Influence of rigid contact lens overall and optic zone diameters on tear pump efficiency

To determine the effects of contact lens overall diameter on tear pump efficiency, oxygen uptake rates were measured for the open-eye condition and following steady-state periods of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear of oxygen-impermeable (polymethylmethacrylate) lenses of 5 overall diameters (8.2, 8.5, 8.8, 9.1, and 9.4 mm). The optic zone diameters were 1.4 mm smaller than the overall diameters in each case, while all other parameters remained constant. Differences in corneal oxygen demand following the static and dynamic conditions are a quantitative index of the tear pump efficiency for each lens size on the same cornea, and the differences were found to increase with reduction in lens overall diameter. This indicated that a more efficient tear pump was associated with small diameter designs. In addition, subjects with large palpebral aperture size, relative to lens overall diameter, were found to demonstrate better tear exchange.     

Effects of rigid contact lens edge lift changes on tear pump efficiency

The effects of rigid contact lens axial edge lift changes on corneal oxygenation and tear exchange with the blink were studied for lenses of five axial edge lifts (0.05 to 0.13 mm in 0.02-mm steps). The overall and optic zone diameters were maintained at 8.8 and 7.4 mm, respectively, and all other parameters were held constant. Oxygen uptake rates were measured for the normal open eye after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear. As expected, no significant differences were found in the oxygen uptake rates associated with the static wear of the contact lenses; however, under dynamic conditions, increasing edge lift was associated with a reduction in oxygen demand. The differences between data obtained under static and dynamic conditions increased with increasing axial edge lift, and subjects with small palpebral aperture sizes seemed to benefit more from increasing axial edge lift.     

Influence of power changes in single cut rigid contact lenses on tear pump efficiency.

Differences between corneal oxygen uptake rates measured after 5 min of static (without blinking) and 5 min of dynamic (with blinking once every 5 s) non-gas permeable (polymethyl methacrylate) contact lens wear, referenced to the oxygen uptake rates of the normal open eye, were used as indications of tear pump efficiencies associated with each of seven contact lens back vertex powers (-9.00 to +9.00 D, in 3.00 D steps). Measurements were made in vivo on six human corneas showing with-the-rule (0.25 to 0.87 D) toricity. Lens overall diameter was 8.8 mm, optic zone diameter was 7.4 mm, and base curve radius was fitted "on K." Lens center thickness was 0.14 mm for all minus-powered lenses, and 0.19, 0.26, 0.34, and 0.41 mm for the plano, +3.00, +6.00, and +9.00 D lenses, respectively, maintaining an edge thickness of 0.11 mm. Under both static and dynamic wearing conditions, no significant differences were found among the corneal oxygen uptake rates associated with the various contact lens back vertex powers; however, the difference between the static and dynamic condition data, an indication of tear pump efficiency, was greatest for the -3.00 D lens. The difference values associated with the +9.00 and +6.00 D lenses were significantly lower than those associated with the minus power lenses.     

Rigid lens tear pump efficiency: effects of overall diameter/base curve combinations

The effect of changes in rigid contact lens overall and optic zone diameters on tear pump efficiency were studied using a polarographic technique. Six overall diameters ranged from 7.6 to 10.6 mm in 0.6-mm steps, with the optic zone diameter always being 1.4 mm smaller. Base curve radii were made flatter as lens overall/optic zone diameters increased in order to maintain a constant tear layer thickness. All other parameters were held constant. Oxygen uptake rates were measured for the normal, open eye, as well as after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) wear of each lens design. Under both static and dynamic conditions, oxygen uptake rates increased with increase in lens overall/optic zone diameter. The differences between the static and dynamic condition data served as an index of tear pump efficiency for each lens. These differences decreased with increase in lens overall/optic zone diameter.     

Measures of tear exchange under rigid contact lenses: Part III. Influence of base curve on post-lens tear exchange

Observations of net tear exchange associated with steeper than K and flatter than K fits of rigid lenses to each of six human corneas revealed within this sample that tear exchange was always reduced in steeper than K cases and always increased in flatter than K cases. Fits within ±0. 05 mm of the K case did not manifest tear-exchange performances significantly different from the K case. Although the net volume of the postlens reservoir exchange associated with on-K cases commonly averaged 5% per blink, exchanges of up to 16.57% were found possible, but rarely practical, with fits of 0.15 mm flatter than K.     

The effects of rigid contact lens overall diameter changes on tear pump efficiency: A comparative study

A series of three studies was performed to evaluate the effects of rigid contact lens overall diameter changes on tear pump efficiency. In all three studies, corneal oxygen-uptake rates were measured on the right eye of six subjects under three conditions: normal open eye, after 5 minutes of static (without blinking) contact lens wear, and after 5 minutes of dynamic (with blinking once every 5 seconds) contact lens wear. Measurements were repeated eight times for each lens/cornea system. In Study I, five lens diameters were used, varying from 8.2 to 9.4 mm in 0.3 mm steps, with optic-zone diameters 1.4 mm less and all other parameters constant. In Study II, seven lens diameters were used, varying from 8.2 to 10.0 mm in 0.3 mm steps, with a constant 7.4 mm optic-zone diameter for all lenses and all other parameters constant. In Study III, six lens diameters were used, varying from 7.6 to 10.6 mm in 0.6 mm steps, optic-zone diameters 1.4 mm less, base-curve radii varying with optic-zone diameter to maintain a constant apical tear-layer thickness, and all other parameters constant. A reduction in corneal oxygen demand with decreasing lens overall diameter was seen under dynamic conditions in all three studies. Greatest and most consistent tear pump efficiencies, as reflected by large differences between static and dynamic measurements, were also found with the smallest lenses in all three studies.     

Rigid gas-permeable contact lens base curve radius and transmissibility effects on corneal oxygen uptake.

PURPOSE: The purpose of this study is to determine the effects of rigid gas-permeable contact lens thickness, base curve radius, and material permeability on corneal hypoxic stress. METHODS: Corneal oxygen uptake rates were measured with a Clark-type polarographic electrode on the right eye of 10 human subjects for the normal open eye (air) and after 5 minutes of static wear of rigid contact lenses of four cornea-to-contact lens base curve fitting relationships: 0.2 mm steeper-than-K (STK), 0.1 mm STK, on K, and 0.1 mm flatter-than-K (FTK). There were also four materials (polymethylmethacrylate [Dk=0], lotifocon B [OP-2, Dk=15.9], lotifocon A [OP-3, Dk=30], lotifocon C [OP-6; Dk=60]) and three center thicknesses (0.14, 0.28, and 0.53 mm for the OP-6 lenses and 0.14 mm for all other materials) with all other parameters being constant. Each subject participated in two identical sessions. A repeated-measures analysis of variance was performed to compare the mean response across lens materials/thicknesses and the four curvature values. RESULTS: Significant differences were found only for lens material/thickness (p<0.0001). Although OP-2 and OP-6 (0.53 mm) were manufactured to have the same Dk/t, post hoc comparisons showed that the oxygen uptake rate with OP-6 (0.53 mm) was significantly lower. The same is also true for OP-3 and OP-6 (0.28 mm), with OP-6 (0.28 mm) having a significantly lower oxygen uptake rate. CONCLUSIONS: In contrast to past studies with PMMA, cornea-to-contact lens base curve fitting relationship, with the lens materials and designs studied here, did not affect corneal hypoxic stress, and thick, high Dk lenses resulted in less change in corneal response than did thin, lower Dk lenses of the same Dk/t. This is attributed to the lens reservoir effect that has been previously described.     

A comparison of the effects of rigid contact lens overall diameter and axial edge lift changes on tear pump efficiency

Corneal oxygen uptake was used to compare the effects of changes in lens overall and optic-zone diameter to changes in lens axial edge lift on tear pump efficiency. Measurements were made on the right cornea of six human subjects under three conditions: (1) normal open eye, (2) after 5 minutes of static (without blinking) wear of a poly(methyl methacrylate) (non-gas permeable) contact lens, and (3) after 5 minutes of dynamic (with blinking once every 5 seconds) wear of the same lens. The differences between the oxygen-uptake rates measured under static and dynamic conditions for a given lens design provided an index of the tear pump efficiency for that design. Two sets of five lenses were worn by each subject. In the first set, lens overall diameter varied from 8.2 to 9.4 mm in 0.3 mm steps, with the optic-zone diameter being 1.4 mm smaller. All other parameters were constant, including a 0.09 mm axial edge lift. In the second set of lenses, axial edge lift varied from 0.05 to 0.13 mm in 0.02 mm steps; overall diameter was 8.8 mm, optic-zone diameter was 7.4 mm, and all other parameters were the same as those in Set 1. Tear pump efficiency was greatest for small lenses and for lenses with large axial edge lifts. A 0.3 mm decrease in overall diameter was approximately equivalent to a 0. 03 mm increase in axial edge lift as a means of enhancing tear pump efficiency.     

Rigid contact lens design: effects of overall diameter changes on tear pump efficiency

The effects of rigid contact lens overall diameter changes on tear pump efficiency were studied for lenses of 7 overall diameters (8.2 to 10.0 mm in 0.3-mm steps) while maintaining all other parameters (including a 7.4-mm optic zone diameter) constant. Oxygen uptake rates were measured in the normal, open eye and after static (without blinking) and dynamic (with blinking once every 5 s) wear of oxygen-impermeable (polymethyl methacrylate) contact lenses. Differences between uptake rates obtained under static vs. dynamic conditions served as an index of tear pump efficiency for each lens. Tear pump efficiency increased with the use of progressively smaller lens diameters and subjects with large palpebral aperture sizes were also found to have better tear exchange (r = 0.934, p = 0.006 for the 9.1-mm overall diameter reference lens).     

Sagittal depth considerations in the selection of the base curve radius of a soft contact lens

Examination of the relationship between the sagittal depth of the cornea and the sagittal depth of an optimally fitting soft contact lens reveals that base curves are not always well chosen. The author reports a new system for selecting the number, range, and distribution of base curve radii such that the range of saggital depths clinically encountered is covered evenly. The saggital increment between adjacent base curves is then 0.1 mm.     

Influence of palpebral aperture height on tear pump efficiency

Oxygen uptake rates were measured on 6 human corneas fitted with a standardized rigid contact lens in order to determine the relation between vertical palpebral aperture height and exchange efficiency of tears in such systems. The contribution provided by tear exchange to the oxygen available to the cornea was isolated through the use of lenses of negligible transmissivity (PMMA). Measurements of corneal oxygen uptake were made on the normal open eye, after 5 min of static (with no blinking) open eye contact lens wear, and after 5 min of dynamic (with blinking every 5 s) contact lens wear. The difference between the oxygen demand associated with static conditions (SC's) and that associated with dynamic conditions (DC's) is an index of the tear pump efficiency for each subject. A statistically significant correlation (p less than 0.0001) was found between the vertical palpebral aperture height and the tear bulk-flow of oxygen to the cornea, confirming that small palpebral apertures can restrict lens motion.     

Dynamic changes in tear meniscus curvature at the rigid contact lens edge

PURPOSE: To investigate dynamic changes in tear meniscus curvature at the edge of a rigid contact lens. METHODS: Four eyes of four men and seven eyes of seven women who were daily wearers of rigid contact lenses were enrolled in the study. A digital video-meniscometer was used to observe real-time changes in tear meniscus curvature. Quantitative measurements have not previously been made at these locations. RESULTS: The meniscus at the lens edge showed stereotypical changes with each blink. The meniscus radius is lowest at the end of the up-phase of the blink and then increases to a steady value 0.213 +/- 0.024 mm (mean +/- SD, n = 6) after about 1 second. CONCLUSIONS: The video-meniscometer permitted observation of real-time changes in meniscus curvature at the contact lens edge during the blink. The short radius at the end of the up-phase suggests that tears are drawn away from the lens during the upward movement of the lid and lens. The restoration of a longer radius of curvature in the period that follows is thought to be due to a flow of fluid into the meniscus from the neighboring tear film.     

Effect of contact lens materials on tear physiology.

PURPOSE: This study measured evaporation rate, thinning characteristics, and lipid layer changes in the prelens tear film (PLTF) associated with wearing of different soft contact lens materials, in an attempt to determine the biocompatibility of the material with the PLTF. METHODS: Twenty habituated contact lens wearers wore five different soft materials in a random order on the left eye at visits separated by at least 24 h. The soft contact lens materials were polymacon (Optima 38), omafilcon A (Proclear Compatibles), phemfilcon A (DuraSoft 2), balafilcon A (PureVision), and etafilcon A (Acuvue). Tear film evaporation rate was measured by a modified Servo Med Evaporimeter and tear thinning time by HirCal grid. Tear film structure, elimination rate, and lens wetting ability were recorded dynamically with a Doane tear film video interferometer and graded according to a new system developed for the study. Baseline measurements were taken of the precorneal tear film before lens insertion, and PLTF was determined 30 min after commencing lens wear. RESULTS: No statistically significant differences were found for any of the baseline (precorneal tear film) data. There was also no significant difference in evaporation rate change (analysis of variance) and in tear thinning time (Friedmann) between the five contact lenses. In the PLTF structure grading, omafilcon A had significantly more stable grades than phemfilcon A (Friedmann, p = 0.0033) and polymacon (p = 0.004). In PLTF observation of tear thinning and elimination rate, there was a significantly slower rate of elimination observed for omafilcon A than phemfilcon A (Friedmann, p = 0.0023) and polymacon (p = 0.0023). There was no significant difference in the overall PLTF wetting ability grading between any of the lenses worn. CONCLUSION: Generally, all soft contact lens materials significantly and adversely affected tear physiology by increasing the evaporation rate and decreasing tear thinning time. The surface wetting ability of all contact lens materials exhibited no significantly difference irrespective of the special surface treatments. Only in PLTF structure and in PLTF elimination rate were differences found from the conventional low water content materials; omafilcon A was better in PLTF structure and in PLTF elimination.     

Rigid contact lens base curve constancy between wet and dry lens storage conditions

The purpose of this study was to investigate dry storage of rigid gas-permeable contact lenses as an alternative to wet storage. This study evaluated the constancy of lens base curve between the two storage methods for lens materials that spanned the spectrum of oxygen permeability values. The differences found in lens base curve between wet and dry storage for any lens material evaluated did not reach a level considered to be clinically significant, although differences in average base curve constancy did exist among the lens materials. Base curve change did not increase with increasing lens material oxygen permeability value (Dk).     

Automated and manual base curve assessment of rigid gas permeable contact lenses

Verification of the base curve radius of a rigid contact lens is important because the base curve affects both the lens fit and power. This study compared the accuracy of base curve measurements obtained from the radiuscope versus the Nikon NRK-8000 autorefractor/keratometer CL attachment. Reliability coefficients were also determined for each technique.Base curve measurements for spherical, aspheric, and back toric rigid gas permeable lenses were obtained by 18 observers with the manual and automated methods in a counterbalanced order. Spherical and aspheric lens results showed the base curve estimates obtained from the radiuscope differed significantly from the automated measurements. The base curve measurements obtained from both methods were flatter than the ordered radius, but were within the conventional 0.025-mm tolerance. The reliability coefficient of the automated unit was approximately twice that of the manual unit. Toric basic curve measurements were statistically the same for both techniques, but the reliability coefficient was less than that obtained with spherical and aspheric base curves.     

Fluorophotometric assessment of tear turnover under rigid contact lenses.

To examine the effect of rigid contact lens design on tear turnover the left and right eyes of 25 healthy volunteers were randomly fitted with either a spherical or an aspherical contact lens. Tear turnover is defined as the percentage of decrease of fluorescein concentration in the tear film per minute as a result of tear flow after instillation of fluorescein and can be determined with a fluorophotometer. A drop of fluorescein (1 microliter, 2%) was instilled in each contact lens-wearing eye, whereafter the decrease of the fluorescein was measured for about 30 min. The test was repeated without contact lenses. No statistically significant difference was found between the tear turnover in the spherical, the aspherical, and the non-contact lens-wearing eyes (mean values +/- SD: 15.2 +/- 4.9, 15.6 +/- 5.9, and 16.9 +/- 6.8%/min, p > 0.20).     

Corneal response to rigid contact lens wear.

Three adult rhesus monkeys were subjected to 2 and 24 hours of polymethylmethacrylate (PMMA) contact lens wear. The induced corneal changes were examined with the electron microscope. Mild epithelial oedema as well as early degenerative cell changes was present already after 2 hours' wear. Rigid lens wear for 24 hours produced more severe oedema and cell alterations together with premature cell loss and ultimately, in areas of lens bearing, corneal denuding. Only the monkeys wearing contact lenses for 24 hours had significant stromal swelling, which was primarily evident in the posterior region, while the anterior limiting lamina remained unaffected. The stromal swelling was patchy and mainly around keratocytes and between lamellae, while fluid within the lamellae was evident only occasionally in posterior stroma. Changes among keratocytes were evident, especially posteriorly, where reaction was frequently severe. Endothelial reaction was restricted to a limited fluid uptake in the 24-hour-wear experiment. In addition there was in these monkeys an apparent loosening of the endothelial adhesion to the posterior limiting lamina. It is concluded that the oedematous epithelium undergoes cell shrinkage and flattening, which is compensated for by an uptake of fluid. The uptake of fluid maintains the overall normal thickness of the epithelium. The conclusion is supported by other studies, where the normal thickness of oedematous epithelium has been shown by pachometry. The results in the present study further suggest that stromal oedema in the contact lens wearer is a result of a relative loss of endothelial function, leading to a swelling that moves in a posterior to anterior direction.     

不同基弧软性角膜接触镜对LASEK的影响

目的：探讨不同基弧软性角膜接触镜对准分子激光上皮下角膜磨镶术(LASEK)的影响。

方法：对186例372眼近视患者施行LASEK手术,随机分为观察组和对照组,激光切削后分别配戴-1.0D基弧为8.7,8.5mm的软性角膜接触镜。术后1,3,5d比较两组术后角膜刺激症状,术后5d比较角膜上皮愈合时间、裸眼视力,术后1mo比较两组裸眼视力,术后3mo比较角膜上皮下雾状混浊(haze)程度。

结果：术后1,3,5d观察组角膜刺激症状明显好于对照组,差异有统计学意义(P<0.05)。术后5d角膜上皮愈合时间、裸眼视力观察组明显好于对照组,差异有统计学意义(P<0.05)。术后1mo两组裸眼视力无明显差别,差异无统计学意义(P>0.05)。术后3mo两组haze比较差异无统计学意义(P>0.05)。

结论：LASEK术后应适当增加需配戴的软性角膜接触镜的基弧,可以减轻角膜刺激症状,促进角膜上皮愈合和视力恢复。     

Comfort-Kone硬性角膜接触镜矫治圆锥角膜

目的观察comfort-kone硬性角膜接触镜(rigid gas-permeable contact lens,RGPCL)控制圆锥角膜发展的效果。方法 52例(94眼)患者采用"三点接触法"配戴comfort-kone RGPCL,分析对比配戴后屈光度、角膜地形图参数及泪液情况的变化,评价RGPCL控制圆锥角膜发展的临床效果。结果配戴RGPCL后最佳矫正视力0.83±0.24,与配戴前最佳矫正视力0.58±0.35比较,差异有显著统计学意义(t=-8.87,P=0.00);RGPCL矫正视力与配戴眼屈光度和角膜曲率半径呈显著相关(P=0.001、0.000),与角膜散光度无显著相关性(P=0.081)。随访期间RGPCL矫正视力稳定,角膜健康状况良好;患者主观适应时间为(11.13±9.10)d;长期配戴后基础泪液分泌量显著降低,差异有显著统计学意义(t=2.867,P=0.007)。结论 Comfort-Kone RGPCL能够显著提高圆锥角膜患者的视觉质量,长时间配戴安全、稳定,有效地控制了圆锥角膜的持续发展。