A paradigm for skeletal strength homeostasis

Additively manufactured porous scaffolds by design for treatment of bone defects

There has been increasing attention to produce porous scaffolds that mimic human bone properties for enhancement of tissue ingrowth, regeneration, and integration. Additive manufacturing (AM) technologies, i.e., three dimensional (3D) printing, have played a substantial role in engineering porous scaffolds for clinical applications owing to their high level of design and fabrication flexibility. To this end, this review article attempts to provide a detailed overview on the main design considerations of porous scaffolds such as permeability, adhesion, vascularisation, and interfacial features and their interplay to affect bone regeneration and osseointegration. Physiology of bone regeneration was initially explained that was followed by analysing the impacts of porosity, pore size, permeability and surface chemistry of porous scaffolds on bone regeneration in defects. Importantly, major 3D printing methods employed for fabrication of porous bone substitutes were also discussed. Advancements of MA technologies have allowed for the production of bone scaffolds with complex geometries in polymers, composites and metals with well-tailored architectural, mechanical, and mass transport features. In this way, a particular attention was devoted to reviewing 3D printed scaffolds with triply periodic minimal surface (TPMS) geometries that mimic the hierarchical structure of human bones. In overall, this review enlighten a design pathway to produce patient-specific 3D-printed bone substitutions with high regeneration and osseointegration capacity for repairing large bone defects.

Structural and Material Determinants Influencing the Behavior of Porous Ti and Its Alloys Made by Additive Manufacturing Techniques for Biomedical Applications

One of the biggest challenges in tissue engineering is the manufacturing of porous structures that are customized in size and shape and that mimic natural bone structure. Additive manufacturing is known as a sufficient method to produce 3D porous structures used as bone substitutes in large segmental bone defects. The literature indicates that the mechanical and biological properties of scaffolds highly depend on geometrical features of structure (pore size, pore shape, porosity), surface morphology, and chemistry. The objective of this review is to present the latest advances and trends in the development of titanium scaffolds concerning the relationships between applied materials, manufacturing methods, and interior architecture determined by porosity, pore shape, and size, and the mechanical, biological, chemical, and physical properties. Such a review is assumed to show the real achievements and, on the other side, shortages in so far research.

The influence of whole-body vibration on heart rate, stride length, and bone mineral content in the mature exercising horse

Research in humans suggests whole-body vibration (WBV) aids in maintaining bone mineral content (BMC) yet results in the horse are less favourable. Anecdotally, WBV is reported to reduce pain and improve performance. This study was designed to test the effect of WBV on exercising horses, hypothesising that WBV would lower heart rate (HR) during treatment, increase BMC, modify markers of bone metabolism, and increase stride length. Eleven horses were randomly assigned into control (CON, n=5) or WBV (VIB, n=6) groups for a 28-day treatment period. Both groups exercised for 1 h, 6 d/wk on a mechanical exerciser. VIB horses received 50 Hz WBV for 45 min, 5 days/wk. Third metacarpal radiographs were taken at 0 and 28 days, and BMC determined via radiographic bone aluminium equivalence (RBAE). Blood samples taken at day 0 and 28 were analysed for serum pyridinoline cross-links (PYD) and osteocalcin (OC). Heart rate was analysed on day 23 for 4 horses per group. Stride length was determined while trotting in hand on day 0 and 28. No influence of WBV on RBAE of any bone cortices, PYD or OC was observed (P>0.10); stride length was also unaffected (P=0.88). A period effect was observed for a decrease in RBAE of the lateral cortex (P=0.01), and a trend towards a decrease was noted in total density (P=0.05), likely an effect of stalling. Compared to baseline, ΔHR declined during treatment (P=0.06) in VIB (-4.8±2.8 bpm) compared to control CON (3.0±2.8 bpm). The results suggest, in normal exercising horses, WBV does not increase BMC, influence markers of bone metabolism, or increase stride length.

An immunofluorescence study on VEGF and extracellular matrix proteins in human periodontal ligament during tooth movement

The periodontal ligament (PDL) is a highly vascularized connective tissue surrounding the root of a tooth. In particular, the PDL is continuously exposed to mechanical stresses during the phases of mastication, and it provides physical, sensory, and trophic functions. It is known that the application of orthodontic force creates a change in periodontal structures. In fact, these forces generate a pressure on the ligament that closes the vessels. The aim of this study is to observe the modifications of vascular endothelial growth factor (VEGF) in the PDL and extracellular matrix proteins after application of a pre-calibrated and constant orthodontic force at different phases of treatment. We used a 50-g NiTi coiled spring and in vivo samples of PDL of maxillary and mandibular premolars of patients subjected to orthodontic treatment. These teeth were extracted at 1, 7, 14, 21, and 30 days, respectively, by application of force. The extraction of the PDL was effectuated by scarifying the radicular surface on the pressure and tension sides. The mechanical stress induced by the application of force caused an increase in the reactive type of metabolism of extracellular matrix proteins and modulation of neoangiogenesis until restoration.

Bone grafts and biomaterials substitutes for bone defect repair: A review

Bone grafts have been predominated used to treat bone defects, delayed union or non-union, and spinal fusion in orthopaedic clinically for a period of time, despite the emergency of synthetic bone graft substitutes. Nevertheless, the integration of allogeneic grafts and synthetic substitutes with host bone was found jeopardized in long-term follow-up studies. Hence, the enhancement of osteointegration of these grafts and substitutes with host bone is considerably important. To address this problem, addition of various growth factors, such as bone morphogenetic proteins (BMPs), parathyroid hormone (PTH) and platelet rich plasma (PRP), into structural allografts and synthetic substitutes have been considered. Although clinical applications of these factors have exhibited good bone formation, their further application was limited due to high cost and potential adverse side effects. Alternatively, bioinorganic ions such as magnesium, strontium and zinc are considered as alternative of osteogenic biological factors. Hence, this paper aims to review the currently available bone grafts and bone substitutes as well as the biological and bio-inorganic factors for the treatments of bone defect.

The role of osteocytes during experimental orthodontic tooth movement: A review

OBJECTIVE

To explore the types of orthodontic force-induced mechanical stimuli that regulate osteocyte function.

DESIGN

In orthodontics, a tooth can be moved through the alveolar bone when an appropriate orthodontic force is applied. These mechanical loads stimulate cells within the bone tissue around the tooth. These cellular responses lead to bone resorption on the side of the tooth where the pressure has been applied and bone deposition on the side of the tooth experiencing tension. Recently, osteocytes were identified to function as mechano-sensory cells in bone tissue that direct bone resorption and bone formation. Based on recent literature, the proposed function of osteocytes during orthodontic tooth movement is explored with better understanding.

RESULTS

Several stimuli regulating osteocyte function have been highlighted, and their potential roles in events initiating osteocyte sensing of orthodontic force have been explored in detail. The most popular hypotheses for osteocyte response include stress-induced bone matrix deformation/microcrack formation and fluid-flow shear stress.

CONCLUSIONS

Understanding osteocyte function under mechanical stress may have profound implications in future orthodontic treatments.

Mechanical and metabolic interactions in cortical bone development

OBJECTIVES

Anthropological studies of cortical bone often aim to reconstruct either habitual activities or health of past populations. During development, mechanical loading and metabolism simultaneously shape cortical bone structure; yet, few studies have investigated how these factors interact. Understanding their relative morphological effects is essential for assessing human behavior from skeletal samples, as previous studies have suggested that interaction effects may influence the interpretation from cortical structure of physical activity or metabolic status.

MATERIAL AND METHODS

This study assesses cross-sectional geometric and histomorphometric features in bones under different loading regimes (femur, humerus, rib) and compares these properties among individuals under different degrees of metabolic stress. The study sample consists of immature humans from a late medieval Lithuanian cemetery (Alytus, 14th-18th centuries AD). Analyses are based on the hypothesis that metabolic bone loss is distributed within the skeleton in a way that optimizes mechanical competency.

RESULTS

Results suggest mechanical compensation for metabolic bone loss in the cross-sectional properties of all three bones (especially ribs), suggesting a mechanism for conserving adequate bone strength for different loads across the skeleton. Microscopic bone loss is restricted to stronger bones under high loads, which may mitigate fracture risk in areas of the skeleton that are more resistive to loading, although alternative explanations are examined.

DISCUSSION

Distributions of metabolic bone loss and subsequent structural adjustments appear to preserve strength. Nevertheless, both mechanics and metabolism have a detectable influence on morphology, and potential implications for behavioral interpretations in bioculturally stressed samples due to this interaction are explored. Am J Phys Anthropol 160:317-333, 2016. © 2016 Wiley Periodicals, Inc.

Role of Osteocyte-derived Insulin-Like Growth Factor I in Developmental Growth, Modeling, Remodeling, and Regeneration of the Bone

The osteocyte has long been considered to be the primary mechanosensory cell in the bone. Recent evidence has emerged that the osteocyte is also a key regulator of various bone and mineral metabolism and that its regulatory effects are in part mediated through locally produced osteocyte-derived factors, such as sclerostin, receptor activator of nuclear factor-kappa B ligand (RANKL), and fibroblast growth factor (FGF)-23. Osteocytes secrete large amounts of insulin-like growth factor (IGF)-I in bone. Although IGF-I produced locally by other bone cells, such as osteoblasts and chondrocytes, has been shown to play important regulatory roles in bone turnover and developmental bone growth, the functional role of osteocyte-derived IGF-I in the bone and mineral metabolism has not been investigated and remains unclear. However, results of recent studies in osteocyte Igf1 conditional knockout transgenic mice have suggested potential regulatory roles of osteocyte-derived IGF-I in various aspects of bone and mineral metabolism. In this review, evidence supporting a regulatory role for osteocyte-derived IGF-I in the osteogenic response to mechanical loading, the developmental bone growth, the bone response to dietary calcium depletion and repletion, and in fracture repair is discussed. A potential coordinated regulatory relationship between the effect of osteocyte-derived IGF-I on bone size and the internal organ size is also proposed.

Non-uniform decay in jumping exercise-induced bone gains following 12 and 24 weeks of cessation of exercise in rats

The effects of deconditioning on exercise-induced bone gains in rats were investigated in 12-week-old female WKY rats performing a standard jumping exercise regimen for either 8, 12 or 24 weeks, followed by sedentary periods of either 24, 12 or 0 weeks, respectively. Age-matched controls received no exercise over the same period. At the end of the training/sedentary period, the tibiae were harvested for analyses of bone parameters. Gains in tibial fat-free dry weight decayed within 12 weeks of deconditioning, but gains in tibial ultimate bending force (strength), maximum diameter and cortical area were still present at 12 weeks of deconditioning. With the exception of cortical area, all other exercise-induced bone gains decayed by the 24th week of deconditioning. It appears that the decay in exercise-induced bone gains in strength, physical and morphological properties is not uniform, and that gains in fat-free dry weight seem to decay earlier.

The distal radius, the most frequent fracture localization in humans: a histomorphometric analysis of the microarchitecture of 60 human distal radii and its changes in aging

BACKGROUND

The distal radius is the most frequent fracture localization in humans. Although younger patients receive a distal radius fracture after an adequate trauma, elderly patients suffer fractures through low-energy mechanisms. Low-energy fractures are hallmarks of osteoporosis. Osteoporotic changes of the distal radius are well described by DXA and peripheral quantitative computed tomography measurements. However, to date, the effects of aging on the microarchitecture of the distal radius have not been investigated.

METHODS

To investigate whether the microarchitecture of the human distal radius shows osteoporotic changes in bone mass and structure during aging, we dissected out 60 complete human distal radii from 30 age- and gender-matched patients at autopsy. Each of the three different age groups (group I: 20-40 years, group II: 41-60 years, group III: 61-80 years) was represented by 10 autopsy cases and 20 specimens (double-sided extraction), respectively. The specimens were analyzed by peripheral quantitative computed tomography, contact-radiography, and histomorphometry.

RESULTS

We observed a significant age-related decrease in bone mass, bone mineral density and an increase in typical osteoporotic changes of the bone microarchitecture in female distal radius specimens. Comparable observations of age-related changes have not been made in male specimens.

CONCLUSIONS

The distal radius is a location of osteoporosis-specific bone changes. Our data provide evidence for the occurrence of typical osteoporotic changes, especially postmenopausal osteoporotic changes, in the distal radius during aging.

Microarchitecture of the radial head and its changes in aging

Fractures of the radial head are common; however, it remains to be determined whether the radial head has to be considered as a typical location for fractures associated with osteoporosis. To investigate whether the human radial head shows structural changes during aging, we analyzed 30 left and 30 right human radial heads taken from 30 individuals. The specimens taken from the left side were analyzed by peripheral quantitative computed tomography (pQCT) and micro-CT. The specimens taken from the right elbow joint were analyzed by radiography and histomorphometry. In these specimens pQCT revealed a significant decrease of total and cortical bone mineral density (BMD(to) BMD(co)) with aging, regardless of sex. Histomorphometry revealed a significant reduction of cortical thickness (Ct.Th), bone volume per tissue volume (BV/TV), and trabecular thickness (Tb.Th) in male and female specimens. In this context, mean BV/TV and mean trabecular number (Tb.N) values were significantly lower and, accordingly, mean trabecular separation (Tb.Sp) was significantly higher in female samples. The presented study demonstrates that the radial head is a skeletal site where different age- and sex-related changes of the bone structure become manifest. These microarchitectural changes might contribute to the pathogenesis of radial head fractures, especially in aged female patients where trabecular parameters (BMD(tr) and Tb.Sp) change significantly for the worse compared to male patients.

Isokinetic training increases ulnar bending stiffness and bone mineral in young women

Numerous studies have investigated the effects of physical activity on bone health; however, little is known about the effects of isokinetic strength training on bone. While bone mineral density (BMD) is widely used to assess bone health and fracture risk, there are several limitations of this measure that warrant new technology development to measure bone strength. The mechanical response tissue analyzer (MRTA) assesses bone strength by measuring maximal bending stiffness (EI). We hypothesized that isokinetic strength training of the elbow flexors and extensors would increase ulnar EI, BMD, and bone mineral content (BMC) in young women. Fifty-four women trained the nondominant arm 3 times per week for 20 weeks; 32 trained concentrically (CON) and 22 trained eccentrically (ECC). Subjects were assessed for the following variables pre- and post-training: CON and ECC peak torque of the elbow flexors and extensors with isokinetic dynamometry, ulnar mineral content and density using dual-energy X-ray absorptiometry, and ulnar EI using MRTA. Isokinetic training increased CON (17%) and ECC (17%) peak torque, even when controlling for changes in the untrained arm. Eccentric training increased CON and ECC peak torque while CON training improved CON peak torque only. Isokinetic training increased ulnar EI 28%, which was statistically greater than the untrained arm. Ulnar EI increased 25% with CON training and 32% with ECC training. Both training modes resulted in greater EI gains compared to the untrained limb. Isokinetic training increased ulnar BMC (2.7%) and BMD (2.3%), even when controlling for untrained ulna changes. Both training modalities resulted in BMC and BMD increases; however, only CON training yielded gains when controlling for changes in the untrained limb. In conclusion, isokinetic strength training increases ulnar EI, BMC, and BMD in young women; no statistical differences were noted between CON and ECC training modes.

Osteocyte lacunae tissue strain in cortical bone

Current theories suggest that bone modeling and remodeling are controlled at the cellular level through signals mediated by osteocytes. However, the specific signals to which bone cells respond are still unknown. Two primary theories are: (1) osteocytes are stimulated via the mechanical deformation of the perilacunar bone matrix and (2) osteocytes are stimulated via fluid flow generated shear stresses acting on osteocyte cell processes within canaliculi. Recently, much focus has been placed on fluid flow theories since in vitro experiments have shown that bone cells are more responsive to analytically estimated levels of fluid shear stress than to direct mechanical stretching using macroscopic strain levels measured on bone in vivo. However, due to the complex microstructural organization of bone, local perilacunar bone tissue strains potentially acting on osteocytes cannot be reliably estimated from macroscopic bone strain measurements. Thus, the objective of this study was to quantify local perilacunar bone matrix strains due to macroscopically applied bone strains similar in magnitude to those that occur in vivo. Using a digital image correlation strain measurement technique, experimentally measured bone matrix strains around osteocyte lacunae resulting from macroscopic strains of approximately 2000 microstrain are significantly greater than macroscopic strain on average and can reach peak levels of over 30,000 microstrain locally. Average strain concentration factors ranged from 1.1 to 3.8, which is consistent with analytical and numerical estimates. This information should lead to a better understanding of how bone cells are affected by whole bone functional loading.

Associations of calcium intake and physical activity with bone density and size in premenopausal and postmenopausal women: a peripheral quantitative computed tomography study

The purpose of this cross-sectional study was to examine the impact of long-term physical activity (PA) and calcium intake on non-weight-bearing radius and weight-bearing tibia. Altogether, 218 healthy, nonsmoking women, [92 premenopausal women, mean age, 32.6 years (SD, 2.2 years), and 126 postmenopausal women, mean age, 67.3 years (SD, 2.0 years)] participated. The subjects were divided according to their habitual levels of physical activity (PA+ or PA-) and calcium intake (Ca+ or Ca-). The distal end and shaft regions of the radius and tibia were evaluated with peripheral quantitative tomography (pQCT). For the shaft regions, bone mineral content (BMC), cortical cross-sectional area (CoA), cortical density (CoD), and bone strength index, that is, 1-11.9% of the density-weighted section modulus (BSI) were determined. For the distal ends, BMC, total cross-sectional area (ToA), trabecular density (TrD), and BSI were determined. The BMC at the distal radius in the young PA+ group was 6.6% (95% CI, 1- to 11.9%) lower than that of the PA- group. A similar nonsignificant trend was found for the radial shaft. The radial shaft showed a mechanically more competent structure among the older subjects with a BSI 8.5% (95% CI, 1.8-15.6%) higher in the older PA+ group than in the older PA- group. The associations between calcium intake and the radial bone characteristics were systematically positive in both age groups. PA seemed to benefit the distal tibia. In the younger age group the TrD was 6.9% (95% CI, 1.8-12.4%) higher in the PA+ group, and in the elderly the BMC was 5% (95% CI, 0.3-9.9%) higher in the PA+ group than in the PA- group. Note that in the younger age group the ToA was 5.1% (95% CI, 0-9.1%) smaller in the PA+ group than in the PA- group, and in the older age group the ToA was 4.2% (95% CI, -0.3-8.9%) greater in the PA+ group than in the PA- group. The association of PA and bone characteristics at the tibial shaft was positive in both age groups (statistically significant for the older subjects). The tibial shaft BSI of the older PA+ group was 8.6% (95% CI, 2.6-14.9%) better than that of the old PA- group. There was no association between calcium intake and the tibial bone characteristics in either age group. In conclusion, high calcium intake was positively associated with a mechanically competent structure in the radius among both younger and older women, whereas the influence of PA did not become apparent until older ages. PA seemed to benefit particularly the weight-bearing tibia, whereas calcium intake was not associated with the tibia.

An approach to estimating bone and joint loads and muscle strength in living subjects and skeletal remains

Skeletal physiology that clarified after 1990 shows that bone modeling normally makes a bone strong enough to keep its loads from causing strains above a "modeling threshold". That arrangement adapts bone strength to the largest loads on a bone, which are usually brief and infrequent. Accordingly, in bone adapted chiefly to uniaxial compression loads, the modeling threshold's value and the cross-sectional amount of that bone could suggest the size of those loads. Bone loaded in that way does support the articular surfaces of synovial joints as their "supporting bone", so its amount could suggest the size of the loads it had adapted to, and therefore the loads on the joint that it supports. During growth a joint's size is proportional, directly but not linearly, to the size of its total loads, so that its size at skeletal maturity could be an index of those loads at that time. Joints cannot decrease in size. Yet throughout life their supporting bone can decrease or increase in strength and "mass" to adapt to changes in a joint's loads. Thus, an adult joint's size could suggest the size of the loads it adapted to by skeletal maturity, while the cross-sectional amount of its supporting bone at any later age could reveal the size of those loads at that later age, and thus suggest any change in those loads that might have occurred after skeletal maturity. Since the bone modeling threshold, and the relationships between bone strain, stress, and unit loads are now known, it is possible with this procedure to estimate the total loads on joints, and how body weight and muscle strength contribute to those loads in both living subjects and skeletal remains. To make a reliable technology of the idea involves some problems which this paper identifies and suggests how to resolve. Am. J. Hum. Biol. 11:437-455, 1999. Copyright 1999 Wiley-Liss, Inc.

From Wolff's law to the Utah paradigm: insights about bone physiology and its clinical applications

Efforts to understand our anatomy and physiology can involve four often overlapping phases. We study what occurs, then how, then ask why, and then seek clinical applications. In that regard, in 1960 views, bone's effector cells (osteoblasts and osteoclasts) worked chiefly to maintain homeostasis under the control of nonmechanical agents, and that physiology had little to do with anatomy, biomechanics, tissue-level things, muscle, and other clinical applications. But it seems later-discovered tissue-level mechanisms and functions (including biomechanical ones, plus muscle) are the true key players in bone physiology, and homeostasis ranks below the mechanical functions. Adding that information to earlier views led to the Utah paradigm of skeletal physiology that combines varied anatomical, clinical, pathological, and basic science evidence and ideas. While it explains in a general way how strong muscles make strong bones and chronically weak muscles make weak ones, and while many anatomists know about the physiology that fact depends on, poor interdisciplinary communication left people in many other specialties unaware of it and its applications. Those applications concern 1.) healing of fractures, osteotomies, and arthrodeses; 2.) criteria that distinguish mechanically competent from incompetent bones; 3.) design criteria that should let load-bearing implants endure; 4.) how to increase bone strength during growth, and how to maintain it afterwards on earth and in microgravity situations in space; 5.) how and why healthy women only lose bone next to marrow during menopause; 6.) why normal bone functions can cause osteopenias; 7.) why whole-bone strength and bone health are different matters; 8.) why falls can cause metaphyseal and diaphyseal fractures of the radius in children, but mainly metaphyseal fractures of that bone in aged adults; 9.) which methods could best evaluate whole-bone strength, "osteopenias" and "osteoporoses"; 10.) and why most "osteoporoses" should not have bone-genetic causes and some could have extraosseous genetic causes. Clinical specialties that currently require this information include orthopaedics, endocrinology, radiology, rheumatology, pediatrics, neurology, nutrition, dentistry, and physical, space and sports medicine. Basic science specialties include absorptiometry, anatomy, anthropology, biochemistry, biomechanics, biophysics, genetics, histology, pathology, pharmacology, and cell and molecular biology. This article reviews our present general understanding of this new bone physiology and some of its clinical applications and implications. It must leave to other times, places, and people the resolution of questions about that new physiology, and to understand the many devils that should lie in its details. (Thompson D'Arcy, 1917).

Does childhood and adolescence provide a unique opportunity for exercise to strengthen the skeleton?

Osteoporosis is a major, and increasing, public health problem. In this review we examine the evidence that childhood physical activity is an important determinant of bone mineral in adult years, and as such, may help to prevent osteoporosis. Animal studies provide incontrovertible evidence that growing bone has a greater capacity to add new bone to the skeleton than does adult bone. Observational studies in children undertaking routine physical activity and cross-sectional athlete studies in young sportspeople both reveal that activity is positively associated with bone mineral density (BMD). Longitudinal studies in pre- and peripubertal gymnasts reveal BMD gains far in excess of those that can be achieved in adulthood. However, such studies permit only limited conclusions as they contain the potential for selection bias and can be confounded by other determinants of bone mineral (e.g. dietary and lifestyle factors). Thus, research comparing inter-individual playing-to-nonplaying arm differences in bone mineral (e.g., in racquet sports) have proven to be extremely useful. These studies suggest that the BMD differences are clearly greater when bone is subjected to mechanical loading prior to the end of puberty and longitudinal growth of the body (in women, before menarche) rather than after it. Tanner stage II and III appears to be the maturational stage when the association between exercise and BMD becomes manifest in most adolescents. Do conclusions drawn from athlete studies apply to the general population? Randomised intervention studies of physical activity and bone mineral accrual in normal children confirm that childhood activity is strongly associated with bone mineral accrual. Furthermore, some retired athlete studies and a detraining study suggest that adolescent bone gain may, at least partly, persist despite reduced adult physical activity. Mechanisms that may underlie the association between childhood physical activity and bone mineral accrual are outlined. Thus, it appears that physical activity during the most active period of maturity (with respect to longitudinal growth of the body) plays a vital role in optimising peak bone mass and that benefits may extend into adulthood.

Comparison of bone mineral content and biochemical markers of bone metabolism in stall- vs. pasture-reared horses

Sixteen Arabian yearlings were assigned randomly to 2 groups, confined to stall and pastured, to investigate the effects of confinement vs. pasture-rearing on bone mineral content and biochemical markers of bone metabolism over a 140 day period. Following an 84 day pretraining period, 6 horses from each group were selected randomly to complete a 56 day training period. Serum osteocalcin concentrations were determined from blood samples collected every 14 days. Urinary deoxypyridinoline concentrations and mineral content of the third metacarpus, as determined by lateral and medial radiographic bone aluminum equivalency (RBAE), were determined every 28 days from 24 h urine samples and radiographs of the left forelimb, respectively. In comparison with starting values, lateral RBAE was lower in the confined horses at Day 28 and remained lower throughout most of the project, while pastured horses had increasing lateral RBAE. Horses kept in stalls had lower medial RBAE at Day 28 than pasture-reared horses. Medial RBAE tended to remain lower in confined horses than in pastured horses throughout most of the project. The onset of training failed to negate the loss of mineral. Serum osteocalcin concentrations were lower and urinary deoxypyridinoline concentrations were higher in the confined horses at Days 14 and 28, respectively, compared with the pastured horses, and subsequently returned to baseline. These results suggest that housing yearling/2-year-old horses in stalls may be associated with a loss of bone mineral content in comparison with horses maintained on pasture.

Joint anatomy, design, and arthroses: insights of the Utah paradigm

This model of joint design argues 1) that excessive fatigue damage (MDx) in articular cartilage collagen can be the "final cause" of an arthrosis; 2) that known responses of a growing joint's anatomy and geometry, and modeling and maintenance activities, to mechanical loads minimize that cause and thus arthroses; 3) and many biomechanical, biochemical, cell-biologic, genetic and traumatic "first causes" of arthroses could lead to that final cause. The model depends partly on the following facts (marked by a single asterisk) and ideas (marked by a double asterisk). A) During growth a joint's total loads can increase over 20 times without causing an arthrosis, yet in adults an equal loading increase would cause one. B) Fatigue damage (MDx) occurs in joint tissues, larger strains increase it, and minimizing strains reduces it. C) Bone can repair amounts of MDx below an "MDx threshold," but larger amounts can escape repair and accumulate. The model assumes articular cartilage has similar features. D) Bone modeling makes bones strong enough to keep their strains below bone's MDx threshold and minimize MDx. Chondral modeling shapes and sizes joints during growth; that would keep articular cartilage strains below the chondral MDx threshold to minimize chondral MDx and arthroses. Normal chondral modeling nearly stops in adults, which might explain point A above. E) Throughout life maintenance activities preserve optimal physical, chemical and biologic properties of a joint's tissues. To past emphases on the biochemical, genetic, cellular and molecular biologic features of adult joint physiology, this model adds organ-level, tissue-level and vital-biomechanical features of growing joints that invite study and understanding at lower levels of biologic organization.

Randomized controlled study of effects of sudden impact loading on rat femur

Physical loading creating high peak strains on the skeleton at high strain rates is suggested to be the most effective type of activity in terms of bone mineral acquisition. This study assessed the effects of sudden impact loading on mineral and mechanical bone properties in 13-week-old Sprague-Dawley rats. The rats were randomly assigned as sedentary controls (SED, n = 10), control animals receiving low-intensity exercise (EX, n = 15), and experimental animals receiving low-intensity exercise combined with sudden impact-loading (EX + IMP, n = 15). In the EX group, the rats walked in a walking mill at a speed of 10 cm/s for 20 minutes/day, 5 days/week for 9 weeks. In the EX + IMP group, the program was identical to the EX group except for the additional sudden impacts administered to their skeleton during the walking exercise. At the start, there were 50 impacts per session, after which their number was gradually increased to 200 impacts per session by week 6 and then kept constant until the end of the experiment, week 9. These horizontally and vertically directed body impacts were produced by a custom-made walking mill equipped with computer-controlled high-pressure air cylinders. After sacrifice, both femora of each rat were removed and their dimensions, bone mineral content (BMC) by dual-energy X-ray absorptiometry, and mechanical properties by femoral shaft three-point bending and femoral neck compression were determined. The cortical wall thickness increased significantly in the EX and EX + IMP groups as compared with SEDs (+7.6%, p = 0.049 and +10%, p = 0.020, respectively). The EX + IMP group showed +9.0% (p = 0.046) higher cross-sectional moment of inertia values than the EX group. No significant intergroup differences were seen in the BMC values, while the breaking load of the femoral shaft (EX + IMP vs. SED +8.8%,p = 0.047) and femoral neck (EX + IMP vs. SED +14.1%, p = 0.013) was significantly enhanced by the impact loading. In conclusion, this study indicates that mechanical loading can substantially improve the mechanical characteristics of a rat femur without simultaneous gain in its mineral mass. If this is true in humans too, our finding gives an interesting perspective to the numerous longitudinal exercise studies (of women) in which the exercise-induced gains in bone mass and density have remained mild to moderate only.

Indirect way to estimate peak joint loads in life and in skeletal remains (insights from a new paradigm)

BACKGROUND

Bone adapts its strength and cross-sectional amount to the loads on it in now partly known ways. This makes it possible to estimate the unit loads on joint surfaces by an indirect method.

METHODS

In essence, multiply the usual largest allowed compression load on a unit cross section area of epiphyseal trabecular bone (now approximately known), by the cross sectional amount of that bone that supports a unit area of the joint surface (partly known and readily measured by histomorphometry). This would equal the usual largest compression unit load on both the joint surface and the articular cartilage supported by that trabecular bone.

RESULTS

Suggested typical peak unit loads on synovial joint surfaces in different joints and/or parts of joints could range from approximately 2 up to approximately 50 megapascals.

CONCLUSIONS

Besides its use in studies of joint development, physiology and osteoarthritis in living subjects, this method could estimate muscle strength and joint loads from skeletal remains in anatomical, anthropologic, forensic-pathological, and even paleontologic studies.

In vivo bone strain patterns in the zygomatic arch of macaques and the significance of these patterns for functional interpretations of craniofacial form

It has been proposed that the mammalian facial skeleton is optimized for countering or dissipating masticatory stress. As optimized load-bearing structures by definition exhibit maximum strength with a minimum amount of material, this hypothesis predicts that during chewing and biting there should be relatively high and near uniform amounts of strain throughout the facial skeleton. If levels of strain in certain areas of the facial skeleton are relatively low during these behaviors, this indicates that the amount of bone mass in these areas could be significantly reduced without resulting in the danger of structural failure due to repeated masticatory loads. Furthermore, and by definition, this indicates that these areas are not optimized for countering masticatory stress, and instead their overall morphology and concentration of bone mass has most likely been selected or influenced mainly by factors unrelated to the dissipation or countering of chewing and biting forces. An analysis of in vivo bone strain along the lateral aspect of the zygomatic arch of macaques indicates the clear absence of a high and near uniform strain environment throughout its extent. Instead, there is a steep strain gradient along the zygomatic arch, with the highest strains along its anterior portion, intermediate strains along its middle portion, and the lowest strains along its posterior portion. These data, in combination with earlier published data (Hylander et al., 1991), indicate that levels of functional strains during chewing and biting are highly variable from one region of the face to the next, and therefore it is unlikely that all facial bones are especially designed so as to minimize bone tissue and maximize strength for countering masticatory loads. Thus, the functional significance of the morphology of certain facial bones need not necessarily bear any important or special relationship to routine and habitual cyclical mechanical loads associated with chewing or biting. Furthermore, the presence of these steep strain gradients within the facial skeleton suggests that the amount of bone mass in the low-strain areas may be largely determined by factors unrelated to processes frequently referred to as "functional adaptation," or conversely, that the "optimal strain environment" of bone varies enormously throughout the facial skeleton (cf., Rubin et al., 1994). Based solely on anatomical considerations, it is likely that the zygomatic arch is bent in both the parasagittal and transverse planes and twisted about its long axis. Due to constraints on rosette position, the strain data are incapable of determining if one or more of these loading conditions predominate. Instead, the strain data simply provide limited support for the possible presence of all of these loading regimes. Finally, as the masseter muscle is concentrated along the anterior portion of the zygomatic arch and as the arch has fixed ends, the largest shearing forces and the largest bending and twisting moments are located along its anterior portion. This in turn explains why the largest strains are found along the anterior portion of the zygomatic arch.

The integrated processes of hard tissue regeneration with special emphasis on fracture healing

When bone is fractured, a sequence of dynamic events ensue to restore form and therefore function. Many key biologic cell regulators for these events have been identified, expressed through recombinant technology, and their roles posited. Moreover, the availability of recombinantly engineered molecules, such as the bone morphogenetic proteins with their potential to benefit patient care, has ushered in an important era in clinical dentistry that may eliminate either autografting or bank bone allografts. Therefore, in this review article, we have highlighted some of the exciting biologic regulators relevant to bone fracture healing and outlined the dynamic elements in this process.

Evidence of structural and material adaptation to specific strain features in cortical bone

BACKGROUND

Functionally induced strains provide epigenetic signaling for bone modeling and remodeling activities. Strain gauge documentation of the equine third metacarpal reveals a neutral axis passing through the craniolateral cortex, resulting in a narrow band of cortex loaded predominantly in tension, with the remainder of the cortex experiencing a wide range of compression strain magnitudes that are maximal in the caudomedial cortex. This predictable strain pattern provides a model for examining the hypothesis that strain mode, magnitude, and strain energy density are potential correlates of compact bone structural and material organization.

METHODS

Structural and material variables were quantified in nine equine (standard breeds) third metacarpals for comparison with the in vivo strain milieu that was evaluated in thoroughbred horses. The variables quantified included secondary osteon population density (OPD), fractional area of secondary bone (FASB), fractional area of porous spaces, collagen fiber orientation, mineral content (% ash), and cortical thickness. Each bone was sectioned transversely at 50% of length, with subsequent quantification of eight radial sectors and three intracortical regions (periosteal, middle, endosteal). Linear regression analysis compared these variables to magnitudes of corresponding regional in vivo longitudinal strain, shear strain, and strain energy density values reported in the literature.

RESULTS

The craniolateral ("tension") cortex of this bone is distinguished by its 30% lower FASB and with the lateral cortex exhibits 20% darker gray level (more longitudinal collagen) compared with the average of all other locations. Conversely, the remaining ("compression") cortices as a group have a high OPD, are more extensively remodeled, and contain more oblique-to-transverse collagen. The caudal cortices (caudomedial, caudal, caudolateral) are significantly thinner (P < 0.01) and have 4% lower mineral content (P < 0.05) than all other locations. Moderately strong correlations exist between collagen fiber orientation and normal strain (r = 0.752) and shear strain (r = 0.555). When normal and shear strains were transformed to their respective absolute values, thus eliminating the effects of strain mode (tension vs. compression), these correlation coefficients decreased markedly.

CONCLUSIONS

Collagen fiber orientation is related to strain mode and may function to accentuate rather than attenuate bending. These differences may represent adaptations that function synergistically with bone geometry to promote a beneficial strain distribution and loading predictability during functional loading.

Adaptation of bone to altered loading environment: a biomechanical approach using X-ray absorptiometric data from the patella of a young woman

Loading induced changes in the bone mineral apparent density (BMAD) and average strain magnitude (strain index) of the patella were estimated using a novel analytic method of dual energy X-ray absorptiometric (DXA) and isometric strength data obtained from repeated measurements of a 26-year-old woman. The strain index was used as a major determinant of bone adaptation to physical loading. The subject's lower limb skeleton was measured 12 times during a 3-year period including a 1-year unilateral strength training intervention, an accidental knee ligament rupture, and a 2-year rehabilitation period. Instead of standard DXA analysis, the patella, a bone practically affected by quadriceps activity only, was analyzed in terms of estimated loading induced stresses and strains. The mechanical stress is directly proportional to the effective force and inversely proportional to the area over which the given force is applied. The strain, in turn, is proportional to the given stress divided by the bone stiffness, which was assumed to be proportional to BMAD. The effective force, the contact area, and the BMAD were estimated from the muscle strength and DXA data, respectively. In addition, post-traumatic bone loss and subsequent recovery with time were assessed using an exponential model. The loading during the 1-year training period increased the average patellar strain index by 47% and did not affect the patellar BMAD. Immediately after the injury, the apparent inability to do muscle work drastically reduced the strain index and likely initiated the patellar bone loss, a loss which continued for about 4 months. During rehabilitation, the imbalance between the patellar stiffness and increased functional stress (muscular performance) became sufficiently large to level off the bone loss and stimulate bone gain. At that time, the strain index indicated that the patellar deformation more than doubled (135%) as compared to baseline level. Accordingly, the BMAD increased until an apparent balance between BMAD and the muscular strength was achieved by the 3-year end point. The exponential models explained well both the post-traumatic bone loss (R2 = 0.89) and bone gain during recovery (R2 = 0.98). In conclusion, the present observations support the concept of the nonlinear nature of the skeletal response to mechanical loading and suggest a potential utility of muscle strength measurements and DXA information for improved understanding of the effects of training, immobilization and remobilization on bone tissue.

Effects of unilateral strength training and detraining on bone mineral mass and estimated mechanical characteristics of the upper limb bones in young women

The aims of this study were to examine the effects of 12 months unilateral high-resistance strength training and 8-month detraining on bone mineral content (BMC), density (BMD) and estimated mechanical characteristics of upper limb bones, and also to estimate consequent loading induced strains on forearm bone shafts. Thirteen female physiotherapy students (mean 23.8 +/- 5.0 yrs, 166 +/- 7 cm, 64.4 +/- 7 cm, 64.4 +/- 13.3 kg) trained their left upper limbs with dumbbells on average 2.8 times per week for 12 months, followed by eight months detraining. Nineteen students served as controls (mean 25.7 +/- 5.2 yrs, 165 +/- 4 cm, 62.1 +/- 7.0 kg). BMC, BMD, and bone width and estimated cortical wall thickness (CWT) were measured at five different sites in both upper extremities (proximal humerus, humeral shaft, radial shaft, ulnar shaft, and distal forearm) using dual energy x-ray absorptiometry (DXA) scanner. In addition, cross-sectional moment of inertia (CSMI) was estimated from DXA data. The maximal isometric strength of the upper extremities was measured with an arm flexion-extension dynamometer. The training increased significantly the flexion strength by 14% (p = 0.001). During the detraining period, all measured strength values in the training group decreased in both limbs with respect to values after training. Despite the clear effect on muscular strength, no significant intergroup differences were observed in BMC, BMD, bone width, CWT, or CSMI values at any measured site after the training or detraining period. The estimated loading-induced strains remained within customary loading, and the change in strain level was only 15%. In conclusion, this study indicated that using high-resistance strength training may not provide an effective osteogenic stimulus for bone formation and geometric changes in upper limb bones of young, healthy, adult women. The interaction of bones and muscles may play an important and relatively unrecognized role in the development of bone strength, suggesting that the entire biomechanical environment should be carefully considered when evaluating the osteogenic efficiency of physical loading.

Intravenous olpadronate restores ovariectomy-affected bone strength. A mechanical, densitometric and tomographic (pQCT) study

Female Wistar rats aged 3 months were ovariectomized (OX, n = 27). Three months later they were given i.v. doses of 150 (6), 300 (7), or 600 (6) ug/kg 2/wk of olpadronate during 12 weeks or left as OX controls (OXc). Bending fracture load of femur diaphyses, reduced in OXc, was recovered by olpadronate. This effect was paralleled by changes in material quality indicators as DEXA-BMD, tomographic (volumetric) BMD, elastic modulus, and maximum elastic stress of cortical bone. No changes were induced by any of the treatments on cross-sectional area or moment of inertia. Diaphyseal stiffness, not reduced by OX, was enhanced to overnormal values by olpadronate at any dose. None of the treatments affected the normal mechanostatic interrelationships between cross-sectional architecture and bone material quality indicators. The positive effects described point out important differences in bisphosphonate action on bone biomechanics according to the experimental conditions assayed.

Perspectives of pQCT technology associated to biomechanical studies in skeletal research employing rat models

Assessment of bone material quality and architectural indicators by means of peripheral quantitative computed tomography (pQCT) offers a wide perspective for skeletal research employing noninvasive procedures. Some mechanically-validated examples of these pQCT applications in animal models are described. They concern (a) the analysis of bone mechanostatical interrelationships as shown by experimental "distribution/quality" curves, and (b) the noninvasive determination of bone strength. An attractive attempt to extrapolate the latter to human bone studies is also discussed.

Southwestern Internal Medicine Conference: prevention of hip fractures in the elderly

Hip fracture is a common, morbid, and costly health problem. Because our population is aging, hip fractures will remain a major health concern as we enter the next century. It has been estimated that by the year 2040, 512,000 hip fractures will occur annually in people 50 years or older. A number of factors common in the elderly increase the risk of falling. Falls and age-related changes that influence bone quality increase susceptibility to fracture. In this article, the author focuses on studies that identified risk factors and strategies to reduce falls as well as pharmacologic agents that may reduce fracture risk. Because of the multifactorial etiology of hip fractures, their prevention will ultimately require a combination of pharmacologic approaches to improve bone strength and strategies to prevent falls and limit injury.

Comparison of areal and estimated volumetric bone mineral density values between older men and women

We compared areal bone mineral density (BMD) of the total body (TBMD), antero-posterior lumbar spine at L3 (APS), lateral spine at L3 (LS) and femoral neck (FN). In order to understand better the effect of gender-related size differences on BMD, we also compared the estimated volumetric BMD at L3 (VLS) and the femoral neck (VFN). Subjects were asymptomatic women (n = 22) and men (n = 44) with an age range of 58-79 years. BMD at each site was measured by dual-energy X-ray absorptiometry using a Hologic 2000 in array mode. Results of the statistical analyses (ANOVA) showed the men to have significantly greater BMD at all areal sites [APS, LS (p < 0.05); FN (p < 0.01); TBMD (p < 0.001)]. The two estimated volumetric comparisons, however, showed no gender differences. Results demonstrate how measures from areal BMD measures can be misleading when comparing groups of different size. In older men and women planar measures may overestimate gender differences in BMD.