Structure and Function of Joints

• Describe the components of the axial versus appendicular skeleton.

• Define the primary components found in bone.

• Describe the five types of bones found in the human skeleton.

• Describe the three primary classifications of joints and give an anatomic example of each.

• Identify the components of a synovial joint.

• Describe the seven different classifications of synovial joints in terms of mobility (degrees of freedom) and stability.

• Provide an anatomic example of each of the seven different classifications of synovial joints.

• Describe the three primary materials found in connective tissue.

• Explain how tendons and ligaments support the structure of a joint.

• Explain how muscles help to stabilize a joint.

• Describe the effects of immobilization on the connective tissues of a joint.

A joint is the articulation, or junction, between two or more bones that acts as a pivot point for bony movement. Motion of the entire body or of a particular body segment generally occurs through the rotation of bones about individual joints. The specific anatomic features of a joint play a large role in determining its range of motion, degrees of freedom, and overall functional potential. This chapter is intended to provide an overview of the basic structure and function of joints as a foundation for understanding the motion of individual body segments and the body as a whole.

Axial versus Appendicular Skeleton

The bones of the skeletal system can be grouped into two categories: the appendicular skeleton and the axial skeleton. The axial skeleton consists of the skull, hyoid bone, sternum, ribs, and vertebral column, including the sacrum and coccyx, forming the central, bony axis of the body. The appendicular skeleton is composed of the bones of the appendages, or extremities. All bones of the upper extremity, including the scapula and clavicle, and all bones in the lower extremity, including the pelvis, are part of the appendicular skeleton. Figure 2-1 differentiates the axial and appendicular skeleton and labels the major bones of the body.

Figure 2-1 An illustration of the human skeleton highlighting the axial skeleton *(red)* and the appendicular skeleton *(white).* A, Anterior view. B, Posterior view. (From Muscolino JE: Kinesiology: the skeletal system and muscle function, St Louis, 2006, Mosby, Figure 4-2.)

Bone: Anatomy and Function

Bone provides the rigid framework of the body and equips muscles with a system of levers. This text describes bone as having two primary types of tissue: cortical (compact) bone and cancellous bone (Figure 2-2).

Figure 2-2 A cross section showing the internal architecture of the proximal femur. Note the thicker areas of compact bone around the shaft and the lattice-like cancellous bone occupying most of the inner regions. (From Neumann DA: An arthritis home study course. The synovial joint: anatomy, function, and dysfunction, Lacrosse, WI, 1998, The Orthopedic Section of the American Physical Therapy Association.)

Cortical (compact) bone is relatively dense and typically lines the outermost portions of bones. This type of bone is extremely strong, especially with regard to absorbing compressive forces through a bone’s longitudinal axis.

Cancellous bone is porous and typically composes the inner portions of a bone. The porous, web-like structure of cancellous bone not only lightens bones but, similar to a series of mechanical struts, redirects forces toward weight-bearing surfaces covered by articular cartilage.

Most bones have common structural features important for maintaining their health and integrity. Figure 2-3 illustrates the primary components found in a bone.

Figure 2-3 The primary components of a bone. (From Muscolino JE: Kinesiology: the skeletal system and muscle function, St Louis, 2006, Mosby, Figure 3-2.)

The diaphysis is the central shaft of the bone. It is similar to a thick, hollow tube and is composed mostly of cortical bone, to withstand the large compressive forces from weight bearing. The epiphyses are the expanded portions of bone that arise from the diaphysis (shaft); each long bone has a proximal and a distal epiphysis. Primarily composed of cancellous (spongy) bone, each epiphysis typically articulates with another bone, forming a joint, and helps transmit weight-bearing forces across regions of the body. Articular cartilage lines the articular surface of each epiphysis, acting as a shock absorber between joints.

Each long bone is covered by a thin, tough membrane called the periosteum. This highly vascular and innervated membrane helps secure the attachments of muscles and ligaments to bone. The medullary canal (cavity) is the central hollow tube within the diaphysis of a long bone. This region is important for storing bone marrow and provides a passageway for nutrient-carrying arteries. The endosteum is a membrane that lines the surface of the medullary canal.

Many of the cells important for forming and repairing bone are housed within the endosteum.

Bone is a dynamic tissue that is constantly being remodeled in response to internal and external forces. Clinically, this is an important fact, because bones will become stronger from forces caused by weight-bearing activities and muscular contractions, or significantly weaker after joint immobilization, periods of restricted weight bearing, or extended inactivity such as is seen in those who have been on bed rest.

Types of Bones

Bones can be classified into five basic categories based on their structure, or shape: long, short, flat, irregular, and sesamoid (Figure 2-4).

Long bones comprise the majority of the appendicular skeleton. As the name implies, they are long and contain obvious longitudinal axes or shafts. Generally, long bones contain an expanded portion of bone at each end of the shaft that articulates with another bone, forming a joint. The femur, humerus, metacarpals, and radius are just some of the numerous examples of long bones found in the body.

Short bones are short, meaning that their lengths, widths, and heights are typically equal. The carpal bones of the hand provide a good example of short bones.

Flat bones such as the scapula or sternum are typically flat or slightly curved. Often the broad surface of these bones provides a wide base for expansive muscular attachments.

Irregular bones, as the name implies, come in a wide variety of shapes and sizes. Examples of irregular bones include vertebrae, most of the bones of the face and skull, and sesamoid bones.

Sesamoid bones are a subcategory of irregular bones, named so because their small, rounded appearance is similar to that of a sesame seed. These bones are encased within the tendon of a muscle, serving to protect the tendon and increase the muscle’s leverage. For example, the patella (knee cap)—the largest sesamoid bone in the body—is embedded within the tendon of the quadriceps muscle. The patella increases the distance (internal moment arm) between the line of force of the quadriceps and the axis of rotation; as a result, the patella augments the torque production of the quadriceps. Also, the patella protects the quadriceps tendon by absorbing some of the compressive and shear forces that occur during flexion and extension of the knee.

Classification of Joints

Joints are commonly classified by their anatomic structure and subsequent movement potential. On the basis of this system, there are three classifications of joints in the body: synarthrosis, amphiarthrosis, and diarthrosis.

Similar to the hinge of a door, the hinge joint (Figure 2-8) allows motion in only one plane about a single axis of rotation. Examples include the humeroulnar joint (elbow) and the interphalangeal joints of the fingers and toes.

Figure 2-8 A, A hinge joint is illustrated as analogous to the humeroulnar joint **(B).** The axis of rotation is represented by the pin. (From Neumann DA: Kinesiology of the musculoskeletal system: foundations for physical rehabilitation, ed 2, St Louis, 2010, Mosby, Figure 2-3.)

Pivot Joint

The pivot joint (Figure 2-9) allows rotation about a single longitudinal axis of rotation, similar to the rotation of a doorknob. Examples include the proximal radioulnar joint and the atlantoaxial joint between the first and second cervical vertebrae.