractures.

Experimental and numerical study on failure mechanisms of bone simulants subjected to projectile impact

Analyses of the human bones failure mechanisms under projectile impact conditions can be made through performing of a large number of ballistic trials. But the amount of data that can be collected during ballistic experiments is limited due to the high dynamics of the process and its destructive character. Numerical analyses may support experimental methodologies allowing to better understand the principles of the phenomenon. Therefore, the main aim of the study was to create and to verify a numerical model of commercially available synthetic bone material-Synbone®. The model could be used in the future as a supporting tool facilitating forensic studies or designing processes of personal protection systems (helmets, bulletproof vests, etc.). Although Synbone® is commonly used in the ballistic experiments, the literature lacks reliable numerical models of this material. In order to define a numerical model of Synbone®, mechanical experiments characterizing the response of the material to the applied loads in a wide range of strains and strain rates were carried out. Based on the mechanical tests results, an appropriate material model was selected for the Synbone® composite and the values of constants in its equations were determined. Material characterization experiments were subsequently reproduced with numerical simulations and a high correlation of the results was obtained. The final validation of the material model was based on the comparison of the ballistic impact experiments and simulation results. High similarity obtained (relative error lower than 10%) demonstrates that the numerical model of Synbone® material was properly defined.

Measuring Drug Therapy Effect on Osteoporotic Fracture Risk by Trabecular Bone Lacunarity: The LOTO Study

An MRI method providing one parameter (TBLβ: trabecular-bone-lacunarity-parameter-β) that is sensitive to trabecular bone architecture (TBA) changes with aging and osteoporosis is under study as a new tool in the early diagnosis of bone fragility fracture. A cross-sectional and prospective observational study (LOTO: Lacunarity Of Trabecular bone in Osteoporosis) on over-50s women, at risk for bone fragility fracture, was designed to validate the method. From the baseline data, we observed that in women with prevalent vertebral fractures (VF+), TBA was differently characterized by TBLβ when osteoporosis treatment is considered. Here we verify the potential of TBLβ as an index of osteoporosis treatment efficacy. Untreated (N = 156) and treated (N = 123) women were considered to assess differences in TBLβ related to osteoporosis treatment. Prevalent VFs were found in 31% of subjects, 63% of which were under osteoporosis medications. The results show that TBLβ discriminates between VF+ and VF− patients (p = 0.004). This result is mostly stressed in untreated subjects. Treatment, drug therapy in particular (89% Bisphosphonates), significantly counteracts the difference between VF+ and VF− within and between groups: TBLβ values in treated patients are comparable to untreated VF− and statistically higher than untreated VF+ (p = 0.014) ones. These results highlight the potential role of TBLβ as an index of treatment efficacy.

Fracture analysis of healthy and osteoporotic femora using clinical CT images, phantomless densitometry, and linear FE method

Computational fracture analysis has become a growing branch of orthopedic research. Particularly, the associated methods provide reliable tools for the analysis of 3D CT-based models of bone. This paper reports the results of such analyses for 15 human femora (healthy and osteoporotic) under different loading orientations (85 different analysis cases). A new method was developed for the calculation of the density distribution in the models from ordinary clinical CT images without calibration phantom. This method, along with a strain-energy-based linear finite element (FE) analysis scheme, was used to predict the fracture strength and pattern of 10 cadaveric femora, for which the mechanical testing results and calibrated FE models were already available. The very good agreement and consistency between different sets of results showed the reliability and accuracy of the new density calibration method, as well as the linear analysis scheme. Accordingly, the method was applied to five new clinical images, gathered from two clinics that used different scanners with different protocols. The strength and fracture pattern of each one of these specimens were analyzed under 15 different loading conditions. A consistent behavior was found for variation of the fracture load and pattern of all specimens with the loading orientations, while very clear contrasts were observed between the strength amplitudes of healthy and osteoporotic specimens. The proposed methods can be easily applied to ordinary daily (even archived) clinical CT scans to conduct fast and reliable fracture analysis of human femora for general bone research and opportunistic studies of osteoporosis and trauma.

Biomechanical and clinical studies on lumbar spine fusion surgery: a review

Low back pain is associated with degenerative disc diseases of the spine. Surgical treatment includes fusion and non-fusion types. The gold standard is fusion surgery, wherein the affected vertebral segment is fused. The common complication of fusion surgery is adjacent segment degeneration (ASD). The ASD often leads to revision surgery, calling for a further fusion of adjacent segments. The existing designs of nonfusion type implants are associated with clinical problems such as subsidence, difficulty in implantation, and the requirement of revision surgeries. Various surgical approaches have been adopted by the surgeons to insert the spinal implants into the affected segment. Over the years, extensive biomechanical investigations have been reported on various surgical approaches and prostheses to predict the outcomes of lumbar spine implantations. Computer models have been proven to be very effective in identifying the best prosthesis and surgical procedure. The objective of the study was to review the literature on biomechanical studies for the treatment of lumbar spinal degenerative diseases. A critical review of the clinical and biomechanical studies on fusion spine surgeries was undertaken. The important modeling parameters, challenges, and limitations of the current studies were identified, showing the future research directions.

Fracture in porous bone analysed with a numerical phase-field dynamical model

A dynamic phase-field fracture finite element model is applied to discretized high-resolution three-dimensional computed tomography images of human trabecular bone to analyse rapid bone fracture. The model is contrasted to quasi-static experimental results and a quasi-static phase-field finite element model. The experiment revealed complex stepwise crack evolution with multiple crack fronts, and crack arrests, as the global tensile displacement load was incrementally increased. The quasi-static phase-field fracture model captures the fractures in the experiment reasonably well, and the dynamic model converges towards the quasi-static model when mechanically loaded at low rates. At higher load rates, i.e., at larger impulses, inertia effects significantly contribute to an increased initial global stiffness, higher peak forces and a larger number of cracks spread over a larger volume. Since the fracture process clearly is different at large impulses compared to small impulses, it is concluded that dynamic fracture models are necessary when simulating rapid bone fracture.

The influence of partial union on the mechanical strength of scaphoid fractures: a finite element study

Assessment of scaphoid fracture union on computed tomography scans is not currently standardized. We investigated the extent of scaphoid waist fracture union required to withstand physiological loads in a finite element model, based on a high-resolution CT scan of a cadaveric forearm. For simulations, the scaphoid waist was partially fused at the radial and ulnar sides. A physiological load of 100 N was transmitted to the scaphoid and the minimal amount of union to maintain biomechanical stability was recorded. The orientation of the fracture plane was varied to analyse the effect on biomechanical stability. The results indicate that the scaphoid is more prone to re-fracture when healing occurs on the ulnar side, where at least 60% union is required. Union occurring from the radial side can withstand loads with as little as 25% union. In fractures more parallel to the radial axis, the scaphoid seems less resistant on the radial side, as at least 50% union is required. A quantitative CT scan analysis with the proposed cut-off values and a consistently applied clinical examination will guide the clinician as to whether mid-waist scaphoid fractures can be considered as truly united.

Soft Hydroxyapatite Composites Based on Triazine-Trione Systems as Potential Biomedical Engineering Frameworks

Composites of triazine-trione (TATO) thiol-ene networks and hydroxyapatite (HA) have shown great potential as topological fixation materials for complex bone fractures due to their high flexural modulus, biocompatibility, and insusceptibility to forming soft-tissue adhesions. However, the rigid mechanical properties of these composites make them unsuitable for applications requiring softness. The scope of these materials could therefore be widened by the design of new TATO monomers that would lead to composites with a range of mechanical properties. In this work, four novel TATO-based monomers, decorated with either ester or amide linkages as well as alkene or alkyne end groups, have been proposed and synthesized via fluoride-promoted esterification (FPE) chemistry. The ester-modified monomers were then successfully formulated along with the thiol TATO monomer tris [2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC) and HA to give soft composites, following the established photo-initiated thiol-ene coupling (TEC) or thiol-yne coupling (TYC) chemistry methodologies. The most promising composite shows excellent softness, with a flexural modulus of 57 (2) MPa and ε_f at maximum σ_f of 11.8 (0.3)%, which are 117 and 10 times softer than the previously developed system containing the commercially available tri-allyl TATO monomer (TATATO). Meanwhile, the surgically convenient viscosity of the composite resins and their excellent cytotoxicity profile allow them to be used in the construction of soft objects in a variety of shapes through drop-casting suitable for biomedical applications.

Does the bone mineral density measured on a preoperative CT scan before total hip arthroplasty reflect the bone's mechanical properties?

INTRODUCTION

No method exists to quantify the bone quality and factors that will ensure osteointegration of total hip arthroplasty (THA) implants. A preoperative CT scan can be used to evaluate the bone mineral density (BMD) when planning a THA procedure. The aim of this study was to validate BMD measurement as a marker of bone quality based on a preoperative CT scan.

HYPOTHESIS

BMD reflects the bone's mechanical properties for the purposes of preoperative THA planning.

METHODS

Patients who underwent primary THA for hip osteoarthritis or dysplasia with cementless implants and 3D preoperative plan were enrolled prospectively. The cortical BMD was calculated on CT scans used in the preoperative planning process. During the surgical procedure, the femoral head and neck were collected. These bone samples were subsequently scanned with a calibrated micro-CT scanner. The BMD was derived from the micro-CT scan and used as input for a finite element model to determine the bone's mechanical properties. Correlations between BMD, apparent moduli of elasticity and porosity were calculated.

RESULTS

The values of cortical BMD measured on the micro-CT and CT scan were significantly correlated (cc=0.52). The mean angular cortical BMD measured with the micro-CT scan was 1472.33mg/cm³ (SD: 357.53mg/cm³, 980.64-2830.6mg/cm³). There was no significant correlation between cortical BMD and the various apparent moduli of elasticity, except for E_yy and G_zy. Cortical BMD and porosity were inversely correlated with a Spearman coefficient of -0.41 (CI₉₅: [-0.71; -0.02], p=0.03). There was also an inverse correlation between the apparent moduli of elasticity (independent of their orientation) and porosity (p<0.01).

DISCUSSION

BMD provides information about porosity, which is a major factor when evaluating the bone's mechanical properties before THA.

LEVEL OF EVIDENCE

IV.

Correlation of mineral density and elastic modulus of dog dentin using μ-CT and nanoindentation

This study sought to 1) investigate the spatial distribution of mineral density of dog dentin using µ-CT and 2) characterize the relationship between the elastic modulus and mineral density of dog dentin using nanoindentation and µ-CT. Maxillary canine teeth of 10 mature dogs were scanned with a µ-CT then sectioned in the transverse and vertical planes and tested using nanoindentation. Spatial distribution of mineral density and elastic modulus was quantified. Results demonstrated significant spatial variation in mineral density and elastic modulus. Mineral density and elastic modulus generally increased from the dentin-pulp interface to the dentino-enamel junction and from the crown base to the crown tip. Significant site dependent correlations between mineral density and elastic modulus were determined (0.021 > R2 > 0.408). The results of this study suggest that while mineral density is a mediator of elastic modulus, other mediators such as collagen content may contribute to the mechanical behavior of dog dentin.

The effects of force application on the compressive properties of femoral spongious bone

BACKGROUND

End artefacts play a major role in uniaxial compression tests with cancellous bone specimens. They lead to misinterpretation of mechanical parameters of bones due to uncontrolled introduction of bending moments into the free ends of trabeculae. This work aims to simplify current methods preventing end-artefacts and furthermore to investigate the influence of end artefacts on plateau stress.

METHODS

176 cylindrical cancellous bone specimens were taken from human femoral condyles and tested in uniaxial compression. The specimens were divided into 2 groups (direct, end-cap) and compressive modulus, maximum stress, plateau stress, energy absorbtion as well as apparent density were evaluated. Density values are from separate specimens which are immediately adjacent to the mechanical specimen.

FINDINGS

All mechanical parameters were significantly higher in the end-cap specimens than in the direct ones by about 30 - 40 %, thus reaching similar differences as the previous studies. Greatest differences between groups were determined for compressive modulus (45 %) and plateau stress (35 %). Energy absorbtion can be explained with great accuracy by plateau stress (P < 0.001; R² = 0.95). Among all parameters plateau stress can be best explained by apparent density using an exponential function (P < 0.001; R² = 0.38).

INTERPRETATION

The end-cap method used here to prevent end artefacts showed variations consistent with the literature when compared to the direct method. Additionally it was shown that the way in which the force is applied to the specimen has a major influence on the failure progression behavior, which was characterized using the plateau stress.

Can guided wave ultrasound predict bone mechanical properties at the femoral neck in patients undergoing hip arthroplasty?

The bone quality of patients undergoing hip replacement surgery is poorly predicted by radiographs alone. With better bone quality information available to a surgeon, the operation can be performed more safely. The aim of this study was to investigate whether ultrasound signals of cortical bone at peripheral sites such as the tibia and radius can be used to predict the compressive mechanical properties of cortical bone at the femoral neck. We recruited 19 patients undergoing elective hip arthroplasty and assessed the radius and tibia of these patients with the Azalée guided wave ultrasound to estimate the porosity and thickness of the cortex. Excess bone tissues were collected from the femoral neck and the compressive mechanical properties of the cortex were characterised under a mechanical loading rig to determine stiffness, ultimate strength, and density. The correlations between the ultrasound measurements and mechanical properties were analysed using linear regression, Pearson correlation statistics, and multiple regression analysis. Cortical mechanical properties were weakly to moderately correlated with the ultrasound measurements at various sites (R2 = 0.00-0.36). The significant correlations found were not consistent across all 4 peripheral measurement sites. Additionally, weak to moderate ability of the ultrasound to predict mechanical properties at the neck of femur with multiple regression analysis was found (R2 = 0.00-0.48). Again, this was inconsistent across the different anatomical sites. Overall, the results demonstrate the need for ultrasound scans to be collected directly from clinically relevant sites such as the femoral neck due to the inconsistency of mechanical properties across various sites.

Strength enhancement and modulus modulation in auxetic meta-biomaterials produced by selective laser melting

Meta-biomaterials are applied to orthopedic implants to avoid stress shielding effects; however, there is no reason for the yield strength to be comparable to that of human bone. In this study, a composite unit cell was designed by combining the positive Poisson's ratio (PPR) and negative Poisson's ratio (NPR) unit cells, inspired by the second-phase strengthening theory. The purpose was to increase the strength while maintaining the elastic modulus. All structures were successfully fabricated from Ti-6Al-4V via selective laser melting. The relative density is between 0.08 and 0.24, which falls within the optimal range for bone growth. Mechanical tests indicated that the center of the inclined rod fractured in a stepwise fracture mode, which was consistent with the predictions of the Johnson-Cook model. The elastic modulus ranged from 0.652 ± 0.016 to 5.172 ± 0.021 GPa, and the yield strength varied from 10.62 ± 0.112 to 87.158 ± 2.215 MPa. An improved Gibson-Ashby law was proposed to facilitate the design of gradient structures. When the re-entrant angle was 40°, a hybrid body-centered cubic NPR structure was formed, resulting in a significant improvement in the mechanical properties. Importantly, the yield strength of the proposed composite structures increased by 43.23%, and the compression strength increased by 44.70% under the same elastic modulus. The strengthening mechanism has been proven to apply to other bending-dominated structures. Overall, this imparts unprecedented mechanical performance to auxetic meta-biomaterials and provides insights into improving the reported porous structures. STATEMENT OF SIGNIFICANCE: : Auxetic meta-biomaterials exhibit auxetic properties that can improve the contact between the bone-implant interface and reduce the risk of aseptic failure. To avoid the stress shielding effect, the elastic modulus has traditionally been decreased by increasing the porosity. However, the strength is simultaneously reduced. Therefore, a composite unit cell was proposed to increase strength rather than modulus by combining the positive and negative Poisson's ratio unit cells, inspired by the second-phase strengthening theory. We observed a 43.23% increase in the yield strength of the composite structure without increasing the elastic modulus. This strengthening mechanism has been proven to apply to other bending-dominated structures. Our approach provides insights into improving other bending-dominated structures and broadening their applications for bone implantation.

Investigation of ultrasonic soft tissue-bone reflection coefficients correlating with curve severity in children with adolescent idiopathic scoliosis

Adolescent idiopathic scoliosis (AIS) is a three-dimensional curvature of spine. Children with AIS and low bone quality have higher chance to get curve progression leading to bigger spinal curvature. In addition, bone quality affects acoustic impedance of bone, thus influencing the reflection coefficient of ultrasound signal from the soft tissue–bone interface. This study aimed to estimate the bone quality of AIS patients based on the reflection coefficients to determine the correlation of the bone quality with curve severity. A simple bone model was used to develop an equation to calculate the reflection coefficient value. Experiments were conducted on five different phantoms. Acrylic was used to design a vertebral shape to study the effect of surface roughness and inclination, including: smooth flat surface (SFS), smooth curved surface (SCS), rough curved surface (RCS), and the rough curved inclined surface (RCIS). A clinical study with 37 AIS patients were recruited. The estimated reflection coefficient values of plate phantoms agreed well with the predicted values and the maximum error was 6.7%. The reflection coefficients measured from the acrylic-water interface for the SFS, SCS, RCS, RCIS (3° and 5°) were 0.37, 0.33, 0.28, (0.23 and 0.12), respectively. The surface roughness and inclination increased the reflection loss. From the clinical data, the average reflection coefficients for children with AIS were 0.11 and 0.07 for the mild curve group and the moderate curve group, respectively. A moderate linear correlation was found between the reflection coefficients and curve severity (r2 = 0.3). Patients with lower bone quality have observed to have larger spinal curvature.

A preliminary study exploring the mechanical properties of normal and Mgp-deficient mouse femurs during early growth

Matrix Gla protein (MGP) is mostly known to be a calcification inhibitor, as its absence leads to ectopic calcification of different tissues such as cartilage or arteries. MGP deficiency also leads to low bone mass and delayed bone growth. In the present contribution, we investigate the effect of MGP deficiency on the structural and material mechanical bone properties by focusing on the elastic response of femurs undergoing three-points bending. To this aim, biomechanical tests are performed on femurs issued from Mgp-deficient mice at 14, 21, 28, and 35 days of postnatal life and compared to healthy control femurs. µCT acquisitions enable to reconstruct bone geometries and are used to construct subject-specific finite element models avoiding some of the reported limitations concerning the use of beam-like assumptions for small bone samples. Our results indicate that MGP deficiency may be associated to differences in both structural and material properties of femurs during early stages of development. MGP deficiency appears to be related to a decrease in bone dimensions, compensated by higher material properties resulting in similar structural bone properties at P35. The search for a unique density-elasticity relationship based on calibrated bone mineral density (BMD) indicates that MGP deficiency may affect bone tissue in several ways, that may not be represented uniquely from the quantification of BMD. Despite of its limitation to elastic response, the present preliminary study reports for the very first time the mechanical skeletal properties of Mgp-deficient mice at early stages of development.

Characterization of regional variation of bone mineral density in the geriatric human cervical spine by quantitative computed tomography

Background

Odontoid process fractures are among the most common in elderly cervical spines. Their treatment often requires fixation, which may include use of implants anteriorly or posteriorly. Bone density can significantly affect the outcomes of these procedures. Currently, little is known about bone mineral density (BMD) distributions within cervical spine in elderly. This study documented BMD distribution across various anatomical regions of elderly cervical vertebrae.

Methods and findings

Twenty-three human cadaveric C1-C5 spine segments (14 males and 9 female, 74±9.3 y.o.) were imaged via quantitative CT-scan. Using an established experimental protocol, the three-dimensional shapes of the vertebrae were reconstructed from CT images and partitioned in bone regions (4 regions for C1, 14 regions for C2 and 12 regions for C3-5). The BMD was calculated from the Hounsfield units via calibration phantom. For each vertebral level, effects of gender and anatomical bone region on BMD distribution were investigated via pertinent statistical tools.

Data trends suggested that BMD was higher in female vertebrae when compared to male ones. In C1, the highest BMD was found in the posterior portion of the bone. In C2, BMD at the dens was the highest, followed by lamina and spinous process, and the posterior aspect of the vertebral body. In C3-5, lateral masses, lamina, and spinous processes were characterized by the largest values of BMD, followed by the posterior vertebral body.

Conclusions

The higher BMD values characterizing the posterior aspects of vertebrae suggest that, in the elderly, posterior surgical approaches may offer a better fixation quality.

Numerical Analysis of the Calcaneal Nail C-NAIL

The presented article investigates the biomechanics of the calcaneal nail C-NAILTM by numerical calculations and, partially, experimentally. This nail is widely used in trauma and orthopaedics. A numerical model of implants directly interacting with the bone tissue model obtained from CT scans was calculated. The material properties of the bone tissue can be described by several models; in this work, a non-homogeneous material model with isotropic elements and prescribed elastic modulus was used to provide a more accurate model of the applied force distribution on the individual parts of the implants. The critical areas of the nail and its fixtures were investigated using finite element strength calculations to verify their strength and reliability, contributing to the safety and faster and easier treatment of patients. These analyses suggest that the strength of the calcaneal nail C-NAIL, as well as the stabilization of bone fragments resulting from its use, are sufficient for clinical practice.

Variations in Strain Distribution at Distal Radius under Different Loading Conditions

Distal radial fractures exhibit various fracture patterns. By assuming that the strain distribution at the distal radius affects the diversification of the fracture pattern, a parameter study using the finite element model of a wrist developed from computed tomography (CT) images was performed under different loading conditions. The finite element model of the wrist consisted of the radius, ulna, scaphoid, lunate, triquetrum, and major carpal ligaments. The material properties of the bone models were assigned on the basis of the Hounsfield Unit (HU) values of the CT images. An impact load was applied to the scaphoid, lunate, and triquetrum to simulate boundary conditions during fall accidents. This study considered nine different loading conditions that combine three different loading directions and three different load distribution ratios. According to the analysis results, the strain distribution at the distal radius changed with respect to the change in the loading condition. High strain concentration occurred in regions where distal radius fractures are commonly developed. The direction and distribution of the load acting on the radius were considered to be factors that may cause variations in the fracture pattern of distal radius fractures.

Sensitivity of the stress field of the proximal femur predicted by CT-based FE analysis to modeling uncertainties

Proximal femur anatomy and bone mineral density vary widely among individuals, precluding the use of one predefined finite element (FE) model to determine the stress field for all proximal femurs. This variability poses a challenge in current prosthetic hip design approach. Given the numerous options for generating computed tomography (CT)-based FE models, selecting the best methods for defining the mechanical behavior of the proximal femur is difficult. In this study, a combination of computational and experimental approaches was used to explore the susceptibility of the predicted stress field of the proximal femur to different combinations of density-elasticity relationships, element type, element size, and calibration error. Our results suggest that FE models with first-order voxelized elements generated by the Keyak and Falkinstein density-elasticity relationship or quadratic tetrahedral elements generated by the Morgan density-elasticity relationship lead to accurate estimations of the mechanical behavior of human femurs. Other combinations of element size, element type, and mathematical relationships produce less accurate results, especially in the cortical bone of the femoral neck and calcar region. The voxelized model was more susceptible to variation of element size and density-elasticity relationships than FE models with quadratic tetrahedral elements. Regardless of element type, the stress fields predicted by the Keyak and Falkinstein and the Morgan relationships were the most robust to calibration error when deriving material density from CT-generated Hounsfield data. These results provide insight into the implementation of a robust platform for designing patient-specific implants capable of maintaining or modifying the stress in bones.

3D bioprinting of osteon-mimetic scaffolds with hierarchical microchannels for vascularized bone tissue regeneration

The integration of three-dimensional (3D) bioprinted scaffold's structure and function for critical-size bone defect repair is of immense significance. Inspired by the basic component of innate cortical bone tissue--osteons, many studies focus on biomimetic strategy. However, the complexity of hierarchical microchannels in the osteon, the requirement of mechanical strength of bone, and the biological function of angiogenesis and osteogenesis remain challenges in the fabrication of osteon-mimetic scaffolds. Therefore, we successfully built mimetic scaffolds with vertically central medullary canals, peripheral Haversian canals, and transverse Volkmann canals structures simultaneously by 3D bioprinting technology using polycaprolactone and bioink loading with bone marrow mesenchymal stem cells (BMSCs) and bone morphogenetic protein-4 (BMP-4). Subsequently, endothelial progenitor cells (EPCs) were seeded into the canals to enhance angiogenesis. The porosity and compressive properties of bioprinted scaffolds could be well controlled by altering the structure and canal numbers of the scaffolds. The osteon-mimetic scaffolds showed satisfactory biocompatibility and promotion of angiogenesis and osteogenesis in vitro and prompted the new blood vessels and new bone formation in vivo. In summary, this study proposes a biomimetic strategy for fabricating structured and functionalized 3D bioprinted scaffolds for vascularized bone tissue regeneration.

Bilateral asymmetry of bone density adjacent to pelvic sarcomas: A retrospective study using computed tomography

Limb-salvaging hemipelvectomy surgeries involving allograft or custom prosthesis reconstruction require high quality remaining pelvic bone for adequate device fixation. Modeling studies of custom pelvis prosthesis designs typically mirror contralateral pelvic bone material properties to the ipsilateral pelvis. However, the extent of bone material property and geometric symmetry, and thus the appropriateness of mirroring, remains unknown and should be considered when designing or analyzing the performance of pelvic prostheses. This study investigates preoperative differences between ipsilateral and contralateral pelvic bone for patients with a pelvic sarcoma. Computed tomography (CT) data were obtained retrospectively from eight patients with a pelvic sarcoma. Subject-specific computational models of the pelvic bones were constructed from the CT data. Bilateral asymmetry of bone material properties and cross-sectional areas between the ipsilateral and contralateral hemipelvis were quantified at points adjacent to the pelvic sarcoma. Large bilateral asymmetry (>20%) in trabecular but not cortical bone density was observed within 20 mm of the tumor location. Differences in trabecular bone density typically declined with increased distance from the tumor. The greatest bilateral difference in cross-sectional area occurred within 10 mm of the tumor boundary for three patients and within 40 mm from the tumor site for four patients. Our results suggest that pelvic sarcomas can cause significant bilateral asymmetries in trabecular bone density for patients with a pelvic sarcoma. These differences should be taken into account when designing custom implants for this patient population.

Structural analysis of hollow versus solid-stemmed shoulder implants of proximal humeri with different bone qualities

Stress shielding of the proximal humerus following total shoulder arthroplasty (TSA) can promote unfavorable bone remodeling, especially for osteoporotic patients. The objective of this finite element (FE) study was to determine if a hollow, rather than solid, titanium stem can mitigate this effect for healthy, osteopenic, and osteoporotic bone. Using a population-based model of the humerus, representative average healthy, osteopenic, and osteoporotic humerus FE models were created. For each model, changes in bone and implant stresses following TSA were evaluated for different loading scenarios and compared between solid versus hollow-stemmed implants. For cortical bone, using an implant decreased von Mises stress with respect to intact values up to 34.4%, with a more pronounced effect at more proximal slices. In the most proximal slice, based on changes in strain energy density, hollow-stemmed implants outperformed solid-stemmed ones through reducing cortical bone volume with resorption potential by 11.7% ± 2.1% (p = .01). For cortical bone in this slice, the percentage of bone with resorption potential for the osteoporotic bone was greater than the healthy bone by 8.0% ± 1.4% using the hollow-stemmed implant (p = .04). These results suggest a small improvement in bone-implant mechanics using hollow-stemmed humeral implants and indicate osteoporosis could exacerbate stress shielding to some extent. The hollow stems maintained adequate strength and using even thinner walls may further reduce stress shielding. After further developing these models, future studies could yield optimized implant designs tuned for varying bone qualities.

Experimental evaluation of the primary fixation stability of uncemented ceramic hip resurfacing implants

Hip resurfacing arthroplasty is associated with increased frictional moments compared to standard heads owing to their large diameter. High frictional moments may harbor the risk of the implant loosening if the frictional moments exceed the fixation stability of the hip resurfacing arthroplasty. Therefore, the aim of this experimental study was to evaluate the fixation stability of ceramic hip resurfacing implants through a turn-off test. The test specimens, made of alumina toughened zirconia (ATZ) ceramics with an inner titanium-coated surface and square base bodies for better application to the test setup, were pushed on artificial bone materials until a predefined seating depth was achieved. Thereafter, the specimens were turned off from the artificial bone material by using a lever-arm and the turn-off moments were calculated. The density of the artificial bone material utilized (15 and 25 pcf), the press-fit (0.4 and 0.8 mm) and the size of the test specimens varied. The push-on forces ranged from 0.6 ± 0.1 kN to 5.6 ± 0.5 kN depending on the press-fit and artificial bone material. The turn-off moments relied on the respective press-fit, artificial bone material and size of the specimen. They belonged between the range of 8.5 ± 0.4 Nm and 105.4 ± 0.2 Nm. Most of the previously described frictional moments are lower compared to the turn-off moments determined in this study. However, in the worst-case scenario, the turn-off moments of the hip resurfacing implants may be reduced, especially when the adjacent bone stock has a low mineral density.

Biomechanical duality of fracture healing captured using virtual mechanical testing and validated in ovine bones

Bone fractures commonly repair by forming a bridging structure called callus, which begins as soft tissue and gradually ossifies to restore rigidity to the bone. Virtual mechanical testing is a promising technique for image-based assessment of structural bone healing in both preclinical and clinical settings, but its accuracy depends on the validity of the material model used to assign tissue mechanical properties. The goal of this study was to develop a constitutive model for callus that captures the heterogeneity and biomechanical duality of the callus, which contains both soft tissue and woven bone. To achieve this, a large-scale optimization analysis was performed on 2363 variations of 3D finite element models derived from computed tomography (CT) scans of 33 osteotomized sheep under normal and delayed healing conditions. A piecewise material model was identified that produced high absolute agreement between virtual and physical tests by differentiating between soft and hard callus based on radiodensity. The results showed that the structural integrity of a healing long bone is conferred by an internal architecture of mineralized hard callus that is supported by interstitial soft tissue. These findings suggest that with appropriate material modeling, virtual mechanical testing is a reliable surrogate for physical biomechanical testing.

Low-Modulus PMMA Has the Potential to Reduce Stresses on Endplates after Cement Discoplasty

Cement discoplasty has been developed to treat patients with advanced intervertebral disc degeneration. In discoplasty, poly(methylmethacrylate) (PMMA) bone cement is injected into the disc, leading to reduced pain and certain spinal alignment correction. Standard PMMA-cements have much higher elastic modulus than the surrounding vertebral bone, which may lead to a propensity for adjacent fractures. A PMMA-cement with lower modulus might be biomechanically beneficial. In this study, PMMA-cements with lower modulus were obtained using previously established methods. A commercial PMMA-cement (V-steady^®, G21 srl) was used as control, and as base cement. The low-modulus PMMA-cements were modified by 12 vol% (LA12), 16 vol% (LA16) and 20 vol% (LA20) linoleic acid (LA). After storage in 37 °C PBS from 24 h up to 8 weeks, specimens were tested in compression to obtain the material properties. A lower E-modulus was obtained with increasing amount of LA. However, with storage time, the E-modulus increased. Standard and low-modulus PMMA discoplasty were compared in a previously developed and validated computational lumbar spine model. All discoplasty models showed the same trend, namely a substantial reduction in range of motion (ROM), compared to the healthy model. The V-steady model had the largest ROM-reduction (77%), and the LA20 model had the smallest (45%). The average stress at the endplate was higher for all discoplasty models than for the healthy model, but the stresses were reduced for cements with higher amounts of LA. The study indicates that low-modulus PMMA is promising for discoplasty from a mechanical viewpoint. However, validation experiments are needed, and the clinical setting needs to be further considered.

Finite Element Evaluation of the Femoral Neck System as Prophylactic Fixation to Prevent Contralateral Hip Fractures

Introduction

Hip fractures cause significant morbidity and mortality for geriatric patients, and incidence is increasing as the population ages. Following a primary hip fracture, up to 20% may suffer a contralateral hip fracture within 5 years despite fracture risk reduction measures, including fall prevention and osteoporosis pharmacologic treatment. The aim of this study is to assess whether insertion of the Femoral Neck System (Depuy Synthes, West Chester, PA) into the contralateral proximal femur may strengthen the bone and decrease the incidence of contralateral hip fractures.

Materials and Methods

ScanIP, an image processing software was used to produce 3-dimensional models of a cadaver femur with the implanted device. Models were meshed and exported to Abaqus for finite element analysis to evaluate the device’s ability to reduce stress in the proximal femur. Results were analyzed for element-wise volume and von-Mises stresses.

Results

The implant reduced peak stress and bone failure at all levels of bone quality. Specifically in osteoporotic bone, the implant decreased peak stress by 27%, proximal femur trabecular bone failure by 5% and cortical bone failure by 100% in the femoral neck.

Conclusions

Our results from computer generated finite element analyses indicate that the Femoral Neck System may strengthen an osteoporotic proximal femur in the event of a lateral fall. Further investigation with expanded finite element analysis and cadaveric biomechanical studies are needed to validate these results.

Subject specific finite element modelling of periprosthetic femoral fractures in different load cases

Periprosthetic femoral fractures (PFF) around total hip replacements are one of the biggest challenges for orthopaedic surgeons. To understand the risk factors and formation of these fractures, the development of a reliable finite element (FE) model incorporating bone failure is essential. Due to the anisotropic and complex hierarchical structure of bone, the mechanical behaviour under large strains is difficult to predict. In this study, a state-of-the-art subject specific FE modelling technique for bone is utilised to generate and investigate PFF. A bilinear constitutive law is applied to bone tissue in subject specific FE models of five human femurs which are virtually implanted with a straight hip stem to numerically analyse PFF. The material parameters of the models are expressed as a function of bone ash density and mapped node wise to the FE mesh. In this way the subject specific, heterogeneous structure of bone is mimicked. For material mapping of the parameters, computed tomography (CT) images of the original fresh-frozen femurs are used. Periprosthetic fractures are generated by deleting elements on the basis of a critical plastic strain failure criterion. The models are analysed under physiological and clinically relevant conditions in two different load cases re-enacting stumbling and a sideways fall on the hip. The results of the analyses are quantified with experimental data from previous work. With regard to fracture pattern, stiffness and failure load the simulations of the load case stumbling delivered the most stable and accurate results. In general, mapping of material properties was found to be an appropriate way to reproduce PFF with finite element models.

Bone Fractures Numerical Analysis in a Femur Affected by Osteogenesis Imperfecta

This work presents a non-invasive methodology to obtain a three-dimensional femur model of three-year-old infants affected with Osteogenesis Imperfecta (OI) type III. DICOM^® Files of a femur were processed to obtain a finite element model to assess the transverse, the oblique, and the comminuted fractures. The model is evaluated under a normal walking cycle. The loads applied were considered the most critical force generated on the normal walking cycle, and the analyses considered anisotropic bone conditions. The outcome shows stress concentration areas in the central zone of the diaphysis of the femur, and the highest levels of stress occur in the case of the comminuted fracture, while the transverse fracture presents the lowest values. Thus, the method can be helpful for determining the bone fracture behavior of certain pathologies, such as osteogenesis imperfecta, osteopenia, and osteoporosis.

Finite Element Analysis of Cannulated Screws as Prophylactic Intervention of Hip Fractures

Introduction

The frequency of hip fractures, a major cause of morbidity and mortality for geriatric patients, is expected to increase exponentially in the next few decades. The aim of this study is to assess the ability of stainless-steel cannulated screws to reduce the risk of a femoral neck fracture, if placed prophylactically prior to a fall.

Materials and Methods

We created finite element models from computed tomography (CT) scan-based 3D models of a geriatric patient through 3D-image processing and model generation software. We used linear finite element simulations to analyze the effect of cannulated screws in the proximal femur in single-leg stance and lateral fall, which were processed for peak von Mises stresses and element failure.

Findings

Prophylactically placed cannulated screws significantly reduced failure in an osteoporotic proximal femur undergoing lateral fall. Three implanted screws in an inverted triangle formation decreased proximal femoral trabecular failure by 21% and cortical failure by 5%. This reduction in failure was achieved with a 55% decrease in femoral neck failure and 14% in lateral cortex failure.

Conclusion

Our results indicate that cannulated hip screws in an inverted triangle formation may strengthen an osteoporotic proximal femur in the event of a lateral fall. Mechanical testing on cadaveric or composite models is required to validate these results.

Cement augmentation of the proximal femoral nail antirotation for the treatment of two intertrochanteric fractures - a comparative finite element study

BACKGROUND

There are concerns regarding initial stability and cutout effect in proximal femoral nail antirotation (PFNA) treating intertrochanteric fractures. No study have used finite element analysis (FEA) to investigate the biomechanics. This study aimed to compare the cutout effect, stress and displacement between stable (AO31-A1.3) and unstable (AO31-A2.2) intertrochanteric fractures treated by cement augmented PFNA.

METHODS

Four femoral finite element models (FEMs) were constructed and tested under the maximum loading during walking. Non-augmented and augmented PFNA in two different intertrochanteric fractures were respectively simulated, assuming Tip Apex Distance (TAD) < 25 mm within each FEM. The cutout effect, stress and displacement between femur and PFNA were compared in each condition.

RESULTS

Cutout effect was observed in both non-augmented femoral head and was more apparently in unstable intertrochanteric fracture model. After reinforced by bone cement, no cutout effect occurred in two models. Stress concentration were observed on medial part of intertrochanteric region and the proximal part of helical blade before augmented while were observed on femoral shaft and the conjunction between blade and nail after augmented in both FEMs. Displacement mainly appeared on femoral head and the helical blade tip before augmented while distributed moderately on intertrochanteric region and the upper part of nail after augmented in both FEMs. The maximum stress and displacement value of femur decreased both in stable and unstable model after augmented but was more significantly in the unstable one. The maximum stress and displacement value of PFNA increased both in stable and unstable model after augmented but was more significantly in the unstable one.

CONCLUSION

Our FEA study indicated that the cement augmentation of the PFNA biomechanically enhances the cutout resistance in intertrochanteric fracture, this procedure is especially efficient for the unstable intertrochanteric fracture.

Density and mechanical properties of vertebral trabecular bone-A review

Being able to predict the mechanical properties of vertebrae in patients with osteoporosis and other relevant pathologies is essential to prevent fractures and to develop the most favorable fracture treatments. Furthermore, a reliable prediction is important for developing more patient- and pathology-specific biomaterials. A plethora of studies correlating bone density to mechanical properties has been reported; however, the results are variable, due to a variety of factors, including anatomical site and methodological differences. The aim of this study was to provide a comprehensive literature review on density and mechanical properties of human vertebral trabecular bone as well as relationships found between these properties. A literature search was performed to include studies, which investigated mechanical properties and bone density of trabecular bone. Only studies on vertebral trabecular bone tissue, reporting bone density or mechanical properties, were kept. A large variation in reported vertebral trabecular bone densities, mechanical properties, and relationships between the two was found, as exemplified by values varying between 0.09 and 0.35 g/cm3 for the wet apparent density and from 0.1 to 976 MPa for the elastic modulus. The differences were found to reflect variations in experimental and analytical processes that had been used, including testing protocol and specimen geometry. The variability in the data decreased in studies where bone tissue testing occurred in a standardized manner (eg, the reported differences in average elastic modulus decreased from 400% to 10%). It is important to take this variability into account when analyzing the predictions found in the literature, for example, to calculate fracture risk, and it is recommended to use the models suggested in the present review to reduce data variability.

Bone Organic-Inorganic Phase Ratio Is a Fundamental Determinant of Bone Material Quality

Background

Bone mineral density is widely used by clinicians for screening osteoporosis and assessing bone strength. However, its effectiveness has been reported unsatisfactory. In this study, it is demonstrated that bone organic-inorganic phase ratio is a fundamental determinant of bone material quality measured by stiffness, strength, and toughness.

Methods and Results

Two-hundred standard bone specimens were fabricated from bovine legs, with a specialized manufacturing method that was designed to reduce the effect of bone anisotropy. Bone mechanical properties of the specimens, including Young's modulus, yield stress, peak stress, and energy-to-failure, were measured by mechanical testing. Organic and inorganic mass contents of the specimens were then determined by bone ashing. Bone density and organic-inorganic phase ratio in the specimens were calculated. Statistical methods were applied to study relationships between the measured mechanical properties and the organic-inorganic phase ratios. Statistical characteristics of organic-inorganic phase ratios in the specimens with top material quality were investigated. Bone organic-inorganic phase ratio had strong Spearman correlation with bone material properties. Bone specimens that had the highest material quality had a very narrow scope of organic-inorganic phase ratio, which could be considered as the “optimal” ratio among the tested specimens.

Conclusion

Bone organic-inorganic phase ratio is a fundamental determinant of bone material quality. There may exist an “optimal” ratio for the bone to achieve top material quality. Deviation from the “optimal” ratio is probably the fundamental cause of various bone diseases. This study suggests that bone organic-inorganic phase ratio should be considered in clinical assessment of fracture risk.

Biomechanics of immature human cortical bone: A systematic review

The whole bone geometry, microstructure, and mechanical properties of mature human bone are widely reported; however, immature bone (0-18 years) has not been similarly robustly characterized. There is an interest in analyzing and predicting the mechanical loading conditions associated with long bone diaphyseal fractures attributed to trauma in children. Thus, understanding the mechanical properties of immature bone in a temporal reference frame is an essential first step to understand diaphyseal fractures of pediatric long bones. The purpose of this systematic review was to ask, what is the state of knowledge regarding the 1) evolution of whole bone geometry and microstructure of immature pediatric bone as a function of maturation and 2) cortical bone density and experimental quasi-static mechanical properties at the tissue level in the diaphyseal region of immature pediatric long bones? The systematic search yielded 36 studies of the whole bone geometry, microstructure, and mechanical properties of immature pediatric long bones. The elastic modulus, yield stress, and ultimate stress were shown to generally increase with maturation, whereas the yield strain was approximately invariant; however, the specific year-to-year progression of these properties could not be characterized from the limited studies available. The results of this systematic search indicate there is a dearth of knowledge associated with the biomechanics of cortical bone from immature pediatric long bones; it also provides a basis for computational studies of immature human long bones. Additional biomechanical studies of immature human bone are necessary to develop a robust catalogue, which can be used in broad applications to understand fracture mechanics, bone pathologies, and athletic injury in the pediatric setting.

Limit analysis of human proximal femur

A limit analysis numerical approach oriented to predict the peak/collapse load of human proximal femur, under two different loading conditions, is presented. A yield criterion of Tsai-Hu-type, expressed in principal stress space, is used to model the orthotropic bone tissues. A simplified human femur 3D model is envisaged to carry on numerical simulation of in-vitro tests borrowed from the relevant literature and to reproduce their findings. A critical discussion, together with possible future developments, is presented.

Single-level subject-specific finite element model can predict fracture outcomes in three-level spine segments under different loading rates

Osteoporosis-related vertebral compression fracture can occur under normal physiological activities. Bone metastasis is another source of vertebral fracture. Different loading rates, either high-energy traumas such as falls or low-energy traumas under normal physiological activities, can result in different fracture outcomes. The aim of the current study was to develop a quantitative computed tomography-based finite element analysis (QCT/FEA) technique for single vertebral bodies to predict fracture strength of three-level spine segments. Developed QCT/FEA technique was also used to characterize vertebral elastic moduli at two loading rates of 5 mm/min, representing a physiologic loading condition, and 12000 mm/min, representing a high-energy trauma. To this end, a cohort of human spine segments divided into three groups of intact, defect, and augmented were mechanically tested to fracture; then, experimental stiffness and fracture strength values were measured. Outcomes of this study showed no significant difference between the elastic modulus equations at the two testing speeds. Areal bone mineral density measured by dual x-ray absorptiometry (DXA/BMD) explained only 53% variability (R² = 0.53) in fracture strength outcomes. However, QCT/FEA could explain 70% of the variability (R² = 0.70) in experimentally measured fracture strength values. Adding disk degeneration grading, testing speed, and sex to QCT/FEA-estimated fracture strength values further increased the performance of our statistical model by 14% (adjusted R² of 0.84 between the prediction and experimental fracture forces). In summary, our results indicated that a single-vertebra model, which is computationally less expensive and more time efficient, is capable of estimating fracture outcomes with acceptable performance (range: 70-84%).