International contact lens clinic (New York, N.Y.)最新文献

The importance of practitioners being able to measure back optic zone radius in verifying rigid gas permeable contact lenses is emphasized. Despite improved standards of manufacturing, errors can occur, and the capacity to verify lenses can be crucial to effective management, especially when new rigid gas permeable materials with unproven stability characteristics are used. Circumstances under which back optic zone radius discrepancies can cause frustration to both patient and practitioner are analyzed as the basis for illustrating the value of either problem anticipation and prevention, or recognition and resolution. Procedures that increase the accuracy of radiuscope measurements are described, including the use of a new lens mount design. The psychology of contact lens verification measurements is discussed, with consideration of procedures that help reduce conflict between laboratory and practitioner on the occasion of questionable quality control.

This article examines the accuracy of the EyeSys 2000 videokeratoscope in measuring aspheric surfaces for calculation of the wavefront aberrations of the anterior cornea. Six rotationally symmetric aspheric surfaces were measured. The surface elevation was computed from the normal measurement files. Both elevation error and relative error were computed from the average of the three maps. The root-mean-square errors for the various surfaces ranged from 1.48 to 6.55 microns, with less error on the oblate and spherical surfaces. The error found was very systematic, increasing monotonically toward the periphery. The article includes a strategy to compensate for the systematic error to meet the required 0.5-micron accuracy needed. An equation was developed that used only the apical radius and shape factor, which improved the accuracy to the required 0.5-micron level.

Incisional refractive surgery for the correction of astigmatism, introduced by Ruiz in the 1980s, was a milestone for the now more sophisticated refractive surgery procedures. Patients who underwent this pioneering technique often suffered from numerous complications, eventually resulting in the need for vision correction with either further surgery or rigid contact lenses. A contact lens case report of one patient, whose procedure resulted in highly irregular and astigmatic corneas, is presented along with an overview of the original Ruiz astigmatic keratotomy technique. This case—as most post-surgical contact lens fitting cases are—was challenging, but resulted in remarkable improvement in the vision quality and lens comfort of the patient.

Truncation is most commonly used for stabilization of bifocal and toric front surface lenses. Lenses with small edge defects also can be truncated and polished rather than being discarded. The purpose of this study was to determine whether corneal physiology is compromised when truncated lenses are worn. Contact lenses were manufactured with three amounts of truncation (0, 0.2, and 0.4 mm) in two contact lens materials (polymethylmethacrylate and itabisfluorofocon A). All lenses were fitted “on K” on the right eye of each of six subjects, with all other parameters kept constant. As expected, central corneal oxygen uptake rates measured with the itabisfluorofocon A material were significantly lower, indicating less hypoxic stress or oxygen deprivation, than those measured with polymethylmethacrylate across conditions (nonblink and blink) and amounts of truncation. No significant differences in central corneal oxygen uptake were found among amounts of truncation across conditions and materials.

Recently, the ongoing drive for still higher lens transmissibilities (Dk/L) has reached new levels of success. Emerging hybrids (e.g., “highly permeable hydrogel” materials1) showing the potential for Dk/Ls of 100 × 10⁻⁹ (cm/sec) (mL O₂/mL mm/Hg), and possibly higher, are joined by evolving generations of rigid gas permeable (RGP) lenses now challenging the “200 barrier” of transmissibility.2

But, with these higher Dk/Ls has come a clearer view of the cornea’s responses to more moderate realms of hypoxia and what transmissibility investments may be required to reduce those chronic levels still further.

Ultraviolet (UV) transmission spectra of UV-absorbing hydrogel contact lenses have been reported in the literature. Few of these studies were done following ANSI or ISO specified procedures. These published procedures do specify an integrating sphere in the detector system and a beam size, but not other instrument settings such as slit width, spectral smoothing functions, and scan speed. This work, done with Acuvue^R and Precision UV^TM contact lenses, shows that an integrating sphere is needed to ensure the accuracy of the data. A smaller beam size than specified was required to obtain accurate spectra. A slit width larger than recommended by the spectrophotometer’s manufacturer was needed to increase precision. Spectra results were unaffected by scan speed and spectral smoothing settings.