Approximately one in five Americans is affected by arthritis, making it one of the most prevalent diseases and the leading cause of disability in the United States. Post-traumatic arthritis occurs after joint injury (e.g., ACL rupture or intraarticular fracture) and makes up a substantial proportion of the population with arthritis. In previous clinical studies, patients suffering from a traumatic joint injury have shown an increased risk in osteoarthritis (OA), independent of surgical intervention to stabilize the joint. Thus, the early events post-injury have an important effect on tissue within the joint in the long term. To understand the processes involved in the onset of OA and factors leading to OA post-traumatic injury, in vitro models have been developed to isolate components of the complex processes occurring in vivo. While in vitro models do not mimic true physiologic conditions in vivo, by isolating the effects of mechanical compression, cytokine treatment, and cartilage co-cultured with adjacent tissue, in vitro models can give insight into key biological and mechanical pathways occurring in vivo.

This study focuses on changes in cartilage gene and protein expression and associated cartilage matrix degradation in response to static or injurious compression of the tissue in the presence or absence of cytokines including TNF-α and IL-6. In addition, normal or injuriously compressed cartilage explants were co-cultured with injured (excised) joint capsule tissue, another in vitro model of post-traumatic cellular behavior. Both young bovine cartilage and human cartilage from a wide range of ages were used. The growth factors insulin-like growth factor-1 (IGF-1) and Osteogenic protein-i (OP-1), as well as the antioxidant, superoxide dismutase mimetic (SODm), were tested to examine if they had the capability to abrogate the negative effects of these injury models. Taking a systems approach, the effects of these stimuli on expression of over 48 genes (in cartilage as well as joint capsule) were quantified, along with measures of chondrocyte viability, biosynthesis, protein expression, and GAG loss.

Chondrocyte gene expression was differentially regulated by 50% static compression or IGF- 1 treatment or the combination of compression and IGF-1. Results showed that IGF-1 stimulated aggrecan biosynthesis in a transcriptionally regulated manner, whereas compression inhibited aggrecan synthesis in a manner not regulated by transcriptional activity. The injury plus co-culture model was examined in detail, and OP-1 and IGF-1 were unable to rescue changes in transcriptional expressions due to injury. However, these growth factors were able to rescue cells from apoptosis, and slightly increase biosynthesis rates. Human tissue was used to further validate the model of mechanical injury (INJ) combined with co-culture (Co). Immunohistochemical analysis of human cartilage explants after INJ+Co treatment revealed changes in versican and aggrecan protein expression, as well as changes in surface tissue morphology, that mimicked certain changes observed in human osteochondral plugs taken from patients at the time of notchplasty surgery (post ACL reconstruction) at 1, 3, or 57 months post-ACL rupture.

The oxidative stress involved in a cytokine plus injury model showed that SODm had no ability to selectively diminish protease transcriptional activity. Cartilage treated with this antioxidant showed significant increases in GAG loss to the medium, but diminished levels of chondrocyte apoptosis. Taken together, this work supports further investigation of the mechanisms of action of OP-1, IGF-1, and SODm in order to elucidate their possible therapeutic value, and demonstrates the usefulness of these complementary in vitro models of cartilage injury