Management of astigmatism in cataract surgery patients is necessary for achieving the best refractive outcomes. Astigmatism correction in these patients can be performed preoperatively (femtosecond laser arcuate incisions [AIs]), intraoperatively (limbal or corneal relaxing incisions [LRIs/CRIs], and toric IOLs), or postoperatively (AIs, LRIs/CRIs, AK, and PRK/LASIK). Here are some recommendations for improving results with these procedures:
- Plan of action: It is important to establish a surgical plan for each patient.
- Accurate measurements: I like to obtain corneal curvature readings from multiple sources (i.e., keratometry, topography, interferometry, wavefront aberrometry). These values should be consistent. Sources of error or poor data include dry eye, epithelial basement membrane dystrophy, contact lens corneal warpage and corneal ectasias. These issues must be addressed before surgery, and contact lens wearers should be out of soft lenses for at least 2 weeks and rigid gas permeable lenses for a minimum of 4 weeks prior to keratometry measurements.
- Surgically induced astigmatism (SIA): Every surgeon should know the SIA created by his or her cataract incision, and this must be taken into account in the surgical plan.
- Posterior corneal astigmatism: The astigmatism from the posterior corneal surface is typically 0.4 D against-the-rule (ATR), and can be directly measured with devices such as the Zeimer Galilei and Oculus Pentacam.
- Zeimer Galilei: This device combines Scheimpflug and placido imaging to obtain pachymetry, elevation, curvature and wavefront values, and has anterior chamber ray-tracing capability to measure the posterior corneal surface.
- Oculus Pentacam: this instrument consists of a slit illumination system and a Scheimpflug camera that images the cornea, iris and lens, and provides topography and elevation data for the anterior and posterior cornea, pachymetry, anterior chamber depth and angle values, lens densitometry, and keratoconus detection.
The Baylor Toric IOL Nomogram, which was developed by Dr. Douglas Koch, recommends a 0.7 D shift in toric IOL threshold to compensate for posterior corneal astigmatism and leave the patient with 0.25-0.5 D residual with-the-rule (WTR) astigmatism.
- Desired correction: Various nomograms and online calculators are helpful for calculating the amount and orientation of astigmatism correction (i.e., www.lricalculator.com, www.acrysoftoriccalculator.com)
- Surgical alignment: Identifying the correct axis of astigmatism consists of two steps. 1. Marking the patient’s eye in an upright position prior to surgery (numerous instruments have been developed for this), and 2. Intraoperative confirmation with corneal axis markers, overlays (TrueVision 3D system, Carl Zeiss Meditec Callisto, SMI Surgical Guidance 5000, Micron Imaging Osher Overlay, Haag-Streit Osher Toric Alignment System), and/or intraoperative wavefront aberrometry (Wavetec ORA System).
Incisional techniques have been the most common method of surgically correcting astigmatism and include AK with an adjustable diamond knife, CRI/LRI with preset diamond or metal blades (typically 500-600 microns), and more recently femtosecond laser AIs. The main drawbacks of AK and CRI/LRI are limited and variable effect. While laser AIs are not surgeon dependent, all of these techniques weaken the cornea and are a contraindication in patients with corneal ectatic disorders. Other complications include over- or under-correction, postop discomfort, rarely infection and corneal perforation.
- AK: Corneal pachymetry, adjustable diamond AK knife and OZ markers are required. AK is a quick, low tech and low cost procedure that can be enhanced at the slit-lamp postoperatively. Because of the smaller optical zone (usually 7 mm), AK is more effective than CRIs/LRIs.
- CRIs/LRIs: Corneal pachymetry can be helpful, but most surgeons empirically treat at 500-600 microns with a preset blade. This technique is even easier, faster and less expensive to perform than AK, but as stated above, has less effect than AK because of the larger (10-11mm) optical zone. The disadvantages and complications of CRI/LRI are the same as those for AK.
- FS laser AIs: Many femtosecond laser devices are now approved for corneal arcuate incisions. These AIs are extremely precise, predictable and reproducible. They are easy to perform and can be titrated postoperatively at the slit-lamp. In addition, the ability to create intrastromal incisions eliminates postop pain and the risk of infection. However, this method of astigmatism correction has the disadvantages of increased cost and transient IOP rise.
Intraocular lens implants offer an excellent option for astigmatism correction in cataract surgery. They are precise, predictable, require no new surgeon skills or equipment, can correct a larger range of astigmatism, and are not contraindicated in patients with corneal ectasias, (although careful selection is necessary when using toric IOLs in patients with irregular astigmatism). The two main disadvantages of toric IOLs are the additional cost of the lens and the risk of misalignment or rotation. There is a 3.3% loss of effect per 1 degree of rotation from the correct axis. In rare instances when the IOL is off axis and needs to be repositioned, then the online Berdahl & Hardten astigmatism fix calculator (www.astigmatismfix.com) is a helpful tool for determining the amount of rotation needed to minimize residual astigmatism. In the United States there are currently two FDA approved toric IOLs.
- STAAR lens: this silicone plate haptic lens has a 6.0 mm optic, is available in two sizes (overall lengths of 10.8 mm or 11.2 mm) and two powers to correct 1.5 D and 2.25 D of astigmatism at the corneal plane.
- Alcon lens: this aspheric, acrylic IOL is available as a one-piece or three-piece lens with a 6 mm optic and 13 mm overall length. It is available in seven powers in 0.5 D increments to correct 1-4 D of astigmatism at the corneal plane.
The advantages of the Alcon toric IOL are: acrylic material, traditional loop haptic design, aspheric optics, larger range of cylinder correction, and less tendency to rotate postoperatively. I use this lens routinely to correct astigmatism and have had great success with it.
Photoablative procedures (i.e., PRK and LASIK) also can be used to treat astigmatism in cataract surgical patients. These techniques are very precise and predictable, and also offer the advantage of being able to correct residual spherical refractive errors. The main disadvantages are access to an excimer laser, cost, and the more involved postoperative care. Complications of PRK/LASIK include over- or under-correction, postop discomfort and/or dry eye, rarely infection, scarring, flap complications and ectasia.