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A Practical Approach to Lens Implant Power
a practical approach to lens implant power Robert C. Drews, M.D. St. Louis, Missouri A number of elegant articles have been written on the determination of lens implant power. Their formulas are carefully derived. Unfortunately, because they make different assumptions, they yield a variety of answers from a given set of data. And more importantly they require data that most of us cannot obtain, or cannot obtain with sufficient accuracy. . Three basic parameters must be measured in order to predict lens implant power: (I) Corneal power
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the phakic chamber angle. Most others assume an average value in their tables. Binkhorst gives correction factors for cases in which the normal value may be assumed to be wrong. One must then make an educated guess about the correct value. Axial Length of the Eye .'
X-ray determination of the actual length of the eye is difficult and of barely adequate precision at best. Ultrasound is the best method today. Again, equipment must be calibrated accurately. But ordinary ultrasound equipment, as expensive as it is, yields an accuracy of only 1 mm, and 1 mm = 3 diopters! Only very high frequency ultrasound can yield measurements to the nearest 0.1 mm. Such equipment is prohibitively expensive and is (therefore) not available to most of us. Indeed many ophthalmologists do not have access even to ordinary ultrasound equipment. When it is avail(2) Aphakic chamber depth ' (distance from the able, however, it can help the surgeon avoid drascornea to the lens implant), and ,., .i., ft- ': . , tic errors in unustial cases, giving him at least ; (3) Axial length of the eye "ball park" limits. It is a tragedy to do an excellent implant and then find the patient is 10 or even 18 And there are two other factors which are not us- diopters myopic. (Such cases exist.) ually mentioned in such discussions: The Lens Implant (4) Distance from lens implant to its second principal plane, and Almost all formulas assume an optically "thin" lens: one whose second principle plane lies at the (5) Astigmatic error center of the lens itself. In the usual lenses this error is truly negligible, especially as compared Corneal Power to the errors inherent in predicting lens position. Only the Troutman lens with its reversed-teleThis is the one parameter which every ophthalphoto design requires formulas in which this facmologist should be able to measure easily and with tor is significant. sufficient accuracy. But how long has it been since your keratometer or ophthalmometer was calibrated? When it reads 41.50 is that really 41.12 or Astigmatism 41.87 perhaps? Satisfactory contact lens fitting What good is the most accurate determination of is no criterion, especially when "fudge factors" lens implant power if the patient has 2.50 to 3.00 such as fitting "steeper than K" are used by some diopters of astigmatism postoperatively?! And and not by others. this is the average amount of astigmatism present in several major implant series (even by surgeons Aphakic Chamber Depth who average much less astigmatism than this in patients who do not receive an implant ... Nor is Most of us cannot predict the postoperative depth this astigmatism in the implant: it can be · meaof the anterior chamber. Galin makes a prediction sured in the cornea; and dislocation of an implant by assuming the position of the chamber angle foot causing gross tilting of the implant in one with respect to the observed limbus (obviously case of mine changed the astigmatic error only subject to variation). He measures the diameter 0.50 diopters.) of the cornea at that level and calculates the chamber depth knowing this and the corneal radius of A Practical Approach curvature. Worst has worked on (but not published What approach can be used practically? First, as far as I know) a goniophotographic method of you must either know the patient's basic refraction measurement, measuring the distance from the (prior to the development of any cataract) or obapex of the cornea to a line joining the recess of
tain some sort of ultrasound measurement to avoid the bitter disappointment of finding a (unilateral) high axial myopia . . The great majority of patients have a basic refraction of about zero and after cataract surgery have a spherical equivalent error of about +10. Patients with significant basic refractive errors yield a postoperative aphakic error which can usually be reasonably predicted by dividing the preoperative spherical equivalent by 2 and adding the result to +10. Thus a 4 diopter hyperope will be +12 aphakic and a 20 diopter myope will be rendered approximately emetropic. Errors in such predictions are seldom greater than 2 diopters. If a lens of 19.5 - 20.0 diopters is implanted, statistically the average patient will be made about 2 diopters myopic. Such an elderly patient will be happy since he can now read without glasses. And if by chance the surgeon is wrong by as much as 2 diopters, the patient may be emetropic (!) or 4 diopters myopic (can still read without glasses). Within this range, most patients will be happy with their visual results: and that's a much more practical goal than emetropia. If the patient is basically somewhat nearsighted, the strength of the implant should be reduced: he should not be made more nearsighted. An 18 diopter lens will, on the average, leave his refractive error where it was. A 4 diopter myope for example might well receive a 16 diopter implant. This should leave him 2 diopters myopic; better than before, but not so much better that he might accidentally wind up hyperopic, unable to see clearly without glasses at any distance, a condition he never experienced before and may not enjoy now. High myopes probably should not have lens implants: they don't need strong glasses postoperatively anyway and they are at risk with respect to retinal detachment. The mildly hyperopic patient can be fitted with the standard lens. It will make him less hyperopic without inducing an inordinate amount of anisometropia. The high hyperope can receive a stronger lens, avoiding too much anisometropia if there is still useful vision in the fellow eye. For example, a patient who is 8 diopters hyperopic in both eyes and has some useful vision in the eye which is not to be operated upon, might well be fitted with a 22 diopter lens. A 20 diopter lens would make him 6 (+2) diopters hyperopic. A 22 diopter lens, 4 (+2) diopters hyperopic. The relative myopia of the pseudophakic eye will reduce aniseikonia.
Summary For the average surgeon, the use of a standard power lens implant, with shifts in power according to the patient's basic refraction - and with considera tion for the fellow eye and the effect of too much anisometropia - and with use of ultrasound in cases of ignorance or suspicion - can result in making the vast majority of his patients very happy with their pseudophakic vision. If better methods are available, they certainly should be used. •
clinical notes
"IF A lens was inserted when your cataract was removed. please phone EX 6-9403 to answer questions?"
Above is an item that appeared in the personals column of a newspaper in California. Upon investigating. it was learned that this ad was placed by a patient interested in having an intraocular lens implantation and wished to find out the results of this procedure from patients who had had it done rather than from their doctor.
vitreous loss The discussion of lens implantation with vitreous loss must be defined. It should be clearly mentioned whether lens implantation was performed after vitreous loss has occurred or whether vitreous was lost after the lens was properly implanted into the eye. There is a great difference between abandoning lens implantation following vitreous loss and removing a lens after vitreous has been lost. This situation should always be mentioned when discussing the subject of intraocular lenses combined with vitreous loss. • 51
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the phakic chamber angle. Most others assume an average value in their tables. Binkhorst gives correction factors for cases in which the normal value may be assumed to be wrong. One must then make an educated guess about the correct value. Axial Length of the Eye .'
X-ray determination of the actual length of the eye is difficult and of barely adequate precision at best. Ultrasound is the best method today. Again, equipment must be calibrated accurately. But ordinary ultrasound equipment, as expensive as it is, yields an accuracy of only 1 mm, and 1 mm = 3 diopters! Only very high frequency ultrasound can yield measurements to the nearest 0.1 mm. Such equipment is prohibitively expensive and is (therefore) not available to most of us. Indeed many ophthalmologists do not have access even to ordinary ultrasound equipment. When it is avail(2) Aphakic chamber depth ' (distance from the able, however, it can help the surgeon avoid drascornea to the lens implant), and ,., .i., ft- ': . , tic errors in unustial cases, giving him at least ; (3) Axial length of the eye "ball park" limits. It is a tragedy to do an excellent implant and then find the patient is 10 or even 18 And there are two other factors which are not us- diopters myopic. (Such cases exist.) ually mentioned in such discussions: The Lens Implant (4) Distance from lens implant to its second principal plane, and Almost all formulas assume an optically "thin" lens: one whose second principle plane lies at the (5) Astigmatic error center of the lens itself. In the usual lenses this error is truly negligible, especially as compared Corneal Power to the errors inherent in predicting lens position. Only the Troutman lens with its reversed-teleThis is the one parameter which every ophthalphoto design requires formulas in which this facmologist should be able to measure easily and with tor is significant. sufficient accuracy. But how long has it been since your keratometer or ophthalmometer was calibrated? When it reads 41.50 is that really 41.12 or Astigmatism 41.87 perhaps? Satisfactory contact lens fitting What good is the most accurate determination of is no criterion, especially when "fudge factors" lens implant power if the patient has 2.50 to 3.00 such as fitting "steeper than K" are used by some diopters of astigmatism postoperatively?! And and not by others. this is the average amount of astigmatism present in several major implant series (even by surgeons Aphakic Chamber Depth who average much less astigmatism than this in patients who do not receive an implant ... Nor is Most of us cannot predict the postoperative depth this astigmatism in the implant: it can be · meaof the anterior chamber. Galin makes a prediction sured in the cornea; and dislocation of an implant by assuming the position of the chamber angle foot causing gross tilting of the implant in one with respect to the observed limbus (obviously case of mine changed the astigmatic error only subject to variation). He measures the diameter 0.50 diopters.) of the cornea at that level and calculates the chamber depth knowing this and the corneal radius of A Practical Approach curvature. Worst has worked on (but not published What approach can be used practically? First, as far as I know) a goniophotographic method of you must either know the patient's basic refraction measurement, measuring the distance from the (prior to the development of any cataract) or obapex of the cornea to a line joining the recess of
tain some sort of ultrasound measurement to avoid the bitter disappointment of finding a (unilateral) high axial myopia . . The great majority of patients have a basic refraction of about zero and after cataract surgery have a spherical equivalent error of about +10. Patients with significant basic refractive errors yield a postoperative aphakic error which can usually be reasonably predicted by dividing the preoperative spherical equivalent by 2 and adding the result to +10. Thus a 4 diopter hyperope will be +12 aphakic and a 20 diopter myope will be rendered approximately emetropic. Errors in such predictions are seldom greater than 2 diopters. If a lens of 19.5 - 20.0 diopters is implanted, statistically the average patient will be made about 2 diopters myopic. Such an elderly patient will be happy since he can now read without glasses. And if by chance the surgeon is wrong by as much as 2 diopters, the patient may be emetropic (!) or 4 diopters myopic (can still read without glasses). Within this range, most patients will be happy with their visual results: and that's a much more practical goal than emetropia. If the patient is basically somewhat nearsighted, the strength of the implant should be reduced: he should not be made more nearsighted. An 18 diopter lens will, on the average, leave his refractive error where it was. A 4 diopter myope for example might well receive a 16 diopter implant. This should leave him 2 diopters myopic; better than before, but not so much better that he might accidentally wind up hyperopic, unable to see clearly without glasses at any distance, a condition he never experienced before and may not enjoy now. High myopes probably should not have lens implants: they don't need strong glasses postoperatively anyway and they are at risk with respect to retinal detachment. The mildly hyperopic patient can be fitted with the standard lens. It will make him less hyperopic without inducing an inordinate amount of anisometropia. The high hyperope can receive a stronger lens, avoiding too much anisometropia if there is still useful vision in the fellow eye. For example, a patient who is 8 diopters hyperopic in both eyes and has some useful vision in the eye which is not to be operated upon, might well be fitted with a 22 diopter lens. A 20 diopter lens would make him 6 (+2) diopters hyperopic. A 22 diopter lens, 4 (+2) diopters hyperopic. The relative myopia of the pseudophakic eye will reduce aniseikonia.
Summary For the average surgeon, the use of a standard power lens implant, with shifts in power according to the patient's basic refraction - and with considera tion for the fellow eye and the effect of too much anisometropia - and with use of ultrasound in cases of ignorance or suspicion - can result in making the vast majority of his patients very happy with their pseudophakic vision. If better methods are available, they certainly should be used. •
clinical notes
"IF A lens was inserted when your cataract was removed. please phone EX 6-9403 to answer questions?"
Above is an item that appeared in the personals column of a newspaper in California. Upon investigating. it was learned that this ad was placed by a patient interested in having an intraocular lens implantation and wished to find out the results of this procedure from patients who had had it done rather than from their doctor.
vitreous loss The discussion of lens implantation with vitreous loss must be defined. It should be clearly mentioned whether lens implantation was performed after vitreous loss has occurred or whether vitreous was lost after the lens was properly implanted into the eye. There is a great difference between abandoning lens implantation following vitreous loss and removing a lens after vitreous has been lost. This situation should always be mentioned when discussing the subject of intraocular lenses combined with vitreous loss. • 51