Which morphology of dry bone articular surfaces suggests so‐called fibrous ankylosis in the elderly human sacroiliac joint?

Usually, joint degeneration with aging results in articular cartilage defects, which results in bony ankylosis. However, the sacroiliac articular cartilage is maintained even in the elderly and the fibrous tissues make so‐called ‘fibrous ankylosis’. Macroscopically and histologically, we observed two sacroiliac joints obtained from one young cadaver as well as 23 sacroiliac joints from 23 elderly cadavers. Each joint was divided into two pieces along the long axis: one half was processed for routine histology after decalcification, whereas the other half was macerated to provide a dry bone specimen. The articular cartilage consistently contained abundant fibers and some of the fibers connected to tight intra‐articular fiber bands. Fiber insertion into the thin subchondral bone displayed a tidemark in a spotty manner. Thus, the calcified fibrocartilage seemed to be present and seemed to provide fine granularity on the dry bone specimen. The joint cavity was sometimes closed with fibrocartilage‐like tissues: we termed this complete fibrous ankylosis. The dry bone specimen corresponding to complete fibrous ankylosis exhibited significant microporosity and granularity because of fragmented subchondral bone. Moreover, bony ankylosis along the sacroiliac joint margin also contained fibrocartilage‐like tissues. Therefore, complete fibrous ankylosis is also likely to be the preliminary step to bony ankylosis in the entire joint area. Consequently, microporosity with granularity seemed to be the most critical anthropological characteristic for estimation of sacroiliac joint movability.