Figure 1

Vascular plasticity in rheumatoid arthritis and osteoarthritis. (a) Vascular distribution in the normal joint. Blood vessels in the synovium (s) are highly organised, with larger vessels sparsely distributed in the sublining region, branching to form dense microvascular networks adjacent to the synovial surface and capsule (cap). An additional vascular network in the subchondral bone (b) does not normally cross into the articular cartilage (cart), which remains avascular. (b) In rheumatoid arthritis (RA), hyperplastic synovial pannus (p) attaches to and invades articular cartilage and adjacent bone. The synovial vascular network is reorganised, leading to reduced vascular densities adjacent to the joint space and increased vascular densities in the deeper synovium. Similar changes in vascular organisation occur in the synovium in osteoarthritis (OA). However, rather than destruction of bone and cartilage by pannus, vascular invasion of cartilage in the developing osteophyte (o) and at the osteochondral junction may lead to advancing endochondral ossification. Innervation of new vessels by fine, unmyelinated sensory nerves may contribute to increased pain sensation.