Trabecular bone adaptation to loading in a rabbit model is not magnitude-dependent

A translational murine model of aseptic loosening with osseointegration failure

An in vivo animal model of a weight-bearing intra-articular implant is crucial to the study of implant osseointegration and aseptic loosening caused by osseointegration failure. Osseointegration, defined as a direct structural and functional attachment between living bone tissue and the surface of a load-carrying implant, is essential for implant stability and considered a prerequisite for the long-term clinical success of implants in total joint arthroplasty. Compared to large animal models, murine models offer extensive genetic tools for tracing cell differentiation and proliferation. The 18- to 22-week-old C57BL/6J background mice underwent either press-fitted or loose implantation of a titanium implant, achieving osseointegration or fibrous integration. A protocol was developed for both versions of the procedure, including a description of the relevant anatomy. Samples were subjected to microcomputed tomography and underwent biomechanical testing to access osseointegration. Lastly, samples were fixed and embedded for histological evaluation. The absence of mineralized tissue and weakened maximum pull-out force in loose implantation samples indicated that these implants were less mechanically stable compared to the control at 4 weeks postoperation. Histological analysis demonstrated extensive fibrotic tissue in the peri-implant area of loose implantation samples and excellent implant osseointegration in press-fitted samples at 4 weeks. Both mechanically stable and unstable hemiarthroplasty models with either osseous ingrowth or a robust periprosthetic fibrosis were achieved in mice. We hope that this model can help address current limitations for in vivo study of aseptic loosening and lead to necessary translational benefits.

Cortical and Trabecular Bone Modeling and Implications for Bone Functional Adaptation in the Mammalian Tibia

Bone modeling involves the addition of bone material through osteoblast-mediated deposition or the removal of bone material via osteoclast-mediated resorption in response to perceived changes in loads by osteocytes. This process is characterized by the independent occurrence of deposition and resorption, which can take place simultaneously at different locations within the bone due to variations in stress levels across its different regions. The principle of bone functional adaptation states that cortical and trabecular bone tissues will respond to mechanical stimuli by adjusting (i.e., bone modeling) their morphology and architecture to mechanically improve their mechanical function in line with the habitual in vivo loading direction. This principle is relevant to various research areas, such as the development of improved orthopedic implants, preventative medicine for osteopenic elderly patients, and the investigation of locomotion behavior in extinct species. In the present review, the mammalian tibia is used as an example to explore cortical and trabecular bone modeling and to examine its implications for the functional adaptation of bones. Following a short introduction and an exposition on characteristics of mechanical stimuli that influence bone modeling, a detailed critical appraisal of the literature on cortical and trabecular bone modeling and bone functional adaptation is given. By synthesizing key findings from studies involving small mammals (rodents), large mammals, and humans, it is shown that examining both cortical and trabecular bone structures is essential for understanding bone functional adaptation. A combined approach can provide a more comprehensive understanding of this significant physiological phenomenon, as each structure contributes uniquely to the phenomenon.

The trabecular architecture of the popliteal sesamoid bone (cyamella) from a New Zealand white rabbit (Oryctolagus cuniculus)

Sesamoid bones are ossified structures that are embedded in tendons near articulation. They consist of an inner trabecular bone architecture surrounded by a thin cortical shell. While the formation of sesamoid bones is probably mainly controlled by genetic factors, the proper development and mineralization of a sesamoid bone depends also on mechanical stimulation. While most sesamoid bones are not loaded directly by other bones during locomotion, they still experience forces directed from the tendon in which they are embedded. In cases when the sesamoid bone is experiencing forces only from a single tendon, such as the cyamella in the rabbit, this may give us a tool to study bone functional adaptation in a relatively simple loading setting. This study investigates the internal trabecular architecture of the popliteal sesamoid bone (cyamellae) in New Zealand white (NZW) rabbits (Oryctolagus cuniculus). Five hind limbs of NZW rabbits were micro-computed tomography scanned and the cortical and trabecular architectures of the cyamellae were evaluated. The results revealed that similar to the patella, the cyamella has a thin cortex and a high trabecular bone volume fraction (BV/TV), which is derived mostly from the high trabecular thickness (Tb.Th). Trabecular BV/TV and Tb.Th were not distributed homogeneously, but they were lower at the periphery and higher closer to the proximal and middle of the cyamella, near the musculotendinous junction. The results also demonstrated that trabeculae tend to align along two recognizable orientations, one with the direction of tensile stresses, in line with the popliteal tendon, and the second bridging the narrow space between the cranial and caudal cortical faces of the bone.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

RNA-seq Analysis of Peri-Implant Tissue Shows Differences in Immune, Notch, Wnt, and Angiogenesis Pathways in Aged Versus Young Mice

The number of total joint replacements (TJRs) in the United States is increasing annually. Cementless implants are intended to improve upon traditional cemented implants by allowing bone growth directly on the surface to improve implant longevity. One major complication of TJR is implant loosening, which is related to deficient osseointegration in cementless TJRs. Although poor osseointegration in aged patients is typically attributed to decreased basal bone mass, little is known about the molecular pathways that compromise the growth of bone onto porous titanium implants. To identify the pathways important for osseointegration that are compromised by aging, we developed an approach for transcriptomic profiling of peri-implant tissue in young and aged mice using our murine model of osseointegration. Based on previous findings of changes of bone quality associated with aging, we hypothesized that aged mice have impaired activation of bone anabolic pathways at the bone-implant interface. We found that pathways most significantly downregulated in aged mice relative to young mice are related to angiogenic, Notch, and Wnt signaling. Downregulation of these pathways is associated with markedly increased expression of inflammatory and immune genes at the bone-implant interface in aged mice. These results identify osseointegration pathways affected by aging and suggest that an increased inflammatory response in aged mice may compromise peri-implant bone healing. Targeting the Notch and Wnt pathways, promoting angiogenesis, or modulating the immune response at the peri-implant site may enhance osseointegration and improve the outcome of joint replacement in older patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Cancellous Bone May Have a Greater Adaptive Strain Threshold Than Cortical Bone

Strain magnitude has a controlling influence on bone adaptive response. However, questions remain as to how and if cancellous and cortical bone tissues respond differently to varied strain magnitudes, particularly at a molecular level. The goal of this study was to characterize the time‐dependent gene expression, bone formation, and structural response of the cancellous and cortical bone of female C57Bl/6 mice to mechanical loading by applying varying load levels (low: −3.5 N; medium: −5.2 N; high: −7 N) to the skeleton using a mouse tibia loading model. The loading experiment showed that cortical bone mass at the tibial midshaft was significantly enhanced following all load levels examined and bone formation activities were particularly elevated at the medium and high loads applied. In contrast, for the proximal metaphyseal cancellous bone, only the high load led to significant increases in bone mass and bone formation indices. Similarly, expression of genes associated with inhibition of bone formation (e.g., Sost) was altered in the diaphyseal cortical bone at all load levels, but in the metaphyseal cortico‐cancellous bone only by the high load. Finite element analysis determined that the peak tensile or compressive strains that were osteogenic for the proximal cancellous bone under the high load were significantly greater than those that were osteogenic for the midshaft cortical tissues under the low load. These results suggest that the magnitude of the strain stimulus regulating structural, cellular, and molecular responses of bone to loading may be greater for the cancellous tissues than for the cortical tissues. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Mechanically superior matrices promote osteointegration and regeneration of anterior cruciate ligament tissue in rabbits

The gold standard treatment for anterior cruciate ligament (ACL) reconstruction is the use of tendon autografts and allografts. Limiting factors for this treatment include donor site morbidity, potential disease transmission, and variable graft quality. To address these limitations, we previously developed an off-the-shelf alternative, a poly(l-lactic) acid (PLLA) bioengineered ACL matrix, and demonstrated its feasibility to regenerate ACL tissue. This study aims to 1) accelerate the rate of regeneration using the bioengineered ACL matrix by supplementation with bone marrow aspirate concentrate (BMAC) and growth factors (BMP-2, FGF-2, and FGF-8) and 2) increase matrix strength retention. Histological evaluation showed robust tissue regeneration in all groups. The presence of cuboidal cells reminiscent of ACL fibroblasts and chondrocytes surrounded by an extracellular matrix rich in anionic macromolecules was up-regulated in the BMAC group. This was not observed in previous studies and is indicative of enhanced regeneration. Additionally, intraarticular treatment with FGF-2 and FGF-8 was found to suppress joint inflammation. To increase matrix strength retention, we incorporated nondegradable fibers, polyethylene terephthalate (PET), into the PLLA bioengineered ACL matrix to fabricate a "tiger graft." The tiger graft demonstrated the greatest peak loads among the experimental groups and the highest to date in a rabbit model. Moreover, the tiger graft showed superior osteointegration, making it an ideal bioengineered ACL matrix. The results of this study illustrate the beneficial effect bioactive factors and PET incorporation have on ACL regeneration and signal a promising step toward the clinical translation of a functional bioengineered ACL matrix.

Murine Axial Compression Tibial Loading Model to Study Bone Mechanobiology: Implementing the Model and Reporting Results

In vivo, tibial loading in mice is increasingly used to study bone adaptation and mechanotransduction. To achieve standardized and defined experimental conditions, loading parameters and animal-related factors must be considered when performing in vivo loading studies. In this review, we discuss these loading and animal-related experimental conditions, present methods to assess bone adaptation, and suggest reporting guidelines. This review originated from presentations by each of the authors at the workshop "Developing Best Practices for Mouse Models of In Vivo Loading" during the Preclinical Models Section at the Orthopaedic Research Society Annual Meeting, San Diego, CA, March 2017. Following the meeting, the authors engaged in detailed discussions with consideration of relevant literature. The guidelines and recommendations in this review are provided to help researchers perform in vivo loading experiments in mice, and thus further our knowledge of bone adaptation and the mechanisms involved in mechanotransduction. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:233-252, 2020.

Bone healing response in cyclically loaded implants: Comparing zero, one, and two loading sessions per day

When bone implants are loaded, they are inevitably subjected to displacement relative to bone. Such micro-motion generates stress/strain states at the interface that can cause beneficial or detrimental sequels. The objective of this study is to better understand the mechanobiology of bone healing at the tissue-implant interface during repeated loading. Machined screw shaped Ti implants were placed in rat tibiae in a hole slightly bigger than the implant diameter. Implants were held stable by a specially-designed bone plate that permits controlled loading. Three loading regimens were applied, (a) zero loading, (b) one daily loading session of 60 cycles with an axial force of 1.5 N/cycle for 7 days, and (c) two such daily sessions with the same axial force also for 7 days. Finite element analysis was used to characterize the mechanobiological conditions produced by the loading sessions. After 7 days, the implants with surrounding interfacial tissue were harvested and processed for histological, histomorphometric and DNA microarray analyses. Histomorphometric analyses revealed that the group subjected to repeated loading sessions exhibited a significant decrease in bone-implant contact and increase in bone-implant distance, as compared to unloaded implants and those subjected to only one loading session. Gene expression profiles differed during osseointegration between all groups mainly with respect to inflammatory and unidentified gene categories. The results indicate that increasing the daily cyclic loading of implants induces deleterious changes in the bone healing response, most likely due to the accumulation of tissue damage and associated inflammatory reaction at the bone-implant interface.

Time course of peri-implant bone regeneration around loaded and unloaded implants in a rat model

The time-course of cancellous bone regeneration surrounding mechanically loaded implants affects implant fixation, and is relevant to determining optimal rehabilitation protocols following orthopaedic surgeries. We investigated the influence of controlled mechanical loading of titanium-coated polyether-ether ketone (PEEK) implants on osseointegration using time-lapsed, non-invasive, in vivo micro-computed tomography (micro-CT) scans. Implants were inserted into proximal tibial metaphyses of both limbs of eight female Sprague-Dawley rats. External cyclic loading (60 or 100 μm displacement, 1 Hz, 60 s) was applied every other day for 14 days to one implant in each rat, while implants in contralateral limbs served as the unloaded controls. Hind limbs were imaged with high-resolution micro-CT (12.5 μm voxel size) at 2, 5, 9, and 12 days post-surgery. Trabecular changes over time were detected by 3D image registration allowing for measurements of bone-formation rate (BFR) and bone-resorption rate (BRR). At day 9, mean %BV/TV for loaded and unloaded limbs were 35.5 ± 10.0% and 37.2 ± 10.0%, respectively, and demonstrated significant increases in bone volume compared to day 2. BRR increased significantly after day 9. No significant differences between bone volumes, BFR, and BRR were detected due to implant loading. Although not reaching significance (p = 0.16), an average 119% increase in pull-out strength was measured in the loaded implants. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:997-1006, 2017.

Interrelation between external oscillatory muscle coupling amplitude and in vivo intramedullary pressure related bone adaptation

Interstitial bone fluid flow (IBFF) is suggested as a communication medium that bridges external physical signals and internal cellular activities in the bone, which thus regulates bone remodeling. Intramedullary pressure (ImP) is one main regulatory factor of IBFF and bone adaptation related mechanotransduction. Our group has recently observed that dynamic hydraulic stimulation (DHS), as an external oscillatory muscle coupling, was able to induce local ImP with minimal bone strain as well as to mitigate disuse bone loss. The current study aimed to evaluate the dose dependent relationship between DHS's amplitude, i.e., 15 and 30mmHg, and in vivo ImP induction, as well as this correlation on bone's phenotypic change. Simultaneous measurements of ImP and DHS cuff pressures were obtained from rats under DHS with various magnitudes and a constant frequency of 2Hz. ImP inductions and cuff pressures upon DHS loading showed a positively proportional response over the amplitude sweep. The relationship between ImP and DHS cuff pressure was evaluated and shown to be proportional, in which ImP was raised with increases of DHS cuff pressure amplitudes (R(2)=0.98). A 4-week in vivo experiment using a rat hindlimb suspension model demonstrated that the mitigation effect of DHS on disuse trabecular bone was highly dose dependent and related to DHS's amplitude, where a higher ImP led to a higher bone volume. This study suggested that sufficient physiological DHS is needed to generate ImP. Oscillatory DHS, potentially induces local fluid flow, has shown dose dependence in attenuation of disuse osteopenia.