Achieving refractive accuracy improves patient satisfaction after cataract surgery

Careful analysis of biometry and lens calculations helps cataract surgeons avoid common pitfalls.

Uday Devgan

Lens calculations are an important part of cataract surgery because even if the surgery is technically perfect, an inaccurate lens power will result in a postoperative refractive surprise and an unhappy patient. When we order lens calculations in our clinic, it is not enough to simply go with the automated printout and hope for the best. We must critically analyze the results and determine if the suggested IOL power makes sense given the clinical picture and patient history.

There are a few guidelines that can help cataract surgeons achieve more accuracy and avoid pitfalls with lens calculations.

1. The keratometry measurement will have almost a 1:1 effect with the IOL power and refraction of the eye. If the corneal power is measured 1 D steeper than it really is, the calculated IOL power will be about 1 D lower than it should be, and the postop refraction will be hyperopic by about 1 D.

2. The axial length measurement will have about a 3:1 effect with the IOL power and refraction of the eye. If the axial length is measured 1 D longer than it really is, the calculated IOL power will be about 3 D lower than it should be, and the postop refraction will be hyperopic by about 3 D. For shorter eyes, 1 mm may equal 4 D, and for longer eyes, 1 mm may equal 2 D.

3. The A-constant of an IOL has about a 1:1 effect with the IOL power and refraction of the eye. If the surgeon changes from an IOL with an A-constant of 119.0 to one that is 118.0, then IOL power should be decreased by 1 D to achieve the same postop refractive goal. The reason why anterior chamber IOLs have a lower A-constant than posterior chamber IOLs is due to their effective lens position, which is much more anterior in the eye.

4. Anisometropia of more than 1 D is unusual in most patients. The exceptions are hyperopic patients with anisometropic amblyopia (the amblyopic eye will be shorter and require a higher IOL power to achieve plano) and highly myopic patients with axial elongation, which can be somewhat asymmetric, and in whom amblyopia is less common.

5. Corneal refractive surgery induces inaccuracy in the lens calculation by giving erroneous keratometry measurements, fooling formulae that assume that a flat cornea is indicative of a shallow anterior chamber and a more anterior effective lens position.

6. Phaco incisions, particularly corneal ones, cause flattening at that meridian and a corresponding steepening at the meridian 90° away. There is approximately a 1:1 coupling, so that flattening of 1 D at one meridian is accompanied by 1 D of steepening at the orthogonal axis.

The following case scenarios are exercises designed to help understand these concepts and put them to use clinically.

Case 1: The capsule breaks and an anterior chamber IOL is required

Right eye

• Keratometry measurement: 43.00 × 90/43.50
• Axial length: 23.5 mm
• Posterior chamber IOL A-constant: 118.5
• Posterior chamber IOL for plano: +21.0

Left eye

• Keratometry measurement: 43.25 × 85/43.75
• Axial length: 23.45mm
• Posterior chamber IOL A-constant: 118.5
• Posterior chamber IOL for plano: +21.0

The right eye has a good surgery, with in-the-bag placement of the posterior chamber IOL and a postop result of plano. During cataract surgery of the left eye, the capsule breaks and an anterior chamber IOL is required. The A-constant of the anterior chamber IOL is 115.0 and you desire a plano result for the left eye as well. What power anterior chamber IOL should you implant?

In this case, the A-constant of the posterior chamber IOL is 118.5 and that of the anterior chamber IOL is 115.0, a difference of –3.5, which means that the anterior chamber IOL power should be 3.5 D lower than the posterior chamber IOL power. For plano, the posterior chamber IOL power is +21.0 and therefore the anterior chamber IOL power is +17.5 D. If you are able to place a three-piece posterior chamber IOL with an A-constant of 118.5 in the ciliary sulcus, the IOL power should be dropped by 0.5 D to 1 D to account for the more anterior lens position. If an optic capture technique is used, where the optic is buttonholed through the capsulorrhexis, the posterior chamber IOL power will be about the same as in-the-bag placement because the effective lens position is similar.

Case 2: History of anisometropic amblyopia in the right eye

Right eye

• Keratometry measurement: 43.00 × 90/43.25
• Axial length: 21.2 mm
• IOL A-constant: 118.5
• IOL for plano: +29.5
• Old glasses: +7.0 D

Left eye

• Keratometry measurement: 43.25 × 85/43.25
• Axial length: 22 mm
• IOL A-constant: 118.5
• IOL for plano: +26.5
• Old glasses: +4.25

You have performed lens calculations for this patient and you have double-checked the results, but you still end up with significantly different IOL calculations, with the right eye getting a +29.5 IOL and the left eye a +26.5 IOL to achieve plano in both eyes. Does this make sense?

Yes. It makes sense because the patient has a known history of anisometropic amblyopia with the right eye being about 3 D more hyperopic than the left eye, which is reflected in the old glasses. In this case, the axial lengths are almost 1 mm different, which corresponds to about 3 D in an average eye (even more in a short eye and a bit less in a long eye). If the patient had no history of anisometropia and the old glasses had similar powers for both eyes, then a red flag should be raised to find the source of error, which is typically in the axial length measurement.

Case 3: Prior PRK/LASIK in both eyes

Right eye

• Keratometry measurement: 40.25 × 180/40.00
• Axial length: 25.55 mm
• IOL A-constant: 118.5
• IOL for plano: +18.5
• Current prescription: plano

Left eye

• Keratometry measurement: 40.25 × 170/40.00
• Axial length: 25.6 mm
• IOL A-constant: 118.5
• IOL for plano: +18.5
• Current prescription: plano

You are in the operating room getting ready to do surgery for this patient when he tells you that he forgot to mention that he had prior PRK in both eyes to treat –6 D of myopia in order to achieve plano in both eyes. The calculations that you have done were with the assumption that he had virgin eyes. If you implant the above-specified lens, using a +18.5 for a goal of plano, what is likely to be the actual postop refractive error?

The patient will likely end up with nearly +2 D of hyperopia if the +18.5 D IOL is implanted. In this case, our machines and our calculations can get confused by the post-refractive surgery corneal shape. The keratometry readings are falsely higher than the true corneal power, and as a result, the IOL calculations call for a lower than expected IOL power, leading to a postop hyperopic surprise. The IOL formulae may also erroneously assume that the effective lens position is more anterior, because the assumption is that flat corneal powers indicate a shallower anterior chamber.

A simple approximation is to adjust the IOL power by one-third of the laser spherical equivalent, in the opposite direction. In this case, because the PRK was for –6 D, one-third in the opposite direction is +2 D, so we can add 2 D to the IOL power of +18.5 to come up with an adjusted IOL power of +20.5 for a goal of about plano. To err on the side of mild myopia, which is useful, and avoid postop hyperopia, we could even add another +0.5 D to the IOL for a power of +21.0 D.

Case 4: What is the effect from the phaco incision?

Right eye

• Keratometry measurement: 44.00 × 180/43.00
• Axial length: 23.20 mm
• Posterior chamber A-constant: 118.5
• Posterior chamber IOL for plano: +21.5

Left eye

• Keratometry measurement: 44.00 × 180/43.00
• Axial length: 23.22 mm
• Posterior chamber A-constant: 118.5
• Posterior chamber IOL for plano: +21.5

Your phaco incision is made at the temporal position (180°) in the clear cornea and it is 2.8 mm wide, which you have determined typically causes 0.5 D of flattening. Will there be an adjustment to the IOL power, and what will the postoperative refraction and keratometry be?

With a 1:1 coupling in the cornea, its spherical equivalent should stay the same and there should be no adjustment needed to the IOL power. While the true coupling is not quite perfect, for the majority of our cases, the 1:1 approximation is accurate. The postop refraction will have a spherical equivalent of plano but will have some degree of corneal astigmatism. Because the incision causes a total of 0.5 D of flattening, this means that the 180° axis will be flatter by 0.25 D and the 90° axis will be steeper by 0.25 D, giving a predicted keratometry of 43.75 × 180/43.25 × 90 and predicted postop refraction of –0.25 +0.50 × 180.

• Uday Devgan, MD, FACS, FRCS(Glasg), is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills and Newport Beach, Calif. He is also chief of ophthalmology at Olive View UCLA Medical Center and associate clinical professor at the UCLA School of Medicine. He can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax 310-388-3028; e-mail: devgan@gmail.com; website: www.devganeye.com.
• Disclosure: No products or companies are mentioned that would require financial disclosure.