To the Editor:
Toric intraocular lenses (IOLs) can neutralize corneal astigmatism. Their efficacy, however, relies on the correct alignment of their cylinder axis counter to the axis of astigmatism.1 Although calculated preoperatively, the correct axis must be easily and accurately identified on the eye at the time of surgery. Marking can be difficult, with errors in alignment resulting in suboptimal correction of astigmatism1 and patient dissatisfaction. This has prompted the development of a growing number of axis-marking methods and instruments,2–5 but none are ideal. Some of the challenges with current methods are their cost, cumbersome manipulations, a considerable learning curve for the surgeon, need for sterilization of instruments, and a lack of adaptability. These issues are barriers to the greater adoption of toric IOL technology. We are pleased to describe a simple, accurate, and easily reproducible method for axis marking that addresses these challenges.
A calibrated decal (TopCon Axis sticker for slit-lamp, Model 71-MESL11014S; TopCon Corp, Tokyo, Japan), indicating angles 0° to 180°, is applied to the vertical surface of the running-plate of the slit-rotator (Fig). This allows for accurate placement of the slit at any angle. The slit beam is set to its narrowest width (∼1 mm) and longest length (∼10 mm), dialed to the desired angle, and projected on the patient’s cornea so that it spans the visual axis and corneal limbus. Using the projected beam as a guide, the axis of astigmatism is marked while the patient is in a seated position with a superficial epithelial scratch using a 30-gauge needle at the corneal limbus in both the upper and lower quadrants. Dual scratches 180° apart maximize the accuracy when dialing the toric IOL into final position. It is recommended that the axis markings be done just before the patient is prepped with povidone-iodine and draped for surgery. This, together with the usual pre-, intra-, and postoperative antibiotic regimen, minimizes the risk of infection, and the superficial scratches are less invasive than the paracenteses and main corneal incision. No additional markings or instruments are required intraoperatively as the superficial limbal abrasion is seen under the operating microscope.
Figure. The calibrated axis-marking sticker applied to the vertical surface of the running plate of a Haag-Streit slit-lamp. This sticker allows the surgeon to set the beam of the slit-lamp at any angle from 0° to 180º. For this example, an angle of 123º was chosen.
We validated this technique in two ways. First, no error occurred between marks made with the calibrated slit-lamp and direct measurements made by an ophthalmic protractor (beveled degree gauge; ASICO, Westmont, Illinois) on Styrofoam balls (mean error= 0°, standard deviation=0, n=10). Second, a blinded independent observer was able to use the calibrated slit-lamp to estimate the angle of marks on mounted cadaveric porcine eyes with an average error of only 3.2° (standard deviation=2.6°, n=10), but a paired samples two-way t test revealed these estimates were not different from initial marks (P>.05, n=10).
The method described herein is accurate and cost effective. The simplicity of the technique allows even novice surgeons to make accurate axis markings. Negating the requirement for any specialized pre- or intraoperative instrumentation reduces operating costs. Independence from sterile instruments also permits multiple toric IOLs to be implanted in a single surgical day in centers where autoclave turnaround time is limiting.
Grayson A. Roumeliotis, MSc
Cindy M.L. Hutnik, MD, PhD, FRCSC
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