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We read with interest the article of Li-Yu et al on synovial rice bodies containing calcium hydroxyapatite crystals.1 From their findings the authors assume that the pathogenesis of this “apparently” rare coincidence of fibrin with bone-like apatite crystals remains unexplained.
A brief case report may give insight into the development of such a coincidence. A specimen taken at operation from the knee joint of a 55 year old woman with longstanding seropositive rheumatoid arthritis and rapid joint destruction owing to ischaemic bone necroses of the femoral and tibial compartment exhibited macroscopically a synovial membrane entirely covered with villous fibrin.2 Light microscopy showed villi of older fibrin including a lot of tissue fragments (fig 1A). Sections stained with haematoxylin and eosin showed that these particles consisted of irregular shaped bone sequesters and basophilic granules as well as shreds of hyaline cartilage. In alizarin red stained sections the bone fragments were usually faintly stained (fig 1B). However, a multitude of tiny granules were characterised by a strong stainability with this “calcium dye” (fig 1C).3
From this observation it can be deduced that bone debris from osteonecrotic areas gained access to the synovial membrane via the synovial fluid. The shreds of bone debris induced a preferentially “fibrinous inflammation”, leading to villi with entrapped cartilage and bone fragments of different size as well as tiny bone particles appearing as alizarin red granules.
With respect to the development of rice bodies of fibrin with enclosed “apatite crystals” it may be assumed that parts of the villous fibrin with bone particles can become detached from their synovial adherence and “reappear” in the synovial fluid with the formation of rice bodies.
The morphological findings described in the case report are an unusual major form of fibrinous debris synovitis due to large bone necroses. However, minor forms of this synovial reaction, originally described by Freund in 1927,4 are not rare, and histological investigations on synovial membranes from patients with advanced rheumatoid arthritis disclosed intrasynovial bone and cartilage fragments in about 50% of patients.5 A fibrinous debris synovitis also often occurs in osteoarthrosis.6 This may indicate that the opportunity for the appearance of rice bodies with bone derived apatite crystals owing to bone “abrasion” in advanced osteoarthrosis or bone infarcts is by no means unusual. Thus, the opinion of Li-Yu et al that “aggregates of apatites may be more common than previously recognised in rice bodies”1 is supported.