Hydrogel (HEMA) contact lenses of increased thickness were worn by owl monkeys (Aotus tivergatus) under open and closed eye conditions to study ultrastructurally the corneal epithelial response to increased stress. The 0.07 mm thick lens with a calculated Dk/L of 4.2-2.2 caused an epithelial thinning without oedema. This thinning was due to a loss of superficial cells and a flattening of the remaining ones. Factors in the mechanism of epithelial thinning without oedema may include anoxia, trauma, lid pressure, and lens weight. The 0.4 mm thick lens offers negligible gas transmission in the closed eye, and after 48 hours much of the epithelium was stripped, while oedema and degenerative cytoplasmic changes were prominent in areas of surviving cells. In such areas the thickness of the epithelium was reduced to one or two layers of cells. The basement membrane was in all experiments unharmed by hydrogel contact lens wear. The epithelial innervation was maintained in all corneas except those of the closed eye, and finding correlates well with the good corneal touch threshold in soft contact lens wearers.
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