The overriding consideration in choosing between soft contact lenses (SCLs) and rigid gas-permeable (RGP) lenses is the severity of the changes each induces in the corneal epithelium, endothelium, and conjunctival structures. Lens-related epithelial changes are largely the consequence of relative oxygen deprivation and consequent corneal edema. Factors such as lower oxygen transmissibility, minimal tear exchange capacity, and large diameters may explain why SCL wearers show a higher rate of corneal infiltrates, sterile ulcers, and irregular staining patterns than users of RGP lenses. The greater association of hydrogels with infectious keratitis may reflect their tendency to accumulate proteinaceous deposits, harbor bacteria in the polymer matrix, and resist easy disinfection. SCL wear has been linked to endothelial polymegethism, a largely irreversible condition that may cause more rapid corneal swelling and slower deswelling after periods of hypoxic stress. Among the conjunctival problems more often seen in SCL wearers are superficial neovascularization, contact lens-related superior limbic keratoconjunctivitis, and gaint papillary conjunctivitis. Mechanical irritation from large-diameter lenses, a tendency to build up mucoproteinaceous deposits, corneal draping, and hypersensitivity to preservatives in SCL care products may play a role in these problems.
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