Pre-operative measurements important in IOL decision-making

By Jared R. Younger, MD, MPH

Refractive cataract surgeons work hard to get "20/happy" patients, and satisfied patients spread the word about a great practice and refer their friends. They don't need extra post-op visits, and can return yearly or go back to see their referring doctor.

This is the case in the majority of patients with the help of accurate biometry, topography measurements, and the variety of available intraocular lens (IOL) technologies. However, how do surgeons minimize those patients who may not be happy with the results of their lens surgery?

Selecting the best presbyopia-correcting IOL for a patient is critical and depends on methodical, preoperative decision-making.

For motivated presbyopic patients who desire minimal dependence on glasses, it is necessary to decide between a multifocal IOL or an extended depth of focus (EDOF) IOL, and determine which one is appropriate. The key to establishing the best candidates for each IOL lies in the pre-operative testing and planning.

Pre-operative assessments

After assessing lifestyle and vision preferences, one approach to choosing which lens is best for a patient is the use of a “decision tree” with corneal analysis metrics.

Assuming the macular optical coherence tomography (OCT) and potential vision are normal, the tree starts with four corneal analysis metrics-corneal higher order aberrations (HOA) values, corneal modular transfer function (MTF), angle alpha measurements, and anterior and posterior astigmatism values.

The first step for reviewing pre-op assessments is to look at the corneal HOA to ensure the values are in an acceptable range. I use the iTrace Ray Tracing Wavefront Aberrometer and Corneal Topographer (Tracey Technologies).

The new topographers also can measure this value. I look for a number that is less than 0.3 µm at a 4-mm pupil. Anything higher than this raises a red flag and should be further examined.

Next, I review the corneal MTF, which is a measure of contrast. If the patient has >40% at 10 cycles per degree, then that patient is a good candidate for either a multifocal IOL or EDOF lens.

MTF curves

Figure 1A: Example of a good MTF curve. (Images courtesy of Jared R. Younger, MD)

Figure 1B: Borderline MTF value with an area under the curve of 40% (An EDOF lens was chosen.).

If the value is borderline and contrast sensitivity already is compromised, I would not recommend a multifocal IOL. Figure 1A is an example of a good MTF curve and Figure 1B illustrates a borderline curve, which shows a value where I would choose an EDOF lens over a multifocal IOL. 

I also look at angle alpha, which is the distance between the center of the limbus and visual axis. While traditional refractive surgery measurements rely on angle kappa measurements, the variable nature of pupil centers makes centering an IOL on the visual axis important.

Lenses have a tendency to settle in the center of the capsular bag, which makes the angle alpha measurement more valuable than angle kappa in this circumstance.

If the angle alpha measurement is less than 0.4 mm, I choose a low-add multifocal IOL. If the measurement is between 0.4 mm to 0.6 mm, I will use an EDOF lens, as the central optic zone is larger.

Of course with higher values, a monofocal IOL is a safe option.  Figure 2 illustrates a borderline angle alpha of 0.547 mm.  In this circumstance, I would consider an EDOF lens.

Figure 2: Borderline Angle Alpha of 0.547mm

Astigmatism values

Finally, when analyzing the keratometry and topography or tomography, the anterior and posterior astigmatism values help in the decision tree. If the total astigmatism is over 1 D, I prefer to use a toric lens implant, and in appropriate patients desiring a range of vision, the Symfony toric EDOF lens (Johnson & Johnson Vision).

Figure 3: Low MTF curve in a patient with moderate regular astigmatism.

Figure 3 illustrates what a low MTF curve in a patient with moderate regular astigmatism looks like. In this case, a monofocal toric IOL would be better than a toric EDOF lens or multifocal IOL.

Fortunately, most patients will have a normal MTF value and can opt for an EDOF toric lens. Having this in our toolbox opens the door for many patients with astigmatism.

Intra-operative assessments

With fourth generation IOL calculation formulas readily available, power selection is accurate in the majority of cases. However, an additional intra-operative data point can be used if calculations are in between two lens powers.

In these cases, I would assess the lens meridian position (LMP) during the femtosecond laser portion of the surgery. The LMP is the distance from the anterior cornea to the central/vertex point of the lens capsule. This distance is measured accurately with the Liquid Optics Interface on the Catalys Precision Laser System (Johnson & Johnson Vision), as there is zero compression on the cornea.

Figure 4: LMP of 3.9 mm (Typical LMP value is around 4.7 mm; therefore, a 0.5 D lower IOL power was chosen in this example.).

I would look at the estimated fornix of the capsular bag and factor this into my IOL power selection if the fornix is very anterior or posterior. This is unique, as other technologies–such as intra-operative aberrometry–do not account for the entire capsular bag anatomy.

Also, using the Scanned Capsule Capsulotomy mode on the femtosecond laser helps with IOL centration. Figures 4 and 5 show a comparison of two different LMP values–3.9 mm and 5.8 mm, respectively. Note the capsular bag anatomy and how the lens fornix is more anterior than in the other.

Working through this decision tree prior to IOL selection can add to the valuable and advanced tools available and helps to ensure each patient is getting the best possible lens implant type for their desired refractive outcome.

Figure 5: LMP of 5.8 mm (A 0.5 D higher power IOL was chosen in this example.).

Jared R. Younger, MD, MPH

e. youngerjared@gmail.com

Dr. Younger is a board certified and fellowship-trained ophthalmologist at the Orange Coast Eye Center, Fountain Valley, CA. He is a consultant to Johnson & Johnson Vision.