Adi Abulafia, MD and Warren E. Hill, MD
Experienced surgeons generally agree that the toric intraocular lens (IOL) is an excellent beginning for those looking to add premium lenses to their practice. A large cataract surgery keratometry database shows that greater than 60% of patients have 0.75 diopters (D) of corneal astigmatism or more (Figure 13-1). Stepping up to lens-based surgery as a refractive procedure, with potentially more than half of patients as instantly eligible candidates, is a wonderful introduction.
Corneal astigmatism can be corrected by 4 different methods (Table 13-1). A widely used approach involves limbal relaxing incisions (LRI). LRIs have the advantage of being inexpensive and relatively easy to do. However, LRIs are considered more for the reduction of astigmatism to some acceptable amount rather than precise correction. LRIs also have a limited effective range, with most surgeons reserving them for 1.50 D of corneal astigmatism or less.
Popularized by Dick Mackool, an innovative variation on the theme of incisional surgery is the use of paired, full-thickness penetrating incisions. By this method, 2 standard phacoemulsification incisions are made 180 degrees apart on the steep axis. The amount of correction is mostly a function of the incision width and location. When guided by a preoperative axial topographic power map, full-thickness penetrating incisions are also effective when the areas of steepening on either side of the corneal vertex are not in a perfectly straight line. Like LRIs, these are also easy to carry out, but they require each surgeon to develop an individual nomogram.
Bioptics is another approach for the correction of corneal astigmatism. By this method, a LASIK flap is cut prior to cataract surgery, which is combined with a myopic target refraction. After the postoperative refraction has stabilized, LASIK is undertaken to correct the residual refractive astigmatism and any remaining spherical equivalent. This has the advantage of offering a relatively precise correction but has the disadvantage of requiring 2 separate procedures, along with 2 opportunities for complications and 2 separate charges to the patient. In general, bioptics is not widely used.
For many surgeons, the correction of corneal astigmatism using a toric IOL has the advantage of requiring a single procedure, and with careful planning, it can be very accurate. If the physician normally charges for LRIs, the physician charge for a toric IOL may be comparable. Because toric IOLs generally come in 0.75 D power steps, implanting a toric IOL can be accomplished with almost the same level of precision as the spherical power of the intraocular lens. In the United States, toric IOLs are available to correct up to 4.00 D of corneal astigmatism. However, in many other countries, it is possible to correct a much wider range of corneal astigmatism using innovative solutions such as toric IOLs that are custom manufactured to the individual needs of a specific patient.
THE MEASUREMENT OF CORNEAL ASTIGMATISM
If you ask 5 surgeons the best way to measure corneal astigmatism preoperatively, you may get 6 different answers. Of course, the best method is the one that delivers the least amount of residual postoperative astigmatism. In general, this requires that we know 2 components: the power difference between principal meridians (which are generally orthogonal, unless the astigmatism is irregular) and the orientation of the steep meridian.
A common misconception that often leads surgeons in the wrong direction is the assumption that the measurement of corneal astigmatism should be the same with multiple instruments. It is helpful to keep in mind that different devices often measure different areas of the astigmatic cornea and may also employ different algorithms. Expecting manual Ks, Placido-based simulated Ks, slit-scanning Ks, Scheimpflug Ks, and the various forms of autokeratometry to all give the same answer is unrealistic. Measurements by 3 or more different instruments will typically produce 3 or more different values. What works best is to first develop a plan.
Aside from the incision location and the surgically induced astigmatism, toric calculators are mostly looking for 2 items. First is the correct identification of the steep meridian, and second is the power difference between principal meridians. This information may actually be different from what we get with a set of Ks. Recall that the toric calculator does not care as much about the absolute powers as it does about the difference between them. Added accuracy comes when we try to think like the toric calculator.
Using a topographic or tomographic axial power map, we first look to see how the power is distributed across the anterior cornea within the central 3 to 4 mm. Regular astigmatism is represented by a pair of symmetrical astigmatic power lobes straddling the corneal vertex (often referred to as a bow tie), with both lobes being aligned along the same meridian (Figure 13-2). If a line is drawn through the center of each lobe and the corneal vertex, where this line intersects the axis scale in the periphery is, by definition, the steep meridian. If the power distribution on the other side of the corneal vertex is not the same, the astigmatism is asymmetric, and if a single meridian cannot be drawn through both lobes, it is irregular. Early in the process, this simple method forces us to carefully look at the cornea when making a decision about the appropriateness of a toric IOL. If the steep meridian cannot be identified, each lobe is aligned with a different meridian, or things are quite asymmetric, the patient may not be a toric IOL candidate.
A topographic axial power map is considered a primary instrument for determining the steep meridian. A primary instrument is one that is well suited to a given task and always provides the correct information when presented in a specific way.
Following the identification of the steep meridian, next we need to determine the power difference between the principal meridians (ie, steep and flat). Here, we use this manually determined steep meridian from the topographic axial power map to validate the power difference. This is often best done using small zone autokeratometry for eyes that have not had prior refractive surgery (see Figure 13-2). Be cautious about using any form of simulated Ks for this exercise, especially for low magnitudes of astigmatism. We look to see that the steep meridian on Ks is the same as what we know to be correct after looking at the topographic axial power map. This is a key concept.
If we are certain about the steep meridian, but the autokeratometer is telling us something different, then the Ks by keratometry are most likely being measured in an incorrect location. This is not uncommon when the power distribution is irregular. When this kind of disconnection occurs, everything stops until the discrepancy can be resolved. One approach is to settle this discrepancy by manual keratometry by setting a manual keratometer to the previously determined steep meridian, taking a measurement, then rotating the same drum 90 degrees and taking a measurement. In this way, we are measuring the power difference; the actual numbers are not as important.
SINGLE-ANGLE AND DOUBLE-ANGLE PLOTS FOR ASTIGMATISM DATA PRESENTATION
Data regarding corneal astigmatism, residual astigmatism prediction errors, etc, can be graphically represented on single and double-angle plots. The traditional single-angle plot is laid out like we see it in the phoropter, in that way the against-the-rule (ATR) eyes are laid separately (Figure 13-3A). The double-angle plot takes the with-the-rule (WTR) eyes and moves them to the left, and it takes the 150 to 180 degrees group of ATR eyes and moves it to the right. All the WTR eyes are grouped to the left, and all the ATR eyes are grouped to the right (Figure 13-3B).