Optical and Visual Assessment of Ophthalmic Plastic Lenses in Hot and Dusty Climate

Abstract

Background: One of the popular spectacle lenses is plastic (CR 39) lenses. However, plastic materials in general inherited some defects. Purpose: This study investigated optical changes in spectacle lenses afier use for a period of one year, the quality and visual perfonnance. Methods: The investigations included the storage of the uncut lenses, thermal exposure lenses, use as spectacles (cleaning and keeping in spectacle case), visual perfomance (visual acuity, contrast sensitivity, stereo acuity, colour vision and central field) and users‘ satisfaction (weight, cost, rate of change and preference). Samples size was 264 of the (CR 39) plastic white lenses, divided into; group A 144 lenses prepared in spectacles to study optical changes and effect on visual perfonnances, group B 30 lenses exposed to indoor environmental conditions for one year and group C 90 lenses exposed to thermal conditions. Lenses parameters investigated were transmission, center thickness, surface curvature (for group A in different meridians) and dioptric powers. In addition, a questionnaire was designed to assess plastic lenses wearers’ satisfaction in Khartoum. Results: Transmission results revealed insignificant changes in group A, B and C after 12 month. Center thickness of group A lenses (R, L) showed a significant change afier 6 month and group C lenses post heat treatment (p< 0.001, = 0.003 and < 0.001) respectively. Only the 180 axis in L front surface group A lenses had shown insignificant change (p =0.174, 0.960; 0.562) at the start point, after 6 month and after l2month. However, no change were detected in the power of these lenses, but a significant change was found in front and back radii of curvature (r|, T1) and power of group C lenses afier exposure to heat (p < 0.001), (p = 0.013) respectively. Stable visual perfonnance was noted by spectacles users, but a significant difference in visual acuity after 12 month (p= 0.037) in lefi eyes was found. Spectacles lenses scratching afier 12 month was found considerable (p< 0.001) attributed to the use of dry cleaner by 70.8 % of spectacles users. Results also showed that 87.5% participants were comfort with plastic lenses, while 91.7 % were preferring plastic for their new spectacles. According to the questionnaire results a significant element of comfort 96% in vision was noted in using CR 39 ophthalmic spectacles lenses in Khartoum, furthermore, 99.7% of the subjects stated that CR 39 lenses have light weight compared with ophthalmic glass lenses; however 89.5% users clearly stated that durability was poor for those type of lenses; while 96% of the participants pointed that the lenses were safer compared to the ophthalmic glass lenses. lt was found that scratches were the main cause of lens colour changes p=0.00l and visual blurring p = 0.007, however; spectacles’ cases were found to be a significant cause of scratches; while 83.1% preferred plastic for their new spectacles. The causes for changing spectacles were found to be scratches (93%) and discomfort (80%). Blurring of vision was found to be in 57.6% and colour changes were 41.1% of the studied group. Conclusion: CR.39 plastic lenses had good stability in transmission, center thickness, vertex power (except when heated, group C), surface curvature showed significant change. Visual perfonnances were found stable with wearing CR 39. Comfort was good, and plastic lenses were preferable and safe for group A wearers and questionnaire participants. However, the main drawback short durability. Scratches were found to have considerable role in vision blurring and discomfort, attributed to the use of improper spectacle cases and the use of dry cleaners to wipe lenses clear.