Innovations in optical biometry optimize effective lens position

Novel IOL technologies and increasing patient expectations for precise refractive results have driven a series of innovations in biometry and IOL power calculation. Notable improvements have been seen in recent years, and about 1,000 citations for optical biometry currently appear in PubMed.

Optical biometry provides detailed images of critical anatomic structures in the anterior and posterior segments of the eye. Intraoperative OCT has shown particular promise in predicting effective lens position, improving on the traditional use of axial length measurement.

ORA intraoperative aberrometry (Alcon) has been shown to improve accuracy in post-myopic LASIK patients, but it does not provide biometric measurements that are helpful in predicting the effective lens position, according to Wendell Scott, MD.

“I think that ORA was an attempt to try to improve results. It was a step in the right direction. It allowed us intraoperatively to confirm tests that we had done preoperatively. The problem, of course, is that intraoperative aberrometry is an optical wavefront measurement and not an anatomic biometric, so it is less helpful in estimating the effective lens position. So, being able to actually do intraoperative OCT, I think, will be the next methodology that really raises the predictability of our implant results,” Scott said.

Image: Findl O

Imaging and measurement of intraocular structures are particularly critical, especially during accommodation. Adrian Glasser, PhD, OSN Optics Board Member, said that accommodation is a complex and multifaceted process, underscoring the difficulty of predicting accommodative optical responses from biometric changes in the eye, the subject of three studies he co-authored.

“True accommodation is an objectively measurable dioptric change in power of the eye, such as might be measured with an autorefractor or an aberrometer. Pseudoaccommodation is a combination of factors that contribute to an increased depth of focus of the eye, but without a change in dioptric power. A pupil constriction alone, for example, increases the depth of focus of the eye. A pupil constriction alone in an eye with negative spherical aberration also acts to increase the optical power of the eye. Accommodation also normally involves both eyes, so binocular summation is also involved. All of these factors occur together,” Glasser said.

Optical biometry systems

Optical biometry has been a major step forward toward improved accuracy because it provides measurements not only of axial length, but also anterior chamber depth and thickness of the cornea, lens and retina. Currently available systems include the IOLMaster 700 (Carl Zeiss Meditec) with swept-source biometry and OCT, the AL-Scan (Nidek) optical biometer, the Lenstar LS 900 (Haag-Streit) with dual zone keratometry and optional Placido topography, the Aladdin biometer (Topcon) and Placido topographer, the Galilei G6 Lens Professional (Ziemer) dual Scheimpflug tomographer and Placido topographer with optical biometry, and the OA-2000 optical biometer and topographer (Tomey).

In a study, Oliver Findl, MD, and colleagues compared the performance of these systems in 57 eyes of 57 cataract patients, including 15 with very dense cataracts.

“We did three measurements with each device, for a total of about 1,000 measurements. The IOLMaster 700 and the OA-2000 had the highest number of successful scans, 96.7% and 98.7%, respectively. These are the two machines I would recommend if you have patients with dense cataract in your population,” Findl said.

The Galilei system had the largest number of dropouts, with only 78% successful scans in a cataract population.

“[Dropouts are] something that needs to be addressed by the manufacturers,” Findl said.

In the same study, axial length measurements of other systems were compared with the IOLMaster, which is currently the gold standard. An excellent correlation was found among all instruments.

“Among all of the potential sources of error in IOL calculation, axial length is the one that has been most effectively reduced by the current technology. We did not expect such a change when we performed the first clinical measurements on a large optical bench with our patients in the early 1990s. In this new study comparing the different new devices, all measurements were similar, as you would expect and hope,” Findl said.

The closest correlation was found between the IOLMaster 700 and the Lenstar LS 900. The AL-Scan had cases with a difference of more than 0.1 mm, which is “also a problem that needs to be addressed,” Findl said.

A study by Stodulka and colleagues showed that swept-source OCT, part of the IOLMaster 700, accurately predicted the orientation of IOL tilt after standard lens implantation.

Intraoperative ORA, OCT

Although the latest OCT and Scheimpflug devices have greatly improved precision in anterior chamber depth measurement, postoperative IOL position cannot be determined preoperatively and remains a cause of refractive surprises.

“Where will the lens sit in this eye for this patient? Postoperative [anterior chamber depth] prediction is the biggest source of IOL power calculation error that we have known for many years,” Findl said.

Intraoperative biometry is a promising new concept. The ORA system, mounted on the operating microscope, is meant to assist the surgeon in selecting IOL power by refracting the eye and measuring aberrometric changes intraoperatively. While it has gained widespread popularity in the U.S., ORA has not yet taken off in Europe and seems to be less unanimously accepted by surgeons there.

According to Erik L. Mertens, MD, ORA is a step forward toward achieving accurate results and lowering the re-treatment rate in the premium IOL segment.

“I use the ORA to place toric IOLs in the correct position and also in post-refractive surgery cases,” he said.

Findl, however, expressed skepticism. He mentioned a study carried out at Hamburg University, where of the 814 measurements taken, only half were successful and a lot of variability was shown.

“A lot of factors change intraoperative aberrometry. The lid speculum puts pressure on the globe and deforms the cornea. The incision and even the anesthetic drops cause changes. These systems can be misleading and are very expensive,” he said.

According to Findl, intraoperative OCT is a more effective and promising way of measuring the eye intraoperatively. He personally uses a prototype anterior segment OCT (Visante, Zeiss) attached to the operating microscope, which produces continuous intraoperative OCT videos. He measures the capsule position during surgery after removal of the crystalline lens and before IOL implantation. The measurements correlate well with the postop IOL position.

“The system is not commercially available and technologically demanding, but I am motivated to go on with this project. IOL prediction problems are greatly reduced, also in longer and shorter eyes,” he said.

Intraoperative OCT 3-D modeling

At the American Society of Cataract and Refractive Surgery meeting in San Diego, Joseph J.K. Ma, MD, FRCSC, presented a study on intraoperative three-dimensional OCT modeling of the eye in the prediction of effective lens position in femtosecond laser-assisted cataract surgery.

Ma found that a 3-D OCT algorithm was superior to current 1-D multivariate formulas in predicting postoperative effective lens position.

“We all know that effective lens position is the greatest source of error for intraocular lens calculations and, therefore, for biometry with respect to lens calculations,” Ma said. “Almost all methods currently use a one-dimensional axial length measurement to use a regression formula to estimate postoperative effective lens position. One of the main findings of my study was that if you used a three-dimensional morphology-based algorithm for predicting effective lens position without the use of axial length, you can get a better prediction than if you used any of the traditional methods.”

In the study, Ma took measurements immediately before surgery, whereas Findl took his measurements immediately after surgery.

“I feel that these immediate preoperative type measurements will theoretically always be a little bit more accurate than something done at the time of surgery,” Ma said. “I think that intraoperative [measurement] will probably in the future have an important role as an adjunct in niche areas, such as post-laser vision correction or post-phakic IOL. I think those are the areas where intraoperative biometry may prove to have a long-standing important niche.”

The 3-D prediction algorithm was more accurate than the Olsen and Haigis IOL power calculation formulas and the ORA in predicting effective lens position, Ma said.

“Our OCT algorithm was more accurate than ORA,” he said. “I don’t believe that the intraoperative aspect helps you determine the postoperative effective lens position. That’s why pseudophakic ORA was not accurate.”

Ma attributed the inaccuracies of pseudophakic ORA to four factors: presence of viscoelastic in the eye, the capsular bag not having contracted to its final position, IOL position in the capsular bag and distortion of the ocular surface.

“The ocular surface is quite different intraoperatively vs. postoperatively. When you’re in the operating room, the eye can be dry, the speculum is in the eye, and the lid has a different relationship with the eye. You have a lot of confounding factors that make it difficult to know. These factors combine to make the ocular surface unpredictable,” Ma said. “Whereas if you do a very good preoperative measurement or intraoperative/immediate preoperative measurement, before you put the drops in, before the speculum is in, before they’re lying down flat, you have a better chance of capturing the ocular surface in its native state.”

Ma and Scott are developing a second formula based on intraoperative effective lens position estimates using intraoperative OCT.

“He and I are working together to try to come up with a better way of intraoperatively estimating the effective lens position and then developing a formula and validating that formula with the use of the intraoperative OCT with femtosecond laser. I think that’s the next step. Compared to ORA, Joseph has already shown better results with this method,” Scott said.

The second formula is designed to refine the first formula in a different population and compare differences, such as those between ethnic groups, Ma said.

“For example, Asian patients tend to have a higher incidence of angle closure and therefore a different relationship between [anterior chamber depth] and axial length,” Ma said.

Both formulas use radial basis function, an algorithm that uses pattern recognition to identify non-linear relationships.

“Radial basis function uses adaptive learning to improve outcomes,” Scott said. “By using radial basis function, we think that we’ll be able to identify these factors that are non-linearly related and then use those to come up with a better formula.”

Ultrasound biomicroscopy

In three recent studies, Glasser and Viswanathan Ramasubramanian, PhD, used ultrasound biomicroscopy (UBM) to predict accommodative optical responses from measured accommodative biometric changes in young adult and pre-presbyopic phakic eyes.

The researchers used the VuMax UBM device (Sonomed Escalon) to measure accommodative changes.

“The strongest linear correlation between objectively measured accommodation and a measured ocular biometry parameter that we found from our UBM studies was the accommodative increase in lens thickness,” Glasser said.

He said that correlations depended somewhat on the accuracy of the parameter being measured and the resolution of the measurement device.

“We did measure lens anterior and posterior surface curvatures with UBM in our study. However, UBM is low in resolution compared to OCT, for example, and more so for the measurement of lens surface curvatures,” Glasser said. “If methods with higher resolution and greater accuracy were used to measure all the ocular biometry parameters, stronger correlations may be found with other of the biometry parameters.”

The authors found that a minimum of about 0.5 D of accommodative optical change can be predicted from UBM biometric measurements.

“The sequential, as opposed to simultaneous, measurements that we used in our studies mean that there is some noise introduced because the two sequentially measured accommodative responses are unlikely to be identical,” Glasser said. “Tighter correlations and smaller standard deviations would be expected if simultaneous measurements could be performed. Also, it is important to bear in mind that this prediction is from a population as a whole, so knowing the relationship between biometric and optical changes for any single individual is less certain. Individual variation in the relationships is certainly expected.”

Use of a similar methodology to test new-concept IOLs such as dual-optic lenses would present major challenges, Glasser said.

“The major consideration in trying to extrapolate our study in phakic eyes to just about any kind of accommodative IOL is that the biometric changes that occur in the eye with a pseudophakic accommodative IOL may be profoundly different from how the phakic eye accommodates,” he said. “For example, in a dual-optic IOL there may be no change in surface curvatures of the IOLs, and all of the change in optical power of the eye comes from a change in the distance of separation between the two optics of the IOL. Obviously this is very different from how the phakic eye accommodates, so the correlations published in our study on phakic eyes are not applicable to any accommodative IOL, unless that accommodative IOL worked in exactly the same way as the human lens.”

Optical and mechanical eye models would help researchers establish relationships between accommodative biometric changes and optical accommodative changes for specific accommodative IOLs, Glasser said.

“As an alternative, a pilot study would have to be performed on patients already implanted with the accommodative IOLs to learn what the correlations were and if these were consistent enough to make predictions. Only then would it be possible to use a biometric method to measure biometric accommodative changes and to use this information to predict that accommodative dioptric response of an eye,” he said. “For now, the optical modeling or the mechanical eye model approaches seem like the lower hanging fruit for getting the information necessary to relate biometric changes to optical accommodative changes for new-generation accommodative IOLs.”

Improving accuracy with toric IOLs

Toric IOL alignment also depends on precise biometric measurement. According to Nino Hirnschall, MD, PhD, OSN Europe Edition Board Member, Young Ophthalmologists Section, toric lenses improve astigmatism in most cases, but “results are not always as good as we would like them to be.”

“There are several sources of error. One of them is misalignment, the deviation between the intended axis of the toric IOL and the really measured astigmatism axis. This error might be due to preoperative marking, intraoperative misalignment or postoperative rotation of the IOL,” he said.

However, the main source of error appears to be the measurement of the cornea before surgery, as shown by a study.

“We measured patients’ corneas twice, 1 year apart, and there were differences in astigmatic axis measurement up to 70°, especially in corneas with a low amount of astigmatism, where the astigmatic axis is more difficult to detect,” Hirnschall said.

He advised surgeons to always use different devices to measure the cornea, and if the amount of astigmatism and the axis of astigmatism are similar, the measurements can usually be trusted.

“If there is a discrepancy, the trouble starts. Remember, if you are 10° off, you kind of lose one-third of your astigmatism reduction effect,” he said.

Determining the amount of astigmatism should include measurements of the entire cornea, including the posterior surface, he said.

“One of the used devices should also be able to detect irregular astigmatism, as these cases are very tricky cases and a special calculation is needed. If one can rule out irregular astigmatism and the different devices show similar results concerning amount and axis of astigmatism, the calculation is relatively easy. It can be done in an Excel file, but one can also use the online calculator of the company that provides the toric IOL of choice. If a company’s calculator is used, it is important to check if it takes the surgically induced astigmatism into account and the postoperative anterior chamber depth. Not all of the calculators do so, and this is a potential source of error,” Hirnschall said.

He also recommended that, at the beginning of the process, the same requirements as for non-toric IOLs should be met.

“This may sound simple, but it is important. Before we think about astigmatism reduction, we should think about the refractive outcome in terms of spherical equivalent. If we end up with a refractive surprise of more than 1 D of [spherical equivalent], the astigmatism correction will not be as beneficial as it could be,” Hirnschall said. – by Michela Cimberle, Matt Hasson and Kristie L. Kahl

• References:
• Behndig A, et al. J Cataract Refract Surg. 2012;doi:10.1016/j.jcrs.2012.02.035.
• Engren AL, et al. J Cataract Refract Surg. 2013;doi:10.1016/j.jcrs.2012.11.019.
• Hahn P, et al. Ophthalmic Surg Lasers Imaging. 2011;doi:10.3928/15428877-20110627-08.
• Hirnschall N, et al. Br J Ophthalmol. 2015;doi:10.1136/bjophthalmol-2013-304731.
• Hirnschall N, et al. Invest Ophthalmol Vis Sci. 2013;doi:10.1167/iovs.13-11991.
• Hirnschall N, et al. J Refract Surg. 2014;doi:10.3928/1081597X-20140429-01.
• Huelle JO, et al. Br J Ophthalmol. 2014;doi:10.1136/bjophthalmol-2013-304786.
• Ianchulev T, et al. Ophthalmology. 2014;doi:10.1016/j.ophtha.2013.08.041.
• Lundström M, et al. J Cataract Refract Surg. 2012;doi:10.1016/j.jcrs.2012.03.006.
• Matz H, et al. Biomed Tech (Berl). 2012;doi:10.1515/bmt-2012-4460.
• Norrby S. J Cataract Refract Surg. 2008;doi:10.1016/j.jcrs.2007.10.031.
• Preussner PR, et al. J Cataract Refract Surg. 2004;doi:10.1016/j.jcrs.2004.07.004.
• Ramasubramanian V, Glasser A. J Cataract Refract Surg. 2015;doi:10.1016/j.jcrs.2014.08.033.
• Ramasubramanian V, Glasser A. J Cataract Refract Surg. 2015;doi:10.1016/j.jcrs.2014.12.049.
• Ramasubramanian V, Glasser A. J Refract Surg. 2015;doi:10.3928/1081597X-20150319-06.
• Stringham J, et al. J Cataract Refract Surg. 2012;doi:10.1016/j.jcrs.2011.09.039.

• For more information:
• Oliver Findl, MD, can be reached at VIROS, Department of Ophthalmology, Hanusch Hospital, Heinrich Collin-Straße 30 1140, Vienna, Austria; email: oliver@findl.at.
• Adrian Glasser, PhD, can be reached at College of Optometry, University of Houston, 4901 Calhoun Rd., Houston, TX 77004; email: aglasser@uh.edu.
• Nino Hirnschall, MD, PhD, can be reached at VIROS, Hanusch Krankenhaus, Heinrich-Collin Straße 30, 1140 Vienna, Austria; email: nino.hirnschall@googlemail.com.
• Joseph J.K. Ma, MD, FRCSC, can be reached at University of Toronto, 500 Sheppard Ave. East, Suite 305, Toronto, Ontario, Canada; email: joseph.j.k.ma@gmail.com.
• Erik L. Mertens, MD, can be reached at Eye Center Medipolis, Boomsesteenweg 223, 2610 Wilrijk, Belgium; email: e.mertens@medipolis.be.
• Wendell Scott, MD, can be reached at Mercy Eye Specialists, 1229 E. Seminole Street, Suite 430, Springfield, MO 65804; email: wendell.scott@mercy.net.

Disclosures: Findl reports he is a consultant to Abbott Medical Optics, Bausch + Lomb, Carl Zeiss Meditec, Croma and Hoya and a patent assignee of intraOP OCT. Glasser reports he is a consultant for Abbott Medical Optics, Encore Vision, Lensar, LensGen, Power Vision and Refocus Group, and is an unpaid member of the AAO/FDA IOL advisory task force. Hirnschall reports he is a patent assignee of intraOP OCT. Ma reports he is a consultant for Abbott Medical Optics, Alcon and Bausch + Lomb. Mertens reports he is a consultant for STAAR Surgical, Ophtec, PhysIOL and Bausch + Lomb. Scott reports he is a consultant for Abbott Medical Optics.

Is intraoperative refractive biometry a worthwhile investment for the cataract surgeon?

Intraoperative aberrometry is a versatile tool

Intraoperative aberrometry is an important tool to improve refractive outcomes in patients undergoing cataract surgery and a good investment for cataract surgeons and their patients. Improvements to the technology have made it an increasingly important tool for selecting the appropriate IOL power and determining the correct axis and magnitude of astigmatism.

This technology is especially important for patients at high risk for refractive surprises. Calculating IOL power in eyes that have previously undergone refractive surgery is imprecise and may result in unplanned postoperative refractive errors for a variety of reasons. Errors such as inaccurate corneal curvature measurements, measured keratometric values that are higher than the actual power, a difference between the visual axis and the center of the cornea, and incorrect anterior chamber depth or IOL position can create inaccuracies in some IOL power formulas.

Intraoperative aberrometry is also very helpful for refractive outliers such as high myopes and hyperopes in whom conventional formulas are not as accurate. Finally, intraoperative aberrometry includes the posterior cornea, which cannot be measured with most devices and for this reason gives a better outcome with toric IOLs. Patients are becoming more demanding of precise refractive outcomes, and intraoperative aberrometry is an important tool in meeting their expectations.

Eric D. Donnenfeld, MD, is an OSN Cornea/External Disease Board Member. Disclosure: Donnenfeld reports he is a consultant for Alcon.

Despite utility, measurements are too unreliable

I agree, first of all, that intraoperative biometry has great utility in certain situations. For example, for a patient who has had previous refractive surgery, in whom you cannot really rely on your actual preop measurements, I think it is great. As the number of post-refractive surgery patients needing cataract surgery continues to grow, this will become a much bigger problem. But other than that, there are so many variables with it that may just confuse the issue.

Now you go into the OR and you are going to base your IOL selection on something that you do on the fly. But that eye that you are treating has a speculum in place, which may alter the shape of the eye, and an incision that is open, which will not demonstrate the final corneal curvature that will evolve once the wound heals. You also have an eye that has been exposed for 5 to 10 minutes and has been subject to other drops, perhaps topical anesthetics and the prep solution, so the epithelium is not likely in its most pristine state. With all of these variables, we are basing a measurement on such a potentially unreliable source that I am not certain that it is any more accurate than my preoperative measurements based on the pristine cornea.

What I find most troubling is, what do you do if your measurements are dramatically different from your preop measurements? Do you ignore your preop measurements? Do you ignore the intraoperative measurements? Do you split the difference? Based on those things, for the routine case, I am really troubled. I cannot figure out how to best utilize this information.

In addition, use of this technology is not instantaneous. Even if the readings were instantaneous, you still have all of those variables. It slows down your flow in the OR. You may also be using femtosecond laser, which adds additional cost and time. So, there are so many variables that I am not convinced that the investment is worthwhile.

Kenneth A. Beckman, MD, FACS, is an OSN Cornea/External Disease Board Member. Disclosure: Beckman reports no relevant financial disclosures.