Relationship between the duration of paralysis and bone structure: a pQCT study of spinal cord injured individuals

Boys with haemophilia have low trabecular bone mineral density and sarcopenia, but normal bone strength at the radius

Although a decreased areal bone mineral density (BMD) has been reported in patients with haemophilia, data are lacking that would reflect the three-dimensional structure of the bone and the muscle-bone relationship. We aimed to assess volumetric BMD, bone geometry and muscle-bone phenotype in boys with haemophilia, and to describe the association between clinical characteristics of haemophilia and bone quality and structure. A cross-sectional study was conducted in 41 boys with haemophilia (mean age 12.4, range 6.6-19.8 years) using peripheral quantitative CT (pQCT) at the nondominant forearm. Results were transformed into Z-scores using previously published reference data. Significant differences were tested by one-sample t-test or sign test. Two-sample t-test and anova were used to compare results between subgroups of patients divided according to the severity of the disease, the fracture history and the number of joint and muscle bleedings. Boys with haemophilia had a decreased trabecular volumetric BMD (mean Z-score -0.5, P < 0.01), while their cortical volumetric BMD was increased (mean Z-score 0.4, P < 0.05). The volumetric bone mineral content and the bone geometry at the radial diaphysis were normal when adjusted for patients' shorter body height. Muscle area was decreased (mean Z-score -1.0, P < 0.001), irrespective of age. No association was observed of bone quality parameters and bone geometry with the disease severity, fracture history or number of bleedings. Bone strength measured at the diaphysis of the radius is not impaired in boys with haemophilia. The finding of the decreased trabecular bone density can be most likely attributed to their sarcopenia.

Enhancing muscle force and femur compressive loads via feedback-controlled stimulation of paralyzed quadriceps in humans

OBJECTIVE

To compare paralyzed quadriceps force properties and femur compressive loads in an upright functional task during conventional constant-frequency stimulation and force feedback-modulated stimulation.

DESIGN

Crossover trial.

SETTING

Research laboratory.

PARTICIPANTS

Subjects (N=13; 12 men, 1 woman) with motor-complete spinal cord injury.

INTERVENTIONS

Subjects performed 2 bouts of 60 isometric quadriceps contractions while supported in a standing frame. On separate days, subjects received constant-frequency stimulation at 20Hz (CONST) or frequency-modulated stimulation triggered by a change in force (FDBCK). During FDBCK, a computer algorithm responded to each 10% reduction in force with a 20% increase in stimulation frequency.

MAIN OUTCOME MEASURES

A biomechanical model was used to derive compressive loads on the femur, with a target starting dose of load equal to 1.5 times body weight.

RESULTS

Peak quadriceps force and fatigue index were higher for FDBCK than CONST (P<.05). Within-train force decline was greater during FDBCK bouts, but mean force remained above CONST values (P<.05). As fatigue developed during repetitive stimulation, FDBCK was superior to CONST for maintenance of femur compressive loads (P<.05).

CONCLUSIONS

Feedback-modulated stimulation in electrically activated stance is a viable method to maximize the physiologic performance of paralyzed quadriceps muscle. Compared with CONST, FDBCK yielded compressive loads that were closer to a targeted dose of stress with known osteogenic potential. Optimization of muscle force with FDBCK may be a useful tactic for future training-based antiosteoporosis protocols.

Anatomical sector analysis of load-bearing tibial bone structure during 90-day bed rest and 1-year recovery

The aim of this study was to investigate whether the bone response to long bed rest-related immobility and during subsequent recovery differed at anatomically different sectors of tibial epiphysis and diaphysis. For this study, peripheral quantitative tomographic (pQCT) scans obtained from a previous 90-day 'Long Term Bed Rest' intervention were preprocessed with a new method based on statistical approach and re-analysed sector-wise. The pQCT was performed on 25 young healthy males twice before the bed rest, after the bed rest and after 1-year follow-up. All men underwent a strict bed rest intervention, and in addition, seven of them received pamidronate treatment and nine did flywheel exercises as countermeasures against disuse-related bone loss. Clearly, 3-9% sector-specific losses in trabecular density were observed at the tibial epiphysis on average. Similarly, cortical density decreased in a sector-specific way being the largest at the anterior sector of tibial diaphysis. During recovery, the bed rest-induced bone losses were practically restored and no consistent sector-specific modulation was observed in any subgroup. It is concluded that the sector-specific analysis of bone cross-sections has potential to reveal skeletal responses to various interventions that cannot be inferred from the average analysis of the whole bone cross-section. This approach is considered also useful for evaluating the bone responses from the biomechanical point of view.

An evidence-based review of aging of the body systems following spinal cord injury

STUDY DESIGN

Systematic review.

OBJECTIVE

To systematically review evidence on aging of the body systems after spinal cord injury (SCI).

SETTING

Toronto, Ontario and Vancouver, British Columbia, Canada.

METHODS

Electronic databases (MEDLINE/PubMed, CINAHL, EMBASE and PsycINFO), were searched for studies published between 1980 and 2009. The search was augmented by reviewing the reference lists of relevant papers. Non-intervention studies that were longitudinal or cross-sectional with able-bodied controls that were at minimum matched on chronological age were included for review. Levels of evidence were assigned to the study design using a modified Sackett scale.

RESULTS

Of the 74 studies selected for inclusion, 16 were longitudinal in design. The hypothesis that SCI represents a model for premature aging is supported by a large proportion of level 5 evidence for the cardiovascular and endocrine systems, level 2, 4 and 5 evidence for the musculoskeletal system, and limited level 5 evidence for the immune system. Only a few level 4 and 5 studies for the respiratory system were found. The evidence on the genitourinary system, gastrointestinal system, and for skin and subcutaneous tissues provide level 4 and 5 evidence that premature aging may not be occurring. The evidence on the nervous system does not provide evidence of premature aging as a result of SCI.

CONCLUSIONS

Premature aging appears to occur in some systems after SCI. Additional longitudinal studies are required to confirm these findings.

Bone and muscle loss after spinal cord injury: organ interactions

Spinal cord injury (SCI) results in paralysis and marked loss of skeletal muscle and bone below the level of injury. Modest muscle activity prevents atrophy, whereas much larger--and as yet poorly defined--bone loading seems necessary to prevent bone loss. Once established, bone loss may be irreversible. SCI is associated with reductions in growth hormone, IGF-1, and testosterone, deficiencies likely to exacerbate further loss of muscle and bone. Reduced muscle mass and inactivity are assumed to be contributors to the high prevalence of insulin resistance and diabetes in this population. Alterations in muscle gene expression after SCI share common features with other muscle loss states, but even so, show distinct profiles, possibly reflecting influences of neuromuscular activity due to spasticity. Changes in bone cells and markers after SCI have similarities with other conditions of unloading, although after SCI these changes are much more dramatic, perhaps reflecting the much greater magnitude of unloading. Adiposity and marrow fat are increased after SCI with intriguing, though poorly understood, implications for the function of skeletal muscle and bone cells.

Evolving concepts in neurogenic osteoporosis

Convincing evidence has accumulated of regulation of bone by the central nervous system. The neural connection between brain and bone is mediated centrally by classic neurotransmitters and several neuropeptides, and peripherally by many of the same neurotransmitters and neuropeptides, albeit with actions opposite to their central effects. Pharmacologic blockade of ß2-adrenergic receptors or disruption of the gene encoding them increases bone mass, whereas increased activity of the sympathetic nervous system (SNS) contributes to bone loss. Brainstem serotonergic neurons regulate SNS activity and its modulation by leptin. Physiologic stimulation of osteoblastic nicotinic receptors results in proliferation and deposition of bone, whereas higher levels inhibit osteoblast function. Activation of sensory nerves has a centrally mediated action on bone, albeit poorly understood. The relative importance of, and interactions between autonomic, sensory, and peripheral nervous system actions on bone mass are also not clear in healthy individuals, and less so in pathologic states.

Structural analysis of the human tibia in men with spinal cord injury by tomographic (pQCT) serial scans

Spinal cord injury (SCI), as a primarily neurological disorder that causes muscular atrophy, is well known to be associated with sub-lesional bone losses. These losses are more pronounced from epiphyseal than from diaphyseal regions. We hypothesized that this discrepancy may be explained by anatomical variation in endocortical circumference. Nine men who had attracted SCI 9 to 32 (mean 21.4) years prior to study inclusion were matched to able bodied control (Ctrl) people by age, height and weight. Serial scans by peripheral quantitative computed tomography were obtained from the tibia at steps corresponding to 5%-steps of the tibias length (s05 to s95, from distal to the proximal end of the tibia). As expected, SCI people had lower total bone mineral content (vBMC.tot) than able bodied control people (P<0.001 at all sites). This group difference (DeltavBMC.tot) was more pronounced at the distal and proximal tibia than in the shaft (P<0.001), and it amounted to 51% at s05, to 22% at s40, and to 47% at s95. Both endocortical and periosteal circumference were better predictors of DeltavBMC.tot (R(2)=0.98 and R(2)=0.97, respectively; P<0.001 in both cases) than vBMC.tot (R(2)=0.58, P<0.001), suggesting that anatomical variation in geometry, rather than in bone mass can explain differential rates of bone loss after SCI. Moreover, the s04:s38 ratio in vBMC.tot was found to be 1.00 (95% confidence interval: 0.95-1.05) in the Ctrl group, and 0.63 in the SCI group (P<0.001, 95% confidence interval: 0.54-0.68). These findings offer a rationale to account for the discrepancy between epiphyseal and diaphyseal bone losses following SCI. The suggestion is that the bone adaptive responses involved are limited in time, and that the reduced surface:volume ratio constitutes a limit within the available time window, in particular in the diaphysis. Finally, the drastically reduced s04:s38 vBMC.tot ratio observed in the SCI group in this study provides a rationale to scrutinize this Capozza index also in other studies as a general indicator of immobilisation-induced bone loss.

Mechanical factors and bone health: effects of weightlessness and neurologic injury

Bone is a dynamic tissue with homeostasis governed by many factors. Among them, mechanical stimuli appear to be particularly critical for bone structure and strength. With removal of mechanical stimuli, a profound bone loss occurs, as best observed in the extreme examples following exposure to space flight or neurologic impairment. This review provides an overview of the changes in bone density and structure that occur during and after space flight as well as following neurologic injury from stroke and spinal cord injury. It also discusses the potential mechanisms through which mechanical stimuli are postulated to act on bone tissue.

Reduced trabecular bone mineral density and cortical thickness accompanied by increased outer bone circumference in metacarpal bone of rheumatoid arthritis patients: a cross-sectional study

Introduction

The objective of this study was to assess three-dimensional bone geometry and density at the epiphysis and shaft of the third meta-carpal bone of rheumatoid arthritis (RA) patients in comparison to healthy controls with the novel method of peripheral quantitative computed tomography (pQCT).

Methods

PQCT scans were performed in 50 female RA patients and 100 healthy female controls at the distal epiphyses and shafts of the third metacarpal bone, the radius and the tibia. Reproducibility was determined by coefficient of varia-tion. Bone densitometric and geometric parameters were compared between the two groups and correlated to disease characteristics.

Results

Reproducibility of different pQCT parameters was between 0.7% and 2.5%. RA patients had 12% to 19% lower trabecular bone mineral density (BMD) (P ≤ 0.001) at the distal epiphyses of radius, tibia and metacarpal bone. At the shafts of these bones RA patients had 7% to 16% thinner cortices (P ≤ 0.03). Total cross-sectional area (CSA) at the metacarpal bone shaft of pa-tients was larger (between 5% and 7%, P < 0.02), and relative cortical area was reduced by 13%. Erosiveness by Ratingen score correlated negatively with tra-becular and total BMD at the epiphyses and shaft cortical thickness of all measured bones (P < 0.04).

Conclusions

Reduced trabecular BMD and thinner cortices at peripheral bones, and a greater bone shaft diameter at the metacarpal bone suggest RA spe-cific bone alterations. The proposed pQCT protocol is reliable and allows measuring juxta-articular trabecular BMD and shaft geometry at the metacarpal bone.

Longitudinal changes in femur bone mineral density after spinal cord injury: effects of slice placement and peel method

SUMMARY

Surveillance of femur metaphysis bone mineral density (BMD) decline after spinal cord injury (SCI) may be subject to slice placement error of 2.5%. Adaptations to anti-osteoporosis measures should exceed this potential source of error. Image analysis parameters likewise affect BMD output and should be selected strategically in longitudinal studies.

INTRODUCTION

Understanding the longitudinal changes in bone mineral density (BMD) after spinal cord injury (SCI) is important when assessing new interventions. We determined the longitudinal effect of SCI on BMD of the femur metaphysis. To facilitate interpretation of longitudinal outcomes, we (1) determined the BMD difference associated with erroneous peripheral quantitative computed tomography (pQCT) slice placement, and (2) determined the effect of operator-selected pQCT peel algorithms on BMD.

METHODS

pQCT images were obtained from the femur metaphysis (12% of length from distal end) of adult subjects with and without SCI. Slice placement errors were simulated at 3 mm intervals and were processed in two ways (threshold-based vs. concentric peel).

RESULTS

BMD demonstrated a rapid decline over 2 years post-injury. BMD differences attributable to operator-selected peel methods were large (17.3% for subjects with SCI).

CONCLUSIONS

Femur metaphysis BMD declines after SCI in a manner similar to other anatomic sites. Concentric (percentage-based) peel methods may be most appropriate when special sensitivity is required to detect BMD adaptations. Threshold-based methods may be more appropriate when asymmetric adaptations are observed.

Skeletal and body composition changes in hemiplegic patients

Hemiplegic patients are prone to bone loss and alterations in fat and lean mass, which ultimately affect their rehabilitation status and propensity in bone fractures. The present study aimed to evaluate body composition and bone mineral density (BMD) in stroke patients within 1st year post-stroke. Fifty-eight hemiplegic patients (36 men and 22 women) were enrolled in this prospective study. Dual-energy X-ray absorptiometry was used to assess total-body and lower-extremity BMDs (g/cm(2)), lean mass (g), and fat mass (g) after 3, 6 and 12 mo of stroke that led to hemiplegia. The Modified Ashworth Scale and the functional ambulation category were used to evaluate spasticity and ambulatory category of patients, respectively. Both sexes exhibited total-body and paretic lower-limb BMD loss, fat mass gain, and lean mass waste during the 1st 12 mo poststroke, and in most cases, statistically significant differences were found between 3 and 6 mo; however, the pattern of changes was different between males and females. Therefore, it is suggested that disability because of hemiplegia led to alterations in muscle function, which triggered skeletal and body composition changes and rendered these patients particularly prone to increased fracture risk.

Regional changes in bone area and bone mineral content in boys with duchenne muscular dystrophy receiving corticosteroid therapy

OBJECTIVE

To examine the functional and skeletal effects of 30 months of steroid treatment in boys with Duchenne muscular dystrophy.

STUDY DESIGN

Lumbar spine (L(2)L(4)) and subcranial dual energy X-ray absorptiometry scanning was performed on 25 boys (mean age 7.4 years) at baseline and after 30 months of steroid treatment.

RESULTS

At baseline, L(2)L(4) bone mineral content (BMC) was significantly low for projected bone area although appropriate for reduced lean body mass (LBM). Subcranial bone area for height and subcranial BMC for area and LBM were all significantly reduced. After 30 months of steroid therapy there was a significant increase in subcranial bone area for height but a significant reduction of subcranial BMC for area. At the lumbar spine there were no significant changes in bone area but small increases in L(2)L(4) BMC both for bone area and LBM.

CONCLUSION

At baseline reduced mechanical load from diminished muscle function results in narrow light bones more noticeable in the subcranial region than the lumbar spine. Increases observed in subcranial bone area at 30 months suggest a gradual adaptation to increased gravitational load whereas at the spine there were no apparent detrimental effects on bone after 30 months of steroid therapy.

Prevention of bone loss during 56 days of strict bed rest by side-alternating resistive vibration exercise

Bed rest is a recognized model for muscle atrophy and bone loss in space flight and in clinical medicine. We hypothesized that whole body vibration in combination with resistive exercise (RVE) would be an effective countermeasure. Twenty healthy male volunteers underwent horizontal bed rest for 56 days and were randomly assigned either to a group that performed RVE 11 times per week or to a group that underwent bed rest only (Ctrl). Bone mineral content (BMC) was assessed by peripheral quantitative computed tomography (pQCT) in the tibia and the radius and by dual x-ray absorptiometry (DXA) in the hip and lumbar spine at baseline and at regular intervals during bed rest and a 12-month follow-up. RVE appeared to protect muscle size and function, and it also prevented bone loss (p-values between <0.001 and 0.01). Bone losses were largest in the distal tibia epiphysis, where BMC declined from 421.8 mg/mm (SD 51.3) to 406.6 mg/mm (SD 52.7) in Ctrl, but only from 411.1 mg/mm (SD 56.6) to 409.6 mg/mm (SD 66.7) in RVE. Most of the BMC losses were recovered by 12-month follow-up. Analyses showed that the epiphyseal cortex, rather than spongiosa, depicted the most pronounced changes during bed rest and recovery. These results suggest that the combined countermeasure applied in this study is effective to prevent bone losses from the tibia. This underlines the importance of mechanical usage for the maintenance of the human skeleton.

Transient muscle paralysis disrupts bone homeostasis by rapid degradation of bone morphology

We have previously shown that transient paralysis of murine hindlimb muscles causes profound degradation of both trabecular and cortical bone in the adjacent skeleton within 3 weeks. Morphologically, the acute loss of bone tissue appeared to arise primarily due to osteoclastic bone resorption. Given that the loss of muscle function in this model is transient, we speculated that the stimulus for osteoclastic activation would be rapid and morphologic evidence of bone resorption would appear before 21 days. We therefore utilized high-resolution in vivo serial micro-CT to assess longitudinal alterations in lower hindlimb muscle volume, proximal tibia trabecular, and tibia mid-diaphysis cortical bone morphology in 16-week-old female C57 mice following transient calf paralysis from a single injection of botulinum toxin A (BtA; 2U/100 g body weight). In an acute study, we evaluated muscle and bone alterations at days 0, 3, 5, and 12 following transient calf paralysis. In a chronic study, following day 0 imaging, we assessed the recovery of these tissues following the maximum observed trabecular degradation (day 12) through day 84 post-paralysis. The time course and degree of recovery of muscle, trabecular, and cortical bone varied substantially. Significant atrophy of lower limb muscle was evident by day 5 of paralysis, maximal at day 28 (-34.1+/-0.9%) and partially recovered by day 84. Trabecular degradation within the proximal tibia metaphysis occurred more rapidly, with significant reduction in BV/TV by day 3, maximal loss at day 12 (-76.8+/-2.9%) with only limited recovery by day 84 (-51.7+/-5.1% vs. day 0). Significant cortical bone volume degradation at the tibia mid-diaphysis was first identified at day 12, was maximal at day 28 (-9.6+/-1.2%), but completely recovered by day 84. The timing, magnitude, and morphology of the observed bone erosion induced by transient muscle paralysis were consistent with a rapid recruitment and prolific activation of osteoclastic resorption. In a broader context, understanding how brief paralysis of a single muscle group can precipitate such rapid and profound bone resorption in an adjacent bone is likely to provide new insight into how normal muscle function modulates bone homeostasis.

Bone rigidity to neuromuscular performance ratio in young and elderly men

Given the adaptation of bone to prevalent loading, bone loss should follow, but lag behind, the decline in physical performance during aging. Furthermore, bone responsiveness to load-induced strains is believed to decrease with aging. However, the relationship between bone and lean body ( approximately muscle) mass appears to remain rather constant throughout adulthood. The purpose of this study was to examine the association between age and bone to neuromuscular performance ratio. Young (N=20, age 24 SD+/-2 years, body mass 77+/-11 kg, height 178+/-6 cm) and elderly (N=25, 72+/-4 years, 75+/-9 kg, 172+/-5 cm) men served as subjects. Bone structural traits were measured at the right distal tibia and tibial mid-shaft with peripheral quantitative computed tomography (pQCT). Maximal section modulus (Z(max50)), total area (ToA(d)), cortical area (CoA(50)), total density (ToD(d)) and cortical density (CoD(50)) were determined from the pQCT images. Neuromuscular performance was measured by recording vertical ground reaction force (GRF) in maximal bilateral hopping. Load-induced strains were estimated by calculating appropriate indices for compressive and tensile loading that took into account both the bone structure and apparent biomechanics of the given bone site. Young subjects had significantly higher maximal GRF compared to older men (4260+/-800 N vs. 3080+/-600 N, P<0.001). They also had smaller ToA(d) (1100+/-170 mm(2) vs. 1200+/-100 mm(2), P=0.028) while their ToD(d) was higher (370+/-46 g/cm(3) vs. 330+/-22 g/cm(3), P=0.002). The Z(max50) did not differ significantly between young (1660+/-320 mm(3)) and elderly men (1750+/-320 mm(3)) (P=0.224). Compressive (0.484+/-0.102 vs. 0.399+/-0.078, P=0.016) and tensile (0.107+/-0.016 vs. 0.071+/-0.018, P<0.001) strain indices were significantly higher in the younger group. In conclusion, the difference in bone to loading ratio at the tibial mid-shaft is bigger than expected from the delay in bone adaptation alone. Potential candidates to explain this phenomenon include a decrease in mechanosensitivity with aging, inability of maximal physical performance to adequately represent the bone loading environment, or the need to maintain constant safety factors to functional strains.

Role of peripheral quantitative computed tomography in identifying disuse osteoporosis in paraplegia

OBJECTIVE

Disuse osteoporosis is a major long-term health consequence of spinal cord injury (SCI) that still needs to be addressed. Its management in SCI should begin with accurate diagnosis, followed by targeted treatments in the most vulnerable subgroups. We present data quantifying disuse osteoporosis in a cross-section of the Scottish paraplegic population to identify subgroups with lowest bone mineral density (BMD).

MATERIALS AND METHODS

Forty-seven people with chronic SCI at levels T2-L2 were scanned using peripheral quantitative computed tomography at four tibial sites and two femoral sites, at the Queen Elizabeth National Spinal Injuries Unit, Glasgow (UK). At the distal epiphyses, trabecular BMD (BMDtrab), total BMD, total bone cross-sectional area (CSA) and bone mineral content (BMC) were determined. In the diaphyses, cortical BMD, total bone CSA, cortical CSA and BMC were calculated. Bone, muscle and fat CSAs were estimated in the lower leg and thigh.

RESULTS

BMDtrab decreased exponentially with time since injury at different rates in the tibia and femur. At most sites, female paraplegics had significantly lower BMC, total bone CSA and muscle CSA than male paraplegics. Subjects with lumbar SCI tended to have lower bone values and smaller muscle CSAs than in thoracic SCI.

CONCLUSION

At the distal epiphyses of the tibia and femur, there is generally a rapid and extensive reduction in BMDtrab after SCI. Female subjects, and those with lumbar SCI, tend to have lower bone values than males or those with thoracic SCI, respectively.

Acute suppression of bone turnover with calcium infusion in persons with spinal cord injury

BACKGROUND

Some people with chronic spinal cord injury (SCI) have low vitamin D levels and secondary hyperparathyroidism.

OBJECTIVE

To determine whether, and to what extent, an acute calcium infusion decreased levels of N-telopeptide (NTx), a marker of osteoclastic activity, in individuals with chronic SCI.

STUDY DESIGN

Case series.

SUBJECTS

Eight men with chronic SCI. A relatively low serum 25 hydroxyvitamin D concentration (25[OH]D < or =20 ng/mL) and/or a high parathyroid hormone (PTH) (>55 pg/mL) was a prerequisite for study inclusion.

METHODS

Calcium gluconate bolus 0.025 mmol elemental calcium/kg over 20 minutes followed by a constant infusion of 0.025 mmol/kg per hour for 6 hours was infused; blood samples were collected every 2 hours for measurement of serum total calcium, creatinine, NTx, and PTH.

RESULTS

All results are expressed as means (+/- SDs). Baseline serum 25-hydroxyvitamin D level was 14.5 +/- 3.5 ng/mL (range: 10.2-19.6 ng/mL); PTH, 70 +/- 25 pg/mL (range: 37-100 pg/mL); and NTx, 21 +/- 7 nM bone collagen equivalents (BCE) (range: 14-34 nM). At 2, 4, and 6 hours after the calcium infusion, serum calcium rose from 9.3 +/- 0.2 to 10.8 +/- 0.9, 10.5 +/- 0.8, and 10.6 +/- 0.6 mg/d; PTH was suppressed from 70 +/- 25 pg/mL to 18 +/- 12, 16 +/- 9, and 15 +/- 9 pg/mL, respectively; NTx fell from 21 +/- 8 nM BCE to 17 +/- 5, 12 +/- 4, and 12 +/- 3 nM BCE, respectively.

CONCLUSIONS

Serum NTx is a marker for bone collagen catabolism, and its reduction suggests that bone turnover was decreased. A relative deficiency of vitamin D associated with chronically elevated levels of PTH would be expected to increase bone turnover and to worsen the bone loss associated with immobilization.

Bisphosphonate use in acute and chronic spinal cord injury: a systematic review

BACKGROUND/OBJECTIVE

Bone density loss occurs rapidly after traumatic spinal cord injury (SCI) and is associated with low-energy fractures below the level of injury, commonly occurring around the knee. Bisphosphonates have been tested as potential agents to prevent bone loss after SCI, but no guidelines exist for clinical use of bisphosphonates in these patients. The objective of this study was to systematically review and evaluate evidence quality in studies of bisphosphonate use in patients with post-treatment follow-up of sublesional bone mineral density.

METHODS

Literature search in MEDLINE/PubMed and ISI database using key words bisphosphonates, spinal cord injury, quadriplegia, paraplegia, and tetraplegia.

RESULTS

The search identified 6 experimental studies and 1 quasi-experimental study of bisphosphonate therapy in patients with acute and chronic SCI. The studies were small and of fair or poor quality, and none included fracture outcomes. Mild attenuation of bone density loss with acute administration of bisphosphonates after SCI was found at some measurement sites but was not always maintained during follow-up.

CONCLUSIONS

Data were insufficient to recommend routine use of bisphosphonates for fracture prevention in these patients. Current studies are limited by heterogeneity of patient populations and outcome measures. Uniform bone density measurement sites with rigorous quality control and compliance monitoring are needed to improve reliability of outcomes. Future studies should address specific populations (acute or chronic SCI) and should assess fracture outcomes.

Six months of disuse during hibernation does not increase intracortical porosity or decrease cortical bone geometry, strength, or mineralization in black bear (Ursus americanus) femurs

Disuse typically uncouples bone formation from resorption, leading to bone loss which compromises bone mechanical properties and increases the risk of bone fracture. Previous studies suggest that bears can prevent bone loss during long periods of disuse (hibernation), but small sample sizes have limited the conclusions that can be drawn regarding the effects of hibernation on bone structure and strength in bears. Here we quantified the effects of hibernation on structural, mineral, and mechanical properties of black bear (Ursus americanus) cortical bone by studying femurs from large groups of male and female bears (with wide age ranges) killed during pre-hibernation (fall) and post-hibernation (spring) periods. Bone properties that are affected by body mass (e.g. bone geometrical properties) tended to be larger in male compared to female bears. There were no differences (p>0.226) in bone structure, mineral content, or mechanical properties between fall and spring bears. Bone geometrical properties differed by less than 5% and bone mechanical properties differed by less than 10% between fall and spring bears. Porosity (fall: 5.5+/-2.2%; spring: 4.8+/-1.6%) and ash fraction (fall: 0.694+/-0.011; spring: 0.696+/-0.010) also showed no change (p>0.304) between seasons. Statistical power was high (>72%) for these analyses. Furthermore, bone geometrical properties and ash fraction (a measure of mineral content) increased with age and porosity decreased with age. These results support the idea that bears possess a biological mechanism to prevent disuse and age-related osteoporoses.

Bone loss in the lower leg during 35 days of bed rest is predominantly from the cortical compartment

Immobilization-induced bone loss is usually greater in the epiphyses than in the diaphyses. The larger fraction of trabecular bone in the epiphyses than in the diaphyses offers an intuitive explanation to account for this phenomenon. However, recent evidence contradicts this notion and suggests that immobilization-induced bone loss from the distal tibia epiphysis is mainly from the cortical compartment. The aim of this study was to establish whether this pattern of bone loss was a general rule during immobilization. We monitored various skeletal sites with different tissue composition during 5 weeks of immobilization. Ten healthy male volunteers with mean age of 24.3 years (SD 2.6 years) underwent strict horizontal bed rest. Bone scans were obtained during baseline data collection, at the end of bed rest and after 14 days of recovery by peripheral Quantitative Computed Tomography (pQCT). Sectional images were obtained from the distal tibia epiphysis (at 4% of the tibia's length), from the diaphysis (at 38%), from the proximal metaphysis (at 93%) and from the proximal epiphysis (at 98%), as well as from the distal femur epiphysis (at 4% of the femur's length) and from the patella. Relative bone losses were largest at the patella, where they amounted to -3.2% (SD 1.8%, p<0.001) of the baseline values, and smallest at the tibia diaphysis, where they amounted to -0.7% (SD 1.0%, p=0.019). The relative losses were generally larger from cortical than from trabecular compartments (p=0.004), and whilst all skeletal sites depicted such cortical losses, substantial trabecular losses were found only from the proximal tibia epiphysis. Results confirm that the differential losses from the various skeletal sites cannot be explained on the basis of trabecular vs. cortical tissue composition differences, but that endocortical circumference can account for the different amounts of bone loss in the tibia. The present study therefore supports the suggestion of the subendocortical layer as a transitional zone, which can readily be transformed into trabecular bone in response to immobilization. The latter will lead to cortical thinning, a factor that has been associated with the risk of fracture and with osteoarthritis.

Evaluation of the femoral midshaft in children with cerebral palsy using magnetic resonance imaging

SUMMARY

Magnetic resonance imaging was used to show that children with quadriplegic cerebral palsy and unable to ambulate independently compared to typically developing children have a remarkably underdeveloped femoral midshaft as indicated by a very thin diameter, a very thin cortical wall, and very low strength estimates.

INTRODUCTION

The femoral shaft is very susceptible to fracture in children with quadriplegic cerebral palsy (QCP); however, its structure and strength have not been evaluated.

METHODS

The volume and width of the middle third of the femur (midfemur) and its cortical wall and medullary cavity were assessed in children with QCP and unable to ambulate independently and typically developing children (n = 10/group) using magnetic resonance imaging (MRI). Estimates of cross-sectional moment of inertia (CSMI), section modulus (Z), and polar moment of inertia (J) were also determined.

RESULTS

Total volume of the midfemur and volume of its cortical wall and medullary cavity were substantially lower in children with QCP than controls (51-55%; p < 0.001). In addition, the total midfemur, its medullary cavity and the anterior, posterior, and lateral sections of its cortical wall were thinner (27-43%) in children with QCP (p < 0.001). The midfemur in children with QCP also had remarkably lower CSMI, Z, and J (60-71%; p < 0.001).

CONCLUSIONS

Children with QCP who lack the ability to ambulate independently have midfemurs that are very thin with very thin cortical walls and very low estimated strength. The disparity can be detected using MRI.

Muscle and bone plasticity after spinal cord injury: review of adaptations to disuse and to electrical muscle stimulation

The paralyzed musculoskeletal system retains a remarkable degree of plasticity after spinal cord injury (SCI). In response to reduced activity, muscle atrophies and shifts toward a fast-fatigable phenotype arising from numerous changes in histochemistry and metabolic enzymes. The loss of routine gravitational and muscular loads removes a critical stimulus for maintenance of bone mineral density (BMD), precipitating neurogenic osteoporosis in paralyzed limbs. The primary adaptations of bone to reduced use are demineralization of epiphyses and thinning of the diaphyseal cortical wall. Electrical stimulation of paralyzed muscle markedly reduces deleterious post-SCI adaptations. Recent studies demonstrate that physiological levels of electrically induced muscular loading hold promise for preventing post-SCI BMD decline. Rehabilitation specialists will be challenged to develop strategies to prevent or reverse musculoskeletal deterioration in anticipation of a future cure for SCI. Quantifying the precise dose of stress needed to efficiently induce a therapeutic effect on bone will be paramount to the advancement of rehabilitation strategies.

Recovery of muscle atrophy and bone loss from 90 days bed rest: results from a one-year follow-up

Earlier studies found the recovery of bone loss after clinical immobilization to be incomplete. It has been argued that this is due to the human skeleton's inability to accrue bone mass once peak bone mass has been attained. However, recent studies suggest that bone losses can fully recover when complete functional rehabilitation is achieved. Accordingly, we hypothesized that bone losses by experimental bed rest would recover within one-year of follow-up. Twenty-five men (mean age 32 years, SD 4.2) were randomly assigned to either bed rest only (Ctrl), resistive flywheel exercise (FW), or to a group receiving 60 mg. i.v pamidronate prior to bed rest (Pam). Calf muscle cross sectional area and bone mineral content of the tibia was measured by peripheral quantitative computed tomography. Calcium, PTH and alkaline phosphatase blood levels were assessed along with urinary desoxypyridinoline excretion. Physical activity was assessed by the Freiburg questionnaire. In Pam and FW, diaphyseal bone losses were completely recovered at a 180-day follow-up, and there was even a small surplus after 1 year (p=0.016). Epiphyseal bone losses were largely, although not completely recovered after 1 year, when they still amounted to -0.6% (SD 1.3%, p=0.034, averaged over all groups). Bone formation and resorption markers had returned to baseline values at this time. However, epiphyseal recovery may still have been on-going, and fitting an exponential model yielded full recovery of the epiphysis within 2 years. Importantly, recovery of calf muscle cross-section and resumption of impact sport activities seemed to precede bone recovery, and bone accrual was closely matching the prior losses on an individual basis. No relationship was found between the epiphyseal BMC deficit at one-year follow-up and the participants' age. Results demonstrate recovery of bed rest induced bone losses in healthy adults. The initial re-accrual rate was remarkably high and is comparable to the accrual of bone mass during the pubertal growth spurt. This and the fact that the recovery of bone appeared to be tightly regulated, and generally followed neuromuscular recovery underline the adult skeleton's capability to adapt to mechanical stimuli.

The influence of severe prolonged exercise restriction on the mechanical and structural properties of bone in an avian model

Many studies have described the effects of exercise restriction on the mammalian skeleton. In particular, human and animal models have shown that reduction in weight bearing leads to generalised bone loss and deterioration of its mechanical properties. The aim of this study was to assess the effect of prolonged exercise restriction coupled with heavy calcium demands on the micro-structural, compositional and mechanical properties of the avian skeleton. The tibiae and humeri of 2-year-old laying hens housed in conventional caging (CC) and free-range (FR) housing systems were compared by mechanical testing and micro-computed tomography (microCT) scanning. Analyses of cortical, cancellous and medullary bone were performed. Mechanical testing revealed that the tibiae and humeri of birds from the FR group had superior mechanical properties relative to those of the CC group, and microCT scanning indicated larger cortical and lower medullary regions in FR group bones. Cancellous bone analysis revealed higher trabecular thickness and a higher bone volume fraction in the FR group, but no difference in mineral density. The biomechanical superiority of bones from the FR group was primarily due to structural rather than compositional differences, and this was reflected in both the cortical and cancellous components of the bones. The study demonstrated that prolonged exercise restriction in laying hens resulted in major structural and mechanical effects on the bird skeleton.

The effect of exercise and estrogen on osteoprotegerin in premenopausal women

BACKGROUND

The benefits of exercise are widely recognized, however physically active women can develop exercise associated menstrual cycle disturbances such as amenorrhea (i.e., estrogen deficiency) secondary to a chronic energy deficiency.

OBJECTIVE

To assess the effects of exercise status and estrogen deficiency on osteoprotegerin (OPG) and its relationship to bone resorption in premenopausal exercising women.

DESIGN

Cross-sectional study of serum OPG, urinary c-telopeptides (uCTX), urinary estrone 3-glucuronide (E1G), urinary pregnanediol 3-glucuronide (PdG) and bone mineral density (BMD) measured on multiple occasions in 67 women. Volunteers were retrospectively grouped: 1) sedentary menstruating group (SedMen n=8), 2) exercising menstruating group (ExMen, n=36), and 3) exercising amenorrheic group (ExAmen, n=23). One-way ANOVAs were performed, and LSD post-hoc tests were performed when differences were detected.

RESULTS

Subjects were similar with respect to age (24.2+/-1.0 years), weight (57.8+/-1.7 kg), and height (164.3+/-1.3 cm) (p>0.05). ExMen and ExAmen groups were more aerobically fit (p=0.003) and had less body fat (p=0.002) than the SedMen group. Resting energy expenditure/fat free mass was lowest (p=0.001) in the ExAmen groups. Mean E1G across the measurement period (p<0.001) and overall E1G exposure as assessed by E1G area under the curve (AUC) (p<0.001) were lower in the ExAmen group vs. the SedMen and ExMen groups. U-CTX-I was elevated (p=0.033) in the ExAmen group (281.8+/-40.3 microg/L/mmCr), compared with the SedMen and ExMen groups (184.5+/-22.4, 197.2+/-14.7 microg/L/mmCr, respectively). OPG was suppressed (p=0.005) in the ExAmen group (4.6+/-0.2 pmol/L) vs. ExMen group (5.2+/-0.2 pmol/L), and OPG was lower in the SedMen group (4.1+/-0.3 pmol/L) compared with the ExMen group. Findings were translated to BMD; the ExAmen group had suppressed total body BMD (p=0.014) and L2-L4 BMD (p=0.015) vs. the ExMen group.

CONCLUSIONS

Our results suggest that OPG responds to the bone loading effect of exercise, and that suppressed OPG may play a role in the etiology of increased bone resorption observed in exercising women with chronic estrogen deficiency secondary to hypothalamic amenorrhea.

Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength

Reduced skeletal loading typically leads to bone loss because bone formation and bone resorption become unbalanced. Hibernation is a natural model of musculoskeletal disuse because hibernating animals greatly reduce weight-bearing activity, and therefore, they would be expected to lose bone. Some evidence suggests that small mammals like ground squirrels, bats, and hamsters do lose bone during hibernation, but the mechanism of bone loss is unclear. In contrast, hibernating bears maintain balanced bone remodeling and preserve bone structure and strength. Differences in the skeletal responses of bears and smaller mammals to hibernation may be due to differences in their hibernation patterns; smaller mammals may excrete calcium liberated from bone during periodic arousals throughout hibernation, leading to progressive bone loss over time, whereas bears may have evolved more sophisticated physiological processes to recycle calcium, prevent hypercalcemia, and maintain bone integrity. Investigating the roles of neural and hormonal control of bear bone metabolism could give valuable insight into translating the mechanisms that prevent disuse-induced bone loss in bears into novel therapies for treating osteoporosis.

Bone steady-state is established at reduced bone strength after spinal cord injury: a longitudinal study using peripheral quantitative computed tomography (pQCT)

Spinal cord injury (SCI) is associated with a marked and rapid sublesional bone loss. So far, reports about the time course of adaptive changes in bone mass and structure in people with chronic and complete SCI are conflicting. Both, a continuous decline of bone parameters throughout the chronic phase of immobilisation as well as stabilisation of bone status on a low level have been documented. In our recently published cross-sectional study we suggested that subjects with a complete SCI reach a new bone steady-state in the paralysed limbs after extensive bone loss was complete. In addition, we described a time loss curve for each measured bone mineral density and geometry parameter and calculated its individual time to reach steady-state (tsteady-state). The aim of the present study was to test the findings of our cross-sectional study in a longitudinal design. Thirty-nine male subjects of the original cross-sectional study with complete SCI and paralysis duration between 0.9 and 34 years were included. Two follow-up pQCT measurements at 15 and 30 months after baseline measurement were performed at the distal epiphyses and mid shafts of the femur, tibia and radius. From the epiphyseal scans, bone mass, trabecular and total BMD were calculated. From the shaft scans, bone mass and cortical BMD, total and cortical cross-sectional areas and cortical thickness were determined. Repeated measures ANOVAs were performed with bone data at baseline, after 15 months and 30 months. Analyses were performed including only subjects with a lesion duration > or =t(steady-state) for each particular bone parameter. Bone parameters of tibial and femoral epi- and diaphyses were found to show no statistically significant differences between the three time points. Relative changes in bone parameters were small and ranged from -1.72% to +0.51% in the femur and from -1.67% to +0.42% in the tibia within 30 months of monitoring. Our data confirm the temporal limitation of the bone loss after complete SCI with stabilisation of BMD and geometric properties on a lower level-a finding of clinical importance considering the treatment strategies of bone loss after SCI with respect to lesion duration.

BMD and bone geometry in transtibial and transfemoral amputees

Prolonged unloading of bone(s) in the residual limb after amputation may cause significant bone loss in the hip and distal bony end of the residual limb. The purpose of this study was to examine the effect of amputation on bone geometry, volumetric BMD (vBMD), and areal BMD (aBMD) by comparing the intact and residual limbs in unilateral transfemoral and transtibial amputees. Amputees (seven above-knee; seven below-knee) and two groups of nonamputee control subjects gave informed consent to participate in this study. aBMD of the dual proximal femur, lumbar spine, and total body was assessed using DXA. Bone geometry and vBMD were assessed at the distal ends of the residual limb and intact limb and at a comparable cross-sectional slice of the intact limb using pQCT (Stratec XCT 3000). There were no significant group differences in age, height, weight, physical activity, time as an amputee, hours wearing a prosthesis per day, or total body and lumbar spine BMD and BMC. There were significant side x group interactions for total hip, femoral neck, and trochanter BMD, with the amputated side having lower BMD, and differences being most severe in above-knee amputees. Total and cortical vBMD and area were significantly lower at the end of the residual limb compared with the similar slice of the intact limb for both above- and below-knee amputees. In conclusion, amputees exhibited large decrements in BMD, both at the hip and at the end of the residual limb, compared with the intact side. These lower BMD values put amputees, particularly the above-knee amputees, at increased risk for osteoporosis and fragility fractures in the hip.

High-volume FES-cycling partially reverses bone loss in people with chronic spinal cord injury

Spinal cord injury (SCI) leads to severe bone loss in the paralysed limbs and to a resulting increased fracture risk thereof. Since long bone fractures can lead to comorbidities and a reduction in quality of life, it is important to improve bone strength in people with chronic SCI. In this prospective longitudinal cohort study, we investigated whether functional electrical stimulation (FES) induced high-volume cycle training can partially reverse the loss of bone substance in the legs after chronic complete SCI. Eleven participants with motor-sensory complete SCI (mean age 41.9+/-7.5 years; 11.0+/-7.1 years post injury) were recruited. After an initial phase of 14+/-7 weeks of FES muscle conditioning, participants performed on average 3.7+/-0.6 FES-cycling sessions per week, of 58+/-5 min each, over 12 months at each individual's highest power output. Bone and muscle parameters were investigated in the legs by means of peripheral quantitative computed tomography before the muscle conditioning (t1), and after six (t2) and 12 months (t3) of high-volume FES-cycle training. After 12 months of FES-cycling, trabecular and total bone mineral density (BMD) as well as total cross-sectional area in the distal femoral epiphysis increased significantly by 14.4+/-21.1%, 7.0+/-10.8% and 1.2+/-1.5%, respectively. Bone parameters in the femoral shaft showed small but significant decreases, with a reduction of 0.4+/-0.4% in cortical BMD, 1.8+/-3.0% in bone mineral content, and 1.5+/-2.1% in cortical thickness. These decreases mainly occurred between t1 and t2. No significant changes were found in any of the measured bone parameters in the tibia. Muscle CSA at the thigh increased significantly by 35.5+/-18.3%, while fat CSA at the shank decreased by 16.7+/-12.3%. Our results indicate that high-volume FES-cycle training leads to site-specific skeletal changes in the paralysed limbs, with an increase in bone parameters at the actively loaded distal femur but not the passively loaded tibia. Thus, we conclude that high-volume FES-induced cycle training has clinical relevance as it can partially reverse bone loss and thus may reduce fracture risk at this fracture prone site.

Doublet stimulation protocol to minimize musculoskeletal stress during paralyzed quadriceps muscle testing

With long-term electrical stimulation training, paralyzed muscle can serve as an effective load delivery agent for the skeletal system. Muscle adaptations to training, however, will almost certainly outstrip bone adaptations, exposing participants in training protocols to an elevated risk for fracture. Assessing the physiological properties of the chronically paralyzed quadriceps may transmit unacceptably high shear forces to the osteoporotic distal femur. We devised a two-pulse doublet strategy to measure quadriceps physiological properties while minimizing the peak muscle force. The purposes of the study were 1) to determine the repeatability of the doublet stimulation protocol, and 2) to compare this protocol among individuals with and without spinal cord injury (SCI). Eight individuals with SCI and four individuals without SCI underwent testing. The doublet force-frequency relationship shifted to the left after SCI, likely reflecting enhancements in the twitch-to-tetanus ratio known to exist in paralyzed muscle. Posttetanic potentiation occurred to a greater degree in subjects with SCI (20%) than in non-SCI subjects (7%). Potentiation of contractile rate occurred in both subject groups (14% and 23% for SCI and non-SCI, respectively). Normalized contractile speed (rate of force rise, rate of force fall) reflected well-known adaptations of paralyzed muscle toward a fast fatigable muscle. The doublet stimulation strategy provided repeatable and sensitive measurements of muscle force and speed properties that revealed meaningful differences between subjects with and without SCI. Doublet stimulation may offer a unique way to test muscle physiological parameters of the quadriceps in subjects with uncertain musculoskeletal integrity.

Dose estimation and surveillance of mechanical loading interventions for bone loss after spinal cord injury

BACKGROUND AND PURPOSE

The interpretation of the results of previous anti-osteoporosis interventions after spinal cord injury (SCI) is undermined by incomplete information about the intervention dose or patient adherence to dose requirements. Rehabilitation research as a whole traditionally has struggled with these same issues. The purpose of this case report is to offer proof of the concepts that careful dose selection and surveillance of patient adherence should be integral components in rehabilitation interventions.

CASE DESCRIPTION

A 21-year-old man with T4 complete paraplegia (7 weeks) enrolled in a unilateral soleus muscle electrical stimulation protocol. Compressive loads applied to the tibia approximated 1.4 times body weight. Over 4.8 years of home-based training, data logging software provided surveillance of adherence. Soleus muscle torque and fatigue index adaptations to training as well as bone mineral density (BMD) adaptations in the distal tibia were measured.

OUTCOMES

The patient performed nearly 8,000 soleus muscle contractions per month, with occasional fluctuations. Adherence tracking permitted intervention when adherence fell below acceptable values. The soleus muscle torque and fatigue index increased rapidly in response to training. The BMD of the untrained tibia declined approximately 14% per year. The BMD of the trained tibia declined only approximately 7% per year. The BMD was preferentially preserved in the posterior half of the tibia; this region experienced only a 2.6% annual decline.

DISCUSSION

Early administration of a load intervention, careful estimation of the loading dose, and detailed surveillance of patient adherence aided in the interpretation of a patient's adaptations to a mechanical load protocol. These concepts possess wider applicability to rehabilitation research and should be emphasized in future physical therapy investigations.

Similar effects of long-term exogenous growth hormone (GH) on bone and muscle parameters: a pQCT study of GH-deficient and small-for-gestational-age (SGA) children

BACKGROUND AND AIMS

Treatment with GH in short children has focused on height development. Little is known about the concomitant changes in muscle mass, bone structure and bone strength.

METHODS

Muscle area as well as parameters of bone architecture (bone mineral content, BMC; volumetric cortical density, total bone area, TBA; cortical area, cortical thickness, CT; and marrow area) were measured by means of pQCT (Stratec) at 65% of the proximal length of the forearm. The strength-strain index (SSI) was calculated as an indicator of bone strength.

RESULTS

Prepubertal children with GHD (mean values: age; 7.2 years; height SDS=-2.9 SDS; GH dose: 30 microg/kg/d) were followed at 0, 6, 12 (n=74) and 24 (n=55) months. Prepubertal children with SGA (mean values: age: 7.1 years; height SDS=-3.4 SDS; GH dose: 55 mug/kg/d) were followed at 0, 6, 12 (n=47) and 24 (n=35) months. Both groups showed a similar increase in height. At GH start, muscle mass and bone characteristics were lower than normal but similar in SGA vs. GHD. Muscle area (mean values, SDS) increased from -3.0 to -1.5 in SGA and from -2.4 to -1.0 in GHD. Bone geometry changed in a biphasic mode, with an increase in total bone area and lowering of bone mineral content (BMC) during the first 12 months, followed by an increase of BMC and CT thereafter. SSI (mean values, mm(3)) improved from 78 to 114 in GHD and from 62 to 101 in SGA after 24 months on GH. The increment in terms of SDS did not reach significance in SGA. SSI correlated positively with muscle area before and during GH treatment.

CONCLUSIONS

Bone strength and muscle mass are impaired in prepubertal children with GHD and SGA. Exogenous GH can indirectly improve bone structure and strength by inducing an increase in muscle mass. Our findings support the assumption that, in SGA, there is impaired tissue responsiveness to GH.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Anemia of immobility: caused by adipocyte accumulation in bone marrow

Anemia of chronic disease has long been used to classify a non-regenerative, low-grade, chronic, normocytic, normochromic anemia that presents with no obvious etiology. Within this group, some patients have a chronic inflammatory condition that limits erythrocyte generation or access to iron stores. This specific type of anemia has been termed anemia of chronic inflammation. However, a substantial remainder of patients diagnosed with anemia of chronic disease present with no active inflammation. These include many clinical populations with reduced limb loading, such as spinal cord injured patients, astronauts, elderly people with limited mobility and experimental bed-rest subjects. In some populations with decreased mobility, accumulation of fat in the bone marrow has been demonstrated. We hypothesize that adipocyte accumulation in bone marrow both passively and actively impairs erythropoiesis and thus defines a new type of anemia called anemia of immobility. The non-specific umbrella term anemia of chronic disease thus becomes obsolete in favour of either the diagnosis of anemia of immobility or anemia of chronic inflammation according to the distinct mechanism involved.

Alendronate prevents bone loss in patients with acute spinal cord injury: a randomized, double-blind, placebo-controlled study

CONTEXT

Patients who sustain an acute spinal cord injury (SCI) experience rapid dramatic reductions in bone mineral density (BMD), especially marked in sublesional areas and sometimes leading to hypercalcemia and hypercalciuria, as well as increased fracture risk.

OBJECTIVE

In this prospective, double-blind, randomized, placebo-controlled study, we evaluated the hypothesis that oral alendronate administration would preserve BMD when administered soon after acute SCI.

PATIENTS AND INTERVENTION

Thirty-one patients with acute SCI were randomly allocated to receive oral alendronate 70 mg/wk or placebo, within 10 d of acute SCI, for 12 months.

MAIN OUTCOME MEASUREMENTS

At entry and at 3, 6, 12, and 18 months, total body bone density, lumbar and hip BMD, ultrasound of the calcaneus, 24-h urinary calcium, and serum C-telopeptide (betaCTX) were measured.

RESULTS

At study entry, patients in the two groups were well matched for age, gender, severity of neurological deficit, BMD, urinary calcium, and betaCTX. BMD indices declined steadily in the placebo group, and this effect was attenuated significantly by alendronate. After 12 months, there was a 5.3% difference (P<0.001) in total body BMD and a 17.6% difference (P<0.001) in the total hip BMD between the two groups. Alendronate compared with placebo induced significant (P<0.001) reductions in urinary calcium excretion and serum betaCTX. No treatment-related side effects were noted.

CONCLUSIONS

We conclude that alendronate therapy, 70 mg/wk, initiated soon after acute SCI, prevents bone loss and is not associated with side effects.

Reduced loading due to spinal-cord injury at birth results in "slender" bones: a case study

INTRODUCTION

The present case study compared bone density, bone geometry and muscle cross-sectional area (CSA) in a male who sustained spinal-cord injury (SCI) at birth (from here called SCI-B) with two matched controls without SCI, and also with four individuals with SCI of similar level and injury completeness but sustained at age 15 or greater.

METHODS

All subjects with SCI were at least 3 years post-injury and had experienced motor incomplete lesions at the cervical level. Computed tomography was used to measure volumetric bone density, indices of bone strength [CSA and maximum, minimum and polar area moments of inertia (I (max), I (min), I (pol))] and muscle CSA at the tibia (66% of tibia length, measured proximally from the distal end).

RESULTS

Lower leg muscle CSA of SCI-B was 63+/-6% of values in non-SCI controls, and 72+/-12% of values in other males with SCI. In SCI-B, bone CSA was roughly half (52+/-4%) that of non-SCI controls and 73+/-16% of bone CSA values in other males with SCI. The magnitudes of the area moment of inertia variables (I (max), I (min), and I (pol)) in SCI-B were approximately 25% of control values. Further, the moment of inertia variables in SCI-B were 27-54% of values obtained in other males with SCI, indicating that experiencing SCI in the early stages of life has a remarkable impact on bone shape. Interestingly, tibia bone density did not appear to be affected; the average difference in bone density between SCI-B and non-SCI controls was -1.2+/-0.7%. The bone densities of other males with SCI were 4-19% lower than in SCI-B.

CONCLUSIONS

Muscle atrophy and bone loss are commonly reported consequences of SCI. This case reveals that important changes in bone geometry occur after SCI, and that mechanical loading during growth plays a vital role in the development of bone size and shape.

Peripheral quantitative computed tomography: measurement sensitivity in persons with and without spinal cord injury

OBJECTIVES

To determine (1) the error attributable to external tibia-length measurements by using peripheral quantitative computed tomography (pQCT) and (2) the effect these errors have on scan location and tibia trabecular bone mineral density (BMD) after spinal cord injury (SCI).

DESIGN

Blinded comparison and criterion standard in matched cohorts.

SETTING

Primary care university hospital.

PARTICIPANTS

Eight able-bodied subjects underwent tibia length measurement. A separate cohort of 7 men with SCI and 7 able-bodied age-matched male controls underwent pQCT analysis.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

The projected worst-case tibia-length-measurement error translated into a pQCT slice placement error of +/-3 mm. We collected pQCT slices at the distal 4% tibia site, 3 mm proximal and 3 mm distal to that site, and then quantified BMD error attributable to slice placement.

RESULTS

Absolute BMD error was greater for able-bodied than for SCI subjects (5.87 mg/cm(3) vs 4.5 mg/cm(3)). However, the percentage error in BMD was larger for SCI than able-bodied subjects (4.56% vs 2.23%).

CONCLUSIONS

During cross-sectional studies of various populations, BMD differences up to 5% may be attributable to variation in limb-length-measurement error.

Bone loss from the human distal tibia epiphysis during 24 days of unilateral lower limb suspension

Bone loss during immobilization is well documented. Currently, the only means of studying this in human beings is bed rest, which is resource intensive and inconvenient for the subjects. Unilateral lower limb suspension (ULLS) has been suggested as an alternative, but has not previously been demonstrated to cause bone loss. The main aim of our study was to test the hypothesis that ULLS would cause bone loss determined by peripheral quantitative computed tomography (pQCT). We investigated eight young healthy volunteers (19.1 +/- 0.7 years; body mass index, 22.4 +/- 2.6 kg m(-2)), who underwent ULLS for 24 days; their right foot was suspended with a strap attached to the shoulder so the knee angle was 10 deg and they wore a left shoe with a 7.5 cm sole to allow clearance of the right foot and used bilateral crutches to perambulate. Bone scans were obtained by pQCT from the distal epiphyses and from the diaphyses of the tibia in each leg twice before suspension, at days 7, 14 and 21 of the ULLS, and at days 4, 9, 35 and 90 of recovery. After 21 days of ULLS, bone mineral content of the peripheral portion of the epiphysis of the suspended tibia was reduced by 0.89 +/- 0.48% (from 280.9 +/- 34.5 to 278.4 +/- 34.2 mg mm(-1), P < 0.001) but no changes were observed in its central portion or in the unsuspended tibia. In the peripheral epiphyseal portion, significant bone loss (by 0.32 +/- 0.54%, P = 0.045) occurred as early as day 7 of ULLS. We have demonstrated, for the first time, that in humans bone is lost during ULLS at rates comparable to those seen with bed rest, without alteration in limb fluid volumes thus validating the technique and raising important questions about the mechanisms involved.

Bone loss in spinal cord-injured patients: from physiopathology to therapy

STUDY DESIGN

Review article on bone metabolism and therapeutic approach on bone loss in patients with spinal cord injury (SCI).

OBJECTIVE

The first part aims to describe the process of bone demineralization and its effects on bone mass in patients with SCI. The second part describes and discusses the therapeutic approaches to limiting the alteration in bone metabolism related to neurological lesions.

SETTING

Propara Rehabilitation Center, Montpellier, France.

RESULTS

During the first 24 months postinjury, demineralization occurs exclusively in the sublesional areas and predominantly in weight-bearing skeletal sites such as the distal femur and proximal tibia, both of which are trabecular-rich sites. Reduced bone mass, in association with a modified bone matrix property and composition, is very likely at the origin of pathological fractures after minor trauma to which these patients are frequently exposed. Since these fractures may be asymptomatic yet may lead to complications, preventing and managing 'neurological osteoporosis' remains a considerable challenge. Two main approaches are considered: the first consists in applying a mechanical stimulus to the bone tissue by standing, orthotically aided walking or functional electrical stimulation (FES). The second uses medications, particularly antiresorptive drugs such as calcitonin or diphosphonates.

CONCLUSION

To develop well-adapted treatments, a more precise understanding of bone loss etiology is needed. The current rehabilitation programs are based on the idea that the bone physiological changes observed in patients with SCI are due to immobility, but results indicate that alterations inherent to neurological damage may play an even greater role in inducing osteoporosis.

Physiological effects of lower extremity functional electrical stimulation in early spinal cord injury: lack of efficacy to prevent bone loss

STUDY DESIGN

Controlled, repeat-measures study.

OBJECTIVES

To determine if functional electrical stimulation (FES) can affect bone atrophy in early spinal cord injury (SCI), and the safety, tolerance and feasibility of this modality in bone loss remediation.

SETTING

Spinal Injuries Units, Royal Adelaide Hospital and Hampstead Rehabilitation Centre, South Australia.

METHODS

Patients with acute SCI (ASIA A-D) were allocated to FES (n=23, 28+/-9 years, C4-T10, 13 Tetra) and control groups (CON, n=10, 31+/-11 years, C5-T12, four Tetra). The intervention group received discontinuous FES to lower limb muscles (15 min sessions to each leg twice daily, over a 5-day week, for 5 months). Dual energy X-ray absorptiometry (DEXA) measured total body bone mineral density (tbBMD), hip, spine BMD and fat mass (FM) within 3 weeks, and 3 and 6 months postinjury.

RESULTS

FES and CON groups' tbBMD differed significantly at 3 months postinjury (P<0.01), but not thereafter. Other DEXA measures (hip, spine BMD, FM) did not differ between groups at any time. No adverse events were identified.

CONCLUSION

Electrically stimulated muscle activation was elicited, and tetanic effects were reproducible; however, there were no convincing trends to suggest that FES can play a clinically relevant role in osteoporosis prevention (or subsequent fracture risk) in the recently injured patient. The lack of an osteogenic response in paralysed extremities to electrically evoked exercise during subacute and rehabilitation/recovery phases cannot be fully explained, and may warrant further evaluation.

Musculoskeletal plasticity after acute spinal cord injury: effects of long-term neuromuscular electrical stimulation training

Maintaining the physiologic integrity of paralyzed limbs may be critical for those with spinal cord injury (SCI) to be viable candidates for a future cure. No long-term intervention has been tested to attempt to prevent the severe musculoskeletal deterioration that occurs after SCI. The purposes of this study were to determine whether a long-term neuromuscular electrical stimulation training program can preserve the physiological properties of the plantar flexor muscles (peak torque, fatigue index, torque-time integral, and contractile speed) as well as influence distal tibia trabecular bone mineral density (BMD). Subjects began unilateral plantar flexion electrical stimulation training within 6 wk after SCI while the untrained leg served as a control. Mean compliance for the 2-yr training program was 83%. Mean estimated compressive loads delivered to the tibia were approximately 1-1.5 times body weight. The training protocol yielded significant trained versus untrained limb differences for torque (+24%), torque-time integral (+27%), fatigue index (+50%), torque rise time (+45%), and between-twitch fusion (+15%). These between-limb differences were even greater when measured at the end of a repetitive stimulation protocol (125 contractions). Peripheral quantitative computed tomography revealed 31% higher distal tibia trabecular BMD in trained limbs than in untrained limbs. The intervention used in this study was sufficient to limit many of the deleterious muscular and skeletal adaptations that normally occur after SCI. Importantly, this method of load delivery was feasible and may serve as the basis for an intervention to preserve the musculoskeletal properties of individuals with SCI.

Bone loss and muscle atrophy in spinal cord injury: epidemiology, fracture prediction, and rehabilitation strategies

Individuals with spinal cord injury (SCI) often experience bone loss and muscle atrophy. Muscle atrophy can result in reduced metabolic rate and increase the risk of metabolic disorders. Sublesional osteoporosis predisposes individuals with SCI to an increased risk of low-trauma fracture. Fractures in people with SCI have been reported during transfers from bed to chair, and while being turned in bed. The bone loss and muscle atrophy that occur after SCI are substantial and may be influenced by factors such as completeness of injury or time postinjury. A number of interventions, including standing, electrically stimulated cycling or resistance training, and walking exercises have been explored with the aim of reducing bone loss and/or increasing bone mass and muscle mass in individuals with SCI. Exercise with electrical stimulation appears to increase muscle mass and/or prevent atrophy, but studies investigating its effect on bone are conflicting. Several methodological limitations in exercise studies with individuals with SCI to date limit our ability to confirm the utility of exercise for improving skeletal status. The impact of standing or walking exercises on muscle and bone has not been well established. Future research should carefully consider the study design, skeletal measurement sites, and the measurement techniques used in order to facilitate sound conclusions.