2023 Volume 72 Issue 3 Pages 440-445
Monosodium urate crystals and calcium pyrophosphate dihydrate crystals are widely known as crystals found in joint fluid, but in rare cases, basic calcium phosphate crystals, such as hydroxyapatite crystals, may cause arthritis. We report a case of massive ectopic calcification in the knee joint, in which a large number of hydroxyapatite crystals was detected in the joint fluid. A 30-year-old man on hemodialysis was referred to our orthopedic surgery department because a mass in his left knee gradually enlarged, self-destroyed, and showed white pus-like drainage for one month. Pyogenic arthritis was initially suspected; therefore, curettage and cleaning of the lesion were performed immediately. On postoperative day 40, a cell count and crystal test of the left knee joint fluid were performed. The cell count was 1,850 cells/μL, and the joint fluid appeared similar to white toothpaste. Microscopic observation using a simple polarizer revealed many small lumpy unpolarized unknown crystals, which were suspected to be hydroxyapatite crystals on the basis of their shape. Subsequently, a positive Kossa stain was confirmed, consistent with the characteristics of these crystals. In this case, high calcium levels due to the use of vitamin D preparations and calcium-containing phosphate binder, hyperphosphatemia due to difficulty in restricting diet, and calcium-phosphate product levels that remained around 80 to 100 for a long period were the possible causes of mass formation by ectopic calcification. Laboratory scientists must understand the patient’s background and the morphologic characteristics and properties of these crystals before testing for joint fluid crystals.
