Sacral insufficiency fractures are not uncommon in elderly patients. We have diagnosed 20 cases in a fiveyear period, and have reviewed the clinical records, radiographs, CT and bone scans. We also assessed the degree of osteoporosis by measuring bone density using dual-energy X-ray absorptiometry and bone histomorphometry, and monitored the patients' functional outcome. Bone scans were positive in all 20 patients, CT showed a fracture or sclerosis in 7 of 12 patients and was useful in excluding malignancy. Plain radiographs were the least helpful, showing sclerosis in only 4 of the 20 patients. Involutional osteoporosis with a reduced bone formation rate was the most common underlying cause. Seventeen patients had complete resolution of pain within nine months, and no patient lost independence in daily activities. Increased awareness of these fractures may help to avoid unnecessary investigation and treatment. Bedrest and analgesia followed by rehabilitation provide good relief of symptoms.

Objective. To determine the distribution of mast cell subsets and their density in synovium from normal subjects and from patients with osteoarthritis (OA) and rheumatoid arthritis (RA). Methods. A sequential double‐immunohisto‐chemical staining technique was used to distinguish mast cells as positive for tryptase only (MCT) or for tryptase plus chymase (MCTC). Synovial tissue was obtained from RA patients (n = 16), OA patients (n = 18), and normal subjects (n = 15). Sections were analyzed to a depth of 1 mm from the synoviocyte lining layer by quantitative histomorphometry. Immunohisto‐chemical data were correlated with histologic findings and clinical indices of disease activity. Results. In normal synovium, the majority of mast cells belonged to the MCTC subset, outnumbering MCT cells by 5:1. The mean density of mast cells was significantly increased in RA synovia (60.9 cells/mm2) compared with OA (21.7 cells/mm2) and with normal (9.4 cells/mm2) synovia. Selective expansion of the MCT subset accounted for the increased mast cell density in OA. In RA, both subsets expanded and were associated with infiltrating inflammatory cells or with regions of highly cellular fibrous tissue (mainly MCTC). An association was noted between clinical parameters of activity or progression of rheumatoid disease and the density of MCTC cells, especially the density in the superficial layer of synovium. In RA synovia, we found no evidence of the chymase only, or MCC, immunophenotype. Conclusion. MCTC mast cells expand in RA but not OA, associate with regions of “active” fibrosis, and correlate with parameters of disease activity or progression of RA. These findings implicate the MCTC subset of mast cells in the pathologic mechanisms that mediate tissue damage in RA.

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