The joints of the skeletal system function to provide for movement of bones and absorb shock through the cushioning provided by joint fluid and cartilage (Levangie & Norkin, 2011). Three main types of joints are found in the human body: immovable, slightly movable, or freely movable joints. Immovable joints function to provide support between bones and will fuse to provide added support and strength. Slightly movable joints contain either hyaline cartilage (i.e. ribs to sternum) or fibrocartilage (i.e. intervertebral discs). With age, hyaline cartilage becomes stiffer from a decrease in water content and an increase in calcium which can lead to and increase in joint stiffness and less elasticity (Levangie & Norkin, 2011). Fibrocartilage also has been shown to lose water during aging, which also stiffens these joints and decreases movement. Collagen in the ligaments of these joints becomes shorter, stiffer and less elastic (Levangie & Norkin, 2011).

Freely movable joints are the most common type of joint in the body; they connect the appendicular skeleton to the axial skeleton. These joints have a synovial membrane that is responsible for the secretion and maintenance of synovial fluid, providing nutrition to articular cartilage and lubricating the joint space through loading and compression (Ahmed et al., 2005). With age, the synovial membrane becomes stiffer and is less able to produce and remove synovial fluid (Levangie & Norkin, 2011). Ligaments and tendons surround these joints, providing additional support and controlling movement. The capsule, ligaments and tendons of the synovial joint are all made up of fibrous connective tissue that provides stability and strength to the joint (Ahmed et al., 2005). There is an increased formation of cross-links and loss of elastic fibers of the capsule and ligaments with age, which increases joint stiffness and decreases the joint’s ability to provide cushioning and movement (Levangie & Norkin, 2011). The loss of mobility of joints can have a significant impact in overall function of an individual (Box 4.1).

Articular cartilage functions to provide the articulating surfaces with a load-bearing and force-distribution structure, and also serves to lower the friction between the joint surfaces (Ahmed et al., 2005). Articular cartilage is absent of vascular elements but is in contact with synovial fluid on the articular surface of joints (Carrington, 2005). It consists of an extracellular matrix, made up of primarily type II collagen, water and proteoglycans, and chondrocytes. Chondrocytes function to maintain cartilage homeostasis through the production of extracellular matrix component (Leong & Sun, 2011). The extracellular matrix makes up the majority of the cartilage volume and is highly hydrated to allow for the diffusion of nutrients from the surrounding synovium, underlying bone, or perichondrium. Water contributes up to 80% of the wet weight of articular cartilage, some of which moves freely in and out of the tissue (Buckwalter et al., 2005). Articular cartilage is dependent on the repetitive compression and release from weight-bearing for nutrition as it allows for the drawing in and exudation of water from the matrix.

The risk of progressive degeneration of cartilage increases with time in individuals older than 40 years of age (Buckwalter et al., 2000); see Box 4.2. Due to its avascular nature, articular cartilage has low metabolic activity and poor regenerative capacity (Pearle et al., 2005). The relative immobility of chondrocytes and limited proliferation of mature chondrocytes contribute to the limited repair capacity (Buckwalter & Mankin, 1997). The concentration of glycosaminoglycans found in cartilage can vary with patient age, cartilage injury and disease (Buckwalter et al., 2005). With aging there is an overall decrease in proteoglycan synthesis and content (DeGroot et al., 2004), in addition the proteoglycans produced are smaller and more irregular (Leong & Sun, 2011). A decrease in water content and decrease in the ability of proteoglycans to absorb and hold water result in a decreased ability to dissipate joint forces.

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