To systematically study the histopathology of sacroiliitis in ankylosing spondylitis (AS) at 5 different stages of the disease.

Two independent observers assessed 75 microscopic features in the sacroiliac (SI) joints in 12 cases of AS (5 biopsies, 7 autopsies) and in 22 control cases (all autopsies).

In AS, synovitis, pannus formation, myxoid marrow, superficial cartilage destruction, enthesitis, intraarticular fibrous strands, new bone formation, and bony ankylosis were significantly more frequent than in control cases, in which there was more endochondral bone within deep‐zone articular cartilage. Cartilaginous fusion occurred in both groups, but much earlier in AS. There was no residual synovium when the joint lumen was totally occluded. Mild but destructive synovitis and myxoid subchondral bone marrow were the earliest changes identified in AS. These lesions destroyed the adjacent articular tissues, a loss that was followed to varying degrees by fibrous scarring, woven bone, and new cartilage. The original cartilages also fused, and chondral fusion was the predominant mode of ankylosis. Both the original and the reparative cartilaginous tissues were replaced by bone. Active enthesitis occurred in 2 advanced and 3 late cases; fibrous scar tissue, presumed to represent previous enthesitis, was observed in all stages except the earliest. Paraarticular bone was at first dense, and later porotic.

In the sacroiliitis of AS, two findings predominate: 1) synovitis and subchondral bone marrow changes offer a more rational explanation for widespread joint destruction than does enthesitis; and 2) an unusual form of chondroid metaplasia contributes to ankylosis.