9.6: Joints

Functions

The bones that constitute the skeletal system are held together by special structures called joints. By furnishing strong attachments, joints contribute to the support provided by the skeletal system. They also protect the body from traumatic injury by absorbing shock and vibration in two ways. One way is by allowing the bones to move somewhat, which permits the skeletal system to yield to sudden physical forces. The other way derives from the cushioning provided by the fluids and resilient cartilage found in many joints. Because of these two features, joints can prevent much of the damage to delicate parts of the body caused by jolting forces from activities such as running and jumping. Assistance with movement derives, of course, from the various movements of bones permitted by the joints. The joints do not help the skeleton store minerals or produce blood cells.

Immovable Joints

There are three main types of joints in the body. An immovable joint consists of tough collagen fibers that bind bones tightly together. The unyielding strength of the collagen, together with the tight fit of the bones, essentially eliminates shifting of the bones. Among the immovable joints are the suture joints between skull bones (Figure 9.1). These joints keep the shieldlike skull bones in place to support and protect the brain.

Age Changes

As people age, the collagen fibers between the bones at immovable joints are coated with bone matrix, and so the space between the bones gets narrower. Eventually the bones may fuse together. Thus, immovable joints improve with aging because they get stronger.

Slightly Movable Joints

The second type of joint is the slightly movable joint. There is a layer of cartilage between the bones joined by these structures. Some of these joints have ligaments, which help hold the bones together. Ligaments are cablelike structures consisting primarily of collagen fibers.

There are two kinds of slightly movable joints. One kind contains hyaline cartilage, which is a smooth, slippery white substance with the consistency of hard rubber. Slightly movable joints with hyaline cartilage join the ribs to the sternum (Figure 9.1). The limitation of movement in these joints allows the ribs to support and protect the lungs, the heart, and other organs in the chest cavity while permitting enough movement of the ribs to allow breathing.

In the other kind of slightly movable joints, symphysis joints, the bones are separated by a pad of fibrocartilage. This type of cartilage is also smooth, slippery, and resilient. It is stronger than hyaline cartilage because it contains many more thick collagen fibers. The fibers add toughness by binding the rubbery cartilage matrix together.

Symphysis joints are located where greater strength is needed, such as between the bodies of the vertebrae (Figure 9.8a), where the intervertebral disks of fibrocartilage permit limited and smooth bending of the vertebral column. Together with the vertebrae, the disks support the weight of the body. Each disk contains a soft center called the nucleus pulposus. The nucleus pulposus helps with support and is important in shock absorption. The intervertebral joints also contain strong ligaments to hold the vertebrae together while allowing a limited amount of bending and twisting of the spine.

Figure 9.8a Vertebrae and slightly movable joints: (a) Young vertebrae and joints. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.

Age Changes

With aging, the hyaline cartilage in slightly movable joints becomes stiffer because of a decrease in water and an increase in hard and rigid calcium salts within the cartilage. The fibers in the ligaments develop more cross-links with age, causing the ligaments to become stiffer and less elastic. The combination of these age changes reduces the movement allowed. For example, such stiffening in the chest area makes breathing more difficult.

Aging causes the fibrocartilage disks in symphysis joints to lose water and gain calcium. These changes may contribute to age-related stiffening of the joints and a decrease in the movement permitted by the vertebral column. The nucleus pulposus becomes weaker and somewhat crumbly, decreasing its ability to provide support for the body and cushioning for the spinal cord and head.

The center region of each vertebral body weakens with aging (Figure 9.8c). The weight of the body then forces the central part of each intervertebral disk to expand into the body of the vertebra, forming a concave region. This alteration in structure seems to place more of the weight of the body onto the outer edge of the intervertebral disk, compressing it somewhat. The net result is a decrease in the height of the body with aging.

Figure 9.8c Vertebrae and slightly movable joints: (c) Age changes (Thinner compact bone, less dense spongy bone, thinner intervertebral disc, nucleous pulposus bulging into weak vertebrae, concave vertebral surfaces, narrower joint spaces). (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Decreasing height with age also has other causes. There is a thinning of the cartilage in other joints, such as the knees and hips. The weakening of muscles and a decrease in muscle tone often lead to poorer posture, further reducing overall height. The rate of loss of height is slow at first and becomes more rapid with age.

Like all collagen, the collagen in the ligaments of the intervertebral joints becomes shorter, stiffer, and less elastic. These changes further reduce the mobility of the vertebral column.

Overall, age changes in symphysis joints reduce the ease and range of motion that these joints provide. This hampers bending and twisting of the vertebral column and makes performing activities such as tying shoes, picking up objects, and dancing more difficult or less enjoyable. The loss is not great enough to be a serious threat to homeostasis. Moreover, as will be discussed below, the decline in movement of symphysis joints can be minimized and even reversed by exercise.

Freely Movable Joints

The third type of joint is the freely movable joint, which is the most common type. These joints make up virtually all the joints in the arms, legs, shoulders, and hips; the joints between the ribs and the vertebrae; and the joints between the vertebrae except the joints between vertebral bodies. The joint between the lower jaw and the skull, the temporomandibular joint (TMJ), is the only freely movable joint in the head.

The bones joined by freely movable joints are separated from each other by a narrow space called the synovial cavity (Figure 9.9). This cavity is surrounded by a thin synovial membrane, which constantly secretes a fluid (synovial fluid) into the cavity. At the same time, the membrane removes the old fluid. Synovial fluid contains water and protein molecules. This mixture is somewhat thick and very slippery, allowing the bones to slide over each other easily. It also absorbs some shock sustained by the joint.

Figure 9.9 Structure of freely movable joints. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The end of each bone is covered by a layer of hyaline cartilage that is very smooth and somewhat resilient. Since the cartilage is lubricated by the synovial fluid, it is very slippery. The synovial fluid also supplies nutrients to the cartilage. The slippery cartilage permits easy movement and cushions the bones and the parts of the body they support.

Surrounding the synovial membrane is the thick sleevelike joint capsule, which consists mostly of flexible strong collagen fibers. The joint capsule helps bind the bones together and encases the synovial membrane for support. The flexibility and slight elasticity of the capsule allow the bones to move freely, though over a limited range.

Outside the joint capsule and extending from one bone to the other are cablelike ligaments, which also consist mostly of collagen fibers. Like joint capsules, ligaments bind the bones together and allow limited movement of the joint.

Motion allowed at a joint is also limited by the shapes of the bones and by muscles and tendons. While the joints must allow the bones to move easily and over enough distance to meet the needs of the body, limiting motion is important in preventing injury to muscles, nerves, and blood vessels. Excessive joint motion (e.g., joint dislocation) stretches, twists, and pinches these soft structures.

Age Changes

With aging, there is an increase in the amount of fibrous material in the synovial membrane, and pieces of cartilage may form in it. These changes make the membrane stiffer and less elastic. The membrane also loses some of its blood vessels so that it is less able to produce and remove synovial fluid. Though there is disagreement about which age changes take place in the synovial fluid and the cartilage on the ends of the bones, it is generally agreed that these changes are slight and have little effect on the functioning of the joint.

More important than these age changes are changes in the joint capsule and ligaments. Because of an increased formation of cross-links among their fibers, these structures become shorter, stiffer, and less able to stretch. These changes make it more difficult to move and reduce the range of movement of the joint. Both changes cause the initiation of movement and the speed of movement to occur more slowly. This results in a reduction of the ability to maintain balance and take action to minimize the force of impact from a fall or another traumatic event. Thus, the aging of freely movable joints substantially reduces the ability of the skeletal system to provide cushioning and movement, resulting in increased injuries and diminished performance of activities.

The functioning of freely movable joints begins to decline at age 20. The joints move less easily and over less of a range as time passes. The decrease in blood vessels in joint structures results in slower healing of injured joints.

All these changes occur very gradually but unremittingly. It seems that only part of the reduction in functioning is due to age changes. Some change may be due to the accumulated effects of the small but repeated injuries sustained by joints during ordinary activities. Distinguishing true age changes from these other changes is difficult.

The progressive decrease in mobility caused by aging in both freely movable and slightly movable joints can be slowed by keeping physically active. Exercises that involve bending, stretching, and turning minimize the restraining effects caused by shortening of the collagen fibers. Some mobility that has been lost over time because of inactivity can be regained by initiating exercises that stretch and increase the flexibility of restrictive joint components such as ligaments. Exercise also seems to increase circulation to the joints. Such exercises reduce the risk of fractures and contribute to better balance, greater independence, and improved psychological well-being. However, when people of advanced age engage in new exercises, care should be taken to avoid injuring the joints.