Changes in Cartilage Structure and Matrix Composition
A number of studies have examined age-related changes in articular cartilage by using tissue collected from various animal species or human tissues obtained either at autopsy or from surgical specimens. When surgical specimens are studied, they usually are obtained from subjects undergoing hip or knee replacement surgery for arthritis, in which case the tissue is taken from grossly normal appearing areas but may not be truly normal. Tissues have also been obtained during hip replacement surgery after femoral neck fracture, which is usually associated with osteoporosis. In these cases, the cartilage is felt more likely to be normal because osteoporosis and OA tend to occur in different patient populations, although this is not always true.
Problems in interpreting the results of cartilage aging studies include translating the biology from lower species of animals to humans and separating disease effects from aging effects. In addition, a number of changes have been reported to occur in cartilage when comparing immature versus middle-aged animals. These changes are probably best considered to be developmentally related rather than aging-related. With the above caveats in mind, there do appear to be a number of changes occurring in cartilage that can be related to aging and, importantly, many of the aging-related changes can be contrasted to changes seen in disease (Table 112-3).
Table 112-3 Contrasting Differences between Aging and Osteoarthritis ||Download (.pdf)
Table 112-3 Contrasting Differences between Aging and Osteoarthritis
Decreased cartilage hydration
Increased cartilage hydration
Ratio of CS 4/6* decreased
Ratio of CS 4/6* increased
Cross-links lost during degradation
No or reduced proliferation
Reduced metabolic activity
Increased metabolic activity
No change in subchondral bone
Increased subchondral bone thinkness
Structurally, fibrillation of the articular cartilage surface becomes more prevalent with age, particularly in the knee joint on the vertical ridge of the patella and in the tibiofemoral areas not covered by the menisci. These changes could represent the presence of early OA, although other pathologic changes of OA are not always present and it is most often asymptomatic. Morphologic changes become more common and more extensive with advancing age (Figure 112-3). Similar to the articular cartilage, age-related changes in the menisci have also been noted. Recent studies that have examined the knee joints of older adults using magnetic resonance imaging have noted that damage to the meniscus is strongly associated with damage to the articular cartilage in the same area and meniscal damage predicts further cartilage loss. This work highlights the importance of the meniscus to the normal function of the knee joint. Since the peripheral areas of the menisci contain nerve fibers, they may also be a source of pain in people with OA.
Effect of age on the prevalence of arthritis. *Knee cartilage degeneration at autopsy is the prevalence of significant histological changes of degeneration. **Radiographic evidence of OA (Kellgren and Lawrence Grade 2 or greater) present in at least one joint site (hands, feet, spine, knees, and hips) in a population survey in northern England. ***Self-reported arthritis and ****activity limitation attributable to arthritis derived from the National Health Interview Survey—US, 1989–1991. (Reproduced with permission from Loeser RF. Aging and the etiopathogenesis and treatment of osteoarthritis. Rheum Dis Clin North Am. 2000;26:547.)
There appears to be a slight decline in the number of chondrocytes present in cartilage with age. Early studies of cartilage from the femoral head noted a 30% fall in cell density between the ages of 30 and 100 years. But more recent studies of knee joints have noted much lower cell loss with normal aging in the range of 1% to 2%. Cartilage from older individuals has also been noted to have microcracks in the calcified layer. The significance of these cracks is not clear, but if they extend to the underlying subchondral bone, they could provide a mechanism for the exchange of cytokines and growth factors between the two tissues; and if vascular invasion occurs, they could mediate remodeling of subchondral bone, which is a characteristic feature of OA.
A consistent biochemical finding in cartilage is an age-related decrease in hydration. A decrease in hydration could explain evidence obtained from magnetic resonance imaging of knee joints showing that some thinning of knee cartilage occurs with age, particularly at the femoral surface, which is more evident in women than men. The decrease in cartilage hydration is likely related to changes with age in the proteoglycans that bind the majority of the water in cartilage. The total proteoglycan content does not appear to change significantly, instead changes in proteoglycan structure have been reported that could affect its biophysical properties. Aggrecan molecules become smaller with age and are structurally altered as the result of proteolytic modification in the core protein as well as changes in the length and abundance of the attached glycosaminoglycan chains. Hyaluronic acid, to which the aggrecan molecules bind to form large aggregates, is also decreased in size with age. In addition, proteolysis of the aggrecan core protein between the G1 and G2 domains results in increased levels of molecules bound to hyaluronic acid that contain only the G1 region and therefore lack the remainder of the aggrecan molecule necessary for normal function. The half-life for the free binding region is calculated to be as long as 25 years, consistent with its accumulation in cartilage with aging. By occupying and competing for space on the hyaluronic acid strands, the bound G1 domains may reduce the number of newly synthesized aggrecan molecules bound to hyaluronic acid.
As with proteoglycans, there does not appear to be a significant reduction in the total amount of collagen present in cartilage with age. However, important changes in collagen structure and function have been noted. The collagen network appears to become stiffer with age. The increased collagen stiffness is thought to be a result of increased collagen cross-linking. There is evidence for nonenzymatic glycosylation in cartilage from older adults that result in the formation of pentosidine residues that can cross-link collagen molecules. An age-related accumulation of AGEs has been noted in cartilage (Figure 112-4). Pentosidine and other AGEs have been found in collagen, as well as in aggrecan. The accumulation of pentosidine is reported to be greater in collagen than aggrecan because of the exceptionally long half-life of collagen in cartilage, which is calculated to be approximately 117 years.
Age-related accumulation of advanced glycation end-products in cartilage. Pentosidine levels were measured in cartilage samples from 36 donors clustered into 10-year age intervals. Results shown are mean ± SEM (standard error of mean). (Reproduced with permission from DeGroot et al. Age-related decrease in proteoglycan synthesis of human articular chondrocytes: The role of nonenzymatic glycation. Arthritis Rheum. 1999;42:1003.)
In addition to increased cross-linking from the accumulation of advanced glycation end products, collagen fibril diameter tends to increase with age, and this may also contribute to changes in collagen stiffness. Increased collagen network stiffness could contribute to the decrease in hydration, as stiffer collagen would tend to cause greater proteoglycan compression and thereby push out more water from the matrix. Biomechanical studies suggest that the stiffer network is more prone to fatigue failure. With age there is a decrease in the tensile strength of cartilage as well as a decrease in overall tensile stiffness. Therefore, age-related changes in the overall composition of the cartilage matrix result in a tissue that is less capable of handling mechanical stress.
In addition to changes in type II collagen and aggrecan, age-related changes in several of the other less-abundant cartilage matrix proteins have been reported. These include a decrease in type IX collagen, a protein that may be important in holding together adjacent collagen fibers and an increase in link protein, the protein that helps bind aggrecan molecules to hyaluronic acid. While link protein is increased with age, it also appears to undergo proteolytic modification with age that could affect its function.
Aging and Chondrocyte Function
Changes in the proliferative and synthetic capacity of chondrocytes with age have been noted. There is evidence for an age-related decreased mitogenic response to serum and growth factor stimulation. There is also evidence for telomere shortening in chondrocytes isolated from older adults but it is not clear if telomere dysfunction is the cause of the reduced mitogenic response. In addition, a reduction in proteoglycan and protein synthesis in response to growth factor stimulation has been in seen in cartilage from older animals, including nonhuman primates. Likewise, decreased proteoglycan synthesis in response to serum stimulation has been noted in human cartilage from older adults. In the human samples, the decreased serum response correlated with the presence of advanced glycation end-products. Other studies also suggest that a decline in cell signaling in response to growth factors is responsible for the decreased mitogenic and synthetic responses.
A reduction in response to growth factor stimulation with age, disease, or both, could be significant in the development of OA, where catabolic processes are greater than anabolic. In addition, there appears to be an age-related increase in the ability of catabolic factors, including the cytokine interleukin-1β (IL-1β) and a fibronectin matrix fragment, to stimulate MMP production by chondrocytes. These age-related changes could contribute to the anabolic–catabolic imbalance that has been observed in OA.
An age-related finding in cartilage, which is also observed in many other soft tissues, is an increased prevalence of crystals and calcification. It is difficult to determine the relative effects of age and disease on cartilage calcification. Calcification or crystal formation within cartilage is a common feature of OA, particularly in advanced disease and, like OA, age is the strongest risk factor for the development of crystal-associated arthritis. Cartilage from older individuals often contains crystals composed of calcium pyrophosphate dihydrate or hydroxyapatite. Studies with animal tissues show that the increase in the formation of calcium pyrophosphate crystals may be caused by an age-related increase in the activity of transglutaminase, an enzyme that is involved in the biomineralization process. Also chondrocytes from older individuals produce more inorganic pyrophosphate in response to TGF-β stimulation, despite a decreased proliferative response to this and other growth factors.
Potential Role of Oxidative Stress in Cartilage Aging
The free radical theory of aging dates back to the 1950s and there is now a large body of evidence that oxidative damage can occur with aging in multiple tissues including musculoskeletal tissues. Oxidative damage in aging cartilage has been detected using antibodies to nitrotyrosine, which recognize a modification occurring after the reaction of protein tyrosine residues with the free-radical peroxynitrite generated from nitric oxide reacting with superoxide. In addition, age-related oxidative stress in human articular chondrocytes has been detected by measuring the ratio of intracellular oxidized to reduced glutathione. A reduction in levels of antioxidant enzymes with aging may contribute to an increase in oxidative stress and damage. As discussed further below, an increased level of reactive oxygen species (ROS) in aging chondrocytes could be important to the age-related increase in OA.