Calcium pyrophosphate crystal deposition in gouty tophi.

BACKGROUND The coexistence of calcium pyrophosphate (CPP) and monosodium urate crystals has been rarely reported in gouty tophi. We investigated CPP crystal deposits in a series of gouty tophi removed by surgery and factors associated with these deposits. METHODS 25 tophi from 22 patients were analyzed by polarized light microscopy, field-emission-scanning electron microscopy (FE-SEM) and µ Fourier transform infrared (FTIR) spectroscopy. RESULTS Tophi consisted of multiple lobules separated by fibrous septa and surrounded by a foreign-body giant cell reaction. CPP crystal aggregates were identified in 9 of 25 tophi from 6 patients. CPP crystals were dispersed or highly compacted, localized at the edge or inside the tophus lobules and filled some of them. Both monoclinic and triclinic CPP crystal phases were identified by FE-SEM and µ FTIR. As compared with patients without CPP, those with CPP-containing tophi were older (mean 60.5 vs 47.2 years, p=0.009), and had longer-term gout (mean 17.0 vs 9.0 years, p<0.05) and tophi (mean 10.0 vs 4.6 years, p<0.01). None had radiological chondrocalcinosis of the knee or wrist. CONCLUSION CPP crystal formation seems to be a late and frequent event of tophus maturation, favored by ageing and could contribute to the speed of tophus dissolution and the apparent persistence of tophus sometimes observed even after efficient, long-lasting urate-lowering therapy.