The mineralogical origin of calcium pyrophosphate dihydrate and carbonate hydroxyapatite depositions in human ligamentum flavum

Abstract

Calcification of ligamentum flavum (CLF) in cervical spine was characterized in terms of crystal morphology, mineral phases and chemical components. The cause of the disease, forming mechanism, as well as the developing process of the calcification were discussed combined with the pathological results. Micro-FTIR, Raman, scanning electron microscope (SEM), transmission electron microscope (TEM) and synchrotron radiation X-ray diffraction (XRD) were applied to two cases of CLF in cervical spine. The results showed that the calcification mainly presented four kinds of morphology, i.e., columnar, granular, flocculent and spherical. Columnar and granular calcification, which was constituted by single crystals with good crystalline shape and sharp boundary, was identified as triclinic-calcium pyrophosphate dihydrate (t-CPPD). Flocculent or spherical calcification, which was aggregated by nano-polycrystalline, was mainly composed by B-type carbonate hydroxyapatite. The results of micro-FTIR and Raman showed that CPPD and CHA unevenly distributed among the calcification aggregates, where the proportion of CHA decreased but CPPD increased from the center to the periphery. Based on the above analysis, we estimated that granular and columnar CPPD crystals were formed in the initial stage of calcification . In the later stage, the decomposition of CPPD and deposition of CHA occurred on the calcification aggregates, in which the former process provided the material source for CHA formation. During the deposition of CHA, nanocrystals firstly formed, followed by their aggregation into flocculent calcification, and finally forming spherical calcification.