Ochronotic arthropathy: structural and ultrastructural features.

Fragments of articular cartilage and synovial membrane in a case of ochronosis were studied by light microscopy (LM), polarized light, and transmission electron microscopy (TEM). Granular and/or shard-shaped pigments were observed in the synovia, cartilage, and subchondral tissue, and dispersed pigment was also seen in the synovial fluid. Zones of the articular cartilage surface showed small erosions near shards, and sometimes, when the degenerative process was in an advanced stage, a substitutive fibrosis of the cartilage edge was demonstrated. LM and TEM observations of the samples studied revealed an alteration of collagen fibrils that appeared wavy and sometimes fragmented with loss of periodicity. They were always mixed with the dispersed pigment. A peculiar finding that characterized this ochronotic case was the complete absence of inflammatory infiltrates or signs of monocyte-macrophage activation. These structural and ultrastructural observations suggest that the pigment deposition in the articular surfaces was due to the synovial fluid circulation and partially to subchondral blood flow, which transports and stores the ochronotic pigments in the synovia and cartilage. These etiopathologic elements associated with the mechanical pathogenesis naturally present in the joints can contribute to the explanation of the pathogenesis and origin of ochronotic arthropathy.