Correcting astigmatism concurrently with cataract or clear lens replacement surgery is no longer a one-size-fits-all procedure. Creating a customized treatment plan depends on many factors, including the amount and location of the patient's astigmatism, previous surgical procedures, presence of corneal pathology, and age, as well as the cost of the procedure. At St. Luke's Cataract and Laser Institute in Tarpon Springs, Florida, the two options we have found to offer the best combination of efficacy, predictability, and relatively low cost are astigmatic keratotomy (AK) and toric IOLs. We use both strategies, either alone or combined, to enhance the patient's surgical outcome by altering the location, length, and type of the cataract incision.
The most important factor when constructing a surgical plan is the amount of pre-existing astigmatism present. Patients with low-to-moderate astigmatism require less intervention but greater precision. Because this population does not tolerate overcorrection, a small reduction of cylinder may not be beneficial if it is accompanied by an axis shift, particularly toward against-the-rule.
The STAAR toric IOL (STAAR Surgical, Monrovia, CA) is a good choice for patients with 1.5 to 3.0 D of astigmatism. It is also an excellent option for younger cataract or clear lens replacement patients who may require longer AK incisions to achieve the desired result. Although the current availability of toric add powers (the 2.0-D add corrects 1.4 D and the 3.5-D add corrects 2.4 D) limits correction, the surgeon can treat any residual refractive error with limbal relaxing incisions (LRIs), either at the time of surgery or postoperatively.
Toric IOLs do not increase the invasiveness of cataract surgery, and they preserve corneal integrity should further astigmatic techniques be required to enhance the postoperative result. One disadvantage of toric IOLs is the possibility of lens rotation in the early postoperative period. Should this occur, the lens can easily be rotated into position within the first 10 to 14 days postoperatively, before fixation occurs.1
We have incorporated three measures to address the problem of rotation. First, because the power is on the anterior surface of the lens, we insert it upside-down to improve its stability as suggested by Stephen Bylsma, MD, of Santa Maria, California. Second, we schedule our toric patients for follow-up 10 days postoperatively to evaluate the IOL's position. Finally, we counsel patients prior to surgery regarding the risk of rotation and prepare for the possibility of a secondary procedure. We typically avoid toric IOLs and prefer LRIs for patients with whom follow-up visits at our office are impossible.
We use LRIs both when the toric IOL is contraindicated and also in conjunction with toric lenses to customize the result. The incisions range from 4 to 10 mm in length and are 0.5 to 0.6 mm in depth at the periphery of the cornea, just anterior to the limbus at the steep meridian (Figure 1). LRIs may be used in conjunction with any type of cataract incision. Our choice of cataract incision depends on the type of lens we implant, as well as the location of the astigmatism. We initially developed this technique in conjunction with a modification of the Langerman three-plane hinge, self-sealing, clear-corneal incision (CCI). By lengthening the initial groove of the incision, we can incorporate the LRI into the cataract incision for against-the-rule cases. For oblique and with-the-rule astigmatism, we also use scleral-limbal-corneal (SLiC) cataract incisions. The SLiC is a single-plane incision that is made at the very edge of the limbus and catches part of the conjunctiva (Figure 2).
LRIs have definite advantages compared with corneal relaxing incisions (CRIs). They produce smoother corneal topographies, cause less corneal distortion or irregularity, and are quite effective for patients with low-to-moderate astigmatism (&Mac178; 3.0 D). In addition to being easier to perform than CRIs, patients experience less discomfort with LRIs, and their vision stabilizes more quickly. Due to the placement and length of the incision, LRIs are also more forgiving. Additionally, precise placement “on-axis” is not as critical because of the longer length. Variations of the depth of the incisions are also less hazardous with LRIs than CRIs. Postoperatively, overcorrections are rare and simple to correct. A 6-mm relaxing incision generally corrects about 1.0 D of astigmatism for the average 73-year-old patient. For qualifying patients with 1.5 to 2.0 D of astigmatism, we use paired LRIs. If the patient requires a correction of between 2.0 and 3.0 D, the LRIs can be extended to 8 mm. Additionally, we can correct or substantially reduce 4.0 D of astigmatism with a pair of 10- to 12-mm LRIs.
Although astigmatism greater than 5.0 D is rare, it can be visually disabling, affecting not only refractive error, but also the quality of vision. Patients with very high astigmatism often have associated corneal pathology, making the result much more unpredictable and the astigmatism more challenging to correct with incisional procedures alone. Before the availability of toric IOLs, correcting high astigmatism involved multiple LRIs and CRIs, which often caused corneal distortion and visual aberrations. Incorporating either one or two toric lenses into the surgical plan can significantly reduce corneal disturbance and provide for safer, less invasive surgery.
Since the maximum correction with a single toric lens is 2.4 D, patients with high astigmatism can only be partially corrected with an IOL and often leave with significant residual cylinder that continues to impact their quality of vision. By using the toric lens as a part of a complete astigmatism-management system, surgeons can customize the amount of cylinder correction for each patient and significantly reduce even high astigmatic errors while minimizing risk.
Toric lenses can be implanted piggyback-style to double the effective correction2 or combined with other astigmatism-reducing techniques for a wide range of correction possibilities. Wound management techniques, AK, and excimer laser surgery can also be used in conjunction with toric lenses. AK often easily corrects residual astigmatism when the pre-existing astigmatism has already been reduced to a manageable level.
Piggybacked Toric Lenses
In our early experience with toric lenses, we avoided implanting them in patients with very high and irregular astigmatism, such as in cases involving corneal pathology or previous surgery. We have since found that toric IOLs are often very useful for this group, and that they provide surprisingly good results, both objectively and subjectively. For patients with astigmatism greater than 5.0 D, two toric lenses may be implanted together, even if the patient would not otherwise require piggyback implantation for high hyperopia. The advantage of this technique is that it provides the ability to correct large amounts of cylinder without treating the cornea. If the patient requires further correction postoperatively, the surgeon can add LRIs to an essentially virgin cornea.
One concern with this strategy is the possibility of interlenticular opacification, although silicone lenses have not shown as frequent or as severe a tendency toward this complication as have acrylic lenses. We carefully polish the posterior capsule with a chalazion curette to reduce this risk. A second initial concern was the possibility of rotation, or even counter-rotation. However, implanting two silicone IOLs completely fills the capsular bag, which inhibits rotation. We also suture the IOLs back-to-back through the fixation holes, thereby stabilizing the piggyback system and completely avoiding the possibility of counter-rotation. To suture toric lenses together, we hold the edges of the lenses with a soft lens grabber. We secure the IOLs with a tying forceps and 9–0 nylon, with one throw and one knot through the fixation holes at each end (Figure 3). Because the cylindrical component is incorporated on the anterior surface of the toric lens, we suture the lenses back-to-back (rather than front-to-back) to decrease the possibility of dimpling the optic. If the optics become compressed, the effective cylinder correction could be reduced.
We have implanted piggyback toric lenses in a relatively small number of patients. We have encountered few postoperative rotation problems, and no counter-rotations have occurred. Suturing the IOLs together necessitates a longer cataract incision, which can actually be useful in correcting some of the astigmatism if the incision is placed at the steep axis. The impact of the cataract incision on the corneal astigmatism must be taken into account when calculating the amount of desired correction.
Toric IOLs Combined with Planned AK
Surgeons can combine relaxing incisions and toric IOLs to correct greater amounts of astigmatism than AK or the toric lens could alone.3 The advantage of using a toric lens over AK independently is a reduction in the amount of incisional surgery required. For many patients with high astigmatism, implanting a toric lens allows the use of limbal rather than corneal relaxing incisions. Even patients with astigmatism high enough to require corneal relaxing incisions may receive fewer and shorter incisions, thereby reducing the risk of corneal distortion.
Evaluating relaxing incisions combined with toric IOL implantation requires the assessment of the keratometric cylinder improvement, as well as final refractive outcome. In many patients with very high astigmatism (5.0 D or more), the refractive cylinder may not accurately reflect the severity of the astigmatism, because adding more cylinder does not diminish the distortion of the cornea or allow the patient to perceive improvement.
We take a conservative approach to astigmatism and prefer a long LRI versus a short CRI. Patients with high astigmatism often have contributing pathology, such as corneal scarring, keratoconus, Pellucid's marginal degeneration, or irregular astigmatism secondary to a previous procedure. In these cases, the surgical effect of AK is less predictable. For keratoconus patients, we have found that toric IOLs correct astigmatism more accurately and with less corneal disruption. When treating patients with corneal pathology such as Salzmann's nodules, Cogan's dystrophy, or superficial scarring, we obtain the best results by first addressing the irregular astigmatism with a superficial keratectomy or phototherapeutic keratectomy. After the cornea has re-epithialized, we can reassess residual astigmatism and correct it as needed in conjunction with cataract surgery. By using a variety of techniques, alone or combined, patients with high astigmatism can enjoy substantially improved quality of vision.
CALCULATING TORIC IOL POWER
We use the Holladay II software, produced by Jack T. Holladay, MD, to calculate IOL power. Initially, we applied our personalized effective lens position constant for the STAAR AA4203 plate lens (STAAR Surgical) to the toric lens. However, early in our experience using the toric IOL, we noticed that our patients' outcomes were consistently more myopic than their target refractions. After analyzing our data, we found that the toric effective lens position should be 5.1 mm rather than 5.5 mm. This difference equates to an adjustment of a 0.5-D reduction of the spherical power for the 2.0-D toric IOL, and a 1.0-D reduction of the spherical power for the 3.5-D toric IOL.
Piggyback toric IOLs present a unique situation, however, because we are concerned about the possibility of hyperopic shift from the compression of the optics. It is better to err on the side of myopia. We subtract half the amount, or 1.0 D total, when implanting two 3.5-D toric IOLs.
Relaxing incisions can also affect the power calculation. Not only do they flatten the meridian of the incision, they can also steepen in the opposite meridian. This effect is called coupling, and the ratio of flattening over steepening comprises the coupling ratio. A coupling ratio of 1:1 has no effect on the IOL power, but a coupling ratio greater than 1:1 causes a hyperopic shift and must be accounted for in the calculation by increasing the IOL power.
DETERMINING THE AXIS
When evaluating the preoperative axis of astigmatism, manual keratometry and corneal topography frequently differ due to the different ways in which the instruments acquire the readings. We identify the steep axis preoperatively with corneal topography and manual keratometry. In the event of a discrepancy, the surgical keratometer is the final arbiter. Using the surgical keratometer before, during, and after inserting the IOL ensures the most accurate placement of the toric lens, AK, and cataract incision. We have found the surgical keratometer to be a vital tool in the OR, and we believe it to be even more accurate than corneal topography. Precise identification of the steep axis is especially critical for patients receiving piggyback toric lenses, because the effect caused by a slightly off-axis placement doubles.
During surgery, we determine the correct orientation of the lens by aligning the axis marked on the lens with the steep axis identified using the surgical keratometer. With the surgical keratometer, we can gauge the actual response to treatment with relaxing incisions, allowing enhancement of the surgical plan intraoperatively. This ability to adapt the surgery to individual variation on the spot is especially important when we are attempting to correct high amounts of cylinder. By starting conservatively and increasing the length and number of incisions as warranted by the surgical keratometer reading, we can avoid large over- and undercorrections.James P. Gills, MD, is Clinical Professor of Ophthalmology at the University of South Florida and is Founder and Director of St. Luke's Cataract and Laser Institute in Tarpon Springs, Florida. Dr. Gills holds no financial interest in any product mentioned herein. He may be reached at (727) 938-2020, email@example.com.
Myra N. Cherchio, COMT, is Manager of the ultrasound department and Director of Clinical Research at St. Luke's Cataract and Laser Institute, Tarpon Springs, Florida. She holds no financial interest in any product mentioned herein. She may be reached at (727) 938-2020, firstname.lastname@example.org.
1. Patel EK, Ormonde S, Rosen PH, et al. Posterior intraocular lens rotation: a randomized comparison of plate and loop haptic implants. Ophthalmology 1999;106:2190-2195.
2. Gills JP, Van Der Karr MA. Correcting high astigmatism with piggyback toric intraocular lens implantation. J Cataract Refract Surg 2002;28(3):547-9.
3. Gills J, Van Der Karr M, Cherchio M. Combined toric intraocular lens implantation and relaxing incisions to reduce high preexisting astigmatism. J Cataract Refract Surg 2002;28(9):1585.