PubMed · 14155440
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N HASEGAWA. 1964. [IN BREAKING UP TINY DIMPLES].. https://pubmed.ncbi.nlm.nih.gov/14155440/
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The instrument oxygen transmissibility (IOT) of organosilicon hydrogels, measured by electrochemical procedures, is 5-10 times larger than that of conventional hydrogels. A method is described that allows the estimation of the oxygen tension at the lens-cornea interface for closed- and open-eyelids situations by combining the IOT of the hydrogels and corneal parameters such as corneal thickness, corneal permeability and oxygen flux across the cornea. From these results the biological oxygen apparent transmissibility (BOAT) is obtained, an important parameter which an multiplication with the pressure of oxygen on the external part of the lens gives the oxygen flux onto the cornea. Contact lenses with oxygen transmissibility higher than 100 Dk/t units [1 Dk/t unit=10(-9) [cm(3) O(2) (STp) cm(-2)s(-1)(mmHg)(-1)] posses a large oxygen tension at the lens-cornea interface that substantially reduces the oxygen flux onto the cornea. Lenses whose oxygen transmissibility is lower than 50 Dk/t units allow a rather small oxygen flux onto the cornea under closed eyelids condition that prevent their use for extended wear.
BACKGROUND: Intrastromal corneal ring segments have been used as a refractive surgery method for treatment of myopia. More recently, attempts have been made to use this surgical procedure in patients with keratoconus. CASE REPORT: A 51-year-old man with keratoconus came to the clinic for a contact lens refit. He had undergone surgical insertion of intrastromal corneal rings in his right eye 7 months earlier in an attempt to obtain acceptable vision without contact lenses. At the time of presentation, the patient was not satisfied with his vision in the right eye and requested an attempt to improve his vision with a contact lens. The resulting corneal shape after surgery presented some complications, but the patient was able to achieve 20/25-vision and adequate comfort in his right eye with a piggyback contact lens system. CONCLUSIONS: Insertion of intrastromal corneal rings may be beneficial in patients with early keratoconus or who are contact lens-intolerant. However, patients with more-advanced keratoconus are not likely to achieve adequate vision after the surgery and will require a contact lens. In addition, changes in the corneal anatomy created by the intrastromal rings may make contact lens fitting more challenging. The corneal irregularity may be exacerbated due to drastic variations in corneal elevation over the intrastromal ring and directly adjacent to the ring. This irregularity in elevation leads to difficulty in centering the lens on the eye and problems with bubbles forming under the contact lens over areas of corneal depression. When successfully fitted, contact lenses over intrastromal corneal rings can improve vision significantly.