This is an excerpt from Structure and Function of the Musculoskeletal System eBook-2nd Edition by James Watkins.
Porosity, Osteopenia, and Osteoporosis
Because of the channels and spaces within compact and cancellous bone, any particular region of a bone consists of certain amounts of bone tissue and nonbone tissue. The term porosity describes the proportion of nonbone tissue. At skeletal maturity the porosity of compact and cancellous bone is approximately 2% and 50%, respectively; the density (amount of bone tissue per unit volume) of compact bone is approximately double that of cancellous bone (Radin 1984, Tortora 2004). The density of bone tissue depends on the degree of mineralization. During ossification, the degree of mineralization of bone tissue gradually increases and reaches a maximum level at skeletal maturity (Bailey et al 1986). However, the amount of bone within the skeleton may continue to increase for 5 to 10 years after skeletal maturity, especially in people who are physically active (Stillman et al 1986, Talmage and Anderson 1984, Frost 2003). Consequently, bone mass peaks in males and females between 25 and 30 years of age. In terms of turnover, this means that from skeletal maturity to the age at which peak bone mass occurs, more new bone is formed than old and damaged bone is absorbed.
Following peak bone mass there is usually a stable period in which the amount of bone in the skeleton remains about the same; there is a balance between bone absorption and bone formation. This stable period is followed by a gradual decrease in bone mass for the rest of the person’s life; the rate of bone absorption exceeds the rate of bone formation. Bone mass is the product of bone volume and bone density. The loss in bone mass that occurs with age following peak bone mass is the result of decreases in bone volume and bone density. Osteopenia refers to a level of bone density below the normal level for a person’s age and sex (Bailey 1995, Frost 1997).
Bone mass starts to decrease earlier and at a greater rate in females than in males. In males, bone loss normally starts between 45 and 50 years of age and proceeds at a rate of 0.4% to 0.75% per year (Bailey et al 1986, Smith 1982). In females, bone loss has three phases. The first phase starts around 30 to 35 years of age and proceeds at a rate of 0.75% to 1% per year until menopause. From menopause until about 5 years after menopause, the rate of bone loss increases to between 2% and 3% per year. During the final phase, the rate of bone loss is approximately 1% per year. Thus, women may lose, on average, about 53% of their peak bone mass by the age of 80 years. In contrast, males may lose, on average, about 18% of their peak bone mass by the age of 80 years (figure 3.32).
Even though body weight tends to decrease with age, the rate of bone loss is usually much greater than the rate at which body weight decreases. Consequently, the effect of bone loss is that the bones, especially weight-bearing bones, become progressively weaker relative to the weight of the rest of the body. In addition to gradually losing strength, the bones also gradually lose their elasticity and become more brittle. In some people, especially women, a loss of bone mass and elasticity is eventually reached when some bones are no longer able to withstand the loads imposed by normal habitual activity. These bones become very susceptible to fracture. This condition, the most common bone disorder in elderly people, is called osteoporosis (Bailey et al 1986, Ferretti et al 2003). Osteoporosis may cause severe disfigurement, especially of the trunk, as a result of fractured or crushed vertebrae. Many deaths in elderly people are due to complications arising from bone fractures that occur as a result of osteoporosis (Kaplan 1983, Kado et al 1999, Cummings and Melton 2002).
Bone loss tends to occur earlier and to proceed at a faster rate in cancellous bone than in compact bone (Bailey et al 1986, Ferretti et al 2003). Consequently, regions of bones with a high proportion of cancellous bone, such as the bodies of the vertebrae, the head and neck of the femur, and the distal end of the radius, are particularly vulnerable to osteoporosis and fracture in elderly people. This vulnerability is reflected in studies that report a rapid increase in the incidence of bone fractures with age, especially in women (Bauer 1960, Chalmers and Ho 1970, Hagino et al 1991, Cummings et al 1993, Court-Brown and Caesar 2006). The results of one study showed that the incidence of fracture to the distal end of the radius was seven times higher in 54-year-old women than in 40-year-old women (Bauer 1960). In another study the incidence of fracture of the neck of the femur was found to be 50 times higher in 70-year-old women than in 40-year-old women (Chalmers and Ho 1970). Bone loss in compact bone occurs mainly on the endosteal surface so that bone width remains relatively unchanged into old age (Smith 1982, Ferretti et al 2003).
Although the cause of osteoporosis is not clear, there is general agreement that four variables are mainly responsible: genetic factors, endocrine status, nutritional factors, and physical activity (Bailey et al 1986, MacKinnon 1988, Ferretti et al 2003). The relative contribution of these variables has not been established, but physical activity seems to be the most important. In the absence of weight-bearing activity, no amount of endocrine or nutritional intervention will prevent rapid bone loss; there must be mechanical stress (Bailey et al 1986, Ferretti et al 2003). Research suggests that regular physical activity throughout life, within the moderate overload range (see chapter 11), can help to prevent osteoporosis in three ways (Bailey 1995, Greene and Naughton 2006, Baxter-Jones et al 2008):
1. Peak bone mass is directly related to the level of physical activity prior to peak bone mass; the higher the peak bone mass, the lower the risk of osteoporosis.
2. An above-average level of physical activity after peak bone mass will delay the onset of bone loss.
3. An above-average level of physical activity after peak bone mass will reduce the rate of bone loss.
Case study 2 discusses whether regular exercise can be used to prevent falls in osteopenic women.
Case Study 2 Using Exercise to Prevent Falls in Osteopenic Women
Hourigan SR, Nitz JC, Brauer S, O’Neill S, Wong J, Richardson CA. 2008. Positive effects of exercise on falls and fracture risk in osteopenic women. Osteoporosis International 19:1077-1086.
The incidence of bone fractures increases with age. As the number of aged people worldwide is increasing, so is the total number of fractures in the aged population (age 65 years and older). Fractures in the aged population are a major health care cost and risk factor for permanent disability and death. Consequently, reducing the risk of fractures in the aged population is a challenge in many countries.
Physical trauma (in particular, falls) and weak bones (due to osteopenia or osteoporosis) are the major causes of fractures in the aged population. Poor balance and weak muscles are risk factors for falls. Osteopenia is defined as bone mineral density (BMD) between 1 and 2.5 standard deviations below the average for young women. Osteoporosis is defined as BMD more than 2.5 standard deviations below the average for young women.
Recent research indicates that muscle strength, bone strength, and balance in the aged population can be improved through properly prescribed physical activity training programs based on resistance exercises involving strength-training machines. The purpose of the present study was to determine the effect of a physical activity program based on weight-bearing activities (rather than strength-training machines) on balance, muscle strength, and bone mineral density in osteopenic women.
Ninety-eight community-dwelling (living more or less independently in their own homes) osteopenic women (age 62.01 ± 8.9; range 41-78 years) were randomly assigned to either a control group (n = 48: no intervention) or an exercise group (n = 50: two 1 hr exercise sessions per week for 20 weeks directed by a trained physiotherapist). Assessments at baseline and post intervention included balance (five measures), strength (quadriceps; hip abductors, adductors, and external rotators; trunk extensors), and BMD (proximal femur and lumbar spine). Baseline assessment showed no significant differences between the exercise and control groups in terms of balance, strength, BMD, or demographics.
The average number of sessions attended by the 42 members of the exercise group who completed the exercise program was 28.2 of a possible 40 sessions (71.2%). Following the intervention, the exercise group showed significantly better performance than the control group in 9 of 11 balance tests (ranging from 10% to 71% better performance) and 7 of 9 strength tests (ranging from 9% to 23% better performance). BMD of the exercise group increased but was not significantly greater than in the control group.
A specific, well-directed program of weight-bearing exercises in a workstation format that emphasizes interaction, discussion, and enjoyment can significantly improve balance and strength in osteopenic women, which in turn is likely to reduce the risk of falling. This type of training may also positively influence BMD, but further research is needed.