low or high dk? spherical or aspherical rigid gas permeable lenses? the gap between theory and...
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Journal of the British Contact Lens Association, Vol. 15, No. 4, pp 159-162, 1992 1992 British Contact Lens Association Printed in Great Britain
LOW OR HIGH Dk? SPHERICAL OR ASPHERICAL RIGID GAS PERMEABLE LENSES? THE GAP BETWEEN THEORY AND
John de Brabander a, Cees J. J. B r inkman b, Jan H. C. Kok b, and Cor van Mi l a
KEYWORDS: Dk, design, aspherical lenses.
Introduction ~)!gid gas permeable (RGP) contact lenses were lXintroduced some 15 years ago. Since that time the oxygen permeability (Dk) has increased from around 10 for the early materials up to 150 or more for the modern high gas permeable (HGP) materials. During the 15 years of development, research has focused on the condition of the cornea with respect to oxygenation. 1 It has been shown clearly that the highest Dk possible would be most beneficial to corneal physiology? One would thus expect that the majority of today's rigid contact lens prescriptions would be a HGP lens, but this is not the case. Sales figures 3 in most countries show that less than 50% of lenses prescribed
Gas permeable, Dk>50, 14% Gas permeable,
Soft, 50% H20, 2()%
Figure 1. Sales figures for contact lenses in the Netherlands, 1990-1991.
are RGPs, and HGPs only account for around 14% (Figure 1). There seems to be a gap between theory and practice? A similar situation exists in the use of aspherical versus spherical contact lenses. An aspher- ical posterior lens geometry has undoubtedly better corneal alignment properties? And yet still the majority of rigid lenses have classical multi-curve designs. Prob- lems encountered with first generation true aspherical
"OD. b MD, PhD.
lenses were decentration, debris, and lens binding. 6 A combination of a HGP material with an aspherical lens design would be the best starting point in optimizing the lens-corneal relationship and physiological condi- tion. Strangely enough, the sales of these lenses are lower than most. Again, theory and practice are not cor- relating.
The First Choice Problem There are a number of arguments used to explain the current discrepancy between the theoretical lens of first choice and that made in daily practice. The foremost argument is that HGPs have provided a number of prac- tical complications. There are many studies that clearly show problems with HGP materials, including changes in base curves, deposits, plaque formation, scratches, cracking, and breakage. 3,4,7 There is strength in the argument that a high Dk lens with a poor surface is worse than a low Dk lens with good wettability, One could even argue that polymethylmethacrylate (PMMA) lenses are better than HGPs with poor sur- faces. With these problems, it is clear why practitioners have become reluctant to fit HGP lenses as their first choice.
A second argument that could be used against the use of high Dk lenses lies within the theory itself. From the classic concept of equivalent oxygen percentage (EOP) curves, it is not difficult to show that there is no need for high Dk lenses under daily wear conditions. Here we encounter a problem of definition. One could say that the minimum required Dk is around 30 in daily wear. But what is daily wear? Are we willing to assume that patients never nap with their lenses in or that they never sleep with them in? We must account for modern life, with discotheques, house parties and late night pas- sion. Today's patients easily run into extended wear sit- uations and then the need for high Dk is obvious.
A third argument is price. Raw materials in the high Dk range are more expensive and the production of these lenses demands high-technology skills and machinery. This results, together with the shorter life span of such lenses, in higher costs for the patients.
All these arguments taken together lead to the situa- tion in which high Dk lenses are not used as lenses of first choice. Practitioners who debate on this subject usually take one of two standpoints. 'High Dk aspher-
JOHN DE BRABANDER ETAL.
ical lenses are the best' or the opposite opinion 'if you want problems fit high Dk aspherical lenses'. The aim of the research described in this article is to quantify the current situation with respect to the clinical func- tioning of high or low Dk materials and spherical or aspherical lens designs. In our research we sought sig- nificant differences between four lens groups in terms of overall success, fitting characteristics, physiological functioning, back curve optical radius (BCOR) stability, and lens condition.
Materials and Methods We used 68 patients in this study. They all had normal healthy eyes and had not worn lenses before. The patients were randomly divided into four groups, which were then fitted with a combination of high (Equa I1) or low (RXD) Dk materials and spherical or aspherical lens design.
The spherical lenses have an E-value of 0 at 30 , and the E-value of the aspherical lenses at 30 is 0.4. Both spherical and aspherical lenses have fixed axial edge lifts of 0.10mm and the same centre thickness- power-diameter relation. All lenses were produced by Oculenti Laboratories, using the latest CNC lathe technique. The physical properties of the materials are given in Table 1.
Table 1. Physical properties of RXD and EQUA II materials.
Physical Properties RXD 45 EQUA H 125
Dk (Fatt units, 35 C) 45 125 Wetting angle 39 30 Hardness (Shore) 122 113 Saline absorption < 1.0% < 1.0% Refractive index 1.435 1.423 Specific gravity 1.27 1.24
The initial BCOR selection was based on central K- readings, with compensation for corneal flattening at 30 (E-value). Follow-up examinations were performed at 6 weeks, 3 months, 0 months, and 1 year after dis- pensing the lenses.
At the first follow-up visit (6 weeks), a careful fitting evaluation was performed by two investigators sepa- rately and without consultation. Criteria for best fit were perfect corneal alignment at the flattest meridian, good centring, adequate movement, and an edge clearance which gave proper tear circulation without air bubbles being trapped or debris. K these criteria were not met the fit was first improved by changing the BCOR for the same lens design. If this result did not meet the criteria, the patient was refitted with a lens of the other design.
At all follow-up visits the eyes and lenses were exam- ined with a slit lamp using routine clinical procedures. If any significant problem occurred the first option was to change the material, but keep the earlier optimised geometry. Also K-readings were taken and the BCORs of the lenses were measured using a radiuscope.
All patients were given the same instructions and all used Unicare Green (produced by ICN Pharma- ceuticals, Holland) for insertion and daily storage plus
Ocuclean (supplied by Oculenti, Holland) for cleaning if desired.
Results Overall Success From the 68 patients that were initially fitted, eight (12%) discontinued wear within 6 months. Table 2 gives the reasons for failure. We were unable to overcome
Table 2. Overall success with 68 patients (=100%) at start of study.
Discontinued Limited wear Successful wear within 6 months after i year after I year
Reason No. Reason No. Criterion No.
Motivation 2 3-9 o'clock 1.5 Wearing 52.5 stain lenses
all waking hours
Discomfort 2 Surface dry 4 Wearing time 2 Wearing time 2 Intolerance 2
8 (12%) 7.5 (11%) 52.5 (77%)
these problems with lenses of other designs or mate- rials. Of the remaining 60 patients (120 eyes), 52.5 patients (105 eyes) wore the lenses for longer than 12 months without problems. Table 2 also details the per- sistent symptoms that occurred with limited wear in 15 eyes. These symptoms could not be diminished by changing to another material or lens design, but after the 1-year follow-up there was no reason to discontinue lens wear.
Fitting Characteristics After careful evaluation it was found that in ten eyes (8.3%) with spherical lenses changing the BCOR could not further improve the fit (Table 3). Typically, in such cases the tear layer is slightly flat in the centre, but the periphery shows a nearly bearing area. Only four out of the ten eyes had symptoms related to wearing time.
Table 3. Fitting characteristics and slit-lamp findings.a
RXD 45 EQUA II 125
Spherical: Spherical: Change fit 6 Change fit 4 Limited wear 0 Limited wear 3 Partial dry plus 3-9 stain 2 Partial dry plus 3-9 stain 4 Good wetting plus Good wetting plus
3-9 o'clock stain 2 3-9 o'clock stain 0
Aspherical: Aspherical: Change fit 2 Change fit 2 Limited wear 3 Limited wear 2 Partial dry plus 3-9 stain 2 Partial dry plus 3-9 stain 2 Good wetting plus Good wetting plus
3-9 stain 1 3-9 stain 2
a Number of eyes are given. No statistically significant difference was found.
LOW OR HIGH Dk? SPHERICAL OR ASPHERICAL RGPs?
Changing to the aspherical design showed an improve- ment in all ten cases.
In the group wearing aspherical lenses, four eyes (3.3%) showed a nearly steep centre, but also too much edge clearance. In this group, all eyes had problems with comfort. Changing from aspherical to spherical lens design could improve the fit in all of these eyes. All of the differences in fitting characteristics are statisti- cally not significant and not related to the material (Chi- square, p ~< 0.5).
Slit-Lamp Findings In none of the cases were serious adverse reactions found. The results from the 120 eyes showed three groups of problems - limited wear, partial dryness with 3-9 o'clock staining, and 3-9 'clock staining with good wetting of the lens. Table 3 shows that these findings are not related to the lens design.