The human skeleton contains many joints, also known as articulations, which are the sites where two or more bones meet. These connections provide mechanical stability to the skeletal structure and enable the movement necessary for daily life. The characteristics of a joint determine the amount of motion possible, ranging from completely stationary to widely mobile. Joints are classified based on two criteria: the degree of movement permitted and the material connecting the bones.
Classification Based on Degree of Movement
The functional classification system categorizes joints based on the extent of movement they allow. Joints that allow no movement are classified as synarthroses, providing a strong, unyielding union between bones. An example is the suture joint found between the flat bones of the skull, which provides robust protection for the brain.
Joints that permit only a small amount of motion are called amphiarthroses, balancing stability with limited flexibility. This slight mobility is observed in structures like the intervertebral discs, which connect adjacent vertebrae and allow the spine to bend and twist. Joints that are freely movable are known as diarthroses, encompassing the majority of the joints in the limbs. Diarthroses are designed for extensive motion and are further sub-classified based on the types of movement they enable.
Classification Based on Connecting Material
The structural classification system organizes joints based on the type of connective tissue that binds the articulating bones together. The three main structural categories are fibrous joints, cartilaginous joints, and synovial joints. Fibrous joints are held together by dense fibrous connective tissue, often rich in collagen fibers, and typically lack a joint cavity.
Examples of fibrous joints include the syndesmoses, such as the ligament connecting the tibia and fibula, and the gomphoses, which anchor the teeth within the jaw sockets. These joints generally correspond functionally to synarthroses or amphiarthroses, providing high stability with little to no movement. Cartilaginous joints are those where the bones are united by cartilage (hyaline or fibrocartilage). These joints generally function as amphiarthroses, allowing for limited movement, such as the pubic symphysis, which joins the two hip bones.
Synovial joints differ because the articulating bone surfaces are not directly connected. Instead, the bones are separated by a fluid-filled space known as the synovial cavity, making them the only joint type to possess this feature. This structural arrangement allows synovial joints to be classified exclusively as diarthroses, or freely movable joints.
The ends of the bones within a synovial joint are covered with articular cartilage, a smooth layer that minimizes friction during movement. The entire joint is enclosed by a tough articular capsule, and the synovial cavity contains synovial fluid, a viscous substance that lubricates the joint surfaces. This structure enables the large range of motion characteristic of these joints, making them the most common and versatile type in the body.
The Six Subtypes of Freely Movable Joints
Synovial joints are further categorized into six specific subtypes based on the shape of the articulating surfaces, which determines the range and type of motion permitted.
Plane Joints
Plane joints, sometimes referred to as gliding joints, feature flat or slightly curved bone surfaces that allow for non-axial, sliding movements. These joints are typically found between the carpal bones of the wrist and the tarsal bones of the ankle, enabling slight shifting motions.
Hinge Joints
Hinge joints are structured so that the cylindrical projection of one bone fits into the trough-shaped surface of another, restricting movement to a single plane. They function as uniaxial joints, permitting only flexion (bending) and extension (straightening), much like a door hinge, and are best exemplified by the elbow joint.
Pivot Joints
Pivot joints allow for rotational movement, where the rounded end of one bone fits into a ring formed by the second bone and surrounding ligaments. These uniaxial joints facilitate rotation around a long axis, such as the joint between the first two cervical vertebrae that allows the head to turn side-to-side.
Condyloid Joints
Condyloid joints, also called ellipsoidal joints, involve an oval-shaped condyle of one bone resting in an elliptical depression of another. This biaxial arrangement allows for movement in two planes, including flexion, extension, abduction, and adduction, but restricts rotation. The joints in the knuckles of the fingers are a common example of this type.
Saddle Joints
Saddle joints are unique in that the articulating surfaces are both convex and concave, resembling a rider’s saddle. Like condyloid joints, they are biaxial, permitting a wide range of motion, including opposition, which is particularly evident in the carpometacarpal joint at the base of the thumb.
Ball-and-Socket Joints
Ball-and-socket joints provide the greatest mobility, featuring a spherical head of one bone fitting into a cup-like socket of another. This multiaxial structure allows for movement in all three anatomical planes, including flexion, extension, rotation, abduction, and adduction, making them highly susceptible to dislocation. The shoulder and hip joints are the body’s primary examples of this highly mobile joint type.