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Acute calcification in alkali-injured rabbit cornea treated with synthetic inhibitor of metalloproteinases (SIMP).

PURPOSE
The aim of this study was to evaluate the ultrastructure of acute calcification observed in alkali-injured rabbit corneas treated with synthetic inhibitor of metalloproteinases (SIMP).


METHODS
A 12-mm alkali burn was made on the right corneas of 16 rabbits. Eight eyes were treated with topical phosphate-buffered SIMP, and eight control eyes were given the buffer only. Six corneas (five in the SIMP group and one in the control group) developed chalky white opacification in the central area of the cornea within 6-30 days. The corneas were excised for the following examinations: (a) light microscopy, (b) energy-dispersive x-ray analysis, (c) electron microscopy, and (d) high-angle x-ray diffraction.


RESULTS
The results confirmed that there was stromal calcification in these corneas, as shown by positive van Kossa staining, needle-shaped calcium crystal aggregates, and increased calcium in the opaque areas of the corneas. Proteoglycans in the corneal stroma of both the SIMP group and the control group significantly decreased after alkali burns. Very few proteoglycan filaments could be identified in the calcified areas of the stroma. X-ray diffraction reflections with a Bragg spacing of 3.44 +/- 0.04 A were observed from all the calcified corneas treated with SIMP. The intermolecular spacings of collagen fibrils in the calcified stroma (1.24 +/- 0.34 nm) were significantly lower than those in the noncalcified stroma (1.64 +/- 0.04 nm) and the control corneas (1.64 +/- 0.02 nm). The results indicate that the collagen molecules within the fibrils of the calcified stroma are packed more closely than normal.


CONCLUSION
The comparison of the x-ray diffraction patterns from the calcified corneas and chicken bone suggests that the calcium deposits in these corneas are in a form similar to those found in bone. The higher rate and more extensive calcification in the SIMP-treated corneas may result from the inhibitory action of SIMP on debris processing, but the mechanism is unknown.