Part III: Basic Biomechanics: Why You Move
Chapter 8: Bone Composition and Function
Bone Composition and Function
In This Chapter
▶ Types of bones and their function
▶ Materials that make up bones
▶ How your bones grow
Ah, the human skeletal system! It’s made up of 206 bones that perform two major functions within the body: The bones support and protect the body and the organs within it, and they provide rigid attachments for muscles and other soft tissues that, altogether, create and support move- ment. Beyond these primary functions, your bones dictate other, very important aspects of your life. For example, the bones play a critical role in hemopoiesis, or the formation of blood cells. And as they model and remodel constantly to accommodate the demands your body places on them, your bones contribute significantly to your daily life.
In this chapter, we discuss the types of bones that exist, identify what they are made of, and explain the means by which they grow and accommodate your activities.
Boning Up on the Basics
Most people think of bones as hard, unforgiving structures. What they often overlook is just how dynamic and accommodating bones really are. Much of a bone’s structure is defined and dictated by its function. As you look closely at a picture of a skeleton, you can see that some bones are intended to assist with load bearing (like walking and running), and others aren’t.
You can generally recognize whether a bone is load bearing or not by its size and shape. Load-bearing bones are typically bigger and thicker, whereas non-load–bearing bones are usually smaller. But the outside appearance of the bone isn’t the only indication of its function; the inside also provides clues.
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Collectively, bones help deliver nutrients, store minerals, protect your organs, support your posture, and make stability and movement possible. A bone’s life is dynamic; it constantly adjusts to its environment and the tasks that you demand of it. Wolff’s law, named after German anatomist Julius Wolff (1836–1902), describes the dynamic adaptation of bone. It states that a bone will adapt to the loads that it encounters — as long as the loads themselves are not so large as to break the bone, of course!
Looking at bone’s composition
The foundations of most bones are calcium carbonate, calcium phosphate, collagen, and water. The percentage of each of these substances vary by the function, age, and relative health of the person and the bone itself. Here’s what you need to know:
✓ Calcium carbonate and calcium phosphate are the minerals primarily responsible for the bone’s strength, and they account for approximately 60 percent to 70 percent of the bone’s weight. They help dictate its stiffness (the extent to which it resists deformation from a force) and compressive strength (its ability to resist being squeezed or otherwise shortened), both of which are important determinants of a bone’s func- tion. Although these materials play a primary role in a bone’s makeup, other materials such as sodium, magnesium, and fluoride also help.
✓ Collagen (a protein) is responsible for the bone’s flexibility and ability to resist tension (a pulling force).
✓ Water enhances the strength of the materials that make up the bone and contributes significantly to nutrient delivery and waste removal. Water accounts for about 25 percent to 30 percent of a healthy bone’s weight.
When the water content decreases, as it does with aging, bones become more brittle.
Think about what happens when you try to break a limb that’s been recently trimmed from a tree. It doesn’t break cleanly; you have to bend it repeatedly back and forth to get it to break. But wait a few days to break the limb, and you can do so easily. The reason is the lack of water. Just as a tree branch becomes easier to break as it dries, bones becomes more brittle and easier to break as they age.
Pouring over porosity: Cortical versus trabecular bones
Bones are characterized based on their amount of porosity. In the context of bone, porosity refers to the number of cavities or pores that exist in the bone, and it determines the bone’s strength. A bone that is porous possesses less calcium carbonate and calcium phosphate.
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Level of porosity also determines whether a bone is classified as a cortical bone or trabecular bone:
✓ Cortical bone: A cortical bone, which has a low porosity, is very dense and mineralized; typically only 5 percent to 30 percent of its volume is non-mineralized. A cortical bone is much stiffer than a bone with high porosity and is able to resist greater stress (pressure or force exerted on the bone).
Consider the femur, the largest long bone in the body. It’s responsible for absorbing the great deal of force generated when you walk or run;
therefore, a large part of its makeup is cortical bone. The femur can absorb a significant amount of pressure and tension (stress), but it can’t be bent very far before it breaks (strain).
✓ Trabecular bone: A trabecular bone has a higher percentage of non- mineralized tissue. Also referred to cancellous or spongy, trabecular bone has a very porous makeup. Bone marrow exists in this type of the bone.
In addition, because of its decreased levels of mineralization, trabecular bone can absorb more strain (the measure of deformation in the bone) than cortical bone before it breaks.
No one bone is exclusively cortical or trabecular. Instead, throughout each bone, various levels of porosity exist. Cortical bone typically resides in the outer shaft of the bone, and the spongy trabecular portion usually exists further within the bone. The location and amount of porosity found in the bone is determined by the amount and type of forces that the bone absorbs.
Because their structural organization is a response to the various forces exerted on them, bones are considered anisotropic. Bottom line: Bone is a dynamic structure that constantly adapts to the forces applied to it.
The structural components of bone
The following list outlines the structural components of bones (see Figure 8-1):
✓ Articular surfaces: The articular surfaces of long bones, the parts that form the joints, are located at the proximal and distal end of the bones and are typically made up of a higher percentage of trabecular bone.
✓ Diaphysis: The long bone’s shaft, called the diaphysis, consists of an outer shell (the cortex ) that is covered by a dense fibrous connective tissue called periosteum.
✓ Periosteum: The periosteum, which covers the diaphysis, is very dense cor- tical bone that is highly sensitized by nociceptors (sensory neurons respon- sible for the perception of pain). That’s why breaking a bone hurts so much.
✓ Cortex: The medullary cavity houses bone marrow. The cells lining the medullary cavity — the periosteum and endosteum — are collectively called the cortex. The density of the cortex is high because this area of the bone needs to be so strong.
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Figure 8-1:
Long bone structure.
✓ Metaphysis: Marrow is a flexible, spongy tissue that’s responsible for creating red blood cells through a process called hematopoiesis. A majority of hematopoietic activity occurs in the metaphysis, or the ends, of the long bone.
✓ Epiphyseal plate: At the point where the diaphysis and metaphysis join is a line of cartilage that separates the sections. This line, referred to as the epiphyseal plate (or, more commonly, the growth plate) is the site of longitudinal bone growth (or the lengthening of the bone).
✓ Apophysis: Where tendons and/or muscles insert onto the bone, a raised section, called an apophysis exists. An apophysis is separated from the cortex by an apophyseal plate that consists of osteoblasts, bone growth cells. The most prominent example of an apophysis is the tibial tuberosity — the slightly raised section on the bottom of your knee that you can feel just by running your hand over your knee and down your
Illustration by Kathryn Born, MA
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shin. The tension that the quadriceps puts on this area by pulling on the tibia via the patellar tendon causes new bone cells to be laid down, forming the raised portion.
You’re stressing me out! Compression, tension, torsion, and shearing
Bone’s internal make up (its mineralization and density) and its shape are dictated by the bone’s function, and normal bone function is characterized by continually adapting to the environment.
Although the primary influence on the architecture and the materials that make up the bone is genetic, adaptations are constantly being made. These adaptations are based on two things: the person’s nutrition and the amount and kind of stresses that the bone is subjected to.
Types of forces
Normal weight-bearing and everyday activities, like walking and standing, exert a variety of forces on the bones, including compression, tension, torsion, and shearing:
✓ Compression: Compression refers to the squeezing force that occurs through normal weight bearing, as your body weight and gravity push down on your frame.
✓ Tension: Tension, which happens during normal weight-bearing and other physical activities, occurs when a muscle or tendon pulls on a bone at its attachment site to create movement or increase stability.
Extending your knee, for example, creates tension on the tibial tuberosity (refer to the preceding section).
✓ Torsion: Torsion is a twisting effect. When you’re standing and twist to change directions, torsion is exerted on the tibia (leg bone). This twisting is usually combated by the strength in the bone, but sometimes the bone may break.
✓ Shearing: Shearing refers to the tearing across the longitudinal axis.
When you stop abruptly and your foot extends in front of you to stop your forward momentum, your leg bone experiences a shear force. Part of the bone wants to continue going in the same direction as your body’s momentum, but the foot stopping your forward movement pushes it back.
The modeling threshold
Each bone has a modeling threshold, which determines how much and what kind of force can be applied to the bone before the bone begins to adapt. A bone is
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able to accommodate a certain type and amount of force; when forces equal the modeling threshold in both direction and amount, no change in bone structure occurs. In essence, the bone has been uniquely designed to handle that activity.
Changing the type or intensity of an activity results in forces exceeding the threshold and stimulating bone development. The development of new bone can enhance mineralization, the addition of osteocytes (bone cells). When forces delivered to the bones decrease and produce less demand, bone mineralization lessens, and osteocytes are reabsorbed. (Note: Osteoblasts and osteoclasts are two types of osteocytes.)
Increases in mineralization occur only in the areas that are experiencing the applied forces. For example, runners often have higher bone density in their lower extremities than in their upper extremities. Likewise, baseball and tennis athletes often demonstrate increases in their throwing or racquet arms while their lower extremities are no different from other athletes’ lower extremities.
The same is true for most active folks in their dominant versus non-dominant sides. The areas that experience the greater forces typically have greater bone diameters, cortical widths, densities, and calcium concentrations.
Knowing the kind of old bone you are
Typically, the skeletal system is divided into two parts, the appendicular skeleton and the axial skeleton, shown in Figure 8-2:
✓ Axial skeleton: This skeleton is made up of the skull, vertebrae,
sternum, and ribs. The axial skeleton is considered the central aspect of the skeletal system.
✓ Appendicular skeleton: This skeleton is made up of the bones that form the appendages.
In addition to being considered part of either of these two skeletons, bones are further characterized by their shapes. When you look at the skeleton, you see that bones are shaped very differently — short, long, flat, irregular, and sesamoid. As we note earlier, the specific function that the bone plays within the body dictates its shape and size. This list has the details:
✓ Short bones: These bones are small, solid, and often cube shaped. They have a relatively large articulating surface and are able to articulate with more than one bone and typically contribute to gliding. Short bones include the carpals (wrist bones) and tarsals (ankle bones).
✓ Flat bones: As the name suggests, these bones are flat or slightly rounded and vary in thickness. Their job is to protect organs within the body. They include the ribs, sternum, ilium (the bone forming the upper part of the pelvis), clavicle (collar bone), and scapula (shoulder blade).
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Figure 8-2:
The appen- dicular and axial skeleton.
Illustration by Kathryn Born, MA
✓ Irregular bones: Irregular bones play a number of different roles, depend- ing on the nature of their irregularities. Typically the irregularities are dictated by muscle and other soft-tissue attachments that result in forces being applied. For example, the vertebrae (the small bones of the spine) possess a tunnel through which the spinal cord travels and also have a number of exterior protrusions where muscle, tendons, and ligaments attach. Irregular bones include the vertebrae, coccyx (tailbone), ischium and pubis (other bones in the pelvis), and sacrum (part of your tailbone).
✓ Long bones: Long bones are the main support of the appendicular skeleton. They’re typically elongated and cylindrical. Given their length and the processes and protuberances where muscles, tendons, and ligaments attach, they play a role in the lever system (read more about this in Chapter 7). Additionally, the long shafts house a medullary canal, which holds bone marrow, and articular cartilage coats the bone’s end to protect it from wear and tear. Examples of long bones include the radius and ulna (bones in the forearm), and the fibula, femur, and tibia (leg bones).
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✓ Sesamoid bones: These bones are embedded within various tendons.
Their role is to protect and to enhance mechanical advantage of the muscle-tendon unit. The patella (kneecap) is the largest and most recognized sesamoid bone.
Growing Up is Hard to Do:
Examining Bone Growth
Your bones begin as cartilaginous masses when you’re a fetus, and progressively morph into the form you’re familiar with today. Bone development continues through your life, as more bone is either added or taken away. Both the loss and gain of bone cells are normal occurrences throughout your life.
The long and the short of it: Longitudinal and circumferential growth
Bone growth occurs in two distinct ways. Longitudinal growth happens as a bone lengthens, and circumferential growth refers to its increase in diameter.
Here are the details (see Figure 8-3):
✓ Longitudinal growth: This type of growth occurs in a longitudinal direction and happens at the epiphyses (the large, knobby ends of long bones). At the location of the epiphyseal plate, the diaphysis continually produces new bone cells, resulting in growth. Longitudinal growth continues through adolescence, until the epiphyseal plates close and are no longer able to contribute. Typically, growth ceases during the later teens, although some growth may continue a few years beyond that.
✓ Circumferential growth: Here, the bone grows in diameter. In circumfer- ential growth, concentric layers of bone develop around what’s already there. Typically, the concentric layers of growth begin around the inner layer of the periosteum and continue to build upon that. As the layers of bone are being laid down on the outer layer (the periphery), the inside of the bone is being reabsorbed. This reabsorption allows for the medullary cavity to continue to grow proportionally and stops the bone from becoming too rigid and heavy (which would happen if the inside bone cells didn’t get reabsorbed).
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Figure 8-3:
Longitudinal versus cir- cumferential
growth.
Illustration by Wiley, Composition Services Graphics
Both types of growth are dependent on specialized cells called osteocytes.
Two types of osteocytes are responsible for the modeling and remodeling that occurs through the lifespan. Osteoblasts are responsible for forming new bone, and osteoclasts resorb old bone. The tireless activity of these cells in response to use and disuse are what makes up bone modeling and remodeling.
Increases and decreases in density
As we note in the earlier section “Pouring over porosity: Cortical versus trabecular bones,” a bone’s density is related to its mineralization and the stresses that are applied to it. Hormones also influence bone density.
During adolescence
During childhood and adolescence, bone hypertrophy occurs in response to activity. That is, bone density increases fairly rapidly as the bones continually ossify (harden and become more dense) to meet demands of the structural system as children walk, run, and otherwise function in their daily activities.
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Mineralization dictates a bone’s strength and ability to accept forces. In the adult years, typical bone development and modeling occurs as bone mineralization takes place in response to forces. As forces increase or are maintained, the more mineralized bone becomes. Conversely, as fewer forces are delivered to the bone, the bone becomes less mineralized and less able to accept stresses.
Although being active can result in increased bone density, through the adult years, beginning in the third or fourth decade of life, a decrease in bone density or demineralization occurs naturally. For women, typically beginning in their mid-20s, and for men, nearly a decade later, bone density typically begins to fall at a progressive rate. As the bone’s density decreases, the mate- rials that make up the bone decrease as well. As the dismantling of the net- work that was the trabeculae occurs and the bone becomes more porous (refer to the earlier section “Pouring over porosity: Cortical versus trabecular bones”), the strength of the framework is significantly compromised. Bones become progressively weaker and less resilient to the forces they once successfully resisted.
Unfortunately, the loss in bone mineralization has been shown to affect women to a larger extent than men. In fact, women lose approximately 0.5 percent to 1 percent of their bone mass each year until menopause.
After menopause, bone mass loss increases to a much larger degree.
Decreases often are most apparent in these groups:
✓ Those who are not physically active: People who are sedentary, bedridden, or not able to ambulate in a way that puts stress on the bones suffer from demineralization.
✓ Those who are not affected by gravity: Yeah, we’re referring to astronauts, especially those living at the international space station for extended periods of time.
During old age
Demineralization is problematic for anyone because when bone density decreases the bone’s ability to resist and accommodate imposed stresses and forces also decreases. People suffering from demineralization tend to experience more fractures, many of which are quite debilitating.
Demineralization is a significant issue in the elderly. First, beginning in a person’s 20s and 30s, a progressive loss of mineralization is natural.
Additionally, bones’ tensile strength and elasticity are also negatively impacted as a person grows older. Some studies have shown that physical activity through the lifespan can help to thwart bone demineralization in men and women, yet the inactive elderly often experience osteoporosis.