Chapter 8: Skeletal System: Articulations

Joint classification is examined in two ways: structurally (fibrous, cartilaginous, synovial, or bony fusion) and functionally based on mobility. Functionally, joints are categorized as synarthroses (immovable, such as sutures found in the skull, or a gomphosis binding teeth to sockets), amphiarthroses (slightly movable, such as a syndesmosis or a symphysis like the pubic symphysis), and diarthroses (freely movable, commonly referred to as synovial joints). Synovial joints possess defining characteristics, including a joint capsule, articular cartilages, a cavity filled with synovial fluid, and a synovial membrane. This fluid is functionally essential as it provides lubrication to reduce friction, nourishment to chondrocytes within the articular cartilages, and acts as a vital shock absorber. Accessory structures, such as pads of fibrous cartilage called menisci (articular discs) that cushion and modify joint surfaces, fat pads, and bursae (fluid-filled pockets that reduce friction), contribute to joint integrity. The strength and mobility of a joint are inversely related; joints allowing a greater range of motion are typically weaker. Synovial joints are classified by the degrees of movement they permit—monaxial (one plane), biaxial (two planes), or triaxial (three planes)—corresponding to structural types like gliding, pivot, hinge, ellipsoid, saddle, and ball-and-socket joints. Essential movements discussed include angular motions like flexion (decreasing the angle), extension (increasing the angle), abduction (moving away from the midline), adduction (moving toward the midline), and circumduction. Rotational movements such as internal and external rotation, along with specialized motions like pronation, supination, dorsiflexion, plantar flexion, inversion, and eversion are also covered. The chapter then analyzes specific complex articulations. The temporomandibular joint (TMJ) is described as a small, complex multiaxial joint utilizing an articular disc, making it unique compared to typical synovial joints. Intervertebral articulations involve plane synovial joints (zygapophysial joints) between articular facets and symphyses between vertebral bodies separated by intervertebral discs. These discs contain a gelatinous core, the nucleus pulposus, surrounded by the tough anulus fibrosus. The highly mobile, triaxial glenohumeral (shoulder) joint is a ball-and-socket joint that sacrifices stability for extensive range of motion and relies heavily on stabilizing ligaments and the rotator cuff muscles. The hip joint, another ball-and-socket articulation, achieves exceptional stability through a deep socket, the acetabular labrum, a dense capsule, and strong ligaments (iliofemoral, pubofemoral, ischiofemoral). The knee joint functions as a complex hinge joint, stabilized by collateral ligaments, menisci, and the vital anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). Clinical discussions include joint dislocation (luxation), the pain associated with herniated discs, and common sports injuries affecting the knee and ankle. The final sections note that aging often leads to joint issues such as rheumatism and arthritis, and emphasizes the structural and functional interdependence of the skeletal and muscular systems.