A histochemical and x-ray diffraction study of keratoconus epikeratoplasty. Report of two cases

Two epikeratoplasty lenticules (Epi-L) with keratoconus host corneas (Epi-H) were removed by penetrating keratoplasty because of poor visual acuity although they were clinically clear. Light microscopy showed that the keratocytes were underpopulated and unevenly distributed in Epi-L. Transmission electron microscopy showed that abnormal sized proteoglycans (PGs) were unevenly accumulated in the stromas of both Epi-L and Epi-H. The low-angle x-ray diffraction results from case 1 showed lower collagen interfibrillar spacings, compared with those from one keratoconus cornea, seven cryolenticules, and four normal human corneas. The fibril diameters and the collagen axial periodicity D were similar in all tissues. Allowing selected samples to swell, the interfibrillar spacing increased by 30.6-45.5% in Epi-H and by 29.8% in the keratoconus control. However, it increased by only 7.6-9.7% in Epi-L and by 11.4% in a cryolenticule. The uneven distribution of PGs and the lower interfibrillar spacings in the swollen lenticules suggest that there is an uneven distribution of water in different parts of the Epi-L and Epi-H stromas. This may lead to a variation of the refractive index or optical properties in the stroma and the deterioration of the visual image, which may explain the poor visual results.