Early osteophyte formation after chemically induced articular cartilage injury.

We examined the early changes that follow injection of sodium iodoacetate (IA) into the guinea pig knee joint, using Safranin-O (SO) to stain the proteoglycan (PG)-rich matrix and the incorporation of Na35 SO4 to determine which cells have maintained, lost, or developed the ability to synthesize significant amounts of PGs. Within 24 hours following injection of IA, articular chondrocytes exhibited a marked loss in ability to synthesize PGs; this inhibition was complete after 2 weeks. Focal loss of SO staining of the interterritorial matrix was noted after 24 hours, progressing to a complete loss of staining after 2 weeks. In sharp contrast to the degenerative changes in the habitually loaded articular cartilage, cells at the chondrosynovial junction began incorporating Na35 SO4 and producing a matrix that stained with SO as early as 72 hours following injection. With time, this resulted in the development of prominent osteophytic cartilage that involved bone remodeling beneath the overlying cartilaginous cap after 3 weeks.