Lens Performance Considerations
Gerald E. Lowther
Contact lenses in general have advantages over spectacles: good peripheral vision, no interference when using optical instruments, no problems in the rain, no reflections from lens surfaces, no breakage while being worn, little likelihood of being knocked off, no lens fogging when going in and out of different environments, and no problems with perspiration (Crosley et al., 1974).
Considerable debate has occurred with respect to wearing contact lenses in adverse conditions such as industrial situations. Ocular trauma is one concern. Surveys and studies have generally indicated greater perceived problems than actual problems (Rengstorff and Black, 1974; Nilsson et al., 1981; Randolph and Zavon, 1987; Logar, 1987).
Subjective surveys of individuals wearing lenses in adverse conditions, including aviation, generally indicate success with and preference for contact lenses, especially hydrogel lenses (Van Norren, 1984; Crosley et al., 1974; Nilsson and Rengstorff, 1979; Backman et al, 1987; Gilchrist, 1980). However, one must be aware that there is often bias on the part of contact lens wearers to want to continue wearing them and they thus downplay any problems.
Advantages of Rigid Lenses
The oxygen transmissibility of newer rigid lens materials is greater than most hydrogel designs. Therefore, corneal edema is seldom a problem even with extended wear and should not be a major problem under low-oxygen-level conditions at higher altitudes.
The tear exchange on each blink is much greater with rigid lenses (about
10 to 15 percent per blink) than with hydrogels (less than 2 percent). Thus, metabolic waste products and cell debris are rapidly removed from under the lens.
Vision is generally sharper and more consistent with rigid lenses. Small amounts of astigmatism due to corneal toricity are corrected with spherical lenses. Toric rigid lenses give good consistent vision on highly toric corneas.
Disadvantages of Rigid Lenses
One disadvantage of rigid lenses is loss or displacement from the cornea when the face is rubbed (i.e., by a helmet or hand). Likewise, foreign bodies can readily get under rigid lenses, causing discomfort or incapacitation (Crosley et al., 1974; Nilsson et al., 1983). A way to minimize these problems is to use large-diameter lenses. Lenses as large as 11 to 12 millimeters may be possible with the high gas-permeable materials. With these diameters it is more difficult to dislodge the lens because the lid(s) will remain over the lens edge. It is also less likely that foreign bodies will get under such lenses. Another potential answer to this and related problems is the use of haptic lenses manufactured from highly permeable materials. Foreign bodies, loss, and visual problems from lens movement would not be problems. Obviously the use of haptics would require techniques and procedures that most practitioners are not presently able to do.
The likelihood of loss or of foreign bodies getting under lenses can also be minimized by using lenses with minimal edge thickness and clearance. Axial edge lifts of less than 0.08 millimeters could be used.
Lens surface drying and front surface coatings causing blurred vision will be a problem with some individuals, especially under low-humidity conditions. Only with the development of materials with better surface properties will this problem be totally eliminated. Surface drying is often worse when a person is concentrating on a visual task, as they may not blink as frequently then. Surface drying does not seem to be as great a problem as with hydrogel lenses.
Lens flexure on toric corneas is a problem with most gas-permeable materials.
When used on an extended-wear basis there may be lens adhesion on awakening. This usually does not cause discomfort or vision problems; however, if it is persistent it could cause physiological problems. Due to that fact an adherent lens will cause temporary corneal distortion, if the lens is removed immediately following adhesion, the vision may be blurred. Proper lens design is necessary to minimize this problem.
Peripheral cornea staining is another problem with rigid lenses. It can be minimized with larger-diameter lenses, thin edges, and other design factors.
A major advantage of hydrogel lenses over rigid lenses under adverse conditions is the decreased chance of lens loss or decentration. Due to the large diameter and close conformance to the eye, these lenses are worn in contact sports such as football, basketball, and hockey without lens loss. In addition, because of their size and fit, there is seldom a problem with foreign bodies getting under them (Crosley et al., 1974; Nilsson et al., 1983). Therefore, they are the lens of choice in dusty environments. By using as large a diameter as possible (e.g., 14.5 to 15.5 millimeters rather than 12.5 to 14), the chance of loss, decentration, or foreign bodies will be minimized.
Hydrogel lenses are generally more comfortable than other lens types.
There would be some concern that the fit of the lenses would change at hypobaric pressures or due to tightening of the lenses under low-humidity conditions. Studies (Polishuk and Raz, 1975; Eng et al., 1987; Tredici and Flynn, 1987) have indicated no change in lens fit or performance flight conditions and hypobaric pressures. However, low humidity in flight is most likely a problem (Eng et al., 1987).
Visual problems with hydrogel lenses due to uncorrected cylinder, surface deposits, surface distortion, and surface drying are a concern under adverse conditions where excellent and consistent vision is required. Visual performance is considered elsewhere in this volume.
Deposits and spoilage of hydrogel lenses have been a major problem with daily and extended wear. Depositing is accelerated by surface drying, increased lens water content, ionic lens materials, and improper cleaning. In addition to proper cleaning, the most feasible way to minimize this problem is frequent lens replacement. With the advent of disposable lenses this is not a major problem.
Dehydration of hydrogel lenses has previously received a lot of attention. It has been shown that hydrogels dehydrate rapidly with wear (Andrasko, 1982, 1983; Wechsler et al., 1983; Kohler and Flanagan, 1985). Thinner lenses lose more water than thicker ones, and higher-water-content lenses dehydrate to such an extent that water is pulled from the epithelium and epithelial damage occurs. Therefore, thin high-water-content lenses should not be used even though they have high oxygen transmission.
Lens dehydration can cause lens to “steepen,” making the lens tighter, which causes vision and fitting problems (Gundel and Cohen, 1986; Fatt and Chaston, 1982; Eng et al., 1982). These changes are generally greater
with high-water-content lenses but can vary with materials of the same water content (Brennan and Efron, 1987).
A study by Finnemore (reported in Lowther and Malinovsky, 1988) found that patients with marginal dry eyes and associated lens-wearing problems preferred thicker lenses (0.12 millimeters) over thinner ones (0.06 millimeters) (38 percent water lenses used). Sixty percent preferred the thicker lenses, 20 percent the thinner lenses, and 20 percent found no difference. If they were fitted with prism ballast lenses, which were even thicker in the inferior portion of the lenses, the prism lenses were preferred to the spherical thicker lenses. Seventy-three percent preferred the prism lenses, 15 percent the spherical, and 12 percent found no difference. Clinical experience has indicated that many of these patients prefer the thicker lenses. Therefore, it may be that under low humidity conditions patients will do better with thicker or spherical prism ballast lenses. Studies are required to determine this.
As previously reported, extended-wear hydrogel lenses have limited oxygen transmissibility. This can present problems if the lenses are to be worn for long time periods in low-oxygen environments, including potential ocular health problems. However, for short periods of 24 to 48 hours or less, even under low-oxygen atmospheres, this should not be a significant problem. If the lenses must be made thicker to combat the dehydration problem, the oxygen and corneal edema problems will be made worse. The answer to these problems may be the new hydrogel materials being developed, which are not dependent on just water content for oxygen transmission. However, hydrogels have been reported to perform well under extended-wear conditions for air force pilots (Nilsson and Rengstorff, 1979).
SOFT SILICONE LENSES
A lens material that has received a lot of attention in the past and is receiving renewed attention now is the soft silicone material. This polymer is by far the highest oxygen permeable material ever used for contact lenses (Hill and Mauger, 1981). With a well-fitted silicone lens there are no oxygen-related corneal problems (LaHood et al., 1988; Sweeney and Holden, 1987). It is used extensively in pediatric aphakic patients, where extended wear is required and the lenses must be thick due to the lens powers.
There have been problems with soft silicone lenses due to discomfort, poor surface wettability, and lens adherence. Some of the problems such as comfort and lens adherence were due to the limited lens parameters (10.5-and 11.3-millimeter diameters) available. Presently new designs with diameters equivalent to hydrogel lenses (14 millimeters) are being studied (Zantos, 1988). Such diameters open the possibility for more comfortable lenses and
decrease the likelihood of lens loss, displacement, and foreign bodies getting under the lenses.
Due to the fact that the lens material is relatively stiffer than hydrogels, visual acuity is generally better than hydrogels. However, they will not correct significant corneal astigmatism.
Since the material does not contain significant water, dehydration is not a problem. However, it has been reported that water moves through the lens material as water vapor.
A previous problem with silicone lenses has been lens adherence. After a period of wear, usually extended wear, the lens does not move, and if worn for sometime without movement corneal health problems occur. This nonmovement occurs with thick lenses and with lenses fitted too steep (tight). New designs will, it is hoped, overcome this problem.
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