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Translating rigid bifocals: Choosing fitting parameters to optimize visual performance
Clinicul Article
Translating Rigid Bifocals: Choosing Fitting Parameters to Optimize Visual Performance Graham
Macalister,
BSc, FBCO, DCLP, and Craig Woods, BSc, MBCO,
Rigid translating bifocal lenses require considerable fitting skills to give optimal results. This study sets out to investigate how two principallens parameters interact on the lens movement characteristics that determine visual performance. A gas permeabk lens with a monocentric design was used. Each of the 10 subjects, all adapted wearers, wore six different lenses to give all permutationsof three radii and two total diameters. Video recordings were made of lens position and mowement, and from the measurements taken, some trends were revealed. Flatter fitting improved translation, but gave more rotation and horizontal decentration on down gaze and more vertical displacement on blink with a longer return time for distance gaze. Smaller lenses gave a more rapid return for distance gaze, but more rotation and horizontaldecentration for down gaze. However, we found considerable intersubject variability, and these findings should only be taken as the first strategy to adopt when faced with a particular fitting problem. There can be no substitute for observing lens dynamics with a trial lens. Keywords: Presbyopia; alternating; video recording; fitting; translation;
bifocal; rigid gas permeable; lens dynamics; parameters
Introduction Practitioners have shown a renewed interest in rigid translating bifocals’ since they became available in gas permeable materials.2 They have a potential for excellent visual acuity for distance and near, providing lens position Address reprint requests to Craig Woods, Institute of Optometry, 56-62 Newington Causeway, Elephant and Castle, London SE1 6DS, United Kingdom. Accepted
for publication
July 13, 1992.
0 1992 Butterworth-Heinemann
DCLP
can give constant pupil coverage by the appropriate part of the lens. However, this ideal can rarely be achieved and a compromise is usually reached. The most common patient complaints3 are distance vision instability, caused by the lens rising too high for too long immediately following the blink, and poor near vision, caused by inadequate translation or translation negated by a combination of lens rotation and horizontal displacement away from the pupil. (Displacement in the absence of rotation has little detrimental effect. Similarly, rotation has little effect if the lens is well centered in the horizontal.) The practitioner needs to know how to choose parameters in order to optimize lens dynamics and positioning. The following parameters will have some effect: 1. Back surface geometry
2. 3. 4. 5.
Lens size Prism Prism axis Truncation
depth (and profile).
Once these parameters have been specified, the segment height is determined on the basis of the expected lens dynamics and positioning. Some scientific work has been done on the interaction between anatomical features, such as the lids, and the lens.4*5 Another study assessed the influence parameter selection has on lens performance.6 Our study sets out to extend this knowledge by using video recording to look in detail at the effect of two lens parameters. These were back optic zone radius and total diameter. All other parameters were kept constant.
ICLC, Vol. 19, September/October
1992
199
Clinical Articles
Method
Vertical Displacement: Distance Gaze
The Subjects
This was taken as the vertical distance the lens moved in the primary gaze position after a blink.
Ten patients (20 eyes) were used in this trial. They were all existing gas permeable lens wearers. None of the patients had more than 1.25 D of cornea1 astigmatism. Eight of them were presbyopes. The remainder were prepresby opes. It was felt that as the purpose of this study was to assess the fitting performance of these lenses and not the visual performance then the introduction of the prepresbyopes would not negate the results.
Return Time This related to the time taken for the lens to return to its base-line position after a blink and was calculated from the number of frames to complete the movement. Horizontal Decent&on:
Down Gape
This was the horizontal distance from the center of the lens to a line passing vertically through the pupil center. Angk of Rotation
Tb Lens The Tangent Streak Bifocal Lens (TSB) was used. This is a solid bifocal made in a rigid gas permeable material. It has a monocentric executive design stabilized by use of truncation and prism. The material is a fluorosilicate providing a Dk of 70. We used two fitting sets, each with a different overall size: 9.40/9.00 mm and 9.9019.50 mm. The sets have positive and negative parameters and fitting steps of 0.1 mm. A lens power of - 2.00 (add + 2.00) was used throughout. The manufacturer’s recommended fitting guide is Cornea1 Toricity Zero 0.50 D Over 1.00 D
Base Curve * 1.00 D flatter than “K” 0.50 D flatter than “K” Not steeper than ‘/4 of toricity
*Modified by steepness of cornea and optical zone size.
The translating bifocal lenses were inserted and left for 15 minutes to settle. The lens movement and position relative to the pupil center was recorded on videotape by a camera mounted on a slit lamp. Recordings were made of the lens in the primary position of gaze and in a fixed downward convergent gaze. The problem of observing the lens in the near position was overcome by the use of a mirror attached to a Hruby lens slit lamp stand. The three radii were chosen as follows:
The degree the lens rotated during down gaze was measured relative to the horizontal. A protractor to assess the degree was used to take this measurement. Translation This was the distance the segment line moved relative to the pupil center, when gaze changed from distance to near gaze. Comfort All patients were asked to grade subjectively the comfort of every lens. The scale was from 0 to 5, 0 being intolerable and 5 being completely unaware.
Results and Discussion The mean of all the measurements taken from all the subjects has been charted in order to indicate the trends found when changing from steeper fitting through recomp mended fitting to flatter fitting and when changing from smaller to larger diameter.
Vertical Displacement: Distance Gaze The vertical displacement induced (Figure 1) by the blink is important to the stability of distance vision. As
1. One fitting step (0.50 D) steeper than recommended according to manufacturer’s guidelines. 2. As recommended by the manufacturer’s guidelines. step flatter than manufacturer’s 3. One fitting guidelines. The Measurements Recordings were made of lens movement and position through successive blinks. The following measurements were taken directly from the monitor screen using the freeze-frame facility. The screen was first calibrated by videoing an eye graticule used on the slit lamp.
200
/UC,
Vol. 19, September/October
1992
Figure 1. Vertical displacement segment line after a blink.
measured in millimeters
of the
Translating rigid bifocals: Macalister and Woods expected, the flatter fittings produced more vertical displacement. There was little difference between the two diameters, except that for lenses fitted flatter than recommended and the larger diameter gave more vertical displacement.
mm
-_-
_-.-.
Return Time We have seen that flatter fittings give higher displacement with blink, but is this compensated by a more rapid speed of downward movement? Figure 2 shows that changing from steeper to flatter does result in an increased return time. Therefore, there is no compensation for the higher displacement. It appears that the free movement required from optimum translation must be balanced against instability of distance vision. If this does prove to be a problem, other strategies must be employed:
Figure 3. Amount of horizontal decentration in millimeters on downward gaze compared to the position in the primary position.
Either 1. Lowering
the segment (if that compromising the near vision)
is possible
without
or 2. Increasing the prism ballast (at the expense of increased thickness and reduced oxygen transmission). Diameter changes had little effect on vertical displacement (except for the flatter lenses, which are unlikely to be used). Therefore, the longer return time shown by the larger lenses is not because they have further to move but because they are moving more slowly. Horizontal Decentration
on Down Gaze
Again, we see that the flatter lenses showed a reversed trend to the recommended and steeper fittings. We can see in Figure 3 that the steeper fitting can reduce the horizontal decentration, as might be expected. However, changing to the larger diameter had an even greater effect, with a combination of steeper fit and larger diameter giving least decentration.
Figure 2. Number of freeze frames for the segment line to return to its preblink position. One frame = % second.
Figure 4. Angle of rotation in degrees from the horizontal downward gaze in comparison to the primary position.
on
Angk of Rotation Again, the flatter lenses showed a reversed trend. Ignoring these, and concentrating on the two other fittings that we have seen are the most use to the practitioner, we find that the same strategies that stabilize horizontal decentration will reduce rotation as well (Figure 4). This is particularly useful since it is the combination of rotation and decentration that causes poor pupil coverage by the near segment. Rotation was much more marked in downward than in distance gaze and it should be noted that even with the optimum pairing of the steeper radius and 9.90 mm diameter the mean rotation angle was still over 25”. Near rotation was almost always in the direction to pull the nasal edge up. Presumably, this is because the lens rotates until its thinnest part, the prism apex, is under the strongest part of the upper lid. Therefore, it may be more profitable to change the base apex line so that the segment line is horizontal when the base apex line is rotated by the lid forces.
ICLC, Vol. 19, September/October
1992
201
Clinical Articles Translation The first consideration in the choice of fitting parameters must be to optimize translation. Poor translation will result in poor vision for near, and increasing the segment height will merely transfer the problem to the distance vision. Figure 5 shows that fitting steeper than the manufacturer’s recommendation does inhibit the free movement of the lens and thus reduces the amount the segment moves up relative to the pupil on down gaze. Going flatter than recommended appears to have no beneficial effect as far as translation is concerned. Changing from one diameter to another had no effect. The recommended fitting gave a mean figure for the 10 subjects of just 3 mm of translation, which gives an indication of the maximum pupil size that can be accommodated before optical quality is compromised with simultaneous vision through both segments.
and steeper gave
Comfort Differences in comfort were not very marked, but there was a trend for the flatter fittings and 9.40 mm diameter lenses to be more comfortable (Figure 6). It is interesting to note the majority of the comfort ratings were close to the completely unaware end of the scale and also that several subjects commented that these lenses were more comfortable than were their own single-vision lenses.
Conclusion
No difference displacement:
was found
for translation
and vertical
gaze)
and larger gave 1. Less rotation (down gaze) 2. Less horizontal decentration
(down gaze).
gave
1. Better translation
Figure 5. Translation of the segment in millimeters relative to the pupil position in downward gaze compared to the primary position.
/UC,
Choice of Diameter
1. More rapid return following blink (distance
Fitting 0.50 D flatter than recommended did not appear to offer any consistent advantages. Comparing recommended and 0.50 D steeper,
202
1. Less horizontal decentration (down gaze) 2. Less rotation (down gaze) 3. Less vertical displacement on blink and more rapid return (distance gaze).
Smaller gave
Choice of Radius
recommended
Figure 6. Results of the comfort rate for the lens on a numeric scale, where 0 is very uncomfortable and 5 is unaware of or found to be as comfortable as their own lenses.
Vol. 19, September/October
1992
It must be stressed that these data can only be taken as showing trends in indicating the type of parameter changes that should be considered first, in order to solve a particular fitting problem. Indeed, the most salient point to come from these data was the great variation in behavior of a given fitting or diameter on different eyes. Occasionally, an individual eye would show a completely different trend to those that have derived from the mean values from all the eyes. Because of wide intersubject variation, we are not able to put any statistical significance on the data presented. These variations, which probably originate in differences in lid position and strength, mean that it is not possible to set up rigid fitting guidelines and that the use of a trial set is essential. Since the effect of changing parameters cannot be predicted with certainty, the opportunity to test the resultant effect on lens dynamics, before ordering the final lens, is very useful. This can be done by borrowing a “loan” lens of approximately the desired specifications from the manufacturer.
Translating rigid bifocals: Macalister and Woods The key to success in this field of contact lens work does not lie in adhering to an inflexible set of fitting rules, but in observing the lens dynamics as well as the static position between blinks. Only if these observations are made can the correct decisions be made on parameter choice. This study illustrates which observations are relevant. They can all be made in the clinical situation, without the precision offered by video recording, which was used here to permit quantitative scientific analysis.
Acknowledgments The authors thank Fused Kontacts of Chicago and the No. 7 Contact Lens Laboratory of London for the supply of the lenses used in this study; Chris Wilson of Barts Medical College, London, for advice on the statistical methods used; Geoff Roberson, Judith Morris, and Eric Papas, Institute of Optometry, London, for their advice and support;
and Eva Macalister for coordinating the trial, which was carried out at the Institute of Optometry, London.
References 1. Stein HA: The management of presbyopia with contact lenses. A review. CLAO 1 1990;16( 1):33-38. 2. Krajenski P: Presbyopic lenses: An update: Contact Lens Forum 1989;14(2):40. 3. De Carle JT: in Stone J, Phillips A (Eds): Bifocal and multifocal confaCt lenses, 3rd ed. Contact Lenses. London: Butterworth, 1989, pp. 571-591. 4. Borish I, Perrigin D: Observations of bifocal contact lenses. Int Eyecare 1985;1(3):241. 5. Borish I, Perrigin D: Relative movement lower lid and line of sight from distance to near fixation. Am J Optom PhysioE Opt 1987;64:881-887. 6. Ames K et al. : Factors influencing vision with rigid gas permeable alternating bifocals. Optom Vis Sci 1989;66( 1):3338.
Clinical Implications
This study points out the importance of analyzing diagnostic lenses to insure that your lens design has the proper base curve and/or proper diameter at the initial fitting, which may help your overall success and ease of fitting rigid gas permeable bifocal contact lenses. The authors have pointed out that this report suggests trends when looking at the overall data and, also, points out the variation present even with a very small sampling. It is known that fitting an RGP bifocal is one of the most challenging and yet most rewarding of all contact lens endeavors. There is no magic formula for successfully fitting these patients. This is truly an art that must be practiced and conquered. When all the necessary parameters are reached: base curve, diameter, secondary curve, edge bevel, segment height, amount of prism, orientation of prism, distance power, near power, central thickness, material, truncation design, etc., and the patient is successful, it is truly a thing of intricate beauty. It would behoove us all to again strive to better understand the mechanisms that influence contact lenses. It would offer our patients better vision and service, which keeps them coming back to us. Jack J. Yager, OD, FAA0 214 East Marks Street Orlando, FL 32803
ICLC, Vol. 19, September/October
1992
203
Clinical Articles
Graham Macalister qualified as an ophthalmic optician in 1978, and in 1982, he joined a London practice to concentrate full time on contact lens work. He gained his DCLP in 1985 and then took up the post of part-time senior optometrist at Moorfields Contact Lens Department. In the following year, he was invited to join Allergan Hydron as a part-time research optometrist, and in 1988, he gave a paper comparing the risks of flexible and extended wear at the International Contact Lens Centenary Congress in London. In 1989, he started a research project on bifocal contact lenses at the Institute of Optometry and presented a paper comparing a translating bifocal to monovision at the BCLA conference in 1991. He has lectured on this general topic at other venues, including the American Academy of Optometry Conference in California. His work is currently divided between work in private practice and his Moorfields post, where he is conducting research on cornea1 grafts, which is registered for a higher degree at The City University, London. Craig Woods qualified from The City University, London, with an honors degree in optometry and visual science. After a period in private practice, he joined the Institute of Optometry as the assistant director in 1989. In 1990, he gained his DCLP. He has presented papers on various subjects including the use of disposable lenses and the use of contact lenses in presbyopia at the BCLA, The Scottish Contact Lens Societies Conference, and other local optical bodies. He has served on the BCLA council for 2 years and is currently their meetings’ secretary. In the summer of 1992, he joined the research organization Eurolens at UMIST, Manchester, as a research optometrist.
204
ICE,
Vol. 19, September/October
1992
Translating Rigid Bifocals: Choosing Fitting Parameters to Optimize Visual Performance Graham
Macalister,
BSc, FBCO, DCLP, and Craig Woods, BSc, MBCO,
Rigid translating bifocal lenses require considerable fitting skills to give optimal results. This study sets out to investigate how two principallens parameters interact on the lens movement characteristics that determine visual performance. A gas permeabk lens with a monocentric design was used. Each of the 10 subjects, all adapted wearers, wore six different lenses to give all permutationsof three radii and two total diameters. Video recordings were made of lens position and mowement, and from the measurements taken, some trends were revealed. Flatter fitting improved translation, but gave more rotation and horizontal decentration on down gaze and more vertical displacement on blink with a longer return time for distance gaze. Smaller lenses gave a more rapid return for distance gaze, but more rotation and horizontaldecentration for down gaze. However, we found considerable intersubject variability, and these findings should only be taken as the first strategy to adopt when faced with a particular fitting problem. There can be no substitute for observing lens dynamics with a trial lens. Keywords: Presbyopia; alternating; video recording; fitting; translation;
bifocal; rigid gas permeable; lens dynamics; parameters
Introduction Practitioners have shown a renewed interest in rigid translating bifocals’ since they became available in gas permeable materials.2 They have a potential for excellent visual acuity for distance and near, providing lens position Address reprint requests to Craig Woods, Institute of Optometry, 56-62 Newington Causeway, Elephant and Castle, London SE1 6DS, United Kingdom. Accepted
for publication
July 13, 1992.
0 1992 Butterworth-Heinemann
DCLP
can give constant pupil coverage by the appropriate part of the lens. However, this ideal can rarely be achieved and a compromise is usually reached. The most common patient complaints3 are distance vision instability, caused by the lens rising too high for too long immediately following the blink, and poor near vision, caused by inadequate translation or translation negated by a combination of lens rotation and horizontal displacement away from the pupil. (Displacement in the absence of rotation has little detrimental effect. Similarly, rotation has little effect if the lens is well centered in the horizontal.) The practitioner needs to know how to choose parameters in order to optimize lens dynamics and positioning. The following parameters will have some effect: 1. Back surface geometry
2. 3. 4. 5.
Lens size Prism Prism axis Truncation
depth (and profile).
Once these parameters have been specified, the segment height is determined on the basis of the expected lens dynamics and positioning. Some scientific work has been done on the interaction between anatomical features, such as the lids, and the lens.4*5 Another study assessed the influence parameter selection has on lens performance.6 Our study sets out to extend this knowledge by using video recording to look in detail at the effect of two lens parameters. These were back optic zone radius and total diameter. All other parameters were kept constant.
ICLC, Vol. 19, September/October
1992
199
Clinical Articles
Method
Vertical Displacement: Distance Gaze
The Subjects
This was taken as the vertical distance the lens moved in the primary gaze position after a blink.
Ten patients (20 eyes) were used in this trial. They were all existing gas permeable lens wearers. None of the patients had more than 1.25 D of cornea1 astigmatism. Eight of them were presbyopes. The remainder were prepresby opes. It was felt that as the purpose of this study was to assess the fitting performance of these lenses and not the visual performance then the introduction of the prepresbyopes would not negate the results.
Return Time This related to the time taken for the lens to return to its base-line position after a blink and was calculated from the number of frames to complete the movement. Horizontal Decent&on:
Down Gape
This was the horizontal distance from the center of the lens to a line passing vertically through the pupil center. Angk of Rotation
Tb Lens The Tangent Streak Bifocal Lens (TSB) was used. This is a solid bifocal made in a rigid gas permeable material. It has a monocentric executive design stabilized by use of truncation and prism. The material is a fluorosilicate providing a Dk of 70. We used two fitting sets, each with a different overall size: 9.40/9.00 mm and 9.9019.50 mm. The sets have positive and negative parameters and fitting steps of 0.1 mm. A lens power of - 2.00 (add + 2.00) was used throughout. The manufacturer’s recommended fitting guide is Cornea1 Toricity Zero 0.50 D Over 1.00 D
Base Curve * 1.00 D flatter than “K” 0.50 D flatter than “K” Not steeper than ‘/4 of toricity
*Modified by steepness of cornea and optical zone size.
The translating bifocal lenses were inserted and left for 15 minutes to settle. The lens movement and position relative to the pupil center was recorded on videotape by a camera mounted on a slit lamp. Recordings were made of the lens in the primary position of gaze and in a fixed downward convergent gaze. The problem of observing the lens in the near position was overcome by the use of a mirror attached to a Hruby lens slit lamp stand. The three radii were chosen as follows:
The degree the lens rotated during down gaze was measured relative to the horizontal. A protractor to assess the degree was used to take this measurement. Translation This was the distance the segment line moved relative to the pupil center, when gaze changed from distance to near gaze. Comfort All patients were asked to grade subjectively the comfort of every lens. The scale was from 0 to 5, 0 being intolerable and 5 being completely unaware.
Results and Discussion The mean of all the measurements taken from all the subjects has been charted in order to indicate the trends found when changing from steeper fitting through recomp mended fitting to flatter fitting and when changing from smaller to larger diameter.
Vertical Displacement: Distance Gaze The vertical displacement induced (Figure 1) by the blink is important to the stability of distance vision. As
1. One fitting step (0.50 D) steeper than recommended according to manufacturer’s guidelines. 2. As recommended by the manufacturer’s guidelines. step flatter than manufacturer’s 3. One fitting guidelines. The Measurements Recordings were made of lens movement and position through successive blinks. The following measurements were taken directly from the monitor screen using the freeze-frame facility. The screen was first calibrated by videoing an eye graticule used on the slit lamp.
200
/UC,
Vol. 19, September/October
1992
Figure 1. Vertical displacement segment line after a blink.
measured in millimeters
of the
Translating rigid bifocals: Macalister and Woods expected, the flatter fittings produced more vertical displacement. There was little difference between the two diameters, except that for lenses fitted flatter than recommended and the larger diameter gave more vertical displacement.
mm
-_-
_-.-.
Return Time We have seen that flatter fittings give higher displacement with blink, but is this compensated by a more rapid speed of downward movement? Figure 2 shows that changing from steeper to flatter does result in an increased return time. Therefore, there is no compensation for the higher displacement. It appears that the free movement required from optimum translation must be balanced against instability of distance vision. If this does prove to be a problem, other strategies must be employed:
Figure 3. Amount of horizontal decentration in millimeters on downward gaze compared to the position in the primary position.
Either 1. Lowering
the segment (if that compromising the near vision)
is possible
without
or 2. Increasing the prism ballast (at the expense of increased thickness and reduced oxygen transmission). Diameter changes had little effect on vertical displacement (except for the flatter lenses, which are unlikely to be used). Therefore, the longer return time shown by the larger lenses is not because they have further to move but because they are moving more slowly. Horizontal Decentration
on Down Gaze
Again, we see that the flatter lenses showed a reversed trend to the recommended and steeper fittings. We can see in Figure 3 that the steeper fitting can reduce the horizontal decentration, as might be expected. However, changing to the larger diameter had an even greater effect, with a combination of steeper fit and larger diameter giving least decentration.
Figure 2. Number of freeze frames for the segment line to return to its preblink position. One frame = % second.
Figure 4. Angle of rotation in degrees from the horizontal downward gaze in comparison to the primary position.
on
Angk of Rotation Again, the flatter lenses showed a reversed trend. Ignoring these, and concentrating on the two other fittings that we have seen are the most use to the practitioner, we find that the same strategies that stabilize horizontal decentration will reduce rotation as well (Figure 4). This is particularly useful since it is the combination of rotation and decentration that causes poor pupil coverage by the near segment. Rotation was much more marked in downward than in distance gaze and it should be noted that even with the optimum pairing of the steeper radius and 9.90 mm diameter the mean rotation angle was still over 25”. Near rotation was almost always in the direction to pull the nasal edge up. Presumably, this is because the lens rotates until its thinnest part, the prism apex, is under the strongest part of the upper lid. Therefore, it may be more profitable to change the base apex line so that the segment line is horizontal when the base apex line is rotated by the lid forces.
ICLC, Vol. 19, September/October
1992
201
Clinical Articles Translation The first consideration in the choice of fitting parameters must be to optimize translation. Poor translation will result in poor vision for near, and increasing the segment height will merely transfer the problem to the distance vision. Figure 5 shows that fitting steeper than the manufacturer’s recommendation does inhibit the free movement of the lens and thus reduces the amount the segment moves up relative to the pupil on down gaze. Going flatter than recommended appears to have no beneficial effect as far as translation is concerned. Changing from one diameter to another had no effect. The recommended fitting gave a mean figure for the 10 subjects of just 3 mm of translation, which gives an indication of the maximum pupil size that can be accommodated before optical quality is compromised with simultaneous vision through both segments.
and steeper gave
Comfort Differences in comfort were not very marked, but there was a trend for the flatter fittings and 9.40 mm diameter lenses to be more comfortable (Figure 6). It is interesting to note the majority of the comfort ratings were close to the completely unaware end of the scale and also that several subjects commented that these lenses were more comfortable than were their own single-vision lenses.
Conclusion
No difference displacement:
was found
for translation
and vertical
gaze)
and larger gave 1. Less rotation (down gaze) 2. Less horizontal decentration
(down gaze).
gave
1. Better translation
Figure 5. Translation of the segment in millimeters relative to the pupil position in downward gaze compared to the primary position.
/UC,
Choice of Diameter
1. More rapid return following blink (distance
Fitting 0.50 D flatter than recommended did not appear to offer any consistent advantages. Comparing recommended and 0.50 D steeper,
202
1. Less horizontal decentration (down gaze) 2. Less rotation (down gaze) 3. Less vertical displacement on blink and more rapid return (distance gaze).
Smaller gave
Choice of Radius
recommended
Figure 6. Results of the comfort rate for the lens on a numeric scale, where 0 is very uncomfortable and 5 is unaware of or found to be as comfortable as their own lenses.
Vol. 19, September/October
1992
It must be stressed that these data can only be taken as showing trends in indicating the type of parameter changes that should be considered first, in order to solve a particular fitting problem. Indeed, the most salient point to come from these data was the great variation in behavior of a given fitting or diameter on different eyes. Occasionally, an individual eye would show a completely different trend to those that have derived from the mean values from all the eyes. Because of wide intersubject variation, we are not able to put any statistical significance on the data presented. These variations, which probably originate in differences in lid position and strength, mean that it is not possible to set up rigid fitting guidelines and that the use of a trial set is essential. Since the effect of changing parameters cannot be predicted with certainty, the opportunity to test the resultant effect on lens dynamics, before ordering the final lens, is very useful. This can be done by borrowing a “loan” lens of approximately the desired specifications from the manufacturer.
Translating rigid bifocals: Macalister and Woods The key to success in this field of contact lens work does not lie in adhering to an inflexible set of fitting rules, but in observing the lens dynamics as well as the static position between blinks. Only if these observations are made can the correct decisions be made on parameter choice. This study illustrates which observations are relevant. They can all be made in the clinical situation, without the precision offered by video recording, which was used here to permit quantitative scientific analysis.
Acknowledgments The authors thank Fused Kontacts of Chicago and the No. 7 Contact Lens Laboratory of London for the supply of the lenses used in this study; Chris Wilson of Barts Medical College, London, for advice on the statistical methods used; Geoff Roberson, Judith Morris, and Eric Papas, Institute of Optometry, London, for their advice and support;
and Eva Macalister for coordinating the trial, which was carried out at the Institute of Optometry, London.
References 1. Stein HA: The management of presbyopia with contact lenses. A review. CLAO 1 1990;16( 1):33-38. 2. Krajenski P: Presbyopic lenses: An update: Contact Lens Forum 1989;14(2):40. 3. De Carle JT: in Stone J, Phillips A (Eds): Bifocal and multifocal confaCt lenses, 3rd ed. Contact Lenses. London: Butterworth, 1989, pp. 571-591. 4. Borish I, Perrigin D: Observations of bifocal contact lenses. Int Eyecare 1985;1(3):241. 5. Borish I, Perrigin D: Relative movement lower lid and line of sight from distance to near fixation. Am J Optom PhysioE Opt 1987;64:881-887. 6. Ames K et al. : Factors influencing vision with rigid gas permeable alternating bifocals. Optom Vis Sci 1989;66( 1):3338.
Clinical Implications
This study points out the importance of analyzing diagnostic lenses to insure that your lens design has the proper base curve and/or proper diameter at the initial fitting, which may help your overall success and ease of fitting rigid gas permeable bifocal contact lenses. The authors have pointed out that this report suggests trends when looking at the overall data and, also, points out the variation present even with a very small sampling. It is known that fitting an RGP bifocal is one of the most challenging and yet most rewarding of all contact lens endeavors. There is no magic formula for successfully fitting these patients. This is truly an art that must be practiced and conquered. When all the necessary parameters are reached: base curve, diameter, secondary curve, edge bevel, segment height, amount of prism, orientation of prism, distance power, near power, central thickness, material, truncation design, etc., and the patient is successful, it is truly a thing of intricate beauty. It would behoove us all to again strive to better understand the mechanisms that influence contact lenses. It would offer our patients better vision and service, which keeps them coming back to us. Jack J. Yager, OD, FAA0 214 East Marks Street Orlando, FL 32803
ICLC, Vol. 19, September/October
1992
203
Clinical Articles
Graham Macalister qualified as an ophthalmic optician in 1978, and in 1982, he joined a London practice to concentrate full time on contact lens work. He gained his DCLP in 1985 and then took up the post of part-time senior optometrist at Moorfields Contact Lens Department. In the following year, he was invited to join Allergan Hydron as a part-time research optometrist, and in 1988, he gave a paper comparing the risks of flexible and extended wear at the International Contact Lens Centenary Congress in London. In 1989, he started a research project on bifocal contact lenses at the Institute of Optometry and presented a paper comparing a translating bifocal to monovision at the BCLA conference in 1991. He has lectured on this general topic at other venues, including the American Academy of Optometry Conference in California. His work is currently divided between work in private practice and his Moorfields post, where he is conducting research on cornea1 grafts, which is registered for a higher degree at The City University, London. Craig Woods qualified from The City University, London, with an honors degree in optometry and visual science. After a period in private practice, he joined the Institute of Optometry as the assistant director in 1989. In 1990, he gained his DCLP. He has presented papers on various subjects including the use of disposable lenses and the use of contact lenses in presbyopia at the BCLA, The Scottish Contact Lens Societies Conference, and other local optical bodies. He has served on the BCLA council for 2 years and is currently their meetings’ secretary. In the summer of 1992, he joined the research organization Eurolens at UMIST, Manchester, as a research optometrist.
204
ICE,
Vol. 19, September/October
1992