Light and resistant: is bone the perfect material?




James F. GRIFFITH,MB, BCh, BAO, MRCP, FRCR
Department of Imaging and Interventional Radiology
The Chinese University of Hong Kong, HONG KONG


Patrick AMMANN,MD
Division of Bone Diseases
Department of Rehabilitation and Geriatrics
University Hospitals
Geneva, SWITZERLAND

Light and resistant: is bone the perfect material?


by P. Ammann, Switzerland



Bone is a fascinating organ and its structure is fully adapted to its function. The presence of cortical and trabecular bone allows perfect mechanical competence with the lowest bone mass, ie, bone weight. Bone is also able to adapt to external constraints, such as repeated stimuli. This is particularly well demonstrated by the modifications of bone structure and geometry occurring in the context of extreme sport performance or in the absence of gravity during space flight. In other examples, the extreme load generated during mastication counteracts the effect of ovariectomy on the mandible. Hormonal modulation leading to progressive decrease in trabecular bone mass and microarchitecture can also be associated with compensatory increase in femoral neck diameter, limiting the decrease in bone strength. Antiosteoporotic treatments restore bone strength by modifying the natural architecture. Modulation of sclerostin action by antisclerostin treatment mimicsmechanical loading of the skeleton and induces formation of new bone and improvement of mechanical properties. All these influences selectively modified all determinants of bone strength, such as bone geometry,microarchitecture, and intrinsic bone tissue quality. Bone is not only able to adapt to the environment, but also to repair microdamage, to heal fracture, and to integrate implants.

Medicographia. 2012;34:178-184 (see French abstract on page 184)



Bone is a fascinating organ. Its structure is fully adapted to its function. Furthermore, it is also able to adapt to new external constraints like repeated stimuli, such as walking, running, and jumping. Bone is not only able to adapt to the environment, but also to repair microdamage, to heal fracture, and to integrate implants. The presence of cortical and trabecular bone allows perfect mechanical competence with the lowest bone mass, ie, bone weight. We now have extensive knowledge on the correlation between bone mechanical properties and bone geometry (diameter of long bone, shape) and microarchitecture (cortical thickness, trabecular bone mass, three-dimensional [3D] distribution, form of the trabeculae [plate vs rods], cortical porosity, microcracks). However, the study of intrinsic bone tissue quality (ie, bone material properties) is only starting to be systematically considered and more investigated at the basic level.

How to evaluate bone mechanical properties and their determinants?

One challenge is to measure bone mechanical properties, which represent an objective measurement of fracture risk. Biomechanical tests of resistance to fracture provide an objective measure of overall bone quality, but these methods are inva- sive and cannot be applied in the clinic. Since the ability of bone to withstand stress is controlled by determinants of bone strength—including mass, geometry, and microarchitecture— and intrinsic bone tissue quality, these determinants are used as surrogate measures in the clinical setting.


Figure 1
Figure 1. Bone fatigue test.

(A) Vertebrae are cyclically loaded in axial compression for 100 cycles. (B) The peak load selected corresponded to 5% of the maximal load (Fmax) of the adjacent vertebra (L3), thus in the domain of elastic deformation (E). The selected peak load induces alteration of post-yield load without any effects on vertebral height, ie, without causing fracture. (C) The cyclically loaded vertebrae were then loaded to failure. We compared the load/displacement curve of the cyclically loaded vertebra (L4) to the adjacent one (L3), not submitted to repeated loading. Bone mechanical properties, including post-yield load and deflection, which characterize post-yield behavior (domain of plastic deformation, P), were investigated.



_ Mechanical properties
Tests are now available to quantify the mechanical properties of bone from different parts of the skeleton.1 Biomechanical properties of intact cortical and trabecular bone are investigated by axial compression of the vertebral body and proximal tibia. Purely cortical bone is tested by flexion applied at three or four points. The load/deflection curve is used tomeasure stiffness (the slope of the linear portion of the curve) and maximal load (load at fracture) (Figure 1). The transition point of the load/deflection curve between elastic (linear) and plastic (nonlinear) deformation is defined as the yield point.1,2 The areas under these sections of the curve represent the energies absorbed during elastic and plastic deformation. Indirect information concerning thematerial properties can be obtained from the load/deflection curve evaluating the plastic deformation. These values quantify the events occurring from the yield point to the fracture, ie, the plastic deformation. The post-yield load corresponds to the load measured after the yield point to the maximal load (fracture). It is also possible to measure the post-yield deflection corresponding to the deformation imposed on the tested bone sample during plastic deformation. The area under the curve from the yield to the fracture is also of interest. These parameters are influenced by the fatigue (cyclical loading) and modification of intrinsic bone tissue quality, eg, due to low protein intake or ovariectomy. Of note, dissipated energy measured by nanoindentation, ie, the measurement of intrinsic bone tissue quality, is correlated with the plastic energy, but not with other parameters, such as stiffness, which is essentially determined by the geometry of the sample. This emphasizes the importance of the post-yield events characterizing intrinsic bone tissue quality. These tests give an excellent quantification of bone mechanical properties and are certainly representative of the load applied on bone during a fall resulting in a fracture. This technical approach allows a very reproducible evaluation of bone strength.

Repeated loading seems to play a role in the genesis of damage and eventually of fracture. In real life, repeated loading could correspond to jumping, running, or hiking. How repeated loading influences mechanical properties or, inversely, how bone tolerates repeated loading, could be of major interest to better understand the risk of fracture occurring not only during bone disease (eg, estrogen deficiency, ovariectomy, and protein malnutrition), but also under the influence of therapeutic agents, particularly those known to influence intrinsic bone tissue quality. We developed an ex vivo dynamic test, for which bone of one site is used as a control value and also to determine the load to apply during the fatigue test (Figure 1). The contralateral bone is cyclically loaded with the load being calculated from the value obtained by measuring the contralateral bone. The load and the number of cycles have to be determined in preliminary studies to induce fatigue, but not fracture, during the test and to mimic a load close to the in vivo situation. Alteration of post-yield load and induced deflection are used to monitor bone tissue alteration induced by fatigue (Figure 1). This test provides a way to investigate other properties of bone and could be used to test cortical bone in the long bone, or to test both trabecular and cortical bone in the vertebrae.


Figure 2
Figure 2. Microfractures induced by in vivo microindentation are similar to fractures observed in cortical and trabecular bone.

Abbreviations: IDI, indentation distance increase; R,
Pearson correlation coefficient; RPI, reference point indentation.
Modified from reference 19: Diez-Perez et al. J Bone Miner Res. 2010;25(8):1877-1885. © 2010 American Society for Bone and Mineral Research.



In patients, the presence of a previous fracture is a risk factor for the occurrence of another fracture.3 As an example, patients suffering a forearmfracture have to be considered at high risk of osteoporosis, as this type of fracture is the first event occurring in this disease. Thus, a forearm fracture occurring in the context of low-energy trauma could be considered an excellent positive biomechanical test requiring a complete investigation for osteoporosis diagnosis.

_ Determinants of bone strength
The most widely used noninvasive method for the diagnosis of early osteoporosis and to establish fracture risk is dual-energy x-ray absorptiometry (DXA), the conventional determinant of mechanical properties, namely “areal” or “surface” (ie, nonvolumetric) bone mineral density (BMD). In the absence of treatment, ex vivo studies show an excellent correlation between proximal femur BMD and the results of biomechanical tests, including neck of femur flexion and vertebral compression4,5; BMD predicts 60% to 74% of mechanical property variance. As a ratio between hydroxyapatite mineral content and the scanned area, BMD incorporates bone dimensions in addition to mineral quantity. Indeed, the propensity of BMD to predict bone strength is due, at least in part, to its incorporation of bone size.

Dimensions such as external diameter and cortical thickness are key determinants of bone strength.1,2 Increasing the external diameter of a long bone substantially increases its resistance to flexion.6-8 Increasing cortical thickness has a lesser effect on bone strength.7 A 3% to 5% change in diameter can strengthen a long bone by 15% to 20%.

The major features of bone microarchitecture are trabecular bone volume, trabecular density, intertrabecular spacing, trabecular morphology (plate versus column ratio), and the parameters of trabecular connectivity. Changes in any of these features can affect bone strength. Histomorphometry, performed on a horizontal transiliac crest core biopsy, offers a two-dimensional (2D) window and provides information on the degree of mineralization and lamellar organization (lamellar or woven bone).9 By offering a three-dimensional (3D) window into bone microarchitecture, microcomputed tomography (μCT) appears to be optimal for the evaluation of trabecular microarchitecture.9 It can also differentiate the trabecular morphology (plates versus columns) that plays a determining role in the transmission and distribution of mechanical stress within bone tissue.9 In addition, it allows finite element analysis to simulate bonemechanical tests and to analyze the role of each determinant.10,11 However, being biopsy-based, it also remains invasive.

Newly-developed μCT systems (extreme CT) have sufficient resolution for the noninvasive in vivo measurement of human wrist and tibia microarchitecture.12 Although the resolution is lower than in ex vivo studies, the technique provides data on trabecular connectivity and morphology. Its major advantage is that it can be used for serial microarchitecture monitoring.13 Reports have confirmed its accuracy, sensitivity, and repro-ducibility. Prospective studies indicate that this measurement is able to monitor treatment efficacy and to predict fracture risk, independent of DXA measurement.11

The study of intrinsic bone tissue quality (ie, the bone tissue material properties) is only starting to be systematically considered and more investigated at the basic level. Bone is a heterogeneous tissue made up of a mineral component (hydroxyapatite) and an organic collagen component. Theoretically, each is capable of influencing the intrinsic quality of bone tissue. The degree of mineralization has been the more studied aspect of bone tissue to date.14 Various techniques are available for assessing and quantifying intrinsic bone tissue quality, with respect to both the bone structural unit (BSU) (microindentation)15,16 and lamella (nanoindentation).17,18 These give overall information on intrinsic quality as influenced by the mineral and organic components, but only nanoindentation selectively evaluates the influence of each. However, all these approaches are invasive and therefore difficult to apply in clinical strategies and do not allow a longitudinal followup. A novel technique is now available for the in vivo measurement of bone tissue strength in a clinical setting.19 This technique is based on creating microfractures and measuring the bone’s overall resistance to their propagation (Figure 2).19 This represents a direct assessment of fracture pathophysiology and potentially of material properties. The major limitation of this new in vivo technology is that we do not know at the present time whether these measurements are related to bone material properties and/or bone strength.

Bone remodeling could also be considered a determinant of bone strength. Bone turnover allows permanent removal of damaged bone and its replacement by new bone of excellent quality (bone remodeling). This process also allows modification of the size and form of trabeculae, and alteration of bone mass and microarchitecture, occurring in pathological bone loss and during treatments. Bone turnover is considered a determinant of fracture risk, independent of DXA measurement.
20
Bone remodeling can modify the size of the bone, eg, during growth and loading, and allows adaptation of the cortical envelope to mechanical demand. The key role of osteocytes, and their response to loading exerted on bone (through sclerostin regulation), in bone formation have been clearly established.21 This process allows bone adaptation to external load.


Figure 3
Figure 3. Bone is able to adapt to mechanical loading.

(A) Cardinals vs Dodgers. October 10, 2004. © Armando/Arorizo/ZUMA/Corbis.
(B) US astronaut in space. © Stocktrek/Corbis.


How do bone structures adapt to hormonal and environmental influences?

_ Mechanical loading and gravity
Through the process of remodeling and modeling, bone is able to adapt its geometry to the demand (Figure 3). Exercise performed at a high level, such as in tennis or baseball, adapts not only the cortical thickness, but also the external diameter of the stimulated site, with the greatest bone gain observed during growth. Studies suggest that exercise-induced gains in bone mass are lost with age.22,23 However, exercise during growth primarily influences bone structure (external diameter), rather than mass, to increase bone strength. As an example, the intense practice of a physical activity such as baseball (pitchers and catchers) induces stimulation of the humerus in torsion, leading to a higher external diameter and increased cortical thickness in men who started playing at a young age. In retired players, a progressive decrease in cortical thickness occurs, whereas the outer diameter remains larger. This is a good example of skeletal adaptation to external solicitation during growth (increased diameter and cortical thickness) and reduced stimulation (decrease in cortical thickness only). This also emphasizes that exercise during youth has lifelong beneficial effects on cortical bone structure and strength, independent of beneficial effects on bone mass.

In contrast, astronauts living for a period of time in space without gravity’s influence lose bone mass in the legs, as they are no longer solicited by any mechanical load (gravity or muscle activity).24 However, in the arms, the BMD remains normal, possibly explained by muscle demand compensating for the absence of gravity, as the muscles of the upper members are extremely solicited to stabilize the body and perform physical activities.

_ Sex hormone deficiency
Estrogen deficiency is associated with increased bone turnover and results in a negative bone balance. Changes in architecture account for the early decline in bone strength after ovariectomy. Significant decreases in vertebral strength antedate any significant decrease in BMD.1 The dissociation between these two variables is due to an early change in microarchitecture such as the perforation and/or disappearance of trabeculae, with no major effect on BMD. In humans, increased vertebral fracture severity, measured semiquantitatively, is associated with deterioration in bone microarchitecture.25 This continuous and progressive modification of microarchitecture accounts for the accelerated cascade of fractures observed in patients with vertebral fracture. Recent studies indicate that microarchitecture evaluated by extreme CT in humans predicts the risk of fracture, independent of BMD.12 This observation is a further argument in favor of the importance of the spatial distribution of bone mass. It also underlines that the perturbation of bone metabolism could interfere with the mechanical adaptation of bone.


Figure 4
Figure 4. Reversibility of isocaloric lowprotein diet effect on bone.

Adult female rats were fed a normal or an isocaloric low-protein diet. Reversibility of the effect was evaluated by giving essential amino acid supplements. Scans of the proximal tibia were obtained at the end of the study. © The author.



In the long bone, compensatory expansion of diameter occurs after ovariectomy, and is related to a transient increase in insulin- like growth factor-I (IGF-I) in the presence of estrogen deficiency.7 In ovariectomized rats, a progressive decrease in bone mass and microarchitecture occurs in the femoral neck,26 but a subsequent compensatory increase in external neck diameter leads to progressive recovery of normal femoral neck strength.26 Though not observed in postmenopausal women, a compensatory increase in femoral neck diameter has been observed in elderly osteoporotic men.27

Estrogen deficiency has different effects on the mandible than other regions of the skeleton,28 but mandible osteoporosis is not always observed after menopause or ovariectomy. This discrepancy is explained by the fact that the extreme load generated during mastication counteracts the effect of ovariectomy. When ovariectomized animals were fed a soft diet (reduction of load), bone loss was observed, but this did not occur in rats fed a hard diet. This shows the capacity of bone to compensate for bone loss and to adapt its structure to environmental conditions.

_ Low protein intake
Protein malnutrition results in increased bone resorption and decreased bone formation, leading to bone loss. In contrast to effects of ovariectomy, no compensatory increment of bone diameter is observed, and alteration of bone strength occurs also in the long bone. The depressed somatotrope axis probably explains the absence of a compensatory periosteal apposition.29 Thus, protein supplementation leads to profound modification of the microarchitecture, geometry, and intrinsic bone tissue quality: thickening of the remaining trabeculae and cortex, and normalization of intrinsic bone tissue quality (Figure 4). All these positive effects on determinants of bone strength restore normal30 bone strength, but the bone architecture is different from that in normal rats. Taken together, this emphasizes potential adaptation of bone geometry and architecture to pathologic situations and the bone’s capacity to recover when environmental conditions are restored.

_ Antiosteoporotic treatments
Osteoporosis is defined as a decreased bone mass and alteration of microarchitecture leading to bone fragility and an increased risk of fracture. In other terms, alteration of the determinants of bone strength represents the phenotype of osteoporosis. Therefore, treatments have to induce a modification of these determinants (bone mass, microarchitecture geometry, and/or intrinsic bone tissue quality) to counteract bone loss and to adapt bone structure tomechanical demand. Different antiosteoporotic treatments are now available. Most of them are classified into two different families according to their cellular bone effects: anticatabolic agents (reduction of bone turnover) and anabolic agents (stimulation of bone formation). Strontium ranelate is a novel compound that can be classified in a third category. Indeed, strontium ranelate influences bone turnover by reducing bone resorption and maintaining a high level of bone formation. The only anabolic agent in clinical use is parathyroid hormone (PTH), which stimulates bone turnover and induces a positive bone balance. Bisphosphonates (anticatabolic agents) reduce bone resorption and, secondarily, bone formation, leading to a prevention of further bone loss and alteration of the microarchitecture and geometry. Strontium ranelate decreases bone resorption, thus maintaining a high level of bone formation and leading to a positive bone balance.

All three treatments decrease fragility and reduce fracture risk. However, they have the opposite effect on bone turnover, which is supposed to be the most important target of antiosteoporotic drugs. They also have variable effects on bone strength determinants, including microarchitecture, geometry, and bone mass.31 A preclinical study comparing the effects of anticatabolic drugs (pamidronate and raloxifen) and PTH in ovariectomized rats indicates that anticatabolic agents prevent further alteration of microarchitecture and geometry and increase bone material properties as evaluated by nanoindentation. By contrast, PTH increases bone mass, geometry, and microstructure, but does not prevent the alteration of bone material properties induced by the ovariectomy. Strontium ranelate reduces the incidence of fractures independently of severity of osteoporosis, bone turnover, and presence of fracture.32-34 Since this efficacy cannot be related only to an extent of bone mass modification, an effect on bone material properties could be suspected. Indeed, the deleterious effect of ovariectomy on bone mechanical properties was fully prevented by strontium ranelate administration in adult rats.32 Thus, microarchitecture deterioration and a decrease in bone mass (both major determinants of bone strength) induced by ovariectomy were prevented by strontium ranelate treatment. Investigation of bone material properties under experimental conditions showed that strontium ranelate treatment markedly improved hardness and working energy in ovariectomized rats in which a decrease in these properties had been observed.18 The valuesmeasured in strontiumranelate–treated ovariectomized rats after one dose were significantly higher than in sham controls. This suggests that the in vivo modulation of material properties by an antiosteoporotic agent could participate in the improvement of bone strength. Similar positive effects of strontium ranelate salt on bone material properties were observed in humans. A recent study of the vertebral bodies of intact rats treated with strontium ranelate or placebo clearly demonstrated by finite element analysis integrating both microarchitecture parameters and intrinsic bone tissue quality that both determinants of bone strength (bone mass and bone material properties) independently and significantly participate in the determination of bone strength.10 Bone strength was simulated in the model and also measured in the adjacent vertebra using a compression test. The contribution of both determinants to the prediction of bone strength was equivalent. These clinical and preclinical observations indicate that these bone strength determinants, characterizing bone mass and its spatial distribution, are not enough to explain changes in bone strength and fracture risk. Bone material properties have to be considered and are of major importance.

Antiosteoporotic treatments restore bone strength by modifying the natural architecture. Modulation of sclerostin action by antisclerostin treatment mimics mechanical loading of the skeleton and induces formation of new bone and improvement of mechanical properties.35 This potential treatment of osteoporosis is an example of therapeutic use of our knowledge of bone adaptation to mechanical loading.

Conclusions

Bone is a fascinating organ and its structure is fully adapted to its function. Furthermore, it is also able to adapt to external constraints, such as repeated stimuli, hormonal modulation, and antiosteoporotic treatment. Modulation of all determinants (geometry, microarchitecture, and intrinsic tissue quality) is implicated in the response to mechanical stimuli and osteoporosis treatments. Hormonal dysregulation partially interferes with skeletal adaptation and further research is warranted to investigate solutions to tailor more adequate treatment options. _

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Keywords: adaptation; bone fatigue; bone geometry; mechanical loading; microarchitecture; remodeling; sex hormone deficiency