Joints are the biological connections formed wherever two or more bones meet in the human body. These articulations are fundamental to the structure and mobility of the skeleton. Joints are categorized based on the material binding the bones together and the extent of movement they permit. The connecting material dictates the joint’s mechanical properties, influencing how much the bones can shift relative to one another. This raises a specific question regarding connections made of cartilage: Do these joints allow any motion?
Joint Classification and Mobility
Anatomists use a dual system to classify joints, providing an understanding of their form and function. Structural classification groups joints based on the material bridging the bones (e.g., fibrous tissue, cartilage, or a fluid-filled cavity). Functional classification addresses mobility, dividing joints into three categories based on the degree of permitted movement.
These functional categories are labeled using Greek roots that describe motion. A synarthrosis is an immovable joint, providing maximum stability (e.g., in the skull). An amphiarthrosis permits slight, limited movement, often serving to absorb shock and provide flexibility. The third type, a diarthrosis, is a freely movable joint allowing a wide range of motion, seen in most limb joints. Cartilaginous joints are functionally classified as either synarthroses or amphiarthroses; they are never freely movable joints.
Structural Components of Cartilaginous Joints
A cartilaginous joint is defined by the presence of cartilage connecting the articulating bones, with no intervening joint cavity or lubricating fluid. This direct physical connection is the primary factor limiting movement. The specific type of cartilage present further differentiates the movement potential within this category.
Two distinct types of cartilage are involved in forming these joints: hyaline cartilage and fibrocartilage. Hyaline cartilage is the more common type, composed of a glassy matrix with fine collagen fibers. Fibrocartilage is the strongest form, characterized by thick, dense bundles of collagen fibers that provide tensile strength. The presence of fibrocartilage allows the joint to withstand greater compressive forces while retaining flexibility.
The absence of a synovial cavity—the fluid-filled space characteristic of freely movable joints—is what fundamentally restricts motion in cartilaginous joints. Instead, the bones are directly united by this tough, flexible connective tissue. The cartilage’s inherent rigidity and high collagen content prevent the broad translational or rotational movements seen in joints like the shoulder or knee.
Synchondroses and Symphyses: Movement Capabilities
The mobility of cartilaginous joints depends entirely on their specific subclass, which is determined by the connecting cartilage. Cartilaginous joints are divided into two main types: synchondroses and symphyses, each having distinct functional mobility.
Synchondroses are joints connected exclusively by hyaline cartilage and are functionally classified as synarthroses (immovable). A prime example is the temporary epiphyseal plate, or growth plate, that unites the shaft and the end of a long bone in children. This immobility facilitates bone lengthening until the cartilage is replaced by bone tissue. The first sternocostal joint, where the first rib meets the sternum, is a permanent example of this immovable union.
Symphyses, by contrast, feature a pad of fibrocartilage uniting the bones, often with a layer of hyaline cartilage covering the bone ends. These joints are classified as amphiarthroses, allowing for slight movement. The fibrocartilage pad acts as a powerful shock absorber and permits minor shifts under stress. The pubic symphysis, which joins the hip bones, allows for slight separation during childbirth, demonstrating this limited mobility. Similarly, the intervertebral discs, which are symphyses between adjacent vertebrae, permit small movements that collectively allow for the spine’s overall flexibility.