Accurate corneal power measurements for IOL calculations
About 30% of patients will need additional measurements before cataract surgery.
Outcomes following cataract surgery with greater than 90% of patients within 0.5 D of the target spheroequivalent refraction are achieved by less than 1% of surgeons. I give these surgeons an “A” for their results, just like in school. “B” surgeons are 80% to 89%, “C” surgeons from 70% to 79%, “D” surgeons from 60% to 69%, and if you are less than 60%, you are failing. An “A” surgeon uses optical biometry and re-measures axial lengths, usually with immersion, if the eyes differ by more than 0.3 mm or have a poor signal-to-noise ratio; uses a five-variable predictor IOL formula, such as Holladay 2, Olsen 2 or Barrett 2; continually personalizes his lens constant; and measures corneal power using the protocol described below.
The first rule in good keratometry is no contact (IOP measurement) and no eye drops (even saline or artificial tears) before taking corneal measurements. Artificial tears or saline drops may generate a reading more easily in a dry eye, but the eye drops often cause steepening and punctate epithelial keratitis, which reduces the accuracy of the measurement. Blinking is much better than saline or artificial tears and is what the patient does normally. The patient should be asked to blink frequently while preparing for the measurement (alignment) and then stop blinking about 1 second before the measurement for the tear film to stabilize. If the patient is found to have dry eye, this must be treated before reliable keratometry is possible. Applanation pressure measurements flatten the cornea centrally and should not be performed before keratometry.
Keratometry with optical biometers is the most common method of corneal power measurements today, replacing manual keratometry. It is satisfactory in approximately 70% of patients. The remaining 30% who need additional measurements are identified on the IOLMaster (Carl Zeiss Meditec) and Lenstar (Haag-Streit) by a standard deviation in corneal radius that exceeds ±0.03 mm (±30 µm), as shown in Figure 1 and Figure 2, respectively. Patients who exceed this limit have irregular astigmatism, and the most common cause is dry eye (Figure 3).
If the dry eye workup is negative or treatment has been successful, topography measurements are now indicated. Keratometry is not accurate in the presence of irregular astigmatism. Two, six or even 24 points (manual, IOLMaster, Lenstar) cannot compare with thousands of points measured with topography and the subsequent analyses in the 3-, 5- and 7-mm zones. Note in Figure 3, the broken lines indicate the steep and flat meridians change, even after the successful treatment of the dry eye. The astigmatism axis varies from one zone to the next as does the average power used for the spheroequivalent IOL calculation. The power from a 3- to 4.5-mm zone from thousands of points is far superior to a 2- to 3.2-mm ring from keratometry. There is no perfect alignment for a toric IOL with irregular astigmatism; it is simply optimal. Even when optimally aligned, there is a decrease in visual performance, which is directly related to the amount of irregular astigmatism that can be quantitated after surgery with aberrometry. Before the cataract surgery, the surgeon should show the topography to the patient and prepare the patient for less than “perfect” vision and the possibility of a second procedure for “fine-tuning” the alignment and/or power of the IOL. In Figure 4, an asymmetric bowtie, early keratoconus and post-LASIK are examples of varying degrees of irregular astigmatism not due to dry eye.
If the patient has had refractive surgery, then it is helpful to perform tomography (Scheimpflug) because the ratio of the back-to-front surface curvature is no longer 82.2% and the astigmatism contribution of the back surface may be at a different orientation from the front. Figure 5 shows the zonal equivalent K-reading (EKR), which includes both surfaces from the 1- to 7-mm zone. In this example, the mean EKR only varies by 0.4 D from 1 mm to 4.5 mm, but the smaller and more central the power, the more important its contribution to vision, so reducing the EKR to the flatter central value reduces the risk of a hyperopic surprise. The 4.5-mm zone has been shown to be the best for patients with normal pupils, but if the pupil is unusually small, then a 4-mm or even 3-mm zone may be used, especially if the mean power or astigmatism is different from and flatter than the 4.5-mm zone.
Figure 6 is the corneal power decision tree that summarizes the discussion above. Notice that the flow of 70% of the cases is not affected because only 30% will need the additional measurements. The additional time determining a more accurate corneal power in these exceptional patients will improve results and if accompanied by the other three attributes of the “A” surgeon will move you to that level.
- For more information:
- Jack T. Holladay, MD, MSEE, FACS, is a clinical professor of ophthalmology at Baylor College of Medicine and the OSN Optics Section Editor. If you have any clinical optics questions, please send them to email@example.com.
Disclosure: Holladay reports he is a consultant to Abbott Medical Optics, AcuFocus, Alcon Laboratories, ArcScan, Calhoun Vision, Carl Zeiss, Elenza, Oculus, Visiometrics and WaveTec.