Muscle and bone follow similar temporal patterns of recovery from muscle-induced disuse due to botulinum toxin injection

Botulinum toxin type A injection into the masticatory muscles and its effects on mandibular bone resorption and density. A systematic review

OBJECTIVE

This systematic review aims to summarize and synthesize the evidence that investigates the secondary effects of the application of botulinum toxin (BT) into the masticatory muscles and its effects on bone density.

MATERIALS AND METHODS

Database searches were conducted until March 19th, 2024. The quality of the studies was assessed by the Cochrane tool risk of bias for the randomized controlled trials and the ROBINS-I tool for non-randomized studies. The Cochrane Grading of Recommendations Assessment Development and Evaluation (GRADE) was used to evaluate the confidence in the overall evidence.

RESULTS

Five studies looking at the effects of botulinum toxin on bone density and resorption when applied to masticatory muscles were found. No significant changes were observed in most of the studies when looking at the effects of botulinum toxin on mandibular condyle volume, density, mandibular angle thickness, and coronoid process volume. The only finding that was statistically and clinically relevant was the difference between patients who received a double application of BT when compared with patients who received a single application (SMD: -0.99 [95%CI: -1.94,-0.05]) on the volume of the mandibular angle.

CONCLUSIONS

There is no clear pattern on whether the application of botulinum toxin is associated with bone resorption or not. Although some studies show statistical significance of the findings, the magnitude of the changes in bone density and their clinical significance are not completely clear.

CLINICAL RELEVANCE

To understand the effectiveness of the use of botulinum toxin into the masticatory muscles and its possible secondary adverse effects on the density of the mandible.

A Pilot Study on Circulating, Cellular, and Tissue Biomarkers in Osteosarcopenic Patients

Aging comes with the loss of muscle and bone mass, leading to a condition known as osteosarcopenia. Circulating, cellular, and tissue biomarkers research for osteosarcopenia is relatively scarce and, currently, no established biomarkers exist. Here we find that osteosarcopenic patients exhibited elevated basophils and TNFα levels, along with decreased aPPT, PT/INR, IL15, alpha-Klotho, DHEA-S, and FGF-2 expression and distinctive bone and muscle tissue micro-architecture and biomarker expressions. They also displayed an increase in osteoclast precursors with a concomitant imbalance towards spontaneous osteoclastogenesis. Similarities were noted with osteopenic and sarcopenic patients, including a lower neutrophil percentage and altered cytokine expression. A linear discriminant analysis (LDA) on models based on selected biomarkers showed a classification accuracy in the range of 61-78%. Collectively, our data provide compelling evidence for novel biomarkers for osteosarcopenia that may hold potential as diagnostic tools to promote healthy aging.

The Interconnection Between Muscle and Bone: A Common Clinical Management Pathway

Often observed with aging, the loss of skeletal muscle (sarcopenia) and bone (osteoporosis) mass, strength, and quality, is associated with reduced physical function contributing to falls and fractures. Such events can lead to a loss of independence and poorer quality of life. Physical inactivity (mechanical unloading), especially in older adults, has detrimental effects on the mass and quality of bone as well as muscle, while increases in activity (mechanical loading) have positive effects. Emerging evidence suggests that the relationship between bone and muscle is driven, at least in part, by bone-muscle crosstalk. Bone and muscle are closely linked anatomically, mechanically, and biochemically, and both have the capacity to function with paracrine and endocrine-like action. However, the exact mechanisms involved in this crosstalk remain only partially explored. Given older adults with lower bone mass are more likely to present with impaired muscle function, and vice versa, strategies capable of targeting both bone and muscle are critical. Exercise is the primary evidence-based prevention strategy capable of simultaneously improving muscle and bone health. Unfortunately, holistic treatment plans including exercise in conjunction with other allied health services to prevent or treat musculoskeletal disease remain underutilized. With a focus on sarcopenia and osteoporosis, the aim of this review is to (i) briefly describe the mechanical and biochemical interactions between bone and muscle; (ii) provide a summary of therapeutic strategies, specifically exercise, nutrition and pharmacological approaches; and (iii) highlight a holistic clinical pathway for the assessment and management of sarcopenia and osteoporosis.

Does Botulinum Toxin Injection Exacerbate Sarcopenia and Bone Mass in Individuals With Cerebral Palsy?

BACKGROUND

Botulinum toxin (BoNT) causes sarcopenia and low bone mass in animal studies. Whether such effect exists in children and adolescents with spastic cerebral palsy (CP) is not clear yet. To investigate the influences of BoNT on grip strength (GS), skeletal muscle mass, and bone mineral density (BMD) in children and adolescents with spastic CP, we conducted this uncontrolled longitudinal study.

METHODS

The body composition of individuals with spastic CP were measured by dual-energy X-ray absorptiometry at preinjection and at 12 and 24 weeks after BoNT intervention. Sarcopenia was defined as meeting both decreased GS and low muscle mass. Twenty-five participants were enrolled (mean age 8.5 years).

RESULTS

Before BoNT intervention, four adolescents had sarcopenia and low bone mass. When the body composition was analyzed as four limbs, trunk, and head, the skeletal muscle mass of the injected limbs, appendicular skeletal muscle mass, and total body less head BMD increased significantly over 24-week follow-up period (P = 0.0117, 0.0032, 0.0229), whereas the GS remained unchanged. When the body composition was analyzed as segments derived from bilateral arms, forearms, hands, thighs, and lower legs, the skeletal muscle mass (P = 0.0113) but not BMD of the injected segments increased significantly over the 24 weeks. The prevalence of low muscle mass, decreased GS, sarcopenia, and low bone mass did not change over 24 weeks.

CONCLUSIONS

The present study showed that BoNT does not exacerbate sarcopenia and low bone mass in individuals with spastic CP.

Association between fat mass by bioelectrical impedance analysis and bone mass by quantitative ultrasound in relation to grip strength and serum 25-hydroxyvitamin D in postmenopausal Japanese women: the Unzen study

Background

Whether fat mass or lean mass affects bone mass in postmenopausal women is controversial. This study aimed to explore the association between body composition measured by bioelectrical impedance analysis (BIA) and bone mass measured by quantitative ultrasound (QUS) in postmenopausal women in Japan.

Methods

We conducted a cross-sectional study, The Unzen Study, on 382 community-dwelling postmenopausal Japanese women (mean (standard deviation) age: 68.2 (7.2) years) who participated in periodic health examinations. The stiffness index (SI) was measured using QUS, and body composition (e.g., fat mass and muscle mass) was measured using BIA. Grip strength was measured. Fasting blood samples were collected, and 25-hydroxyvitamin D (25(OH)D), tartrate-resistant acid phosphatase-5b (TRACP-5b), and parathyroid hormone (PTH) levels were measured. Data on current smoking, alcohol consumption, exercise, and any comorbidities (heart disease, lung disease, stroke, or diabetes mellitus) were collected.

Results

The SI increased with increasing quartiles of fat mass and muscle mass (both p for trend <  0.001), respectively. There were positive correlations between SI and log (25(OH)D) or grip strength. Fat mass significantly correlated with grip strength. Multiple linear regression analysis showed that higher fat mass was independently and significantly associated with higher SI after adjusting for age, height, comorbidity, current smoking, alcohol consumption, exercise, log (25(OH)D), log (TRACP-5b), log (PTH), and grip strength (p = 0.001). In contrast, no association was observed between muscle mass and SI.

Conclusions

Fat mass, but not muscle mass, was a significant determinant of SI in community-dwelling postmenopausal Japanese women.

Botulinum Toxin A and Osteosarcopenia in Experimental Animals: A Scoping Review

We conducted a scoping review to investigate the effects of intramuscular injection of Botulinum Toxin A (BoNT-A) on bone morphology. We investigated if the muscle atrophy associated with Injection of BoNT-A had effects on the neighboring bone. We used the search terms: osteopenia, bone atrophy, Botulinum Toxin A, Micro-CT, mice or rat. The following databases were searched: Medline, Embase, PubMed and the Cochrane Library, between 1990 and 2020. After removal of duplicates, 228 abstracts were identified of which 49 studies satisfied our inclusion and exclusion criteria. The majority of studies (41/49) reported a quantitative reduction in at least one measure of bone architecture based on Micro-CT. The reduction in the ratio of bone volume to tissue volume varied from 11% to 81% (mean 43%) according to the experimental set up and study time points. While longer term studies showed muscle recovery, no study showed complete recovery of all bone properties at the termination of the study. In experimental animals, intramuscular injection of BoNT-A resulted in acute muscle atrophy and acute degradation of the neighboring bone segment. These findings may have implications for clinical protocols in the use of Botulinum Toxin in children with cerebral palsy, with restraint recommended in injection protocols and consideration for monitoring bone density. Clinical studies in children with cerebral palsy receiving injections of Botulinum are indicated.

Rodent model of disuse-induced bone loss by hind limb injection with botulinum toxin A

Bone loss materializes rapidly after immobilization or mechanical unloading. Hind limb injection with botulinum toxin A (BTX) is a highly reproducible animal model for disuse-induced bone loss. Here we describe an easy-to-use and enhanced version of the method employing multiple hind limb injections with BTX to induce a pervasive muscle paralysis and thereby disuse of the hind limb. Thirty-six 12-14-week-old female Wistar rats were stratified into three groups: Baseline (Base), Control (Ctrl), and BTX. Disuse was achieved by injecting BTX directly into the right quadriceps femoris muscle, the hamstring muscles, and the posterior calf muscles. The rats were sacrificed after six weeks, and the right rectus femoris muscle and femur were isolated and analyzed. Hind limb disuse resulted in a significant and substantial loss of both muscle mass and bone mass. The loss of bone mass was accompanied by a reduction of trabecular bone mass and a deterioration of the trabecular micro-architecture with a reduction of trabecular thickness and trabecular number compared to Ctrl. In addition, the trabeculae changed from a more plate-like towards a more rod-like shape as indicated by an increase in the structure model index.•Multiple injections with BTX targeting muscles on both the anterior and posterior thigh and the calf ensure a uniform and pervasive muscle paralysis and hind limb disuse.•Hind limb injections with BTX results in a substantial loss of muscle and bone mass and deterioration of the trabecular micro-architecture.•The induction of hind limb disuse with BTX is highly reproducible.

Changes in mechanical loading affect arthritis-induced bone loss in mice

Arthritis induces bone loss by inflammation-mediated disturbance of bone homeostasis. On the other hand, pain and impaired locomotion are highly prevalent in arthritis and result in reduced general physical activity and less pronounced mechanical loading. Bone is affected by mechanical loading, directly through impact with the ground during movement and indirectly through muscular activity. Mechanical loading in its physiological range is essential for maintaining bone mass, whereas disuse leads to bone loss. The aim of this study was to investigate the impact of mechanical loading on periarticular bone as well as inflammation during arthritis. Mechanical loading was either blocked by botulinum neurotoxin A (Botox) injections before induction of arthritis, or enhanced by cyclic compressive loading, three times per week during arthritis induction. Arthritis was verified and evaluated histologically. Trabecular and cortical bone mass were investigated using micro-computed tomography (μCT), subchondral osteoclastogenesis and bone turnover was assessed by standard methods. Inhibition of mechanical loading enhanced arthritis-induced bone loss while it did not affect inflammation. In contrast, enhanced mechanical loading mitigated arthritis-induced bone loss. Furthermore, the increase in bone resorption markers by arthritis was partly blocked by mechanical loading. In conclusion, enhanced arthritic bone loss after abrogation of mechanical loading suggests that muscle forces play an essential role in preventing arthritic bone loss. In accordance, mechanical loading of the arthritic joints inhibited bone loss, emphasizing that weight bearing activities may have the potential to counteract arthritis-mediated bone loss.

Complicated Muscle-Bone Interactions in Children with Cerebral Palsy

PURPOSE OF REVIEW

The goal of this review is to highlight the deficits in muscle and bone in children with cerebral palsy (CP), discuss the muscle-bone relationship in the CP population, and identify muscle-based intervention strategies that may stimulate an improvement in their bone development.

RECENT FINDINGS

The latest research suggests that muscle and bone are both severely underdeveloped and weak in children with CP, even in ambulatory children with mild forms of the disorder. The small and low-performing muscles and limited participation in physical activity are likely the major contributors to the poor bone development in children with CP. However, the muscle-bone relationship may be complicated by other factors, such as a high degree of fat and collagen infiltration of muscle, atypical muscle activation, and muscle spasticity. Muscle-based interventions, such as resistance training, vibration, and nutritional supplementation, have the potential to improve bone development in children with CP, especially if they are initiated before puberty. Studies are needed to identify the muscle-related factors with the greatest influence on bone development in children with CP. Identifying treatment strategies that capitalize on the relationship between muscle and bone, while also improving balance, coordination, and physical activity participation, is an important step toward increasing bone strength and minimizing fractures in children with CP.

Architectural Changes in the Medial Gastrocnemius on Sonography after Nerve Ablation in Healthy Adults

Architectural changes in healthy muscle after denervation have not yet been reported. This study aimed to investigate architectural changes in the medial head of the gastrocnemius muscle (GCM) after aesthetic tibial nerve ablation in healthy adults using ultrasonography (US). The effects of tibial nerve ablation were verified by visual observation and surface electromyography analysis. US images of medial GCMs were taken by one trained physician using B-mode and real-time US with a linear-array probe before nerve ablation, at 1 week after nerve ablation and at 3 months after nerve ablation in an anatomic standing position with the feet about shoulder-width apart in 19 healthy adults (17 females and 2 males). Muscle thickness was significantly reduced on the left side at 1 week and 3 months after the procedure and on the right side at 3 months after the procedure (p<0.050). Although fascicle length was not significantly changed, pennation angle was significantly reduced on both sides at 3 months after the procedure (p<0.050). Muscle thickness and pennation angle of the muscle fascicle were significantly reduced, although fascicle length was not significantly changed, after tibial nerve ablation in the medial GCM of healthy adults.

Botox-induced muscle paralysis alters intracortical porosity and osteocyte lacunar density in skeletally mature rats

Reduced mechanical loading can lead to disuse osteoporosis, resulting in bone fragility. Disuse models report macroscopic bone loss due to muscle inactivity and immobilization, yet only recently has there been quantification of the effects of disuse on the vascular pores and osteocyte network, which are believed to play an important role in mechanotransduction via interstitial fluid flow. The goal of this study was to perform a high-resolution analysis of the effects of muscle inactivity on intracortical porosity and osteocyte lacunar density in skeletally mature rats. Muscle paralysis was induced in 20-week-old female Sprague Dawley rats by injection of botulinum neurotoxin. Rats were injected in the right hindlimb muscles with either Botox (BTX, n = 8) or saline solution (CTRL, n = 8), with a third group used as baseline controls (n = 8). Four weeks after injection, Botox caused a ∼60% reduction in hindlimb muscle mass. High-resolution micro-CT analysis showed that Botox-induced muscle paralysis increased vascular canal porosity and reduced osteocyte lacunar density within the tibial metaphysis cortex. Cortical thickness and other areal properties were diminished in the proximal tibial metaphysis, whereas no differences were found in the mid-diaphysis. Within the BTX group, the injected limbs showed a lower cancellous bone volume fraction relative to the contralateral limb. These results indicate that diminished muscle activity alters the vascular canal porosity and osteocyte lacunar density in cortical bone, which could alter interstitial fluid flow, affecting molecular transport and the transmission of mechanical signals to osteocytes. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Long-Term Quantitative Evaluation of Muscle and Bone Wasting Induced by Botulinum Toxin in Mice Using Microcomputed Tomography

Muscle and bone masses are highly correlated and muscles impose large loads on bone. Muscle wasting that accompanies bone loss has been poorly investigated. 21 female mice were spread into seven groups. At day 0, 18 mice received Botulinum toxin (BTX) injection in the quadriceps muscle to induce paralysis of the right hind limb; the left contralateral side was used as control. Mice were sacrificed at 7, 14, 21, 28, 56 and 90 days post-injection. A remaining group was sacrificed at day 0. Trabecular bone volume was determined by microcomputed tomography (microCT) at the distal femur and tibia proximal metaphyses on both sides. Limbs were immersed in an HgCl₂ solution allowing muscle visualization by microCT. On 2D sections, the cross-sectional areas and form-factors were measured for the quadriceps at mid-thigh and gastrocnemius at mid-leg and these muscles were dissected and weighed. Bone volume decreased in the paralysed side. Bone loss was maximal at 56 days followed by recuperation at 90 days. The cross-sectional areas of gastrocnemius and quadriceps were significantly lower in the paralysed limb from 7 days; the decrease was maximum at 21 days for the gastrocnemius and 28 days for the quadriceps. No difference in form-factors was found between the two limbs. Similar results were obtained with the anatomical method and significant correlations were obtained between the two methods. Quantitative analysis of muscle loss and recovery was possible by microCT after using a metallic contrast agent. Loss of bone secondary to muscle wastage induced by BTX and recovery showed a parallel evolution for bone and muscles.

Disuse osteopenia induced by botulinum toxin is similar in skeletally mature young and aged female C57BL/6J mice

Osteopenia and osteoporosis predominately occur in the fully grown skeleton. However, it is unknown whether disuse osteopenia in skeletally mature, but growing, mice resembles that of fully grown mice. Twenty-four 16-week-old (young) and eighteen 44-week-old (aged) female C57BL/6J mice were investigated. Twelve young and nine aged mice were injected with botulinum toxin in one hind limb; the remaining mice served as controls. The mice were euthanized after 3 weeks of disuse. The femora were scanned by micro-computed tomography (µCT) and bone strength was determined by mechanically testing the femoral mid-diaphysis and neck. At the distal femoral metaphysis, the loss of trabecular bone volume fraction (BV/TV) differed between the young and aged mice. However, at the distal femoral epiphysis, no age-dependent differences were observed. Thinning of the trabeculae was not affected by the age of the mice at either the distal femoral metaphysis or the epiphysis. Furthermore, the aged mice lost more bone strength at the femoral mid-diaphysis, but not at the femoral neck, compared to the young mice. In general, the bone loss induced by botulinum toxin did not differ substantially between young and aged mice. Therefore, the loss of bone in young mice resembles that of aged mice, even though they are not fully grown.

Muscle-Bone Crosstalk: Emerging Opportunities for Novel Therapeutic Approaches to Treat Musculoskeletal Pathologies

Osteoporosis and sarcopenia are age-related musculoskeletal pathologies that often develop in parallel. Osteoporosis is characterized by a reduced bone mass and an increased fracture risk. Sarcopenia describes muscle wasting with an increasing risk of injuries due to falls. The medical treatment of both diseases costs billions in health care per year. With the impact on public health and economy, and considering the increasing life expectancy of populations, more efficient treatment regimens are sought. The biomechanical interaction between both tissues with muscle acting on bone is well established. Recently, both tissues were also determined as secretory endocrine organs affecting the function of one another. New exciting discoveries on this front are made each year, with novel signaling molecules being discovered and potential controversies being described. While this review does not claim completeness, it will summarize the current knowledge on both the biomechanical and the biochemical link between muscle and bone. The review will highlight the known secreted molecules by both tissues affecting the other and finish with an outlook on novel therapeutics that could emerge from these discoveries.

Hypodynamia Alters Bone Quality and Trabecular Microarchitecture

Disuse induces a rapid bone loss in humans and animals; hypodynamia/sedentarity is now recognized as a risk factor for osteoporosis. Hypodynamia also decreases bone mass but its effects are largely unknown and only few animal models have been described. Hypodynamic chicken is recognized as a suitable model of bone loss but the effects on the quality have not been fully explored. We have used ten chickens bred in a large enclosure (FREE group); ten others were confined in small cages with little space to move around (HYPO group). They were sacrificed at 53 days and femurs were evaluated by microcomputed tomography (microCT) and nanoindentation. Sections (4 µm thick) were analyzed by Fourier Transform InfraRed Microspectroscopy (FTIR) to see the effects on mineralization and collagen and quantitative backscattered electron imaging (qBEI) to image the mineral of the bone matrix. Trabecular bone volume and microarchitecture were significantly altered in the HYPO group. FTIR showed a significant reduction of the mineral-to-matrix ratio in the HYPO group associated with an increase in the carbonate content and an increase in crystallinity (calculated as the area ratio of subbands located at 1020 and 1030 cm^-1) indicating a poor quality of the mineral. Collagen maturity (calculated as the area ratio of subbands located at 1660 and 1690 cm^-1) was significantly reduced in the HYPO group. Reduced biomechanical properties were observed at the tissue level. Confined chicken represents a new model for the study of hypodynamia because bone changes are not created by a surgical lesion or a traumatic method. Animals have a reduced bone mass and present with an altered bone matrix quality which is less mineralized and whose collagen contains less crosslinks than in control chicken.

Effects of Sex and Notch Signaling on the Osteocyte Cell Pool

Osteocytes play a fundamental role in mechanotransduction and skeletal remodeling. Sex is a determinant of skeletal structure, and female C57BL/6J mice have increased osteoblast number in cancellous bone when compared to male mice. Activation of Notch in the skeleton causes profound cell-context dependent changes in skeletal physiology. To determine the impact of sex and of Notch signaling on the osteocyte cell pool, we analyzed cancellous and cortical bone of 1-6-month-old C57BL/6J or 129SvJ/C57BL/6J mice and determined the osteocyte number/area. There was an age-dependent decline in osteocyte number in cancellous bone of male but not female mice, so that 6-month-old female mice had a greater number of osteocytes than male littermates. Although differences between male and female mice were modest, female mice had ∼10-15% greater number of osteocytes/area. RNA sequence analysis of osteocyte-rich preparations did not reveal differences between sexes in the expression of genes known to influence bone homeostasis. Neither the activation of Notch1 nor the concomitant inactivation of Notch1 and Notch2 in Osterix (Sp7) or Dentin matrix protein 1 (Dmp1) expressing cells had a pronounced and consistent effect on cancellous or cortical bone osteocyte number in either sex. Moreover, inactivation of Notch1 and Notch2 in Dmp1 expressing cells did not influence the bone loss in a muscle immobilization model of skeletal unloading. In conclusion, cancellous bone osteocytes decline with age in male mice, cortical osteocytes are influenced by sex in younger mice, but osteocyte cell density is not affected substantially by Notch signaling. J. Cell. Physiol. 232: 363-370, 2017. © 2016 Wiley Periodicals, Inc.

Missense Mutations in LRP5 Associated with High Bone Mass Protect the Mouse Skeleton from Disuse- and Ovariectomy-Induced Osteopenia

The low density lipoprotein receptor-related protein-5 (LRP5), a co-receptor in the Wnt signaling pathway, modulates bone mass in humans and in mice. Lrp5 knock-out mice have severely impaired responsiveness to mechanical stimulation whereas Lrp5 gain-of-function knock-in and transgenic mice have enhanced responsiveness to mechanical stimulation. Those observations highlight the importance of Lrp5 protein in bone cell mechanotransduction. It is unclear if and how high bone mass-causing (HBM) point mutations in Lrp5 alter the bone-wasting effects of mechanical disuse. To address this issue we explored the skeletal effects of mechanical disuse using two models, tail suspension and Botulinum toxin-induced muscle paralysis, in two different Lrp5 HBM knock-in mouse models. A separate experiment employing estrogen withdrawal-induced bone loss by ovariectomy was also conducted as a control. Both disuse stimuli induced significant bone loss in WT mice, but Lrp5 A214V and G171V were partially or fully protected from the bone loss that normally results from disuse. Trabecular bone parameters among HBM mice were significantly affected by disuse in both models, but these data are consistent with DEXA data showing a failure to continue growing in HBM mice, rather than a loss of pre-existing bone. Ovariectomy in Lrp5 HBM mice resulted in similar protection from catabolism as was observed for the disuse experiments. In conclusion, the Lrp5 HBM alleles offer significant protection from the resorptive effects of disuse and from estrogen withdrawal, and consequently, present a potential mechanism to mimic with pharmaceutical intervention to protect against various bone-wasting stimuli.

Immobilization induced osteopenia is strain specific in mice

Immobilization causes rapid and massive bone loss. By comparing Botulinum Toxin A (BTX)-induced bone loss in mouse strains with different genetic backgrounds we investigated whether the genetic background had an influence on the severity of the osteopenia. Secondly, we investigated whether BTX had systemic effects on bone. Female mice from four inbred mouse strains (BALB/cJ, C57BL/6 J, DBA/2 J, and C3H/HeN) were injected unilaterally with BTX (n = 10/group) or unilaterally with saline (n = 10/group). Mice were euthanized after 21 days, and the bone properties evaluated using μCT, DXA, bone histomorphometry, and mechanical testing. BTX resulted in substantially lower trabecular bone volume fraction (BV/TV) and trabecular thickness in all mouse strains. The deterioration of BV/TV was significantly greater in C57BL/6 J (− 57%) and DBA/2 J (− 60%) than in BALB/cJ (− 45%) and C3H/HeN (− 34%) mice. The loss of femoral neck fracture strength was significantly greater in C57BL/6 J (− 47%) and DBA/2 J (− 45%) than in C3H (− 25%) mice and likewise the loss of mid-femoral fracture strength was greater in C57BL/6 J (− 17%), DBA/2 J (− 12%), and BALB/cJ (− 9%) than in C3H/HeN (− 1%) mice, which were unaffected. Using high resolution μCT we found no evidence of a systemic effect on any of the microstructural parameters of the contralateral limb. Likewise, there was no evidence of a systemic effect on the bone strength in any mouse strain. We did, however, find a small systemic effect on aBMD in DBA/2 J and C3H/HeN mice. The present study shows that BTX-induced immobilization causes the greatest loss of cortical and trabecular bone in C57BL/6 J and DBA/2 J mice. A smaller loss of bone microstructure and fracture strength was seen in BALB/cJ mice, while the bone microstructure and fracture strength of C3H/HeN mice were markedly less affected. This indicates that BTX-induced loss of bone is mouse strain dependent. We found only minimal systemic effects of BTX.

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Botulinum Toxin A (Botox) causes only minimal systemic effects in mice.

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The degree of immobilization induced osteopenia is highly strain specific in mice.

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The greatest degree of bone loss was observed with C57BL/6 J and DBA/2 J mice followed by BALB/cJ mice after Botox-injection.

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C3H/HeN mice had the smallest bone loss following Botox-injection.

Biomechanical aspects of the muscle-bone interaction

There is growing interest in the interaction between skeletal muscle and bone, particularly at the genetic and molecular levels. However, the genetic and molecular linkages between muscle and bone are achieved only within the context of the essential mechanical coupling of the tissues. This biomechanical and physiological linkage is readily evident as muscles attach to bone and induce exposure to varied mechanical stimuli via functional activity. The responsiveness of bone cells to mechanical stimuli, or their absence, is well established. However, questions remain regarding how muscle forces applied to bone serve to modulate bone homeostasis and adaptation. Similarly, the contributions of varied, but unique, stimuli generated by muscle to bone (such as low-magnitude, high-frequency stimuli) remains to be established. The current article focuses upon the mechanical relationship between muscle and bone. In doing so, we explore the stimuli that muscle imparts upon bone, models that enable investigation of this relationship, and recent data generated by these models.

Bone mass and bone quality are altered by hypoactivity in the chicken

Disuse induces a rapid bone loss in adults; sedentarity is now recognized as a risk factor for osteoporosis. Hypoactivity or confinement also decrease bone mass in adults but their effects are largely unknown and only few animal models have been described. We have used 10 chickens of the rapidly growing strain 857K bred in a large enclosure (FREE group); 10 others were confined in small cages with little space to move around (HYPO group). They were sacrificed at 53 days and femurs and tibias were evaluated by texture analysis, dual energy X-ray densitometry, microcomputed tomography (microCT) and histomorphometry. Hypoactivity had no effect on the length and diameter of the bones. Bone mineral density (BMD), microCT (trabecular bone volume and trabecular microarchitecture) and texture analysis were always found significantly reduced in the animals of the HYPO group. BMD was reduced at both femur and tibia diaphysises; BMD of the metaphysis was significantly reduced in the femur but not in the tibia. An increase in osteoid volume and surfaces was noted in the HYPO group. However, there was no alteration of the mineral phase as the osteoid thickness did not differ from control animals. Bone loss was much more pronounced at the lower femur metaphysis than at the upper metaphysis of the tibia. At the tibia, only microarchitectural changes of trabecular bone could be evidenced. The confined chicken represents a new method for the study of hypodynamia since these animals do not have surgical lesions.

Architectural changes of the gastrocnemius muscle after botulinum toxin type A injection in children with cerebral palsy

PURPOSE

This study used ultrasonography (US) to investigate the architectural changes in gastrocnemius muscles (GCM) after botulinum toxin injection (BoNT-A) in children with cerebral palsy (CP).

MATERIALS AND METHODS

Thirteen children with CP who received a BoNT-A injection into their GCM to treat equinus were recruited (9 males and 4 females). Architectural changes in both the medial and lateral heads of the GCM from a total of 20 legs were assessed using B-mode, real-time US. Muscle thickness (MT), fascicle length (FL), and fascicle angle (FA) were measured over the middle of the muscle belly in both a resting and neutral ankle position. Measures at 1 and 3 months after the injection were compared with baseline data taken before the injection.

RESULTS

The mean age of the subjects was 5.8 (±1.6) years. Spasticity was significantly reduced when measured by both the modified Tardieu scale and the modified Ashworth scale at 1 and 3 months after injection (p<0.05). The MT and FA of both the medial and lateral heads of the GCM were significantly reduced for both neutral and resting ankle positions at 1 and 3 months after the injection. The FL of both the medial and lateral heads of the GCM were significantly increased in a resting position (p<0.05), but not in a neutral position.

CONCLUSION

Our results demonstrated muscle architectural changes induced by BoNT-A injection. The functional significances of these changes were discussed.

Physiological effects of microgravity on bone cells

Life on Earth developed under the influence of normal gravity (1g). With evidence from previous studies, scientists have suggested that normal physiological processes, such as the functional integrity of muscles and bone mass, can be affected by microgravity during spaceflight. During the life span, bone not only develops as a structure designed specifically for mechanical tasks but also adapts for efficiency. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to evaluate bone cell responses. One of the most serious problems induced by long-term weightlessness is bone mineral loss. Results from in vitro studies that entailed the use of bone cells in spaceflights showed modification in cell attachment structures and cytoskeletal reorganization, which may be involved in bone loss. Humans exposed to microgravity conditions experience various physiological changes, including loss of bone mass, muscle deterioration, and immunodeficiency. In vitro models can be used to extract valuable information about changes in mechanical stress to ultimately identify the different pathways of mechanotransduction in bone cells. Despite many in vivo and in vitro studies under both real microgravity and simulated conditions, the mechanism of bone loss is still not well defined. The objective of this review is to summarize the recent research on bone cells under microgravity conditions based on advances in the field.

Osteopenic consequences of botulinum toxin injections in the masticatory muscles: a pilot study

Patients with temporomandibular muscle and joint disorder (TMJD) increasingly seek and receive treatment for their pain with botulinum toxin (BoNTA; botulinum toxin A). Used intramuscularly in therapeutic doses, it produces localised paresis. Such paresis creates risk of reduced bone mineral density, or 'disuse osteopenia'. Animal studies have frequently used BoNTA as a model of paralysis to induce bone changes within short periods. Osteopenic effects can be enduring in animals but have yet to be studied in humans. This is the first study in humans to examine bone-related consequences of BoNTA injections in the masticatory muscles, comparing oral and maxillofacial radiologists' ratings of trabecular bone patterns in the condyles of patients with TMJD exposed to multiple masticatory muscle injection sessions with BoNTA to a sample of patients with TMJD unexposed to masticatory muscle injections with BoNTA. Cone-beam computed tomography (CBCT)-derived images of bilateral condyles were evaluated in seven patients with TMJD receiving 2+ recent BoNTA treatment sessions for facial pain and nine demographically matched patients with TMJD not receiving BoNTA treatment. Two oral and maxillofacial radiologists evaluated CBCT images for evidence of trabecular changes consistent with osteopenia. Both evaluators noted decreased density in all participants exposed to BoNTA and in none of the unexposed participants (P < 0.001). No other abnormalities associated with reduced loading were detected. These findings need replication in a larger sample and over a longer time period, to ensure safety of patients with TMJD receiving multiple BoNTA injections for their pain.

Combined effects of botulinum toxin injection and hind limb unloading on bone and muscle

Bone receives mechanical stimulation from two primary sources, muscle contractions and external gravitational loading; but the relative contribution of each source to skeletal health is not fully understood. Understanding the most effective loading for maintaining bone health has important clinical implications for prescribing physical activity for the treatment or prevention of osteoporosis. Therefore, we investigated the relative effects of muscle paralysis and reduced gravitational loading on changes in muscle mass, bone mineral density, and microarchitecture. Adult female C57Bl/6J mice (n = 10/group) underwent one of the following: unilateral botulinum toxin (BTX) injection of the hind limb, hind limb unloading (HLU), both unilateral BTX injection and HLU, or no intervention. BTX and HLU each led to significant muscle and bone loss. The effect of BTX was diminished when combined with HLU, though generally the leg that received the combined intervention (HLU+BTX) had the most detrimental changes in bone and muscle. We found an indirect effect of BTX affecting the uninjected (contralateral) leg that led to significant decreases in bone mineral density and deficits in muscle mass and bone architecture relative to the untreated controls; the magnitude of this indirect BTX effect was comparable to the direct effect of BTX treatment and HLU. Thus, while it was difficult to definitively conclude whether muscle force or external gravitational loading contributes more to bone maintenance, it appears that BTX-induced muscle paralysis is more detrimental to muscle and bone than HLU.

Metaphyseal and diaphyseal bone loss in the tibia following transient muscle paralysis are spatiotemporally distinct resorption events

When the skeleton is catabolically challenged, there is great variability in the timing and extent of bone resorption observed at cancellous and cortical bone sites. It remains unclear whether this resorptive heterogeneity, which is often evident within a single bone, arises from increased permissiveness of specific sites to bone resorption or localized resorptive events of varied robustness. To explore this question, we used the mouse model of calf paralysis induced bone loss, which results in metaphyseal and diaphyseal bone resorption of different timing and magnitude. Given this phenotypic pattern of resorption, we hypothesized that bone loss in the proximal tibia metaphysis and diaphysis occurs through resorption events that are spatially and temporally distinct. To test this hypothesis, we undertook three complimentary in vivo/μCT imaging studies. Specifically, we defined spatiotemporal variations in endocortical bone resorption during the 3weeks following calf paralysis, applied a novel image registration approach to determine the location where bone resorption initiates within the proximal tibia metaphysis, and explored the role of varied basal osteoclast activity on the magnitude of bone loss initiation in the metaphysis using μCT based bone resorption parameters. A differential response of metaphyseal and diaphyseal bone resorption was observed throughout each study. Acute endocortical bone loss following muscle paralysis occurred almost exclusively within the metaphyseal compartment (96.5% of total endocortical bone loss within 6days). Using our trabecular image registration approach, we further resolved the initiation of metaphyseal bone loss to a focused region of significant basal osteoclast function (0.03mm(3)) adjacent to the growth plate. This correlative observation of paralysis induced bone loss mediated by basal growth plate cell dynamics was supported by the acute metaphyseal osteoclastic response of 5-week vs. 13-month-old mice. Specifically, μCT based bone resorption rates normalized to initial trabecular surface (BRRBS) were 3.7-fold greater in young vs. aged mice (2.27±0.27μm(3)/μm(2)/day vs. 0.60±0.44μm(3)/μm(2)/day). In contrast to the focused bone loss initiation in the metaphysis, diaphyseal bone loss initiated homogeneously throughout the long axis of the tibia predominantly in the second week following paralysis (81.3% of diaphyseal endocortical expansion between days 6 and 13). The timing and homogenous nature are consistent with de novo osteoclastogenesis mediating the diaphyseal resorption. Taken together, our data suggests that tibial metaphyseal and diaphyseal bone loss induced by transient calf paralysis are spatially and temporally discrete events. In a broader context, these findings are an essential first step toward clarifying the timing and origins of multiple resorptive events that would require targeting to fully inhibit bone loss following neuromuscular trauma.

Increasing the Number of Unloading/Reambulation Cycles does not Adversely Impact Body Composition and Lumbar Bone Mineral Density but Reduces Tissue Sensitivity

A single exposure to hindlimb unloading leads to changes in body mass, body composition and bone, but the consequences of multiple exposures are not yet understood. Within a 18wk period, adult C57BL/6 male mice were exposed to one (1x-HLU), two (2x-HLU) or three (3x-HLU) cycles of 2 wk of hindlimb unloading (HLU) followed by 4 wk of reambulation (RA), or served as ambulatory age-matched controls. In vivo µCT longitudinally tracked changes in abdominal adipose and lean tissues, lumbar vertebral apparent volumetric bone mineral density (vBMD) and upper hindlimb muscle cross-sectional area before and after the final HLU and RA cycle. Significant decreases in total adipose tissue and vertebral vBMD were observed such that all unloaded animals reached similar values after the final unloading cycle. However, the magnitude of these losses diminished in mice undergoing their 2^nd or 3^rd HLU cycle. Irrespective of the number of HLU/RA cycles, total adipose tissue and vertebral vBMD recovered and were no different from age-matched controls after the final RA period. In contrast, upper hindlimb muscle cross-sectional area was significantly lower than controls in all unloaded groups after the final RA period. These results suggest that tissues in the abdominal region are more resilient to multiple bouts of unloading and more amenable to recovery during reambulation than the peripheral musculoskeletal system.

Notch signaling in osteocytes differentially regulates cancellous and cortical bone remodeling

Notch receptors play a role in skeletal development and homeostasis, and Notch activation in undifferentiated and mature osteoblasts causes osteopenia. In contrast, Notch activation in osteocytes increases bone mass, but the mechanisms involved and exact functions of Notch are not known. In this study, Notch1 and -2 were inactivated preferentially in osteocytes by mating Notch1/2 conditional mice, where Notch alleles are flanked by loxP sequences, with transgenics expressing Cre directed by the Dmp1 (dentin matrix protein 1) promoter. Notch1/2 conditional null male and female mice exhibited an increase in trabecular bone volume due to an increase in osteoblasts and decrease in osteoclasts. In male null mice, this was followed by an increase in osteoclast number and normalization of bone volume. To activate Notch preferentially in osteocytes, Dmp1-Cre transgenics were crossed with Rosa(Notch) mice, where a loxP-flanked STOP cassette is placed between the Rosa26 promoter and Notch1 intracellular domain sequences. Dmp1-Cre(+/-);Rosa(Notch) mice exhibited an increase in trabecular bone volume due to decreased bone resorption and an increase in cortical bone due to increased bone formation. Biomechanical and chemical properties were not affected. Osteoprotegerin mRNA was increased, sclerostin and dickkopf1 mRNA were decreased, and Wnt signaling was enhanced in Dmp1-Cre(+/-);Rosa(Notch) femurs. Botulinum toxin A-induced muscle paralysis caused pronounced osteopenia in control mice, but bone mass was preserved in mice harboring the Notch activation in osteocytes. In conclusion, Notch plays a unique role in osteocytes, up-regulates osteoprotegerin and Wnt signaling, and differentially regulates trabecular and cortical bone homeostasis.

Reduced gravitational loading does not account for the skeletal effect of botulinum toxin-induced muscle inhibition suggesting a direct effect of muscle on bone

Intramuscular injection of botulinum toxin (botox) into rodent hindlimbs has developed as a useful model for exploring muscle-bone interactions. Botox-induced muscle inhibition rapidly induces muscle atrophy and subsequent bone loss, with the latter hypothesized to result from reduced muscular loading of the skeleton. However, botox-induced muscle inhibition also reduces gravitational loading (as evident by reduced ground reaction forces during gait) which may account for its negative skeletal effects. The aim of this study was to investigate the skeletal effect of botox-induced muscle inhibition in cage control and tail suspended mice, with tail suspension being used to control for the reduced gravitational loading associated with botox. Female C57BL/6J mice were injected unilaterally with botox and contralaterally with vehicle, and subsequently exposed to tail suspension or normal cage activities for 6 weeks. Botox-induced muscle inhibition combined with tail suspension had the largest detrimental effect on the skeleton, causing the least gains in midshaft tibial bone mass, cortical area and cortical thickness, greatest gains in midshaft tibial medullary area, and lowest proximal tibial trabecular bone volume fraction. These data indicate botox-induced muscle inhibition has skeletal effects over and above any effect it has in altering gravitational loading, suggesting that muscle has a direct effect on bone. This effect may be relevant in the development of strategies targeting musculoskeletal health.

Effects of botulinum toxin A on fracture healing in rats: an experimental study

INTRODUCTION

Fracture healing is a complex process influenced by intrinsic and extrinsic factors. The aim of the present study was to evaluate the effects of botulinum toxin (BTX) A on fracture healing.

MATERIALS AND METHODS

Following the induction of bilateral standard closed femoral fractures and relative fixation in 18 Wistar albino rats, 8 IU of BTX A were injected into the right femoral region. After 28 days, all of the rats were sacrificed, the diameter of the callus was measured, and fracture healing was assessed by biomechanical and histopathologic evaluation.

RESULTS

While an increase in biomechanical and histopathologic healing was noted on the side injected with BTX A, a decrease in callus diameter was observed.

CONCLUSION

Botulinum toxin A administration increases the healing power in a relatively fixated fracture and decreases the callus diameter, just as if rigid fixation had been performed. The beneficial effect of BTX A on fracture healing might be associated with increased fixation rigidity.

Automated rodent in situ muscle contraction assay and myofiber organization analysis in sarcopenia animal models

Age-related sarcopenia results in frailty and decreased mobility, which are associated with increased falls and long-term disability in the elderly. Given the global increase in lifespan, sarcopenia is a growing, unmet medical need. This report aims to systematically characterize muscle aging in preclinical models, which may facilitate the development of sarcopenia therapies. Naïve rats and mice were subjected to noninvasive micro X-ray computed tomography (micro-CT) imaging, terminal in situ muscle function characterizations, and ATPase-based myofiber analysis. We developed a Definiens (Parsippany, NJ)-based algorithm to automate micro-CT image analysis, which facilitates longitudinal in vivo muscle mass analysis. We report development and characterization of translational in situ skeletal muscle performance assay systems in rat and mouse. The systems incorporate a custom-designed animal assay stage, resulting in enhanced force measurement precision, and LabVIEW (National Instruments, Austin, TX)-based algorithms to support automated data acquisition and data analysis. We used ATPase-staining techniques for myofibers to characterize fiber subtypes and distribution. Major parameters contributing to muscle performance were identified using data mining and integration, enabled by Labmatrix (BioFortis, Columbia, MD). These technologies enabled the systemic and accurate monitoring of muscle aging from a large number of animals. The data indicated that longitudinal muscle cross-sectional area measurement effectively monitors change of muscle mass and function during aging. Furthermore, the data showed that muscle performance during aging is also modulated by myofiber remodeling factors, such as changes in myofiber distribution patterns and changes in fiber shape, which affect myofiber interaction. This in vivo muscle assay platform has been applied to support identification and validation of novel targets for the treatment of sarcopenia.

High-frequency, low-magnitude vibration does not prevent bone loss resulting from muscle disuse in mice following botulinum toxin injection

High-frequency, low-magnitude vibration enhances bone formation ostensibly by mimicking normal postural muscle activity. We tested this hypothesis by examining whether daily exposure to low-magnitude vibration (VIB) would maintain bone in a muscle disuse model with botulinum toxin type A (BTX). Female 16–18 wk old BALB/c mice (N = 36) were assigned to BTX-VIB, BTX-SHAM, VIB, or SHAM. BTX mice were injected with BTX (20 µL; 1 U/100 g body mass) into the left hindlimb posterior musculature. All mice were anaesthetized for 20 min/d, 5 d/wk, for 3 wk, and the left leg mounted to a holder. Through the holder, VIB mice received 45 Hz, ±0.6 g sinusoidal acceleration without weight bearing. SHAM mice received no vibration. At baseline and 3 wk, muscle cross-sectional area (MCSA) and tibial bone properties (epiphysis, metaphysis and diaphysis) were assessed by in vivo micro-CT. Bone volume fraction in the metaphysis decreased 12±9% and 7±6% in BTX-VIB and BTX-SHAM, but increased in the VIB and SHAM. There were no differences in dynamic histomorphometry outcomes between BTX-VIB and BTX nor between VIB and SHAM. Thus, vibration did not prevent bone loss induced by a rapid decline in muscle activity nor produce an anabolic effect in normal mice. The daily loading duration was shorter than would be expected from postural muscle activity, and may have been insufficient to prevent bone loss. Based on the approach used in this study, vibration does not prevent bone loss in the absence of muscle activity induced by BTX.

Micro and macroarchitectural changes at the tibia after botulinum toxin injection in the growing rat

The aim of this study was to analyze bone microarchitecture and macroarchitecture of tibia in a disuse model in growing rats. Eight-weeks-old Copenhagen rats were injected intramuscularly with 1.5 units BTX in the quadriceps muscle of the right hind limb. Saline injection was done at the left hind limb to serve as control. Five rats were killed at day 1 and represented the baseline group (D1), 5 rats were killed at day 14 (D14), 5 at day 21 (D21), 5 at day 28 (D28) and 5 at day 35 (35). For each group, muscle surface, parameters of bone microarchitecture and macroarchitecture (including length, width and curvature of the tibia) were measured using microtomography. Paralysis occurred as soon as day 2. At the left hind limb, muscle surface area, cortical thickness, cross sectional total area and growth in length significantly increased during the time study. At the right hind limb, muscle surface area, bone trabecular volume, and cortical thickness decreased as soon as day 14 associated with an increased cortical porosity. Growth in length did not differ from left side; cross sectional total area did not increase and the diaphyseal cross section acquired a more rounded shape. There was no modification of the curvature between right and left hind limbs during the time study. In this murine model of unilateral muscle paralysis in growing animals, we showed a rapid muscle loss leading to a decreased growth in width; however growth in length and curvature were unaltered.

Loss of bone strength is dependent on skeletal site in disuse osteoporosis in rats

Intramuscular injection with botulinum toxin A (BTX) leads to a transient paralysis of the muscles, resulting in a rapid loss of muscle mass and function as well as rapid bone loss (disuse osteoporosis). The purpose of this study was to investigate the temporal development and the site specificity of BTX-induced immobilization on bone strength at five skeletal sites. Three-month-old rats (n = 108) were randomized into nine groups: one served as baseline, while four were injected with BTX and four with saline in the right hind-limb musculature. Animals were killed after 1, 2, 3, or 4 weeks. BTX-induced a significant loss of rectus femoris muscle mass (-61%) and muscle cell cross-sectional area (-59%) as well as bone strength at the femoral neck (-31%), femoral diaphysis (-6%), distal femoral metaphysis (-17%), proximal tibial metaphysis (-31%), and tibial diaphysis (-13%) after 4 weeks. Muscle atrophy occurred in parallel with the bone loss at the femoral neck and proximal tibia, whereas it occurred earlier than the bone loss at the other skeletal sites. At the proximal tibial metaphysis BTX significantly decreased BV/TV (-10%), trabecular thickness (-13%), and bone formation (MS/BS -25%, BFR/BS -50%) and increased osteoclast covered surfaces (+97%) after 4 weeks. In conclusion, BTX-induced a time-dependent loss of bone strength. Moreover, the loss of bone strength differed significantly at the five tested skeletal sites.

Botulinum toxin in masticatory muscles: short- and long-term effects on muscle, bone, and craniofacial function in adult rabbits

Paralysis of the masticatory muscles using botulinum toxin (BTX) is a common treatment for cosmetic reduction of the masseters as well as for conditions involving muscle spasm and pain. The effects of this treatment on mastication have not been evaluated, and claims that the treatment unloads the jaw joint and mandible have not been validated. If BTX treatment does decrease mandibular loading, osteopenia might ensue as an adverse result. Rabbits received a single dose of BTX or saline into one randomly chosen masseter muscle and were followed for 4 or 12 weeks. Masticatory muscle activity was assessed weekly, and incisor bite force elicited by stimulation of each masseter was measured periodically. At the endpoint, strain gages were installed on the neck of the mandibular condyle and on the molar area of the mandible for in vivo bone strain recording during mastication and muscle stimulation. After termination, muscles were weighed and mandibular segments were scanned with micro CT. BTX paralysis of one masseter did not alter chewing side or rate, in part because of compensation by the medial pterygoid muscle. Masseter-induced bite force was dramatically decreased. Analysis of bone strain data suggested that at 4 weeks, the mandibular condyle of the BTX-injected side was underloaded, as were both sides of the molar area. Bone quantity and quality were severely decreased specifically at these underloaded locations, especially the injection-side condylar head. At 12 weeks, most functional parameters were near their pre-injection levels, but the injected masseter still exhibited atrophy and percent bone area was still low in the condylar head. In conclusion, although the performance of mastication was only minimally harmed by BTX paralysis of the masseter, the resulting underloading was sufficient to cause notable and persistent bone loss, particularly at the temporomandibular joint.

Transient muscle paralysis degrades bone via rapid osteoclastogenesis

A unilateral injection of botulinum toxin A (BTxA) in the calf induces paralysis and profound loss of ipsalateral trabecular bone within days. However, the cellular mechanism underlying acute muscle paralysis-induced bone loss (MPIBL) is poorly understood. We hypothesized that MPIBL arises via rapid and extensive osteoclastogenesis. We performed a series of in vivo experiments to explore this thesis. First, we observed elevated levels of the proosteoclastogenic cytokine receptor activator for nuclear factor-κB ligand (RANKL) within the proximal tibia metaphysis at 7 d after muscle paralysis (+113%, P<0.02). Accordingly, osteoclast numbers were increased 122% compared with the contralateral limb at 5 d after paralysis (P=0.04) and MPIBL was completely blocked by treatment with human recombinant osteoprotegerin (hrOPG). Further, conditional deletion of nuclear factor of activated T-cells c1 (NFATc1), the master regulator of osteoclastogenesis, completely inhibited trabecular bone loss (-2.2±11.9%, P<0.01). All experiments included negative control assessments of contralateral limbs and/or within-animal pre- and postintervention imaging. In summary, transient muscle paralysis induced acute RANKL-mediated osteoclastogenesis resulting in profound local bone resorption. Elucidation of the pathways that initiate osteoclastogenesis after paralysis may identify novel targets to inhibit bone loss and prevent fractures.

Connexin43 deficiency reduces the sensitivity of cortical bone to the effects of muscle paralysis

We have shown previously that the effect of mechanical loading on bone depends in part on connexin43 (Cx43). To determine whether Cx43 is also involved in the effect of mechanical unloading, we have used botulinum toxin A (BtxA) to induce reversible muscle paralysis in mice with a conditional deletion of the Cx43 gene in osteoblasts and osteocytes (cKO). BtxA injection in hind limb muscles of wild-type (WT) mice resulted in significant muscle atrophy and rapid loss of trabecular bone. Bone loss reached a nadir of about 40% at 3 weeks after injection, followed by a slow recovery. A similar degree of trabecular bone loss was observed in cKO mice. By contrast, BtxA injection in WT mice significantly increased marrow area and endocortical osteoclast number and decreased cortical thickness and bone strength. These changes did not occur in cKO mice, whose marrow area is larger, osteoclast number higher, and cortical thickness and bone strength lower relative to WT mice in basal conditions. Changes in cortical structure occurring in WT mice had not recovered 19 weeks after BtxA injection despite correction of the early osteoclast activation and a modest increase in periosteal bone formation. Thus BtxA-induced muscle paralysis leads to rapid loss of trabecular bone and to changes in structural and biomechanical properties of cortical bone, neither of which are fully reversed after 19 weeks. Osteoblast/osteocyte Cx43 is involved in the adaptive responses to skeletal unloading selectively in the cortical bone via modulation of osteoclastogenesis on the endocortical surface.

Increased bone strength is associated with improved bone microarchitecture in intact female rats treated with strontium ranelate: a finite element analysis study

Strontium ranelate has been previously shown to act on bone metabolism and to be effective in postmenopausal osteoporosis treatment by preventing vertebral and non-vertebral fractures. Animal studies explicitly demonstrated that bone strength was improved with strontium ranelate treatment, but the contribution of either improved bone microarchitecture or intrinsic quality of the bone tissue is not clear. Therefore, the purpose of this research was to address this issue by using the unique capability of finite element (FE) analysis to integrate both intrinsic bone quality properties from nano-indentation and microarchitecture measured by micro-computed tomography (μCT). The two groups included intact female Fischer rats fed a normal diet (controls, N=12) or a diet containing strontium ranelate (900mg/kg/day; N=12) for a period of 104weeks. The L(5) vertebra was scanned by μCT and a morphological analysis of the vertebral body was performed. Subsequently, those μCT data were the basis of FE models with added virtual endcaps that simulated axial compression tests. The FE models were solved with the vertebral bodies only and repeated with the vertebral processes intact. In the initial stages, the intrinsic bone properties were kept constant between the control and the treated animals in order to independently study the impact of microarchitectural changes on bone strength. Morphological data indicated a significant improvement in bone microarchitecture associated with strontium ranelate compared to controls, including a 40% (p<0.01) higher trabecular thickness, a 28% (p<0.01) higher cortical thickness, and no significant change in the number of trabeculae (p=0.56). The poor correlation of bone strontium content against bone volume fraction (BV/TV) (R(2)=0.013, p=0.74) and BMD (R(2)=0.153, p=0.23) indicated that the morphological data were not biased by the presence of strontium in bone. The FE simulations demonstrated a 22% (p<0.01) increase of stiffness and 29% (p<0.01) increase in strength compared to controls. The magnitudes were greater, but the relative differences were similar when the entire intact vertebra was modeled compared to the vertebral body alone. Adjusting the FE models to account for differences in intrinsic bone tissue quality between control and treated animals resulted in an even higher bone strength with strontium ranelate. Furthermore, load transfer in strontium ranelate treated animals shifted from an equal distribution between cortical and trabecular compartments to more load being supported by the trabecular bone (a shift of 8%, p<0.02). Tissue-level stresses were reduced on average (-7%, p<0.01) and more homogeneously distributed. Together, these findings indicated that, independently from bone strontium content, microarchitectural adaptations played a major role in the increased bone strength associated with strontium ranelate exposure and that the changes in load distribution resulted in patterns that were more favorable to resisting fracture.

Muscle changes can account for bone loss after botulinum toxin injection

Studies to date have assumed that botulinum toxin type A (BTX) affects bone indirectly, through its action on muscle. We hypothesized that BTX has no discernable effect on bone morphometry, independent of its effect on muscle. Therefore, we investigated whether BTX had an additional effect on bone when combined with tenotomy compared to tenotomy in isolation. Female BALB/c mice (n = 73) underwent one of the following procedures in the left leg: BTX injection and Achilles tenotomy (BTX-TEN), BTX injection and sham surgery (BTX-sham), Achilles tenotomy (TEN), or sham surgery (sham). BTX groups were injected with 20 μL of BTX (1 U/100 g) in the posterior lower hindlimb. At 4 weeks, muscle cross-sectional area (MCSA) and tibial bone morphometry were assessed using micro-CT. Each treatment, other than sham, resulted in significant muscle and bone loss (P < 0.05). BTX-TEN experienced the greatest muscle loss (23-45% lower than other groups) and bone loss (20-30% lower bone volume fraction than other groups). BTX-sham had significantly lower MCSA and bone volume fraction than TEN and sham. After adjusting for differences in MCSA, there were no significant between-group differences in bone properties. We found that BTX injection resulted in more adverse muscle and bone effects than tenotomy and that effects were amplified when the procedures were combined. However, between-group differences in bone could be accounted for by MCSA. We conclude that any independent effect of BTX on bone morphometry is likely small or negligible compared with the effect on muscle.

Current concepts in rehabilitation following ulnar collateral ligament reconstruction

Injuries to the ulnar collateral ligament (UCL) in throwing athletes frequently occurs from the repetitive valgus loading of the elbow during the throwing motion, which often results in surgical reconstruction of the UCL requiring a structured postoperative rehabilitation program. Several methods are currently used and recommended for UCL reconstruction using autogenous grafts in an attempt to reproduce the stabilizing function of the native UCL. Rehabilitation following surgical reconstruction of the UCL begins with range of motion and initial protection of the surgical reconstruction, along with resistive exercise for the entire upper extremity kinetic chain. Progressions for resistive exercise are followed that attempt to fully restore strength and local muscular endurance in the rotator cuff and scapular stabilizers, in addition to the distal upper extremity musculature, to allow for a return to throwing and overhead functional activities. Rehabilitation following UCL reconstruction has produced favorable outcomes, allowing for a return to throwing in competitive environments.