Severe periprosthetic cortical atrophy in the skeletally immature: a report of three cases

We describe three patients who developed cortical bone atrophy around cemented endoprostheses used for partial femur reconstructions after resection of Ewing's sarcoma. We believe this to be related to remodeling secondary to stress-shielding. Rather than increased porosity and decreased mineral density, the stress-shielding in these skeletally immature patients resulted in altered morphology of the cortical bone, with apparent maintenance of density.

Articular fractures

Although injuries to articular cartilage may lead to radiographic osteoarthritis, pain, and decreased joint function, the actual effects of such injury and of its treatment on joint function are not completely understood. The mechanisms of repair after impact loading are different from those after frank disruption by fracture of the articular cartilage, but basic and clinical research both indicate that the resultant articular surface is prone to degeneration. The sensitivity of a joint to resultant incongruity varies considerably, depending on the thickness and modulus of the articular cartilage and the geometry of the joint. Also, factors other than articular congruity play a substantial role in determining outcomes after treatment. For these reasons, defining a single threshold for articular displacement that correlates with outcomes in all joints is not practical. Some articular fractures injure cartilage so severely that the joint will degenerate even with an accurate articular reduction. Also, radiographic evidence of osteoarthritis does not necessarily correlate with poor function. More reliable measurement techniques are needed to accurately assess how treatment affects arthritis, and factors other than articular congruity are needed to predict posttraumatic arthritis.

The impact of osteoarthritis: implications for research

Osteoarthritis, the clinical syndrome of joint pain and dysfunction caused by joint degeneration, affects more people than any other joint disease. There are no consistently effective methods for preventing osteoarthritis or slowing its progression, and symptomatic treatments provide limited benefit for many patients. Osteoarthritis disables about 10% of people who are older than 60 years, compromises the quality of life of more than 20 million Americans, and costs the United States economy more than $60 billion per year. The incidence of osteoarthritis rises precipitously with age; as a result, the prevalence and burden of this disorder is increasing rapidly. Study of the patterns osteoarthritis incidence and prevalence shows that it occurs frequently in the hand, foot, knee, spine and hip, but rarely in the ankle, wrist, elbow, and shoulder, and the most important universal risk factors are age, excessive joint loading, and joint injury. Analysis of the impact of osteoarthritis raises questions that include: Why does the incidence increase progressively with age? Why are some joints rarely affected? How do mechanical forces cause joint degeneration? What biologic and mechanical factors slow or accelerate the rate of joint degeneration? Answering these questions could lead to effective methods of preventing osteoarthritis and slowing its progression.

Chondrocyte senescence, joint loading and osteoarthritis

cellular level is not completely understood, but both aging and loading-induced stresses have been shown to undermine cell functions related to the maintenance and restoration of the cartilage matrix. Based on precedents set by studies of other age-related degenerative diseases, we have focused our laboratory work on senescence as the cause of age-dependent decline in chondrocytes and on the impact of excessive mechanical stresses in promoting senescence. We hypothesized that senescent chondrocytes accumulate with age in articular cartilage and we propose that excessive mechanical stress plays a role in this process by promoting oxidative damage in chondrocytes that ultimately causes them to senesce. To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes, and determined the effects of chronic exposure to oxidative stress on chondrocyte growth and senescence. In addition, we measured the effects of abnormally high levels of mechanical shear stress on the release of oxidants in cartilage explants. We found that senescent chondrocytes accumulated with age in articular cartilage. In vitro studies showed that chronic oxidative stress caused by repeated exposure to peroxide, or by growth under superphysiologic oxygen tension caused chondrocyte populations to senesce prematurely, before extensive telomere erosion occurred. Mechanical shear stress applied to cartilage explants considerably increased the production of oxidants. These observations support the hypothesis that senescence accounts for age-related decline in chondrocyte function and indicate that mechanically induced oxidative damage plays a role in this process. This suggests that new efforts to prevent the development and progression of osteoarthritis should include strategies that slow the progression of chondrocyte senescence or replace senescent cells.

Telomerase reverse transcriptase subunit expression is associated with chondrosarcoma malignancy

Expression of the telomerase reverse transcriptase subunit telomerase reverse transcriptase gene is associated with most human malignancies. Because telomerase reverse transcriptase is rarely expressed in normal tissue, its presence in pathologic specimens is considered a marker of transformed cells. Moreover, high levels of expression have been correlated with poor prognosis in many cancers. Although telomerase activity has been found in chondrosarcomas, its prognostic significance in these malignant cartilage tumors is unknown. Malignancy in cartilage-derived tumors is assessed routinely by histomorphologic grading, but even well differentiated, low-grade lesions can metastasize. This unpredictable behavior greatly complicates the clinical treatment of cartilage tumors, making better prognostic indicators desirable. To address this issue we used immunohistochemistry to compare telomerase reverse transcriptase expression in a collection of 61 tumors consisting of malignant chondrosarcomas of varying grade and benign enchondromas. Associated case histories were reviewed to test the hypothesis that telomerase reverse transcriptase expression levels correlated with subsequent tumor recurrence. We found that the relative abundance of telomerase reverse transcriptase-expressing cells correlated significantly with grade and recurrence. These findings indicate that telomerase reverse transcriptase immunostaining may be a useful adjunct to the conventional three-level grading system.

Sports and osteoarthritis

PURPOSE OF REVIEW

Participation in sports improves general health but increases the risk of osteoarthritis. This review analyzes the relationships among increased joint use, joint injuries, and injury-induced joint degeneration that causes posttraumatic osteoarthritis. The purpose is to help people who participate in sports minimize their risk of joint degeneration.

RECENT FINDINGS

Participation in sports that cause minimal joint impact and torsional loading by people with normal joints and neuromuscular function may cause osteophyte formation, but it has minimal, if any, effect on the risk of osteoarthritis. In contrast, participation in sports that subject joints to high levels of impact and torsional loading increases the risk of injury-induced joint degeneration. People with abnormal joint anatomy or alignment, previous joint injury or surgery, osteoarthritis, joint instability, articular surface incongruity or dysplasia, disturbances of joint or muscle innervation, or inadequate muscle strength have increased risk of joint damage during participation in athletics.

SUMMARY

Gaining the benefits of participation in athletics while minimizing the risk of osteoarthritis requires understanding of the relationships between sports participation and joint injury and the relationships between joint injury and joint degeneration. People who wish to participate in sports should have an evaluation of their joint structure and function, muscle strength, and neuromuscular function, and people with a history of joint injury or mild osteoarthritis should select sports that have limited risk of accelerating joint degeneration.

Joint injury, repair, and remodeling: roles in post-traumatic osteoarthritis

Joint injuries, especially intraarticular fractures, frequently lead to progressive joint degeneration that causes the clinical syndrome of posttraumatic osteoarthritis. Orthopaedists try to prevent this disease by attempting to restore joint congruity, alignment, and stability; however, many patients have crippling joint pain and dysfunction develop despite optimal current treatment. The pathophysiology of posttraumatic osteoarthritis has not been explained. It is not simply the magnitude and type of injury that determines whether an injured articular surface will repair and remodel or undergo progressive degeneration. For these reasons, clinically significant progress in preventing posttraumatic osteoarthritis depends on advances in understanding of the pathogenesis of this disease that will make it possible to decrease the risk of articular surface degeneration and facilitate articular surface repair and remodeling. We examine the relationships between joint injury, repair and remodeling, and joint degeneration; the factors that increase the risk of posttraumatic joint degeneration; and, the questions that need additional investigation to develop treatments of joint injuries that will decrease the risk or severity of posttraumatic osteoarthritis.

Effects of Oxidative Damage and Telomerase Activity on Human Articular Cartilage Chondrocyte Senescence

Senescence compromises the ability of chondrocytes to maintain and repair articular cartilage. We hypothesized that oxidative stress and telomere loss contribute to chondrocyte senescence. To test this hypothesis, we compared the growth of human articular cartilage chondrocytes incubated in 5% O2 and 21% O2. Cells grown in 5% O2 reached 60 population doublings (PD) before senescing, but growth in 21% O2 induced DNA damage and premature senescence at less than 40 PD. Human telomerase reverse transcriptase (hTERT)-transduction failed to prevent chondrocyte senescence in 21% O2, but allowed 1 of 3 chondrocyte strains to exceed 90 PD in 5% O2. These results show that oxidative stress causes premature chondrocyte senescence. They may help explain the increased risk of osteoarthritis with age and after joint trauma and inflammation, and suggest that minimizing oxidative damage will help produce optimal results for chondrocyte transplantation.

Post-traumatic osteoarthritis: the role of accelerated chondrocyte senescence

Joint injuries frequently lead to progressive joint degeneration that causes the clinical syndrome of post-traumatic osteoarthritis. The pathogenesis of osteoarthritis remains poorly understood, but patient age is a significant risk factor for progressive joint degeneration. We have found that articular cartilage chondrocytes show strong evidence of senescence with increasing age, including synthesis of smaller more irregular aggrecans; increased expression of lysosomal beta-galactosidase and telomere erosion; and decreased proteoglycan synthesis, response to the anabolic cytokine IGF-I, proliferative capacity, and mitochondrial function. These observations help explain the strong association between age and joint degeneration, but they do not explain how joint injury increases the risk of joint degeneration in younger individuals. We hypothesized that excessive loading of articular surfaces due to acute joint trauma or post-traumatic joint instability, incongruity or mal-alignment increases release of reactive oxygen species, and that the increased oxidative stress on chondrocytes accelerates chondrocyte senescence thereby decreasing the ability of the cells to maintain or restore the tissue. To test this hypothesis, we exposed human articular cartilage chondrocytes from young adults to mechanical and oxidative stress. We found that shear stress applied to cartilage explants in a triaxial pressure vessel increased release of reactive oxygen species and oxidative stress induced chondrocyte senescence (as measured by expression of lysosomal beta-galactosidase, nuclear and mitochondrial DNA damage and decreased mitochondrial function). These observations support the hypothesis that joint injury accelerates chondrocyte senescence and that this acceleration plays a role in the joint degeneration responsible for post-traumatic osteoarthritis.

Brucella osteomyelitis of the proximal tibia: a case report

Brucellosis is a disease of domestic and wild animals that is transmittable to humans. Although endemic in some parts of the world, brucellosis is an uncommon human pathogen in the United States. The clinical presentation of brucellosis is nonspecific, and brucella osteomyelitis can produce lytic lesions on radiographs that resemble neoplasm. Diagnosis can therefore be difficult unless a high index of suspicion is maintained. We present a case of brucella osteomyelitis of the proximal tibia that demonstrates these features.

Oxygen effects on senescence in chondrocytes and mesenchymal stem cells: consequences for tissue engineering

Primary isolates of chondrocytes and mesenchymal stem cells are often insufficient for cell-based autologous grafting procedures, necessitating in vitro expansion of cell populations. However, the potential for expansion is limited by cellular senescence, a form of irreversible cell cycle arrest regulated by intrinsic and extrinsic factors. Intrinsic mechanisms common to most somatic cells enforce senescence at the so-called "Hayflick limit" of 60 population doublings. Termed "replicative senescence", this mechanism prevents cellular immortalization and suppresses oncogenesis. Although it is possible to overcome the Hayflick limit by genetically modifying cells, such manipulations are regarded as prohibitively dangerous in the context of tissue engineering. On the other hand, senescence associated with extrinsic factors, often called "stress-induced" senescence, can be avoided simply by modifying culture conditions. Because stress-induced senescence is "premature" in the sense that it can halt growth well before the Hayflick limit is reached, growth potential can be significantly enhanced by minimizing culture related stress. Standard culture techniques were originally developed to optimize the growth of fibroblasts but these conditions are inherently stressful to many other cell types. In particular, the 21% oxygen levels used in standard incubators, though well tolerated by fibroblasts, appear to induce oxidative stress in other cells. We reasoned that chondrocytes and MSCs, which are adapted to relatively low oxygen levels in vivo, might be sensitive to this form of stress. To test this hypothesis we compared the growth of MSC and chondrocyte strains in 21% and 5% oxygen. We found that incubation in 21% oxygen significantly attenuated growth and was associated with increased oxidant production. These findings indicated that sub-optimal standard culture conditions sharply limited the expansion of MSC and chondrocyte populations and suggest that cultures for grafting purposes should be maintained in a low-oxygen environment.

Effect of chemotherapy on segmental bone healing enhanced by rhBMP-2

Segmental bone defects are challenging clinical problems, and current surgical solutions are associated with high complication rates. In oncologic reconstructive surgery, bone healing will occur coincidently with the administration of chemotherapy to treat the underlying disease. Effective methods of graft modification or bone graft alternatives can be of great help clinically. A series of osteoinductive proteins (bone morphogenetic proteins or BMPs) has been described and shown to enhance bone formation in animal models. This study was designed to evaluate the effect of chemotherapy on bone healing enhanced by rhBMP-2. We used a critical-sized bone-defect rabbit model. Histological and radiological analysis showed that chemotherapy affects both the quantity and the quality of the bone enhanced by the addition of rhBMP-2. These results suggest that the effect of chemotherapy on bone formation could be related to inhibition in a specific pathway stimulated by the rhBMP-2.

Sports, joint injury, and posttraumatic osteoarthritis

Participation in sports increases the risk of joint injuries that can lead to posttraumatic osteoarthritis, a clinical syndrome caused by trauma-initiated joint degeneration that results in permanent and often progressive joint pain and dysfunction. Minimizing the risk of joint injuries and helping people with osteoarthritis participate in regular physical activity, including some sports, requires understanding of the relationships between joint use, joint injury, and joint degeneration. Lifelong participation in sports that cause minimal joint impact and torsional loading by individuals with normal joints and neuromuscular function does not increase the risik of posttraumatic osteoarthritis. In contrast, participation in sports that subject joints to high levels of impact and torsional loading increases the risk of joint injury and subsequent joint degeneration. Immediate diagnosis and appropriate treatment and rehabilitation following joint injuries decrease the risk of subsequent injuries and posttraumatic osteoarthritis. Individuals with abnormaljoint anatomy or alignment, previous significant joint injury, osteoarthritis, joint surgery, joint instability, disturbances of joint or muscle innervation or inadequate muscle strength have increased risk of joint damage during participation in athletics. These individuals can benefit from regular exercise, including selected sports, but they should have an evaluation of their joint structure and function, muscle strength, and neuromuscular function before participating in vigorous physical activity.

Industrial support of orthopaedic research in the academic setting

Industry support provides critical resources for researchers in departments of orthopaedic surgery, and affords research that otherwise likely would not be possible. However, in contrast to sponsorship from the federal agencies or most foundations, corporate sponsorship raises ethical, practical, and legal issues for the individual researcher, the department, the academic institution, the scientific community at large, and industry. Most of these issues relate to ownership of intellectual property, confidentiality, disclosure of results, and apparent bias. For the public the issues involve ethical issues, including trust. Academic institutions have evolved approaches for contracts with industry, which minimize, but not eliminate these problems. Given appropriate contracts, corporate sponsorship of research is not only mutually beneficial, but for many departments, critical.

The role of chondrocyte senescence in the pathogenesis of osteoarthritis and in limiting cartilage repair

BACKGROUND

With increasing age, the prevalence of osteoarthritis increases and the efficacy of articular cartilage repair decreases. As chondrocytes age, they synthesize smaller, less uniform aggrecan molecules and less functional link proteins, their mitotic and synthetic activity decline, and their responsiveness to anabolic mechanical stimuli and growth factors decreases. These observations led us to hypothesize that progressive cell senescence decreases the ability of chondrocytes to maintain and to restore articular cartilage.

METHODS

To test this hypothesis, we measured cell senescence markers (beta-galactosidase expression, mitotic activity, and telomere length) in human articular cartilage chondrocytes from twenty-seven donors ranging in age from one to eighty-seven years. We also assessed mitochondrial DNA, membrane potential, and numerical density. To determine if chondrocyte age changes are reversible, we transfected human articular cartilage chondrocytes with the human telomerase gene (hTERT) and human papilloma virus oncogenes (E6 and E7).

RESULTS

Beta-galactosidase expression increased with age (r = 0.84, p = 0.0001), while mitotic activity and telomere length declined (r = -0.77, p = 0.001 and r = -0.71, p = 0.0004, respectively). Decreasing telomere length was closely correlated with increasing expression of beta-galactosidase and decreasing mitotic activity. As the number of population doublings increased, mitochondrial DNA was degraded, mitochondrial membrane potential was lost, and the number of mitochondria per cell declined. Transfection of human articular cartilage chondrocytes from a forty-seven-year-old donor with hTERT and human papilloma virus proto-oncogenes E6 and E7 created a cell line that has completed more than 300 population doublings as compared with an upper limit of twenty-five population doublings for normal cells. Telomere length increased in cells transduced with hTERT.

CONCLUSIONS

These findings help to explain the previously reported age-related declines in chondrocyte synthetic activity, mitotic activity, and responsiveness to anabolic cytokines and mechanical stimuli. They also suggest that in vivo chondrocyte senescence contributes to the age-related increase in the prevalence of osteoarthritis and decrease in the efficacy of cartilage repair. The creation of immortal cells with increased telomere length suggests that the progression of human chondrocytes toward senescence is not inevitable.

The role of tenascin-C in adaptation of tendons to compressive loading

Although most tendon regions are subjected primarily to high tensile loads, selected regions, primarily those that directly contact bones that change the direction of the tendon, must withstand high compressive loads as well. Compressed tendon regions differ from regions subjected to primarily tensile loads: they have a fibrocartilaginous structure with spherical cells surrounded by a matrix containing aggrecan and collagen types I and II, in contrast regions not exposed to compression have a fibrous structure with spindle shaped fibroblasts surrounded by a matrix of dense, longitudinally oriented type I collagen fibrils. The spherical shape of cells in fibrocartilagenous regions indicates these cells are more loosely attached to the matrix than their spindle-shaped counterparts in fibrous regions, a feature that may help to minimize cell deformation during tendon compression. We hypothesized that expression of tenascin-C, an anti-adhesive protein, is part of the adaptation of tendon cells to compression that helps establish and maintain fibrocartilaginous regions. To test this hypothesis we compared tenascin-C content and expression in compressed (distal) versus uncompressed (proximal) segments of bovine flexor tendons. Immunohistochemistry and immunoblot analyses showed that tenascin-C content was increased in the distal tendon where it co-distributed with type II collagen and aggrecan. Tendon cells from the distal segments expressed more tenascin-C than did cells from the proximal segments for up to four days in cell culture, indicating that increased tenascin-C expression is a relatively stable feature of the distal cells. These observations support the hypothesis that tenascin-C expression is a cellular adaptation to compression that helps establish and maintain fibrocartilagenous regions of tendons.

Osteochondral repair of primate knee femoral and patellar articular surfaces: implications for preventing post-traumatic osteoarthritis

The risk of post-traumatic osteoarthritis following an intra-articular fracture is determined to large extent by the success or failure of osteochondral repair. To measure the efficacy of osteochondral repair in a primate and determine if osteochondral repair differs in the patella (PA) and the medial femoral condyle (FC) and if passive motion treatment affects osteochondral repair, we created 3.2 mm diameter 4.0 mm deep osteochondral defects of the articular surfaces of the PA and FC in both knees of twelve skeletally mature cynomolgus monkeys. Defects were treated with intermittent passive motion (IPM) or cast-immobilization (CI) for two weeks, followed by six weeks of ad libitum cage activity. We measured restoration of the articular surface, and the volume, composition, type II collagen concentration and in situ material properties of the repair tissue. The osteochondral repair response restored a mean of 56% of the FC and 34% of the PA articular surfaces and filled a mean of 68% of the chondral and 92% of the osseous defect volumes respectively. FC defect repair produced higher concentrations of hyaline cartilage (FC 83% vs. PA 52% in chondral defects and FC 26% vs. PA 14% in osseous defects) and type II collagen (FC 84% vs. PA 71% in chondral defects and FC 37% vs. PA 9% in osseous defects) than PA repair. IPM did not increase the volume of chondral or osseous repair tissue in PA or FC defects. In both PA and FC defects, IPM stimulated slightly greater expression of type II collagen in chondral repair tissue (IPM 81% vs. CI 74%); and, produced a higher concentration of hyaline repair tissue (IPM 62% vs. CI 42%), but IPM produced poorer restoration of PA articular surfaces (IPM 23% vs. CI 45%). Normal articular cartilage was stiffer, and had a larger Poisson's ratio and less permeability than repair cartilage. Overall Cl treated repair tissue was stiffer and less permeable than IPM treated repair tissue. The stiffness, Poisson's ratio and permeability of femoral condyle cast immobilized (FC CI) treated repair tissue most closely approached the normal values. The differences in osteochondral repair between FC and PA articular surfaces suggest that the mechanical environment strongly influences the quality of articular surface repair. Decreasing the risk of post-traumatic osteoarthritis following intra-articular fractures will depend on finding methods of promoting the osteochondral repair response including modifying the intra-articular biological and mechanical environments.

The Vienna heritage of Iowa orthopaedics

Strong traditions of basic research, clinical innovation, teaching and integrating science and evaluation of outcomes into clinical practice have characterized University of Iowa orthopaedics for ninety years. These traditions were brought to Iowa City from Vienna when Iowa City was a town of fewer than 10,000 people in a sparcely populated rural state. In the last third of the 19th century, surgeons at the University of Vienna, led by Theodore Billroth (1829-1894), helped transform the practice of surgery. They developed new more effective procedures, analyzed the results of their operations, promoted the emergence and growth of surgical specialties and sought understanding of tissue structure, physiology and pathophysiology. Their efforts made Vienna one of the world's most respected centers for operative treatment, basic and clinical research and surgical education. Two individuals who followed Billroth, Eduard Albert (1841-1900) and Adolf Lorenz (1854-1946) focussed their research and clinical practice on orthopaedics. Their successes in the study and treatment of musculoskeletal disorders led one of their students, Arthur Steindler (1878-1959), a 1902 graduate of the Vienna Medical School, to pursue a career in orthopaedics. Following medical school, he worked in Lorenz's orthopaedic clinic until 1907 when he joined John Ridlon (1852-1936) at the Chicago Home for Crippled Children. In 1910, Steindler became Professor of Orthopaedics at the Drake Medical School in Des Moines, Iowa, and, in 1913, John G. Bowman, the President of the University of Iowa, recruited him to establish an orthopaedic clinical and academic program in Iowa City. For the next third of a century he guided the development of the University of Iowa Orthopaedics Department, helped establish the fields of orthopaedic biomechanics and kinesiology and tirelessly stressed the importance of physiology, pathology and assessment of the outcomes of operations. From the legacy of Billroth, Albert and Lorenz, Arthur Steindler created an internationally recognized center for orthopaedic care, research and teaching in Iowa City.

Iowa and Eugene, Oregon, orthopaedics

Over the last 50 years, the commitment of orthopaedic surgeons to basic and clinical research and evaluation of treatment outcomes has made possible remarkable improvements in the care of people with injuries and diseases of the limbs and spine. A group of Oregon orthopaedic surgeons has had an important role in these advances, especially in the orthopaedic specialties of sports medicine and hip reconstruction. Since Don Slocum (Iowa Orthopaedic Resident, 1934-1937), started practice in Eugene, Oregon, in 1939, three orthopaedic surgeons, Denny Collis, Craig Mohler and Paul Watson, who received their orthopaedic residency education at the University of Iowa, and three orthopaedic surgeons, Stan James, Tom Wuest and Dan Fitzpatrick, who received their undergraduate, medical school and orthopaedic residency education at the University of Iowa, have joined the group Dr. Slocum founded. These individuals, and their partners, established and have maintained a successful growing practice that serves the people of the Willamette valley, but in addition, they have made important contributions to the advancement of orthopaedics.

Aneurysmal bone cyst following tibial fracture: a case report

A 14 year-old boy developed a tibial aneurysmal bone cyst (ABC) following a closed tibia fracture. The tumor formed in a site remote from the fracture and was not radiographically apparent until one year following the traumatic event. Most ABC's present due to fracture or pain, but this lesion was discovered during routine follow up films of the tibial shaft fracture. This case lends support to the debated theory that ABC's are reactive bone lesions and provides a rare radiographic glimpse at the lesion's early rate of development.

Aging, articular cartilage chondrocyte senescence and osteoarthritis

The incidence of osteoarthritis (OA), the disease characterized by joint pain and loss of joint form and function due to articular cartilage degeneration, is directly correlated with age. The strong association between age and increasing incidence of osteoarthritis (OA) marks OA as an age related disease. Yet, like many other age related diseases, OA is not an inevitable consequence of aging; instead, aging increases the risk of OA. Articular cartilage aging changes that may lead to articular cartilage degeneration include fraying and softening of the articular surface, decreased size and aggregation of proteoglycan aggrecans and loss of matrix tensile strength and stiffness. These changes most likely are the result of an age related decrease in the ability of chondrocytes to maintain and repair the tissue manifested by decreased mitotic and synthetic activity, decreased responsiveness to anabolic growth factors and synthesis of smaller less uniform aggrecans and less functional link proteins. Our recent work suggests that progressive chondrocyte senescence marked by expression of the senescence associated enzyme beta-galactosidase, erosion of chondrocyte telomere length and mitochondrial degeneration due to oxidative damage causes the age related loss of chondrocyte function. New efforts to prevent the development and progression of OA might include strategies that slow the progression of chondrocyte senescence or replace senescent cells.