Article

Knowing when to use intraoperative aberrometry

IA can offer greater precision even in challenging refractive cases.

The popularity and adoption of intraoperative aberrometry (IA) has increased since the introduction of Optiwave Refractive Analysis (ORA, Alcon) in 2012. Previously known as ORange and originally introduced by WaveTec Vision, the device utilizes Talbot moiré interferometry to provide real-time wavefront refraction sphere, cylinder and axis measurements during cataract surgery. With improvement in corneal measurement devices and technologies as well as the new formulas for IOL calculations now available, there have been some questions as to the role of IA in our cataract surgical armamentarium today. However, IA provides additional information that may allow for more precise refractive outcomes, especially in challenging patients. Here, I review those situations in which IA is particularly beneficial.

REFRACTIVE TARGETING

The benefit of aberrometry in post-refractive cases has been clearly demonstrated in literature, with Ianchulev et al finding that it provided significantly better refractive outcomes in patients undergoing cataract surgery who had previously undergone myopic laser refractive surgery.1 However, newer formulas, smaller incisions and femtosecond or other automated forms of capsulotomy creation have continued to improve our ability to provide more consistent refractive targeting, especially in straightforward eyes without prior refractive surgery. In normal patients, does the addition of IA add higher potential for refractive targeting?

In the largest series of cataract surgery patients undergoing surgery to date, Cionni et al evaluated the outcomes of IA in 32,189 eyes undergoing uncomplicated surgery. When the preoperative selected IOL differed from the implanted lens recommended by IA, significantly more eyes had an absolute prediction error of 0.5 D or less (81.3% vs 68.8%, P < .0001).2 This could result in a substantially reduced potential enhancement rate — or at least in higher patient satisfaction, especially in patients with high expectations such as those electing for refractive cataract surgery.

ASTIGMATIC CORRECTION

IA may also be a useful adjunct in incisional astigmatic correction. The use of limbal relaxing incisions during cataract surgery, under guidance of IA, was demonstrated in 2010 by Packer to reduce postoperative enhancement rates.3 Surgeons now more frequently utilize the femtosecond laser to perform corneal relaxing incisions (CRI), especially in low cylinder cases. In some cases, adequate astigmatic effect may be obtained without opening femtosecond CRIs. IA may be utilized to guide opening of these femtosecond CRI.

There is variability in incisional astigmatic surgery based on age, incision placement and tissue response. IA allows surgeons the option to measure the impact of the CRI while in surgery and augment the effect, if needed, by manually extending a femtosecond CRI with a diamond blade under IA guidance. I have found this to be helpful in many cases, in the same way that a surgeon would potentially increase a toric lens power based on IA measurements in a high cylinder patient.

For example, Figure 1 (page 38) shows topography and wavefront aberrometry measurement of the right eye of a 65-year-old white male patient undergoing cataract surgery, with distance targeting and femtosecond CRI.

Figure 1. Topography and wavefront aberrometry measurement of the right eye of a 65-year-old white male patient undergoing cataract surgery, with distance targeting and femtosecond CRI.

Preoperative refraction showed +0.50 + 1.00 x 180, yielding 20/25- acuity with disabling glare complaint. Topography demonstrated +0.76 D of ATR astigmatism. Femto CRI was planned at 85% depth with staggered incisions, 30° temporal (the primary incision was placed through this incision) and 40° nasal. After the aphakic measurement showed +1.62 D of ATR astigmatism, both CRI were opened under IA guidance. However, pseudophakic residual astigmatism of 1.13 D was measured by IA in the preoperative axis (Figure 2). The temporal CRI was then extended 15° with a diamond blade under IA guidance to augment the astigmatic effect. Figure 2 shows the initial pseudophakic measurement and final IA measurement demonstrating residual astigmatism of 0.36 D.

Figure 2. Initial pseudophakic measurement and final IA measurement demonstrating residual astigmatism of 0.36 D.

Figure 3 (page 40) shows postoperative topography and wavefront. Final visual acuity postop was 20/20 with plano refraction. This type of case demonstrates the power of utilizing IA to provide real-time enhancements for our refractive cataract surgery patients.

Figure 3. Postoperative topography and wavefront aberrometry.

PLANNING FOR PCA

Obtaining accurate lens power calculations is even more challenging when attempting to correct both sphere and cylinder with a toric IOL. Since the presence and importance of posterior corneal astigmatism (PCA) was identified and reported in 2012, we now know how important this factor is in surgical planning for astigmatism correction.4

Surgeons should either utilize formulas that theoretically account for PCA or incorporate direct or indirect measurements of PCA into our astigmatic nomograms. Most online toric IOL calculators now compensate for the theoretical population averages of PCA. Additionally, there are numerous Scheimpflug devices that have the capability of measuring both anterior and posterior corneal power. However, the reproducibility of this data may not yet achieve the level of consistency for surgeons to rely on this alone, due to the inherent difficulty in measuring the posterior cornea directly.5

The newer formulas, such as the Barrett Toric calculator, that employ theoretical population-based nomograms to account for PCA should also be utilized; however, individual patient measurements may not always adhere to the expected nomogram. Although the average amount of PCA in human corneas is 0.3 D, the range of PCA may vary from 0 to 1.1 D depending on the individual, and 9% of eyes have greater than 0.5 D of PCA.4 Therefore, in surgical planning for astigmatism management, there may still be a role for IA in providing measurement of aphakic and pseudophakic refraction during cataract surgery, accounting for all sources of astigmatism, including the posterior cornea. Surgeons who use IA have seen cases in which the IA measurements differ from preoperative planning, with patients who may have more with-the-rule astigmatism or less against-the-rule than expected, allowing them to adjust for outliers and avoid these potential refractive misses.

MANAGING SIA

IA allows surgeons the ability to evaluate immediate effects of our incisional surgical maneuvers in real time. Although most surgeons employ temporal clear cornea incisions as a routine, some surgeons employ on-axis incisions, and others choose oblique or superior incisions. With the advent of centroid SIA (surgically induced astigmatism), we have learned that SIA is typically lower than we think in most cases with modern small incisions. Therefore, use of a very low SIA has become mainstream for IOL calculations. However, studies show that there can be extreme case-to-case variability in SIA based on a more anterior or posterior location, as well as incision size.6,7 In a toric IOL case, especially a low cylinder case, a higher than expected SIA can alter the toric IOL requirement significantly, and in some cases obviate the need for a toric IOL altogether.

Our assumed low centroid SIA value should have minimal impact on the resultant vector of astigmatism for all but the lowest amounts of preoperative astigmatism. However, if the SIA in a given patient is higher than anticipated, the impact on the resultant vector of ideal toric lens placement may be significantly impacted as well, unless the incision is on-axis or perpendicular to the preoperative axis. This should be considered when making corneal incisions, and temporal clear cornea incisions made as limbal as possible, just inside the limbal vessels, will induce consistent results with very low SIA. Consistent preoperative marking, or surgical guidance systems, allow us to ensure the true location of our incisions on the 0/180° meridian, as cyclorotation induces another potential for errors in planning and vector calculations. The ability to have some idea of the relative astigmatic impact of our phaco incision, by measuring intraoperatively with IA, allows us to make adjustments during surgery as needed.

VARIABLES

One of the purported limitations of IA by some surgeons is the variability of measurements. A learning curve does exist, and surgeons must learn to control or minimize many variables to achieve consistent, reproducible measurements. Of course, there are limitations in obtaining preoperative biometry measurements as well, including ocular surface disease, patient cooperation, fixation and relative ptosis.

In surgery, adjusting the temporal clear corneal incision to be as limbal as possible not only minimizes its astigmatic impact but also minimizes its impact on IA measurements. In a prolonged case, para-incisional edema can creep anteriorly into the measurement zone of the aberrometer. Femtosecond laser incisions may have a tendency to be more anterior, with more impact on IA measurements, especially if placed obliquely or superiorly.7

There is an art to hydrating corneal incisions to allow for pressurization during pseudophakic measurements, which are performed with the chamber filled with balanced salt solution. Hydration should be minimal at this stage of the surgery; if necessary to stabilize the chamber for measurement, the lateral aspect of the incision should be minimally hydrated. Hydration of the anterior lip of the incision should be avoided until the end of the case to prevent increasing anterior para-incisional edema. Level head position in parallax with the aberrometer, consistent patient fixation, holding the speculum away from the globe, watching for pooling of fluid and ensuring even hydration of the cornea are all important factors to ensure good capture.

With newer generation IA platforms, live streaming data is imbedded in the microscope ocular, which allows for an estimation of axis and cylinder magnitude prior to taking a static measurement. This can facilitate toric lens rotation, as well.

Even with all of these considerations, in 5% to 10% of cases the data may not be consistent enough to make changes in preoperative plans. Surgeon judgement is important in deciding how much weight to place on IA measurements. When there are differences between preop and intraoperative measurements, it is important to consider the intraoperative variables, including corneal irregularity or dryness, patient fixation, and poor exposure to name a few.

IA is another powerful data point to consider, just as we consider the topography, refraction, post-refractive calculators and optical biometers and decide how much weight to place on each of these components based on the quality of the data and consistency between measurements. After using ORA for many cases and tracking cases in which there are differences between the recommended treatment and preop data, surgeons are able to become more comfortable with gauging how much weight to place on intraoperative data, just as they would with adding any other tool to their surgical planning algorithm. As with any new technique, it may also be useful to observe another surgeon with extensive experience in using aberrometry at some point in the learning process.

CONCLUSION

Our ability to anticipate and plan for our cataract surgery patients has increased with the advent of better imaging devices and IOL formulas. However, IA addresses a need to assess and adapt to changing conditions intraoperatively that preoperative diagnostics cannot anticipate. For refractive cataract surgeons, the ability to verify IOL power in challenging cases and make astigmatic adjustments in real time allows us to move patients from “20/happy” to “20/ecstatic,” driving more referrals and reducing potential downstream refractive enhancements. OM

REFERENCES

  1. Ianchulev T, Hoffer KJ, Yoo SH, et al. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014;121:56-60.
  2. Cionni R, Dimalanta R, Breen M, Hamilton C. A large retrospective database analysis comparing outcomes of intraoperative aberrometry with conventional preoperative planning. J Cataract Refract Surg. 2018;44:1230-1235.
  3. Packer M. Effect of intraoperative aberrometry on the rate of postoperative enhancement: retrospective study. J Cataract Refract Surg. 2010;36:747-755.
  4. Koch D, Shazia F, Weikert M, et al. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38: 2080-2087.
  5. Cao DM, Al-Mohtaseb Z, Wang L, Weikert MP, Koch DD. Chapter 185: Incisional Keratotomy. In: Cornea, 5th Ed, Mannis M, Holland E. Elsevier; 2020 (In Press).
  6. Wang, L, Xiao, X, Zhao, L, et al. Comparison of efficacy between coaxial microincision and standard-incision phacoemulsification in patients with age-related cataracts: a meta-analysis. BMC Ophthalmol. 2017;17:267.
  7. Zhu S, Qu N, Wang W, et al. Morphologic features and surgically induced astigmatism of femtosecond laser versus manual clear corneal incisions. J Cataract Refract Surg. 2017;43:1430-1435.

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