Measurement of fracture callus material properties via nanoindentation

Quality assessment of regenerated bone in intraosseous and intramuscular scaffolds by spectroscopy and nanoindentation

The efficiency of synthetic bone grafts can be evaluated either in osseous sites, to analyze osteoconduction or ectopically, in intramuscular or subcutaneous sites, to assess osteoinduction. Bone regeneration is usually evaluated in terms of the presence and quantity of newly formed bone, but little information is normally provided on the quality of this bone. Here, we propose a novel approach to evaluate bone quality by the combined use of spectroscopy techniques and nanoindentation. Calcium phosphate scaffolds with different architectures, either foamed or 3D-printed, that were implanted in osseous or intramuscular defects in Beagle dogs for 6 or 12 weeks were analyzed. ATR-FTIR and Raman spectroscopy were performed, and mineral-to-matrix ratio, crystallinity, and mineral and collagen maturity were calculated and mapped for the newly regenerated bone and the mature cortical bone from the same specimen. For all the parameters studied, the newly-formed bone showed lower values than the mature host bone. Hardness and elastic modulus were determined by nanoindentation and, in line with what was observed by spectroscopy, lower values were observed in the regenerated bone than in the cortical bone. While, as expected, all techniques pointed to an increase in the maturity of the newly-formed bone between 6 and 12 weeks, the bone found in the intramuscular samples after 12 weeks presented lower mineralization than the intraosseous counterparts. Moreover, scaffold architecture also played a role in bone maturity, with the foamed scaffolds showing higher mineralization and crystallinity than the 3D-printed scaffolds after 12 weeks.

Surfactant-assisted photo-crosslinked silk fibroin sponges: A versatile platform for the design of bone scaffolds

Critical size bone defects cannot heal without aid and current clinical approaches exhibit some limitations, underling the need for novel solutions. Silk fibroin, derived from silkworms, is widely utilized in tissue engineering and regenerative medicine due to its remarkable properties, making it a promising candidate for bone tissue regeneration in vitro and in vivo. However, the clinical translation of silk-based materials requires refinements in 3D architecture, stability, and biomechanical properties. In earlier research, improved mechanical resistance and stability of chemically crosslinked methacrylate silk fibroin (Sil-Ma) sponges over physically crosslinked counterparts were highlighted. Furthermore, the influence of photo-initiator and surfactant concentrations on silk properties was investigated. However, the characterization of sponges with Sil-Ma solution concentrations above 10 % (w/V) was hindered by production optimization challenges, with only cell viability assessed. This study focuses on the evaluation of methacrylate sponges' suitability as temporal bone tissue regeneration scaffolds. Sil-Ma sponge fabrication at a fixed concentration of 20 % (w/V) was optimized and the impact of photo-initiator (LAP) concentrations and surfactant (Tween 80) presence/absence was studied. Their effects on pore formation, silk secondary structure, mechanical properties, and osteogenic differentiation of hBM-MSCs were investigated. We demonstrated that, by tuning silk sponges' composition, the optimal combination boosted osteogenic gene expression, offering a strategy to tailor biomechanical properties for effective bone regeneration. Utilizing Design of Experiment (DoE), correlations between sponge composition, porosity, and mechanical properties are established, guiding tailored material outcomes. Additionally, correlation matrices elucidate the microstructure's influence on gene expressions, providing insights for personalized approaches in bone tissue regeneration.

Application of Antioxidant Compounds in Bone Defect Repair

Bone defects caused by trauma, tumor resection, and infections are significant clinical challenges. Excessive reactive oxygen species (ROS) usually accumulate in the defect area, which may impair the function of cells involved in bone formation, posing a serious challenge for bone repair. Due to the potent ROS scavenging ability, as well as potential anti-inflammatory and immunomodulatory activities, antioxidants play an indispensable role in the maintenance and protection of bone health and have gained increasing attention in recent years. This narrative review aims to give an overview of the main research directions on the application of antioxidant compounds in bone defect repair over the past decade. In addition, the positive effects of various antioxidants and their biomaterial delivery systems in bone repair are summarized to provide new insights for exploring antioxidant-based strategies for bone defect repair.

A Gellan Gum, Polyethylene Glycol, Hydroxyapatite Composite Scaffold with the Addition of Ginseng Derived Compound K with Possible Applications in Bone Regeneration

Engineered bone scaffolds should mimic the natural material to promote cell adhesion and regeneration. For this reason, natural biopolymers are becoming a gold standard in scaffold production. In this study, we proposed a hybrid scaffold produced using gellan gum, hydroxyapatite, and Poly (ethylene glycol) within the addition of the ginseng compound K (CK) as a candidate for bone regeneration. The fabricated scaffold was physiochemically characterized. The morphology studied by scanning electron microscopy (SEM) and image analysis revealed a pore distribution suitable for cells growth. The addition of CK further improved the biological activity of the hybrid scaffold as demonstrated by the MTT assay. The addition of CK influenced the scaffold morphology, decreasing the mean pore diameter. These findings can potentially help the development of a new generation of hybrid scaffolds to best mimic the natural tissue.

Effect of uncertain clinical conditions on the early healing and stability of distal radius fractures

BACKGROUND AND OBJECTIVES

The healing outcomes of distal radius fracture (DRF) treated with the volar locking plate (VLP) depend on surgical strategies and postoperative rehabilitation. However, the accurate prediction of healing outcomes is challenging due to a range of certainties related to the clinical conditions of DRF patients, including fracture geometry, fixation configuration, and physiological loading. The purpose of this study is to investigate the influence of uncertainty and variability in fracture/fixation parameters on the mechano-biology and biomechanical stability of DRF, using a probabilistic numerical approach based on the results from a series of experimental tests performed in this study.

METHODS

Six composite radius sawboneses fitted with titanium VLP (VLP 2.0, Austofix) were loaded to failure at a rate of 2 N/s. The testing results of the elastic and plastic behaviour of the VLP were used as inputs for a probabilistic-based computational model of DRF, which simulated mechano-regulated tissue differentiation and fixation elastic capacity at the fracture site. Finally, the probability of success in early indirect healing and fracture stabilisation was predicted.

RESULTS

The titanium VLP is a strong and ductile fixation whose flexibility and elastic capacity are governed by flexion working length and bone-to-plate distance, respectively. A fixation with optimised designs and configurations is critical to mechanically stabilising the early fracture site. Importantly, the uncertainty and variability in fracture/fixation parameters could compromise early DRF healing. The physiological loading uncertainty is the most adverse factor, followed by the negative impact of uncertainty in fracture geometry.

CONCLUSIONS

The VRP 2.0 fixation made of grade II titanium is a desirable fixation that is strong enough to resist irreparable deformation during early recovery and is also ductile to deform plastically without implant failure at late rehabilitation.

Influence of Crosslinking Methods on Biomimetically Mineralized Collagen Matrices for Bone-like Biomaterials

To assist in bone defect repair, ideal bone regeneration scaffolds should exhibit good osteoconductivity and osteoinductivity, but for load-bearing applications, they should also have mechanical properties that emulate those of native bone. The use of biomimetic processing methods for the mineralization of collagen fibrils has resulted in interpenetrating composites that mimic the nanostructure of native bone; however, closely matching the mechanical properties of bone on a larger scale is something that is still yet to be achieved. In this study, four different collagen crosslinking methods (EDC-NHS, quercetin, methacrylated collagen, and riboflavin) are compared and combined with biomimetic mineralization via the polymer-induced liquid-precursor (PILP) process, to obtain bone-like collagen-hydroxyapatite composites. Densified fibrillar collagen scaffolds were fabricated, crosslinked, and biomimetically mineralized using the PILP process, and the effect of each crosslinking method on the degree of mineralization, tensile strength, and modulus of the mineralized scaffolds were analyzed and compared. Improved modulus and tensile strength values were obtained using EDC-NHS and riboflavin crosslinking methods, while quercetin and methacrylated collagen resulted in little to no increase in mechanical properties. Decreased mineral contents appear to be necessary for retaining tensile strength, suggesting that mineral content should be kept below a percolation threshold to optimize properties of these interpenetrating nanocomposites. This work supports the premise that a combination of collagen crosslinking and biomimetic mineralization methods may provide solutions for fabricating robust bone-like composites on a larger scale.

A standardized extract of Coleus forskohlii root protects rats from ovariectomy-induced loss of bone mass and strength, and impaired bone material by osteogenic and anti-resorptive mechanisms

INTRODUCTION

In obese humans, Coleus forskohlii root extract (CF) protects against weight gain owing to the presence of forskolin, an adenylate cyclase (AC) activator. As AC increases intracellular cyclic adenosine monophosphate (cAMP) levels in osteoblasts that has an osteogenic effect, we thus tested the skeletal effects of a standardized CF (CFE) in rats.

METHODS

Concentrations of forskolin and isoforskolin were measured in CFE by HPLC. CFE and forskolin (the most abundant compound present in CFE) were studied for their osteogenic efficacy in vitro by alkaline phosphatase (ALP), cAMP and cyclic guanosine monophosphate (cGMP) assays. Femur osteotomy model was used to determine the osteogenic dose of CFE. In growing rats, CFE was tested for its osteogenic effect in intact bone. In adult ovariectomized (OVX) rats, we assessed the effect of CFE on bone mass, strength and material. The effect of forskolin was assessed in vivo by measuring the expression of osteogenic genes in the calvarium of rat pups.

RESULTS

Forskolin content in CFE was 20.969%. CFE increased osteoblast differentiation and intracellular cAMP and cGMP levels in rat calvarial osteoblasts. At 25 mg/kg (half of human equivalent dose), CFE significantly enhanced calcein deposition at the osteotomy site. In growing rats, CFE promoted modeling-directed bone formation. In OVX rats, CFE maintained bone mass and microarchitecture to the level of sham-operated rats. Moreover, surface-referent bone formation in CFE treated rats was significantly increased over the OVX group and was comparable with the sham group. CFE also increased the pro-collagen type-I N-terminal propeptide: cross-linked C-telopeptide of type-I collagen (PINP : CTX-1) ratio over the OVX rats, and maintained it to the sham level. CFE treatment decreased the OVX-induced increases in the carbonate-to-phosphate, and carbonate-to-amide-I ratios. CFE also prevented the OVX-mediated decrease in mineral crystallinity. Nanoindentation parameters, including modulus and hardness, were decreased by OVX but CFE maintained these to the sham levels. Forskolin stimulated ALP, cAMP and cGMP in vitro and upregulated osteogenic genes in vivo.

CONCLUSION

CFE, likely due to the presence of forskolin displayed a bone-conserving effect via osteogenic and anti-resorptive mechanisms resulting in the maintenance of bone mass, microarchitecture, material, and strength.

Evaluation of bone healing process after intramedullary nailing for femoral shaft fracture by quantitative computed tomography-based finite element analysis

BACKGROUND

There is no proven method for quantitative evaluation of bone healing progress or decision to remove the nail after intramedullary nailing for femoral shaft fractures. Finite element analysis has become commonly utilized in bone analysis, but it may also be used to evaluate callus. The goal of this study was to use quantitative CT-based finite element analysis to assess the bone healing process and predict bone strength with the nail removed.

METHODS

Quantitative CT-based finite element analysis was conducted on CT images from patients who had intramedullary nailing after a femoral shaft fracture at 6, 12, and 15 months postoperatively. The failure risk of the callus was evaluated with maximal load throughout the gait cycle. The tensile failure ratio was calculated using the volume ratio of the callus element with a tensile failure risk ≥100%. A virtual model with the nail removed was built for bone strength study, and the strength was calculated using the displacement-load curve.

FINDINGS

The tensile failure ratio reduced with time, reaching 11.6%, 2.6%, and 0.5% at 6, 12, and 15 months postoperatively, respectively, consistent with bone healing inferred from imaging results. At 15 months, the bone strength at nail removal grew to 212, 2670, and 3385 N, surpassing the healthy side's 2766 N.

INTERPRETATION

Quantitative CT-based finite element analysis enables mechanical assessment during the bone healing process and is expected to be applied to the selection of revision surgery. It is also applicable to the nail removal decision.

Towards mechanobiologically optimized mandible reconstruction: CAD/CAM miniplates vs. reconstruction plates for fibula free flap fixation: A finite element study

Due to their advantages in applicability, patient-specific (CAD/CAM) reconstruction plates are increasingly used in fibula free flap mandible reconstruction. In addition, recently, CAD/CAM miniplates, with further advantages in postoperative management, have been introduced. However, biomechanical conditions induced by CAD/CAM systems remain partially unknown. This study aimed to evaluate the primary fixation stability of CAD/CAM fixators. For a patient-specific scenario, the biomechanical conditions induced in a one segmental fibula free flap stabilized using either a CAD/CAM reconstruction plate or CAD/CAM miniplates were determined using finite element analysis. The main output parameters were the strains between intersegmental bone surfaces and stresses in the fixation systems due to different biting scenarios. CAD/CAM miniplates resulted in higher mechanical strains in the mesial interosseous gap, whereas CAD/CAM reconstruction plate fixation resulted in higher strains in the distal interosseous gap. For all investigated fixation systems, stresses in the fixation systems were below the material yield stress and thus material failure would not be expected. While the use of CAD/CAM miniplates resulted in strain values considered adequate to promote bone healing in the mesial interosseous gap, in the distal interosseous gap CAD/CAM reconstruction plate fixation might result in more beneficial tissue straining. A mechanical failure of the fixation systems would not be expected.

Smart fracture plate for quantifying fracture healing: Preliminary efficacy in a biomechanical model

The diagnosis of fracture nonunion following plate osteosynthesis is subjective and frequently ambiguous. Initially following osteosynthesis, loads applied to the bone are primarily transmitted through the plate. However, as callus stiffness increases, the callus is able to bear load proportional to its stiffness while forces through the plate decrease. The purpose of this study was to use a "smart" fracture plate to distinguish between phases of fracture healing by measuring forces transmitted through the plate. A wireless force sensor and small adapter were placed on the outside of a distal femoral locking plate. The adapter converts the slight bending of the plate under axial load into a transverse force which is measurable by the sensor. An osteotomy was created and then plated in the distal femur of biomechanical Sawbones. Specimens were loaded to simulate single-leg stance first with the osteotomy defect empty (acute healing), then sequentially filled with silicone (early callus) and then polymethyl methacrylate (hard callus). There was a strong correlation between applied axial load and force measured by the "smart" plate. Data demonstrate statistically significant differences between each phase of healing with as little as 150 N of axial load applied to the femur. Forces measured in the plate were significantly different between acute (100%), early callus (66.4%), and hard callus (29.5%). This study demonstrates the potential of a "smart" fracture plate to distinguish between phases of healing. These objective data may enable early diagnosis of nonunion and enhance outcomes for patients.

Mechanical Characterization at the Microscale of Mineralized Bone Callus after Bone Lengthening

Distraction osteogenesis (DO) involves several processes to form an organized distracted callus. While bone regeneration during DO has been widely described, no study has yet focused on the evolution profile of mechanical properties of mineralized tissues in the distracted callus. The aim of this study was therefore to measure the elastic modulus and hardness of calcified cartilage and trabecular and cortical bone within the distracted callus during the consolidation phase. We used a microindentation assay to measure the mechanical properties of periosteal and endosteal calluses; each was subdivided into two regions. Histological sections were used to localize the tissues. The results revealed that the mechanical properties of calcified cartilage did not evolve over time. However, trabecular bone showed temporal variation. For elastic modulus, in three out of four regions, a similar evolution profile was observed with an increase and decrease over time. Concerning hardness, this evolves differently depending on the location in the distracted callus. We also observed spatial changes in between regions. A first duality was apparent between regions close to the native cortices and the central area, while latter differences were seen between periosteal and endosteal calluses. Data showed a heterogeneity of mechanical properties in the distracted callus with a specific mineralization profile.

Novel approach to estimate distraction forces in distraction osteogenesis and application in the human lower leg

PURPOSE

In distraction osteogenesis (DO) of long bones, new bone tissue is distracted to lengthen limbs or reconstruct bone defects. However, mechanical boundary conditions in human application such as arising forces are mainly based on limited empirical data. Our aim was the numerical determination of the callus distraction force (CDF) and the total distraction force (TDF) during DO in the tibia of adults to advance the understanding of callus tissue behavior and optimize DO procedures.

METHOD

We implemented a mathematical model based on an animal experiment to enable the calculation of forces arising while distracting callus tissue, excluding the influence of surrounding soft tissue (muscles, skin etc.). The CDF progression for the distraction period was calculated using the implemented model and varying distraction parameters (initial gap, area, step size, time interval, length). Further, we estimated the CDF based on reported forces in humans and compared the results to our model predictions. In addition, we calculated the TDF based on our CDF predictions in combination with reported resisting forces due to soft tissue presence in human cadavers. Finally, we compared the progressions to in vivo TDF measurements for validation.

RESULTS

Due to relaxation, a peak and resting CDF is observable for each distraction step. Our biomechanical results show a non-linear degressive increase of the resting and peak CDF at the beginning and a steady non-linear increase thereafter. The calculated resting and peak CDF in the tibial metaphysis ranged from 0.00075 to 0.0089 N and 0.22-2.6 N at the beginning as well as 20-25 N and 70-75 N at the end of distraction. The comparison to in vivo data showed the plausibility of our predictions and resulted in a 10-33% and 10-23% share of resting CDF in the total resting force for bone transport and elongation, respectively. Further, the percentage of peak CDF in total peak force was found to be 29-58% and 27-55% for bone transport and elongation, respectively. Moreover, our TDF predictions were valid based on the comparison to in vivo forces and resulted in a degressive increase from 6 to 125 N for the peak TDF and from 5 to 76 N for the resting TDF.

CONCLUSION

Our approach enables the estimation of forces arising due to the distraction of callus tissue in humans and results in plausible force progressions as well as absolute force values for the callus distraction force during DO. In combination with measurements of resisting forces due to the presence of soft tissue, the total distraction force in DO may also be evaluated. We thus propose the application of this method to approximate the behavior of mechanical callus properties during DO in humans as an alternative to in vivo measurements.

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.

Time-Dependent Collagen Fibered Structure in the Early Distraction Callus: Imaging Characterization and Mathematical Modeling

Collagen is a ubiquitous protein present in regenerating bone tissues that experiences multiple biological phenomena during distraction osteogenesis until the deposition of phosphate crystals. This work combines fluorescence techniques and mathematical modeling to shed light on the mechano-structural processes behind the maturation and accommodation-to-mineralization of the callus tissue. Ovine metatarsal bone calluses were analyzed through confocal images at different stages of the early distraction osteogenesis process, quantifying the fiber orientation distribution and mean intensity as fiber density measure. Likewise, a mathematical model based on the experimental data was defined to micromechanically characterize the apparent stiffening of the tissue within the distracted callus. A reorganization of the fibers around the distraction axis and increased fiber density were found as the bone fragments were gradually separated. Given the degree of significance between the mathematical model and previous in vivo data, reorganization, densification, and bundle maturation phenomena seem to explain the apparent mechanical maturation observed in the tissue theoretically.

A numerical bone regeneration model incorporating angiogenesis, considering oxygen-induced secretion of vascular endothelial growth factor and vascular remodeling

Angiogenesis is considered playing an important role in bone regeneration. Studies have shown that angiogenesis is affected by biological factors, oxygen tension, and blood flow. In this paper, we propose a bone regeneration model with angiogenesis based on the theories of mechanobiology regulation, vascular network modeling, oxygen-induced secretion of vascular endothelial growth factor (VEGF), and vascular remodeling. The results showed that this model can describe the distribution and concentration of vascular endothelial growth factor induced by oxygen tension during bone regeneration, the growth and remodeling of vascular tissue under the influence of vascular endothelial growth factor and mechanical loading, and the correspondence between vascular tissue and bone regeneration.

Balance Between Mechanical Stability and Mechano-Biology of Fracture Healing Under Volar Locking Plate

The application of volar locking plate (VLP) is promising in the treatment of dorsally comminuted and displaced fracture. However, the optimal balance between the mechanical stability of VLP and the mechanobiology at the fracture site is still unclear. The purpose of this study is to develop numerical models in conjunction with experimental studies to identify the favourable mechanical microenvironment for indirect healing, by optimizing VLP configuration and post-operative loadings for different fracture geometries. The simulation results show that the mechanical behaviour of VLP is mainly governed by the axial compression. In addition, the model shows that, under relatively large gap size (i.e., 3-5 mm), the increase of FWL could enhance chondrocyte differentiation while a large BPD could compromise the mechanical stability of VLP. Importantly, bending moment produced by wrist flexion/extension and torsion moment produced from forearm rotation could potentially hinder endochondral ossification at early stage of healing. The developed model could potentially assist orthopaedic surgeons in surgical pre-planning and designing post-operation physical therapy for treatment of distal radius fractures.

Strategies to Improve Bone Healing: Innovative Surgical Implants Meet Nano-/Micro-Topography of Bone Scaffolds

Successful fracture healing is dependent on an optimal mechanical and biological environment at the fracture site. Disturbances in fracture healing (non-union) or even critical size bone defects, where void volume is larger than the self-healing capacity of bone tissue, are great challenges for orthopedic surgeons. To address these challenges, new surgical implant concepts have been recently developed to optimize mechanical conditions. First, this review article discusses the mechanical environment on bone and fracture healing. In this context, a new implant concept, variable fixation technology, is introduced. This implant has the unique ability to change its mechanical properties from “rigid” to “dynamic” over the time of fracture healing. This leads to increased callus formation, a more homogeneous callus distribution and thus improved fracture healing. Second, recent advances in the nano- and micro-topography of bone scaffolds for guiding osteoinduction will be reviewed, particularly emphasizing the mimicry of natural bone. We summarize that an optimal scaffold should comprise micropores of 50–150 µm diameter allowing vascularization and migration of stem cells as well as nanotopographical osteoinductive cues, preferably pores of 30 nm diameter. Next to osteoinduction, such nano- and micro-topographical cues may also reduce inflammation and possess an antibacterial activity to further promote bone regeneration.

Using Sensitivity Analysis to Develop a Validated Computational Model of Post-operative Calvarial Growth in Sagittal Craniosynostosis

Craniosynostosis is the premature fusion of one or more sutures across the calvaria, resulting in morphological and health complications that require invasive corrective surgery. Finite element (FE) method is a powerful tool that can aid with preoperative planning and post-operative predictions of craniosynostosis outcomes. However, input factors can influence the prediction of skull growth and the pressure on the growing brain using this approach. Therefore, the aim of this study was to carry out a series of sensitivity studies to understand the effect of various input parameters on predicting the skull morphology of a sagittal synostosis patient post-operatively. Preoperative CT images of a 4-month old patient were used to develop a 3D model of the skull, in which calvarial bones, sutures, cerebrospinal fluid (CSF), and brain were segmented. Calvarial reconstructive surgery was virtually modeled and two intracranial content scenarios labeled “CSF present” and “CSF absent,” were then developed. FE method was used to predict the calvarial morphology up to 76 months of age with intracranial volume-bone contact parameters being established across the models. Sensitivity tests with regards to the choice of material properties, methods of simulating bone formation and the rate of bone formation across the sutures were undertaken. Results were compared to the in vivo data from the same patient. Sensitivity tests to the choice of various material properties highlighted that the defined elastic modulus for the craniotomies appears to have the greatest influence on the predicted overall skull morphology. The bone formation modeling approach across the sutures/craniotomies had a considerable impact on the level of contact pressure across the brain with minimum impact on the overall predicated morphology of the skull. Including the effect of CSF (based on the approach adopted here) displayed only a slight reduction in brain pressure outcomes. The sensitivity tests performed in this study set the foundation for future comparative studies using FE method to compare outcomes of different reconstruction techniques for the management of craniosynostosis.

Core-binding factor beta is required for osteoblast differentiation during fibula fracture healing

Background

Growing evidence has implicated core-binding factor beta (Cbfb) as a contributor to osteoblast differentiation, which plays a key role in fracture healing. Herein, we aimed to assess whether Cbfb affects osteoblast differentiation after fibula fracture.

Methods

Initially, we established a Cbfb conditional knockout mouse model for subsequent studies. Immunohistochemical staining was conducted to detect the expression of proliferating cell nuclear antigen (PCNA) and collagen II in the fracture end. Next, we isolated and cultured osteoblasts from specific Cbfb conditional knockout mice for BrdU analysis, alkaline phosphatase (ALP) staining, and von Kossa staining to detect osteoblast viability, differentiation, and mineralization, respectively. Western blot analysis and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) were used to detect the expression of osteoblast differentiation-related genes.

Results

The Cbfb conditional knockout mice exhibited downregulated expression of PCNA and collagen II, reduced ALP activity, and mineralization, as well as diminished expression of osteoblast differentiation-related genes. Further, Cbfb knockout exerted no obvious effects on osteoblast proliferation.

Conclusions

Overall, these results substantiated that Cbfb could promote fibula fracture healing and osteoblast differentiation and thus provided a promising therapeutic target for clinical treatment of fibula fracture.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-021-02410-9.

Biomechanical Assessment of the Validity of Sheep as a Preclinical Model for Testing Mandibular Fracture Fixation Devices

Mandibular fracture fixation and reconstruction are usually performed using titanium plates and screws, however, there is a need to improve current fixation techniques. Animal models represent an important step for the testing of new designs and materials. However, the validity of those preclinical models in terms of implant biomechanics remains largely unknown. In this study, we investigate the biomechanics of the sheep mandible as a preclinical model for testing the mechanical strength of fixation devices and the biomechanical environment induced on mandibular fractures. We aimed to assess the comparability of the biomechanical conditions in the sheep mandible as a preclinical model for human applications of fracture fixation devices and empower analyses of the effect of such defined mechanical conditions on bone healing outcome. We developed 3D finite element models of the human and sheep mandibles simulating physiological muscular loads and three different clenching tasks (intercuspal, incisal, and unilateral). Furthermore, we simulated fractures in the human mandibular body, sheep mandibular body, and sheep mandibular diastema fixated with clinically used titanium miniplates and screws. We compared, at the power stroke of mastication, the biomechanical environment (1) in the healthy mandibular body and (2) at the fracture sites, and (3) the mechanical solicitation of the implants as well as the mechanical conditions for bone healing in such cases. In the healthy mandibles, the sheep mandibular body showed lower mechanical strains compared to the human mandibular body. In the fractured mandibles, strains within a fracture gap in sheep were generally not comparable to humans, while similar or lower mechanical solicitation of the fixation devices was found between the human mandibular body fracture and the sheep mandibular diastema fracture scenarios. We, therefore, conclude that the mechanical environments of mandibular fractures in humans and sheep differ and our analyses suggest that the sheep mandibular bone should be carefully re-considered as a model system to study the effect of fixation devices on the healing outcome. In our analyses, the sheep mandibular diastema showed similar mechanical conditions for fracture fixation devices to those in humans.

Effect of fatigue loading and rest on impact strength of rat ulna

Stress fracture is a common injury among athletes and military personnel and is associated with fatigue-initiated damage and impact loading. The recovery of bending strength has been shown to be a function of the rest days allowed after fatigue loading in rodents and the aim of this study was to investigate if similar results would occur under impact conditions. In this study, cyclic axial compression load was applied in vivo on the right forelimbs while left forelimbs served as controls. Two rest groups were used: one day of rest and seven days of rest. Afterwards, all ulnae were scanned using micro-Computed Tomography followed by impact testing. The micro-CT scan confirmed the formation of woven bone on loaded ulnae after seven days rest. The peak impact force was 37.5% higher in the control (mean = 174.96 ± 33.25 N) specimens compared to the loaded bones (mean = 130.34 ± 22.37 N). Fourier-transformed infrared spectroscopy analyses suggested no significant change of chemical composition in the cortical region between the loaded and control ulnae, but woven bone region had lower carbonate and amide I content than contralateral controls (p < 0.05). We find that cyclic fatigue loading had a negative effect on bone's impact response. Bones that experienced fatigue loading became less stiff, weaker, and more prone to fracture when subjected to impact. The formation of woven bone after seven days of rest did not restore the stiffness upon impact and confirm that rest time is crucial to the recovery of fatigue damage.

Overview of Skeletal Repair (Fracture Healing and Its Assessment)

The study of postnatal skeletal repair is of immense clinical interest. Optimal repair of skeletal tissue is necessary in all varieties of elective and reparative orthopedic surgical treatments. However, the repair of fractures is unique in this context in that fractures are one of the most common traumas that humans experience and are the end-point manifestation of osteoporosis, the most common chronic disease of aging. In the first part of this introduction the basic biology of fracture healing is presented. The second part discusses the primary methodological approaches that are used to examine repair of skeletal hard tissue and specific considerations for choosing among and implementing these approaches.

The investigation of bone fracture healing under intramembranous and endochondral ossification

After trauma, fractured bone starts healing directly through bone union or indirectly through callus formation process. Intramembranous and endochondral ossification are two commonly known mechanisms of indirect healing. The present study investigated the bone fracture healing under intramembranous and endochondral ossification by developing theoretical models in conjunction with performing a series of animal experiments. Using experimentally determined mean bone densities in sheep tibia stabilized by the Locking Compression Plate (LCP) fixation system, the research outcomes showed that intramembranous and endochondral ossification can be described by Hill Function with two unique sets of function parameters in mechanical stimuli mediated fracture healing. Two different thresholds exist within the range of mechanical simulation index which could trigger significant intramembranous and endochondral ossification, with a relatively higher bone formation rate of endochondral ossification than that of intramembranous ossification. Furthermore, the increase of flexibility of the LCP system and the use of titanium LCP could potentially promote uniform bone formation across the fracture gap, ultimately better healing outcomes.

Mechanobiology of Bone Consolidation During Distraction Osteogenesis: Bone Lengthening Vs. Bone Transport

Bone lengthening and bone transport are regeneration processes that commonly rely on distraction osteogenesis, a widely accepted surgical procedure to deal with numerous bony pathologies. Despite the extensive study in the literature of the influence of biomechanical factors, a lack of knowledge about their mechanobiological differences prevents a clinical particularization. Bone lengthening treatments were performed on sheep metatarsus by reproducing the surgical and biomechanical protocol of previous bone transport experiments. Several in vivo monitoring techniques were employed to build an exhaustive comparison: gait analysis, radiographic and CT assessment, force measures through the fixation, or mechanical characterization of the new tissue. A significant initial loss of the bearing capacity, quantified by the ground reaction forces and the limb contact time with the ground, is suffered by the bone lengthening specimens. The potential effects of this anomaly on the musculoskeletal force distribution and the evolution of the bone callus elastic modulus over time are also analyzed. Imaging techniques also seem to reveal lower bone volume in the bone lengthening callus than in the bone transport one, but an equivalent mineralization rate. The simultaneous quantification of biological and mechanical parameters provides valuable information for the daily clinical routine and numerical tools development.

Alternate soaking enables easy control of mineralized collagen scaffold mechanics from nano- to macro-scale

The mechanical properties of biologic scaffolds are critical to cellular interactions and hence functional response within the body. In the case of scaffolds for bone tissue regeneration, engineered scaffolds created by combining collagen with inorganic mineral are increasingly being explored, due to their favourable structural and chemical characteristics. Development of a method for controlling the mechanics of these scaffolds could lead to significant additional advantages by harnessing the intrinsic mechnotransduction pathways of stem cells via appropriate control of scaffold mechanical properties. Here we present a method for controlling the macroscale flexural modulus of mineralized collagen sheets, and the radial indentation modulus of the sheets' constituent collagen fibrils. Scaffolds were created starting with sheets of highly aligned, natively structured collagen fibrils, prepared via cryosectioning of decellularized tendon. Sheets underwent an alternate soaking mineralization procedure, with sequential exposure to citrate-doped calcium and carbonate-containing phosphate solutions, both of which included poly aspartic acid. The extent of scaffold mineralization was controlled via number of repeated mineralization cycles: 0 (unmineralized), 5, 10, and 20 cycles were trialed. Following scaffold preparation, ultrastructure, macroscale flexural modulus, and nanoscale indentation modulus were assessed. Surface architecture studied by SEM, and inspection of individual extracted fibrils by TEM and AFM confirmed that fibrils became increasingly laden with mineral as the number of mineralization cycles increased. Measurements of collagen fibril nanomechanics using AFM showed that the radial modulus of collagen fibrils increased linearly with mineralization cycles completed, from 215 ± 125 MPa for fibrils from unmineralized (0 cycle) scaffolds to 778 ± 302 MPa for fibrils from the 20 mineralization cycle scaffolds. Measurements of scaffold macromechanics via flexural testing also showed a linear increase in flexural modulus with increasing number of mineralization cycles completed, from 18 ± 7 MPa for the 5 cycle scaffolds to 156 ± 50 MPa for the 20 cycle scaffolds. The process detailed herein provides a way to create mineralized collagen scaffolds with easily controllable mechanical properties.

Mechanical Influence of Surrounding Soft Tissue on Bone Regeneration Processes: A Bone Lengthening Study

Bone lengthening is a bone regeneration technique with multiple clinical applications. One of the most common complications of this treatment is the lack of adaptation of the surrounding soft tissue to their extension. A better understanding of the mechanobiology of the tissues involved in distraction osteogenesis would allow better control of the clinical cases. Bone lengthening treatments were performed in vivo in the metatarsus of Merino sheep, measuring the distraction forces by means of an instrumented fixator. The tissue relaxation after distraction was analyzed in this study. A viscoelastic model was also applied to distraction data to assess the mechanical behavior of the tissues during the distraction phase. Tissue relaxation is similar to other bone regeneration processes which do not imply surrounding soft tissue extension, e.g. bone transport. The effects of this tissue on distraction forces are limited to the first minutes of distraction and elongations above 4% of the original length with the protocol applied. Moreover, the surrounding soft tissue initially loses some of its viscoelasticity and subsequently suffers strain hardening from day 5 of distraction until the end of the distraction phase, day 15. Finally, anatomical changes were also evidenced in the elongated limb of our specimens.

Real-Time Wireless Platform for In Vivo Monitoring of Bone Regeneration

For the monitoring of bone regeneration processes, the instrumentation of the fixation is an increasingly common technique to indirectly measure the evolution of bone formation instead of ex vivo measurements or traditional in vivo techniques, such as X-ray or visual review. A versatile instrumented external fixator capable of adapting to multiple bone regeneration processes was designed, as well as a wireless acquisition system for the data collection. The design and implementation of the overall architecture of such a system is described in this work, including the hardware, firmware, and mechanical components. The measurements are conditioned and subsequently sent to a PC via wireless communication to be in vivo displayed and analyzed using a developed real-time monitoring application. Moreover, a model for the in vivo estimation of the bone callus stiffness from collected data was defined. This model was validated in vitro using elastic springs, reporting promising results with respect to previous equipment, with average errors and uncertainties below 6.7% and 14.04%. The devices were also validated in vivo performing a bone lengthening treatment on a sheep metatarsus. The resulting system allowed the in vivo mechanical characterization of the bone callus during experimentation, providing a low-cost, simple, and highly reliable solution.

Regional Nanoindentation Properties in Different Locations on the Mouse Tibia From C57BL/6 and Balb/C Female Mice

The local spatial heterogeneity of the material properties of the cortical and trabecular bone extracted from the mouse tibia is not well-known. Nevertheless, its characterization is fundamental to be able to study comprehensively the effect of interventions and to generate computational models to predict the bone strength preclinically. The goal of this study was to evaluate the nanoindentation properties of bone tissue extracted from two different mouse strains across the tibia length and in different sectors. Left tibiae were collected from four female mice, two C57BL/6, and two Balb/C mice. Nanoindentations with maximum 6 mN load were performed on different microstructures, regions along the axis of the tibiae, and sectors (379 in total). Reduced modulus (Er) and hardness (H) were computed for each indentation. Trabecular bone of Balb/C mice was 21% stiffer than that of C57BL/6 mice (20.8 ± 4.1 GPa vs. 16.5 ± 7.1 GPa). Moreover, the proximal regions of the bones were 13-36% less stiff than the mid-shaft and distal regions of the same bones. No significant differences were found for the different sectors for E _r and H for Balb/C mice. The bone in the medial sector was found to be 8-14% harder and stiffer than the bone in the anterior or posterior sectors for C57BL/6 mice. In conclusion, this study showed that the nanoindentation properties of the mouse tibia are heterogeneous across the tibia length and the trabecular bone properties are different between Balb/C and C57BL/6 mice. These results will help the research community to identify regions where to characterize the mechanical properties of the bone during preclinical optimisation of treatments for skeletal diseases.

Computational modeling of human bone fracture healing affected by different conditions of initial healing stage

BACKGROUND

Bone healing process includes four phases: inflammatory response, soft callus formation, hard callus development, and remodeling. Mechanobiological models have been used to investigate the role of various mechanical and biological factors on bone healing. However, the effects of initial healing phase, which includes the inflammatory stage, the granulation tissue formation, and the initial callus formation during the first few days post-fracture, are generally neglected in such studies.

METHODS

In this study, we developed a finite-element-based model to simulate different levels of diffusion coefficient for mesenchymal stem cell (MSC) migration, Young's modulus of granulation tissue, callus thickness and interfragmentary gap size to understand the modulatory effects of these initial phase parameters on bone healing.

RESULTS

The results quantified how faster MSC migration, stiffer granulation tissue, thicker callus, and smaller interfragmentary gap enhanced healing to some extent. However, after a certain threshold, a state of saturation was reached for MSC migration rate, granulation tissue stiffness, and callus thickness. Therefore, a parametric study was performed to verify that the callus formed at the initial phase, in agreement with experimental observations, has an ideal range of geometry and material properties to have the most efficient healing time.

CONCLUSIONS

Findings from this paper quantified the effects of the initial healing phase on healing outcome to better understand the biological and mechanobiological mechanisms and their utilization in the design and optimization of treatment strategies. It is also demonstrated through a simulation that for fractures, where bone segments are in close proximity, callus development is not required. This finding is consistent with the concepts of primary and secondary bone healing.

Bone-Inspired Mineralization with Highly Aligned Cellulose Nanofibers as Template

Bioinspired by the aligned structure and building blocks of bone, this work mineralized the aligned bacterial cellulose (BC) through in situ mineralization using CaCl₂ and K₂HPO₄ solutions. The cellulose nanofibers were aligned by a scalable stretching process. The aligned and mineralized bacterial cellulose (AMBC) homogeneously incorporated hydroxyapatite (HAP) with a high mineral content and exhibited excellent mechanical strength. The ordered 3D structure allowed the AMBC composite to achieve a high elastic modulus and hardness and the development of a nanostructure inspired by natural bone. The AMBC composite exhibited an elastic modulus of 10.91 ± 3.26 GPa and hardness of 0.37 ± 0.18 GPa. Compared with the nonaligned mineralized bacterial cellulose (NMBC) composite with mineralized crystals of HAP randomly distributed into the BC scaffolds, the AMBC composite possessed a 210% higher elastic modulus and 95% higher hardness. The obtained AMBC composite had excellent mechanical properties by mimicking the natural structure of bone, which indicated that the organic BC aerogel with aligned nanofibers was a promising template for biomimetic mineralization.

Comparison of methods for assigning the material properties of the distraction callus in computational models

In silico models of distraction osteogenesis and fracture healing usually assume constant mechanical properties for the new bone tissue generated. In addition, these models do not always account for the porosity of the woven bone and its evolution. In this study, finite element analyses based on computed tomography (CT) are used to predict the stiffness of the callus until 69 weeks after surgery using 15 CT images obtained at different stages of an experiment on bone transport, technique in which distraction osteogenesis is used to correct bone defects. Three different approaches were used to assign the mechanical properties to the new bone tissue. First, constant mechanical properties of the hard callus tissue and no porosity were assumed. Nevertheless, this approach did not show good correlations. Second, random variations in the elastic modulus and porosity of the woven bone were taken from previous experimental studies. Finally, the elastic properties of each element were assigned depending on gray scale in CT images. The numerically predicted callus stiffness was compared with previous in vivo measurements. It was concluded firstly that assignment depending on gray scale is the method that provides the best results and secondly that the method that considers a random distribution of porosity and elastic modulus of the callus is also suitable to predict the callus stiffness from 15 weeks after surgery. This finding provides a method for assigning the material properties of the distraction callus, which does not require CT images and may contribute to improve current in silico models.

Non-radiative healing assessment techniques for fractured long bones and osseointegrated implant

The paper provides an overview of the fracture healing process of long bones, a review of work that proposed appropriate physical parameters for the assessment of healing and highlights some recent work that reported on the development of non-radiative technique for healing assessment. An overview of the development and monitoring of osseointegration for trans-femoral osseointegrated implant is also presented. The state of healing of a fractured long bone and the stability of osseointegrated implants can be seen as engineering structural components where the mechanical properties are restored to facilitate their desired function. To this end, this paper describes non-radiative techniques that are useful for healing assessment and the stability assessment of osseointegrated implants. The achievement of non-radiative quantitative assessment methodologies to determine the state of healing of fractured long bones and to assess the stability of osseointegrated implant will shorten the patient's rehabilitation time, allowing earlier mobility and return to normal activities. Recent work on the development of assessment techniques supported by the Office of Naval Research as part of the Monitoring of Osseointegrated Implant Prosthesis program is highlighted.

Direct electromagnetic coupling for non-invasive measurements of stability in simulated fracture healing

Diagnostic monitoring and prediction of bone fracture healing is critical for the detection of delayed union or non-union and provides the requisite information as to whether therapeutic intervention or timely revision are warranted. A promising approach to monitor fracture healing is to measure the mechanical load-sharing between the healing callus and the implanted hardware used for internal fixation. The objectives of this study were to evaluate a non-invasive measurement system in which an antenna electromagnetically couples with the implanted hardware to sense deflections of the hardware due to an applied load and to investigate the efficacy of the system to detect changes in mechanical load-sharing in an ex vivo fracture healing model. The measurement system was applied to ovine metatarsal bones treated with osteotomies, resulting in four different levels of bone stability which simulated various degrees of fracture healing. Computational finite element simulations supplemented these ex vivo experiments to compare the osteotomy model of fracture healing to a more clinically applicable callus stiffening model of healing. In the ex vivo experiments, the electromagnetic coupling system detected significant differences between the four simulated degrees of healing with good repeatability. Computational simulations indicated that the experimental model of fracture healing provided a good surrogate for studying healing during the early time period as the callus stiffness is increasing as well as when diagnostic monitoring of the healing process is most critical. Based upon the data reported herein, the direct electromagnetic coupling method holds strong potential for clinical assessments and predictions of fracture healing. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Biomimetic hydroxyapatite/collagen composite drives bone niche recapitulation in a rabbit orthotopic model

Synthetic osteoinductive materials that mimic the human osteogenic niche have emerged as ideal candidates to address this area of unmet clinical need. In this study, we evaluated the osteoinductive potential in a rabbit orthotopic model of a magnesium-doped hydroxyapatite/type I collagen ​(MHA/Coll) composite. The composite was fabricated to exhibit a highly fibrous structure of carbonated MHA with 70% (±2.1) porosity and a Ca/P ratio of 1.5 (±0.03) as well as a diverse range of elasticity separated to two distinct stiffness peaks of low (2.35 ​± ​1.16 ​MPa) and higher (9.52 ​± ​2.10 ​MPa) Young's Modulus. Data suggested that these specific compositional and nanomechanical material properties induced the deposition of de novo mineral phase, while modulating the expression of early and late osteogenic marker genes, in a 3D in vitro model using human bone marrow-derived mesenchymal stem cells (hBM-MSCs). When tested in the rabbit orthotopic model, MHA/Col1 scaffold induction of new trabecular bone mass was observed by DynaCT scan, only 2 weeks after implantation. Bone histomorphometry at 6 weeks revealed a significant amount of de novo bone matrix formation. qPCR demonstrated MHA/Coll scaffold full cellularization in vivo and the expression of both osteogenesis-associated genes (Spp1, Sparc, Col1a1, Runx2, Dlx5) as well as hematopoietic (Vcam1, Cd38, Sele, Kdr) and bone marrow stromal cell marker genes (Vim, Itgb1, Alcam). Altogether, these data provide ​evidence of the solid osteoinductive potential of MHA/Coll and its suitability for multiple approaches of bone regeneration.

Characterizing and Modeling Bone Formation during Mouse Calvarial Development

The newborn mammalian cranial vault consists of five flat bones that are joined together along their edges by soft fibrous tissues called sutures. Early fusion of these sutures leads to a medical condition known as craniosynostosis. The mechanobiology of normal and craniosynostotic skull growth is not well understood. In a series of previous studies, we characterized and modeled radial expansion of normal and craniosynostotic (Crouzon) mice. Here, we describe a new modeling algorithm to simulate bone formation at the sutures in normal and craniosynostotic mice. Our results demonstrate that our modeling approach is capable of predicting the observed ex vivo pattern of bone formation at the sutures in the aforementioned mice. The same approach can be used to model different calvarial reconstruction in children with craniosynostosis to assist in the management of this complex condition.

Novel In Situ Gelling Hydrogels Loaded with Recombinant Collagen Peptide Microspheres as a Slow-Release System Induce Ectopic Bone Formation

New solutions for large bone defect repair are needed. Here, in situ gelling slow release systems for bone induction are assessed. Collagen-I based Recombinant Peptide (RCP) microspheres (MSs) are produced and used as a carrier for bone morphogenetic protein 2 (BMP-2). The RCP-MSs are dispersed in three hydrogels: high mannuronate (SLM) alginate, high guluronate (SLG) alginate, and thermoresponsive hyaluronan derivative (HApN). HApN+RCP-MS forms a gel structure at 32 ºC or above, while SLM+RCP-MS and SLG+RCP-MS respond to shear stress displaying thixotropic behavior. Alginate formulations show sustained release of BMP-2, while there is minimal release from HApN. These formulations are injected subcutaneously in rats. SLM+RCP-MS and SLG+RCP-MS loaded with BMP-2 induce ectopic bone formation as revealed by X-ray tomography and histology, whereas HApN+RCP-MS do not. Vascularization occurs within all the formulations studied and is significantly higher in SLG+MS and HApN+RCP-MS than in SLM+RCP-MS. Inflammation (based on macrophage subset staining) decreases over time in both alginate groups, but increases in the HApN+RCP-MS condition. It is shown that a balance between inflammatory cell infiltration, BMP-2 release, and vascularization, achieved in the SLG+RCP-MS alginate condition, is optimal for the induction of de novo bone formation.

Comparison of the volumetric composition of lamellar bone and the woven bone of calluses

Woven tissue is mainly present in the bone callus, formed very rapidly either after a fracture or in distraction processes. This high formation speed is probably responsible for its disorganized microstructure and this, in turn, for its low stiffness. Nonetheless, the singular volumetric composition of this tissue may also play a key role in its mechanical properties. The volumetric composition of woven tissue extracted from the bone transport callus of sheep was investigated and compared with that of the lamellar tissue extracted from the cortical shell of the same bone. Significant differences were found in the mineral and water contents, but they can be due to the different ages of both tissues, which affects the mineral/water ratio. However, the content in organic phase remains more or less constant throughout the mineralization process and has proven to be a good variable to measure the different composition of both tissues, being that content significantly higher in woven tissue. This may be linked to the abnormally high concentration of osteocytes in this tissue, which is likely a consequence of the more abundant presence of osteoblasts secreting osteoid and burying other osteoblasts, which then differentiate into osteocytes. This would explain the high formation rate of woven tissue, useful to recover the short-term stability of the bone. Nonetheless, the more abundant presence of organic phase prevents the woven tissue from reaching a stiffness similar to that of lamellar tissue in the long term, when it is fully mineralized.

Matrix architecture plays a pivotal role in 3D osteoblast migration: The effect of interstitial fluid flow

Osteoblast migration is a crucial process in bone regeneration, which is strongly regulated by interstitial fluid flow. However, the exact role that such flow exerts on osteoblast migration is still unclear. To deepen the understanding of this phenomenon, we cultured human osteoblasts on 3D microfluidic devices under different fluid flow regimes. Our results show that a slow fluid flow rate by itself is not able to alter the 3D migratory patterns of osteoblasts in collagen-based gels but that at higher fluid flow rates (increased flow velocity) may indirectly influence cell movement by altering the collagen microstructure. In fact, we observed that high fluid flow rates (1 µl/min) are able to alter the collagen matrix architecture and to indirectly modulate the migration pattern. However, when these collagen scaffolds were crosslinked with a chemical crosslinker, specifically, transglutaminase II, we did not find significant alterations in the scaffold architecture or in osteoblast movement. Therefore, our data suggest that high interstitial fluid flow rates can regulate osteoblast migration by means of modifying the orientation of collagen fibers. Together, these results highlight the crucial role of the matrix architecture in 3D osteoblast migration. In addition, we show that interstitial fluid flow in conjunction with the matrix architecture regulates the osteoblast morphology in 3D.

Novel systems for the application of isolated tensile, compressive, and shearing stimulation of distraction callus tissue

Background

Distraction osteogenesis is a procedure widely used for the correction of large bone defects. However, a high complication rate persists, likely due to insufficient stability during maturation. Numerical fracture healing models predict bone regeneration under different mechanical conditions allowing fixation stiffness optimization. However, most models apply a linear elastic material law inappropriate for the transient stresses/strains present during limb lengthening or segment transport. They are also often validated using in vivo osteotomy models lacking precise mechanical regulation due to the unavoidable stimulation of secondary interfragmentary motion during ambulation under finitely stiff fixation. Therefore, in order to create a robust numerical model of distraction osteogenesis, it is necessary to both characterize the new tissue’s viscoelasticity during distraction and determine the influence of strictly isolated stimulation in each loading mode (tension, compression, and shear) to account for potential differences in mechanical and histological response.

Aim

Two electromechanical fixators with integrated load cells were designed to precisely perform and monitor in vivo lateral distraction and isolated stimulation in sheep tibiae using a mobile, hydroxyapatite-coated titanium plate. The novel surgical procedure circumvents osteotomy, eliminating the undesirable and unquantifiable mechanical stimulation during ambulation.

Methods

After a 10-day post-surgery latency period, two 0.275 mm distraction steps were performed daily for 10 days. The load cell collected data before, during, and after each distraction step and was terminated after no less than one minute from the time of distraction. A 7-day consolidation period separated the distraction phase and 18-day stimulation phase. Stimulation was carried out in isolated tension, compression, or shear while recording force/time data. Each stimulation session consisted of 120 cycles with a magnitude of either 0.1 mm or 0.6 mm in the tension and compression groups and 1.0 mm in the shear group. The animals were euthanized after a 3-day holding period following stimulation.

Results

Our initial results show that the tissue progressively stiffens and maintains an increasingly large residual traction. The force curves during compressive stimulation show a progressive drift from compression toward tension. We hypothesize that this behavior may be due to the preferential flow of fluid outward from the tissue and a greater resistance to reabsorption during the plate’s return to the starting position.

Finite element analysis of the effect of medullary contact on fracture healing and remodeling in the intramedullary interlocking nail-fixed tibia fracture

Intramedullary interlocking nail is an effective treatment for tibial diaphyseal fracture. The contact between medullary rod and diaphyseal cortex is able to enhance fracture stability. However, how and to what degree the contact affects fracture healing and subsequent bone remodeling is still unclear. To investigate this, fracture healing and remodeling algorithms were combined, improved, and used to simulate the healing and remodeling processes in a transverse tibial diaphyseal fracture fixed with an intramedullary interlocking nail device. Two different diaphyseal fracture statuses, three different initial loading levels, and two nail materials were considered. The results showed that the medullary contact could significantly enhance the fixation stability; the strain reduction was up to 80% in the initial granulation callus. However, low initial loading level was found to be a more potential risk factor for the insufficient loading-induced nonunion other than medullary contact and stiffer nail material. Furthermore, the stabilizing effect of medullary contact diminished when stiff bone tissue formed in the callus; thus, the remodeling in the long-term was not affected by medullary contact. Copyright © 2016 John Wiley & Sons, Ltd.

Micromotion-induced peri-prosthetic fluid flow around a cementless femoral stem

Micromotion-induced interstitial fluid flow at the bone-implant interface has been proposed to play an important role in aseptic loosening of cementless implants. High fluid velocities are thought to promote aseptic loosening through activation of osteoclasts, shear stress induced control of mesenchymal stem cells differentiation, or transport of molecules. In this study, our objectives were to characterize and quantify micromotion-induced fluid flow around a cementless femoral stem using finite element modeling. With a 2D model of the bone-implant interface and full-factorial design, we first evaluated the relative influence of material properties, and bone-implant micromotion and gap on fluid velocity. Transverse sections around a femoral stem were built from computed tomography images, while boundary conditions were obtained from experimental measurements on the same femur. In a second step, a 3D model was built from the same data-set to estimate the shear stress experienced by cells hosted in the peri-implant tissues. The full-factorial design analysis showed that local micromotion had the most influence on peak fluid velocity at the interface. Remarkable variations in fluid velocity were observed in the macrostructures at the surface of the implant in the 2D transverse sections of the stem. The 3D model predicted peak fluid velocities extending up to 2.2 mm/s in the granulation tissue and to 3.9 mm/s in the trabecular bone. Peak shear stresses on the cells hosted in these tissues ranged from 0.1 to 12.5 Pa. These results offer insight into mechanical stimuli encountered at the bone-implant interface.

Nanoindentation analysis of the micromechanical anisotropy in mouse cortical bone

Studies investigating micromechanical properties in mouse cortical bone often solely focus on the mechanical behaviour along the long axis of the bone. Therefore, data on the anisotropy of mouse cortical bone is scarce. The aim of this study is the first-time evaluation of the anisotropy ratio between the longitudinal and transverse directions of reduced modulus and hardness in mouse femurs by using the nanoindentation technique. For this purpose, nine 22-week-old mice (C57BL/6) were sacrificed and all femurs extracted. A total of 648 indentations were performed with a Berkovich tip in the proximal (P), central (C) and distal (D) regions of the femoral shaft in the longitudinal and transverse directions. Higher values for reduced modulus are obtained for indentations in the longitudinal direction, with anisotropy ratios of 1.72 ± 0.40 (P), 1.75 ± 0.69 (C) and 1.34 ± 0.30 (D). Hardness is also higher in the longitudinal direction, with anisotropic ratios of 1.35 ± 0.27 (P), 1.35 ± 0.47 (C) and 1.17 ± 0.19 (D). We observed a significant anisotropy in the micromechanical properties of the mouse femur, but the correlation for reduced modulus and hardness between the two directions is low (r² < 0.3) and not significant. Therefore, we highly recommend performing independent indentation testing in both the longitudinal and transverse directions when knowledge of the tissue mechanical behaviour along multiple directions is required.

Nanoindentation analysis of the micromechanical anisotropy in mouse cortical bone

Studies investigating micromechanical properties in mouse cortical bone often solely focus on the mechanical behaviour along the long axis of the bone. Therefore, data on the anisotropy of mouse cortical bone is scarce. The aim of this study is the first-time evaluation of the anisotropy ratio between the longitudinal and transverse directions of reduced modulus and hardness in mouse femurs by using the nanoindentation technique. For this purpose, nine 22-week-old mice (C57BL/6) were sacrificed and all femurs extracted. A total of 648 indentations were performed with a Berkovich tip in the proximal (P), central (C) and distal (D) regions of the femoral shaft in the longitudinal and transverse directions. Higher values for reduced modulus are obtained for indentations in the longitudinal direction, with anisotropy ratios of 1.72 ± 0.40 (P), 1.75 ± 0.69 (C) and 1.34 ± 0.30 (D). Hardness is also higher in the longitudinal direction, with anisotropic ratios of 1.35 ± 0.27 (P), 1.35 ± 0.47 (C) and 1.17 ± 0.19 (D). We observed a significant anisotropy in the micromechanical properties of the mouse femur, but the correlation for reduced modulus and hardness between the two directions is low (r² < 0.3) and not significant. Therefore, we highly recommend performing independent indentation testing in both the longitudinal and transverse directions when knowledge of the tissue mechanical behaviour along multiple directions is required.

Hierarchically Staggered Nanostructure of Mineralized Collagen as a Bone-Grafting Scaffold

A hierarchical, intrafibrillarly mineralized collagen (HIMC) is achieved through a selective mineralization progress in the collagenous gap regions mediated by poly(acrylic acid) with appropriate molecular weight. The associated topographical features directly correlate with nanomechanical heterogeneities of the HIMC to accommodate a broad range of external loads. Moreover, this hierarchically staggered nanostructure provides an optimized microenvironment to improve bone regeneration by instructing host cells.

Computational characterization of fracture healing under reduced gravity loading conditions

The literature is deficient with regard to how the localized mechanical environment of skeletal tissue is altered during reduced gravitational loading and how these alterations affect fracture healing. Thus, a finite element model of the ovine hindlimb was created to characterize the local mechanical environment responsible for the inhibited fracture healing observed under experimental simulated hypogravity conditions. Following convergence and verification studies, hydrostatic pressure and strain within a diaphyseal fracture of the metatarsus were evaluated for models under both 1 and 0.25 g loading environments and compared to results of a related in vivo study. Results of the study suggest that reductions in hydrostatic pressure and strain of the healing fracture for animals exposed to reduced gravitational loading conditions contributed to an inhibited healing process, with animals exposed to the simulated hypogravity environment subsequently initiating an intramembranous bone formation process rather than the typical endochondral ossification healing process experienced by animals healing in a 1 g gravitational environment. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1206-1215, 2016.