PubMed · 9120870
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
P L Rakow. Solving soft lens fitting problems.. https://pubmed.ncbi.nlm.nih.gov/9120870/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
PURPOSE: Empirical soft toric contact lens fitting based on manifest refraction and keratometry often presents unanticipated fitting and power errors upon initial lens dispensing. However, corneal topography may provide features that influence soft toric lens performance, flexure, and back vertex power in situ, which may assist in improved fitting guidelines. In this study, quantitative topographic descriptors were generated and analyzed as potential variables in predicting soft toric fitting success. METHODS: One hundred five eyes of 54 patients were empirically fit with back surface toric, prism ballasted, soft contact lenses after videokeratography was performed with the EyeSys 2000 (v. 4.0) or Humphrey Atlas (v. A6) instrument. Custom software was written to generate 54 separate quantitative descriptors of shape and astigmatism from the raw data files. A logistic regression was used to determine which variables significantly contributed to a successful or failed fit. RESULTS: Two types of empirical fitting failures were identified: loose fit (n = 15) and power errors (n = 17). The following variables were associated with a fitting failure: flat simulated keratometry (SimKf2b) within the central 3 mm zone, steep simulated keratometry (SimKs2b) within the central 3 mm zone, a difference between central and peripheral flat meridian axis (DIFFAXIS), and a difference between central and peripheral astigmatism (DIFFASTIG). For fitting failures caused by power errors, a larger steep SimKs2b (p< 0.01) and smaller DIFFAXIS (p< 0.05) were associated with a failed fit. For failures caused by physical fit of a selected base curve, a smaller DIFFAXIS (p< 0.05), larger steep SimKs2b (p< 0.05), and larger DIFFASTIG (p< 0.01) were associated with a failed fit. CONCLUSIONS: Novel quantitative descriptors of corneal shape and toricity derived from topography are associated with empirical soft toric contact lens fitting failures. Future algorithms or recommendations for improved soft toric lens selection may be derived from such indices to develop a predictive model for successful soft toric lens fitting using corneal topography data.
PURPOSE: To quantify the quality of vision achieved with multifocal and bifocal contact lenses. METHODS: We analyzed differential light sensitivity by computerized automatic perimetry in 21 patients wearing monofocal soft contact lenses (group 1, controls) and multifocal and bifocal contact lenses (groups 2 and 3, respectively). Seven patients each were fitted with multifocal or bifocal contact lenses; seven patients were without contact lenses (without correction for testing the visual periphery and with near-vision correction using monofocal contact lens for testing the central 30 degrees of vision). The type of correction was randomly changed in a crossover fashion so that each eye was examined at different times with different corrections. Humphrey 640 VFA computerized automated perimetry was used to test visual fields at baseline, 45 days, and 3, 4.5, and 6 months. RESULTS: A statistically significant difference was found between the global sensitivities (GS) of the central visual field in patients with near-vision monofocal contact lenses and with bifocal contact lenses (P=0.0273) and between the GS of the central visual fields with multifocal contact lenses and with bifocal contact lenses (P=0.0261). In both cases, the GS were significantly reduced with bifocal contact lenses (total GS: group 1, 11256 dB (Decibels); group 2, 11154 dB; group 3, 10679 dB). CONCLUSIONS: The results indicate that there is reduced differential light sensitivity in the central 30 deg of the visual field with bifocal contact lenses compared with multifocal contact lenses and monofocal contact lenses (controls).
BACKGROUND: International Organization for Standardization (ISO) document ISO 9913-1 details procedures for oxygen permeability coefficient (Dk) measurement of hydrogel contact lens materials up to 100 barrer. A draft ISO document 9913-2 describing the coulometric technique states that the recommended procedures are suitable for nonhydrogel materials. New generation hydrogel lens materials exceed 100 barrer, which is outside the scope of both documents. METHODS: A range of rigid gas-permeable materials from Dr. W. Benjamin's standard repository was used as a baseline to assess the accuracy of the polarographic and the coulometric methods. We also measured the permeability of a new generation hydrogel, lotrafilcon A, to explore the suitability of the polarographic and coulometric techniques for high-Dk hydrogel lenses. We modified equipment for both methods that incorporate several improvements including front surface masking to eliminate "edge effect." RESULTS: The coulometric technique provided results similar to those previously reported for the standard rigid gas-permeable materials and yielded relative standard errors typically <10%. The polarographic technique provided results similar to the accepted values for the standard rigid gas-permeable materials Dk <70 barrer only after the edge effect correction algorithm was applied. The polarographic results showed poor precision and significant, systematic differences from accepted values for the rigid gas-permeables with Dk > 70 barrer. CONCLUSIONS: The coulometric method is preferable for the measurement of contact lens materials with permeability >70 barrer. The coulometric method was successfully modified to include hydrogels.