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The effects of rigid contact lens overall diameter changes on tear pump efficiency: A comparative study
Clinical Article
The Effects of Rigid Contact Lens Overall Diameter Changes on Tear Pump Efficiency: A Comparative Study Barbara A. Fink, OD, PhD, Leo G. Camey,
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, cornea1 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 kns 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 fur 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 I. 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. Keywords: Contact lens; cornea; diameter; tear exchange; oxygen
Address reprint requests to Dr. Fink at The Ohio State University, College of Optometry, Columbus, OH 43210. Submitted for publication February 22, 1991; accepted for publication March 22, 1991.
0 1991 Butterworth-Heinemann
OD, PhD, and Richard M. Hill, OD, PhD
Introduction Over the last 40 years, considerable evidence has accumulated to suggest that the rigid contact lens fitting techniques (extrapalpebral or intrapalpebral aperture) that best provide for (1) adequate tear exchange, (2) a good bearing relationship between the contact lens and the cornea, and (3) proper lens movement and positioning are alignment in nature. ld When applying this fitting philosophy, however, several factors must be taken into consideration in order to arrive at an optimum lens size (i.e., overall diameter). Among them are palpebral aperture size, lid-to-cornea geometry, cornea1 diameter, pupil diameter, lid tension, corneal topography, and the power of the contact lens.2*5,6 A small lens may be appropriate when the patient has a high upper lid, a high 1ower lid, a small palpebral aperture, or a steep cornea. 5,7 The optic-zone diameter usually varies with the overall diameter, being commonly 1.0-1.5 mm smaller.6 The optic-zone diameter that most closely parallels cornea1 contour when fitted “on K” is 7 mm, assuming an average amount of peripheral flattening, a 43.00 D apical curvature, and no cornea1 toricity. When the optic zone is made larger than 7 mm, the base-curve radius must be made flatter-than-K in order to maintain a contour fit and prevent clearance between the lens and the comea.5 Both overall and optic-zone diameters of rigid gas permeable lenses are typically larger than those used in polymethylmethacrylate (PMMA) lenses, with overall diameters of 9.2 mm and optic-zone diameters of 7.8 mm being common. * How do these variations in overall and optic-zone diam-
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95
Clinical Art& eter affect comeal physiology? In this series of three studies, rigid lens overall diameter was varied in three different ways: (1) both overall and optic-zone diameters varied, maintaining peripheral curve width and all other parameters constant; (2) only overall diameter varied, optic-zone diameter was set at 7.4 mm, and all other parameters were maintained constant; and (3) as overall and optic-zone diameters varied, the base-curve radius of the lens also varied to maintain a constant tear-layer thickness between the cornea and the contact lens, while all other parameters remained constant. Cornea1 oxygen uptake was measured after periods of blinking and nonblinking with each lens, and the differences between these measurements indicated the tear pump efficiency for each lens design. It was the purpose of these comparative studies to determine how these variations in rigid lens diameters affect the pumping (bulk flow movement) of tears beneath the lens with the blink, this function being critical to the clearance of debris and metabolic wastes from beneath the lens and to the maintenance of an adequate oxygen supply to the cornea.
Materials and Methods Subjects Cornea1 oxygen uptake was measured on the right eye only of six different subjects in each of the three studies.
Table 1. Age, Sex, Flattest Keratometry Reading, Cornea1 Toricity, of the Three Studies Sex
Age
Table 1 lists the age, sex, flattest keratometry reading, corneal toricity, and vertical palpebral aperture height for each subject in each study, as well as the means. All subjects had with-the-rule cornea1 toricity of between 0.25 and 1.00 D; all had good ocular and systemic health, and none had ever worn contact lenses.
Contact Lenses For all three studies, the contact lens material was poly methylmethacrylate (PMMA), which was chosen specifically for its negligible oxygen permeability so that the oxygen provided to the cornea would be from the pumping of tears beneath the lens with each blink rather than from transmission through the lens itself.’ This permitted isolation of those changes in the physiological response of the cornea (i.e., oxygen-uptake rate) due solely to lens design. Lens parameters other than overall diameter, optic-zone diameter, and base-curve radius were constant in all three studies. Axial edge lift was 0.09 mm, back vertex power was - 3.00 D, and center thickness was 0.14 mm. Table 2 lists the constant and varying parameters for each of the three studies. In Study I (constant peripheral curve width), two parameters varied: overall diameter and optic-zone diameter. Five diameters were used for each eye, varying from 8.2 to 9.4 mm in 0.3 mm steps.
The optic-zone
and Vertical Palpebral Aperture
Flattest “K” (D)
Toricity
(D)
diameter
Height for Each Subject in Each
Palpebral aperture (mm)
Study I subjects 1
24
Male
42.50
0.62
12.0
f :
25 23 28
Female Male Male Female
41.62 42.50 43.87 41.87
0.75 0.87 0.37 0.87
11.5 10.0 10.0 8.5
6
;:
Male
41.37
1.00
11.0
42.29
0.75
11.75
Mean Study II subjects 1
26
Male
42.50
0.62
12.0
s !
25 21 46
Male Male
43.87 42.50 43.00 44.12
0.37 0.87 0.37 0.75
10.0 10.5 9.5
6
22
Male
Mean Study III subjects
96
24.5
27.5
41.50
0.50
10.5
42.92
0.66
10.4
:
25
Female Male
43.00
0.25 0.87
13.0 12.0
: 5 6
:: 22
Male Female Male Male
43.00 44.00 43.00 42.25
0.87 0.50 0.37 0.50
12.0 11.5 10.5 12.5
Mean
23.7
43.04
0.56
11.9
;:
ICLC, Vol. 16, May/June 1991
for each
RCL diameter on tear pump efficiency: Fink et al. Table 2. Constant and Variable Contact Lens Design Parameters for Study I (Constant Optic-zone
Diameter),
and Study III (Constant
Peripheral Curve Width),
Study II (Constant
Tear-layer Thickness)
Nonvarying parameters Material Back vertex power Center thickness Axial edge lift Varying parameters Overall diameter (mm) Optic-zone diameter (mm) Peripheral-curve width (mm) Base-curve radius Fit (base curve vs. cornea1 curvature) Tear-layer thickness (from sagittal calculations)
lens was 1.4 mm smaller, and the base-curve radius was fitted to the flattest cornea1 meridian (“on K”). In Study II (constant optic-zone diameter), only overall diameter varied, from 8.2 to 10.0 mm in 0.3 mm steps, for a total of seven lenses for each subject. The optic zone for all lenses was 7.4 mm, and the lenses were fitted “on K.” In Study III (constant tear-layer thickness), six contact lenses had overall diameters ranging from 7.6 to 10.6 mm in 0.6 mm steps and optic-zone diameters 1.4 mm smaller. The base-curve radius was fitted to maintain a constant apical tear-layer thickness, assuming an average cornea1 asphericity of -0.26.’
Equipment A Clark-type polarographic electrode was used to measure the oxygen-uptake rates of the cornea. It was calibrated in saline baths bubbled with air (for 155 mmHg) and nitrogen (for 0 mmHg) at 36°C. Oxygen to the 25 pm platinum cathode was supplied from a 12 Frn-thick poly ethylene membrane. As the electrode was touched perpendicularly to the central cornea, oxygen was taken up from the membrane, and a reduction in oxygen tension with time was displayed on the gas analyzer and chart recorder. Oxygen-uptake rates were calculated from the response curves over the range of 140 to 40 mmHg, as was the time constant, which was subtracted from all measurements. Procedure For each of the three studies, oxygen-uptake rates were measured for the normal open eye, as well as after wearing each contact lens for 5 minutes under static (without blinking while the experimenter held the lids open and the subject blotted tears from the inner and outer canthi) and dynamic (with blinking once every 5 seconds to a computer-generated tone) conditions. Measurements for each condition (static and dynamic) and for each lens-cornea combination were repeated eight times for each subject. The increase in oxygen-uptake rate measured under both static and dynamic conditions over that of the normal open
Study I
Study II
Study III
PMMA -3.00 D 0.14 mm 0.09 mm
PMMA -3.00 D 0.14 mm 0.09 mm
PMMA -3.00 D 0.14 mm 0.09 mm
8.2-9.4 6.8-8.0 0.7 Constant
8.2-10.0 7.4 0.4-1.3 Constant On K Constant
7.6-10.6 6.4-9.2 0.7 Variable Variable Constant
Eitble
eye were calculated by dividing all measurements by the oxygen uptake rate of the normal open eye. Differences between the static and dynamic condition data, relative to that of the normal open eye, were then determined for each lens design and each subject, as an index of the tear pump efficiency associated with each lens design.
Results Table 3 shows the means and standard errors of the mean for the static condition, dynamic condition, and difference data (all relative to mean nonwearing condition) of the six subjects for each lens design from the three studies. Figure 1 summarizes the means of the static condition data for each lens design from the three studies; Fig-we 2 summarizes the means of the dynamic condition data for each lens design from the three studies; and the means of the difference data for each lens design from the three studies are presented in Figure 3.
Discussion Static Condition Data
In Study I (constant peripheral-curve width), both overall and optic-zone diameters varied. As the lens was made larger, there was an increase in the pooling of tears between the cornea and the lens. Under static conditions, this pooling of tears served as a limited reservoir of oxygen for the cornea. The lens with the 8.8 mm diameter resulted in the maximum mean relative oxygen-uptake rate (6.5 times that of the normal open eye). As larger lenses were used, the rates generally decreased, and the largest lens used was associated with a mean relative oxygen-uptake rate that was significantly lower than those measured for any of the other lenses. In Study II (constant optic-zone diameter), the opticzone diameter was 7.4 mm for all lenses as the overall diameter varied from 8.2 to 10.0 mm in 0.3 mm steps. The tear volume over the central cornea did not vary with overall diameter. This is reflected in the oxygen-uptake rates
ICLC, Vol. 18, May/June 1991
97
Clinical Articles
Table 3. Mean Oxygen Demand Response* Study I: Constant peripheral-curve width Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM Study II: Constant optic-zone diameter Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM Study III: Constant tear-layer thickness Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM
-a.2
-8.5
-8.8
-9.1
-9.4
6.25 0.13
6.44 0.14
6.5 0.14
6.4 0.15
5.95 0.12
4.96 0.10
5.42 0.10
5.59 0.10
5.72 0.10
5.66 0.10
1.29 0.14
1.02 0.21
0.91 0.21
0.69 0.27
0.29 0.05
Ia2
-8.5
-8.8
9.1 -
-9.4
9.7 -
6.29 0.12
6.36 0.13
6.30 0.13
6.31. 0.12
6.34 0.14
6.37 0.12
6.33 0.13
5.50 0.14
5.56 0.12
5.70 0.12
5.86 0.12
5.99 0.11
6.07 0.12
6.17 0.11
0.78 0.10
0.81 0.08
0.60 0.11
0.46 0.09
0.35 0.09
0.30 0.07
0.15 0.08
10.0
-7.6
-8.2
-8.8
-9.4
5.38 0.14
5.94 0.13
6.33 0.13
6.40 0.13
6.48 0.13
6.54 0.11
3.92 0.14
4.82 0.13
5.28 0.14
5.76 0.12
6.00 0.13
6.13 0.13
1.46 0.11
1.12 0.10
1.05 0.13
0.64 0.13
0.47 0.13
0.41 0.11
10.0
210 6
Mean oxygen demand response (mmHg/s rate of oxygen demand) of six human corneas, each observed eight times, to 5 minutes of lens wear under either static or dynamic wearing conditions relative to the average of the nonlens-wearing open-eye oxygen-uptake rate for each of 18 lens designs (five from Study I, seven from Study II. and six from Study 111).The differences between the means of the ratioed static and dynamic condition values are also listed. l
measured under static conditions, which were not significantly different from one another. In Study III, a constant tear-layer thickness was maintained as overall and optic-zone diameters varied, by vary ing the base-curve radius accordingly. Although the tear pool over the central cornea should not have varied, higher oxygen-uptake rates resulted as the lens diameter increased. This is particularly remarkable since the total tear volume over the cornea increased with larger lenses. The two smallest lenses were associated with significantly lower oxygen-uptake rates than those obtained with the two largest lenses, indicating that central cornea1 oxygen demand increases as more of the cornea is covered. Dynamic Condition Data For all three studies, there was an increase in cornea1 oxygen uptake (greater oxygen deprivation) with increasing
98
ICLC, Vol. 18, May/June 1991
overall diameter. This result was seen regardless of opticzone diameter or base-curve radius. In Study II, the oxygenuptake rates associated with the two smallest lenses (8.2 and 8.5 mm) were significantly lower than those obtained with the largest lens (10.0 mm), and in Study III, the smallest lens once again brought about significantly lower oxygen-uptake rates than did any of the larger lenses. Difference Data Difference values reflect a reduction in cornea1 oxygen demand between static and dynamic wearing conditions, as oxygen-containing tears are pumped beneath the lens with the blink. Larger difference values reflect greater tear exchange or tear pump efficiency. Once again, smaller lenses were shown to be associated with a better physiological response. In all three studies, reductions in lens overall diameter were associated with improved tear pump efficien-
RCL diameter on tear STATIC CONDITION DATA
700
efficiency: Fink et al.
pump
DIFFERENCE DATA ::]I
. .
l
.
.
.
.
0
.
0
. 0 *
0
0 013
7.6
7.9
82
85
LENS
88
OVERALL
9.1
9.4
DIAMETER
9 7
10.0
103
106
109
76
79
82
,
,
85
8.8
LENS
(mm\
Figure 1. The mean comeal oxygen demands of six human eyes after static wear of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and basecurve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
OVERALL
,
91
94
DIAMETER
,
97
100
,
103
106
109
(mm)
Figure 3. The difference between the mean cornea1 oxygen demands of six human eyes obtained under static and dynamic wearing conditions of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and basecurve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
DYNAMIC CONDITION DATA
.
* .
,L 76
79
82
85
LENS
8.8
OVERALL
9 1
9.4
DIAMETER
9 7
100
103
106
109
(mm)
Figure 2. The mean comeal oxygen demands of six human eyes after the dynamic wear of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and base-curve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
ties or higher difference values. This was true even when the optic-zone diameter remained constant and when the base-curve radius varied with overall diameter to maintain a constant tear-layer thickness. Nevertheless, when fitting a patient with contact lenses, all design parameters must be considered, since they are
interrelated. There is an interaction between contact lens area, base curve, circumference, and diameter in determining the length and nature of the edge tear meniscus, the contact area, and the thickness of tears under the contact lens, all of which affect the fluid forces acting to position the lens and controlling tear flow beneath the lens during blinking. The physiological advantages of smaller diameter lenses were brought to the fore with these studies, which demonstrated that overall diameters smaller than those that are commonly used in gas permeable materials are more likely to provide for adequate tear exchange and an appropriate cornea1 environment (i.e., pH, oxygen content, osmolality, and absence of metabolic wastes). More study is needed on the influence of the lids and peripheral cornea1 topography on contact lens positioning and movement to assure proper lens performance, particularly with larger lenses.
References 1. Phillips AJ, Stone J: Contact Lenses: A Textbook for Practitioner and Student. Boston, Butterworths, 1989, pp. 333-381. 2. Mandell RB: Contact Lens Practice. Springfield, IL; Charles C. Thomas, 1981, pp. 125-247. 3. Kiely PM, Smith G, Camey LG: The mean shape of the human cornea. Opt Acta 1982;29:1027-1040. 4. Kiely PM, Smith G, Camey LG: Meridional variations of cornea1 shape. Am J Optom Physiol Opt 1984;61:619-626. 5. Bier N, Lowther GE: Contact Lens Correction. Boston, Butterworths, 1979, pp. 274-298. 6. Stein HA, Slatt BJ, Stein RM: Fitting Guide fur Rigid and Soft
ICLC, Vol. 18, May/June 1991
99
Clinical Articles Contact Lenses: A Practical Approach. St. Louis: C.V. Mosby, 1990, pp. 164-183. 7. Fink F%, Camey LG, Hill RM: Influence of palpebral aperture height on tear pump efficiency. Oprom Vis Sci
8. Bennett ES, Grohe RM: RigidGas-PermeableContact Lenses. New York, Professional Press Books, Fairchild Publications, 1986, pp. 189-224. 9. American National Standards. New York, American National
Standards, 1976, p. 7.
1990;67(4):287-290.
Clinical
Implications
The results of this study confirm what the clinician has known for many years: as you increase overall diameter, tear exchange is decreased resulting in a reduction in cornea1 oxygen supply. However, it is important to note that selection of smaller than average (i.e., traditional PMMA) overall diameter also increases the likelihood of both flare and possibly lens edge-lid interaction. Although a risk-benefit ratio may be associated with selecting larger overall lens diameters, the availability of higher oxygen permeable rigid lens materials, in addition to the obvious benefit of minimizing flare-induced visual disturbances and, in some cases, increased subjective comfort, result in the selection of a larger overall diameter in the great majority of cases. Edward S. Bennett, OD, MSJZd College of Optometry University of Missouri--St. Louis 8001 Natural Bridge Road St. Louis, MO 63 121
100
/CL C, Vol. 18, May/June 1991
RCL diameter on tear pump efficiency: Fink et al.
Barbara A. Fink, OD, PhD, received her BS and OD degrees from Indiana University. Between 1981 and 1984, she worked as an assistant professor and director of the Contact Lens Clinic at Inter American University of Puerto Rico School of Optometry. She earned her MS and PhD degrees from The Ohio State University, where she is currently working as an assistant professor. Her research interests are in the areas of cornea1 physiology, contact lens design, and anterior segment pathology.
Leo G. Camey, OD, PhD, completed the optometry program at the University of Melbourne, Australia, and received his MSc and PhD degrees from the same institution. He is currently professor and associate dean at the College of Optometry at The Ohio State University. His research interests include the relationship between contact lenses and the cornea.
Richard M. Hill, OD, PhD, completed his optometry, MOpt, and PhD programs at the University of California, Berkeley. He is currently dean of the College of Optometry at The Ohio State University in Columbus, Ohio, and directs the Eye Physiology Laboratory of the college. His research interests are (1) the testing of oxygen (EOP) performances of new hard gas permeable and hydrophilic contact lens materials, (2) the measurement and quality control of contact lens care systems and their compatibilities with materials and the eye, and (3) the enhancement of contact lens wearability through improved lens designs (i.e., by increasing tear exchange and oxygen availability to the cornea). Dr. Hill publishes a column in International Contact Lens Clinic, “The Eye and the Contact Lens,” and has authored two books: Curiosities of the Contact Lens and Contact Lens Perspectives.
ICLC, Vol. 18, May/June 1991
101
The Effects of Rigid Contact Lens Overall Diameter Changes on Tear Pump Efficiency: A Comparative Study Barbara A. Fink, OD, PhD, Leo G. Camey,
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, cornea1 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 kns 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 fur 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 I. 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. Keywords: Contact lens; cornea; diameter; tear exchange; oxygen
Address reprint requests to Dr. Fink at The Ohio State University, College of Optometry, Columbus, OH 43210. Submitted for publication February 22, 1991; accepted for publication March 22, 1991.
0 1991 Butterworth-Heinemann
OD, PhD, and Richard M. Hill, OD, PhD
Introduction Over the last 40 years, considerable evidence has accumulated to suggest that the rigid contact lens fitting techniques (extrapalpebral or intrapalpebral aperture) that best provide for (1) adequate tear exchange, (2) a good bearing relationship between the contact lens and the cornea, and (3) proper lens movement and positioning are alignment in nature. ld When applying this fitting philosophy, however, several factors must be taken into consideration in order to arrive at an optimum lens size (i.e., overall diameter). Among them are palpebral aperture size, lid-to-cornea geometry, cornea1 diameter, pupil diameter, lid tension, corneal topography, and the power of the contact lens.2*5,6 A small lens may be appropriate when the patient has a high upper lid, a high 1ower lid, a small palpebral aperture, or a steep cornea. 5,7 The optic-zone diameter usually varies with the overall diameter, being commonly 1.0-1.5 mm smaller.6 The optic-zone diameter that most closely parallels cornea1 contour when fitted “on K” is 7 mm, assuming an average amount of peripheral flattening, a 43.00 D apical curvature, and no cornea1 toricity. When the optic zone is made larger than 7 mm, the base-curve radius must be made flatter-than-K in order to maintain a contour fit and prevent clearance between the lens and the comea.5 Both overall and optic-zone diameters of rigid gas permeable lenses are typically larger than those used in polymethylmethacrylate (PMMA) lenses, with overall diameters of 9.2 mm and optic-zone diameters of 7.8 mm being common. * How do these variations in overall and optic-zone diam-
ICLC, Vol. 18, May/June 1991
95
Clinical Art& eter affect comeal physiology? In this series of three studies, rigid lens overall diameter was varied in three different ways: (1) both overall and optic-zone diameters varied, maintaining peripheral curve width and all other parameters constant; (2) only overall diameter varied, optic-zone diameter was set at 7.4 mm, and all other parameters were maintained constant; and (3) as overall and optic-zone diameters varied, the base-curve radius of the lens also varied to maintain a constant tear-layer thickness between the cornea and the contact lens, while all other parameters remained constant. Cornea1 oxygen uptake was measured after periods of blinking and nonblinking with each lens, and the differences between these measurements indicated the tear pump efficiency for each lens design. It was the purpose of these comparative studies to determine how these variations in rigid lens diameters affect the pumping (bulk flow movement) of tears beneath the lens with the blink, this function being critical to the clearance of debris and metabolic wastes from beneath the lens and to the maintenance of an adequate oxygen supply to the cornea.
Materials and Methods Subjects Cornea1 oxygen uptake was measured on the right eye only of six different subjects in each of the three studies.
Table 1. Age, Sex, Flattest Keratometry Reading, Cornea1 Toricity, of the Three Studies Sex
Age
Table 1 lists the age, sex, flattest keratometry reading, corneal toricity, and vertical palpebral aperture height for each subject in each study, as well as the means. All subjects had with-the-rule cornea1 toricity of between 0.25 and 1.00 D; all had good ocular and systemic health, and none had ever worn contact lenses.
Contact Lenses For all three studies, the contact lens material was poly methylmethacrylate (PMMA), which was chosen specifically for its negligible oxygen permeability so that the oxygen provided to the cornea would be from the pumping of tears beneath the lens with each blink rather than from transmission through the lens itself.’ This permitted isolation of those changes in the physiological response of the cornea (i.e., oxygen-uptake rate) due solely to lens design. Lens parameters other than overall diameter, optic-zone diameter, and base-curve radius were constant in all three studies. Axial edge lift was 0.09 mm, back vertex power was - 3.00 D, and center thickness was 0.14 mm. Table 2 lists the constant and varying parameters for each of the three studies. In Study I (constant peripheral curve width), two parameters varied: overall diameter and optic-zone diameter. Five diameters were used for each eye, varying from 8.2 to 9.4 mm in 0.3 mm steps.
The optic-zone
and Vertical Palpebral Aperture
Flattest “K” (D)
Toricity
(D)
diameter
Height for Each Subject in Each
Palpebral aperture (mm)
Study I subjects 1
24
Male
42.50
0.62
12.0
f :
25 23 28
Female Male Male Female
41.62 42.50 43.87 41.87
0.75 0.87 0.37 0.87
11.5 10.0 10.0 8.5
6
;:
Male
41.37
1.00
11.0
42.29
0.75
11.75
Mean Study II subjects 1
26
Male
42.50
0.62
12.0
s !
25 21 46
Male Male
43.87 42.50 43.00 44.12
0.37 0.87 0.37 0.75
10.0 10.5 9.5
6
22
Male
Mean Study III subjects
96
24.5
27.5
41.50
0.50
10.5
42.92
0.66
10.4
:
25
Female Male
43.00
0.25 0.87
13.0 12.0
: 5 6
:: 22
Male Female Male Male
43.00 44.00 43.00 42.25
0.87 0.50 0.37 0.50
12.0 11.5 10.5 12.5
Mean
23.7
43.04
0.56
11.9
;:
ICLC, Vol. 16, May/June 1991
for each
RCL diameter on tear pump efficiency: Fink et al. Table 2. Constant and Variable Contact Lens Design Parameters for Study I (Constant Optic-zone
Diameter),
and Study III (Constant
Peripheral Curve Width),
Study II (Constant
Tear-layer Thickness)
Nonvarying parameters Material Back vertex power Center thickness Axial edge lift Varying parameters Overall diameter (mm) Optic-zone diameter (mm) Peripheral-curve width (mm) Base-curve radius Fit (base curve vs. cornea1 curvature) Tear-layer thickness (from sagittal calculations)
lens was 1.4 mm smaller, and the base-curve radius was fitted to the flattest cornea1 meridian (“on K”). In Study II (constant optic-zone diameter), only overall diameter varied, from 8.2 to 10.0 mm in 0.3 mm steps, for a total of seven lenses for each subject. The optic zone for all lenses was 7.4 mm, and the lenses were fitted “on K.” In Study III (constant tear-layer thickness), six contact lenses had overall diameters ranging from 7.6 to 10.6 mm in 0.6 mm steps and optic-zone diameters 1.4 mm smaller. The base-curve radius was fitted to maintain a constant apical tear-layer thickness, assuming an average cornea1 asphericity of -0.26.’
Equipment A Clark-type polarographic electrode was used to measure the oxygen-uptake rates of the cornea. It was calibrated in saline baths bubbled with air (for 155 mmHg) and nitrogen (for 0 mmHg) at 36°C. Oxygen to the 25 pm platinum cathode was supplied from a 12 Frn-thick poly ethylene membrane. As the electrode was touched perpendicularly to the central cornea, oxygen was taken up from the membrane, and a reduction in oxygen tension with time was displayed on the gas analyzer and chart recorder. Oxygen-uptake rates were calculated from the response curves over the range of 140 to 40 mmHg, as was the time constant, which was subtracted from all measurements. Procedure For each of the three studies, oxygen-uptake rates were measured for the normal open eye, as well as after wearing each contact lens for 5 minutes under static (without blinking while the experimenter held the lids open and the subject blotted tears from the inner and outer canthi) and dynamic (with blinking once every 5 seconds to a computer-generated tone) conditions. Measurements for each condition (static and dynamic) and for each lens-cornea combination were repeated eight times for each subject. The increase in oxygen-uptake rate measured under both static and dynamic conditions over that of the normal open
Study I
Study II
Study III
PMMA -3.00 D 0.14 mm 0.09 mm
PMMA -3.00 D 0.14 mm 0.09 mm
PMMA -3.00 D 0.14 mm 0.09 mm
8.2-9.4 6.8-8.0 0.7 Constant
8.2-10.0 7.4 0.4-1.3 Constant On K Constant
7.6-10.6 6.4-9.2 0.7 Variable Variable Constant
Eitble
eye were calculated by dividing all measurements by the oxygen uptake rate of the normal open eye. Differences between the static and dynamic condition data, relative to that of the normal open eye, were then determined for each lens design and each subject, as an index of the tear pump efficiency associated with each lens design.
Results Table 3 shows the means and standard errors of the mean for the static condition, dynamic condition, and difference data (all relative to mean nonwearing condition) of the six subjects for each lens design from the three studies. Figure 1 summarizes the means of the static condition data for each lens design from the three studies; Fig-we 2 summarizes the means of the dynamic condition data for each lens design from the three studies; and the means of the difference data for each lens design from the three studies are presented in Figure 3.
Discussion Static Condition Data
In Study I (constant peripheral-curve width), both overall and optic-zone diameters varied. As the lens was made larger, there was an increase in the pooling of tears between the cornea and the lens. Under static conditions, this pooling of tears served as a limited reservoir of oxygen for the cornea. The lens with the 8.8 mm diameter resulted in the maximum mean relative oxygen-uptake rate (6.5 times that of the normal open eye). As larger lenses were used, the rates generally decreased, and the largest lens used was associated with a mean relative oxygen-uptake rate that was significantly lower than those measured for any of the other lenses. In Study II (constant optic-zone diameter), the opticzone diameter was 7.4 mm for all lenses as the overall diameter varied from 8.2 to 10.0 mm in 0.3 mm steps. The tear volume over the central cornea did not vary with overall diameter. This is reflected in the oxygen-uptake rates
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Clinical Articles
Table 3. Mean Oxygen Demand Response* Study I: Constant peripheral-curve width Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM Study II: Constant optic-zone diameter Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM Study III: Constant tear-layer thickness Overall diameter Static condition data Mean SEM Dynamic condition data Mean SEM Difference data Mean SEM
-a.2
-8.5
-8.8
-9.1
-9.4
6.25 0.13
6.44 0.14
6.5 0.14
6.4 0.15
5.95 0.12
4.96 0.10
5.42 0.10
5.59 0.10
5.72 0.10
5.66 0.10
1.29 0.14
1.02 0.21
0.91 0.21
0.69 0.27
0.29 0.05
Ia2
-8.5
-8.8
9.1 -
-9.4
9.7 -
6.29 0.12
6.36 0.13
6.30 0.13
6.31. 0.12
6.34 0.14
6.37 0.12
6.33 0.13
5.50 0.14
5.56 0.12
5.70 0.12
5.86 0.12
5.99 0.11
6.07 0.12
6.17 0.11
0.78 0.10
0.81 0.08
0.60 0.11
0.46 0.09
0.35 0.09
0.30 0.07
0.15 0.08
10.0
-7.6
-8.2
-8.8
-9.4
5.38 0.14
5.94 0.13
6.33 0.13
6.40 0.13
6.48 0.13
6.54 0.11
3.92 0.14
4.82 0.13
5.28 0.14
5.76 0.12
6.00 0.13
6.13 0.13
1.46 0.11
1.12 0.10
1.05 0.13
0.64 0.13
0.47 0.13
0.41 0.11
10.0
210 6
Mean oxygen demand response (mmHg/s rate of oxygen demand) of six human corneas, each observed eight times, to 5 minutes of lens wear under either static or dynamic wearing conditions relative to the average of the nonlens-wearing open-eye oxygen-uptake rate for each of 18 lens designs (five from Study I, seven from Study II. and six from Study 111).The differences between the means of the ratioed static and dynamic condition values are also listed. l
measured under static conditions, which were not significantly different from one another. In Study III, a constant tear-layer thickness was maintained as overall and optic-zone diameters varied, by vary ing the base-curve radius accordingly. Although the tear pool over the central cornea should not have varied, higher oxygen-uptake rates resulted as the lens diameter increased. This is particularly remarkable since the total tear volume over the cornea increased with larger lenses. The two smallest lenses were associated with significantly lower oxygen-uptake rates than those obtained with the two largest lenses, indicating that central cornea1 oxygen demand increases as more of the cornea is covered. Dynamic Condition Data For all three studies, there was an increase in cornea1 oxygen uptake (greater oxygen deprivation) with increasing
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ICLC, Vol. 18, May/June 1991
overall diameter. This result was seen regardless of opticzone diameter or base-curve radius. In Study II, the oxygenuptake rates associated with the two smallest lenses (8.2 and 8.5 mm) were significantly lower than those obtained with the largest lens (10.0 mm), and in Study III, the smallest lens once again brought about significantly lower oxygen-uptake rates than did any of the larger lenses. Difference Data Difference values reflect a reduction in cornea1 oxygen demand between static and dynamic wearing conditions, as oxygen-containing tears are pumped beneath the lens with the blink. Larger difference values reflect greater tear exchange or tear pump efficiency. Once again, smaller lenses were shown to be associated with a better physiological response. In all three studies, reductions in lens overall diameter were associated with improved tear pump efficien-
RCL diameter on tear STATIC CONDITION DATA
700
efficiency: Fink et al.
pump
DIFFERENCE DATA ::]I
. .
l
.
.
.
.
0
.
0
. 0 *
0
0 013
7.6
7.9
82
85
LENS
88
OVERALL
9.1
9.4
DIAMETER
9 7
10.0
103
106
109
76
79
82
,
,
85
8.8
LENS
(mm\
Figure 1. The mean comeal oxygen demands of six human eyes after static wear of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and basecurve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
OVERALL
,
91
94
DIAMETER
,
97
100
,
103
106
109
(mm)
Figure 3. The difference between the mean cornea1 oxygen demands of six human eyes obtained under static and dynamic wearing conditions of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and basecurve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
DYNAMIC CONDITION DATA
.
* .
,L 76
79
82
85
LENS
8.8
OVERALL
9 1
9.4
DIAMETER
9 7
100
103
106
109
(mm)
Figure 2. The mean comeal oxygen demands of six human eyes after the dynamic wear of gas impermeable rigid contact lenses, relative to the mean nonwearing eye condition, associated with 18 combinations of lens overall diameter, optic-zone diameter, and base-curve radius. Each data point is the mean of 48 measurements, eight for each of the six corneas.
ties or higher difference values. This was true even when the optic-zone diameter remained constant and when the base-curve radius varied with overall diameter to maintain a constant tear-layer thickness. Nevertheless, when fitting a patient with contact lenses, all design parameters must be considered, since they are
interrelated. There is an interaction between contact lens area, base curve, circumference, and diameter in determining the length and nature of the edge tear meniscus, the contact area, and the thickness of tears under the contact lens, all of which affect the fluid forces acting to position the lens and controlling tear flow beneath the lens during blinking. The physiological advantages of smaller diameter lenses were brought to the fore with these studies, which demonstrated that overall diameters smaller than those that are commonly used in gas permeable materials are more likely to provide for adequate tear exchange and an appropriate cornea1 environment (i.e., pH, oxygen content, osmolality, and absence of metabolic wastes). More study is needed on the influence of the lids and peripheral cornea1 topography on contact lens positioning and movement to assure proper lens performance, particularly with larger lenses.
References 1. Phillips AJ, Stone J: Contact Lenses: A Textbook for Practitioner and Student. Boston, Butterworths, 1989, pp. 333-381. 2. Mandell RB: Contact Lens Practice. Springfield, IL; Charles C. Thomas, 1981, pp. 125-247. 3. Kiely PM, Smith G, Camey LG: The mean shape of the human cornea. Opt Acta 1982;29:1027-1040. 4. Kiely PM, Smith G, Camey LG: Meridional variations of cornea1 shape. Am J Optom Physiol Opt 1984;61:619-626. 5. Bier N, Lowther GE: Contact Lens Correction. Boston, Butterworths, 1979, pp. 274-298. 6. Stein HA, Slatt BJ, Stein RM: Fitting Guide fur Rigid and Soft
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Clinical Articles Contact Lenses: A Practical Approach. St. Louis: C.V. Mosby, 1990, pp. 164-183. 7. Fink F%, Camey LG, Hill RM: Influence of palpebral aperture height on tear pump efficiency. Oprom Vis Sci
8. Bennett ES, Grohe RM: RigidGas-PermeableContact Lenses. New York, Professional Press Books, Fairchild Publications, 1986, pp. 189-224. 9. American National Standards. New York, American National
Standards, 1976, p. 7.
1990;67(4):287-290.
Clinical
Implications
The results of this study confirm what the clinician has known for many years: as you increase overall diameter, tear exchange is decreased resulting in a reduction in cornea1 oxygen supply. However, it is important to note that selection of smaller than average (i.e., traditional PMMA) overall diameter also increases the likelihood of both flare and possibly lens edge-lid interaction. Although a risk-benefit ratio may be associated with selecting larger overall lens diameters, the availability of higher oxygen permeable rigid lens materials, in addition to the obvious benefit of minimizing flare-induced visual disturbances and, in some cases, increased subjective comfort, result in the selection of a larger overall diameter in the great majority of cases. Edward S. Bennett, OD, MSJZd College of Optometry University of Missouri--St. Louis 8001 Natural Bridge Road St. Louis, MO 63 121
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RCL diameter on tear pump efficiency: Fink et al.
Barbara A. Fink, OD, PhD, received her BS and OD degrees from Indiana University. Between 1981 and 1984, she worked as an assistant professor and director of the Contact Lens Clinic at Inter American University of Puerto Rico School of Optometry. She earned her MS and PhD degrees from The Ohio State University, where she is currently working as an assistant professor. Her research interests are in the areas of cornea1 physiology, contact lens design, and anterior segment pathology.
Leo G. Camey, OD, PhD, completed the optometry program at the University of Melbourne, Australia, and received his MSc and PhD degrees from the same institution. He is currently professor and associate dean at the College of Optometry at The Ohio State University. His research interests include the relationship between contact lenses and the cornea.
Richard M. Hill, OD, PhD, completed his optometry, MOpt, and PhD programs at the University of California, Berkeley. He is currently dean of the College of Optometry at The Ohio State University in Columbus, Ohio, and directs the Eye Physiology Laboratory of the college. His research interests are (1) the testing of oxygen (EOP) performances of new hard gas permeable and hydrophilic contact lens materials, (2) the measurement and quality control of contact lens care systems and their compatibilities with materials and the eye, and (3) the enhancement of contact lens wearability through improved lens designs (i.e., by increasing tear exchange and oxygen availability to the cornea). Dr. Hill publishes a column in International Contact Lens Clinic, “The Eye and the Contact Lens,” and has authored two books: Curiosities of the Contact Lens and Contact Lens Perspectives.
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