Simultaneous occurrence of calcium pyrophosphate dihydrate and basic calcium phosphate (hydroxyapatite) crystals in a knee.

An 82-year-old man developed periarticular roentgenographic calcifications of the knee joint. Fragments of meniscus and peritendinous tissue were collected during total knee arthroplasty. Crystals were released from tissues by collagenase digestion. Calcium pyrophosphate dihydrate crystals were identified in the meniscus, and basic calcium phosphate crystals were identified in the peritendinous tissue. Methods of crystal identification included compensated polarized light microscopy, scanning electron microscopy with X-ray energy dispersive analysis, and X-ray diffraction.