A joint, also known as an articulation, is a point in the body where two or more bones connect. These connections serve the fundamental purpose of linking the skeletal system into a functional whole. Joints provide the necessary support and flexibility that enable various body movements, from simple actions like bending and stretching to complex activities such as running and jumping. They are integral to daily activities, allowing for a wide range of motion and contributing to the overall mobility and stability of the human body.
Classification by Structure
Joints are categorized structurally based on the material binding bones and the presence of a joint cavity. This classification divides joints into three primary types: fibrous, cartilaginous, and synovial joints. The nature of the connecting material largely dictates the degree of movement permitted at each joint.
Fibrous joints are united by dense, fibrous connective tissue, allowing little to no movement. An example includes the sutures found in the skull, such as the coronal suture, which tightly interlock the skull bones. Another type, syndesmoses, connects bones with a band of fibrous tissue, like the tibiofibular joint in the lower leg, permitting slight give. Gomphoses represent specialized fibrous joints, anchoring teeth within their bony sockets in the jaw.
Cartilaginous joints form where bones are joined by cartilage, allowing for limited movement. Synchondroses involve hyaline cartilage connecting bones, as seen in epiphyseal plates of growing bones or the first sternocostal joint. Symphyses, another cartilaginous type, use fibrocartilage to unite bones, exemplified by the pubic symphysis in the pelvis or the intervertebral discs between spinal vertebrae.
Synovial joints are the most common type, distinguished by a fluid-filled joint cavity between articulating bones. These joints feature articular cartilage covering bone ends, a joint capsule enclosing the cavity, and synovial fluid within the cavity that lubricates and reduces friction. Ligaments often reinforce the joint capsule, providing stability.
Classification by Movement
Joints are also classified by the amount of movement they allow, which directly correlates with their structural composition. This functional classification system groups joints into synarthroses, amphiarthroses, and diarthroses.
Synarthroses are joints that permit little to no movement, providing strong connections between bones. These immobile joints often serve to protect internal organs or provide structural stability. The sutures of the skull are examples of synarthroses.
Amphiarthroses are joints that allow for limited mobility. This slight movement is crucial for absorbing shock and distributing forces across the skeletal system. The intervertebral discs in the spine illustrate amphiarthroses.
Diarthroses are freely movable joints, enabling a wide range of motions. All synovial joints fall into this category. Joints such as the shoulder or knee are classic examples of diarthroses, allowing complex actions like rotation, flexion, and extension.
Specific Synovial Joint Types
Synovial joints are further subdivided based on the shapes of their articulating surfaces and the types of movement they facilitate. There are six types of synovial joints.
Plane, or gliding, joints feature flat or slightly curved articular surfaces that allow for simple gliding or sliding movements. These joints enable non-axial motion. Examples include the intercarpal joints in the wrist and the intertarsal joints in the ankle.
Hinge joints operate like a door hinge, permitting movement primarily in one plane. The convex surface of one bone fits into the concave trough of another, allowing for flexion and extension. The elbow joint, the knee joint, and the interphalangeal joints of the fingers and toes are all examples of hinge joints.
Pivot joints allow for rotational movement around a central axis. The rounded end of one bone fits into a ring formed by another bone and ligaments. The atlantoaxial joint between the first two cervical vertebrae, which allows head rotation, and the radioulnar joint, facilitating forearm pronation and supination, are pivot joints.
Condyloid, or ellipsoidal, joints possess an oval-shaped articular surface that fits into a complementary oval depression. These joints allow for biaxial movement, including flexion, extension, abduction, and adduction. The radiocarpal joint in the wrist and the metacarpophalangeal joints, or knuckles, are examples of condyloid joints.
Saddle joints are uniquely shaped, resembling a saddle, with both concave and convex surfaces on each bone that interlock. This configuration allows for biaxial movement with a greater range of motion. The carpometacarpal joint at the base of the thumb is a classic saddle joint, enabling the thumb’s opposable movement.
Ball-and-socket joints offer the greatest range of motion, allowing multiaxial movement. A spherical head of one bone fits into a cup-like socket of another bone. The shoulder joint and the hip joint are prime examples, facilitating extensive movements like flexion, extension, abduction, adduction, circumduction, and rotation.