Mechanical microenvironments and protein expression associated with formation of different skeletal tissues during bone healing

Mechanobiology-informed biomaterial and tissue engineering strategies for influencing skeletal stem and progenitor cell fate

Skeletal stem and progenitor cells (SSPCs) are the multi-potent, self-renewing cell lineages that form the hematopoietic environment and adventitial structures of the skeletal tissues. Skeletal tissues are responsible for a diverse range of physiological functions because of the extensive differentiation potential of SSPCs. The differentiation fates of SSPCs are shaped by the physical properties of their surrounding microenvironment and the mechanical loading forces exerted on them within the skeletal system. In this context, the present review first highlights important biomolecules involved with the mechanobiology of how SSPCs sense and transduce these physical signals. The review then shifts focus towards how the static and dynamic physical properties of microenvironments direct the biological fates of SSPCs, specifically within biomaterial and tissue engineering systems. Biomaterial constructs possess designable, quantifiable physical properties that enable the growth of cells in controlled physical environments both in-vitro and in-vivo. The utilization of biomaterials in tissue engineering systems provides a valuable platform for controllably directing the fates of SSPCs with physical signals as a tool for mechanobiology investigations and as a template for guiding skeletal tissue regeneration. It is paramount to study this mechanobiology and account for these mechanics-mediated behaviors to develop next-generation tissue engineering therapies that synergistically combine physical and chemical signals to direct cell fate. Ultimately, taking advantage of the evolved mechanobiology of SSPCs with customizable biomaterial constructs presents a powerful method to predictably guide bone and skeletal organ regeneration.

Postnatal Osterix but not DMP1 lineage cells significantly contribute to intramembranous ossification in three preclinical models of bone injury

Murine models of long-bone fracture, stress fracture, and cortical defect are used to discern the cellular and molecular mediators of intramembranous and endochondral bone healing. Previous work has shown that Osterix (Osx⁺) and Dentin Matrix Protein-1 (DMP1⁺) lineage cells and their progeny contribute to injury-induced woven bone formation during femoral fracture, ulnar stress fracture, and tibial cortical defect repair. However, the contribution of pre-existing versus newly-derived Osx⁺ and DMP1⁺ lineage cells in these murine models of bone injury is unclear. We addressed this knowledge gap by using male and female 12-week-old, tamoxifen-inducible Osx Cre_ERT2 and DMP1 Cre_ERT2 mice harboring the Ai9 TdTomato reporter allele. To trace pre-existing Osx⁺ and DMP1⁺ lineage cells, tamoxifen (TMX: 100 mg/kg gavage) was given in a pulse manner (three doses, 4 weeks before injury), while to label pre-existing and newly-derived lineage Osx⁺ and DMP1⁺ cells, TMX was first given 2 weeks before injury and continuously (twice weekly) throughout healing. TdTomato positive (TdT⁺) cell area and cell fraction were quantified from frozen histological sections of injured and uninjured contralateral samples at times corresponding with active woven bone formation in each model. We found that in uninjured cortical bone tissue, Osx Cre_ERT2 was more efficient than DMP1 Cre_ERT2 at labeling the periosteal and endosteal surfaces, as well as intracortical osteocytes. Pulse-labeling revealed that pre-existing Osx⁺ lineage and their progeny, but not pre-existing DMP1⁺ lineage cells and their progeny, significantly contributed to woven bone formation in all three injury models. In particular, these pre-existing Osx⁺ lineage cells mainly lined new woven bone surfaces and became embedded as osteocytes. In contrast, with continuous dosing, both Osx⁺ and DMP1⁺ lineage cells and their progeny contributed to intramembranous woven bone formation, with higher TdT⁺ tissue area and cell fraction in Osx⁺ lineage versus DMP1⁺ lineage calluses (femoral fracture and ulnar stress fracture). Similarly, Osx⁺ and DMP1⁺ lineage cells and their progeny significantly contributed to endochondral callus regions with continuous dosing only, with higher TdT⁺ chondrocyte fraction in Osx⁺ versus DMP1⁺ cell lineages. In summary, pre-existing Osx⁺ but not DMP1⁺ lineage cells and their progeny make up a significant amount of woven bone cells (particularly osteocytes) across three preclinical models of bone injury. Therefore, Osx⁺ cell lineage modulation may prove to be an effective therapy to enhance bone regeneration.

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.

Extracellular matrix compression temporally regulates microvascular angiogenesis

Mechanical cues influence tissue regeneration, and although vasculature is known to be mechanically sensitive, little is known about the effects of bulk extracellular matrix deformation on the nascent vessel networks found in healing tissues. Previously, we found that dynamic matrix compression in vivo potently regulated revascularization during bone tissue regeneration; however, whether matrix deformations directly regulate angiogenesis remained unknown. Here, we demonstrated that load initiation time, magnitude, and mode all regulate microvascular growth, as well as upstream angiogenic and mechanotransduction signaling pathways. Immediate load initiation inhibited angiogenesis and expression of early sprout tip cell selection genes, while delayed loading enhanced microvascular network formation and upstream signaling pathways. This research provides foundational understanding of how extracellular matrix mechanics regulate angiogenesis and has critical implications for clinical translation of new regenerative medicine therapies and physical rehabilitation strategies designed to enhance revascularization during tissue regeneration.

Wireless sensor enables longitudinal monitoring of regenerative niche mechanics during rehabilitation that enhance bone repair

Mechanical loads exerted on the skeleton during activities such as walking are important regulators of bone repair, but dynamic biomechanical signals are difficult to measure inside the body. The inability to measure the mechanical environment in injured tissues is a significant barrier to developing integrative regenerative and rehabilitative strategies that can accelerate recovery from fracture, segmental bone loss, and spinal fusion. Here we engineered an implantable strain sensor platform and longitudinally measured strain across a bone defect in real-time throughout rehabilitation. The results showed that load-sharing permitted by a load-sharing fixator initially delivered a two-fold increase in deformation magnitude, subsequently increased mineralized bridging by nearly three-fold, and increased bone formation by over 60%. These data implicate a critical role for early mechanical cues on the long term healing response as strain cycle magnitude at 1 week (before appreciable healing occurred) had a significant positive correlation with the long-term bone regeneration outcomes. Furthermore, we found that sensor readings correlated with the status of healing, suggesting a role for strain sensing as an X-ray-free healing assessment platform. Therefore, non-invasive strain measurements may possess diagnostic potential to evaluate bone repair and reduce clinical reliance on current radiation-emitting imaging methods. Together, this study demonstrates a promising framework to quantitatively develop and exploit mechanical rehabilitation strategies that enhance bone repair.

Comparison of bone surface and trough fixation on bone-tendon healing in a rabbit patella-patellar tendon injury model

BACKGROUND

Many orthopedic surgical procedures involve reattachment between tendon and bone. Whether bone-tendon healing is better facilitated by tendon fixation on a bone surface or within a tunnel is unknown. The purpose of this study was to comparatively evaluate the effects of bone surface versus bone trough fixation on bone-tendon healing in a rabbit patella-patellar tendon (PPT) injury model.

METHODS

The rabbits underwent partial patellectomy with patellar-tendon fixation on the osteotomy surface (bone surface fixation, BSF group) (n = 28) or within a bone trough (bone trough fixation, BTF group) (n = 28). The PPT interface was evaluated by macroscopic observation, micro-computed tomography scanning, histological analysis, and biomechanical testing at postoperative week 8 or week 16.

RESULTS

Macroscopically, no signs of infection or osteoarthritis were observed, and the regenerated tissue bridging the residual patella and patellar tendon showed no obvious difference between the two groups. There were significantly higher bone mineral density ​and trabecular thickness ​in BSF group compared with BTF group at week 8 (p ​< ​0.05 for both). However, the bone volume fraction (BVF), bone mineral density and trabecular thickness in BSF group were significantly lower than those in BTF group (p ​< ​0.05 for all) at week 16. Histological analysis demonstrated that new bone was formed at the proximal patella and reattached to the residual patellar tendon through a regenerated fibrocartilage-like tissue in both groups. There was more formation and better remodelling of fibrocartilage-like tissue in BTF group than BSF group at week 8 and week 16 (p ​< ​0.05 for both). Biomechanical testing revealed that there was higher failure load and stiffness at the PPT interface in BTF group than BSF group at week 16 (p ​< ​0.05 for both).

CONCLUSIONS

These results suggested that raptured tendon fixation in a bone trough resulted in superior bone-tendon healing in comparison with tendon fixation on bone surface in a rabbit PPT injury model.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Although the structural and functional difference of knee joint between human and rabbit limit the results to be directly used in clinical, our research does offer a valuable reference for the improvement of reattachment between bone and tendon.

The association between mineralised tissue formation and the mechanical local in vivo environment: Time-lapsed quantification of a mouse defect healing model

An improved understanding of how local mechanical stimuli guide the fracture healing process has the potential to enhance clinical treatment of bone injury. Recent preclinical studies of bone defect in animal models have used cross-sectional data to examine this phenomenon indirectly. In this study, a direct time-lapsed imaging approach was used to investigate the local mechanical strains that precede the formation of mineralised tissue at the tissue scale. The goal was to test two hypotheses: 1) the local mechanical signal that precedes the onset of tissue mineralisation is higher in areas which mineralise, and 2) this local mechanical signal is independent of the magnitude of global mechanical loading of the tissue in the defect. Two groups of mice with femoral defects of length 0.85 mm (n = 10) and 1.45 mm (n = 9) were studied, allowing for distinct distributions of tissue scale strains in the defects. The regeneration and (re)modelling of mineralised tissue was observed weekly using in vivo micro-computed tomography (micro-CT), which served as a ground truth for resolving areas of mineralised tissue formation. The mechanical environment was determined using micro-finite element analysis (micro-FE) on baseline images. The formation of mineralised tissue showed strong association with areas of higher mechanical strain (area-under-the-curve: 0.91 ± 0.04, true positive rate: 0.85 ± 0.05) while surface based strains could correctly classify 43% of remodelling events. These findings support our hypotheses by showing a direct association between the local mechanical strains and the formation of mineralised tissue.

Dynamically Crystalizing Liquid-Crystal Elastomers for an Expandable Endplate-Conforming Interbody Fusion Cage

Degenerative disc disease (DDD) is the leading cause of low back pain and radiating leg pain. DDD is commonly treated surgically using spinal fusion techniques, but in many cases failure occurs due to insufficient immobilization of the vertebrae during fusion. The fabrication and demonstration of a 3D-printed semi-crystalline liquid crystal elastomer (LCE) spinal fusion cage that addresses these challenges in particular subsidence are described. During implantation of the fusion cage, the LCE is rubbery and capable of deforming around and conforming to delicate anatomy. In the hours following implantation, the device crystallizes into a rigid, structural material with the modulus increasing tenfold from 8 to 80 MPa. In the crystalline regime, a 3D-printed prototype device is capable of enduring 1 million cycles of physiologic compressive loading with minimal creep-induced ratcheting. Effects of LCE molecular architecture on the rate and magnitude of modulus increase, material processability, and mechanical properties are explored. This fundamental characterization informs a proof-of-concept device-the first bulk 3D printed LCE demonstrated to date. Moreover, the novel deployment strategy represents an exciting new paradigm of spinal fusion cages, which addresses real clinical challenges in expandable interbody fusion cages.

The Size of Intramedullary Fixation Affects Endochondral-Mediated Angiogenesis During Fracture Repair

OBJECTIVES

To explore the effect of intramedullary pin size on the biology of a healing fracture, specifically endochondral angiogenesis. We hypothesized that fracture fixation with a smaller pin would permit greater interfragmentary strain resulting in increased total amount of vascular endothelial growth factor within the callus and greater angiogenesis compared to fixation with a larger pin.

METHODS

Transverse mid-shaft femur fractures in 8-week-old mice were fixed with either a 23-gauge (G) or 30-G pin. Differences in interfragmentary strain at the fracture site were estimated between cohorts. A combination of histology, gene expression, serial radiography, and microcomputed tomography with and without vascular contrast agent were used to assess fracture healing and vascularity for each cohort.

RESULTS

Larger soft-tissue callus formation increased vascular endothelial growth factor-A expression, and a corresponding increase in vascular volume was observed in the higher strain, 30-G cohort. Radiographic analysis demonstrated earlier hard callus formation with greater initial interfragmentary strain, similar rates of union between pin size cohorts, yet delayed callus remodeling in mice with the larger pin size.

CONCLUSIONS

These findings suggest that the stability conferred by an intramedullary nail influences endochondral angiogenesis at the fracture.

Effect of porous orthopaedic implant material and structure on load sharing with simulated bone ingrowth: A finite element analysis comparing titanium and PEEK

Osseointegration of load-bearing orthopaedic implants, including interbody fusion devices, is critical to long-term biomechanical functionality. Mechanical loads are a key regulator of bone tissue remodeling and maintenance, and stress-shielding due to metal orthopaedic implants being much stiffer than bone has been implicated in clinical observations of long-term bone loss in tissue adjacent to implants. Porous features that accommodate bone ingrowth have improved implant fixation in the short term, but long-term retrieval studies have sometimes demonstrated limited, superficial ingrowth into the pore layer of metal implants and aseptic loosening remains a problem for a subset of patients. Polyether-ether-ketone (PEEK) is a widely used orthopaedic material with an elastic modulus more similar to bone than metals, and a manufacturing process to form porous PEEK was recently developed to allow bone ingrowth while preserving strength for load-bearing applications. To investigate the biomechanical implications of porous PEEK compared to porous metals, we analyzed finite element (FE) models of the pore structure-bone interface using two clinically available implants with high (> 60%) porosity, one being constructed from PEEK and the other from electron beam 3D-printed titanium (Ti). The objective of this study was to investigate how porous PEEK and porous Ti mechanical properties affect load sharing with bone within the porous architectures over time. Porous PEEK substantially increased the load share transferred to ingrown bone compared to porous Ti under compression (i.e. at 4 weeks: PEEK = 66%; Ti = 13%), tension (PEEK = 71%; Ti = 12%), and shear (PEEK = 68%; Ti = 9%) at all time points of simulated bone ingrowth. Applying PEEK mechanical properties to the Ti implant geometry and vice versa demonstrated that the observed increases in load sharing with PEEK were primarily due to differences in intrinsic elastic modulus and not pore architecture (i.e. 4 weeks, compression: PEEK material/Ti geometry = 53%; Ti material/PEEK geometry = 12%). Additionally, local tissue energy effective strains on bone tissue adjacent to the implant under spinal load magnitudes were over two-fold higher with porous PEEK than porous Ti (i.e. 4 weeks, compression: PEEK = 784 ± 351 microstrain; Ti = 180 ± 300 microstrain; and 12 weeks, compression: PEEK = 298 ± 88 microstrain; Ti = 121 ± 49 microstrain). The higher local strains on bone tissue in the PEEK pore structure were below previously established thresholds for bone damage but in the range necessary for physiological bone maintenance and adaptation. Placing these strain magnitudes in the context of literature on bone adaptation to mechanical loads, this study suggests that porous PEEK structures may provide a more favorable mechanical environment for bone formation and maintenance under spinal load magnitudes than currently available porous 3D-printed Ti, regardless of the level of bone ingrowth.

Mechanical Loading Promotes the Expansion of Primitive Osteoprogenitors and Organizes Matrix and Vascular Morphology in Long Bone Defects

Elucidating the effects of mechanical stimulation on bone repair is crucial for optimization of the healing process. Specifically, the regulatory role that mechanical loading exerts on the osteogenic stem cell pool and vascular morphology during healing is incompletely understood. Because dynamic loading has been shown to enhance osteogenesis and repair, we hypothesized that loading induces the expansion of the osteoprogenitor cell population within a healing bone defect, leading to an increased presence of osteogenic cells. We further hypothesized that loading during the repair process regulates vascular and collagen matrix morphology and spatial interactions between vessels and osteogenic cells. To address these hypotheses, we used a mechanobiological bone repair model, which produces a consistent and reproducible intramembranous repair response confined in time and space. Bilateral tibial defects were created in adult C57BL/6 mice, which were subjected to axial compressive dynamic loading either during the early cellular invasion phase on postsurgical days (PSDs) 2 to 5 or during the matrix deposition phase on PSD 5 to 8. Confocal and two-photon microscopy was used to generate high-resolution three-dimensional (3D) renderings of longitudinal thick sections of the defect on PSD 10. Endomucin (EMCN)-positive vessels, Paired related homeobox 1 (Prrx1+) stem cell antigen-1 positive (Sca-1+) primitive osteoprogenitors (OPCs), and osterix positive (Osx+) preosteoblasts were visualized and quantified using deep tissue immunohistochemistry. New bone matrix was visualized with second harmonic generation autofluorescence of collagen fibers. We found that mechanical loading during the matrix deposition phase (PSD 5 to 8) increased vessel volume and number, and aligned vessels and collagen fibers to the load-bearing direction of bone. Furthermore, loading led to a significant increase in the proliferation and number of Prrx1+ Sca-1+ primitive OPCs, but not Osx+ preosteoblasts within the defect. Together, these data illustrate the adaptation of both collagen matrix and vascular morphology to better withstand mechanical load during bone repair, and that the mechanoresponsive cell population consists of the primitive osteogenic progenitors. © 2019 American Society for Bone and Mineral Research.

Wireless Implantable Sensor for Noninvasive, Longitudinal Quantification of Axial Strain Across Rodent Long Bone Defects

Bone development, maintenance, and regeneration are remarkably sensitive to mechanical cues. Consequently, mechanical stimulation has long been sought as a putative target to promote endogenous healing after fracture. Given the transient nature of bone repair, tissue-level mechanical cues evolve rapidly over time after injury and are challenging to measure noninvasively. The objective of this work was to develop and characterize an implantable strain sensor for noninvasive monitoring of axial strain across a rodent femoral defect during functional activity. Herein, we present the design, characterization, and in vivo demonstration of the device's capabilities for quantitatively interrogating physiological dynamic strains during bone regeneration. Ex vivo experimental characterization of the device showed that it possessed promising sensitivity, signal resolution, and electromechanical stability for in vivo applications. The digital telemetry minimized power consumption, enabling extended intermittent data collection. Devices were implanted in a rat 6 mm femoral segmental defect model, and after three days, data were acquired wirelessly during ambulation and synchronized to corresponding radiographic videos, validating the ability of the sensor to noninvasively measure strain in real-time. Together, these data indicate the sensor is a promising technology to quantify tissue mechanics in a specimen specific manner, facilitating more detailed investigations into the role of the mechanical environment in dynamic bone healing and remodeling processes.

Effects of mechanical loading on cortical defect repair using a novel mechanobiological model of bone healing

Mechanical loading is an important aspect of post-surgical fracture care. The timing of load application relative to the injury event may differentially regulate repair depending on the stage of healing. Here, we used a novel mechanobiological model of cortical defect repair that offers several advantages including its technical simplicity and spatially confined repair program, making effects of both physical and biological interventions more easily assessed. Using this model, we showed that daily loading (5N peak load, 2Hz, 60 cycles, 4 consecutive days) during hematoma consolidation and inflammation disrupted the injury site and activated cartilage formation on the periosteal surface adjacent to the defect. We also showed that daily loading during the matrix deposition phase enhanced both bone and cartilage formation at the defect site, while loading during the remodeling phase resulted in an enlarged woven bone regenerate. All loading regimens resulted in abundant cellular proliferation throughout the regenerate and fibrous tissue formation directly above the defect demonstrating that all phases of cortical defect healing are sensitive to physical stimulation. Stress was concentrated at the edges of the defect during exogenous loading, and finite element (FE)-modeled longitudinal strain (ε_zz) values along the anterior and posterior borders of the defect (~2200με) was an order of magnitude larger than strain values on the proximal and distal borders (~50-100με). It is concluded that loading during the early stages of repair may impede stabilization of the injury site important for early bone matrix deposition, whereas loading while matrix deposition and remodeling are ongoing may enhance stabilization through the formation of additional cartilage and bone.

Cartilage to bone transformation during fracture healing is coordinated by the invading vasculature and induction of the core pluripotency genes

Fractures heal predominantly through the process of endochondral ossification. The classic model of endochondral ossification holds that chondrocytes mature to hypertrophy, undergo apoptosis and new bone forms by invading osteoprogenitors. However, recent data demonstrate that chondrocytes transdifferentiate to osteoblasts in the growth plate and during regeneration, yet the mechanism(s) regulating this process remain unknown. Here, we show a spatially-dependent phenotypic overlap between hypertrophic chondrocytes and osteoblasts at the chondro-osseous border in the fracture callus, in a region we define as the transition zone (TZ). Hypertrophic chondrocytes in the TZ activate expression of the pluripotency factors [Sox2, Oct4 (Pou5f1), Nanog], and conditional knock-out of Sox2 during fracture healing results in reduction of the fracture callus and a delay in conversion of cartilage to bone. The signal(s) triggering expression of the pluripotency genes are unknown, but we demonstrate that endothelial cell conditioned medium upregulates these genes in ex vivo fracture cultures, supporting histological evidence that transdifferentiation occurs adjacent to the vasculature. Elucidating the cellular and molecular mechanisms underlying fracture repair is important for understanding why some fractures fail to heal and for developing novel therapeutic interventions.

Strains caused by daily loading might be responsible for delayed healing of an incomplete atypical femoral fracture

Atypical femoral fractures are insufficiency fractures in the lateral femoral diaphysis or subtrochanteric region that mainly affect older patients on bisphosphonate therapy. Delayed healing is often seen in patients with incomplete fractures (cracks), and histology of bone biopsies shows mainly necrotic material inside the crack. We hypothesized that the magnitude of the strains produced in the soft tissue inside the crack during normal walk exceeds the limit for new bone formation, and thereby inhibit healing. A patient specific finite element model was developed, based on clinical CT images and high resolution μCT images of a biopsy from the crack site. Strain distributions in the femur and inside the crack were calculated for load cases representing normal walk. The models predicted large strains inside the crack, with strain levels above 10% in more than three quarters of the crack volume. According to two different tissue differentiation theories, bone would only form in less than 1-5% of the crack volume. This can explain the impaired healing generally seen in incomplete atypical fractures. Furthermore, the microgeometry of the crack highly influenced the strain distributions. Hence, a realistic microgeometry needs to be considered when modeling the crack. Histology of the biopsy showed signs of remodeling in the bone tissue adjacent to the fracture line, while the crack itself contained mainly necrotic material and signs of healing only in portions that seemed to have been widened by resorption. In conclusion, the poor healing capacity of incomplete atypical femoral fractures can be explained by biomechanical factors, and daily low impact activities are enough to cause strain magnitudes that prohibit bone formation.

DC-STAMP: A Key Regulator in Osteoclast Differentiation

Osteoimmunology research is a new emerging research field that investigates the links between the bone and immune responses. Results from osteoimmunology studies suggest that bone is not only an essential component of the musculoskeletal system, but is also actively involved in immune regulation. Many important factors involved in immune regulation also participate in bone homeostasis. Bone homeostasis is achieved by a coordinated action between bone-synthesizing osteoblasts and bone-degrading osteoclasts. An imbalanced action between osteoblasts and osteoclasts often results in pathological bone diseases: osteoporosis is caused by an excessive osteoclast activity, whereas osteopetrosis results from an increased osteoblast activity. This review focuses on dendritic cell-specific transmembrane protein (DC-STAMP), an important protein currently considered as a master regulator of osteoclastogenesis. Of clinical relevance, the frequency of circulating DC-STAMP+ cells is elevated during the pathogenesis of psoriatic diseases. Intriguingly, recent results suggest that DC-STAMP also plays an imperative role in bone homeostasis by regulating the differentiation of both osteoclasts and osteoblasts. This article summarizes our current knowledge on DC-STAMP by focusing on its interacting proteins, its regulation on osteoclastogenesis-related genes, its possible involvement in immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated signaling cascade, and its potential of developing therapeutics for clinical applications. J. Cell. Physiol. 231: 2402-2407, 2016. © 2016 Wiley Periodicals, Inc.