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OBJECTIVE: Two typical clinical courses of calcific periarthritis in the shoulder are known: acute, with severe inflammation, and chronic, in the form of impingement syndrome with secondary subacromial bursitis. It remains unclear what factors determine these clinical courses. Our objectives were to clarify whether the calcified deposits that occur in both acute and chronic cases are composed of carbonate apatite; and to compare the Ca:P molar ratio in the 2 forms and to determine if there was any correlation in this respect with the intensity of inflammation induced by basic calcium phosphate crystals. METHODS: Ten samples were aspirated from 10 women (ages 42-65 yrs) with acute inflammation. The average time from first attack to aspiration was 2.3 days. A further 10 samples were operatively removed from 10 women (ages 35-58 yrs) with refractory chronic subacromial bursitis, among whom an average of 7.8 months had passed since the onset of symptoms. All samples were analyzed by x-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy, and Ca:P molar ratios were measured by x-ray fluorescence spectrometry. RESULTS: Calcified deposits from both acute and chronic cases were identified as carbonate apatite, and not hydroxyapatite, octacalcium phosphate, tricalcium phosphate, or dicalcium phosphate dihydrate. The average Ca:P molar ratio of calcified deposits was calculated as 1.71 +/- 0.16 in acute cases and 1.71 +/- 0.16 in chronic cases (statistically nonsignificant). CONCLUSION: Deposits around the shoulder in both acute and chronic calcific periarthritis are composed of carbonate apatite, Ca:P molar ratios being almost identical in the 2 forms. The results suggest that some factor other than the composition of the crystalline deposits may determine clinical course in calcific periarthritis of the shoulder.