What Are Joints In The Skeletal System – The point where two or more bones meet is called a joint or joint. Joints are responsible for movement, such as limb movement, and stability, such as the stability found in the bones of the skull.

Here are two ways to classify joints: based on their structure or based on their function. Structural classification divides joints into bony, fibrous, cartilaginous and synovial joints, depending on the material of which the joint is made and the presence or absence of a cavity in the joint.

What Are Joints In The Skeletal System

The bones of fibrous joints are held together by fibrous connective tissue. There is no cavity or space between the bones, so most fibrous joints do not move at all or are only capable of small movements. There are three types of fibrous joints: sutures, syndesmose, and gomophose. Sutures are found only in the skull and have short fibers of connective tissue that hold the skull bones tightly together (Figure 19.23).

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Syndesmoses are joints where the bones are connected by a band of connective tissue, allowing for more movement than with a suture. An example of a syndesmosis is the joint of the tibia and fibula in the ankle. The amount of movement in these types of joints is determined by the length of the connective tissue fibers. Gomophoses occur between the teeth and their sockets; the term refers to the way the tooth fits into the socket like a wedge (Figure 19.24). The tooth is connected to the socket by a connective tissue called the periodontal ligament.

Figure 19.24. Gomophoses are fibrous joints between teeth and their sockets. (credit: Gray’s Anatomy part modification)

Cartilaginous joints are joints in which bones are connected by cartilage. There are two types of cartilaginous joints: synchondroses and symphyses. In synchondrosis, the bones are connected by hyaline cartilage. Synchondroses are found in the epiphyseal plates of growing bones in children. In symphyses, hyaline cartilage covers the end of the bone, but the connection between the bones is via fibrocartilage. Symphyses are located in the joints between the vertebrae. Each type of cartilaginous joint allows very little movement.

Synovial joints are the only joints that have space between adjacent bones (Figure 19.25). This space is called the synovial (or joint) cavity and is filled with synovial fluid. Synovial fluid lubricates the joint, reduces friction between the bones and allows greater movement. The ends of the bones are covered with articular cartilage, hyaline cartilage, and the entire joint is surrounded by a joint capsule consisting of connective tissue that allows the joint to move while resisting dislocation. Joint capsules may also have ligaments that hold the bones together. Synovial joints are capable of the greatest range of motion of the three structural types of joints; however, the more flexible the joint, the weaker the joint. Knees, elbows, and shoulders are examples of synovial joints.

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Functional classification divides joints into three categories: synarthroses, amphiarthroses and diarthroses. A synarthrosis is a joint that is immovable. This includes sutures, gomophores, and synchondroses. Amphiarthroses are joints that allow slight movement, including syndesmose and symphysis. Diarthroses are joints that allow free movement of the joint, as in synovial joints.

The wide range of motion allowed by synovial joints results in different types of motion. Synovial joint motion can be classified into one of four different types: sliding, angular, rotational, or special motion.

Gliding movements occur when relatively flat bone surfaces move past each other. Gliding motions cause very little rotation or angular movement of the bones. The carpal and tarsal joints are examples of joints that cause sliding motions.

Angular movements occur when the angle between the bones of a joint changes. There are several different types of angular movements, including flexion, extension, hyperextension, abduction, adduction, and circumduction. Flexion or bending occurs when the angle between the bones decreases. Moving the forearm up at the elbow or moving the wrist to move the hand toward the forearm are examples of flexion. Extension is the opposite of flexion as it increases the angle between the bones of the joint. Straightening a limb after flexion is an example of extension. Extension past the regular anatomical position is called hyperextension. This includes rolling the neck back to look up or flexing the wrist so the hand moves away from the forearm.

Skeletal System (hesi) Diagram

Abduction occurs when a bone moves away from the midline of the body. Examples of abduction are moving the arms or legs laterally to lift straight out to the side. Adduction is the movement of bones towards the midline of the body. The inward movement of the limbs after abduction is an example of adduction. Circumduction is the movement of a limb in a circular motion, as in moving an arm in a circular motion.

Rotational motion is the motion of a bone as it rotates around its longitudinal axis. The rotation can be towards the midline of the body, which is called medial rotation, or away from the midline of the body, which is called lateral rotation. Moving the head from side to side is an example of rotation.

Certain motions that cannot be defined as sliding, angular, or rotational are called special motions. Inversion involves moving the soles of the feet inward, toward the midline of the body. Eversion is the opposite of inversion, moving the sole of the foot outward, away from the midline of the body. Protraction is the movement of a bone forward in a horizontal plane. Retraction occurs when a joint moves back into position after protraction. Protraction and retraction can be seen in the movement of the lower jaw when the jaw is pushed outward and then back inward. Lifting is the upward movement of the bones, such as when we shrug our shoulders and lift our shoulder blades. Depression is the opposite of elevation—a downward movement of the bones, such as after shrugging the shoulders and returning the shoulder blades to their normal position from an elevated position. Dorsal flexion is bending at the ankle so that the toes rise toward the knee. Plantar flexion is the bending in the ankle when the heel rises, such as when standing on the toes. Supination is the movement of the radius and ulna of the forearm so that the palm faces forward. Pronation is the opposite movement in which the palm faces backwards. Opposition is the movement of the thumb against the fingers of the same hand, which allows objects to be grasped and held.

Synovial joints are further classified into six different categories based on the shape and structure of the joint. The shape of the joint affects the type of movement that the joint allows (Figure 19.26). These joints can be described as plane, hinge, rotator, condyloid, saddle or ball joints.

The Human Skeletal System

Different types of joints allow for different types of movement. Planar, hinge, swivel, condyloid, saddle, and ball joints are all types of synovial joints.

Planar joints have bones with articular surfaces that are flat or slightly curved. These joints allow for gliding movements, which is why the joints are sometimes called gliding joints. The range of motion in these joints is limited and does not include rotation. Planar joints are located in the carpal bones of the hand and the tarsal bones of the foot and between the vertebrae (Figure 19.27).

The joints of the carpal bones in the wrist are examples of planar joints. (credit: modification of work by Brian C. Goss)

In hinge joints, the slightly rounded end of one bone fits into the slightly hollow end of another bone. In this way, one bone moves while the other remains stationary, such as the hinge of a door. The elbow is an example of a hinge joint. The knee is sometimes classified as a modified hinge joint (Figure 19.28).

Skeletal System Anatomy And Physiology

The elbow joint, where the radius articulates with the humerus, is an example of a hinge joint. (credit: modification of work by Brian C. Goss)

Articulating joints consist of a rounded end of one bone that fits into a ring formed by another bone. This structure allows for rotational movement as the rounded bone moves around its own axis. An example of a swivel joint is the joint of the first and second cervical vertebrae, which allows the head to move forward and backward (Figure 19.29). The wrist joint that allows the palm to turn up and down is also a pivot joint.

Figure 19.29. The joint in the neck that allows the head to move back and forth is an example of a swivel joint.

Condyloid joints consist of an oval end of one bone that fits into a similar oval cavity of another bone (Fig. 19.30). This is sometimes called an ellipsoid joint. This type of joint allows for angular movement along two axes, as seen in the wrist and finger joints, which can move from side to side and up and down.

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The metacarpophalangeal joints in the finger are examples of condyloid joints.