Ординатура / Офтальмология / Английские материалы / Wavefront Customized Visual Correction The Quest for Super Vision II_Krueger, Applegate, MacRae_2003

The negative spherical aberration of the young lens gradually approaches zero at about age 40 and then continues to become increasingly positive as aging continues.

The Tecnis Z-9000 is a typical example of an IOL developed with the help of wavefront technology,16 and details are provided in Chapter 34. Terwee et al measured the wavefront aberrations of the Tecnis Z-9000 and other commercially available lenses in a model eye with a Shack-Hartmann wavefront sensor.17 The model eye also had a cornea that exhibited the spherical aberration of an average human cornea. The eye with the Tecnis IOL showed less aberrations than the eyes with other IOLs.

Initial clinical results with these lenses are encouraging, with significant improvement in contrast sensitivity under low luminance and high spatial frequencies when compared with fellow eyes implanted with conventional IOLs. In a randomized study including 40 patients, Neuhann and Mester compared the quality of vision after phacoemulsification and implantation of the CeeOn Edge model 911A in one eye and the Tecnis Z-9000 in the contralateral eye.18 Control examinations at 1 to 2, 30 to 60, and

90 to 120 days postoperatively included contrast sensitivity, visual acuity assessment under different conditions, and wavefront analysis. The authors found that spherical aberration was markedly reduced in eyes implanted with the Tecnis Z-9000 IOL. They also found that low-contrast visual acuity and mesopic contrast sensitivity were significantly better in eyes implanted with this lens.

Corydon et al also compared the contrast sensitivity of the same lens designs in a prospective randomized study including 10 patients.19 At 30 to 60 days postoperatively, visual acuity for near and far as well as contrast sensitivity were measured, the latter with low-contrast charts and the Contrast Sensitivity Tester model 1800 (Vision Sciences Research Co, San Ramon, Calif) for near and far. The authors found that the pupil size had an influence on the results, with patients having small pupils showing no difference between the two lenses, while patients with larger pupils had a better contrast sensitivity in the eyes implanted with the Tecnis Z-9000 IOL. Pupil size apparently plays an important role, and, taking into account the modified prolate profile of the Tecnis, no significant advantages would be expected when the pupil size is less than 3 mm.

Figure 32-12. Gross photograph showing a CeeOn Edge lens experimentally implanted in a human eye obtained postmortem, prepared according to the Miyake-Apple posterior videophotographic technique.

The design of the CeeOn Edge lens appears appropriate for incorporation of the Z-Sharp Optic Technology. The CeeOn Edge IOL model 911 is a foldable three-piece lens; the diameter of the biconvex optic is 6 mm and the overall diameter of the lens is 12 mm (Figure 32-12). The CeeOn Edge is also available with an overall diameter of 13 mm, as will the Tecnis Z-9001 lens. The optic component is manufactured from a third-generation silicone material, polydimethyl diphenyl siloxane, developed and manufactured by Pfizer with a high refractive index (1.46). The optic rim has square truncated edges. The haptics are manufactured from polyvinylidene fluoride (PVDF). The haptic design is a cap C with a 90-degree exit and an angulation of 6 degrees. The lens has an incorporated ultraviolet absorber (benzotriazole) to protect the retina from radiation in the 300to 400-nm range.

The 911 model is the first silicone IOL design with a square truncated optic edge. This has proven to provide good properties regarding posterior capsule opacification (PCO) prevention. Schauersberger et al compared the performance of the 911 and the three-piece AcrySof lens in terms of capsular bag opacification.20 Twenty-five eyes operated on by the same surgeon were included in each lens group. After a follow up of 3 years, there was no statistically significant difference between both groups of lenses regarding PCO.

Although this lens design has square optic edges, optical phenomena are generally not associated with this lens. Indeed, these phenomena depend, among other factors, on the refractive index of the lens material. The refractive index of this lens is relatively low compared to hydrophobic acrylic lenses, which have a markedly higher refractive index. Also, opacification of the anterior capsule, which was demonstrated to be highly associated with silicone lenses, will help in the prevention of any edge-glare phenomena if the capsulorrhexis is smaller than the IOL optic diameter.

The haptic material, PVDF, was found to present good rigidity and retentive memory. Izak et al compared the shape recovery ratios of silicone lenses having different haptic materials (PMMA, polypropylene, polyimide, and PVDF) after compression (Figure 32-13).21 Silicone-PMMA, silicone-polyimide, and silicone-PVDF lenses presented similar loop memories, which were significantly better than with silicone-polypropylene lenses

Figure 32-13. Chemical structures of the four haptic materials currently used in the manufacture of IOLs and gross photographs showing details of the optic-haptic junctions of four different threepiece silicone lenses. (A) CeeOn (912) silicone optic-PMMA haptic lens. (B) PhacoFlex II SI30 NB (Advanced Medical Optics, Santa Ana, Calif) silicone optic-Prolene haptic lens. (C) ELASTIMIDE (STAAR Surgical, Monrovia, Calif) silicone optic-Elastimide haptic lens. (D) CeeOn Edge silicone optic-PVDF haptic lens.

(P < 0.05). The haptic cap C design is stated to maintain the shape of the capsular bag, offering more clock hours of contact between the haptics and the capsule. These characteristics help in the prevention of lens decentration and tilt in cases of capsular bag contraction. This is very important because, qualitatively, any aberration correction is sensitive to decentration and tilt. Patients will benefit from the advanced technology of Tecnis within the normal clinical limits of lens decentration inferior to 0.40 mm and lens tilt inferior to 7 degrees.

SUMMARY

We have briefly reviewed the biomaterials currently used for the manufacture of the optic component of IOLs and presented two examples of “customized” lenses. The refractive power of the Calhoun LAL can be noninvasively adjusted in the postoperative period, correcting the major cause of postoperative explantation of modern foldable lens designs. Implementation of wavefront aberrometry on the Calhoun LAL technology will allow full customization of the lens, with correction of different ocular aberrations. The Tecnis Z-9000 lens with the Z-Sharp Optic Technology has a modified prolate profile that produces a negative spherical aberration compensating the positive aberration of the cornea. Eyes implanted with this lens presented significantly reduced spherical aberration and better contrast sensitivity in comparison to standard IOLs. We will certainly witness, in the near future, an increasing influence of wavefront aberrometry on the manufacture of different IOL designs. This will provide patients with higher quality of vision, well beyond the quality obtained with currently available lenses.

ACKNOWLEDGMENTS

The authors would like to thank N.E. Sverker Norrby, PhD (Pfizer) and Christian Sandstedt, PhD (Calhoun Vision) for their help in the preparation of this text.

Supported in part by a grant from Research to Prevent Blindness, Inc., New York, NY, to the Department of Ophthalmology and Visual Sciences, University of Utah.

REFERENCES

1.Werner L, Apple DJ, Schmidbauer JM. Ideal IOL (PMMA and foldable) for year 2002. In: Buratto L, Werner L, Zanini M, Apple DJ, eds.

Phacoemulsification: Principles and Techniques. 2nd ed. Thorofare: NJ: SLACK Incorporated; 2002;435-451.

2.Werner L, Apple DJ, Izak A, Pandey SK, Trivedi RH, Macky TA. Phakic anterior chamber intraocular lenses. In: Werner L, Apple DJ, eds. Complications of Aphakic and Refractive Intraocular Lenses. International Ophthalmology Clinics. Philadelphia: Lippincott Williams & Wilkins; 2001:133-152.

3.Werner L, Apple DJ, Pandey SK, Trivedi RH, Izak A, Macky TA. Phakic posterior chamber intraocular lenses. In: Werner L, Apple DJ, eds. Complications of Aphakic and Refractive Intraocular Lenses. International Ophthalmology Clinics. Philadelphia: Lippincott Williams & Wilkins; 2001:153-174.

4.Applegate RA, Thibos LN, Hilmantel G. Optics aberroscopy and super vision. J Cataract Refract Surg. 2001;27:1093-1107.

5.Christ FR, Buchen SY, Deacon J, et al. Biomaterials used for intraocular lenses. In: Wise DL, Trantolo DJ, Altobelli DE, et al, eds.

Encyclopedic Handbook of Biomaterials and Bioengineering. Part B: applications. Vol 2. New York: Marcel Dekker Inc; 1995:1261-1313.

6.Glazer LC, Shen TT, Azar DT, Murphy E. Intraocular lens material. In: Azar DT, ed. Intraocular Lenses in Cataract and Refractive Surgery.

Philadelphia: WB Saunders; 2001:39-49.

7.Werner L, Legeais JM. Les matériaux pour implants intraoculaires. Partie I: les implants intraoculaires en polyméthylméthacrylate et modifications de surface. J Fr Ophthalmol. 1998;21:515-524.

8.Werner L, Legeais JM. Les matériaux pour implants intraoculaires. Partie II: les implants intraoculaires souples, en silicone. J Fr Ophthalmol. 1999;22:492-501.

9.Legeais JM, Werner L, Werner LP, Renard G. Les matériaux pour implants intraoculaires. Partie III: les implants intraoculaires acryliques souples. J Fr Ophthalmol. 2001;24:309-318.

10.Maloney RK, Jethmalani J, Sandstedt C, et al. Intraocular lens with light-adjustable power. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

11.Mamalis N, Chang SH, Sandstedt CA, et al. Evaluation of a lightadjustable intraocular lens. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

12.Mamalis N, Spencer TS. Complications of foldable intraocular lenses requiring explantation or secondary intervention—2000 survey update. J Cataract Refract Surg. 2001;7:1310-1317.

13.Norrby S. Conception of Z-sharp optic technology. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

14.Artal P, Berrio E, Guirao A, Piers P. Contribution of the cornea and internal surfaces to the change of ocular aberrations with age. J Opt Soc Am A Opt Image Sci Vis. 2002;19:137-143.

15.Bellucci R. Optical aberrations and intraocular lens design. In: Buratto L, Werner L, Zanini M, Apple DJ, eds. Phacoemulsification: Principles and Techniques. 2nd ed. Thorofare, NJ: SLACK Incorporated; 2002:454-455.

16.Holladay JT, Piers P, Koranyi G, et al. IOL design to reduce the ocular wavefront aberration in aphakic eyes. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

17.Terwee T, Barkhof J, Weeber H, Piers P. Influence of the Tecnis model Z-9000 and other IOLs on wavefront aberrations. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

18.Neuhann T, Mester U. Improvement of optical and visual quality by an intraocular lens with a correcting optical design. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

19.Corydon L, Dam-Johansen M, Winther-Nielsen A, Lundkvist T. Contrast sensitivity with the Pharmacia Tecnis Z-9000 IOL. Paper presented at: the ASCRS Symposium on Cataract, IOL, and Refractive Surgery; June 2002; Philadelphia, Pa.

20.Schauersberger J, Amon M, Kruger A, et al. Comparison of the biocompatibility of 2 foldable intraocular lenses with sharp optic edges. J Cataract Refract Surg. 2001;27:1579-1585.

21.Izak AM, Werner L, Apple DJ, Macky TA, Trivedi RH, Pandey SK. Loop memory of different haptic materials used in the manufacture of posterior chamber intraocular lenses. J Cataract Refract Surg. 2002; 28:1229-1235.

The presence of higher-order aberrations in the eye has been known for several decades.1-3 Primarily identified as spherical aberration, the significant role of nonrotationally symmetrical aberrations, such as coma, has been studied and identified more recently.4 Current measurements using Shack-Hartmann–based wavefront sensors have opened the door to identifying the magnitude and form of higher-order wavefront aberrations in large, preoperative, physiological populations.5

Early descriptions of spherical aberration as the predominant monochromatic aberration of the human eye were particularly appealing to contact lens manufacturers. As early as the 1960s, contact lenses themselves were identified as having inherent spherical aberration due to their steep curvatures and relatively constant alignment with the visual axis of the eye.6 Many manufacturers believed that it was a simple matter of using their lathing technologies to form an aspheric correcting surface on one or both sides of the contact lens to not only correct the optical aberrations of the lens itself, but also to enhance the retinal image of the eye—the first attempts at super vision.7 Indeed, a number of manufacturers sell products today based on this concept.

WAVEFRONT ABERRATIONS

AND THE GENERAL POPULATION

Initial inspection of the average distribution of higher-order wavefront aberrations across a typical prepresbyopic population requiring refractive correction shows a distinct deviation from zero for the spherical aberration Zernike term, while all other higher-order Zernike terms have an average close to zero—a value expected of a biological optical system attempting to optimize itself across a population (Figure 33-1). This would suggest that an appropriate aspheric correcting surface would significantly reduce the wavefront aberration of the eye if it were incorporated into a contact lens. However, as Figure 33-2 shows, there is a wide range of spherical aberration values across the patient population that are not related to another variable, such as degree of ametropia. To realistically correct more than 38% of the population, spherical aberration would have to be introduced as an additional parameter in the contact lens rather than an average level of correction added into every lens. Perhaps more importantly, spherical aberration, while significant in the hierarchy of higher-order wavefront aberration of the eye, is typically not the dominant aberration in most eyes. Third-order Zernike

terms, such as coma and trefoil, are the largest magnitude high- er-order wavefront aberrations found in any given eye in the general population. These aberrations must be corrected by a rotationally stable contact lens and manufactured using a process capable of creating nonrotationally symmetrical surfaces. In fact, Figure 33-3 shows the Strehl ratio for a large sample of the general ophthalmic population when corrected alternatively with sphere and cylinder only; with sphere, cylinder, and spherical aberration; and with sphere, cylinder, third-order Zernike terms, and spherical aberration. Clearly, while some patients benefit from correction of spherical aberration in addition to their myopia and astigmatism, there is a substantially larger increase in retinal image quality when the third-order Zernike terms are also corrected. Therefore, a contact lens must be designed to correct both symmetrical and nonrotationally symmetrical higherorder aberrations of the eye if a true visual benefit is to be realized across a substantial proportion of the population.

Third-order Zernike terms, such as coma and trefoil, are the largest magnitude higher-order wavefront aberrations found in a normal eye.

There is a substantially larger increase in retinal image quality when the third-order Zernike terms are also corrected.

RIGID GAS PERMEABLE VS SOFT CONTACT LENSES

FOR CUSTOMIZED CORRECTION

This leads us to the question: Which type of contact lens would be ideal for neutralizing higher-order wavefront aberrations? Rigid gas permeable (RGP) lenses have been utilized for years as a method for correcting eyes with pathology or postsurgical wavefront aberration, where standard spherocylindrical spectacle lenses do not provide adequate visual acuity. However, it is the rigid nature of the lens itself, rather than innovative optics, that provides the wavefront aberration correction of these lenses. Vision of eyes with significantly distorted corneas can be improved with RGP lenses because the anterior surface of the contact lens forms the new refracting surface, with the tear film

Figure 33-1. Distribution of wavefront aberrations in the patient population. Mean values of all Zernike modes in the population across a 5.7 millimeter (mm) pupil. The error bars represent plus and minus one standard deviation from the mean value. The variability of the higher-order modes is shown in the inset of the figure, which excludes all second-order modes (Z-22, Z02, and Z22) and expands the ordinate. (Reprinted with permission from Porter J, Guirao A, Cox IG, Williams DR. Monochromatic aberrations of the human eye in a large population. Journal of the Optical Society of America. 2001;18[8]:1793-1803.)

Figure 33-3. Distribution of Strehl ratios in a large physiological preoperative patient population following correction of sphere and cylinder only (blue); sphere, cylinder, and spherical aberration (red); and sphere, cylinder, coma, trefoil, and spherical aberration (green).

filling in the difference between the irregular cornea and the regular back surface of the lens. Unfortunately, the physical nature of RGP lenses—and the way they must be fit to ensure tear exchange and mobility on the eye—renders them less desirable as a method for correcting wavefront aberrations of the eye through complex optical surfaces. RGP lenses are designed to be very mobile on the eye, moving several millimeters with each blink and finding a position of rest with up to 1 millimeter (mm) difference relative to the optical axis of the eye following each blink.8 Hence, stabilizing

Figure 33-2. Distribution of spherical aberration (Zernike term Z04) for a 5.70 mm pupil in a large physiological preoperative patient population.

an RGP lens, such that it repeatedly returns to the same horizontal and vertical location relative to the visual axis without rotating around this axis and maintaining physiologically desirable tear exchange behind the lens, is very difficult.

The physical nature of RGP lenses, and the way they must be fit to ensure tear exchange and mobility on the eye, renders them less desirable as a method for correcting wavefront aberrations of the eye through complex optical surfaces.

Soft lenses are held in position relative to the cornea by an entirely different set of forces. These lenses are always fit with a sagittal depth greater than the cornea/sclera beneath the lens. In this way, the lens is squeezed onto the cornea with the first blink and deformed to take the shape of the tissue beneath. The deformation that the lens undergoes during this process generates radial stress in the lens, and it is these forces combined with gravity that center the lens on the cornea at the position of equilibrium. When the lid blinks, the soft lens is moved away from this position of equilibrium in a vertical direction through interaction with the eyelid. As the lens is moved farther away from the center of the cornea, the radial stress within the lens increases to the point where it is greater in magnitude than the eyelid interaction. This causes the lens to reverse direction and return to its position of equilibrium on the corneal surface. Because there is only one optimal position on the corneal/scleral surface that provides the least radial stress, clinicians find that well-fitted soft lenses relocate to the same position on the cornea within 0.10 to 0.40 mm after every blink.9 Although the lens is not centered on the visual axis of the eye, it does relocate consistently relative to that axis after every blink. Control of rotation with soft lenses has been realized for over a decade, and the current generation of sophisticated prism-ballasted designs provide rotational stability within 5 degrees between any series of blinks (Figure 33-4). It is this ability to relocate with great precision that makes soft lenses more desirable than RGP lenses for correcting higher-order wavefront aberrations.

Figure 33-4. Soft toric lens rotational stability values (change from original position) based on biomicroscopic reticule measurements on 20 eyes. Lenses were misrotated 45 degrees from their position of equilibrium, and their position was remeasured 1 minute later.

Soft lenses that fit well relocate to the same position on the cornea within 0.10 to 0.40 mm after every blink.9

The current generation of sophisticated prism-ballasted designs provide rotational stability within 5 degrees between any series of blinks.

In theory, even slight changes in centration and rotation of a lens designed to correct up to fifth-order Zernike terms will significantly reduce the visual benefits experienced by that correction. Calculations to understand the tolerance:benefit ratio to decentration and rotational alignment of correcting surfaces typical of those found in the general ophthalmic population have been performed by Guirao et al.10 They have found that Zernike terms with higher (rotational) angular orders were more sensitive to rotation of the correcting surface from the ideal position. Hence, coma is most tolerant to misrotation of the correcting surface, offering a visual benefit with misrotation up to 60 degrees; astigmatism is the next most tolerant aberration, with visual benefit with rotation up to 30 degrees; trefoil is the next most tolerant aberration, with visual benefit with rotation up to 20 degrees; and so on. Rotation from the ideal correction position that is greater than these values would provide a retinal image quality that is poorer than leaving the Zernike term uncorrected.

Decentration of the ideal correcting lens from the ideal correcting axis results in the higher-order aberrations generating a larger magnitude of lower-order aberrations. Hence, a correcting lens with coma will generate astigmatism and defocus, spherical aberration will produce coma and tilt, while defocus or astigmatism will produce only tilt (prism). In general, higher-order Zernike terms are more intolerant to decentration than lower terms. Figures 33-5A and 33-5B show the reduction in visual benefit from lenses designed to correct wavefront aberrations of a 10 eye sample population with increasing lens misrotation and decentration, respectively. Clearly the visual benefit of a lens designed to correct both lowerand higher-order wavefront aberrations is dependent on repeatable lens centration and rota-

Feasibility of Wavefront Customized Contact Lenses 281

tion following each blink or eye movement. The values generated by Guirao’s analysis suggest that a soft lens is capable of remaining within these limits.10

It is apparent that a contact lens designed to correct the high- er-order wavefront aberrations of the eye needs to be a soft lens, prism ballasted to control rotation, with the higher order correction on the anterior surface of the lens. Because it would be extremely difficult to predict the manner in which the soft lens would distort as it is squeezed onto the cornea by the lid, the most pragmatic method would be to measure the wavefront aberrations with the contact lens in situ. Hence, a trial lens—with all the physical properties of the final custom-correcting lens but without the custom-correcting optical surface—would be placed on the eye and allowed to settle, at which time wavefront sensor measurements would be taken through the lens-eye combination. In this way, the final custom-correcting lens will compensate for any variations in the eye’s higher-order wavefront aberrations introduced by the lens itself or by the tear film between the lens and the cornea.

It is apparent that a contact lens designed to correct the higher-order wavefront aberrations of the eye needs to be a soft lens, prism ballasted to control rotation, with the high- er-order correction on the anterior surface of the lens.

Alignment of the wavefront measurement is critical, and it is well known that soft lenses typically find their position of equilibrium centered on the corneal apex, which is most typically temporal and superior to the line of sight. Therefore, the line of sight as defined by the center of the pupil will be decentered relative to the geometric center of the lens. As a result, the correcting wavefront applied to the contact lens needs to compensate for the difference between these two axes. To achieve this, a marking scheme needs to be implemented on the trial lenses used for these measurements, such as that shown in Figure 33-6. These markings provide an indication of the center of the lens, as well as the rotational orientation of the lens during wavefront aberration measurements.

CUSTOMIZED CONTACT LENSES

IN A DISPOSABLE WORLD

Having established the desired measurement procedure, another question that arises is how to provide a wavefront aberration correcting soft contact lens in today’s paradigm of affordable frequent replacement and disposable lenses. These lenses need to be customized to an individual eye, yet the physiological needs of patients and demands of clinicians make it necessary to provide up to a year’s supply of weekly to monthly replacement lenses at any one time. Figure 33-7 demonstrates one proposed model for ordering and delivery of a wavefront aberration correcting soft lens customized to an individual eye. First, the patient’s eye is measured with the trial lens in place and the Zernike coefficients are uploaded to a central remote server along with other necessary data through a modem in the wavefront sensor computer. The lathing parameters necessary to cut the nonrotationally symmetrical front surface using a three-axis computer numeric controlled (CNC) lathe are calculated by the central server and downloaded into the lathe, generating the prescribed number of lenses—from a single trial lens to a 1-year supply. These lenses are then processed, packaged, sterilized,

Figure 33-6. One possible marking scheme that could be used on a soft trial lens to locate the center of the lens and the rotational orientation of the lens during measurements of the wavefront aberration of the lens-eye combination.

labeled, and returned to the prescribing clinician’s office within a few days. If necessary, the process can be repeated with the new lens in situ to refine the wavefront aberration correction.

Do the currently available contact lens lathes have the capability of creating the surface profiles necessary to correct higherorder aberrations found in both the physiologic and pathologic ophthalmic population? Figure 33-8 shows a test surface created using a currently available CNC contact lens lathe for pentafoil representing a correction for fifth-order Zernike terms. This interferogram demonstrates that the CNC lathes have sufficient rotational resolution to generate the required surfaces. While this shows its capability of creating normal surfaces, there might be concern that the complex surfaces generated by pathologies, such as keratoconus, may be beyond the reach of these lathes.

Figure 33-7. Customized contact lens order and delivery model. Wavefront aberrations are measured in the clinician’s office (A), and uploaded to a remote server (B), where lathing parameters are calculated automatically and downloaded into the lathe (C). The lens is processed, packaged, sterilized, labeled, and sent back to the clinician for dispensing to the patient (D).

Figure 33-9 shows the outcome of creating a plexiglass cylinder with a single front surface that mimics the wavefront aberration of a patient with keratoconus. Polymethylmethacrylate (PMMA) lenses with second-order Zernike correction only—and with secondthrough fifth-order Zernike correction—were manufactured and placed on the model eye, with the lenses centered and aligned empirically. Clearly, the majority of the higher-order wavefront aberrations were corrected by the surfaces generated by the CNC lathe manufacturing process.

A

B

C

Figure 33-9. Measured wavefront aberrations (A), point spread function (PSF) (B), and image convolution (C) of a keratoconic plexiglass model eye generated by a three-axis CNC lathe. The second column represents the model eye corrected by a plexiglass contact lens generated by the same lathing technology. The third column represents the model eye corrected by a plexiglass contact lens with a front surface designed to correct up through fifth-order Zernike terms. All measurements calculated for a 5.70 mm pupil.

Experiments conducted at our research lab using this prescribing and manufacturing model have demonstrated the feasibility of a custom soft contact lens designed to correct wavefront aberrations up to and including the fifth-order Zernike terms. Measurements are made using a wavefront sensor in upstate New York, and the information is uploaded to a server in Florida where the lenses are lathed and processed. Figure 33-10 shows the results of correcting a single eye with soft, custom-correcting lenses with a variety of measured Zernike terms: second-order only (defocus and astigmatism); second-order and spherical aberration; second and third order; second-order, third-order and spherical aberration; and secondthrough fifth-order. Higherorder RMS, PSF, Strehl ratios, image convolutions, and highand

Figure 33-8. Interferogram demonstrating rotational resolution of CNC lathing technology.

low-contrast logMAR visual acuities are presented. As anticipated, the results show that as the higher-order aberrations are corrected sequentially, the wavefront RMS is reduced, the Strehl ratio increases, and the low-contrast visual acuity improves up to one line with a dilated pupil. Perhaps, not surprisingly, the highcontrast visual acuity does not improve dramatically because the retinal image quality enhancement primarily affects contrast rather than resolution. These results are exciting and encouraging, although expanded studies have shown that lens alignment and rotational stability are critical to ensure the visual benefit reported in this case study.

As anticipated, the results show that as the higher-order aberrations are corrected sequentially, the wavefront RMS is reduced, the Strehl ratio increases, and the low-contrast visual acuity improves up to one line with a dilated pupil.

The concept of a customized soft contact lens designed to correct the wavefront aberration of the eye up through the fifth Zernike terms is feasible. Manufacturing technologies are available to create the complex surfaces necessary to achieve this, and a business model that provides the means of communicating the necessary individual parameters to the lab and delivering lenses within an acceptable timeframe has been demonstrated. The ability to improve wavefront aberrations, both lowand high-order Zernike terms, in a typical presurgical eye has been demonstrated, and the commensurate improvements in large pupil vision are detectable. What remains to be confirmed is the visual benefit obtained with these lenses across a wide range of physiologic and pathologic eyes, and whether this type of correction will be desirable for improved night vision to the general contact lens wearing population or confined in its appeal to that segment of the population that has significantly greater higher-order wavefront aberrations resulting from pathological or postsurgical conditions.

DEFINITIONS

CNC lathe manufacturing: A computer numeric controlled lathe that is used in the optical industry, often for manufacturing soft contact lenses.

customized soft contact lens: Soft contact lenses that correct the higher-order wavefront aberrations of an individual eye.

284 Chapter 33

Figure 33-10. Higher-order RMS, PSFs, Strehl ratios, image convolutions, and logMAR visual acuity of an eye with a customized contact lens.

keratoconus: Noninflammatory, bilateral ectasia of the axial portion of the cornea. It is characterized by progressive thinning and steepening of the central cornea and often results in visual impairment.

nonrotationally symmetrical aberrations: Wavefront aberrations that are not symmetric around the optical axis of the optical system (eg, coma).

prism-ballasting: A design feature incorporating additional material mass into the inferior portion of a soft contact lens in order to stabilize its rotation and mislocation.

rigid gas permeable (RGP) contact lens: Rigid gas permeable contact lenses, often known as oxygen permeable lenses, are made of durable silicone or fluorosilicone-acrylate materials and provide a high level of oxygen transmissibility to the cornea.

Strehl ratio: A metric representing the quality of the PSF at the image plane of an optical system.

REFERENCES

1.Ivanoff A. Letter to the editor: about the spherical aberration of the eye. J Opt Soc Am A. 1953;46(10):901-903.

2.Jenkins TCA. Aberrations of the eye and their effect upon vision. Part II. British Journal of Physiological Optics. 1963;20:161-201.

3.Koomen M, Tousey R, Scolnik R. The spherical aberration of the eye. J Opt Soc Am A. 1949;39(5):370-376.

4.Howland HC, Howland B. A subjective method for the measurement of the monochromatic aberrations of the eye. J Opt Soc Am A. 1977;67(11):1508-1518.

5.Porter J, Guirao A, Cox IG, Williams DR. Monochromatic aberrations of the human eye in a large population. J Opt Soc Am A. 2001;18(8):1793-1803.

6.Westheimer, G. Aberrations of contact lenses. American Journal of Optometry and Archives of the American Academy of Optometry. 1961;38:445-448.

7.Oxenberg LD, Carney LG. Visual performance with aspheric rigid contact lenses. Optom Vis Sci. 1989;66(12):818-821.

8.Knoll HA, Conway HD. Analysis of blink-induced vertical motion of contact lenses. Am J Optom Physiol Opt. 1987;64(2):153-155.

9.Young, G. Soft lens fitting reassessed. Contact Lens Spectrum. 1992;56-61.

10.Guirao A, Williams DR, Cox IG. Effect of rotation and translation on the expected benefit of an ideal method to correct the eye’s higher order aberrations. J Opt Soc Am A. 2001;18(5), 1003-1015.

INTRODUCTION

When a patient undergoes cataract surgery, the cataractous lens is removed and replaced with an intraocular lens (IOL). During the last decade, extensive progress in the reliability and effectiveness of cataract surgery has been due not only to the introduction of new surgical techniques and new instrumentation but also to the development of next-generation IOL designs. The optical quality of isolated IOLs has received considerable attention.1-3 However, the optical quality of the pseudophakic eye, as a whole, has only recently been examined. For example, recent studies of the contrast sensitivity of pseudophakic eyes have shown that patients implanted with spherical IOLs have contrast vision that is comparable with that of a healthy control population of the same age.4 Similarly, Guirao et al measured the modulation transfer function (MTF) of 20 IOL patients and 20 patients with healthy control eyes of the same age and found that the MTF measured in the IOL population was similar to that measured in the normal population.5 Similarities in outcomes are observed despite the fact that the human crystalline lens is optically inferior to an IOL. One explanation for these observations lies in the optical aberrations of the human cornea and spherical IOLs.

Recent advances in wavefront technology have enabled measurements that quantitatively describe the optical performance and aberrations of both the cornea6,7 and the entire refractive system of the eye.8-10 Studies have shown that the young human eye is a system in which aberrations introduced by the cornea are at least partially compensated for by the lens.11-13 Major changes that do occur in the lens with age, such as hardening of the nucleus,14 changes in the internal refractive index gradient,15 and changes in lens shape,16 produce changes in lens aberration. As a result of these structural changes in the lens, aberration compensation is gradually lost with age, leading to an increase in total ocular aberrations10 and a corresponding loss in optical quality.17 This reduction in ocular optical quality is at least partially responsible for a measured reduction in visual quality with age.18

Wavefront sensing, the technology that was used to study the aging eye, can also be applied to the on-axis optics of the pseudophakic eye. Positive spherical aberration exhibited by the cornea6,7 occurs when rays entering the periphery of the pupil are focused in front of rays entering near the center of the pupil.

Because most monofocal IOLs have one or two spherical surfaces, they also contribute positive spherical aberration,19 and thus increase the total positive ocular spherical aberration of the average cataract patient. In a number of studies, spherical aberration data were collected in pseudophakic eyes with foldable lenses using a Shack-Hartmann wavefront sensor. Figure 34-1 shows the average and standard deviation of the Zernike coefficient Z(4,0) of the wavefront aberration (representative of fourthorder spherical aberration) measured for a 4.00 millimeter (mm) pupil in 27 eyes implanted with the AcrySof MA60BM (Alcon Laboratories, Fort Worth, Tex), 48 eyes implanted with the PhakoFlex II SI40NB (Advanced Medical Optics, Santa Ana, Calif), and 82 eyes implanted with the CeeOn Edge 911A (Pfizer, New York, NY). For these three spherical lenses of different materials and optical designs, very little difference was found in the measured pseudophakic ocular spherical aberration. The average spherical aberration present in the pseudophakic eye implanted with spherically surfaced monofocal IOLs is overwhelmingly positive.

The average spherical aberration present in the pseudophakic eye implanted with a conventional spherically surfaced monofocal IOL is overwhelmingly positive. The Tecnis lens (Pfizer, New York, NY) uses a negative spherical aberration design to compensate for this.

Based upon these observations, a logical solution to the problem of increased positive spherical aberration in the pseudophakic eye is an IOL that compensates for the positive spherical aberration introduced by the cornea. This can be done by modifying one or both surfaces of the IOL to produce a lens that introduces negative spherical aberration to the system. To this end, Pfizer has designed the Tecnis Z-9000 lens, in which the peripheral power of the IOL is reduced by modifying the front surface, producing an IOL with negative spherical aberration. The expected result is a pseudophakic eye with very little remaining spherical aberration.

As an example of the potential benefit of aberration-correcting IOLs, the sections that follow describe the design and clinical performance of the Tecnis Z-9000.