8.3.3 Special Lenses: Aspheric

8.3.3 Special Lenses: Aspheric

Mark Packer, I. Howard Fine,

and Richard S. Hoffman

Core Messages

ßAspheric intraocular lenses represent an innovative design for enhanced image formation in

the eye.

ßAging changes in the crystalline lens cause increasing spherical aberration throughout

life.

ßAspheric IOLs offer the cataract surgeons, the opportunity to improve patients’ functional

vision by minimizing spherical aberration.

In the current era of presbyopia-correcting intraocular lenses (IOLs), toric IOLs and aspheric IOL technology, the practice milieu is changing. Informed consent takes on a new meaning when the surgeon and the patient decide together which IOL technology represents the best fit for a particular lifestyle and its visual demands. Customizing IOL choice is no longer optional; it is essential to the practice of refractive lens surgery.

Because the positive spherical aberration of a spherical pseudophakic IOL tends to increase the total optical aberrations, attention has turned to the development of aspheric IOLs [1]. These designs are intended to reduce or eliminate the spherical aberration of the eye and improve functional vision as compared with that of a spherical pseudophakic implant. Three aspheric IOL designs are currently marketed in the United States: the Tecnis Z9000/2/3 IOLs (AMO, Santa Ana, CA), the AcrySof IQ IOL (Alcon, Ft. Worth, TX) and the SofPort AO IOL (Bausch & Lomb, San Dimas, CA). Other aspheric IOL designs, which are not available in the United States as yet, also promise to reduce spherical aberration [2].

M. Packer ( )

Oregon Health & Science University, Drs. Fine, Hoffman and Packer, 1550 Oak Street, Suite 5, Eugene, OR 97401, USA e-mail: mpacker@finemd.com

The Tecnis IOL was designed with a modified prolate anterior surface to compensate for the average corneal spherical aberration found in the adult eye. It introduces −0.27 mm of spherical aberration to the eye measured at the 6 mm optical zone. The clinical investigation of the Tecnis IOL submitted to the US Food and Drug Administration (FDA) demonstrated the elimination of mean spherical aberration as well as significant improvement in functional vision, when compared to a standard spherical IOL [3].

The AcrySof IQ shares the ultraviolet (UV) and blue light-filtering chromophores found in the singlepiece acrylic AcrySof Natural IOL. The special feature of the IQ IOL is the posterior aspheric surface, which is designed to reduce spherical aberration by addressing the effects of over-refraction at the periphery. It adds −0.20 mm of spherical aberration to the eye at the 6 mm optical zone. The SofPort Advanced Optics (LI61AO) IOL is an aspheric IOL that has been specifically designed with zero spherical aberration so that it will not contribute to any preexisting higherorder aberrations.

Multiple peer-reviewed, prospective, randomized scientific publications have demonstrated the reduction or elimination of spherical aberrations with the Tecnis modified prolate IOL, when compared to a variety of spherical IOLs [4–13]. Data show that the mean spherical aberration in the eyes implanted with the Tecnis IOL is, in the words approved by the FDA, “not different from zero.” Studies have also documented superior functional vision with the Tecnis IOL. Subjects in the FDA monitored randomized double-masked night driving simulation study of the Tecnis IOL, performed functionally better in 20 out of 24 driving conditions (and statistically better in ten conditions), when using best-spectacle correction with the eye implanted with the Tecnis IOL, as compared to best-spectacle correction with the eye implanted with the AcrySof spherical IOL [2]. Data from the night-driving simulation showed a significant correlation between the reduction of spherical aberration and the detection distance for the pedestrian target under rural conditions with glare (the most difficult target to discern).

More recently, peer-reviewed published clinical studies have also supported the reduction of spherical aberration and superior functional vision with the AcrySof IQ when compared with spherical IOLs [14– 17]. In fact, the optical advantages of aspheric IOL technology have become fairly well accepted although

some controversy remains in the areas of functional benefit as it relates to pupil size, IOL decentration, depth of focus and customization [18]. Some studies have shown little or no benefit of aspheric IOLs with smaller pupils [12, 13], while one laboratory study showed that the SofPort AO provides better optical quality than either a negatively aspheric or a spherical IOL, under conditions of significant decentration [19]. One study has shown diminished distance-corrected near visual acuity, a surrogate measure for the depth of focus, with the AcrySof IQ aspheric IOL as compared to that of the AcrySof SN60AT spherical IOL [17].

Regarding customization of the aspheric correction, it has been suggested that achieving zero total spherical aberration postoperatively provides the best quality of vision. Piers and coauthors utilized an adaptive optics simulator to assess letter acuity and contrast sensitivity for two different values of spherical aberration. The first condition was the average amount of spherical aberration measured in pseudophakic patients with spherical IOLs. The second condition represented the complete correction of the individual’s spherical aberration (Z [4,0] = 0). The researchers found an average improvement in visual acuity associated with the correction of spherical aberration of 10 and 38% measured in white and green light, respectively. Similarly, average contrast sensitivity measurements improved to 32 and 57% in white and green light, respectively. When spherical aberration was corrected, visual performance was as good as or better than the normal spherical aberration case for defocus as large as ±1 D. Therefore, these researchers concluded that completely correcting ocular spherical aberration improves spatial vision in the best-focus position, without compromising the subjective tolerance to defocus [20].

On the other hand, it has alternatively been suggested that providing Z [4,0] = +0.1 micron of postoperative spherical aberration represents a better choice [21]. This line of reasoning originated from a study demonstrating that 35 young subjects with uncorrected visual acuity of 20/15 or better had a mean total spherical aberration of Z [4,0] = +0.110 ± 0.077 mm [22]. However, there is no logical basis to infer that the spherical aberration is responsible for the supernormal visual acuity. In fact, the authors of this study concluded that “The amount of ocular HOAs in eyes with natural supernormal vision is not negligible, and is comparable to the reported amount of HOAs in myopic eyes” [22]. This conclusion is brought out by a study

performed by Wang and Koch demonstrating a mean total spherical aberration of Z [4,0] = +0.128 ± 0.074 mm in a series of 532 eyes of 306 subjects, presenting for refractive surgery [23]. Nevertheless, Beiko [21] used the Easygraph corneal topographer (Oculus, Lynnwood, WA) to select patients with corneal spherical aberration of +0.37 mm, thus targeting a postoperative total ocular spherical aberration of +0.10 mm following implantation of the Tecnis IOL with −0.27 mm (the Easygraph includes an optional software package that provides Zernike analysis). The selected patient group demonstrated a significantly better contrast sensitivity than an unselected group of control patients, under both mesopic and photopic conditions.

Recently, Beiko, Haigis and Steinmueller presented data from a series of 696 eyes confirming the mean corneal spherical aberration of +0.27 mm used in the design of the Tecnis IOL [24]. They found a wide standard deviation of 0.089 mm, with a range from +0.041 to +0.632 mm, and significantly different corneal spherical aberration means in men and women. In some cases the corneal spherical aberration differed significantly between fellow eyes. The authors concluded that “individuals’ eyes should be measured to determine their unique value when considering correction of this aberration” [24]. In addition, they noted that keratometry and the corneal Q value do not correlate well with spherical aberration, and therefore, corneal spherical aberration must be measured directly with a topographer.

One method of proceeding with customized selection of aspheric IOLs involves the following protocol:

1.Preoperative testing to include corneal topography as well as axial length determination, anterior chamber depth, phakic lens thickness and corneal white-to-white diameter

2.Application of a software package such as VOL-CT (Sarver and Associates, Carbondale, IL) or the use of corneal topographer such as the i-Trace (Tracey Technologies, Houston, TX) to transform the topography elevation data into preoperative corneal Zernike coefficients, with special attention to Z [4,0], fourth order spherical aberration at the 6 mm optical zone

3.Application of an IOL calculation formula, such as the Holladay 2 (available as part of the Holladay IOL Consultant & Surgical Outcomes Assessment Program,JackT.Holladay,Houston,TX)to determine

the correct IOL power for desired postoperative spherical equivalent

4.Determination of desired postoperative total ocular spherical aberration and selection of the IOL type

For example, if the desired postoperative total ocular spherical aberration is zero, and the preoperative corneal spherical aberration measures about + 0.27 mm, the Tecnis with −0.27 mm would be selected. In general, the aspheric IOL that comes closest to providing the desired correction should be selected.

Initial results for customizing the selection of aspheric IOLs have shown promise.

A prospective study was undertaken to determine the feasibility of selectively targeting zero total postoperative spherical aberration, by selecting the best fit aspheric IOL, based on preoperative topographically derived corneal spherical aberration (Fig. 8.45). Candidates for unilateral or bilateral cataract surgery who did not desire presbyopia-correcting IOLs were offered customized selection of aspheric lenses. Subjects with a history of previous keratorefractive surgery, ocular pathology, judged to limit potential visual acuity to 20/30 or less, or sufficient corneal astigmatism to indicate peripheral corneal relaxing incisions, were excluded. Informed consent for cataract surgery was obtained from all subjects. Preoperative evaluation included a complete ophthalmologic examination, including manifest refraction, best-corrected visual acuity and brightness acuity testing, if indicated.

Fig. 8.45 Aspheric IOL selection chart based on the frequency distribution of corneal spherical aberration in the population (for background information, see [1, 24]

In some cases, a visual function questionnaire was administered to help ascertain whether cataract surgery was indicated. Axial length measurement, keratometry, anterior chamber depth and corneal white-to-white measurements were obtained with the IOL Master (Carl Zeiss Meditec, Inc., Dublin, CA). The preoperative corneal topographic spherical aberration (Z [4,0]) was measured at the 6 mm optical zone (iTrace, Tracey Technologies) (Fig. 8.46).

IOL power calculation was performed with the Holladay IOL Consultant using the Holladay II formula (Jack Holladay, Bellaire, TX). An aspheric IOL was selected for implantation in each eye based on the preoperative corneal spherical aberration and the labeled IOL spherical aberration, such that the arithmetic sum of these two values was closest to zero. Thus, for corneal spherical aberration less than + 0.1 mm, the LI61A0 was selected; for corneal spherical aberration greater than + 0.1 mm but less than +0.235 mm, the SN60WF was selected; and, for corneal spherical aberration greater than +0.235 mm, the Tecnis Z9000 or Tecnis Z9002 was selected.

At 4–6 weeks postoperatively, the subjects returned for evaluation, including manifest refraction, best-cor- rected visual acuity, pupillometry and dilated wavefront aberrometry (WASCA, Wavefront Sciences, Inc.). The measured total ocular spherical aberration Z [4,0] for a maximal 6 mm pupil size was compared with the predicted value given mathematically by the sum of the preoperative corneal spherical aberration and the labeled IOL spherical aberration.

Data from 30 eyes of 18 consecutive subjects (9 men and 9 women) were available for analysis. The mean age of the subjects was 72.8 ± 6.2 years (range: 62–86 years). The mean preoperative corneal spherical aberration, measured at the 6 mm zone for the entire population was +0.26 ± 0.089 mm. One eye had Z [4,0] less than + 0.1 mm and therefore was selected to receive the LI61AO IOL. The corneal spherical aberration of this eye measured +0.065 mm. Eleven eyes had Z [4,0] greater than +0.1 mm, but less than +0.235 mm, and therefore were selected to receive the SN60WF. The mean corneal spherical aberration of this group measured +0.18 ± 0.037 mm. Eighteen eyes had corneal spherical aberration greater than +0.235 mm, and therefore received the Tecnis IOL. The mean corneal spherical aberration of this group measured +0.31 ± 0.063 mm. The frequency distribution of the preoperative corneal spherical aberration is shown in Fig. 8.47.

Fig. 8.47 Frequency distribution of corneal topographic wavefront spherical aberration Z [4,0] in the study population (mean +0.26 ± 0.089 mm)

All patients underwent uneventful phacoemulsification and IOL implantation by one surgeon (MP) utilizing a biaxial micro incision technique [25]. The capsulorhexis diameter was intentionally sized at

approximately 4.5–5.0 mm in all cases in order to facilitate the “shrink wrap effect” of the IOL for a long-term prevention of posterior capsular opacification and maintenance of stable centration (Fig. 8.48) [26]. Of the 12 subjects implanted bilaterally, 10 received the same IOL in both the eyes.

The postoperative best-corrected visual acuity measured 20/20 or better in 27 of 30 eyes; it measured 20/25 in 2 eyes and 20/30 in 1 eye. The mean spherical equivalent measured −0.32 ± 0.54 D; 93.3% of the eyes measured within ±0.50 D of the targeted postoperative refraction. The mean postoperative cylinder was 0.375; no eye had greater than 0.75 D of refractive astigmatism. The mean postoperative mesopic pupil size measured 3.58 ± 0.78 mm. There was no significant tilt or decentration greater than 0.25 mm of any IOL, as measured by Guyton’s method [27, 28].

The total postoperative ocular spherical aberration for the entire population measured −0.013 ± 0.072 mm. The total ocular spherical aberration for the eye

implanted with the LI61AO IOL measured +0.025 mm. This single eye was excluded from further comparative statistical analysis, but included in the descriptive

Fig. 8.48 The anterior capsule margin overlies the edge of the Tecnis Z9000 IOL, effectively reducing the aperture (despite wide pupil dilation) to 4.5 mm

statistics for the entire population. For the eyes implanted with the SN60WF, the mean total postoperative ocular spherical aberration measured + 0.010 ± 0.053 mm. For the eyes implanted with the Tecnis, the mean total postoperative ocular spherical aberration measured −0.015

± 0.052 mm. There was no statistically significant difference between the postoperative ocular spherical aberration of the SN60WF group vs. the Tecnis group (two sample t-test assuming equal variances, p = 0.22).

Examination of the difference between the predicted and measured postoperative spherical aberration for the entire group shows that the mean absolute error measured 0.058 ± 0.056 mm. For the eyes implanted with the SN60WF, the mean absolute error measured 0.052 ± 0.040 mm. For the eyes implanted with the Tecnis, the mean absolute error measured 0.063 ± 0.066 mm. A two sample t-test assuming equal variances did not reveal any statistically significant difference between the mean absolute errors for the SN60WF and Tecnis eyes (p = 0.63). Figure 8.49 shows a scatter plot of predicted vs. measured Z [4,0] for all the eyes. As expected from the values and standard deviations

Fig. 8.49 Predicted vs. measured post operative wavefront spherical aberration Z [4,0] for the entire study population. 86.7% of the paired data points lie within ±0.1 mm of zero, and 53.3% lie within ±0.05 mm of zero

Measured WF Z [4,0]

Predicted v Measured Post Op Wavefront Z [4,0]

0.3

0.25

0.2

0.15

0.1

0.05

0

-0.05

-0.1

-0.15

-0.2

-0.25

-0.3

Predicted WF Z [4,0]

Pre Op Corneal Z [4,0] VPost Op Total Z [4,0]

Pre OP Cornea Z [4,0]

Fig. 8.50 The plot of preoperative corneal spherical aberration (x-axis) and postoperative total wavefront spherical aberration (y-axis) shows the distribution of IOLs among the subjects

for mean absolute error, almost all the points lie within 0.10 mm of zero, and approximately half the points lie within 0.050 mm of zero. Figure 8.50 demonstrates the relationship between the preoperative corneal spherical aberration and the postoperative total spherical aberration, with reference to the selected aspheric IOLs.

The value of customizing any refractive or wavefront parameter can only reside in enhanced outcomes. On an average, for all the eyes, the customization procedure outlined here achieved a mean total ocular spherical aberration of −0.013 ± 0.072 mm; 93.3% of the eyes achieved total ocular spherical aberration of less than ±0.10 mm.

Other investigators have examined postoperative spherical aberration without selection of subjects, based on preoperative corneal measurements. For example, Padmanabhan et al found that the mean spherical aberration was statistically, significantly lower in eyes with a Tecnis Z9000 IOL (Z [4,0] = +0.07 ± 0.12 mm) compared with the eyes with an Acrysof MA60BM IOL (Z [4,0] = +0.29 ± 0.20 mm, p < 0.001) and, with eyes with a Sensar Opti Edge AR40e IOL (Z [4,0] = +0.20 ± 0.09 mm, p = 0.002) [29]. Denoyer et al found that spherical aberration was lower in patients with the Tecnis IOL (mean Z [4,0] = +0.03

± 0.06 mm) [30]. Kasper et al reported a median Z [4,0] = +0.017 mm, for eyes with the Tecnis IOL [13]. Bellucci et al found that, for a 4 mm optical zone, the ocular spherical aberration was +0.0054 ± 0.0172 mm root-mean-square (RMS) in eyes implanted with the Tecnis lens, and was +0.0562 to +0.0974 mm RMS in eyes implanted with four other conventional IOLs [31]. Awwad et al reported that SN60WF eyes had less mean

absolute spherical aberration than SN60AT eyes, both at 4 mm (+0.04 ± 0.03 vs. +0.11 ± 0.03 RMS, p < 0.0001) and 6 mm pupils (+0.09 ± 0.04 vs. +0.43 ± 0.12 RMS, p < 0.0001) [14]. Rocha et al found a mean spherical aberration of +0.03 ± 0.05 mm in their AcrySof IQ groups in two separate publications [16, 17]. Caporossi et al found that the mean total spherical aberration with aspheric IOLs measured 0.05 ± 0.06, 0.11 ± 0.1, and 0.19 ± 0.08 mm for the Tecnis Z9000, Acrysof IQ SN60WF, and Sofport L161AO, respectively, for a 5 mm pupil diameter [32]. The results presented here, in general, compare favorably with these outcomes, although direct comparisons are difficult, given various testing conditions and equipment and variations in surgical technique.

As a feasibility study, the accuracy of our customization procedure is reflected in the difference between predicted and measured spherical aberration. The mean absolute error measured 0.058 ± 0.056 mm for all the eyes. Several factors such as, effective postoperative pupil size, tilt or decentration of the IOL and surgically induced spherical aberration may influence these results, and should be considered in our evaluation.

The predicted postoperative spherical aberration is based on simple addition of the 6mm preoperative corneal Z [4,0] value and the labeled 6mm Z [4,0] for each IOL. However, the aperture size of the postoperative wavefront measurement is limited by the reduction of pupillary diameter following dilation and capsulorhexis diameter. These vary from case to case and will produce variations in the predictive accuracy of the customization process. Additionally, an important limitation of the functional benefit of the reduction of spherical aberration depends upon the correction of defocus and astigmatism, i.e., lower order aberrations. Furthermore, while a perfectly aspherical optical system may correct for the spherical aberration of incoming parallel light rays, a near target will remain blurred, because of defocus.

Regarding the performance limitations of aspherical IOLs, due to tilt and decentration, Piers et al recently demonstrated that customized correction of ocular wavefront aberrations with an IOL is relatively insensitive to as much as 0.8 mm decentration, 10° of tilt and 15° of rotation [33]. Fortunately, with continuous curvilinear capsulorhexis and in-the-bag IOL fixation, very few IOL would be expected to fall outside these criteria [34].

Some effect of the surgery should be expected to occur on the corneal spherical aberration and therefore,