Calcium pyrophosphate crystal deposition in hyaline cartilage. Ultrastructural analysis and implications for pathogenesis.

Using transmission electron microscopy, selected area electron diffraction, and micro x-ray diffraction techniques, we studied calcium pyrophosphate dihydrate (CPPD) deposits from 23 patients with chondrocalcinosis affecting the femoral head to delineate the cellular and matrix environment in which CPPD crystals form, to determine the sequence of crystal deposition, and to address the question of coexistent calcium apatite crystal deposition. We found 2 types of CPPD crystal deposits with few transitional forms. First, small collections of crystals were seen at the border of the territorial matrix at the articular and subarticular poles of the chondron. CPPD crystal deposits were unassociated with collagen or matrix vesicles. Second, and more frequently, large collections of randomly arranged crystals (agglomerates) were observed, the smallest replacing the chondrocyte and adjacent pericellular matrix. Chondrocytes adjacent to crystal deposits were intact. Coexistent apatite crystal deposition was demonstrated in only 2 of 23 cartilages ultrastructurally examined, providing evidence that mixed crystal deposits are possible but not common in CPPD crystal arthropathy.