Charles H. Turner

Basic biomechanical measurements of bone: A tutorial

Although bone densitometry is often used as a surrogate to evaluate bone fragility, direct biomechanical testing of bone undoubtedly provides more information about mechanical integrity. Like any other specialized field, biomechanics contains its own techniques and vocabulary. This article serves as a guide to biomechanical principles and testing techniques for bone specimens.

Mechanical Stimulation of Bone in Vivo Reduces Osteocyte Expression of Sost/Sclerostin

Sclerostin, the protein product of the Sost gene, is a potent inhibitor of bone formation. Among bone cells, sclerostin is found nearly exclusively in the osteocytes, the cell type that historically has been implicated in sensing and initiating mechanical signaling. The recent discovery of the antagonistic effects of sclerostin on Lrp5 receptor signaling, a crucial mediator of skeletal mechanotransduction, provides a potential mechanism for the osteocytes to control mechanotransduction, by adjusting their sclerostin (Wnt inhibitory) signal output to modulate Wnt signaling in the effector cell population. We investigated the mechanoregulation of Sost and sclerostin under enhanced (ulnar loading) and reduced (hindlimb unloading) loading conditions. Sost transcripts and sclerostin protein levels were dramatically reduced by ulnar loading. Portions of the ulnar cortex receiving a greater strain stimulus were associated with a greater reduction in Sost staining intensity and sclerostin-positive osteocytes (revealed via in situ hybridization and immunohistochemistry, respectively) than were lower strain portions of the tissue. Hindlimb unloading yielded a significant increase in Sost expression in the tibia. Modulation of sclerostin levels appears to be a finely tuned mechanism by which osteocytes coordinate regional and local osteogenesis in response to increased mechanical stimulation, perhaps via releasing the local inhibition of Wnt/Lrp5 signaling.

Mechanotransduction and the functional response of bone to mechanical strain

Mechanotransduction plays a crucial role in the physiology of many tissues including bone. Mechanical loading can inhibit bone resorption and increase bone formation in vivo. In bone, the process of mechanotransduction can be divided into four distinct steps: (1) mechanocoupling, (2) biochemical coupling, (3) transmission of signal, and (4) effector cell response. In mechanocoupling, mechanical loads in vivo cause deformations in bone that stretch bone cells within and lining the bone matrix and create fluid movement within the canaliculae of bone. Dynamic loading, which is associated with extracellular fluid flow and the creation of streaming potentials within bone, is most effective for stimulating new bone formation in vivo. Bone cells in vitro are stimulated to produce second messengers when exposed to fluid flow or mechanical stretch. In biochemical coupling, the possible mechanisms for the coupling of cell-level mechanical signals into intracellular biochemical signals include force transduction through the integrin-cytoskeleton-nuclear matrix structure, stretch-activated cation channels within the cell membrane. G protein-dependent pathways, and linkage between the cytoskeleton and the phospholipase C or phospholipase A pathways. The tight interaction of each of these pathways would suggest that the entire cell is a mechanosensor and there are many different pathways available for the transduction of a mechanical signal. In the transmission of signal, osteoblasts, osteocytes, and bone lining cells may act as sensors of mechanical signals and may communicate the signal through cell processes connected by gap junctions. These cells also produce paracrine factors that may signal osteoprogenitors to differentiate into osteoblasts and attach to the bone surface. Insulin-like growth factors and prostaglandins are possible candidates for intermediaries in signal transduction. In the effector cell response, the effects of mechanical loading are dependent upon the magnitude, duration, and rate of the applied load. Longer duration, lower amplitude loading has the same effect on bone formation as loads with short duration and high amplitude. Loading must be cyclic to stimulate new bone formation. Aging greatly reduces the osteogenic effects of mechanical loading in vivo. Also, some hormones may interact with local mechanical signals to change the sensitivity of the sensor or effector cells to mechanical load.

Young's modulus of trabecular and cortical bone material: Ultrasonic and microtensile measurements

An ultrasonic technique and microtensile testing were used to determine the Youngs modulus of individual trabeculae and micro-specimens of cortical bone cut to similar size as individual trabeculae. The average trabecular Youngs modulus measured ultrasonically and mechanically was 14.8 GPa (S.D. 1.4) and 10.4 (S.D. 3.5) and the average Youngs modulus of microspecimens of cortical bone measured ultrasonically and mechanically was 20.7 GPa (S.D. 1.9) and 18.6 GPa (S.D. 3.5). With either testing technique the mean trabecular Youngs modulus was found to be significantly less than that of cortical bone (p < 0.0001). However, the specimens were dried before microtensile testing so Youngs modulus values may have been greater than those of trabeculae in vivo. Using Youngs modulus measurements obtained from 450 cubes of cancellous bone and 256 cubes of cortical bone, Wolffs hypothesis that cortical bone is simply dense cancellous bone was tested. A multiple regression analysis that controlled for group membership showed that Youngs modulus of cortical bone cannot be extrapolated from the Youngs modulus vs density relationship for cancellous bone, yet the Youngs modulus of trabeculae can be predicted by extrapolation from the relationship between Youngs modulus vs density of the cancellous bone. These results suggest that when considered mechanically, cortical and trabecular bone are not the same material.

Three rules for bone adaptation to mechanical stimuli

The primary mechanical function of bones is to provide rigid levers for muscles to pull against, and to remain as light as possible to allow efficient locomotion. To accomplish this bones must adapt their shape and architecture to make efficient use of material. Bone adaptation during skeletal growth and development continuously adjusts skeletal mass and architecture to changing mechanical environments. There are three fundamental rules that govern bone adaptation: (1) It is driven by dynamic, rather than static, loading. (2) Only a short duration of mechanical loading is necessary to initiate an adaptive response. (3) Bone cells accommodate to a customary mechanical loading environment, making them less responsive to routine loading signals. From these rules, several mathematical equations can be derived that provide simple parametric models for bone adaptation.

Suppressed Bone Turnover by Bisphosphonates Increases Microdamage Accumulation and Reduces Some Biomechanical Properties in Dog Rib

It has been hypothesized that suppression of bone remodeling allows microdamage to accumulate, leading to increased bone fragility. This study evaluated the effects of reduced bone turnover produced by bisphosphonates on microdamage accumulation and biomechanical properties of cortical bone in the dog rib. Thirty‐six female beagles, 1–2 years old, were divided into three groups. The control group (CNT) was treated daily for 12 months with saline vehicle. The remaining two groups were treated daily with risedronate (RIS) at a dose of 0.5 mg/kg per day or alendronate (ALN) at 1.0 mg/kg per day orally. After sacrifice, the right ninth rib was assigned to cortical histomorphometry or microdamage analysis. The left ninth rib was tested to failure in three‐point bending. Total cross‐sectional bone area was significantly increased in both RIS and ALN compared with CNT, whereas cortical area did not differ significantly among groups. One‐year treatment with RIS or ALN significantly suppressed intracortical remodeling (RIS, 53%; ALN, 68%) without impairment of mineralization and significantly increased microdamage accumulation in both RIS (155%) and ALN (322%) compared with CNT. Although bone strength and stiffness were not significantly affected by the treatments, bone toughness declined significantly in ALN (20%). Regression analysis showed a significant nonlinear relationship between suppressed intracortical bone remodeling and microdamage accumulation as well as a significant linear relationship between microdamage accumulation and reduced toughness. This study showed that suppression of bone turnover by high doses of bisphosphonates is associated with microdamage accumulation and reduced some mechanical properties of bone.

Bone Microdamage and Skeletal Fragility in Osteoporotic and Stress Fractures

The accumulation of bone microdamage has been proposed as one factor that contributes to increased skeletal fragility with age and that may increase the risk for fracture in older women. This paper reviews the current status and understanding of microdamage physiology and its importance to skeletal fragility. Several questions are addressed: Does microdamage exist in vivo in bone? If it does, does it impair bone quality? Does microdamage accumulate with age, and is the accumulation of damage with age sufficient to cause a fracture? The nature of the damage repair mechanism is reviewed, and it is proposed that osteoporotic fracture may be a consequence of a positive feedback between damage accumulation and the increased remodeling space associated with repair.

Effects of Suppressed Bone Turnover by Bisphosphonates on Microdamage Accumulation and Biomechanical Properties in Clinically Relevant Skeletal Sites in Beagles

We recently demonstrated that suppression of bone remodeling allows microdamage to accumulate, leading to reduced bone toughness in the rib cortex of dogs. This study evaluates the effects of reduced bone turnover produced by bisphosphonates on microdamage accumulation and biomechanical properties at clinically relevant skeletal sites in the same dogs. Thirty-six female beagles, 1-2 years old, were divided into three groups. The control group was treated daily for 12 months with saline vehicle (CNT). The remaining two groups were treated daily with risedronate at a dose of 0.5 mg/kg per day (RIS), or alendronate at 1.0 mg/kg per day (ALN) orally. The doses of these bisphosphonates were six times the clinical doses approved for treatment of osteoporosis in humans. After killing, the L-1 vertebra was scanned by dual-energy X-ray absorptiometry (DXA), and the L-2 vertebra and right ilium were assigned to histomorphometry. The L-3 vertebra, left ilium, Th-2 spinous process, and right femoral neck were used for microdamage analysis. The L-4 vertebra and Th-1 spinous process were mechanically tested to failure in compression and shear, respectively. One year treatment with risedronate or alendronate significantly suppressed trabecular remodeling in vertebrae (RIS 90%, ALN 95%) and ilium (RIS 76%, ALN 90%) without impairment of mineralization, and significantly increased microdamage accumulation in all skeletal sites measured. Trabecular bone volume and vertebral strength increased significantly following 12 month treatment. However, normalized toughness of the L-4 vertebra was reduced by 21% in both RIS (p = 0.06) and ALN (p = 0.05) groups. When the two bisphosphonate groups were pooled in a post hoc fashion for analysis, this reduction in toughness reached statistical significance (p = 0.02). This study demonstrates that suppression of trabecular bone turnover by high doses of bisphosphonates is associated with increased vertebral strength, even though there is significant microdamage accumulation and a reduction in the intrinsic energy absorption capacity of trabecular bone.

Improved Bone Structure and Strength After Long‐Term Mechanical Loading Is Greatest if Loading Is Separated Into Short Bouts

Mechanical loading presents a potent osteogenic stimulus to bone cells, but bone cells desensitize rapidly to mechanical stimulation. Resensitization must occur before the cells can transduce future mechanical signals effectively. Previous experiments show that mechanical loading protocols are more osteogenic if the load cycles are divided into several discrete bouts, separated by several hours, than if the cycles are applied in a single uninterrupted bout. We investigated the effect of discrete mechanical loading bouts on structure and biomechanical properties of the rat ulna after 16 weeks of loading. The right ulnas of 26 adult female rats were subjected to 360 load cycles/day, delivered in a haversine waveform at 17 N peak force, 3 days/week for 16 weeks. One‐half of the animals (n = 13) were administered all 360 daily cycles in a single uninterrupted bout (360 × 1); the other half were administered 90 cycles four times per day (90 × 4), with 3 h between bouts. A nonloaded baseline control (BLC) group and an age‐matched control (AMC) group (n = 9/group) were included in the experiment. The following measurements were collected after death: in situ mechanical strain at the ulna midshaft; ulnar length; maximum and minimum second moments of area (IMAX and IMIN) along the entire length of the ulnas (1‐mm increments); and ultimate force, energy to failure, and stiffness of whole ulnas. Qualitative observations of bone morphology were made from whole bone images reconstructed from microcomputed tomography (μCT) slices. Loading according to the 360 × 1 and 90 × 4 schedules improved ultimate force by 64% and 87%, energy to failure by 94% and 165%, IMAX by 13% and 26% (in the middistal diaphysis), IMIN by 69% and 96% (in the middistal diaphysis), and reduced peak mechanical strain by 40% and 36%, respectively. The large increases in biomechanical properties occurred despite very low 5–12% gains in areal bone mineral density (aBMD) and bone mineral content (BMC). Mechanical loading is more effective in enhancing bone biomechanical and structural properties if the loads are applied in discrete bouts, separated by recovery periods (90 × 4 schedule), than if the loads are applied in a single session (360 × 1). Modest increases in aBMD and BMC can improve biomechanical properties substantially if the new bone formation is localized to the most biomechanically relevant sites, as occurs during load‐induced bone formation.

Biomechanics of Bone: Determinants of Skeletal Fragility and Bone Quality

Abstract: Bone fragility can be defined by biomechanical parameters, including ultimate force (a measure of strength), ultimate displacement (reciprocal of brittleness) and work to failure (energy absorption). Bone fragility is influenced by bone size, shape, architecture and tissue ‘quality’. Many osteoporosis treatments build bone mass but also change tissue quality. Antiresorptive therapies, such as bisphosphonates, substantially reduce bone turnover, impairing microdamage repair and causing increased bone mineralization, which can increase the brittleness of bone. Anabolic therapies, such as parathyroid hormone (PTH-(1–84)) or teriparatide (PTH-(1–34)), increase bone turnover and porosity, which offset some of the positive effects on bone strength. Osteoporosis therapies may also affect bone architecture by causing the redistribution of bone structure. Restructuring of bone during treatment may change bone fragility, even in the absence of drug effects on bone mineral density (BMD). This effect may explain why some drugs can affect fracture incidence disproportionately to changes in BMD. For instance, in a recent clinical trial, PTH-(1–34) therapy caused a dose-related increase in spinal BMD without any dose-dependent effect on the observed decrease in spinal fracture incidence. This apparent disassociation between spinal BMD and bone fragility is probably due to effects of PTH-(1–34) on bone architecture within vertebral bodies. While it has been shown that BMD is highly heritable, bone mineral distribution and architecture are also under strong genetic influence. Recent findings suggest that different genes regulate trabecular and cortical structures within lumbar vertebrae, producing a wide range of bone architectural designs. These findings suggest that there is no single optimal bone architecture; instead many different architectural solutions produce adequate bone strength.