Pathologic Calcification of Biomaterials

Yashwant Patbak, Frederick J. Schoen, and Robert ]. Levy

The failure of certain clinical devices, particularly in the cardiovascular system, is frequently caused by the formation of nodular deposits of calcium phosphate or other calcium-containing compounds, a process known as calcification or mineralization. Although deposition of mineral salts of calcium occurs as a normal process in bones and teeth, the biomaterials that comprise medical devices are not intended to calcify since mineral deposits can interfere with function. Therefore, calcification of biomaterials is abnormal or pathologic. Pathologic calcification can be either dystrophic or metastatic. Dystrophic calcification is the deposition of calcium salts in damaged or diseased tissues or biomaterials in individuals with normal calcium metabolism. In contrast, metastatic calcification is the deposition of calcium salts, in previously normal tissues, as a result of deranged mineral metabolism (usually elevated blood calcium levels). Dystrophic and metastatic calcification can be synergistic; in the presence of abnormal mineral metabolism, calcification associated with biomaterials or injured tissues is enhanced.

Calcification of biomaterials can affect a variety of prostheses implanted into the circulatory system, within connective tissues, or at other sites (Table 1). For example, dystrophic calcification has been encountered as degeneration of biopros-thetic or homograft cardiac valve replacements, calcification in blood pumps used as cardiac assist devices, mineralization of intrauterine contraceptive devices, encrustation of urinary prostheses, and mineral deposition within soft contact lenses.

Calcification has been associated with both synthetic and biologically derived biomaterials. The mature mineral phase of most biomaterial calcifications is a poorly crystalline calcium phosphate, known as apatite, which is related to calcium hy-droxyapatite, the mineral that provides the structural rigidity for bone and has the chemical formula Ca10(PO4)6(OH)2- In general, the determinants of biomaterial mineralization includes factors related to both hosr metabolism and implant structure and chemistry (Fig. 1). Furthermore, mineralization of a biomaterial is generally enhanced at the sites of intense mechanical deformations, such as the flexing points in circulatory devices. Calcification can be potentiated in the presence of implant infection. Of additional importance is the fact that calcification may occur on the surface of an implant (extrinsic calcification), where it is often associated with attached tissue or cells, or within the structural components (intrinsic calcification). Finally, the underlying mechanism of pathologic calcification shares many similarities with that of normal bone mineralization. In fact, some investigators have attempted to exploit the calcification of biomaterial implants to create new-hard tissue.

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