Histopathologic evidence that sacroiliitis in ankylosing spondylitis is not merely enthesitis - Systematic study of specimens from patients and control subjects

Objective. To systematically study the histopathology of sacroiliitis in an kylosing spondylitis (AS) at 5 different stages of the disease. Methods. Two independent observers assessed 75 microscopic features in the sacroiliac (SI) joints in 12 cases of AS (5 biopsies, 7 autopsies) and in 2 2 control cases (all autopsies). Results. In AS, synovitis, pannus formation, myxoid marrow, superficial car tilage destruction, enthesitis, intraarticular fibrous strands, new bone fo rmation, and bony ankylosis were significantly more frequent than in contro l cases, in which there was more endochondral bone within deep-zone articul ar cartilage. Cartilaginous fusion occurred in both groups, but much earlie r in AS. There was no residual synovium when the joint lumen was totally oc cluded. Mild but destructive synovitis and myxoid subchondral bone marrow w ere the earliest changes identified in AS. These lesions destroyed the adja cent articular tissues, a loss that was followed to varying degrees by fibr ous scarring, woven bone, and new cartilage. The original cartilages also f used, and chondral fusion was the predominant mode of ankylosis. Both the o riginal and the reparative cartilaginous tissues were replaced by bone. Act ive enthesitis occurred in 2 advanced and 3 late cases; fibrous scar tissue , presumed to represent previous enthesitis, was observed in all stages exc ept the earliest. Paraarticular bone was at first dense, and later porotic. Conclusion. In the sacroiliitis of AS, two findings predominate: 1) synovit is and subchondral bone marrow changes offer a more rational explanation fo r widespread joint destruction than does enthesitis; and 2) an unusual form of chondroid metaplasia contributes to ankylosis.