Kinetics and Mechanisms of Dissolution and Growth of Acicular Triclinic Calcium Pyrophosphate Dihydrate Crystals

Formation of calcium pyrophosphate (diphosphate) dihydrate (CPPD) crystals in articular cartilage and synovial fluid leads to CPPD deposition disease and pseudogout. Rates of dissolution and growth in the pH range of 5−7 of triclinic CPPD (t-CPPD) microcrystals of acicular (needlelike) shape are reported here. As previously found for columnar t-CPPD, the mechanism of dissolution of acicular t-CPPD is best described by the polynuclear model. The rates of dissolution per unit area of acicular t-CPPD are about a factor of 2 faster than the corresponding rates for columnar t-CPPD. Growth of acicular t-CPPD also appears to be explained by the polynuclear mechanism, as previously found for growth of columnar t-CPPD. At low pH (4.5), the rates of growth per unit area of acicular and columnar t-CPPD are similar. At higher pH (5.5 and 6.5), the rates of growth per unit area of acicular t-CPPD are slower than corresponding rates for columnar t-CPPD. At constant pH, this effect increases as the supersaturation decreases. The main difference between the two morphologies is the edge length per unit area. The larger solution volume around edges may facilitate acicular crystal dissolution but lead to inhibition of acicular growth by chelation of pyrophosphate to calcium ions blocking normal growth.