CHAPTER 2 Structure and Function of Joints TABLE 2.1 Skeletal System Functions Support Protection Storage


Blood Cell Production


Movement


Provides a rigid framework and transfers weight of the body to the ground


Provides external protection of the brain, spinal cord, heart, and lungs


Stores reservoirs of calcium


Produces red and white blood cells and platelets


Forms rigid levers that provide the attachment sites of muscles for movement


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and proportion of these three basic materials determine whether the connective tissue forms bone; articular car- tilage; or other periarticular joint structures, such as a ligament, tendon, bursa, joint capsule, or meniscus.


TISSUE AND CELLULAR COMPONENTS Fibrous Proteins


Clinical Connection 2.1


An understanding of the composition and structural anatomy of joints and their optimal biomechanics is essential before studying the pathology of these structures and interventions to address joint dys- function. The clinician must understand the char- acteristics of the articular cartilage that covers joint surfaces, including its responses to high stress loads and poor ability to heal, to understand the effects of cartilage erosion in osteoarthritis. The erosion of articular cartilage surfaces of joints that occurs in osteoarthritis occurs over a period of time and can be due to excessive stresses on the joint, to genetics, or to idiopathic causes. As the cartilage wears away, the underlying subchondral bone can become the weight-bearing surface for the joint. Because bone is covered with highly innervated periosteum, the bone of one surface rubbing on bone of the oppos- ing articulating surface often results in severe pain and dysfunction. This abnormal weight-bearing pattern, along with thickening of the joint peri- articular structures, can result in marked stiffness, discomfort, and decreased function. Rehabilitation interventions such as increasing muscle strength around the joint and decreasing stresses through the joint often address many of the impairments resulting from this joint dysfunction.


JOINT COMPOSITION


Connective tissue forms the basic structure of joint tissues and can change in response to functional demands, stresses, aging, and injury. Three basic materials make up connective tissue throughout the body—fi brous pro- teins, ground substance, and cells. The arrangement


Figure 2.2 Type I collagen fibrils. (From Scanlon VC, Sanders T. Essentials of Anatomy and Physiology, 7th ed. Philadelphia, PA. F.A. Davis company, 2014; p. 90, with permission.)


Fibrous proteins, one of the components of connective tissue, can be subclassifi ed into collagen and elastin. Approximately one third of the proteins in the body are collagen. Collagen has remarkable tensile strength and gives connective tissue structures the ability to resist tension forces. The basic structure of collagen is a triple helix of three polypeptide chains. These spiraled chains are called tropocollagen and combine to form fi brils. As fi brils group together, they form collagen fi bers. Although many types of collagen have been identifi ed in the connective tissue throughout the body, most peri- articular connective tissue consists of type I and type II collagen. Type I collagen is found primarily in ligaments, tendons, menisci, fi brocartilage, labra, fascia, fi brous joint capsules and synovium, and skin. The fi bers are thick and stiff and provide little elongation when stretched to withstand the tension produced during musculoskel- etal motion (Fig. 2.2). The fi bers in type II collagen are thinner and have less tensile strength. This property helps maintain the shape of a structure by providing a solid framework such as that seen in hyaline cartilage and intervertebral discs.


Periarticular structures also contain rubber-like elastin


fi bers that give more than collagen fi bers when stretched. These deforming fi bers return to their original shape after being stretched and are found in joint structures, skin, arterial walls, and the tracheobronchial tree. For example, the ligamentum fl avum, a spinal ligament, is composed of 75% elastin and 15% collagen. The elastin helps realign the vertebra as a person returns to an erect standing position after bending forward.


Fibrous tissue Fibroblasts


Collagen fibers


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