The junction between two bones or between a bone and a tooth forms an articulation, or joint. Joints allow varying degrees of movement and are categorised as immovable, slightly movable, or freely movable.
Bones forming an immovable joint, or synarthrosis (sin-ar-thro’-sis), are tightly joined and are separated by a thin band of dense connective tissue or a thin layer of hyaline cartilage. For example, skull bones, except the mandible, are joined by dense connective tissue called sutures because they resemble stitches. The joints between bones and teeth are also immovable joints separated by dense connective tissue. The epiphysial plates in growing bones are composed of hyaline cartilage, are also immovable joints.
Slightly Movable Joints
Bones forming a slightly movable joint, or amphiarthrosis (am-fe-ar-th-ro’-sis), are separated by a layer of cartilage or dense connective tissue. For example, the joints formed by adjacent vertebrae contain intervertebral discs formed of fibrocartilage. The limited flexibility of the discs allows slight movement between adjacent vertebrae. Other examples include the pubic symphysis and sacroiliac joints.
Freely Movable Joints
Most articulations are freely movable. The structure of a freely movable joint, or diarthrosis (di-ar-thro ‘-sis), is more complex. These joints are also called synovial (si- no’-ve-al) joints. The ends of the bones forming the joint are bound together by an articular, or joint, capsule. The thick external layer of the capsule, called fibrous membrane, is composed of dense irregular connective tissue. The thin internal layer of the capsule, called synovial (si-no ‘-ve-al) membrane, secretes a synovial fluid that lubricates the joint. The ends of the bones are covered with articular cartilage, which protects bones and reduces friction. Ligaments, the cords or bands of dense regular connective tissue that connect bones together, reinforce the joints. Freely movable joints are categorised into several types based on their structure and types of movements.
A plane joint occurs between two flat articular surfaces that slide over each other and allows for movement in one plane. Some examples of plane joints are the joints between carpal bones between tarsal bones, and between clavicle and scapula.
A condylar joint is formed between an oval articular surface and an oval socket and allows for movements in two planes. The joints between carpal bones and radius and between metacarpals and proximal phalanges are examples of condylar joints.
A saddle joint occurs where a saddle-like articular surface fits into a complementary depression, allowing movement in two planes. This type of joint occurs between the trapezium (a carpal bone) and metacarpal I.
A hinge joint involves a cylindrical articular surface and a complementary depression. It allows for movement similar to opening and closing a door. The elbow, knee, and joints between phalanges are all hinge joints.
A pivot joint involves a cylindrical articular surface and a complementary depression. It allows for rotation movements along a longitudinal axis. Examples of a pivot joint are the joint between atlas and axis and the joint between the radius head and the ulna.
In a ball-and-socket joint, a rounded head fits into a rounded socket. It allows for movements in all planes and provides the greatest range of movement of all types of freely movable joints. The ball-and-socket joints in the human body are the shoulder and hip joints.
Movements At Freely Movable Joints
Movement at a joint result from the contraction of skeletal muscles that span across the joint. The type of movement that occurs is determined by the type of joint and the location of the muscle or muscles involved.