Trabecular bone adaptation to variations in porous-coated implant topology

Challenges and perspectives on functional interpretations of australopith postcrania and the reconstruction of hominin locomotion

In 1994, Hunt published the 'postural feeding hypothesis'-a seminal paper on the origins of hominin bipedalism-founded on the detailed study of chimpanzee positional behavior and the functional inferences derived from the upper and lower limb morphology of the Australopithecus afarensis A.L. 288-1 partial skeleton. Hunt proposed a model for understanding the potential selective pressures on hominins, made robust, testable predictions based on Au. afarensis functional morphology, and presented a hypothesis that aimed to explain the dual functional signals of the Au. afarensis and, more generally, early hominin postcranium. Here we synthesize what we have learned about Au. afarensis functional morphology and the dual functional signals of two new australopith discoveries with relatively complete skeletons (Australopithecus sediba and StW 573 'Australopithecus prometheus'). We follow this with a discussion of three research approaches that have been developed for the purpose of drawing behavioral inferences in early hominins: (1) developments in the study of extant apes as models for understanding hominin origins; (2) novel and continued developments to quantify bipedal gait and locomotor economy in extant primates to infer the locomotor costs from the anatomy of fossil taxa; and (3) novel developments in the study of internal bone structure to extract functional signals from fossil remains. In conclusion of this review, we discuss some of the inherent challenges of the approaches and methodologies adopted to reconstruct the locomotor modes and behavioral repertoires in extinct primate taxa, and notably the assessment of habitual terrestrial bipedalism in early hominins.

Bone Ingrowth Around an Uncemented Femoral Implant Using Mechanoregulatory Algorithm: A Multiscale Finite Element Analysis

The primary fixation and long-term stability of a cementless femoral implant depend on bone ingrowth within the porous coating. Although attempts were made to quantify the peri-implant bone ingrowth using the finite element (FE) analysis and mechanoregulatory principles, the tissue differentiation patterns on a porous-coated hip stem have scarcely been investigated. The objective of this study is to predict the spatial distribution of evolutionary bone ingrowth around an uncemented hip stem, using a three-dimensional (3D) multiscale mechanobiology-based numerical framework. Multiple load cases representing a variety of daily living activities, including walking, stair climbing, sitting down, and standing up from a chair, were used as applied loading conditions. The study accounted for the local variations in host bone material properties and implant-bone relative displacements of the macroscale implanted FE model, in order to predict bone ingrowth in microscale representative volume elements (RVEs) of 12 interfacial regions. In majority RVEs, 20-70% bone tissue (immature and mature) was predicted after 2 months, contributing toward a progressive increase in average Young's modulus (1200-3000 MPa) of the interbead tissue layer. Higher bone ingrowth (mostly greater than 60%) was predicted in the anterolateral regions of the implant, as compared to the posteromedial side (20-50%). New bone tissue was formed deeper inside the interbead spacing, adhering to the implant surface. The study helps to gain an insight into the degree of osseointegration of a porous-coated femoral implant.

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.

First metatarsal trabecular bone structure in extant hominoids and Swartkrans hominins

Changes in first metatarsal (MT1) morphology within the hominin clade are crucial for reconstructing the evolution of a forefoot adapted for human-like gait. Studies of the external morphology of the MT1 in humans, non-human apes, and fossil hominins have documented changes in its robusticity, epiphyseal shape and its articulation with the medial cuneiform. Here, we test whether trabecular structure in the MT1 reflects different loading patterns in the forefoot across extant large apes and humans, and within this comparative context, infer locomotor behavior in two fossil hominins from Swartkrans, South Africa. Microtomographic scans were collected from the MT1 of Pongo sp. (n = 6), Gorilla gorilla (n = 10), Pan troglodytes (n = 10), Homo sapiens (n = 11), as well as SKX 5017 (Paranthropus robustus), and SK 1813 (Hominin gen. sp. indet.). Trabecular structure was quantified within the head and base using a 'whole-epiphysis' approach with medtool 4.2. We found that modern humans displayed relatively higher bone volume fraction (BV/TV) in the dorsal region of each epiphysis and a higher overall degree of anisotropy (DA), whereas great apes showed higher BV/TV in the plantar regions, reflecting dorsiflexion at the metatarsophalangeal (MTP) joint in the former and plantarflexion in the latter. Both fossils displayed low DA, with SKX 5017 showing a hyper-dorsal concentration of trabecular bone in the head (similar to humans), while SK 1813 showed a more central trabecular distribution not seen in either humans or non-human apes. Additionally, we found differences between non-human apes, modern humans, and the fossil taxa in trabecular spacing (Tb.Sp.), number (Tb.N.), and thickness (Tb.th.). While low DA in both fossils suggests increased mobility of the MT1, differences in their trabecular distributions could indicate variable locomotion in these Pleistocene hominins (recognizing that the juvenile status of SK 1813 is a potential confounding factor). In particular, evidence for consistent loading in hyper-dorsiflexion in SKX 5017 would suggest locomotor behaviors beyond human-like toe off during terrestrial locomotion.

Cancellous bone and theropod dinosaur locomotion. Part II-a new approach to inferring posture and locomotor biomechanics in extinct tetrapod vertebrates

This paper is the second of a three-part series that investigates the architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and therefore has the potential to provide insight into locomotor biomechanics in extinct tetrapod vertebrates such as dinosaurs. Here in Part II, a new biomechanical modelling approach is outlined, one which mechanistically links cancellous bone architectural patterns with three-dimensional musculoskeletal and finite element modelling of the hindlimb. In particular, the architecture of cancellous bone is used to derive a single 'characteristic posture' for a given species-one in which bone continuum-level principal stresses best align with cancellous bone fabric-and thereby clarify hindlimb locomotor biomechanics. The quasi-static approach was validated for an extant theropod, the chicken, and is shown to provide a good estimate of limb posture at around mid-stance. It also provides reasonable predictions of bone loading mechanics, especially for the proximal hindlimb, and also provides a broadly accurate assessment of muscle recruitment insofar as limb stabilization is concerned. In addition to being useful for better understanding locomotor biomechanics in extant species, the approach hence provides a new avenue by which to analyse, test and refine palaeobiomechanical hypotheses, not just for extinct theropods, but potentially many other extinct tetrapod groups as well.

Trabecular health of vertebrae based on anisotropy in trabecular architecture and collagen/apatite micro-arrangement after implantation of intervertebral fusion cages in the sheep spine

Healthy trabecular bone shows highly anisotropic trabecular architecture and the preferential orientation of collagen and apatite inside a trabecula, both of which are predominantly directed along the cephalocaudal axis. This makes trabecular bone stiff in the principally loaded direction (cephalocaudal axis). However, changes in these anisotropic trabecular characteristics after the insertion of implant devices remain unclear. We defined the trabecular architectural anisotropy and the preferential orientation of collagen and apatite as parameters of trabecular bone health. In the present study, we analyzed these parameters after the implantation of two types of intervertebral fusion cages, open and closed box-type cages, into sheep spines for 2 and 4months. Alteration and evolution of trabecular health around and inside the cages depended on the cage type and implantation duration. At the boundary region, the values of trabecular architectural anisotropy and apatite orientation for the closed-type cages were similar to those for isotropic conditions. In contrast, significantly larger anisotropy was found for open-type cages, indicating that the open-type cage tended to maintain trabecular anisotropy. Inside the open-type cage, trabecular architectural anisotropy and apatite orientation significantly increased with time after implantation. Assessing trabecular anisotropy might be useful for the evaluation of trabecular health and the validation and refinement of implant designs.

High-strength, surface-porous polyether-ether-ketone for load-bearing orthopedic implants

Despite its widespread clinical use in load-bearing orthopedic implants, polyether-ether-ketone (PEEK) is often associated with poor osseointegration. In this study, a surface-porous PEEK material (PEEK-SP) was created using a melt extrusion technique. The porous layer was 399.6±63.3 μm thick and possessed a mean pore size of 279.9±31.6 μm, strut spacing of 186.8±55.5 μm, porosity of 67.3±3.1% and interconnectivity of 99.9±0.1%. Monotonic tensile tests showed that PEEK-SP preserved 73.9% of the strength (71.06±2.17 MPa) and 73.4% of the elastic modulus (2.45±0.31 GPa) of as-received, injection-molded PEEK. PEEK-SP further demonstrated a fatigue strength of 60.0 MPa at one million cycles, preserving 73.4% of the fatigue resistance of injection-molded PEEK. Interfacial shear testing showed the pore layer shear strength to be 23.96±2.26 MPa. An osseointegration model in the rat revealed substantial bone formation within the pore layer at 6 and 12 weeks via microcomputed tomography and histological evaluation. Ingrown bone was more closely apposed to the pore wall and fibrous tissue growth was reduced in PEEK-SP when compared to non-porous PEEK controls. These results indicate that PEEK-SP could provide improved osseointegration while maintaining the structural integrity necessary for load-bearing orthopedic applications.

Interstitial fluid flow in canaliculi as a mechanical stimulus for cancellous bone remodeling: in silico validation

Cancellous bone has a dynamic 3-dimensional architecture of trabeculae, the arrangement of which is continually reorganized via bone remodeling to adapt to the mechanical environment. Osteocytes are currently believed to be the major mechanosensory cells and to regulate osteoclastic bone resorption and osteoblastic bone formation in response to mechanical stimuli. We previously developed a mathematical model of trabecular bone remodeling incorporating the possible mechanisms of cellular mechanosensing and intercellular communication in which we assumed that interstitial fluid flow activates the osteocytes to regulate bone remodeling. While the proposed model has been validated by the simulation of remodeling of a single trabecula, it remains unclear whether it can successfully represent in silico the functional adaptation of cancellous bone with its multiple trabeculae. In the present study, we demonstrated the response of cancellous bone morphology to uniaxial or bending loads using a combination of our remodeling model with the voxel finite element method. In this simulation, cancellous bone with randomly arranged trabeculae remodeled to form a well-organized architecture oriented parallel to the direction of loading, in agreement with the previous simulation results and experimental findings. These results suggested that our mathematical model for trabecular bone remodeling enables us to predict the reorganization of cancellous bone architecture from cellular activities. Furthermore, our remodeling model can represent the phenomenological law of bone transformation toward a locally uniform state of stress or strain at the trabecular level.

Preclinical evaluation of a novel implant for treatment of a full-thickness distal femoral focal cartilage defect

A novel, nonresorbable, monolithic composite structure ceramic, developed using a partially stabilized zirconia ceramic common to implantable devices, was used in a cementless weight-bearing articular implant to test the feasibility of replacing a region of degenerated or damaged articular cartilage in the knee as part of a preclinical study using male mongrel dogs lasting up to 24 weeks. Gross/histological cartilage observations showed no differences among control, 12-week and 24-week groups, while pull-out tests showed an increase in maximum pull-out load over time relative to controls. Hence, the use of a novel ceramic implant as a replacement for a focal cartilage defect leads to effective implant fixation within 12 weeks and does not cause significant degradation in opposing articular cartilage in the time frame evaluated.

NUMERICAL SIMULATIONS OF CHANGE IN TRABECULAR STRUCTURE DUE TO BONE REMODELING UNDER ULTRASOUND PROPAGATION

Bone remodeling is defined as the coupling of bone formation and resorption on the bone surface. Numerical simulations of the remodeling in cancellous bone were performed to reproduce the change in the trabecular structure. Assuming that the formation/resorption in cancellous bone could be generated on the trabecular surface, where the local stress under the mechanical load was larger/smaller than the averaged stress on the surrounding surface, voxel trabecular elements in a numerical model of bovine cancellous bone were added/removed. An ultrasound continuous wave in the frequency range 0.1–1.0 MHz was applied as the mechanical load, and then, the local stress was analyzed using a finite-difference time-domain (FDTD) method. Using the remodeling simulations, both changes in the trabecular structure could be reproduced with decreasing and increasing porosity. In changes, the trabecular elements and the pore spaces became strongly oriented in the direction of ultrasound propagation. In addition, the remodeling simulations indicated that both bone formation and resorption lessened as the frequency increased.

A paradigm for the development and evaluation of novel implant topologies for bone fixation: in vivo evaluation

While contemporary prosthetic devices restore some function to individuals who have lost a limb, there are efforts to develop bio-integrated prostheses to improve functionality. A critical step in advancing this technology will be to securely attach the device to remnant bone. To investigate mechanisms for establishing robust implant fixation in bone while undergoing loading, we previously used a topology optimization scheme to develop optimized orthopedic implants and then fabricated selected designs from titanium (Ti)-alloy with selective laser sintering (SLS) technology. In the present study, we examined how implant architecture and mechanical stimulation influence osseointegration within an in vivo environment. To do this, we evaluated three implant designs (two optimized and one non-optimized) using a unique in vivo model that applied cyclic, tension/compression loads to the implants. Eighteen (six per implant design) adult male canines had implants surgically placed in their proximal, tibial metaphyses. Experimental duration was 12 weeks; daily loading (peak load of ±22 N for 1000 cycles) was applied to one of each animal's bilateral implants for the latter six weeks. Following harvest, osseointegration was assessed by non-destructive mechanical testing, micro-computed tomography (microCT) and back-scatter scanning electron microscopy (SEM). Data revealed that implant loading enhanced osseointegration by significantly increasing construct stiffness, peri-implant trabecular morphology, and percentages of interface connectivity and bone ingrowth. While this experiment did not demonstrate a clear advantage associated with the optimized implant designs, osseointegration was found to be significantly influenced by aspects of implant architecture.

Effects of loading frequency on the functional adaptation of trabeculae predicted by bone remodeling simulation

The process of bone remodeling is regulated by metabolic activities of many bone cells. While osteoclasts and osteoblasts are responsible for bone resorption and formation, respectively, activities of these cells are believed to be controlled by a mechanosensory system of osteocytes embedded in the extracellular bone matrix. Several experimental and theoretical studies have suggested that the strain-derived interstitial fluid flow in lacuno-canalicular porosity serves as the prime mover for bone remodeling. Previously, we constructed a mathematical model for trabecular bone remodeling that interconnects the microscopic cellular activities with the macroscopic morphological changes in trabeculae through the mechanical hierarchy. This model assumes that fluid-induced shear stress acting on osteocyte processes is a driving force for bone remodeling. The validity of this model has been demonstrated with a remodeling simulation using a two-dimensional trabecular model. In this study, to investigate the effects of loading frequency, which is thought to be a significant mechanical factor in bone remodeling, we simulated morphological changes of a three-dimensional single trabecula under cyclic uniaxial loading with various frequencies. The results of the simulation show the trabecula reoriented to the loading direction with the progress of bone remodeling. Furthermore, as the imposed loading frequency increased, the diameter of the trabecula in the equilibrium state was enlarged by remodeling. These results indicate that our simulation model can successfully evaluate the relationship between loading frequency and trabecular bone remodeling.

Trabecular bone remodelling simulation considering osteocytic response to fluid-induced shear stress

In bone functional adaptation by remodelling, osteocytes in the lacuno-canalicular system are believed to play important roles in the mechanosensory system. Under dynamic loading, bone matrix deformation generates an interstitial fluid flow in the lacuno-canalicular system; this flow induces shear stress on the osteocytic process membrane that is known to stimulate the osteocytes. In this sense, the osteocytes behave as mechanosensors and deliver mechanical information to neighbouring cells through the intercellular communication network. In this study, bone remodelling is assumed to be regulated by the mechanical signals collected by the osteocytes. From the viewpoint of multi-scale biomechanics, we propose a mathematical model of trabecular bone remodelling that takes into account the osteocytic mechanosensory network system. Based on this model, a computational simulation of trabecular bone remodelling was conducted for a single trabecula under cyclic uniaxial loading, demonstrating functional adaptation to the applied mechanical loading as a load-bearing construct.

The Effect of the Loading Condition Corresponding to Functional Shoulder Activities on Trabecular Architecture of Glenoid

There is little information on bone morphology as it relates to shoulder activities. This study investigated how loads corresponding to functional shoulder activities affect the trabecular architecture of the glenoid. Two different protocols were used. Protocol 1 investigated the material and morphological characteristics of the glenoid by analyzing digitized trabecular bone images obtained from 12 cadaver scapula specimens. Protocol 2 used a finite element analysis (FEA) to compute the principal stress trajectories acting within the glenoid. The principal stresses were derived for five loading conditions, which represent typical functional shoulder activities. The study showed that shoulder activity involved in carrying a light load makes the greatest contribution to the trabecular architecture compared with other shoulder activities considered in this study (p<0.05). With all of the activities considered in this study, the lateral region, particularly in the anterior and posterior portions, showed greater deviation and greater sensitivity to variation under loading conditions than did the other regions (p<0.05). These results suggest that owing to the extra sensitivity of the anterior and posterior parts of the lateral region, these regions may be more informative in the analysis of the trabecular architecture following shoulder musculoskeletal injuries. These results may provide essential design information for shoulder prostheses and contribute to an understanding of morphological changes resulting from injury.

Spatial and temporal regulation of cancellous bone structure: characterization of a rate equation of trabecular surface remodeling

A computer simulation of trabecular surface remodeling was carried out to investigate the spatial and temporal regulation of the cancellous bone structure caused by bone cellular activities responding to a local mechanical environment. In the remodeling simulation, the rate of trabecular surface movement was directly related to stress at the trabecular level. Two model parameters, the threshold value of the lazy zone and the sensing distance of the mechanical environment, were introduced into the remodeling rate equation to express the sensitivity of bone cells to mechanical stimuli. A rectangular cancellous bone model under simple and nonuniform compressive loads was constructed using pixel finite elements. A simulation result revealed that the trabecular structure underwent a temporal and spatial change depending on the loading condition. It was found that the threshold value of the lazy zone regulates the rate of structural changes in time, and that sensing distance regulates the spatial distribution of the trabecular structure. The results demonstrate the possibility that the spatial and temporal regulation of the trabecular structure is determined by the sensitivities of bone cells to mechanical stimuli.

Volume-based non-continuum modeling of bone functional adaptation

BACKGROUND

Bone adapts to mechanical strain by rearranging the trabecular geometry and bone density. The common finite element methods used to simulate this adaptation have inconsistencies regarding material properties at each node and are computationally demanding. Here, a volume-based, non-continuum formulation is proposed as an alternative. Adaptive processes corresponding to various external mechanical loading conditions are simulated for the femur.

RESULTS

Bone adaptations were modeled for one-legged stance, abduction and adduction. One-legged stance generally results in higher bone densities than the other two loading cases. The femoral head and neck are the regions where densities change most drastically under different loading conditions while the distal area always contains the lowest densities regardless of the loading conditions. In the proposed formulation, the inconsistency of material densities or strain energy densities, which is a common problem to finite element based approaches, is eliminated. The computational task is alleviated through introduction of the quasi-binary connectivity matrix and linearization operations in the Jacobian matrix and is therefore computationally less demanding.

CONCLUSION

The results demonstrated the viability of the proposed formulation to study bone functional adaptation under mechanical loading.

Mediation of bone ingrowth in porous hydroxyapatite bone graft substitutes

Previous investigations have shown that both the early biological response and the mechanical properties of a porous hydroxyapatite bone graft substitute are highly sensitive to its pore structure. The objective of this study was to evaluate whether the pore structure continued to influence bone integration in the medium to long term. Two screened batches of porous hydroxyapatite (PHA) designated as batch A and batch B, with porosities of approximately 60 and 80%, respectively, were selected for this study and implanted for periods of 5, 13, and 26 weeks into the lower femur of New Zealand White rabbits. Histomorphometric analysis of the absolute volume of bone ingrowth within batch A and B implants from 5 to 26 weeks showed that the absolute volume of bone ingrowth was consistently lower in batch A (10-21%), compared to batch B implants (24-31%). However, when the volume of bone ingrowth was normalised for the available pore space, this difference was reduced (23-47% and 32-42% for batches A and B, respectively). These observations suggest that differences in the volume of bone ingrowth initially depended on pore interconnectivity rather than pore size, whereas the volume or morphology of the PHA influenced the volume and morphology of bone ingrowth at later time points. Compression testing showed that bone ingrowth had a strong reinforcing effect on PHA bone graft substitutes, and a strong correlation was identified between mechanical properties and the absolute volume of ingrowth for both batches A and B. Furthermore, at 13 and 26 weeks, there was no significant variation in the ultimate compressive strength of integrated batch A and B implants. This similarity in ultimate mechanical properties indicated that the absolute volume of ingrowth may be mediated by the PHA structure through its impact on the dynamics of the local biomechanical environment. The results of push-out testing showed that fixation of PHA bone graft substitutes was independent of density within the range studied, with no significant difference in the interfacial shear stress between batches A and B at each time point throughout the study.

Changes in the Fabric and Compliance Tensors of Cancellous Bone due to Trabecular Surface Remodeling, Predicted by a Digital Image-based Model

Fabric and compliance tensors of a cube of cancellous bone with a complicated three-dimensional trabecular structure were obtained for trabecular surface remodeling by using a digital image-based model combined with a large-scale finite element method. Using mean intercept length and a homogenization method, the fabric and compliance tensors were determined for the trabecular structure obtained in the computer remodeling simulation. The tensorial quantities obtained indicated that anisotropic structural changes occur in cancellous bone adapting to the compressive loading condition. There were good correlations between the fabric tensor, bone volume fraction, and compliance tensor in the remodeling process. The result demonstrates that changes in the structural and mechanical properties of cancellous bone are essentially anisotropic and should be expressed by tensorial quantities.

Biological adaptive control model: a mechanical analogue of multi-factorial bone density adaptation

The mechanism of how bone adapts to every day demands needs to be better understood to gain insight into situations in which the musculoskeletal system is perturbed. This paper offers a novel multi-factorial mathematical model of bone density adaptation which combines previous single-factor models in a single adaptation system as a means of gaining this insight. Unique aspects of the model include provision for interaction between factors and an estimation of the relative contribution of each factor. This interacting system is considered analogous to a Newtonian mechanical system and the governing response equation is derived as a linear version of the adaptation process. The transient solution to sudden environmental change is found to be exponential or oscillatory depending on the balance between cellular activation and deactivation frequencies.

Functional adaptation of cancellous bone in human proximal femur predicted by trabecular surface remodeling simulation toward uniform stress state

Two-dimensional simulation of trabecular surface remodeling was conducted for a human proximal femur to investigate the structural change of cancellous bone toward a uniform stress state. Considering that a local mechanical stimulus plays an important role in cellular activities in bone remodeling, local stress nonuniformity was assumed to drive trabecular structural change to seek a uniform stress state. A large-scale pixel-based finite element model was used to simulate structural changes of individual trabeculae over the entire bone. As a result, the initial structure of trabeculae changed from isotropic to anisotropic due to trabecular microstructural changes caused by surface remodeling according to the mechanical environment in the proximal femur. Under a single-loading condition, it was shown that the apparent structural property evaluated by fabric ellipses corresponded to the apparent stress state in cancellous bone. As is observed in the actual bone, a distributed trabecular structure was obtained under a multiple-loading condition. Through these studies, it was concluded that trabecular surface remodeling toward a local uniform stress state at the trabecular level could naturally bring about functional adaptation phenomenon at the apparent tissue level. The proposed simulation model would be capable of providing insight into the hierarchical mechanism of trabecular surface remodeling at the microstructural level up to the apparent tissue level.

Development of magnetic particle techniques for long-term culture of bone cells with intermittent mechanical activation

Magnetic particles were coated with RGD and adhered to primary human osteoblasts. During a 21-day culture, the osteoblasts plus adhered magnetic particles underwent a daily exposure to a time-varying magnetic field via a permanent NdFeB magnet, thus applying a direct mechanical stress to the cells (Bmax approximately 60 mT). After 21 days, preliminary results show that the cells plus magnetic particles were viable and had proliferated. A von-kossa stain showed mineralized bone matrix produced at 21 days in the experimental group whereas the control groups showed no mineralized matrix production. Real-time reverse transcription-polymerase chain reaction at 21 days showed an upregulation of osteopontin from the experimental group in comparison to the control group of cells with adhered particles and no magnet applied. These preliminary results indicate that adherence of RGD-coated 4.5 microm ferromagnetic particles to primary human osteoblasts does not initiate cell necrosis up to 21 days in vitro. Also, mechanical stimulation of human osteoblasts by magnetic particle technology appears to have an influence on osteoblastic activity.

Why mechanobiology? A survey article

The central paradigm of skeletal mechanobiology is that mechanical forces modulate morphological and structural fitness of the skeletal tissues-bone, cartilage, ligament and tendon. Traditionally, skeletal biomechanics has focussed on how these tissues perform the structural and locomotory functions of the vertebrate skeleton. In mechanobiology the central question is how these same load-bearing tissues are produced, maintained and adapted by cells as an active response to biophysical stimuli in their environment. The idea that 'form follows function' is not new, but we now believe that the scientific community has the knowledge and tools to prove, understand and use functional adaptation to benefit medicine and human health. In this Survey Article the philosophy and progress of skeletal mechanobiology are discussed. The revival of this science, with roots dating back to the 19th Century, is now driven by new developments in cellular, molecular and computational technologies. These developments are still in an early stage of application, but if modern mechanobiology fulfills the promises of its ambitions, the results will bring great benefits to tissue engineering and to the treatment and prevention of skeletal conditions such as congenital deformities, osteoporosis, osteoarthritis and bone fractures.

Weight-bearing alters the expression of collagen types I and II, BMP 2/4 and osteocalcin in the early stages of distraction osteogenesis

This study was performed to investigate the effect of loading on the biology of newly forming bone during limb lengthening. Unilateral 2.0 mm femoral lengthenings were performed in 20 male Sprague Dawley rats. Half (n = 10) of the animals were allowed to bear weight freely, while the other half were prevented from weight-bearing via an ipsilateral through-knee amputation. The animals in each group were sacrificed after one (n = 5) or four (n = 5) days of consolidation (post-operative days seven and 10, respectively). In situ hybridization for osteocalcin and collagen I, and antibody staining for collagen II and BMP 2/4 were used to evaluate the molecular influence of loading. There was more new bone in the distraction gap of the weight-bearing animals than there was in the non-weight-bearing animals. BMP 2/4 expression, and the messages for collagen I and osteocalcin, were more abundant in tissue from the weight-bearing animals; collagen II was higher in the non-weight-bearing animals. This suggests that early regenerate tissue is capable of responding to loading, and that weight-bearing appears to stimulate intramembranous ossification. These findings support the concept of early weight-bearing after limb lengthening.

Trabecular surface remodeling simulation for cancellous bone using microstructural voxel finite element models

A computational simulation method for three-dimensional trabecular surface remodeling was proposed, using voxel finite element models of cancellous bone, and was applied to the experimental data. In the simulation, the trabecular microstructure was modeled based on digital images, and its morphological changes due to surface movement at the trabecular level were directly expressed by removing/adding the voxel elements from/to the trabecular surface. A remodeling simulation at the single trabecular level under uniaxial compressive loading demonstrated smooth morphological changes even though the trabeculae were modeled with discrete voxel elements. Moreover, the trabecular axis rotated toward the loading direction with increasing stiffness, simulating functional adaptation to the applied load. In the remodeling simulation at the trabecular structural level, a cancellous bone cube was modeled using a digital image obtained by microcomputed tomography (microCT), and was uniaxially compressed. As a result, the apparent stiffness against the applied load increased by remodeling, in which the trabeculae reoriented to the loading direction. In addition, changes in the structural indices of the trabecular architecture coincided qualitatively with previously published experimental observations. Through these studies, it was demonstrated that the newly proposed voxel simulation technique enables us to simulate the trabecular surface remodeling and to compare the results obtained using this technique with the in vivo experimental data in the investigation of the adaptive bone remodeling phenomenon.

Loading improves anchorage of hydroxyapatite implants more than titanium implants

Roentgen Stereophotogrammetric Analysis (RSA) studies have shown that the quality of the early fixation of implants has a dominant effect on their long-term function. To evaluate methods to improve their fixation, we examined the influence of mechanical loading and surface coating on the quality of the bone-implant interface. We compared the fixation of a cylindrical, stable 6.0 mm implant initially surrounded by a 0.75 mm concentric gap, after 4 weeks of loaded or unloaded conditions. Two types of surfaces were analyzed: plasma sprayed hydroxyapatite (HA) and plasma sprayed titanium (Ti). The histomorphometric evaluation showed that HA implants had greater bone coverage than Ti implants, and this coverage was further increased under loaded conditions only for HA. Furthermore, loading reduced the fibrous tissue coverage for the HA implants, while it increased fibrous tissue coverage for Ti implants. These findings were in agreement with pushout results showing that HA implants had greater shear strength, stiffness, and energy than Ti implants, and (except for energy) these parameters were further increased under loaded conditions only for HA. In addition, because the two implant surfaces exhibited a different relative response to load, it is important to evaluate new surfaces under the more clinically relevant loaded condition.

An in vivo model for investigations of mechanical signal transduction in trabecular bone

The premise that bone cells are able to perceive and respond to mechanical forces is well accepted. This article describes the use of an in vivo hydraulic bone chamber for investigations of mechanical signal transduction. The servohydraulic loading mechanism was activated to apply a controlled compressive load to the woven trabecular bone that formed in one chamber, while the contralateral chamber served as an unloaded control. Specimens were harvested at a series of postload time points, and the cellular response to loading was evaluated by cytochemical, histomorphometric, and Northern blot analysis. A repetitive daily load stimulus elicited osteoblast biosynthetic activity characterized by an initial increase in type I procollagen by day 3 and a subsequent rise in alkaline phosphatase (ALP) activity after the sixth daily load episode. Application of a single load episode induced a biphasic pattern of c-fos and zif-268 gene expression with up-regulation at 30 minutes, down-regulation at 12 h, and up-regulation 24 h after the mechanical stimulus. The results show that a synchronized pattern of bone cell activity and gene expression occurs in response to controlled mechanical stimulation and that candidate load-responsive molecular mediators can be evaluated easily by this model.

Mechanical stimulation of tissue repair in the hydraulic bone chamber

A hydraulically activated bone chamber model was utilized to investigate cellular and microstructural mechanisms of mechanical adaptation during bone repair. Woven trabecular bone and fibrotic granulation tissue filled the initially empty chambers by 8 weeks postimplantation into canine tibial and femoral metaphyses. Without mechanical stimulation, active bone remodeling to lamellar trabecular bone and reconstitution of marrow elements were observed between 8 and 24 weeks. In subsequent loading studies, the hydraulic mechanism was activated on one randomly chosen side of 10 dogs following 8 weeks of undisturbed bone repair. The loading treatment applied an intermittent compressive force (18 N, 1.0 Hz, 1800 cycles/day) for durations of a few days up to 12 weeks. Stereological analysis of three-dimensional microcomputed tomography images revealed an increase in trabecular plate thickness and connectivity associated with the loaded repair tissue microstructure relative to unloaded contralateral controls. These microstructural alterations corresponded to an over 600% increase in the apparent modulus of the loaded bone tissue. A significant increase in the percentage of trabecular surfaces lined by osteoblasts immunopositive for type I procollagen after a few days of loading provided further evidence for mechanical stimulation of bone matrix synthesis. The local principal tissue strains associated with these adaptive changes were estimated to range from approximately -2000 to +3000 mustrain using digital image-based finite element methods. This study demonstrates the sensitivity of bone tissue and cells to a controlled in vivo mechanical stimulus and identifies microstructural mechanisms of mechanical adaptation during bone repair. The hydraulic bone chamber is introduced as an efficient experimental model to study the effects of mechanical and biological factors on bone repair and regeneration.