Cortical and trabecular bone adaptation to incremental load magnitudes using the mouse tibial axial compression loading model

Lnk Deficiency Leads to TPO-Mediated Osteoclastogenesis and Increased Bone Mass Phenotype

The Lnk adapter protein negatively regulates the signaling of thrombopoietin (TPO), the main megakaryocyte (MK) growth factor. Lnk-deficient (-/-) mice have increased TPO signaling and increased MK number. Interestingly, several mouse models exist in which increased MK number leads to a high bone mass phenotype. Here we report the bone phenotype of these mice. MicroCT and static histomorphometric analyses at 20 weeks showed the distal femur of Lnk^-/- mice to have significantly higher bone volume fraction and trabecular number compared to wild-type (WT) mice. Notably, despite a significant increase in the number of osteoclasts (OC), and decreased bone formation rate in Lnk^-/- mice compared to WT mice, Lnk^-/- mice demonstrated a 2.5-fold greater BV/TV suggesting impaired OC function in vivo. Additionally, Lnk^-/- mouse femurs exhibited non-significant increases in mid-shaft cross-sectional area, yet increased periosteal BFR compared to WT femurs was observed. Lnk^-/- femurs also had non-significant increases in polar moment of inertia and decreased cortical bone area and thickness, resulting in reduced bone stiffness, modulus, and strength compared to WT femurs. Of note, Lnk is expressed by OC lineage cells and when Lnk^-/- OC progenitors are cultured in the presence of TPO, significantly more OC are observed than in WT cultures. Lnk is also expressed in osteoblast (OB) cells and in vitro reduced alkaline phosphatase activity was observed in Lnk^-/- cultures. These data suggest that both direct effects on OB and OC as well as indirect effects of MK in regulating OB contributes to the observed high bone mass. J. Cell. Biochem. 118: 2231-2240, 2017. © 2017 Wiley Periodicals, Inc.

Different bone remodeling levels of trabecular and cortical bone in response to changes in Wnt/β-catenin signaling in mice

Trabecular bone and cortical bone have different bone remodeling levels, and the underlying mechanisms are not fully understood. In the present study, the expression of Wnt/β-catenin signaling and its downstream molecules along with bone mass in trabecular and cortical bone were compared in wild-type mice, constitutive activation of β-catenin (CA-β-catenin) mice and β-catenin deletion mice. It was found that the expression level of most of the examined genes such as Wnt3a, β-catenin, osteocalcin and RANKL/OPG ratio were significantly higher in trabecular bone than in cortical bone in wild-type mice. CA-β-catenin resulted in up-regulated expression of the above-mentioned genes except for RANKL/OPG ratio, which were down-regulated. Also, CA-β-catenin led to increased number of osteoblasts, decreased number of osteoclasts and increased bone mass in both the trabecular bone and cortical bone compared with wild-type mice; however, the extent of changes was much greater in the trabecular bone than in the cortical bone. By contrast, null β-catenin led to down-regulated expression of the above-mentioned genes except for RANKL/OPG ratio. Furthermore, β-catenin deletion led to decreased number of osteoblasts, increased number of osteoclasts and decreased bone mass when compared with wild-type mice. Again, the extent of these changes was more significant in trabecular bone than cortical bone. Taken together, we found that the expression level of Wnt/β-catenin signaling and bone remodeling-related molecules were different in cortical bone and trabecular bone, and the trabecular bone was more readily affected by changes in the Wnt/β-catenin signaling pathway. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:812-819, 2017.

Localized tissue mineralization regulated by bone remodelling: A computational approach

Bone is a living tissue whose main mechanical function is to provide stiffness, strength and protection to the body. Both stiffness and strength depend on the mineralization of the organic matrix, which is constantly being remodelled by the coordinated action of the bone multicellular units (BMUs). Due to the dynamics of both remodelling and mineralization, each sample of bone is composed of structural units (osteons in cortical and packets in cancellous bone) created at different times, therefore presenting different levels of mineral content. In this work, a computational model is used to understand the feedback between the remodelling and the mineralization processes under different load conditions and bone porosities. This model considers that osteoclasts primarily resorb those parts of bone closer to the surface, which are younger and less mineralized than older inner ones. Under equilibrium loads, results show that bone volumes with both the highest and the lowest levels of porosity (cancellous and cortical respectively) tend to develop higher levels of mineral content compared to volumes with intermediate porosity, thus presenting higher material densities. In good agreement with recent experimental measurements, a boomerang-like pattern emerges when plotting apparent density at the tissue level versus material density at the bone material level. Overload and disuse states are studied too, resulting in a translation of the apparent-material density curve. Numerical results are discussed pointing to potential clinical applications.

Effects of Loading Duration and Short Rest Insertion on Cancellous and Cortical Bone Adaptation in the Mouse Tibia

The skeleton's osteogenic response to mechanical loading can be affected by loading duration and rest insertion during a series of loading events. Prior animal loading studies have shown that the cortical bone response saturates quickly and short rest insertions between load cycles can enhance cortical bone formation. However, it remains unknown how loading duration and short rest insertion affect load-induced osteogenesis in the mouse tibial compressive loading model, and particularly in cancellous bone. To address this issue, we applied cyclic loading (-9 N peak load; 4 Hz) to the tibiae of three groups of 16 week-old female C57BL/6 mice for two weeks, with a different number of continuous load cycles applied daily to each group (36, 216 and 1200). A fourth group was loaded under 216 daily load cycles with a 10 s rest insertion after every fourth cycle. We found that as few as 36 load cycles per day were able to induce osteogenic responses in both cancellous and cortical bone. Furthermore, while cortical bone area and thickness continued to increase through 1200 cycles, the incremental increase in the osteogenic response decreased as load number increased, indicating a reduced benefit of the increasing number of load cycles. In the proximal metaphyseal cancellous bone, trabecular thickness increased with load up to 216 cycles. We also found that insertion of a 10 s rest between load cycles did not improve the osteogenic response of the cortical or cancellous tissues compared to continuous loading in this model given the age and sex of the mice and the loading parameters used here. These results suggest that relatively few load cycles (e.g. 36) are sufficient to induce osteogenic responses in both cortical and cancellous bone in the mouse tibial loading model. Mechanistic studies using the mouse tibial loading model to examine bone formation and skeletal mechanobiology could be accomplished with relatively few load cycles.

Femoral subchondral bone properties of patients with cam-type femoroacetabular impingement

OBJECTIVE

Morphological deformities of the hip, such as femoroacetabular impingement (FAI) may be responsible for up to 80% of hip osteoarthritis. In cam type FAI, the pathomechanism has been attributed to repeated abnormal contact between the femur and the antero-superior acetabular rim, resulting in cartilage and labrum degeneration. Subchondral bone stiffness likely plays a major role in the process, but little is known of the mechanical properties of the cam deformity. The purpose of this study was to determine tissue modulus and the trabecular micro-architecture of the subchondral bone of the cam deformity of patients undergoing resection surgery as well as comparing these parameters to healthy aged matched controls.

DESIGN

Twelve osteochondral bone biopsies were obtained from symptomatic FAI patients and ten osteochondral control specimens were harvested from cadaveric femurs. A combination of mechanical testing, micro-CT and finite element (FE) analysis were used to determine tissue modulus, bone volume fraction, trabecular thickness, trabecular and spacing, and trabecular number.

RESULTS

The mean tissue modulus of the cam-type FAI deformities (E = 5.4 GPa) was significantly higher than normal controls (E = 2.75 GPa, P = 0.038), but no statistically significant differences were found in bone micro-architectural parameters.

CONCLUSIONS

The data suggests that subchondral bone of the cam deformity consists of older secondary mineralized bone. This supports the notion that the cam deformity is a primary malformation with intrinsic biomechanical abnormalities rather than a secondary deformity as part of the degenerative process of the covering cartilage or remodeling due to repeated impingement.

Predicting cortical bone adaptation to axial loading in the mouse tibia

The development of predictive mathematical models can contribute to a deeper understanding of the specific stages of bone mechanobiology and the process by which bone adapts to mechanical forces. The objective of this work was to predict, with spatial accuracy, cortical bone adaptation to mechanical load, in order to better understand the mechanical cues that might be driving adaptation. The axial tibial loading model was used to trigger cortical bone adaptation in C57BL/6 mice and provide relevant biological and biomechanical information. A method for mapping cortical thickness in the mouse tibia diaphysis was developed, allowing for a thorough spatial description of where bone adaptation occurs. Poroelastic finite-element (FE) models were used to determine the structural response of the tibia upon axial loading and interstitial fluid velocity as the mechanical stimulus. FE models were coupled with mechanobiological governing equations, which accounted for non-static loads and assumed that bone responds instantly to local mechanical cues in an on–off manner. The presented formulation was able to simulate the areas of adaptation and accurately reproduce the distributions of cortical thickening observed in the experimental data with a statistically significant positive correlation (Kendall's τ rank coefficient τ = 0.51, p < 0.001). This work demonstrates that computational models can spatially predict cortical bone mechanoadaptation to a time variant stimulus. Such models could be used in the design of more efficient loading protocols and drug therapies that target the relevant physiological mechanisms.

Rest intervals reduce the number of loading bouts required to enhance bone formation

PURPOSE

As our society becomes increasingly sedentary, compliance with exercise regimens that require numerous high-energy activities each week become less likely. Alternatively, given an osteogenic exercise intervention that required minimal effort, it is reasonable to presume that participation would be enhanced. Insertion of brief rest intervals between each cycle of mechanical loading holds potential to achieve this result because substantial osteoblast function is activated by many fewer loading repetitions within each loading bout. Here, we examined the complementary hypothesis that the number of bouts per week of rest-inserted loading could be reduced from three bouts per week without loss of osteogenic efficacy.

METHODS

We conducted a series of 3-wk in vivo experiments that noninvasively exposed the right tibiae of mice to either cyclic (1 Hz) or rest-inserted loading interventions and quantified osteoblast function via dynamic histomorphometry.

RESULTS

Although reducing loading bouts from three bouts per week (i.e., nine total bouts) to one bout per week (i.e., three total bouts) effectively mitigated the osteogenic benefit of cyclic loading, the same reduction did not significantly reduce periosteal bone formation parameters induced by rest-inserted loading. The osteogenic response was robust to the timing of the rest-inserted loading bouts (three bouts in the first week vs one bout per week for 3 wk). However, elimination of any single bout of the three one-bout-per-week bouts mitigated the osteogenic response to rest-inserted loading. Finally, periosteal osteoblast function assessed after the 3-wk intervention was not sensitive to the timing or number of rest-inserted loading bouts.

CONCLUSIONS

We conclude that rest-inserted loading holds potential to retain the osteogenic benefits of mechanical loading with significantly reduced frequency of bouts of activity while also enabling greater flexibility in the timing of the activity.

Experimental studies of bone mechanoadaptation: bridging in vitro and in vivo studies with multiscale systems

Despite advancements in technology and science over the last century, the mechanisms underlying Wolff's law-bone structure adaptation in response to physical stimuli-remain poorly understood, limiting the ability to effectively treat and prevent skeletal diseases. A challenge to overcome in the study of the underlying mechanisms of this principle is the multiscale nature of mechanoadaptation. While there exist in silico systems that are capable of studying across these scales, experimental studies are typically limited to interpretation at a single dimension or time point. For instance, studies of single-cell responses to defined physical stimuli offer only a limited prediction of the whole bone response, while overlapping pathways or compensatory mechanisms complicate the ability to isolate critical targets in a whole animal model. Thus, there exists a need to develop experimental systems capable of bridging traditional experimental approaches and informing existing multiscale theoretical models. The purpose of this article is to review the process of mechanoadaptation and inherent challenges in studying its underlying mechanisms, discuss the limitations of traditional experimental systems in capturing the many facets of this process and highlight three multiscale experimental systems which bridge traditional approaches and cover relatively understudied time and length scales in bone adaptation.

Preclinical mouse models for assessing axial compression of long bones during exercise

The aim of this laboratory method is to describe two approaches for the investigation of bone responses to mechanical loading in mice in vivo. The first is running exercise, because it is easily translatable clinically, and the second is axial compression of the tibia, because it is precisely controllable. The effects of running exercise, and in general physical activity, on bone tissue have been shown to be both direct through mechanical loading (ground impact and muscle tension) and indirect through metabolic changes. Therefore, running exercise has been considered the most convenient preclinical model for demonstrating the general idea that exercise is good for bone health, either early in age for increasing peak bone mass or later in age by slowing down bone loss. However, numerous combinations of protocols have been reported, which makes it difficult to formulate a simple take-home message. This laboratory method also provides a detailed description of in vivo direct mechanical axial compression of the mouse tibia. The effects of mechanical loading depend on the force (strain), frequency, waveform and duration of application, and they range from bone anabolism with low bone remodeling, inducing lamellar bone accumulation, to bone catabolism with high bone remodeling, leading to microdamage, woven bone formation and bone loss. Direct in vivo loading models are extensively used to study mechanotransduction pathways, and contribute by this way to the development of new bone anabolism treatments. Although it is particularly difficult to assemble an internationally adopted protocol description, which would give reproducible bone responses, here we have attempted to provide a comprehensive guide for best practice in performing running exercise and direct in vivo mechanical loading in the laboratory.

Mechanical loading causes site-specific anabolic effects on bone following exposure to ionizing radiation

During spaceflight, astronauts will be exposed to a complex mixture of ionizing radiation that poses a risk to their health. Exposure of rodents to ionizing radiation on Earth causes bone loss and increases osteoclasts in cancellous tissue, but also may cause persistent damage to stem cells and osteoprogenitors. We hypothesized that ionizing radiation damages skeletal tissue despite a prolonged recovery period, and depletes the ability of cells in the osteoblast lineage to respond at a later time. The goal of the current study was to test if irradiation prevents bone accrual and bone formation induced by an anabolic mechanical stimulus. Tibial axial compression was used as an anabolic stimulus after irradiation with heavy ions. Mice (male, C57BL/6J, 16 weeks) were exposed to high atomic number, high energy (HZE) iron ions ((56)Fe, 2 Gy, 600 MeV/ion) (IR, n=5) or sham-irradiated (Sham, n=5). In vivo axial loading was initiated 5 months post-irradiation; right tibiae in anesthetized mice were subjected to an established protocol known to stimulate bone formation (cyclic 9N compressive pulse, 60 cycles/day, 3 day/wk for 4 weeks). In vivo data showed no difference due to irradiation in the apparent stiffness of the lower limb at the initiation of the axial loading regimen. Axial loading increased cancellous bone volume by microcomputed tomography and bone formation rate by histomorphometry in both sham and irradiated animals, with a main effect of axial loading determined by two-factor ANOVA with repeated measure. There were no effects of radiation in cancellous bone microarchitecture and indices of bone formation. At the tibia diaphysis, results also revealed a main effect of axial loading on structure. Furthermore, irradiation prevented axial loading-induced stimulation of bone formation rate at the periosteal surface of cortical tissue. In summary, axial loading stimulated the net accrual of cancellous and cortical mass and increased cancellous bone formation rate despite prior exposure to ionizing radiation, in this case, HZE particles. Our findings suggest that mechanical stimuli may prove an effective treatment to improve skeletal structure following exposure to ionizing radiation.

C-Mpl Is Expressed on Osteoblasts and Osteoclasts and Is Important in Regulating Skeletal Homeostasis

C-Mpl is the receptor for thrombopoietin (TPO), the main megakaryocyte (MK) growth factor, and c-Mpl is believed to be expressed on cells of the hematopoietic lineage. As MKs have been shown to enhance bone formation, it may be expected that mice in which c-Mpl was globally knocked out (c-Mpl(-/-) mice) would have decreased bone mass because they have fewer MKs. Instead, c-Mpl(-/-) mice have a higher bone mass than WT controls. Using c-Mpl(-/-) mice we investigated the basis for this discrepancy and discovered that c-Mpl is expressed on both osteoblasts (OBs) and osteoclasts (OCs), an unexpected finding that prompted us to examine further how c-Mpl regulates bone. Static and dynamic bone histomorphometry parameters suggest that c-Mpl deficiency results in a net gain in bone volume with increases in OBs and OCs. In vitro, a higher percentage of c-Mpl(-/-) OBs were in active phases of the cell cycle, leading to an increased number of OBs. No difference in OB differentiation was observed in vitro as examined by real-time PCR and functional assays. In co-culture systems, which allow for the interaction between OBs and OC progenitors, c-Mpl(-/-) OBs enhanced osteoclastogenesis. Two of the major signaling pathways by which OBs regulate osteoclastogenesis, MCSF/OPG/RANKL and EphrinB2-EphB2/B4, were unaffected in c-Mpl(-/-) OBs. These data provide new findings for the role of MKs and c-Mpl expression in bone and may provide insight into the homeostatic regulation of bone mass as well as bone loss diseases such as osteoporosis.

Structural and Mechanical Improvements to Bone Are Strain Dependent with Axial Compression of the Tibia in Female C57BL/6 Mice

Strain-induced adaption of bone has been well-studied in an axial loading model of the mouse tibia. However, most outcomes of these studies are restricted to changes in bone architecture and do not explore the mechanical implications of those changes. Herein, we studied both the mechanical and morphological adaptions of bone to three strain levels using a targeted tibial loading mouse model. We hypothesized that loading would increase bone architecture and improve cortical mechanical properties in a dose-dependent fashion. The right tibiae of female C57BL/6 mice (8 week old) were compressively loaded for 2 weeks to a maximum compressive force of 8.8N, 10.6N, or 12.4N (generating periosteal strains on the anteromedial region of the mid-diaphysis of 1700 με, 2050 με, or 2400 με as determined by a strain calibration), while the left limb served as an non-loaded control. Following loading, ex vivo analyses of bone architecture and cortical mechanical integrity were assessed by micro-computed tomography and 4-point bending. Results indicated that loading improved bone architecture in a dose-dependent manner and improved mechanical outcomes at 2050 με. Loading to 2050 με resulted in a strong and compelling formation response in both cortical and cancellous regions. In addition, both structural and tissue level strength and energy dissipation were positively impacted in the diaphysis. Loading to the highest strain level also resulted in rapid and robust formation of bone in both cortical and cancellous regions. However, these improvements came at the cost of a woven bone response in half of the animals. Loading to the lowest strain level had little effect on bone architecture and failed to impact structural- or tissue-level mechanical properties. Potential systemic effects were identified for trabecular bone volume fraction, and in the pre-yield region of the force-displacement and stress-strain curves. Future studies will focus on a moderate load level which was largely beneficial in terms of cortical/cancellous structure and cortical mechanical function.

GATA-1 deficiency rescues trabecular but not cortical bone in OPG deficient mice

GATA-1(low/low) mice have an increase in megakaryocytes (MKs) and trabecular bone. The latter is thought to result from MKs directly stimulating osteoblastic bone formation while simultaneously inhibiting osteoclastogenesis. Osteoprotegerin (OPG) is known to inhibit osteoclastogenesis and OPG(-/-) mice have reduced trabecular and cortical bone due to increased osteoclastogenesis. Interestingly, GATA-1(low/low) mice have increased OPG levels. Here, we sought to determine whether GATA-1 knockdown in OPG(-/-) mice could rescue the observed osteoporotic bone phenotype. GATA-1(low/low) mice were bred with OPG(-/-) mice and bone phenotype assessed. GATA-1(low/low) × OPG(-/-) mice have increased cortical bone porosity, similar to OPG(-/-) mice. Both OPG(-/-) and GATA-1(low/low) × OPG(-/-) mice, were found to have increased osteoclasts localized to cortical bone, possibly producing the observed elevated porosity. Biomechanical assessment indicates that OPG(-/-) and GATA-1(low/low) × OPG(-/-) femurs are weaker and less stiff than C57BL/6 or GATA-1(low/low) femurs. Notably, GATA-1(low/low) × OPG(-/-) mice had trabecular bone parameters that were not different from C57BL/6 values, suggesting that GATA-1 deficiency can partially rescue the trabecular bone loss observed with OPG deficiency. The fact that GATA-1 deficiency appears to be able to partially rescue the trabecular, but not the cortical bone phenotype suggests that MKs can locally enhance trabecular bone volume, but that MK secreted factors cannot access cortical bone sufficiently to inhibit osteoclastogenesis or that OPG itself is required to inhibit osteoclastogenesis in cortical bone.

Activated leukocyte cell adhesion molecule (ALCAM or CD166) modulates bone phenotype and hematopoiesis

Activated Leukocyte Cell Adhesion Molecule (ALCAM/CD166), is expressed on osteoblasts (OB) and hematopoietic stem cells (HSC) residing in the hematopoietic niche, and may have important regulatory roles in bone formation. Because HSC numbers are reduced 77% in CD166(-/-) mice, we hypothesized that changes in bone phenotype and consequently the endosteal niche may partially be responsible for this alteration. Therefore, we investigated bone phenotype and OB function in CD166(-/-) mice. Although osteoclastic measures were not affected by loss of CD166, CD166(-/-) mice exhibited a modest increase in trabecular bone fraction (42%), and increases in osteoid deposition (72%), OB number (60%), and bone formation rate (152%). Cortical bone geometry was altered in CD166(-/-) mice resulting in up to 81% and 49% increases in stiffness and ultimate force, respectively. CD166(-/-) OB displayed elevated alkaline phosphatase (ALP) activity and mineralization, and increased mRNA expression of Fra 1, ALP, and osteocalcin. Overall, CD166(-/-) mice displayed modestly elevated trabecular bone volume fraction with increased OB numbers and deposition of osteoid, and increased OB differentiation in vitro, possibly suggesting more mature OB are secreting more osteoid. This may explain the decline in HSC number in vivo because immature OB are mainly responsible for hematopoiesis enhancing activity.

Characterization of cancellous and cortical bone strain in the in vivo mouse tibial loading model using microCT-based finite element analysis

The in vivo mouse tibial loading model has been increasingly used to understand the mechanisms governing the mechanobiological responses of cancellous and cortical bone tissues to physical stimuli. Accurate characterization of the strain environment throughout the tibia is fundamental in relating localized mechanobiological processes to specific strain stimuli in the skeleton. MicroCT-based finite element analysis, together with diaphyseal strain gauge measures, was conducted to quantify the strain field in the tibiae of 16-wk-old female C57Bl/6 mice during in vivo dynamic compressive loading. Despite a strong correlation between the experimentally-measured and computationally-modeled strains at the gauge site, no correlations existed between the strain at the gauge site and the peak strains in the proximal cancellous and midshaft cortical bone, indicating the limitations of using a single diaphyseal strain gauge to estimate strain in the entire tibia. The peak compressive and tensile principal strain magnitudes in the proximal cancellous bone were 10% and 34% lower than those in the midshaft cortical bone. Sensitivity analyses showed that modeling bone tissue as a heterogeneous material had a strong effect on cancellous strain characterization while cortical strain and whole-bone stiffness were primarily affected by the presence of the fibula and the proximal boundary conditions. These results show that microCT-based finite element analysis combined with strain gauge measures provides detailed resolution of the tissue-level strain in both the cancellous and cortical bones of the mouse tibia during in vivo compression loading, which is necessary for interpreting localized patterns of modeling/remodeling and, potentially, gene and protein expression in skeletal mechanobiology studies.

Aging diminishes lamellar and woven bone formation induced by tibial compression in adult C57BL/6

Aging purportedly diminishes the ability of the skeleton to respond to mechanical loading, but recent data show that old age did not impair loading-induced accrual of bone in BALB/c mice. Here, we hypothesized that aging limits the response of the tibia to axial compression over a range of adult ages in the commonly used C57BL/6. We subjected the right tibia of old (22 month), middle-aged (12 month) and young-adult (5 month) female C57BL/6 mice to peak periosteal strains (measured near the mid-diaphysis) of -2200 με and -3000 με (n=12-15/age/strain) via axial tibial compression (4 Hz, 1200 cycles/day, 5 days/week, 2 weeks). The left tibia served as a non-loaded, contralateral control. In mice of every age, tibial compression that engendered a peak strain of -2200 με did not alter cortical bone volume but loading to a peak strain of -3000 με increased cortical bone volume due in part to woven bone formation. Both loading magnitudes increased total volume, medullary volume and periosteal bone formation parameters (MS/BS, BFR/BS) near the cortical midshaft. Compared to the increase in total volume and bone formation parameters of 5-month mice, increases were less in 12- and 22-month mice by 45-63%. Moreover, woven bone incidence was greatest in 5-month mice. Similarly, tibial loading at -3000 με increased trabecular BV/TV of 5-month mice by 18% (from 0.085 mm3/mm3), but trabecular BV/TV did not change in 12- or 22-month mice, perhaps due to low initial BV/TV (0.032 and 0.038 mm3/mm3, respectively). In conclusion, these data show that while young-adult C57BL/6 mice had greater periosteal bone formation following loading than middle-aged or old mice, aging did not eliminate the ability of the tibia to accrue cortical bone.

Cortical and trabecular bone benefits of mechanical loading are maintained long term in mice independent of ovariectomy

Skeletal loading enhances cortical and trabecular bone properties. How long these benefits last after loading cessation remains an unresolved, clinically relevant question. This study investigated long-term maintenance of loading-induced cortical and trabecular bone benefits in female C57BL/6 mice and the influence of a surgically induced menopause on the maintenance. Sixteen-week-old animals had their right tibia extrinsically loaded 3 days/week for 4 weeks using the mouse tibial axial compression loading model. Left tibias were not loaded and served as internal controls. Animals were subsequently detrained (restricted to cage activities) for 0, 4, 8, 26, or 52 weeks, with ovariectomy (OVX) or sham-OVX surgery being performed at 0 weeks detraining. Loading increased midshaft tibia cortical bone mass, size, and strength, and proximal tibia bone volume fraction. The cortical bone mass, area, and thickness benefits of loading were lost by 26 weeks of detraining because of heightened medullary expansion. However, loading-induced benefits on bone total area and strength were maintained at each detraining time point. Similarly, the benefits of loading on bone volume fraction persisted at all detraining time points. The long-term benefits of loading on both cortical and trabecular bone were not influenced by a surgically induced menopause because there were no interactions between loading and surgery. However, OVX had independent effects on cortical bone properties at early (4 and 8 weeks) detraining time points and trabecular bone properties at all detraining time points. These cumulative data indicate loading has long-term benefits on cortical bone size and strength (but not mass) and trabecular bone morphology, which are not influenced by a surgically induced menopause. This suggests skeletal loading associated with physical activity may provide long-term benefits by preparing the skeleton to offset both the cortical and trabecular bone changes associated with aging and menopause.

Experimental and finite element analysis of strains induced by axial tibial compression in young-adult and old female C57Bl/6 mice

Axial compression of the mouse tibia is used to study strain-adaptive bone (re)modeling. In some studies, comparisons between mice of different ages are of interest. We characterized the tibial deformation and force-strain relationships in female C57Bl/6 mice at 5-, 12- and 22-months age. A three-gauge experimental method was used to determine the strain distribution at the mid-diaphysis, while specimen-specific finite element analysis was used to examine strain distribution along the tibial length. The peak strains in the tibial mid-diaphyseal cross-section are compressive and occur at the postero-lateral apex. The magnitudes of these peak compressive strains are 1.5 to 2 times those on the opposite, antero-medial face (a site often used for strain gauge placement). For example, -10 N force applied to a 5-months old mouse engenders a peak compressive strain of -2800 µε and a tensile strain on the antero-medial face of +1450 µε. The orientation of the neutral axis at the mid-diaphysis did not differ with age (p=0.46), indicating a similar deformation mode in young and old tibiae. On the other hand, from 5- to 22-months there is a 25% reduction in cortical thickness and moment of inertia (p<0.05), resulting in significantly greater tibial strain magnitudes in older mice for equivalent applied force (p<0.05). We conclude that comparisons of tibial loading responses in young-adult and old C57Bl/6 tibiae are facilitated by similar deformation pattern across ages, but that modest adjustment of force levels is required to engender matching peak strains.

Diminished response to in vivo mechanical loading in trabecular and not cortical bone in adulthood of female C57Bl/6 mice coincides with a reduction in deformation to load

Bone loss occurs during adulthood in both women and men and affects trabecular bone more than cortical bone. The mechanism responsible for trabecular bone loss during adulthood remains unexplained, but may be due at least in part to a reduced mechanoresponsiveness. We hypothesized that trabecular and cortical bone would respond anabolically to loading and that the bone response to mechanical loading would be reduced and the onset delayed in adult compared to postpubescent mice. We evaluated the longitudinal adaptive response of trabecular and cortical bone in postpubescent, young (10 week old) and adult (26 week old) female C57Bl/6J mice to axial tibial compression using in vivo microCT (days 0, 5, 10, and 15) and dynamic histomorphometry (day 15). Loading elicited an anabolic response in both trabecular and cortical bone in young and adult mice. As hypothesized, trabecular bone in adult mice exhibited a reduced and delayed response to loading compared to the young mice, apparent in trabecular bone volume fraction and architecture after 10 days. No difference in mechanoresponsiveness of the cortical bone was observed between young and adult mice. Finite element analysis showed that load-induced strain was reduced with age. Our results suggest that trabecular bone loss that occurs in adulthood may in part be due to a reduced mechanoresponsiveness in this tissue and/or a reduction in the induced tissue deformation which occurs during habitual loading. Therapeutic approaches that address the mechanoresponsiveness of the bone tissue may be a promising and alternate strategy to maintain trabecular bone mass during aging.

Resonance in the mouse tibia as a predictor of frequencies and locations of loading-induced bone formation

To enhance new bone formation for the treating of patients with osteopenia and osteoporosis, various mechanical loading regimens have been developed. Although a wide spectrum of loading frequencies is proposed in those regimens, a potential linkage between loading frequencies and locations of loading-induced bone formation is not well understood. In this study, we addressed a question: Does mechanical resonance play a role in frequency-dependent bone formation? If so, can the locations of enhanced bone formation be predicted through the modes of vibration? Our hypothesis is that mechanical loads applied at a frequency near the resonant frequencies enhance bone formation, specifically in areas that experience high principal strains. To test the hypothesis, we conducted axial tibia loading using low, medium, or high frequency to the mouse tibia, as well as finite element analysis. The experimental data demonstrated dependence of the maximum bone formation on location and frequency of loading. Samples loaded with the low-frequency waveform exhibited peak enhancement of bone formation in the proximal tibia, while the high-frequency waveform offered the greatest enhancement in the midshaft and distal sections. Furthermore, the observed dependence on loading frequencies was correlated to the principal strains in the first five resonance modes at 8.0-42.9 Hz. Collectively, the results suggest that resonance is a contributor to the frequencies and locations of maximum bone formation. Further investigation of the observed effects of resonance may lead to the prescribing of personalized mechanical loading treatments.