Chapter 4. The Biomechanics of Human Bone Growth and Development

After completing this chapter, you will be able to:

• Explain how the material constituents and structural organization of bone affect its ability to withstand mechanical loads.
• Describe the processes involved in the normal growth and maturation of bone.
• Describe the effects of exercise and of weightlessness on bone mineralization.
• Explain the significance of osteoporosis and discuss current theories on its prevention.
• Explain the relationship between different forms of mechanical loading and common bone injuries.

What determines when a bone stops growing? How are stress fractures caused? Why does space travel cause reduced bone mineral density in astronauts? What is osteoporosis and how can it be prevented?

The word bone typically conjures up a mental image of a dead bone—a dry, brittle chunk of mineral that a dog would enjoy chewing. Given this picture, it is difficult to realize that living bone is an extremely dynamic tissue that is continually modeled and remodeled by the forces acting on it. Bone fulfills two important mechanical functions for human beings: (a) It provides a rigid skeletal framework that supports and protects other body tissues, and (b) it forms a system of rigid levers that can be moved by forces from the attaching muscles (see Chapter 12). This chapter discusses the biomechanical aspects of bone composition and structure, bone growth and development, bone response to stress, osteoporosis, and common bone injuries.

The material constituents and structural organization of bone influence the ways in which bone responds to mechanical loading. The composition and structure of bone yield a material that is strong for its relatively light weight.

Material Constituents

The major building blocks of bone are calcium carbonate, calcium phosphate, collagen, and water. The relative percentages of these materials vary with the age and health of the bone. Calcium carbonate and calcium phosphate generally constitute approximately 60–70% of dry bone weight. These minerals give bone its stiffness and are the primary determiners of its compressive strength. Other minerals, including magnesium, sodium, and fluoride, also have vital structural and metabolic roles in bone growth and development (62). Collagen is a protein that provides bone with flexibility and contributes to its tensile strength.

The water content of bone makes up approximately 25–30% of total bone weight. The water present in bone tissue is an important contributor to bone strength. For this reason, scientists and engineers studying the material properties of different types of bone tissue must ensure that the bone specimens they are testing do not become dehydrated. The flow of water through bones also carries nutrients to and waste products away from the living bone cells within the mineralized matrix. In addition, water transports mineral ions to and from bone for storage and subsequent use by the body tissues when needed (18).

Structural Organization

The relative percentage of bone mineralization varies not only with the age of the individual but also with the specific bone in the body. Some bones are more porous than others. The more porous the bone, the smaller the proportion of calcium phosphate and calcium carbonate, and the greater the proportion of nonmineralized tissue. Bone tissue has been classified into two categories based on porosity (Figure 4-1). If the porosity is low, with 5–30% of bone volume occupied by nonmineralized tissue, the tissue is termed cortical bone. Bone tissue with a relatively high porosity, with 30% to greater than 90% of bone volume occupied by nonmineralized tissue, is known as spongy, cancellous, or trabecular bone. Trabecular bone has a honeycomb structure with mineralized vertical and horizontal bars, called trabeculae, forming cells filled with marrow and fat.

The porosity of bone is of interest because it directly affects the mechanical characteristics of the tissue. With its higher mineral content, cortical bone is stiffer, so that it can withstand greater stress, but less strain or relative deformation, than trabecular bone. Because trabecular bone is spongier than cortical bone, it can undergo more strain before fracturing.

The function of a given bone determines its structure. The shafts of the long bones are composed of strong cortical bone. The relatively high trabecular bone content of the vertebrae contributes to their shock-absorbing capability. Trabecular bone develops four types of structure, depending on whether it must withstand relatively high or relatively low forces and whether the primary loading is axial (tension or compression) or asymmetric (bending) (31). Because of this, the strength and elasticity of trabecular bone vary considerably with location in the body, as well as with the age and health of the individual (47).

Both cortical and trabecular bone are anisotropic; that is, they exhibit different strength and stiffness in response to forces applied from different directions. Bone is strongest in resisting compressive stress and weakest in resisting shear stress (Figure 4-2).

Types of Bones

The structures and shapes of the 206 bones of the human body enable them to fulfill specific functions. The skeletal system is nominally subdivided into the central or axial skeleton and the peripheral or appendicular skeleton (Figure 4-3). The axial skeleton includes the bones that form the axis of the body, which are the skull, the vertebrae, the sternum, and the ribs. The other bones form the body appendages, or the appendicular skeleton. Bones are also categorized according to their general shapes and functions.

Short bones, which are approximately cubical, include only the carpals and the tarsals (Figure 4-4). These bones provide limited gliding motions and serve as shock absorbers.

Flat bones are also described by their name (Figure 4-4). These bones protect underlying organs and soft tissues, and also provide large areas for muscle and ligament attachments. The flat bones include the scapulae, sternum, ribs, patellae, and some of the bones of the skull.

Irregular bones have different shapes to fulfill special functions in the human body (Figure 4-4). For example, the vertebrae provide a bony, protective tunnel for the spinal cord; offer several processes for muscle and ligament attachments; and support the weight of the superior body parts while enabling movement of the trunk in all three cardinal planes. The sacrum, coccyx, and maxilla are other examples of irregular bones.

Long bones form the framework of the appendicular skeleton (Figure 4-4). They consist of a long, roughly cylindrical shaft (also called the body, or diaphysis) of cortical bone, with bulbous ends known as condyles, tubercles, or tuberosities. A self-lubricating articular cartilage protects the ends of long bones from wear at points of contact with other bones. Long bones also contain a central hollow area known as the medullary cavity or canal.

The long bones are adapted in size and weight for specific biomechanical functions. The tibia and femur are large and massive to support the weight of the body. The long bones of the upper extremity, including the humerus, radius, and ulna, are smaller and lighter to promote ease of movement. Other long bones include the clavicle, fibula, metatarsals, metacarpals, and phalanges.

Bone growth begins early in fetal development, and living bone is continually changing in composition and structure during the life span. Many of these changes represent normal growth and maturation of bone.

Longitudinal Growth

Longitudinal growth of a bone occurs at the epiphyses, or epiphyseal plates (Figure 4-5). The epiphyses are cartilaginous discs found near the ends of the long bones. The diaphysis (central) side of each epiphysis continually produces new bone cells. During or shortly following adolescence, the plate disappears and the bone fuses, terminating longitudinal growth. Most epiphyses close around age 18, although some may be present until about age 25.

Circumferential Growth

Long bones grow in diameter throughout most of the life span, although the most rapid bone growth occurs before adulthood. The internal layer of the periosteum builds concentric layers of new bone tissue on top of existing ones. At the same time, bone is resorbed or eliminated around the circumference of the medullary cavity, so that the diameter of the cavity is continually enlarged. This occurs in such a way that both bending stresses and torsional stresses on the bones remain relatively constant (91).

These changes in bone size and shape are the work of specialized cells called osteoblasts and osteoclasts, which respectively form and resorb bone tissue. In healthy adult bone, the activity of osteoblasts and osteoclasts is largely balanced.

Adult Bone Development

There is a progressive loss of collagen and increase in bone brittleness with aging. Thus, the bones of children are more pliable than the bones of adults.

Bone mineral normally accumulates throughout childhood and adolescence, reaching a peak at about age 25–28 in women and age 30–35 in men (80). Following this peak, researchers disagree as to the length of time that bone density remains constant (88). However, an age-related, progressive decline in bone density and bone strength in both men and women may begin as soon as the early twenties (7, 68). This involves a progressive diminishment in the mechanical properties and general toughness of bone, with increasing loss of bone substance and increasing porosity (23). Trabecular bone is particularly affected, with progressive disconnection and disintegration of trabeculae compromising the integrity of the bone’s structure and seriously diminishing bone strength (60).

These changes are much more pronounced in women than in men, however. In women, there is a notable decrease in both volume and density of cortical bone, and a decrease in the density of trabecular bone with aging (94). Approximately 0.5–1.0% of bone mass is lost each year, until women reach about age 50 or menopause (7, 88). Following menopause, there appears to be an increased rate of bone loss, with values as high as 6.5% per year reported during the first five to eight years (52). Although similar changes occur in men, they do not become significant before a more advanced age. Women at all ages tend to have smaller bones and less cortical bone area than do men (91), although volumetric bone mineral density is similar for both genders (69).

Other changes that occur in living bone throughout the life span are unrelated to normal growth and development. Bone responds dynamically to the presence or absence of different forces with changes in size, shape, and density. This phenomenon was originally described by the German scientist Julius Wolff in 1892:

The form of a bone being given, the bone elements place or displace themselves in the direction of functional forces and increase or decrease their mass to reflect the amount of the functional forces (112).

Bone Modeling and Remodeling

According to Wolff’s law, the densities and, to a much lesser extent, the shapes and sizes of the bones of a given human being are a function of the magnitude and direction of the mechanical stresses that act on the bones. Dynamic mechanical loading causes bones to deform or strain, with larger loads producing higher levels of strain. These strains are translated into changes in bone shape and strength through a process known as remodeling. Remodeling involves resorption of fatigue-damaged older bone and subsequent formation of new bone (71). Bone modeling is the term given to formation of new bone that is not preceded by resorption, and is the process by which immature bones grow.

Adult bones gain or lose mass in accordance with Wolff’s law. When strain on a bone exceeds a certain threshold, new bone is laid down at the strain sites, and overall bone mass and density are increased. When strain magnitudes stay below a lower threshold, bone remodeling occurs, with bone removed close to the marrow (28). Strain magnitudes in between these two thresholds occur in what is termed the “lazy zone” and do not trigger bone adaptation (104). Remodeling can occur in either “conservation mode,” with no change in bone mass, or “disuse mode,” with a net loss of bone mass characterized by an enlarged marrow cavity and thinned cortex (28). Bone is a very dynamic tissue, with the modeling and remodeling processes continuously acting to increase, decrease, or reshape bone.

The modeling and remodeling processes are directed by osteocytes, cells embedded in bone that are sensitive to changes in the flow of interstitial fluid through the pores resulting from strain on the bone (11, 14, 95). Dynamic loading resulting from high-level impact produces a high rate of deformation that best pushes fluid through the bone matrix (87). It is for this reason that activities involving high levels of impact are best at stimulating bone formation. In response to the motion of fluid within the bone matrix, osteocytes trigger the actions of osteoblasts and osteoclasts, the cells that respectively form and resorb bone (87). A predominance of osteoblast activity produces bone modeling, with a net gain in bone mass. Bone remodeling involves a balance of osteoblast and osteoclast action or a predominance of osteoclast activity, with associated maintenance or loss of bone mass. Approximately 25% of the body’s trabecular bone is remodeled each year through this process (42). Strains resulting from an activity such as walking are sufficient to provoke bone turnover and new bone formation (14, 113).

Thus, bone mineralization and bone strength in both children and adults are a function of stresses producing strains on the skeleton. Since body weight provides the most constant mechanical stress to bones, bone mineral density generally parallels body weight, with heavier individuals having more massive bones. Adults who gain or lose weight tend to also gain or lose bone mineral density (25, 84). However, a given individual’s physical activity profile, diet, lifestyle, and genetics can also dramatically influence bone density. Factors such as lean body mass, muscle strength, and regular participation in weight-bearing exercise have been shown to exert stronger influences on bone density than weight, height, and race (33, 58, 93, 103). Even in young, nonathletic children, bone appears to remodel in response to the presence or absence of physical activity (46).

The malleability of bone is dramatically exemplified by the case of an infant who was born in normal physical condition but missing one tibia, the major weight-bearing bone of the lower extremity. After the child was walking for a time, X rays revealed that modeling of the fibula in the abnormal leg had occurred to the extent that it could not be distinguished from the tibia of the other leg (2).

Another interesting case is that of a construction worker who had lost all but the fifth finger of one hand in a war injury. After 32 years, the metacarpal and phalanx of the remaining finger had been modeled to resemble the third finger of the other hand (82).

Bone Hypertrophy

Although cases of complete changes in bone shape and size are unusual, there are many examples of bone modeling, or bone hypertrophy, in response to regular physical activity. The bones of physically active individuals tend to be denser and therefore more mineralized than those of sedentary individuals of the same age and gender. Moreover, the results of several studies indicate that occupations and sports particularly stressing a certain limb or region of the body produce accentuated bone hypertrophy in the stressed area. For example, professional tennis players display not only muscular hypertrophy in the tennis arm but also hypertrophy of that arm’s radius (75). Similar bone hypertrophy has been observed in the arms of baseball pitchers and racquetball players (51, 108).

It also appears that the greater the forces or loads habitually encountered, the more dramatic the increased mineralization of the bone. In one interesting study, researchers measured the density of the femur among 64 nationally ranked athletes representing different sports (70). The femurs displaying the greatest density were those of the weight lifters, followed by throwers, runners, soccer players, and swimmers. In a similar study involving collegiate female athletes in basketball, volleyball, soccer, track, and swimming, the athletes participating in high-impact sports (basketball and volleyball) were found to have higher bone mineral densities and bone formation values than the swimmers (19). In another investigation, the bone mineral densities of trained runners and cyclists were compared to those of sedentary individuals of the same age (92). Compared to the non-exercisers, the runners were found to have increased bone density, although this was not true for the cyclists. Among older women, both yard work and weight training have been found to be strong predictors for bone density, with jogging, swimming, and calisthenics being weak predictors (76). On the whole, the research evidence suggests that physical activity involving impact forces is necessary for maintaining or increasing bone mass (74). Competitive swimmers, who spend a lot of time in the water where the buoyant force counteracts gravity, may have bone mineral densities lower than those of sedentary individuals (81).

Bone Atrophy

Whereas bone hypertrophies in response to increased mechanical stress, it displays the opposite response to reduced stress. When the normal stresses exerted on bone by muscle contractions, weight bearing, or impactforces are reduced, bone tissue atrophies through remodeling. When bone atrophy occurs, the amount of calcium contained in the bone diminishes, and both the weight and the strength of the bone decrease. Loss of bone mass due to reduced mechanical stress has been found in bedridden patients, sedentary senior citizens, and astronauts. Four to six weeks of bed rest can result in significant decrements in bone mineral density that are not fully reversed after six months of normal weight-bearing activity (8).

Bone demineralization is a potentially serious problem. From a biomechanical standpoint, as bone mass diminishes, strength and thus resistance to fracture also decrease, particularly in trabecular bone.

The results of calcium loss studies conducted during the Skylab flights indicate that urinary calcium loss is related to time spent out of the earth’s gravitational field. The pattern of bone loss observed is highly similar to that documented among patients during periods of bed rest, with greater bone loss in the weight-bearing bones of the lumbar spine and lower extremity than in other parts of the skeleton (55, 66). During one month in space, astronauts lose 1–3% of bone mass, or approximately as much bone mass as postmenopausal women lose in a year (13, 37).

It is not yet clear what specific mechanism or mechanisms are responsible for bone loss outside of the gravitational field. Research has consistently documented a negative calcium balance in astronauts and experimental animals during space flight, with reduced intestinal absorption of calcium and increased excretion of calcium (99). It is not known, however, whether this is caused by an increase in bone remodeling, a decrease in bone remodeling, or an imbalance between osteoblast and osteoclast activity (99). It appears that the normal balance between formation and resorption of bone becomes disturbed, with an initial increase in osteoclast activity followed by a prolonged decrease in osteoblast activity (59). One hypothesis is that these changes in bone remodeling are precipitated by changes in bone blood flow related to being outside of the gravitational field (16). More research on this topic is clearly needed.

It remains to be seen if measures other than the artificial creation of gravity can effectively prevent bone loss during space travel. Astronauts’ current exercise programs during flights in space are designed to prevent bone loss by increasing the mechanical stress and strain placed on bones using muscular force. However, the muscles of the body exert mainly tensile forces on bone, whereas gravity provides a compressive force. Therefore, it may be that no amount of physical exercise alone can completely compensate for the absence of gravitational force.

In the future, astronauts visiting other planets might maintain bone mass by carrying extra loads while walking on the planet surface. Researchers have calculated that individuals in average physical condition could walk for eight hours on the moon while carrying as much as 170% of body mass and on Mars while carrying 50% of body mass without undue fatigue (109). According to scientific projections, however, the daily walking times on these planets necessary for maintaining bone mass are unrealistically long (109).

Another important related question is to what extent bone mass can be recovered when astronauts reenter the earth’s gravitational field. Research shows that some recovery of bone mass can be expected, but that the rate and extent of recovery will vary with the individual and with the location of the bone (54, 55). Bones that are directly involved in weight bearing during ambulation can be expected to recover most quickly; one study of subjects who underwent 17 weeks of continuous bed rest showed an almost complete reversal of a 10.4% loss of bone density in the calcaneus after six months (55). An example of loading on a weight-bearing bone is shown in Sample Problem 4.1.

Sample Problem 4.1

The tibia is the major weight-bearing bone in the lower extremity. If 88% of body mass is proximal to the knee joint, how much compressive force acts on each tibia when a 600 N person stands in anatomical position? How much compressive force acts on each tibia if the person holds a 20 N sack of groceries?

Solution

Given: wt = 600 N

(It may be deduced that weight = compressive force, Fc.)

Formula: Fc on knees = (600 N) (0.88)

Bone atrophy is a problem not only for astronauts and bedridden patients but also for a growing number of senior citizens and female athletes. Osteoporosis is found in most elderly individuals, with earlier onset in women, and is becoming increasingly prevalent with the increasing mean age of the population. The condition begins as osteopenia, reduced bone mass without the presence of a fracture, but often progresses to osteoporosis, a condition in which bone mineral mass and strength are so severely compromised that daily activities can cause bone pain and fracturing (28).

Postmenopausal and Age-Associated Osteoporosis

The majority of those affected by osteoporosis are postmenopausal and elderly women, although elderly men are also susceptible, with more than half of all women and about one-third of men developing fractures related to osteoporosis (83). Although it was once regarded as primarily a health concern for women, with the increasing age of the population, osteoporosis is now also emerging as a serious health-related concern for men (24, 45, 48). Risk factors for osteoporosis include being female, white or Asian ethnicity, older age, small stature or frame size, and family history of osteoporosis (71).

Type I osteoporosis, or postmenopausal osteoporosis, affects approximately 40% of women after age 50 (56). The first osteoporotic fractures usually begin to occur about 15 years postmenopause, with women suffering approximately three times as many femoral neck fractures, three times as many vertebral fractures, and six times as many wrist fractures as men of the same age (56).

This discrepancy occurs partially because men reach a higher peak of bone mass and strength than women in early adulthood, and partially because of a greater prevalence of disconnections in the trabecular network among postmenopausal women than among men (67).

Type II osteoporosis, or age-associated osteoporosis, affects most women and also affects men after age 70 (49). After age 60, about 90% of all fractures in both men and women are osteoporosis-related, and these fractures are one of the leading causes of death in the elderly population (79).

Although the radius and ulna, femoral neck, and spine are all common sites of osteoporotic fractures, the most common symptom of osteoporosis is back pain derived from fractures of the weakened trabecular bone of the vertebral bodies. Crush fractures of the lumbar vertebrae resulting from compressive loads created by weight bearing during activities of daily living frequently cause reduction of body height. Because most body weight is anterior to the spine, the resulting fractures often leave the vertebral bodies wedge-shaped, accentuating thoracic kyphosis (see Chapter 9). This disabling deformity is known as dowager’s hump. Vertebral compression fractures are extremely painful and debilitating, and affect physical, functional, and psychosocial aspects of the person’s life (83). As spinal height is lost, there is added discomfort from the rib cage pressing on the pelvis. An estimated 26% of women age 50 and over have vertebral compression fractures (81). Alarmingly, the incidence of these vertebral fractures has increased three- to fourfold in women and more than fourfold in men over the past 30 years, even when figures are adjusted for age (67). This underscores the increasing prevalence of lifestyle factors that negatively affect bone health.

As the skeleton ages in men, there is an increase in vertebral diameter that serves to reduce compressive stress during weight bearing (67). Thus, although osteoporotic changes may be taking place, the structural strength of the vertebrae is not reduced. Why the same compensatory change does not occur in women is unknown.

Female Athlete Triad

The desire to excel at competitive sports causes some young female athletes to strive to achieve an undesirably low body weight. This dangerous practice commonly involves a combination of disordered eating, amenorrhea, and osteoporosis, a combination that has come to be known as the “female athlete triad” (86). This condition often goes unrecognized, but because the triad can result in negative consequences ranging from irreversible bone loss to death, friends, parents, coaches, and physicians need to be alert to the signs and symptoms (39).

As many as 62% of female athletes in certain sports display disordered eating behaviors, with those participating in endurance or artistic sports such as gymnastics and figure skating most likely to be involved (86). Prolonged disordered eating can lead to anorexia nervosa or bulimia nervosa, illnesses that affect 1–10% of all adolescent and college-age women (34). Symptoms of anorexia nervosa in girls and women include body weight 15% or more below minimal normal weight for age and height, an intense fear of gaining weight, a disturbed body image, and amenorrhea. Symptoms of bulimia nervosa are a minimum of two eating binges a week for at least three months, a feeling of lack of control during binges, regular use of self-induced vomiting, laxatives, diuretics, strict dieting, or exercise to prevent weight gain, and excessive concern with body image and weight (34). Disordered eating behavior has been found to be strongly associated with both menstrual irregularity and low bone mineral density (15).

The relationship between disordered eating and amenorrhea appears to be related to a decrease in hypothalamus secretion of gonadotrophin-releasing hormone, which in turn decreases the secretion of luteinizing hormone and follicle-stimulating hormone, with subsequent shutting down of stimulation of the ovary (106). A reported 2–5% of premenopausal women in the United States have amenorrhea, but the prevalence in female athletes is higher. Studies of competitive women athletes in different sports indicate that 3.4–66% have either primary amenorrhea, with menarche delayed beyond 16 years of age, or secondary amenorrhea, the absence of three to six consecutive menstrual cycles (86).

The link between cessation of menses and osteoporosis is a low level of endogenous estrogen (7). Energy deficiency resulting from disordered eating is also likely to independently contribute to altered bone metabolism and reduced bone density (22). Although the incidence of osteoporosis among female athletes is unknown, the consequences of this disorder in young women are potentially tragic. Among one group of over 200 premenopausal female runners, those with amenorrhea had 10% less lumbar bone density than those with normal menses (38). In other research, young women with anorexia nervosa were found to have marked trabecular and cortical bone loss on the order of 4–10% per year (65). This pattern is of particular concern for adolescent athletes, because roughly 50% of bone mineralization and 15% of adult height are normally established during the teenage years (4, 107). Not surprisingly, amenorrheic premenopausal female athletes have a high rate of stress fractures, with more fractures related to later onset of menarche (107). Moreover, the loss of bone that occurs may be irreversible, and osteoporotic wedge fractures can ruin posture for life (86).

Preventing and Treating Osteoporosis

Osteoporosis is neither a disease with acute onset nor an inevitable accompaniment of aging, but is the result of a lifetime of habits that are erosive to the skeletal system (10). Early detection of low bone mineral density is advantageous, because once osteoporotic fractures begin to occur, there has been irreversible loss of trabecular structure (83). Although proper diet, hormone levels, and exercise can work to increase bone mass at any stage in life, evidence suggests that it is easier to prevent osteoporosis than it is to treat it. The single most important factor for preventing or prolonging the onset of osteoporosis is the optimization of peak bone mass during childhood and adolescence (6, 9, 20, 36, 50, 100). Researchers hypothesize that weight-bearing exercise is particularly crucial during the prepubertal years, because the presence of high levels of growth hormone may act with exercise in a synergistic fashion to increase bone density (6, 36, 50, 100). Activities involving osteogenic impact forces, such as jumping, have been shown to be effective in increasing bone mass in children (29).

Weight-bearing physical activity is necessary for maintaining skeletal integrity in both humans and animals. Importantly, studies show that a regular program of weight-bearing exercise, such as walking, can increase bone health and strength even among individuals with osteoporosis (5).

The American College of Sports Medicine pronouncement on “Physical Activity and Bone Health” (53) makes recommendations relative to the role of exercise in preventing and treating osteoporosis. Children and adolescents should engage in impact activities such as gymnastics, plyometrics, jumping, and moderate-intensity resistance training (< 60% of 1 RM), with other sports that involve running and jumping likely to also be beneficial. The recommended frequency and duration is at least three times per week for at least 10–20 minutes, with two or more sessions per day possibly being more effective. In order to maintain bone mass, adults should participate in weight-bearing endurance activities such as tennis, stair climbing, and jogging; activities that involve jumping, such as volleyball and basketball; and resistance exercise. Intensity should be moderate to high in terms of bone-loading forces, and weight-bearing endurance activities should be done 3–5 times per week and resistance exercise 2–3 times per week. Jumping in place, with 50–100 jumps done in sets of 10 with a frequency of 3–5 times per week, is also recommended for maintenance of bone mass (110).

Research on animals, however, has shown that the positive effect of jumping on bone formation saturates after about 40 jumps, with 100 jumps resulting in no greater stimulus for bone formation than 40 (101). A related recent research finding is that the insertion of a rest interval between impacts greatly stimulates bone formation, possibly by enhancing the flow of fluid within the bone matrix and the related stimulation of osteocytes (35). In fact, it has been shown that the insertion of these “rest” periods between impacts reduces the effects of impact saturation and can double the effects of mechanical loading on bone building (76). In practical terms, incorporating a 10- to 15-second pause between hops into the childhood game of hopscotch could be much more effective in building skeletal mass than hopping without the pause.

Although regular physical exercise promotes bone mineralization and bone health, it is clear that hormonal factors also exert a powerful influence, given the prevalence of osteoporosis in amenorrheic female athletes. High bone mineral density has been associated with early menarche, late menopause, and a long reproductive period, all of which support the effectiveness of estrogen in maintaining bone (30). Conversely, low levels of estrogen in females and low levels of testosterone in males promote bone loss in both children and adults. Estrogen deficiency potentially damages bone by diminishing the efficiency with which calcium is absorbed in the body and facilitating the resorption of bone by osteoclasts (88). Bone is also more responsive to mechanical loading when the number of estrogen receptors in bone cells is high, as occurs during adolescence, and less responsive when the number is low, as occurs during amenorrhea or postmenopausally (57). Estrogen replacement therapy acts to maintain bone density by reducing bone turnover, with high doses actually stimulating osteoblast activity and formation of new bone (105). Because of its effectiveness and relatively low cost, estrogen replacement therapy is considered the first-line pharmacologic therapy for preventing osteoporosis in postmenopausal and amenorrheic women (1). Although the mechanism by which testosterone influences bone mass is not understood, testosterone replacement therapy also stimulates bone building (88).

Increased dietary calcium intake exerts a positive influence on bone mass for women with a dietary deficiency, with the amount of calcium absorbed influenced positively by calcitriol (the active form of vitamin D) and negatively by dietary fiber (76, 90). Although adequate dietary calcium is particularly important during the teenage years, unfortunately the median American girl falls below the recommended daily intake of 1200 mg per day by age 11 (20). A modified diet or calcium supplementation can be critical for the development of peak bone mass among adolescent females at a dietary deficiency. The role of vitamin D in enabling absorption of calcium by bone is also important, with over half the women receiving treatment for low bone density in North America having a vitamin D deficiency (41). Clinicians are now recognizing that a predisposition for osteoporosis can begin in childhood and adolescence when a poor diet interferes with bone mass development (12).

Other lifestyle factors also affect bone mineralization. Known risk factors for developing osteoporosis include physical inactivity; weight loss or excessive thinness; tobacco smoking; deficiencies in estrogen, calcium, and vitamin D; and excessive consumption of protein and caffeine (21, 78, 89, 102, 111). A study of female twins, one of whom smoked more heavily than the other, showed that women who smoke one pack of cigarettes a day through adulthood will have a reduction in bone density of 5–10% by the time of menopause, which is sufficient to increase the risk of fracture (40). Although caffeine consumption may negatively affect bone mineral density among postmenopausal women who consume low amounts of dietary calcium, it has been shown not to affect bone mineral density among young women (17). Genetic factors also influence bone mass, but do not appear to be as important as diet and exercise (26).

Research indicates that in the future the use of pharmacologic agents that stimulate bone formation may enable the prevention and eventually the elimination of osteoporosis (3, 73). Recent clinical trials suggest that low doses of growth factors can stimulate osteoblast recruitment and promote bone formation, and researchers have begun to identify genetic markers for assessment of fracture risk (64, 77). However, until much more is understood about osteoporosis, young women in particular are encouraged to maximize peak bone mass and to minimize its loss by engaging in regular physical activity and avoiding the lifestyle factors that negatively affect bone health.

Because of the important mechanical functions performed by bone, bone health is an important part of general health. Bone health can be impaired by injuries and pathologies.

Fractures

A fracture is a disruption in the continuity of a bone. The nature of a fracture depends on the direction, magnitude, loading rate, and duration of the mechanical load sustained, as well as the health and maturity of the bone at the time of injury. Fractures are classified as simple when the bone ends remain within the surrounding soft tissues and compound when one or both bone ends protrude from the skin. When the loading rate is rapid, a fracture is more likely to be comminuted, containing multiple fragments (Figure 4-6).

Avulsions are fractures caused by tensile loading in which a tendon or ligament pulls a small chip of bone away from the rest of the bone. Explosive throwing and jumping movements may result in avulsion fractures of the medial epicondyle of the humerus and the calcaneus.

Excessive bending and torsional loads can produce spiral fractures of the long bones (Figure 4-6). The simultaneous application of forces from opposite directions at different points along a structure such as a long bone generates a torque known as a bending moment, which can cause bending and ultimately fracture of the bone. A bending moment is created on a football player’s leg when the foot is anchored to the ground and tacklers apply forces at different points on the leg in opposite directions. When bending is present, the structure is loaded in tension on one side and in compression on the opposite side, as discussed in Chapter 3. Because bone is stronger in resisting compression than in resisting tension, the side of the bone loaded in tension will fracture first.

Torque applied about the long axis of a structure such as a long bone causes torsion, or twisting of the structure. Torsion creates shearstress throughout the structure, as explained in Chapter 3. When a skier’s body rotates with respect to one boot and ski during a fall, torsional loads can cause a spiral fracture of the tibia. In such cases, a combined loading pattern of shear and tension produces failure at an oblique orientation to the longitudinal axis of the bone (72).

Since bone is stronger in resisting compression than in resisting tension and shear, acute compression fractures of bone (in the absence of osteoporosis) are rare. However, under combined loading, a fracture resulting from a torsional load may also be impacted by the presence of a compressive load. An impacted fracture is one in which the opposite sides of the fracture are compressed together. Fractures that result in depression of bone fragments into the underlying tissues are termed depressed.

Since the bones of children contain relatively larger amounts of collagen than do adult bones, they are more flexible and more resistant to fracture under day-to-day loading than are adult bones. Consequently, greenstick fractures, or incomplete fractures, are more common in children than in adults (Figure 4-6). A greenstick fracture is an incomplete fracture caused by bending or torsional loads.

Stress fractures, also known as fatigue fractures, result from low-magnitude forces sustained on a repeated basis. Any increase in the magnitude or frequency of bone loading produces a stress reaction, which may involve microdamage (32). Bone responds to microdamage by remodeling: First, osteoclasts resorb the damaged tissue; then, osteoblasts deposit new bone at the site. When there is not time for the repair process to complete itself before additional microdamage occurs, the condition can progress to a stress fracture. Stress fractures begin as a small disruption in the continuity of the outer layers of cortical bone, but can worsen over time, eventually resulting in complete cortical fracture.

In runners, a group particularly prone to stress fractures, about 50% of fractures occur in the tibia and approximately 20% of fractures are in the metatarsals, with fractures of the femoral neck and pubis also reported (44, 64). Increases in training duration or intensity that do not allow enough time for bone remodeling to occur are the primary culprits. Other factors that predispose runners to stress fractures include muscular fatigue and abrupt changes in either the running surface or the running direction (64).

Epiphyseal Injuries

About 10% of acute skeletal injuries in children and adolescents involve the epiphysis (61). Epiphyseal injuries include injuries to the cartilaginous epiphyseal plate, the articular cartilage, and the apophysis. The apophyses are the sites of tendon attachments to bone, where bone shape is influenced by the tensile loads to which these sites are subjected. The epiphyses of long bones are termed pressure epiphyses and the apophyses are called traction epiphyses, after the types of physiological loading present. Both acute and repetitive loading can injure the growth plate, potentially resulting in premature closure of the epiphyseal junction and termination of bone growth.

Another form of epiphyseal injury, osteochondrosis, involves disruption of blood supply to an epiphysis, with associated tissue necrosis and potential deformation of the epiphysis. The cause of the condition is poorly understood. Osteochondrosis occurs most commonly between the ages of 3 and 10 and is more prevalent among boys than among girls (65).

Osteochondrosis of an apophysis, known as apophysitis, is often associated with traumatic avulsions. Common sites for apophysitis are the calcaneus and the tibial tubercle at the site of the patellar tendon attachment, where the disorder is referred to respectively as Sever’s disease and Osgood-Schlatter’s disease.

Bone is an important and dynamic living tissue. Its mechanical functions are to support and protect other body tissues and to act as a system of rigid levers that can be manipulated by the attached muscles.

Bone’s strength and resistance to fracture depend on its material composition and organizational structure. Minerals contribute to a bone’s hardness and compressive strength, and collagen provides its flexibility and tensile strength. Cortical bone is stiffer and stronger than trabecular bone, whereas trabecular bone has greater shock-absorbing capabilities.

Bone is an extremely dynamic tissue that is continually being modeled and remodeled in accordance with Wolff’s law. Although bones grow in length only until the epiphyseal plates close at adolescence, bones continually change in density, and to some extent in size and shape, through the actions of osteoblasts and osteoclasts.

Osteoporosis, a disorder characterized by excessive loss of bone mineral mass and strength, is extremely prevalent among the elderly. It affects women at an earlier age and more severely than men. It is also present in an alarming frequency among young, eating-disordered, amenorrheic female athletes. Although the cause of osteoporosis remains unknown, the condition can often be improved through hormone therapy, avoidance of negative lifestyle factors, and a regular exercise program.

1. 1. Explain why the bones of the human body are stronger in resisting compression than in resisting tension and shear.

2. 2. In the human femur, bone tissue is strongest in resisting compressive force, approximately half as strong in resisting tensile force, and only about one-fifth as strong in resisting shear force. If a tensile force of 8000 N is sufficient to produce a fracture, how much compressive force will produce a fracture? How much shear force will produce a fracture? (Answer: compressive force = 16,000 N; shear force = 3200 N)

3. 3. Explain why bone density is related to an individual’s body weight.

4. 4. Rank the following activities according to their effect on increasing bone density: running, backpacking, swimming, cycling, weight lifting, polo, tennis. Write a paragraph providing the rationale for your ranking.

5. 5. Why is bone tissue organized differently (cortical versus trabecular bone)?

6. 6. What kinds of fractures are produced by compression, tension, and shear, respectively?

7. 7. What kinds of fractures are produced only by combined loading? (Identify fracture types with their associated causative loadings.)

8. 8. Approximately 56% of body weight is supported by the fifth lumbar vertebra. How much stress is present on the 22 cm2 surface area of that vertebra in an erect 756 N man? (Assume that the vertebral surface is horizontal.) (Answer: 19.2 N/cm)

9. 9. In Problem 8, how much total stress is present on the fifth lumbar vertebra if the individual holds a 222 N weight bar balanced across his shoulders? (Answer: 29.3 N/cm)

10. 10. Why are men less prone than women to vertebral compression fractures?

1. 1. Hypothesize about the way or ways in which each of the following bones is loaded when a person stands in anatomical position. Be as specific as possible in identifying which parts of the bones are loaded.

   • a. Femur

   • b. Tibia

   • c. Scapula

   • d. Humerus

   • e. Third lumbar vertebra

2. 2. Outline a six-week exercise program that might be used with a group of osteoporotic elderly persons who are ambulatory.

3. 3. Speculate about what exercises or other strategies may be employed in outer space to prevent the loss of bone mineral density in humans.

4. 4. Hypothesize as to the ability of bone to resist compression, tension, and shear, compared to the same properties in wood, steel, and plastic.

5. 5. How are the bones of birds and fish adapted for their methods of transportation?

6. 6. Why is it important to correct eating disorders?

7. 7. Hypothesize as to why men are less prone to osteoporosis than are women.

8. 8. When an impactforce is absorbed by the foot, the soft tissues at the joints act to lessen the amount of force transmitted upward through the skeletal system. If a ground reaction force of 1875 N is reduced 15% by the tissues of the ankle joint and 45% by the tissues of the knee joint, how much force is transmitted to the femur? (Answer: 750 N)

9. 9. How much compression is exerted on the radius at the elbow joint when the biceps brachii, oriented at a 30˚ angle to the radius, exerts a tensile force of 200 N? (Answer: 173 N)

10. 10. If the anterior and posterior deltoids both insert at an angle of 60˚ on the humerus and each muscle produces a force of 100 N, how much force is acting perpendicular to the humerus? (Answer: 173.2 N)

• Collagen resists tension and provides flexibility to bone.

• Because cortical bone is stiffer than trabecular bone, it can withstand greater stress but less strain.

• Bone is strongest in resisting compression and weakest in resisting shear.

• Most epiphyses close around age 18, although some may be present until about age 25.

• Wolff’s law indicates that bone strength increases and decreases as the functional forces on the bone increase and decrease.

• The processes causing bone remodeling are not fully understood and continue to be researched by scientists.

• Osteoporosis is a serious health problem for most elderly individuals, with women affected more dramatically than men.

• Painful, deforming, and debilitating crush fractures of the vertebrae are the most common symptom of osteoporosis.

• Disordered eating, amenorrhea, and osteoporosis constitute a dangerous and potentially lethal triad for young female athletes.

• Anorexia nervosa and bulimia nervosa are life-threatening eating disorders.

• Regular exercise has been shown to be effective to some extent in mediating age-related bone loss.

• Estrogen and testosterone deficiencies promote the development of osteoporosis.

• Under excessive bending loads, bone tends to fracture on the side loaded in tension.

• Injuries of an epiphyseal plate can terminate bone growth early.

Cowin SC: Bone mechanics handbook (2nd ed), Boca Raton, FL, 2001, CRC Press.

Summarizes current understanding of the mechanical properties of bone, as well as clinically related issues such as bone prostheses, implants, and imaging of bone structure.

Currey JD: Bones: structure and mechanics, Princeton, NJ, 2002, Princeton University Press.

Reviews bone structure and mechanics, with numerous insights on form following function.

Frost HM: From Wolff’s law to the Utah paradigm: insights about bone physiology and its clinical applications, Anat Rec 262:398, 2001.

Reviews current thinking about the biology of bone quality and related clinical applications.

Turner RT: Physiology of a microgravity environment invited review: what do we know about the effects of spaceflight on bone? J Appl Physiol 89:840, 2000.

Reviews the scientific literature on the effects of spaceflight on bone.

American Academy of Orthopaedic Surgeons

• www.aaos.org/
• Includes information for the public, as well as for orthopods, with links to current news, a library, research, and a quiz with flashcard photos of orthopedic conditions.

American Orthopaedic Society for Sports Medicine

• www.sportsmed.org
• Contains resources related to orthopedic sports medicine education and research, including selected abstracts of presentations from the most recent annual AOSSM meeting.

Clinical Orthopaedics and Related Research

• http://www.clinorthop.org/index.html
• The online version of Clinical Orthopaedics and Related Research; contains peer-reviewed, original articles on general orthopedics, specialty topics, the basic sciences, and pathology reports on the latest advances in research and practice.

duPont Hospital for Children

• http://gait.aidi.udel.edu/res695/homepage/pd_ortho/educate/clincase/clcasehp.htm
• Contains numerous clinical case presentations.

Medscape Orthopaedics

• www.medscape.com/orthopaedicshome
• Includes descriptions of medical conferences, news, a bookstore, specialty pages, and Medline access.

National Institutes of Health Osteoporosis and Related Bone Diseases, National Resource Center

• www.osteo.org/
• Includes fact sheets, research, newsletters, the Surgeon General’s Report, bone links, and a search engine for bone-related resources.

Orthopaedic Research Laboratories

• http://orl-inc.com/
• Provides links to orthopedic biomechanics publications and presentations.

Osteoporosis and Bone Physiology

• http://courses.washington.edu/bonephys/
• Provides information on osteoporosis prevention and treatment, bone density, secondary osteomalacia, cases, images, FAQs, and advice.

Wheeless’ Textbook of Orthopaedics

• www.wheelessonline.com/
• A comprehensive online medical textbook; contains 11,000 pages with more than 5000 images, with each topic fully searchable by alphabetical, anatomical, and keyword searches; updated daily.

World Ortho

• www.worldortho.com
• Includes links to over 3000 pictures with accompanying text, quizzes, and online books, directed at students, junior doctors, paramedics, and other health professionals.