Crystalline Arthropathies

Top, Urate crystals from a tophus are viewed under high-power polarized microscopy. Under optimal circumstances of having the microscope condenser racked down as low as possible and the appropriate light intensity, monosodium urate monohydrate crystals can be seen with the ordinary light microscope. The crystals are very weakly refractile and exhibit no characteristic coloring. This method is not diagnostic, nor is it adequate for the routine evaluation of joint fluid in which crystals are less numerous and many are within leukocytes and therefore more difficult to find. Bottom, Urate crystals from the same field are viewed under high-power compensated polarized light microscopy. Monosodium urate monohydrate crystals are strongly negatively birefringent. They are bright yellow when parallel to the orienting line (slow vibration orientation) of the microscope’s compensator and bright blue when perpendicular to this line. The crystals are almost always in a fine needle shape and extinguish exactly on the axis of the polarizer and analyzer (see slide 1).

This posteroanterior projection of the knee demonstrates calcification of the menisci and articular cartilage that is typical of chondrocalcinosis. The calcification is due to focal deposits of calcium pyrophosphate dihydrate crystals in articular cartilage. In this view the deposits are more marked in the lateral compartment of the joint. Classically, both knees (especially menisci) are involved, and the calcifications, although linear, are not smooth and continuous but are interrupted by multiple focal deposits, as seen here. Statistically, the knee is the most frequent site of such calcification and attacks of pseudogout. However, other joints may be involved. Many patients with chondrocalcinosis on radiograph have not had clinical attacks of pseudogout, and some patients with proven attacks of pseudogout do not have radiographic evidence of chondrocalcinosis on routine radiographs.